TURBULENCE REDUCTION IN A BOUNDARY-LAYER BY A LOCAL SPANWISE OSCILLATING SURFACE

This Letter describes an experimental investigation of the response of a turbulent boundary layer on a flat plate to a local spanwise oscill ation of the wall, with a nondimensional frequency f+ varying between 0.0033 and 0.0166. The investigation has been carried out for a wall m otion amplitude DELTAz+ = 160. The three components of the turbulence intensities and the Reynolds stress prove to be a decreasing function of the frequency. This reduction affects almost the whole boundary lay er in a cross section located at the middle of the oscillating wall. T he mean streamwise velocity UBAR is reduced throughout the region y+ < 30. The velocity profiles exhibit a well-defined log region when plot ted against y, nondimensionalized with the friction velocity of the un perturbed boundary layer. The weighted probability density functions o f u and upsilon exhibit an increase in intensities of wallward motion related to changes in the structure of the oscillatory flow. The fine structure of the turbulence is also affected by the spanwise oscillati on as shown by the reduction of Taylor's microscale.