CHARACTERISTICS OF TURBULENT BOUNDARY LAYER OVER SILICON GEL SURFACE AND BEHAVIOR OF SURFACE DISPLACEMENT

ABSTRACT This paper describes a study on interaction between compliant coatings and turbulent boundary layer flows. The use of compliant coatings has been known as the drag reduction means of the skin-friction and was studied by many researchers for 50 years. However the mechanism of drag reduction is not clarified. In this study, experimental study was conducted using compliant walls made of silicone gel to investigate the influence of compliant coating on the flow field. Boundary layer characteristics and displacement of the walls were simultaneously measured using hot-wire anemometer and a laser displacement sensor. The experimental results showed the suppression of the turbulent intensity turbulent skin-friction drag reduction over the compliant wall. Characteristics of the turbulent energy cascade process over the solid and compliant walls are investigated using the Wavelet transform and the FFT. The compliant coatings suppress the turbulent energy production and enhance the turbulent energy dissipation compared to those of the solid wall. The cross correlation between the displacement of the compliant wall and the velocity fluctuation in the turbulent boundary layer are investigated in detail. The phase difference angle of the motion of the compliant wall to the velocity fluctuation is about 60 degrees. This wall motion has the roll to suppress the turbulent vortex motion near the wall. The drag reduction mechanism on the compliant coating can be explained as philosophy based on the Vcontrol algorithm