Dual Cylindrical Wave Laser-doppler Method for Measurement of Skin Friction in Fluid Flow

The objective of the present work is to overcome the inadequacies of conventional skin friction measuring methods in terms of temporal and spatial resolution. An instrument has been developed for measurement of the instantaneous velocity gradient in a boundary layer at a solid surface, from which the instantaneous wall shear stress can be deduced. The principle of the device is akin to that of dual beam Laser Doppler Anemometer. Instead of crossing two laser beams to produce the measuring volume, the present technique uses interference between two closely spaced cylindrical waves emanating from the solid surface. The interference fringes which constitute the measuring volume appear to be radiating from a point on surface. Keywords: Skin friction; Fluid flow; Lasers; Boundary layers; Doppler method; Wall shear stress; Laser beams; Cylindrical waves.