This work was supported by the Israel Science Foundation, grant # 306/15.

1. System Preparation
<ol>
	<li>Fill the tank with tap water up to a depth of about 20 cm to satisfy deep-water condition; clean the water surface of any contaminants that may affect the surface tension.</li>
	<li>Position the instrument carriage at the desired fetch.
	<ol>
		<li>Mount the Pitot tube and position it at the center of the airflow part of the test section.</li>
		<li>Mount the wave gauge on a computer-controlled vertical stage to enable its static calibration.</li>
	</ol>
	</li>
	<li>Position the LSG assembly at the desired fetch and at the lateral distance of about 7 cm from the wave gauge to eliminate interference of the wave gauge assembly with the optical path. 
	NOTE: The use of an opaque curtain is recommended to prevent the exposure of the PSD to ambient light, as well as to protect the environment from spurious reflections of the laser beam.
	<ol>
		<li>Align the laser positioned below the water tank so that the beam is directed vertically, and focus the beam.</li>
		<li>Position the fresnel lens within the test section as high as possible above the water surface to minimize the lens&#39;s disturbance of the air flow.</li>
		<li>Make sure that the deflected laser beam hits the lens in its central part under the extreme wind conditions planned in the experimental session.</li>
		<li>Mount the diffusive screen exactly at the focal plane of the lens, then check the horizontal and vertical alignment of both the lens and the screen.</li>
		<li>Make sure that any two parallel vertical laser beams hit the diffusive screen exactly at the center when the water surface is still. 
		NOTE: This can be tested using two identical lasers positioned at some distance from each other.</li>
		<li>Position the PSD making sure that the whole area of the diffusive screen is within the effective area of the detector. Perform focusing of the PSD lens by adjusting the lens settings to the actual distance between the lens and the screen.</li>
	</ol>
	</li>
</ol>
2. Calibration and Operation of Sensors
<ol>
	<li>Calibration of the wave gauge
	<ol>
		<li>Perform wave gauge calibration for each measuring location and each maximum wind velocity expected in the experimental run.
		<ol>
			<li>Set the vertical position of the sensor so that the mean water level is approximately in the middle of the sensing wires&#39; length.</li>
			<li>Set the blower speed to the desired value, and allow the wind to blow steadily for a sufficiently long time (2 - 3 min).</li>
			<li>Using an oscilloscope, manually adjust sensibility, gain, and offset of the wave gauge using the conditioner unit to ensure that the voltage values corresponding to the highest crest and the lowest trough expected in the wave field are within the range of the A/D converter (+/- 10 V).</li>
			<li>Shut down the blower for several minutes, until the water&#39;s surface becomes completely undisturbed.</li>
			<li>Verify that the submerged length is within expected maximum crest and trough values by moving the wave gauge vertically.</li>
			<li>Perform automatic calibration of the wave gauge using a custom-made routine in still water submerging the gauge at a number of specified depths and recording the mean voltage output during 5 s for each depth.</li>
			<li>Fit a quadratic calibration polynomial to the recorded data to obtain the dependence H(V), where H is the submergence depth (corresponding to the instantaneous surface elevation), as a function of the gauge output voltage V.</li>
			<li>Verify visually the quality of the fitted calibration polynomial (Figure 3).</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56480/56480fig3.jpg" /> 
Figure 3. Calibration curve of the wave gauge. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="2">
	<li>Calibration and adjustment of LSG
	<ol>
		<li>Verify the performance of the LSG after each displacement of the sensor assembly.
		<ol>
			<li>Using an optical wedge prism placed on a horizontal glass sheet, deviate the laser beam with respect to the optical axis simulating a known water surface slope.</li>
			<li>Sample the PSD outputs of the deviated laser beam spot at the diffusive screen using an oscilloscope or a custom-built data acquisition program.</li>
			<li>Compute the beam deflection angle and the slope from the measured coordinate laser beam spot coordinate; compare the result with the known wedge angle.</li>
			<li>Repeat the procedure for several deflection angles using one or more prisms. 
			NOTE: Wedge prisms with deflection angles ranging from 2.5&#176;&#160;to 17.5&#176;&#160;were used; if the test fails due to misalignment of PSD with the diffusive screen, adjust the PSD manually, to correct for misalignment. This procedure is performed manually using a 2D horizontal translation stage and a level and is very time consuming.</li>
		</ol>
		</li>
		<li>Verification of linearity of PSD and calibration procedure
		<ol>
			<li>Place an equally spaced grid that has been printed on a transparent sheet on the diffusive screen and orient it so that its axes, x and y, are aligned with the down and cross wind directions, respectively (Figure 4). 
			NOTE: The grid facilitates directing the laser beam to the desired locations on the diffusive screen conveniently and accurately either using a set of prisms or moving the laser below the diffusive screen in along-wind and crosswind directions.</li>
			<li>Using the set of prisms, deflect the vertical laser beam to obtain multiple radial positions of the laser beam spot on the diffusive screen while maintaining a constant azimuthal angle. 
			NOTE: Resolution of 1 cm and maximal radius of 7 cm is used for each of the 9 azimuthal angles.</li>
			<li>Move the laser spot to multiple positions in the x-direction, while keeping y coordinate constant, then change the direction of the movement to y, and keep x constant. 
			NOTE: Range and resolution used are similar to those from the previous section.</li>
			<li>Collect about 50 points on the grid in every calibration. 
			NOTE: The laser beam spot coordinates are acquired by PSD and evaluated using a standard two-channel oscilloscope connected to the PSD.
			<ol>
				<li>For each direction, use linear fit of the data to yield the calibration coefficients to convert the coordinates of the laser beam on the PSD sensor to the corresponding coordinates on the diffusive screen. 
				NOTE: An example of the PSD calibration is plotted in Figure 5 for a set of points taken along the centerline of the test section. The response of the sensor, and thus the calibration coefficients, is nearly identical in all directions when the axes of the diffusive screen and the sensor are aligned properly. The grid facilitates the calibration procedure, allowing an easy determination of the laser slope coordinates on the diffusive screen.</li>
			</ol>
			</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56480/56480fig4.jpg" /> 
Figure 4. The diffusive screen grid. The grid facilitates directing the laser beam to the desired locations on the diffusive screen conveniently and accurately, either using a set of prisms or moving the laser below the diffusive screen in along-wind and crosswind directions <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56480/56480fig5.jpg" /> 
Figure 5. PSD calibration curve. The figure demonstrates that the translation of the PSD output voltages to coordinates yields adequate results. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
3. Experimental Procedure and Data Acquisition
NOTE: See Supplementary Figure 1 for the user interface used in the following steps.
<ol>
	<li>Set blower frequency using a custom-built program user interface. 
	NOTE: Nearly stepwise increase of the wind velocity over initially undisturbed water surface was applied, followed by a steady airflow rate for a prescribed duration (120 s), and a nearly impulsive shut down of the blower.</li>
	<li>Determine the number of different steady wind flow rates and the required blower settings.</li>
	<li>Adjust the pressure transducer settings to the expected range of dynamic pressure variations detected by the Pitot tube.</li>
	<li>Make sure that at the beginning of each realization, there is no wind and the water surface is undisturbed (mirror smooth). Start data acquisition synchronously with operation of the blower.</li>
	<li>Record the instantaneous surface elevation, surface slope components in along- and crosswind directions, the Pitot tube output monitoring the mean wind velocity U, and the voltage variation from the blower controller at the prescribed sampling rate (300 Hz/channel were used). 
	NOTE: The voltage from wave-gauge acquired by the program is converted automatically to surface elevation using the calibration coefficients from the fit presented in Figure 3.</li>
	<li>Continue sampling for sufficient time to record the decaying wave field after the shutdown of the blower.</li>
	<li>Upon completion of the sampling, make sure that the automatic experimental procedure allows for sufficient time (depending on the system) to bring water surface to undisturbed condition prior to the initiation of the next run.</li>
	<li>Save all the recorded data for subsequent processing.</li>
	<li>Perform the prescribed number of realizations (usually 100 independent runs were found sufficient).</li>
	<li>Compute the ensemble-averaged parameters of the recorded data as a function of the time elapsed since the initiation of the blower.</li>
	<li>Repeat the whole procedure for the next setting of the blower corresponding to the selected target wind velocity in the test section.</li>
</ol>

