Abstract
Effects of external turbulence in a free stream on a convective boundary layer over a flat boundary are investigated by means of laboratory experiments using a wind tunnel. Turbulence-generating grid is installed upstream of turbulent boundary layer to generate nearly isotropic, external turbulence. The bottom wall of the test section is heated and thus convective boundary layer with unstable thermal stratification (i.e., negative vertical temperature gradient) is developed. Instantaneous velocities, U and V, and instantaneous temperature T are measured simultaneously using a hot wire anemometry with X-probe and a constant current resistance thermometer. The results show that rms value of velocity fluctuations and Reynolds stress are strongly suppressed by the external grid turbulence as in the isothermal case. In the convective boundary layer, rms value of temperature fluctuation and vertical turbulent heat flux are also strongly suppressed by the grid turbulence. The power spectra of u, v and θ and their cospectra show that almost all scale motions are suppressed by the grid turbulence as in the isothermal case.
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