Experimental investigation of a salt water turbulent boundary layer modified by an applied streamwise magnetohydrodynamic body force

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Abstract
Single-component velocity field measurements, mean and fluctuating wall shear stress measurements, and photographic flow visualizations have been made of a magnetohydrodynamic (MHD) body-force modified turbulent boundary layer. The turbulent boundary layer flowed over a flat plate in salt water at zero pressure gradient; the MHD force was created by the interaction of a permanent magnetic field and an applied electric field from a magnet/electrode array integral to the surface of the plate. A MHD force, when applied to an electroconducting fluid and acting in a streamwise direction, can generate a near-wall jet, decreasing the boundary layer thickness and suppressing the intensity of the turbulent fluctuations across the boundary layer. At very high interactions, the force causes an increase in mean wall shear and in turbulence; in the zero free-stream velocity limit, the force acts as a pump. An increase in local skin friction, however, is offset by a grain in thrust due to the force. At moderate interactions, mean quantities are unaffected, but fluctuating wall shear stress and turbulence intensity are suppressed by up to 30% of their unperturbed values across the lower part of the boundary layer. At very low interactions, effects are seen only near the wall. An interaction parameter is derived that characterizes these regimes. The effects likely occur because the MHD force pumps high momentum fluid along the wall, disrupting the liftup of shear-generated wall vorticity. This jet effect is associated with increased convection of turbulent kinetic energy by the mean flow. With the force directed axial upstream, turbulence amplification is seen, along with a reduced mean velocity.