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Authors have nothing to disclose.

The present experimental protocol is aimed at quantitative characterization of a wave field under unsteady wind forcing that evolves in time and space. Since wind-waves are essentially random and three-dimensional, and thus vary quickly in time and space, records of individual realizations of a growing wind-wave field under time-dependent wind forcing can only provide qualitative estimates of the governing wave parameters. To achieve the goal of this protocol and obtain statistically reliable time- and fetch-dependent wave characteristics, time-resolved ensemble averaging of numerous experimental runs with an identical pattern of wind variation in time has to be applied. Since accumulation of such detailed and extensive information for different target wind velocities and fetches is extremely time consuming, the experimental procedure has to be automated and sufficiently flexible to allow necessary adjustments for different wind forcing conditions. To the best of our knowledge, the experimental procedure described in this manuscript that is capable of providing diverse statistical wave field parameters under variable wind forcing with temporal and spatial resolution was largely unavailable until now.
Note that the duration of experiments required to carry out wind-wave measurements in a laboratory wave tank increases significantly with the size of the facility. This is partially because the duration of the wave growth stage increases with fetch (see Figure 6). The wave lengths and periods also grow with fetch17,21,23,24, thus for accumulating statistically representative data, the duration of the experiment in each run has to be long compared to the corresponding dominant wave period. Individual realizations at fetches significantly longer than those employed in the present study have thus to be lengthier. Even more important, the longest wavelengths excited by wind in any experimental facility are of the order of the characteristic length of the tank. The residual waves remaining in the tank after the shutdown of the wind in a larger facility thus require longer times for their decay. The interval between the consecutive runs in a bigger wind-wave tank required to return to undisturbed water surface therefore must be considerable longer than the six-minute interval employed in our experiments.
The general approach described above can be applied for a variety of steady and unsteady wind forcing conditions. In this manuscript, the temporal variation of wind forcing was selected with the goal of enabling the accumulation of a sufficient body of data to study unsteady wave field under effectively impulsive wind forcing and shutdown, as well as under steady wind forcing. To this end, care was taken to ensure that the water surface was calm and undisturbed prior to the initiation of each experimental run. In each iteration, the wind was nearly impulsively started by the blower, then the airflow rate remained constant for a sufficiently long time (2 min in the present experiments), after which the blower was shut down. This approach allows studying separately temporally evolving wind-wave fields growing from the initially calm water surface to the quasi-steady state at the given fetch and wind velocity, then the characteristic of waves under steady wind forcing, and finally the decay of waves once wind forcing is abruptly shut down.
The duration of each individual realization in the present study that contains the calm down period thus exceeds 8 min. Therefore, experiments in which data are accumulated for 100 independent runs at a single fetch and target wind velocity last nearly 15 hours (including the duration of the wave gauge calibration procedure). It is obvious that for such an endeavor to be successful, it is imperative that the entire experimental procedure can be carried out automatically, i.e. completely without human intervention.
It thus should be stressed that both the modest size of our facility and the fully automated measuring and calibration procedures were crucial for implementation of the adopted experimental approach. While detailed results on the wind-wave field evolution under unsteady forcing will be presented in the future elsewhere, it is apparent from Figure 6, Figure 7, and Figure 8 that previously unavailable information on fine details of the wind-wave evolution is accumulated in the present study. This information will be used to validate different theoretical models that deal with excitation of waves by wind1,2,3,4,5,24.

