Effect of Bubbles on Wall Shear in a Horizontal Channel

The effect of bubbles on the wall shear in a horizontal channel is examined by direct numerical simulations. The volume flux is kept constant by dynamically adjusting the pressure gradient and gravity is adjusted to control the average distance of the bubbles from the top wall at steady state. The shear stress at the wall is determined by the velocity gradient at the wall and in turbulent flows the mixing of the low velocity fluid at the wall and the high velocity fluid away from the wall generally changes the velocity profile. Near the wall, mixing in turbulent flow takes place predominantly by vortices oriented in the streamwise direction, resulting from realignment and stretching of spanwise vorticity due to the mean shear. Injecting bubbles into a turbulent boundary layer is known to modify the mixing by disrupting the streamwise vorticity and thus change the velocity profile. Here, the channel Reynolds number is 145 and gravity was chosen to result in a Froude number comparable to that of non-planning vessels. Several bubbles of a diameter of approximately 50 wall units, at We=0.467 and 0.236 were studied, allowing us to examine the effect of bubble deformability on the structure of the flow and the resulting changes in wall stress.