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

Summary

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.

Abstract

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' 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.

Introduction

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 years^1,2,3,4, 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....

Protocol

1. System Preparation

1. 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.
2. Position the instrument carriage at the desired fetch.
   1. Mount the Pitot tube and position it at the center of the airflow part of the test section.
   2. Mount the wave gauge on a computer-controlled vertical stage to enable its static calibration.
3. 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 ....

Results

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 <η²>^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. .......

Discussion

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 wa.......

Materials

List of materials used in this article

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