Since ancient times, it has been well known that waves on water surfaces are excited by wind. The current understanding of the physical mechanisms that govern this process is far from satisfactory. Numerous theories attempting to describe wind-wave generation were proposed over the years1,2,3,4, however their reliable experimental validation is not yet available. Measurements of random wind-waves in the ocean are extremely challenging due to unpredictable wind that may vary quickly in direction as well as in magnitude. Laboratory experiments have the advantage of controllable conditions that enable prolonged and repeatable measurements.
Under steady wind forcing in the laboratory environment, wind-waves evolve in space. Early laboratory experiments on waves under steady forcing performed decades ago were limited to instantaneous surface elevation measurements5,6,7,8. More recent studies also employed various optical techniques to measure instantaneous water surface inclination angle, such as LSG9,10. Those measurements allowed getting some limited qualitative information on the three-dimensional structure of wind-wave fields. When wind forcing is unstable, as it is in field experiments, additional complexity is introduced to the problem of water waves&#39; excitation by wind, since the statistical parameters of the resulting wave field vary not just in space but in time as well. The attempts made so far to describe wave evolution patterns qualitatively and quantitatively under time-dependent forcing were only partially successful11,12,13,14,15,16. The relative contribution of different plausible physical mechanisms that may lead to excitation and growth of waves due to wind action remains largely unknown.
Our experimental facility was designed with the purpose of enabling the accumulation of accurate and diverse statistical information on the variation of wind-wave field characteristics under either steady or unsteady wind forcing. Two major factors facilitated carrying out these detailed studies. First, the modest size of the facility results in relatively short characteristic evolution scales in time and space. Second, the whole experiment is fully controlled by a computer, thus enabling the performance of experimental runs under different experimental conditions automatically and practically without human intervention. These features of the experimental set-up are of crucial importance in performing experiments on waves excited from rest by impulsive wind.
Spatial growth of wind-waves under steady forcing has been studied in our facility for a range of wind velocities17. Results were compared with growth rate estimates based on the Miles18 theory as presented by Plant19. The comparison revealed that the experimental results differ notably from the theoretical predictions. Additional important parameters were also obtained in17, such as mean pressure drop in the test section, as well as the absolute values and phases of characteristic static pressure fluctuations. The shear stress at the air-water interface is essential for characterization of momentum and energy transfer between wind and waves17,19. Therefore, detailed measurements of the logarithmic boundary layer and the turbulent fluctuations in the air flow above water waves were performed at numerous fetches and wind velocities20. The values of the friction velocity u* at the air-water interface determined in this study were used to obtain dimensionless statistical parameters of the wind-waves measured in our facility21. These values were compared with the corresponding dimensionless parameters obtained in larger experimental installations and field experiments. It was demonstrated previously21 that with proper scaling, the important characteristics of the wind-wave field obtained in our small-scale facility do not differ significantly from the corresponding data accumulated in larger laboratory installations and open sea measurements. These parameters include spatial growth of the representative wave height and wave length, the shape of the frequency spectrum of the surface elevation, as well as the values of higher statistical moments.
The subsequent studies carried out in our facility22,23 showed that wind waves are essentially random and three-dimensional. To get a better insight into the 3D structure of wind waves, an attempt was made to perform quantitative time-dependent measurements of water surface elevation over an extended area using stereo video imaging22. Due to inadequate computer power available at present and processing algorithms that are not yet sufficiently effective, these attempts proved to be only partially successful. However, it was demonstrated that combined use of a conventional capacitance-type wave gauge and the LSG provides valuable information on the spatial structure of wind waves. Simultaneous application of both those instruments enables independent measurements with high temporal resolution of the instantaneous surface elevation and of the two components of the instantaneous surface slope23. These measurements allow estimation of both the dominant frequency and dominant wave length of the waves, as well as providing insight into the wave structure in the direction normal to the wind. A pitot tube, which can be moved vertically by a computer-controlled motor, complements the set of sensors and is used for measurements of wind velocity.
