Drag reduction by compressible bubbles

Drag reduction by bubbles in stationary turbulent flows is sensitive to the compressibility of the bubbles. Without this dynamical effect the bubbles only renormalize the fluid density and viscosity, an effect that by itself can only lead to a small percentage of drag reduction. We show in this paper that the dynamics of bubbles and their effect on the compressibility of the mixture can lead to a much higher drag reduction.

Figure 1

Predicted values of drag reduction with $\alpha =2$ and different values of $A$. In a dashed line we reproduce the predictions of Legner’s model which suffer from an unphysical drag enhancement at $C=0$. For $A=0$ (rigid spheres) we find only increasing drag enhancement as a function of $C$. For small values of $A$ we have first a slight drag enhancement, and then modest drag reduction. For large values of $A$, associated with strong bubble volume variations, we find significant values of drag reduction.