Experiments on the electromagnetic control of turbulence

Abstract

The possibility of controlling the wall shear stress in a turbulent boundary layer, by applying a Lorentz force jxB perpendicular to the wall, was demonstrated in the paper by Nosenchuck and Brown. (1) Large reductions in the Reynolds stress were found. While the measurements and flow visualization showed a substantial effect, the results were from an early experiment and raised a number of questions. Two particularly important issues to be resolved were: firstly, the role that 3-dimensionality had played due to the tile arrangement of electrodes and magnetic poles; and secondly, the non-dimensional scaling to much larger free stream velocities and to boundary layers of different thickness. The present experiments are being pursued in an effort to more clearly illuminate the underlying physics of a body force acting on near wall turbulence and to explore the effects of scaling. For these experiments, a special purpose water tunnel has been built. The tunnel has been arranged to produce a two-dimenionsal channel flow with wall injection of an electrolyte or a fluid of different density from water. The particular advantages of a fully developed channel flow are that the shear stress is a linear function of the distance between the walls and measurements of the velocity profile and pressure drop along the channel can be used in principal to infer the eddy viscosity. Thus, the direct effects of buoyancy and/or Lorentz force acting on the injected flow near one wall can be measured. In the absence of buoyancy or Lorentz force effects Preston tubes can be accurately calibrated against the shear stress calculated from the measured pressure drop. A channel flow also has the advantage that a wide range of maximum flow velocities and Reynolds numbers can be achieved so that the scaling of the phenomena can be explored. In these experiments, we have focused on what Nosenchuck and Brown called type I electromagnetic turbulence control in which the effect is produced by a gradient in conductivity so that the action of the Lorentz force is analogous to a buoyancy force with a corresponding density gradient. The facility can also be used for type II electromagnetic turbulence control in which the conductivity is uniform but the Lorentz force has a spatial and temporal variation in a way such that the vorticity produced interacts non-linearly with the turbulence production in the near wall region.