All those studies made clear that randomness and three-dimensionality of wind waves result in significant variability of the measured parameters even for steady wind forcing and a single measuring location. Thus, prolonged measurements with duration commensurate with the characteristic time scales of the measured wave field are needed to accumulate sufficient information for extracting reliable statistical quantities. To gain valuable physical insight into the mechanisms governing spatial variation of the wave field, it is imperative to carry out measurements at numerous locations and for as many values of the wind flow rate as possible in the test section. To achieve this goal, it is thus highly desirable to apply an automated experimental procedure.
Experiments on waves excited by unsteady wind forcing introduce an additional level of complexity. In such studies, it is imperative to relate the instantaneous measured parameters to the instantaneous level of the wind speed. Consider experiments on waves excited from rest by a nearly impulsive wind forcing as an important example. In this case, numerous independent measurements are needed of the wind-wave field evolving under the action of wind that varies in time following the same prescribed pattern24. Meaningful statistical parameters, expressed as a function of time elapsed since the initiation of air flow, are then calculated by averaging the data extracted from the accumulated ensemble of independent realizations. This undertaking may involve tens and hundreds of hours of continuous sampling. The total duration of experimental sessions required to accomplish such an ambitious task renders the whole approach unfeasible, unless the experiment is fully automated. No such fully computerized experimental procedure in wind-wave facilities has been developed until recently. That is among the main reasons for the lack of reliable statistical data on wind waves under unsteady forcing.
Since the facility used for the experiment is not constructed from commercially available, off-the-shelf hardware, a brief description of its main parts is provided here.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56480/56480fig1.jpg" /> 
Figure 1. Schematic (not to scale) view of the experimental facility. 1 - blower; 2 - inflow settling chamber; 3 - outflow settling chamber; 4 - silencer boxes; 5 - test section; with a 6 - beach; 7 - heat exchanger; 8 - honeycomb; 9 - nozzle; 10 - wavemaker; 11 - flap; 12 - instrument carriage; 13 - wave gauge driven by a stepper motor; 14 - Pitot tube driven by a stepper motor. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The experimental facility consists of a closed loop wind tunnel mounted over a wave tank (a schematic view is shown in Figure 1). The test section is 5 m long, 0.4 m wide, and 0.5 m deep. The sidewalls and floor are made of 6 mm thick glass plates and are enclosed within a frame made of aluminum profiles. A 40-cm long flap provides a smooth expansion of the airflow cross-section from the nozzle to the water surface. Wave energy absorbing beach made of porous packing material is located at the far end of the tank. A computer-controlled blower allows attaining mean air flow velocity in the test section up to 15 m/s.
The custom-made capacitance-type 100 mm-long wave gauge is made of anodized tantalum. 0.3 mm wire is mounted on a vertical stage driven by a PC-controlled step motor designed for wave gauge calibration. A Pitot tube with a diameter of 3 mm is used for measuring the dynamic pressure in the central airflow part of the test section.
The LSG, measuring instantaneous 2D water surface slope, is installed on a frame detached from the test section that can be positioned at any location along the tank (Figure 2). LSG consists of four main parts: a laser diode, a Fresnel lens, a diffusive screen, and a Position Sensing Detector (PSD) assembly. The laser diode generates a 650 nm (red), 200 mW focusable laser beam with diameter of about 0.5 mm. The 26.4 cm diameter Fresnel lens with focal length of 22.86 cm directs the incoming laser beam to the 25 x&#160;25 cm2 diffusive screen located in the back focal plane of the lens.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56480/56480fig2.jpg" /> 
Figure 2. Schematic view of the Laser Slope Gauge (LSG). 1 - laser diode; 2 - Fresnel lens; 3 - diffusive screen; 4 - Position Sensor Detector (PSD). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
This protocol describes the procedure that allows performing experiments in which numerous parameters characterizing unsteady waves are measured simultaneously under time-dependent wind forcing. The procedure can be adjusted to any desired dependence of wind velocity on time that can be attained in view of the technical limitations of the experimental facility. The present protocol describes specifically experiments in which in every realization, wind starts nearly impulsively over initially calm water. The steady wind forcing then lasts for long enough that the wind-wave field everywhere in the test section attains quasi-steady state. The wind eventually is shut down, again nearly impulsively. At all stages, multiple wave parameters are recorded. The procedure that allows computation of numerous statistically representative ensemble-averaged quantities characterizing the instantaneous local wind-wave field is novel, and was developed in the course of recent experiments carried out in our facility22,23,24.

