Visualization of Surface Flow over a Blade of Rotating Machinery by Oil-Film Technique and Measurement of Shear Stress Distribution by Image Analysis

Abstract

The method for measuring the shear stress distributions over a blade of rotating machinery is proposed by using the oil-film visualization technique combined with the image analysis. The visualization study is carried out in a water tank and the surface velocity of the oil-film over the blade surface is measured by applying the cross-correlation algo-rithm to a series of images with a certain pattern displacement. The transformation of surface velocity to shear stress is made by a well known shear stress distribution over a rotating disk in relation to the experimental surface velocity of oil film. The surface velocity of the oil-film and the centrifugal force effect are studied by using the flow over a rotating disk. The measurement of shear stress distribution over the fan blade is carried out over the fan blade at various operating conditions. The results indicate that the high-est shear stress appears along the blade tip at the design condition, while it is moved to the leading edge of the blade.