Turbulent boundary layer control utilizing the Lorentz force

Direct Numerical Simulations (DNS) of a turbulent channel flow at low Reynolds number (Re_τ = 100, 200, 400) are performed to examine the effectiveness of using the Lorentz force to reduce skin-friction drag and pressure fluctuations. A Lorentz force is created by embedding electrodes and permanent magnets in the flat surface over which the flow passes. Both open-loop and closed-loop control schemes are examined. It is found that skin-friction drag can be reduced by approximately 40% if a temporally oscillating spanwise Lorentz force is applied to an Re_τ = 100 channel flow. However, the power to generate the required Lorentz force is an order of magnitude larger than the power saved due to the reduced drag. Simulations are carried out at higher Reynolds number (Re_τ = 200, 400) to determine whether efficiency, defined as the ratio of the power saved to the power used, improves with increasing Reynolds number. It is found that the efficiency decreases with increasing Reynolds number.; An idealized wall-normal Lorentz force is effected by detecting the near-wall turbulent events responsible for high-skin friction. It is found that the skin-friction drag can be significantly reduced with a greater efficiency than that produced by the spanwise open-loop control approach. Extending the idealized case to a realistic one, a closed-loop control scheme using a three-dimensional Lorentz force distribution is effected on a turbulent channel flow by activating an alternating array of tiles in a checkerboard pattern. No statistically significant drag reduction is achieved using this approach.; Control of outer-layer structures is implemented by using a three-dimensional Lorentz force. This approach is based on the reports of other researchers that a greater efficiency may be attained by controlling structures found in the outer portion of a turbulent boundary layer. Using both idealized and realistic force distributions yields no skin-friction drag reduction.; The effect of the Lorentz force on wall-pressure fluctuations is analyzed. Significant reductions in the root-mean-square values of pressure fluctuations at the control wall are found for divergence-free Lorentz force distributions that reduced both skin-friction drag and all three root-mean-square velocity components near the control wall. No reduction is observed for those cases that involve nondivergence-free force distributions.