<table>
	<tbody>
		<tr>
			<td>PSD</td>
			<td>THORLABS</td>
			<td>PDP90A</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser Diode</td>
			<td></td>
			<td></td>
			<td>any laser pointer &#8804; 200 mW</td>
		</tr>
		<tr>
			<td>Aspheric Fresnel Lens</td>
			<td>EDMUND OPTICS</td>
			<td>#46-390</td>
			<td>Diameter 10.4&#39;&#39;, Focal length 9&#39;&#39;</td>
		</tr>
		<tr>
			<td>Wave-gauge</td>
			<td></td>
			<td></td>
			<td>custom made</td>
		</tr>
		<tr>
			<td>Pressure Transducer</td>
			<td>MAMAC SYSTEMS</td>
			<td>PR-274-R2-VDC</td>
			<td></td>
		</tr>
		<tr>
			<td>Signal Conditioner</td>
			<td></td>
			<td></td>
			<td>custom made</td>
		</tr>
		<tr>
			<td>Diffusive screen</td>
			<td>EDMUND OPTICS</td>
			<td>#02-147</td>
			<td></td>
		</tr>
		<tr>
			<td>Water tank</td>
			<td></td>
			<td></td>
			<td>custome made</td>
		</tr>
		<tr>
			<td>A/D card PCI-6221</td>
			<td>National Instruments</td>
			<td>779066-01</td>
			<td></td>
		</tr>
		<tr>
			<td>Pitot tube</td>
			<td>KIMO Instruments</td>
			<td>12971</td>
			<td></td>
		</tr>
		<tr>
			<td>15&#176; Nom. VIS-NIR Coated, Wedge Prism</td>
			<td>EDMUND OPTICS</td>
			<td>#47-624</td>
			<td></td>
		</tr>
		<tr>
			<td>10&#176; Nom. VIS 0&#176; Coated, Wedge Prism</td>
			<td>EDMUND OPTICS</td>
			<td>#49-444</td>
			<td></td>
		</tr>
		<tr>
			<td>2.5&#176; Nom. Fused Silica Wedge Prism Uncoated</td>
			<td>EDMUND OPTICS</td>
			<td>#84-863</td>
			<td></td>
		</tr>
		<tr>
			<td>4&#176; Nom. Uncoated, Wedge Prism</td>
			<td>EDMUND OPTICS</td>
			<td>#43-650</td>
			<td></td>
		</tr>
		<tr>
			<td>5.0&#176; Nom. Fused Silica Wedge Prism Uncoated</td>
			<td>EDMUND OPTICS</td>
			<td>#84-865</td>
			<td></td>
		</tr>
		<tr>
			<td>LabView Full Development System</td>
			<td>National Instruments</td>
			<td>776670-35</td>
			<td></td>
		</tr>
	</tbody>
</table>

measurements of waves in a wind-wave tank under steady and time-varying wind forcing

This manuscript describes an experimental procedure that allows obtaining diverse quantitative information on temporal and spatial evolution of water waves excited by time-dependent and steady wind forcing. Capacitance-type wave gauge and Laser Slope Gauge (LSG) are used to measure instantaneous water surface elevation and two components of the instantaneous surface slope at a number of locations along the test section of a wind-wave facility. The computer-controlled blower provides airflow over the water in the tank whose rate can vary in time. In the present experiments, the wind speed in the test section initially increases quickly from rest to the set value. It is then kept constant for the prescribed duration; finally, the airflow is shut down. At the beginning of each experimental run, the water surface is calm and there is no wind. Operation of the blower is initiated simultaneously with the acquisition of data provided by all sensors by a computer; data acquisition continues until the waves in the tank fully decay. Multiple independent runs performed under identical forcing conditions allow determining statistically reliable ensemble-averaged characteristic parameters that quantitatively describe wind-waves&#39; variation in time for the initial development stage as a function of fetch. The procedure also allows characterizing the spatial evolution of the wave field under steady wind forcing, as well as decay of waves in time, once the wind is shut down, as a function of fetch.

