This paper describes an experimental study on the friction drag reduction on the compliant wall. In the fluid dynamics, the research on the turbulent friction drag reduction is one of the important subjects. The use of the compliant wall has the possibilities of the friction drag reduction. Many researchers have investigated the effects of the compliant wall since 1960’s. However, the past studies are focused on those effects such as dependency of the material properties and flow conditions. Those effects are intimately related with the turbulent flow fields. In order to clarify the mechanism of the drag reduction, the coherent structures near the wall surface must be investigated. The purpose of the present study is to investigate the vortical semi-periodic motions over the solid and compliant walls in detail. The experiments are conducted at Re = 5.0 × 105 based on unit length and the free stream velocity using the NPL type wind tunnel. The compliant wall is made of silicone resin with Young’s modulus E = 2.83 MPa. The boundary layer flows are measured using the single and X-array hot wire probe. The VITA and four quadrant analyses are applied to investigate the bursting phenomena in the turbulent flow structures. The experimental results show that the skin friction drag over the compliant wall is smaller than that on the solid wall about 15%. Further the bursting frequencies over the compliant wall are greater than those over the solid wall. The four quadrant analysis is also applied to investigate the turbulent coherent motions. These results show that the ejection event over the compliant wall is increased. The contribution rate of the ejection event to the Reynolds stress over the compliant wall is greater than that of the sweep event. The increment of the bursting frequency is consistent with that of the ejection event. The friction drag decreases because the velocity near the wall is decelerated when the contribution rates in second and third quadrants to the Reynolds stress increase. These tendencies are also confirmed over the compliant wall with the various thicknesses. The present results show that the friction drag reduction over the compliant wall is related with the increment of contribution rate of the ejection event in the bursting phenomena.
Effect of Bed Load Motion on Bursting Phenomena in Open Channel Flows.