DIRECT NUMERICAL SIMULATION OF DRAG REDUCTION IN TURBULENT BOUNDARY LAYERS CONTROLLED BY INCLINED BLOWING AND SUCKING

Reynolds shear stress (RSS) is an important source of high frictional resistance in wall turbulence. A theory suggests that the distribution of Reynolds shear stress in turbulent flow fields can be weakened through negative Reynolds stress (net positive) on walls in order to achieve drag reduction. However, integrals of the Reynolds−averaged Navier−Stokes equations indicate that the positive Reynolds stress (net negative) generated on a wall has a negative contribution to the skin friction coefficient of the wall. In this study, a series of inclined slits are being set up at the bottom of the control region for the turbulent boundary layer flow. Positive or negative wall Reynolds stress is generated by periodic blowing and sucking achieved by this device. Direct numerical simulation method is used to validate and explore the drag reduction theory described above. The turbulent boundary flow model used here has a Reynolds number (based on the outer flow velocity and momentum loss thickness) from 300 to 860. Through multiple sets of numerical simulations, the influences of jet strength and frequency on skin friction coefficient have been explored and the effects of positive and negative wall-generated Reynolds stress on the flow have been compared. Results show that the drag reduction rate associated with positive wall-generated Reynolds stress can reach 3.26, which is higher than that associated with negative Reynolds stress. It is concluded that the positive wall-generated Reynolds stress has a negative contribution to the skin friction, while the negative Reynolds stress has a positive contribution to the skin friction. Based on the gain-loss ratio, this control strategy is not able to obtain a net energy gain.