The representative ensemble-averaged results are plotted in Figure 6, Figure 7, and Figure 8. The variation of the RMS values of the instantaneous surface elevation &#60;&#951;2&#62;1/2 that characterizes the amplitude of random wind waves as presented in Figure 6 as a function of time elapsed since initiation of the blower. Results are presented for 3 distances from the wavemaker, x, and for three target wind velocities, U.
For fixed fetch x, the equilibrium quasi-steady state characteristic wave amplitudes increase with the wind velocity U; however, the duration needed to attain the quasi steady value of &#60;&#951;2&#62;1/2 after initiation of the blower does not seem to depend strongly on U at any given x. The equilibrium values of the characteristic wave amplitudes for a constant target value of the wind forcing U increase with fetch. Note also that variation in the rate of change of &#60;&#951;2&#62;1/2 is identifiable in each curve plotted in Figure 6, clearly suggesting that distinct stages exist in the wind-waves growth process. The ensemble-averaged RMS values of the downwind and crosswind slope components, &#60;&#951;x2&#62;1/2 and &#60;&#951;y2&#62;1/2, are plotted in Figure 7 for two fetches and two values of the wind velocity U.
It is evident from the comparison of Figure 6 and Figure 7 that the characteristic time scales of variation of both surface slope components are notably shorter than the corresponding scales of the surface elevation variation. The quasi-steady values of &#60;&#951;x2&#62;1/2 and &#60;&#951;y2&#62;1/2 are of the same order of magnitude, although the characteristic slopes in the crosswind direction are smaller than the slopes in the along-wind direction. These results indicate that wind-waves are short-crested and three-dimensional. The characteristic slope values in both directions under quasi-steady wind forcing seem to be essentially independent of fetch x, but increase with the wind velocity U. A closer look at the temporal variation of the two slope components for fixed x and U reveals that the initial increase in &#60;&#951;x2&#62;1/2 is consistently and notably faster than that of &#60;&#951;y2&#62;1/2. Thus, during the very early stage of the growth of the initial ripples that appear on the calm water surface with the activation of wind, they can be seen as approximately two-dimensional. This stage lasts only for a fraction of one second; nevertheless, it is important to emphasize that the essential three-dimensionality of the wave field develops with a certain delay.
The behavior of the wave field after the shutdown of the blower is shown in Figure 8. The waves remaining in the tank decayed fast, effectively vanishing after approximately 1 min.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56480/56480fig6.jpg" /> 
Figure 6. Temporal variation of the RMS of the surface elevation. The figure demonstrates that the time scales of variation of the characteristic wave height represented by &#60;&#951;2&#62;1/2 depend on the target wind velocity U and on the fetch x. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56480/56480fig7.jpg" /> 
Figure 7. Variation with time of the RMS of the downwind/cross-wind surface slope components. The ensemble-averaged RMS values of the downwind and crosswind slope components, &#60;&#951;x2&#62;1/2 and &#60;&#951;y2&#62;1/2, are plotted here. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56480/56480fig8.jpg" /> 
Figure 8. Decay of the wind-wave filed after the shutdown of the blower. The ensemble-averaged RMS values of the downwind and crosswind slope components, &#60;&#951;x2&#62;1/2 and &#60;&#951;y2&#62;1/2, are plotted in Figure 7 for two fetches and two values of the wind velocity U. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/56480fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary Figure 1: Custom-built software user-interface for data acquisition. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56480/Print_screen_Labview_transient_case.jpg">Please click here to download this file.</a>

Watch this Scientific Journal Video about Measurements of Waves in a Wind-wave Tank Under Steady and Time-varying Wind Forcing at JoVE.com

Measurements of Waves in a Wind-wave Tank Under Steady and Time-varying Wind Forcing

This manuscript describes a fully computer-controlled procedure that allows obtaining reliable statistical parameters from experiments of water waves excited by steady and unsteady wind forcing in a small-scale facility.

This manuscript describes an experimental procedure that allows obtaining diverse quantitative information on temporal and spatial evolution of water ...

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8.6K 조회수.  Tel-Aviv University. This manuscript describes a fully computer-controlled procedure that allows obtaining reliable statistical parameters from experiments of water waves excited by steady and unsteady wind forcing in a small-scale facility.

비디오: Measurements of Waves in a Wind-wave Tank Under Steady and Time-varying Wind Forcing

Measuring Material Microstructure Under Flow Using 1-2 Plane Flow-Small Angle Neutron Scattering

A new small-angle neutron scattering (SANS) sample environment optimized for studying the microstructure of complex fluids under simple shear flow is presented. The SANS shear cell consists of a concentric cylinder Couette geometry that is sealed and rotating about a horizontal axis so that the vorticity direction of the flow field is aligned with the neutron beam enabling scattering from the 1-2 plane of shear (velocity-velocity gradient, respectively). This approach is an advance over previous shear cell sample environments as there is a strong coupling between the bulk rheology and microstructural features in the 1-2 plane of shear. Flow-instabilities, such as shear banding, can also be studied by spatially resolved measurements. This is accomplished in this sample environment by using a narrow aperture for the neutron beam and scanning along the velocity gradient direction. Time resolved experiments, such as flow start-ups and large amplitude oscillatory shear flow are also possible by synchronization of the shear motion and time-resolved detection of scattered neutrons. Representative results using the methods outlined here demonstrate the useful nature of spatial resolution for measuring the microstructure of a wormlike micelle solution that exhibits shear banding, a phenomenon that can only be investigated by resolving the structure along the velocity gradient direction. Finally, potential improvements to the current design are discussed along with suggestions for supplementary experiments as motivation for future experiments on a broad range of complex fluids in a variety of shear motions.

