Skin friction determination by Laser Doppler Velocimeter (LDV) measurements in a viscous sublayer. Analysis of systematic errors

The feasibility of skin friction determination by direct laser measurement of the slope of the velocity profile in the linear viscous sublayer of one dimensional flows is discussed. A dual beam configuration with grazing incidence on a plexiglas plate with forward off-axis light collection was selected. Low speed measurements suggest that a systematic error is present. Skin friction coefficients are 3% to 9% lower than reference values. Velocity bias error is discussed; a distinction between measurement procedures for large and short scale flows is made. The error introduced by a velocity gradient across the sensing volume is analyzed; models of the shape of the measured velocity profile under wall truncation and wall reflection are presented. Qualitative agreement with the model is found for a processor controlled effective volume.