Effects of Free-stream Turbulence on a Turbulent Boundary Layer (1st Report, Effects of Grid Turbulence on an Isothermal Turbulent Boundary Layer)

Abstract

Effects of external turbulence in a free stream on an isothermal turbulent boundary layer over a flat boundary are investigated by means of laboratory experiments using a wind tunnel. A turbulence-generating grid is installed upstream of the turbulent boundary layer to generate nearly isotropic, external turbulence having 1.3-2.4% turbulence levels. Instantaneous velocities, U and V, are measured using a hot wire anemometry with an X-probe. The results show that the log-law for the time-averaged mean velocity holds for turbulent boundary layer with external grid turbulence. However, turbulence intensities and the Reynolds stress are strongly suppressed by the external grid turbulence under the present conditions. The suppressions are caused by the decrease of production terms in the respective transport equations. The power spectra of u, v and the cospectra of u and v show that almost all scale motions are suppressed by the external grid turbulence.