A shear cell is developed for small-angle neutron scattering measurements in the velocity-velocity gradient plane of shear and is used to characterize complex fluids. Spatially resolved measurements in the velocity gradient direction are possible for studying shear-banding materials. Applications include investigations of colloidal dispersions, polymer solutions, and self-assembled structures.

1-2 평면 흐름 - 작은 각 중성자 산란을 이용하여 재료 미세 조직에서 유량을 측정

A new small-angle neutron scattering (SANS) sample environment optimized for studying the microstructure of complex fluids under simple shear flow is ...

연구

공학

Preparation and Friction Force Microscopy Measurements of Immiscible, Opposing Polymer Brushes

Solvated polymer brushes are well known to lubricate high-pressure contacts, because they can sustain a positive normal load while maintaining low friction at the interface. Nevertheless, these systems can be sensitive to wear due to interdigitation of the opposing brushes. In a recent publication, we have shown via molecular dynamics simulations and atomic force microscopy experiments, that using an immiscible polymer brush system terminating the substrate and the slider surfaces, respectively, can eliminate such interdigitation. As a consequence, wear in the contacts is reduced. Moreover, the friction force is two orders of magnitude lower compared to traditional miscible polymer brush systems. This newly proposed system therefore holds great potential for application in industry. Here, the methodology to construct an immiscible polymer brush system of two different brushes each solvated by their own preferred solvent is presented. The procedure how to graft poly(N-isopropylacrylamide) (PNIPAM) from a flat surface and poly(methyl methacrylate) (PMMA) from an atomic force microscopy (AFM) colloidal probe is described. PNIPAM is solvated in water and PMMA in acetophenone. Via friction force AFM measurements, it is shown that the friction for this system is indeed reduced by two orders of magnitude compared to the miscible system of PMMA on PMMA solvated in acetophenone.

The methodology to perform friction force microscopy experiments for contacting brushes is presented: Two polymer brushes that are grafted from (a) substrates and (b) colloidal probes are slid to show that, by using two contacting immiscible brush systems, friction in sliding contacts is reduced compared to miscible brush systems.

고분자 브러쉬를 반대 섞이지의 준비 및 마찰 힘 현미경 측정,

Solvated polymer brushes are well known to lubricate high-pressure contacts, because they can sustain a positive normal load while maintaining low ...

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.

A protocol for conducting thermopower wave experiments is presented. The synthesis of hybrid composites of a chemical fuel and micro/nanostructured material, manufacturing of a thermopower wave generator, and methods for measuring the corresponding physical phenomena are described.

열 전력 (thermopower) 파도의 연구에서 화학 연료 및 다중 벽 탄소 나노 튜브의 하이브리드 복합 재료의 제조 및 평가

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs ...

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can potentially contribute to the exposure of biota and humans. The diffusive gradients in thin films technique is introduced here for in-situ measurements of Pu bioavailability and speciation. A diffusion cell constructed for laboratory experiments with Pu and the newly developed protocol make it possible to simulate the environmental behavior of Pu in model solutions of various chemical compositions. Adjustment of the oxidation states to Pu(IV) and Pu(V) described in this protocol is essential in order to investigate the complex redox chemistry of plutonium in the environment. The calibration of this technique and the results obtained in the laboratory experiments enable to develop a specific DGT device for in-situ Pu measurements in freshwaters. Accelerator-based mass-spectrometry measurements of Pu accumulated by DGTs in a karst spring allowed determining the bioavailability of Pu in a mineral freshwater environment. Application of this protocol for Pu measurements using DGT devices has a large potential to improve our understanding of the speciation and the biological transfer of Pu in aquatic ecosystems.

The technique of diffusive gradients in thin films (DGT) is proposed for speciation studies of plutonium. This protocol describes diffusion experiments probing the behavior of Pu(IV) and Pu(V) in presence of organic matter. DGTs deployed in a karstic spring allow assessment of the bioavailability of Pu.

박막의 확산을 사용하여 환경 플루토늄의 분화 및 생체 이용률 측정

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can ...

