Modification of turbulent structure in channel flows by microbubble injection close to the wall
An investigation of turbulent structure modification of a boundary layer for a fully developed channel flow by microbubble injection close to the upper wall was carried out using Particle Image Velocimetry (PIV). Two-dimensional velocity components in an x-y plane at Reynolds number of 5128 based on the half height of the channel and bulk velocity were measured. Microbubbles, with an average diameter of 30 ??m were produced by electrolysis and injected in the buffer layer. Different values of the void fraction were attained and used to evaluate the effects of the presence of microbubbles and their concentration within the boundary layer. A reduction in drag was observed due to the injection of microbubbles. Drag reduction augments as the value of the void fraction increases. Furthermore, increases in both the non-dimensional values of streamwise and normal turbulent intensities, normalized by the friction velocity were observed with the void fraction growth. A gradual decrease in the Reynolds shear stresses was achieved as the void fraction increases. This effect is due to a ??decorrelation?? or ??decoupling?? between the streamwise and normal fluctuating velocities. Modifications in the length and time scales due to the presence of microbubbles were detected by calculating two-point correlation coefficients in one and two dimensions and the autocorrelation coefficient at various locations within the measurement zone. Streamline length and time scales were increased. On the contrary, the normal length and time scales were decreased. The vorticity and strain rate values decreased with the injection of microbubbles. Turbulent energy production was also decreased within the boundary layer. Quadrant analysis was used to find out the contribution of the u?? and v?? fluctuating velocity components to the Reynolds stress. The presence of microbubbles reduces the contribution to the Reynolds stresses by Q4 events (sweeps), which are responsible for the production of skin friction. Vortical structure detection in the measurement area was pursued. The structure with and without the microbubble injection is compared. In this study the presence of microbubbles within the boundary layer has produced several modifications in the flow structure as well as reduction in the drag.