Effects of external body forcing on the flow physics in turbulent channel flow

Direct numerical simulations are carried out to investigate the controlled effects of external body forcing in a turbulent channel flow. The transitional Reynolds number studied is Re_τ = 85, and the analysis is performed by distinguishing new asymmetric states using a 20% threshold from the standard deviation of their symmetric counterparts. The numerical results reveal a maximum drag reduction of 25% in the total wall shear stress of the controlled channel with a penetration depth of 0.03 and an oscillation period of 10. From statistical probability, it is determined that not only does the shear stress decrease but becomes narrower, which indicates improved accuracy. During the symmetric states of the uncontrolled case, the wall shear is found to be close to the mean value, while the controlled channel exhibits shear stress lower than the mean. In addition, results show that the average bulk velocity for the controlled channel is higher near the uncontrolled wall, rather than the wall where force is applied. The total number of symmetric states for the uncontrolled and controlled channel was found to be 10% and 3%, respectively. These essential observations enhance a fundamental understanding of the drag reduction process, and the dependence of Reynolds number is further discussed.