(181u) Turbulence Structures In Poiseuille-Couette Flow That Lead to Drag Reduction

Turbulent structures near the wall in Poiseuille and Poiseuille – Couette flows are studied by employing direct numerical simulation. The channel walls in Poiseuille – Couette flow are moving with a specified velocity in opposite directions simulating, thus, a specified slip velocity at the channel walls. Such slip can be observed in cases of flow over superhydrophobic surfaces. The goal is to study the effect of the slip velocity on the flow structures close to the walls, and to understand the mechanism by which drag reduction is obtained in cases of drag reduction due to superhydrophobic surfaces.

The simulations were conducted in a computational box of dimensions 4ïh x 2h x 2ïh in x, y, z, where the half channel height in viscous wall units was h = 300. The number of grid points in x, y and z directions was 256, 129, 256, respectively. The time step used was 0.2 in dimensionless units. A pseudo-spectral fractional step method has been employed to solve the Navier-Stokes equations.

Poiseuille flow is equivalent to flow with no slip boundary condition. In Poiseuille – Couette flow, both walls were moving in opposite directions with finite velocity.  This value was increased from 0 to 2 in increments of 0.1 after 500 iterations. At slip values of interest, stationary state of the flow must first be obtained before any other calculations. This was done by taking an average over 8000 viscous wall units at the same slip velocity. The velocity profile, the turbulent kinetic energy production, higher order statistics and correlation coefficients will be presented and discussed in order to characterize the mechanism by which drag reduction is obtained.