The objective of this paper is to examine the effectiveness of wall oscillation as a control scheme of Drag reduction. Two flow configurations are considered: constant flow rate and constant mean pressure gradient. The Navier-Stokes equations are solved using Fourier-Chebyshev spectral methods and the oscillation in sinusoidal form is enforced on the walls through boundary conditions for the spanwise and streamwise (for the case of inclination cycle) velocity components. Results include the effects of oscillation frequency, amplitude, oscillation orientation, and peak wall speed on Drag reduction at a Reynolds number of 180 based on wall-shear velocity and channel half-width as well as the Reynolds number dependency in both flow configurations. Drag reduction as a function of peak wall speed is compared with both experimental and numerical data and the agreement is good in the trend and in the quantity. Comparison between these two flow configurations in the transient response to the sudden start of wall oscillation, turbulence statistics, and instantaneous flow fields is detailed and differences are clearly shown. This analysis and comparison have allowed some light to be shed on the way that oscillations interact with wall turbulence.

Research has shown when a rectangular cylinder is located near a flat plate, the flat plate skin friction coefficient changes as a function of the rectangular aspect ratio, gap height between the rectangular and flat plate, distance of rectangular from the flat plate leading edge, and speed of free stream. However, there is no comprehensive experimental study on the comparison of the results of the flat plate skin friction coefficient for all the interactions between effective variables in the presence and absence of the obstacle. On the other hand, testing all possible combinations of effective variables will not be reasonable. In this paper, maximum and minimum ratios of the flat plate skin friction coefficients with and without the rectangular cylinder were determined using robust Taguchi design. Design of experiments method was applied for decreasing the number of experiments without losing the required information in the first step. Then, experimentation was done in a wind tunnel, the maximum speed of which was 13 m/s. Finally, the flat plate skin friction coefficient was optimized using Taguchi method and Minitab software. Results showed that presence of the rectangular cylinder near the flat plate decreased the average skin friction coefficient of the flat plate for all the possible combinations of the effective variables. Additionally, maximum value of the flat plate skin friction reduction was about 40%.

A series of subsonic wind tunnel tests was conducted on an ogive-cylinder-flare configuration at zero angle of attack to study the effects of small helical protuberances on viscous Drag reduction behavior. The experiments have been carried out on a smooth model and two wire-wrapped models with different spacing between the helical riblet rings, known as pitch length. In the present experiments, the pitch length to model diameter ratio for the wire-wrapped models were 0.5 and 1.0 and the velocity profiles at 19 streetwise locations as well as the pressure distribution on the model at the same positions have been measured and compared for the smooth model and the two wire-wrapped ones. The results show about 10% reduction in the local skin friction Drag for the wire-wrapped model in comparison with the smooth one. The investigations also showed that depending on the boundary layer thickness, optimum values could be found for the riblet diameter and its pitch length for which, the maximum reduction would be achieved in the local friction Drag.