Experimental Protocol to Investigate Particle Aerosolization of a Product Under Abrasion and Under Environmental Weathering

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering, which is a fundamental element to the approach of nanosafety-by-design of nanostructured products for their durable development. This approach is basically a preemptive one in which the focus is put on minimizing the emission of engineered nanomaterials' aerosols during the usage phase of the product's life cycle. This can be attained by altering its material properties during its design phase without compromising with any of its added benefits. In this article, an experimental protocol is presented to investigate the nanosafety-by-design of three commercial nanostructured products with respect to their mechanical solicitation and environmental weathering. The means chosen for applying the mechanical solicitation is an abrasion process and for the environmental weathering, it is an accelerated UV exposure in the presence of humidity and heat. The eventual emission of engineered nanomaterials is studied in terms of their number concentration, size distribution, morphology and chemical composition. The purpose of the protocol is to study the emission for test samples and experimental conditions which are corresponding to real life situations. It was found that the application of the mechanical stresses alone emits the engineered nanomaterials' aerosols in which the engineered nanomaterial is always embedded inside the product matrix, thus, a representative product element. In such a case, the emitted aerosols comprise of both nanoparticles as well as microparticles. But if the mechanical stresses are coupled with the environmental weathering, the experimental protocol reveals then the eventual deterioration of the product, after a certain weathering duration, may lead to the emission of the free engineered nanomaterial aerosols too.

In this article, an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental weathering is presented. Results on the emission of engineered nanomaterials, in the form of aerosols are presented. The specific experimental set-up is described in detail.

실험 프로토콜 마모에서 환경 풍화에서 제품의 입자 에어로졸을 조사하기

The present article presents an experimental protocol to investigate particle aerosolization of a product under abrasion and under environmental ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

A protocol to detect and automate mode locking in a pre-adjusted nonlinear polarization rotation fiber laser is presented. The detection of a sudden change in the output polarization state when mode locking occurs is used to command the alignment of an intra-cavity polarization controller in order to find mode-locking conditions.

출력 편광 측정을 통해 비선형 편광 회전 광섬유 레이저의 모드 잠금의 자동화

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

제조 및 Acoustofluidic 장치의 작동은 입자의 초점 쉬스에 대한 대량 음향 서 파도 지원

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

We describe the use of micro-thermocouples to estimate local temperature gradients in steady laminar boundary layer diffusion flames. By extension of the Reynolds Analogy, local temperature gradients can be further used to estimate the local mass burning rates and heat fluxes in such flames with high accuracy.

정상 층류 경계층 확산 불길에 지역 열 플럭스와 불타는 환율의 평​​가에 대한 실험 방법론

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex ...

In Situ Visualization of the Phase Behavior of Oil Samples Under Refinery Process Conditions

To help address production issues in refineries caused by the fouling of process units and lines, we have developed a setup as well as a method to visualize the behavior of petroleum samples under process conditions. The experimental setup relies on a custom-built micro-reactor fitted with a sapphire window at the bottom, which is placed over the objective of an inverted microscope equipped with a cross-polarizer module. Using reflection microscopy enables the visualization of opaque samples, such as petroleum vacuum residues, or asphaltenes. The combination of the sapphire window from the micro-reactor with the cross-polarizer module of the microscope on the light path allows high-contrast imaging of isotropic and anisotropic media. While observations are carried out, the micro-reactor can be heated to the temperature range of cracking reactions (up to 450 &#176;C), can be subjected to H2 pressure relevant to hydroconversion reactions (up to 16 MPa), and can stir the sample by magnetic coupling. 
Observations are typically carried out by taking snapshots of the sample under cross-polarized light at regular time intervals. Image analyses may not only provide information on the temperature, pressure, and reactive conditions yielding phase separation, but may also give an estimate of the evolution of the chemical (absorption/reflection spectra) and physical (refractive index) properties of the sample before the onset of phase separation.

In Situ Visualization of the Phase Behavior of Oil Samples Under Refinery Process Conditions

This article describes a setup and method for the in situ visualization of oil samples under a variety of temperature and pressure conditions that aim to emulate refining and upgrading processes. It is primarily used for studying isotropic and anisotropic media involved in the fouling behavior of petroleum feeds.

정유 공정 조건에서 오일 샘플의 위상 행동 제자리 시각화에서

To help address production issues in refineries caused by the fouling of process units and lines, we have developed a setup as well as a method to ...

Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of gamma rays created when neutrons interact with soil elements. The main parts of the INS system are a pulsed neutron generator, NaI(Tl) gamma detectors, split electronics to separate gamma spectra due to INS and thermo-neutron capture (TNC) processes, and software for gamma spectra acquisition and data processing. This method has several advantages over other methods in that it is a non-destructive in situ method that measures the average carbon content in large soil volumes, is negligibly impacted by local sharp changes in soil carbon, and can be used in stationary or scanning modes. The result of the INS method is the carbon content from a site with a footprint of ~2.5 - 3 m2 in the stationary regime, or the average carbon content of the traversed area in the scanning regime. The measurement range of the current INS system is &#62;1.5 carbon weight % (standard deviation &#177; 0.3 w%) in the upper 10 cm soil layer for a 1 hmeasurement.

Here, we present the protocol for in situ measurement of soil carbon using the neutron-gamma technique for single point measurements (static mode) or field averages (scanning mode). We also describe system construction and elaborate data treatment procedures.

측정 토양 탄소 고정 및 검색 모드에서 중성자-감마선 분석에 의해

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of ...