Studies on impingement effects of low density jets on surfaces-determination of shear force

This paper presents the details of the Laser Reflection Method (LRM) that has been successfully implemented for determination of shear stress due to impingement of low-density flows on surfaces. For thin oil film moving under the action of aerodynamic boundary layer the shear stress at the air-oil interface is equal to the shear stress between the surface and air. Direct and dynamic measurements of the oil film slope is measured by determining the position change of the reflected beam of a coherent light from the top surface of the oil film, which undergoes thinning due to shear stress. From the oil film slope history, direct calculation of the shear stress is done using a three-point formula. The shear stress values for low-density flows in the transition regime are thus obtained using LRM and the measured values of shear show fair agreement with those obtained by other methods. Results of the flow field measurements done in low density jets by using Sankowich impact pressure probe are also presented in the paper. The pressure profiles obtained show the characteristic features of low density jets such as shocks at higher pressure. ratios and rapid expansion of isentropic core near the nozzle exit plane.