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EFFECTS OF FREE-STREAM TURBULENCE ON TURBULENT BOUNDARY LAYER SUBJECTED TO ADVERSE PRESSURE GRADIENT

Tomoya Houra
Department of Electronical and Mechanical Engineering, Department of Environmental Technology, Nagoya Institute of Technology, Gokiso-cho, Showa-ки, Nagoya 466-8555, JAPAN

Yasutaka Nagano
Nagoya Industrial Science Research Institute; Department of Mechanical Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan

Аннотация

We have investigated the effects of free-stream turbulence on a turbulent boundary layer developing on a flat plate with and without streamwise pressure gradient. An active type turbulence grid located in the settling chamber is used to generate the free-stream turbulence; its intensity is about 1%, and the length scale is about 1.0 times the boundary layer thickness. Despite the slight increase in the streamwise turbulence intensity at the outer edge of the boundary layer, the intensity becomes much larger in the middle of the layer, and these tendencies are more evident in an adverse-pressure-gradient flow. The absolute value of the third-order moment vuv in the outer region increases remarkably in the adverse-pressure-gradient flow with free-stream turbulence. Thus, the wallward transfer observed in the adverse-pressure-gradient flow is enhanced by imposing free-stream turbulence. Moreover the local wavelet spectra reveal that a strong interaction, consisting of the near-wall turbulence with high frequency and free-stream turbulence with low frequency, occurs intermittently.