Experimental Investigation On Wind-induced Shallow Water Wave And Flow

The wind-induced wave in lakes are caused by the wind friction shearstress and the wind pressure on the back of waves. Still water surface isperturbed by the wind stress which stimulates the growth of the wind wave.Due to the viscous force, the moving of the surface water makes themovement of lower water along the wind direction, thus the water swell inthe lake. Because of the mass conservation, there existing a compensate flownear the bottom in the contrary direction of the wind, which results in thegeneration of the wind-induced flow on vertical direction. In China, a lot oflakes belong shallow-water lakes. It is under the action of shallow waterwind-induced wave and flow that the movement of the shallow lake wateris complicated. The sludge, contaminant, etc. suspend with the turbulenceof the shallow wind-induced surface wave. And the resuspended sludge andcontaminant transport with the wind-induced flow. It can be concluded thatthe shallow water wind-induced wave and flow are the direct force of theresuspension and transport. In order to investigate the hydrodynamicproperty, the contaminant transportation characteristic and the interaction oflake water and river flowing into lake in Lake Taihu, Nanjing HydraulicResearch Institute is building a3-D physical Taihu Model.In this thesis, a physical model is used to investigate the shallow waterwind-induced wave and flow. The main work of the thesis is as follows: 1) A height-flexible artificial bottom is adopted to simulate the shallow-water effect of lakes in the Shanghai Jiaotong University wind-inducedwave and flow flume. The characteristics of wind-induced waves in shallowwater with different water depths are studied, the frequency spectrum, waveheight, wave length and swell identity are investigated in the serialexperiments. The results of the wave length, wave height and swell obtainedin the experiments are compared with the empirical equation, indicating thatthe wind-induced wave produced in the experiments is apparently affectedby the shallow-water effect. Moreover there is a critical wind velocity, towhich the variation characteristics of the wave frequency spectrum and thewave height are related.2) In the wind-induced wave and flow flume, the tracer particle is usedto measure the surface velocity of the wind-induced flow in different featurewind velocity and water depth. The measured surface velocitiy of the wind-induced flow linearly grows with the increase of wind velocity, anddecreases with the increase of water depth. The Particle Image Velocimetry(PIV) and Acoustic Doppler Velocimetry (ADV) are used to measure thevertical average velocity distribution in the given measuring section. Fromthe comparison between the measured results (PIV and ADV) and thetheoretical result, it can be concluded that the measured result andtheoretical result are basically identical, the wind direction and the directionof wind-induced surface flow are the same. The more close to the watersurface, the greater the wind-induced flow velocity is. And the movementof the lower water is contrary to the wind direction, which is considered asa compensate flow. Near the bottom, the wind-induced flow velocitydecreases rapidly and is close to zero on the bottom. And there is a symbolicshift near the1/3water depth, which is regarded as the wind-induced flowvelocity shear layer. The wind-induced wave and flow flume can be used tosimulate the wind-induced flow.3) There are two methods to simulate the shallow water wind-inducedwave. One is to increase the surface roughness, the other is to increase thebottom roughness. In the first method, the wave maker is used to generate the surface wave which increases the surface roughness. In the secondmethod, a roughness equipment is installed on the bottom of flume toincrease the bottom roughness. The methods are applied to decrease theenergy consumption of the fan, to meet the requirements of the swell in theflume experiment, which can give a direction to the construction of “TaihuModel”.4) According to the swell rate changes with the water depth and featurewind velocity in the flume experiment of the wind-induced wave, this thesisgives the relationship curve of the water depth and feature wind velocity.The curve shows the relationship that: when the water depth is fixed, theswell rate grows with the increasing wind velocity; when wind velocity isfixed, the swell rate falls off with the increasing water depth. And therelationship curve of the mean wave height (fetch length F=140m) and thewater depth is given in this thesis. These two relationship curves are of greatsignificance and practical value in forecasting the mean wave height and theswell in “Taihu” Model.