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Abstract

An optical technique was developed to make possible a study of the instantaneous structure of the turbulent wall-pressure field. The approach involved the use of real-time laser-holographic-moiré interferometry. A moiré fringe pattern generated by the holographic method was superimposed on the surface of a specially-fabricated compliant pipe wall. The compliant surface, in response to wall-pressure changes, introduces optical path length changes which are manifested by distortions in the fringe field. The fringe distortions, observed during flow, were recorded (framed area, 11 mm x 34 mm) by means of medium-speed motion photography. The amplitude of fringe distortion provides a measure of the pressure magnitude at the wall. A 26.3 mm ID horizontal glass pipline (7.0 m long) supplied with distilled water from a constant head reservoir was used in the study. Photographs taken of the fringe patterns observed at a flow velocity (U) of 0.47 m/sec (Re[sub=d] = 12,300) were analysed. Results show that the wall-pressure field consists of a positive and negative pressure region. A statistical analysis reveals that the wall-pressure distribution is asymmetrical (Skewness = -0.29). From an analysis of the pressure patterns, a relationship between the generation of wall-pressure fluctuations and known wall-layer flow characteristics is inferred. A flow model is proposed to explain some aspects of the wall region dynamics and a mechanism for particle detachment from a wall, during turbulent flow, is also presented.