Numerical Simulation and Analysis of Turbulence Control Using Lorentz Forces

This study focuses on the response of a boundary layer to perturbation by periodic electromagnetic forces in seawater. The time-dependent Navier-Stokes equations are solved coupled with an unsteady Lorentz force actuator. The Lorentz force actuator is comprised of a pair of magnets and a pair of electrodes and produces a volumetric body force. An electromagnetic solver provides the spatial variation of the electric and magnetic fields, which are then cycled in time. First, both simulations and experiments of an isolated actuator in an environment with no mean flow are conducted. A pressurized water vessel contains a uniform water/electrolyte solution. Particle image velocimetry and laser sheet flow visualization are used to obtain snapshots of the induced flow for different orientations of the laser sheet. Simulations are also performed in a static water/electrolyte solution with the same current levels as in the experiments. Flow is induced upward at the center of the actuator, even though a downward Lorentz force is applied there. Both the simulations and the experiments show the Lorentz force actuator creates a complex three-dimensional interaction resulting in an upward flow over the actuator. Two wall jets directed toward one another are created by the forces over the electrodes that impinge in the center of the actuator and fluid is pushed up and away from the wall. In terms of vorticity production, the actuator exhibits local maxima in the curl of the Lorentz force away from the wall in a region above the magnets and the electrodes. Comparison between simulation and experiment is remarkably good. Simulations of the interaction of an array of time-dependent actuators and both laminar and turbulent boundary layers are also performed. Rotational structures in a thickening boundary layer are observed. Regions of increased and reduced skin friction are found. Aft of the acuators, the skin friction is reduced. The simulations capture many essential features of the flowfield that have been experimentally observed by Nosenchuck, including rotational flow structures, frequency dependence, significant reductions in local skin friction, and effect of magnet orientation. A Lorentz force distribution, in the form of actuator disks, has also been analyzed through numerical simulation of the Navier-Stokes equations. It is designed to capture the essential features of previous experimental investigations of electromagnetic turbulence control. The Lorentz force introduces pairs of vorticity sources of opposite sign in the boundary layer. The sources are phased such that resonant reinforcement occurs as the structures convect downstream. Drag reduction has been achieved in a turbulent boundary layer over a variety of flow conditions and Lorentz force actuator designs. Using flow visualization, roll-cell like structures are formed that travel at a convective speed of 60% of the edge velocity. These structures are seen to move negative vorticity away from the wall on their upstream edges resulting in a net drag reduction.