Engineering
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Measurements of Waves in a Wind-wave Tank Under Steady and Time-varying Wind Forcing
Chapters
Summary February 13th, 2018
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.
Transcript
The overall goal of this experimental procedure is to enable accumulation of diverse reliable statistical parameters that characterize spatial and temporal evolution of water waves under study and time-dependent wind forcing. This method can clarify mechanism that govern excitation of water waves by wind. These mechanisms are not yet understood.
Proper modeling of wind input is essential for wave climate forecasting. The main advantage of this technique is that it is fully automated and computer-controlled. The experimental procedure can be easily adjusted for study wide variety of process and wind-wave generation.
The experiments take place in this facility, which consists of a closed-loop wind tunnel above a wave tank. This is a schematic of the facility and some of its features. The direction of airflow is indicated by arrows.
The tank is five meters long, 1/2 meters tall and 4/10 of a meter wide. The tank side walls and floor are made of glass supported by an aluminum frame. There is a flap and wave generator at one end of the tank.
The flap smoothly expands the airflow cross-section as it enters the test area and serves as a reference for distance measurements. A wave energy absorber is at the opposite end of the tank. This consists of porous packing materials used to form a beach.
There is a moveable carriage above the tank to hold instruments. For the experiment, have the tank filled to a depth of 20 centimeters and manually precision the carriage so the instruments will be at the desired fetch. Next, prepare some of the instruments for the experiment.
Have a Pitot tube that can reach the center of the airflow. In addition, have a wave gauge that is mounted on a computer-controlled vertical stage. Use the carriage to mount the Pitot tube at the center of the test section's airflow.
Then, mount the wave gauge on the carriage. Next, work with a laser slope gauges part. These consist of a laser diode, a Fresnel lens, a diffusive screen and a precision-sensing detector.
Align the laser diode at the desired fetch below the water tank. Direct the beam vertically upward. The beam should be about seven centimeters from the wave gauge.
After positioning the Fresnel lens and diffusive screen, use a level to check the horizontal and vertical alignment of the lens and screen before proceeding. Adjust the lens setting to the actual distance between the lens and the screen. To calibrate the wave gauge, set the vertical precision of the sensor.
The mean water level should be approximately in the middle of the sensing wire's length. Set the blower speed to one of the desired levels and allow the wind to blow steadily for two to three minutes. As the wind blows, monitor the wave gauge's signal on an oscilloscope.
Manually adjust the sensibility, gain and offset to ensure the voltage values corresponding to the highest crest and lowest trough are in the range of the AD converter. When done, shut down the blower and wait until the water surface is undisturbed. Repeat the calibration steps for each measuring location and maximum wind velocity expected in the experimental run.
Now, calibrate the laser slope gauge. For this, use an optical wedge prism on a transparent support. View the output of the precision-sensing detector on an oscilloscope to check the calibration.
Use different prisms to repeat this for several deflection angles. Once again, use a prism to deflect the vertical laser beam. In this case, deflect the beam to a radial position on the diffusive screen.
Do this with several prisms while maintaining a constant azimuthal angle. For each position, record the output of the precision-sensing detector on the oscilloscope. When done, collect data by moving the laser spot to multiple positions in the X direction keeping the Y-coordinate constant.
Repeat the same steps moving the Y-position and keeping the X-coordinate constant. Prepare the data acquisition system to record the desired quantities. To begin the experiment, make sure there is no wind.
The water surface should be smooth and undisturbed. Activate the blower and start data acquisition at the same time. For this experiment, apply steady forcing for 200 seconds.
Over time, the system will achieve a steady state. Use the oscilloscope to display data as it is being collected. This is an example of the surface slope data.
Downwind on the left, cross-wind on the right. At the conclusion, shut down the blower and continue to collect data to record the decaying wave field. Allow sufficient time for the water to return to an undisturbed state before continuing.
In each realization, three distinct stages can be identified. The initial wave growth with a start of the blower. The quasi-steady wind-wave field under constant wind forcing, and the decay of the waves following shutdown of the blower.
Numerous independent realizations of wave field under impulsive wind forcing were recorded. And sample averaging provides results as a function of time relative to the instant of blower initiation. Here are the data for the RMS values of the instantaneous surface elevation as a function of time.
The data were taken 120 centimeters from the flap and for three different wind velocities. The equilibrium quasi-steady-state characteristic wave amplitude increases with the wind velocity. Note the time to obtain the quasi-steady-state value does not appear to depend on the velocity.
Data for different fetch values and the same three wind velocities show similar behavior. These plots are for instantaneous surface elevations. 220 centimeters from the flap, and 340 centimeters from the flap.
Use the same data to explore the characteristic wave amplitudes for a constant target value of the wind velocity. This plot, for 6.5 meters per second is representative and demonstrates the equilibrium values of the RMS values of the surface elevation increases with fetch. The experiment also provides measurements of the RMS downwind and cross-wind components of the surface slope.
These data are for two values of the fetch, 120 centimeters and 340 centimeters. For a target wind velocity of 6.5 meters per second, and also for a target wind velocity of 10.5 meters per second. Turn off the blower and the characteristic wave amplitude together with the surface slope components decay over about one minute independent of the fetch.
Once mastered, a single realization using this procedure takes about eight minutes. The accumulation of 100 realizations requires about 13 hours of continuous experiment for a single wind target velocity and a fixed fetch. While attempting this procedure, it's important to remember that water evaporates and should be replenished from time to time.
After its development, this technique paved the way for quantitative investigation water waves and steady wind forcing. After watching the video, you should have a good understanding of how quantitative results on unsteady and inhomogeneous wind-wave fields can be obtained.
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