Study On The Wind Drag Coefficient Of Shallow Lakes Considering Wind-wave-flow Characteristics

In recent decades,many lakes are facing water pollution and ecological degradation problems.Wave and flow induced by wind are the two main dynamic factors for shallow lakes,which have decisive influences on the lake water environment and water ecological pattern.Due to the limited fetch characteristics of lakes,"wind-wave-flow" dynamic system constituted by wind,wind-induced wave and wind-induced flow gradually evolves with the increase of wind blowing distance.Particularly,for shallow lakes,wind-wave-flow system is easily restricted by lakebed,which presents a more complex gas-liquid-solid three phase dynamic system.Great achievements on marine wind-wave-flow characteristics have been reported,while,due to the differences between oceans and lakes,related research achievements of oceans cannot be applied directly to the lakes.In this paper,the wind-wave-flow characteristics and wind drag coefficient expression of water area with limited blowing range and water depth are systematically studied by means of theoretical analysis,wind tunnel experiment,field monitoring and numerical simulation.The major conclusions have been achieved:(1)The simulation measurement system of wave flow in shallow lake is constructed and optimized.Aiming to overcome the difficulty of data acquisition in the wind tunnel tank,"automatic filtering method of wave surface interference spot" and "recognition and tracking method of wave surface floating particle" were proposed based on image recognition technology,which can obtain more detailed and abundant experimental data and effectively improved the efficiency of data processing.(2)The evolution characteristics of the wind dynamics upon land surface and water surface are confirmed,and the relationship between the friction wind speed,relative friction wind speed and the characteristic roughness of different underlying surface(smooth surface,rough surface and water surface),wind speed and blowing distance are systematically analyzed.Air flow above the wave has the "same frequency,different phase" mutual feed pattern with waves.The wind dominant frequency is negatively correlated with wind speed and blowing range,and the height of the wave-affected area is positively correlated with wind speed and blowing range.The total wind turbulence energy(TWTE)and the wind turbulence energy directly affected by wave(WTEAW)attenuate as a power function with the elevation,and the attenuation rate is positively correlated with the wind speed.However,WTEAW attenuation rate with dimensionless height is independent of wind speed and positively correlated with the blowing distance.(3)The instantaneous flow field characteristics of wind-induced flow indicate that the overall velocity decreases with the increase of water depth,and the flow direction at the crest is consistent with the wave propagation direction,while the flow direction at the trough is opposite to the wave propagation direction.The average surface flow velocity,which positively correlated with the wind speed,gradually decreases with the increase of the blowing distance and tends to be stable.Ratio between the surface flow velocity and the wind speed after stability is between 1.1%and 1.7%.Ratio of slip flow velocity to surface flow velocity is negatively correlated with the wind speed and positively correlated with the water depth.The water body in flume presents a typical wind-induced double-layer circulation structure.The position of the velocity inflection point moves down with the increase of the wind speed,and moves down with the increase of the blowing distance as well,and finally stabilizes at 5/6 times of the water depth.(4)The relationship between wave height,wave period,nonlinear parameters,wind speed,blowing distance,water depth and wind direction is clarified.We found that when the wind speed of Lake Taihu is less than 3m/s,waves in the lake are basically deep-water waves.With the increase of wind speed,deep water wave and finite water depth wave coexist in the lake.Wind wave in Lake Taihu tends to saturate with the increase of wind speed and blowing range,and the dominant frequency is about 0.3 5Hz after saturation.The dominant frequency of wind waves decreases nonlinearly with the increase of wind speed and blowing distance,and the energy of the dominant frequency increases nonlinearly with the increase of wind speed,blowing range and water depth.While the relationship between spectral width and nonlinear parameter varies with wind speed and blowing distance.Surface flow can promote the wave celerity and wavelength,while the wave nonlinearity can suppress the wave celerity and wavelength.The empirical relation of wave celerity and wave number and the logarithmic empirical relation of dominant frequency wave number and nonlinear parameter are established.(5)Two dimensionless numbers,(?)and u10F/vw,were constructed to characterize the combined action of water and gas in finite waters.The expression of wind drag coefficient considering wind speed,blowing distance and water depth is proposed for the first time.It improves the traditional wind stress coefficient considering only wind speed,overcomes its limitation in limited blowing distance and water depth,and maintains the overall consistency with the existing formula.The sensitivity coefficients of wind speed,blowing distance and water depth to wind drag coefficient are 0.92?0.22 and 0.14,respectively.The characteristic analysis of the expression shows that it is also applicable to the ocean environment.(6)The characteristics of wind-induced flow in Lake Taihu is simulated using a threedimensional hydrodynamic mathematical model.The results show that by considering the influence of water depth and blowing distance,the modeling results are more consistent with the field measurements and experimental results.Cyclone-free wind can also produce swirling flows in the lakes.It is also found that there are three main types of vertical flow velocity distributions:unidirectional flow in whole layer,double-layer circulation and more complex three-layer flow.The relationship between the characteristics of wind-generated flow and the pattern of lake water environment is discussed.