The Response of the Boundary Layer to Weak Forcing

The aim of this research is to study experimentally how turbulent kinetic energy dissipation rate-TKED or ε and temperature dissipation rate- TD or ε_θ change at the surface boundary layer and streamwise direction of the tank equipped with the heater grid and the turbulence-generating grid. The value of TD increasing by approaching to the water surface because of the constant surface heat flux. We have observed three surface boundary layers for TKED which are separated by depths of z/d_t= 0.07 and 0.21, z shows the depth of the collecting data and d_t is the total water depth. The grid-generated turbulence has the key role under the depth of z/d_t= 0.21. The convection created by air-water heat transfer has the dominant role on the TKED at depth of z/d_t> 0.21. From the depth of z/d_t= 0.07 up to the water surface, called wave boundary layer depth here, the surface wave increases the TKED in addition to convection and the TKED gets a uniform value for wavelength, λ > 0.035 in this region. The relation of the wave boundary layer depth, dw ~ 0.035 or z/d_t= 0.07, and the wavelength, λ, is 0.53λ <d_w< 1.0λ for apparent wind velocity range of 0.125 m/s to 0.0183 m/s. The surface perturbation created by grids decreases the minimum wind velocity for the wind-wave generation to the 0.066 m/s in our experiment. The power-law exponent decay of the tank for TD, m=1.25, and TKED, 1.01<n<1.18, is observed for different mean velocities.