Actuation and control of a turbulent channel flow using Lorentz forces

Abstract

Results concerning the design and fabrication of electromagnetic actuators, and their application to affect the wall shear stress in a fully turbulent channel flow are discussed. The actuators utilize a Lorentz force to induce fluid motion due to the interaction between a magnetic field and a current density. The actuators are comprised of spanwise-aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction. Problems commonly associated with electromagnetic flow control-electrolysis, bubble formation, and electrode corrosion are substantially reduced, and in most cases eliminated by the use of a conductive polymer coating. The actuators generate velocity profiles with a penetration depth into the flow of approximately 1 mm ͑set by the electrode/magnet pitch͒ and maximum velocities of approximately 4 cm/s. The actuation velocities are found to scale linearly with forcing voltage and frequency. The electrical to mechanical efficiency is found to be very low (Ϸ10 Ϫ4 ), primarily due to the limitations on the magnetic field strength and the low conductivity of the working fluid ͑saltwater͒. The actuators are used in a fully turbulent low Reynolds number channel flow and their effect on the turbulent skin friction is measured using a direct measurement of drag. Maximum drag reductions of approximately 10% are measured when the flow is forced using a spanwise oscillating Lorentz force. A scaling argument for the optimal amplitude of the current density is developed. The efficiency of this method for drag reduction, and its application at higher Reynolds numbers is also discussed.

Turbulent boundary-layer control with plasma actuators

“…The Lorentz forcing was used for turbulent boundary-layer control in many DNS studies [26,27]. There was also a lot of experimental work using electromagnetic actuators [28][29][30] to complement these studies. Since the Lorentz forcing requires electrically conductive media such as sea water, it is not easily applicable to aeronautics.…”

Experiments on rapidly rotating turbulent flows

“…In this sense the present setup to investigate 3D rotating turbulence resembles the oscillating grid experiment by Hopfinger et al 3 The turbulence generation by electromagnetic forcing has previously been proven useful in various experiments on twodimensional ͑2D͒ turbulence, in a few experiments on Lagrangian aspects of nonrotating 3D turbulence, 4,5 and in actuation and control of boundary layers in turbulent channel flow. [6][7][8] Stereoscopic particle image velocimetry ͑SPIV͒ measurements have been performed to obtain twodimensional three-component ͑2D3C͒ velocity fields. Unlike conventional PIV measurements, which give 2D2C velocity fields, no stereoscopic PIV measurements have been reported for rotating turbulence.…”

Plasma Virtual Actuators for Flow Control

“…The spanwise travelling waves had a profound effect on the nearwall turbulence: they amalgamated the sublayer streaks into wide ribbons of low-speed fluid, which caused significant disruption to the turbulence regeneration cycle. Experimental investigations of spanwise travelling waves with electromagnetic actuation soon followed [18] [19], where Xu and Choi [19] observed the formation of the wide ribbons of low-speed fluid within the viscous sublayer and measured a 30% reduction in skin-friction drag.…”