APPLICATION OF AN ELECTROCHEMICAL METHOD TO MEASURE THE WALL SHEAR STRESS IN A TRANSPIRED TURBULENT BOUNDARY LAYER

Abstract

An electrochemical method was applied to measure the local wall shear stress in the transpired turbulent boundary layer.
In the case of injection or suction it was determined by comparing the wall shear stresses obtained by a meter with those by the momentum balance that the wall shear stress meter cannot directly measure the real value at the permeable wall due to the impermeable region around the imbedded meter.
A correction method to obtain the actual shear stress at the permeable wall from the measured one at the meter is derived from an assumption of an inner boundary layer developing over this impermeable region. The validity of this method has been confirmed by measuring the variation of wall shear stress over the impermeable wall, and also by comparing the corrected shear stresses with those obtained by the momentum balance.
The friction factors calculated from the corrected shear stresses were in good agreement with those by Simpson and Baker, and with the analytical prediction by Rubesin in the case of injection. The simple film theory is also applicable to the estimation of the variation of friction factors in a small range of injection or suction rate (|2v_w/C_f0u_∞|<1).