High-Frame-Rate Oil Film Interferometry

TL;DR

This paper demonstrates high-frame-rate oil film interferometry to visualize and measure the dynamic response of a laminar boundary layer to sudden disturbances, providing detailed insights into skin friction variations.

Contribution

It introduces a high-speed interferometry method for visualizing boundary layer responses with high temporal resolution, enabling detailed analysis of skin friction dynamics.

Findings

Successful visualization of boundary layer response at over 1000 frames/sec

Correlation between fringe spacing rate and skin friction established

Different puff strengths show varied boundary layer behaviors

Abstract

The fluid dynamics video to which this abstract relates contains visualization of the response of a laminar boundary layer to a sudden puff from a small hole. The boundary layer develops on a flat plate in a wind tunnel; the hole is located at a streamwise Reynolds number of 100,000. The visualization of the boundary layer response is accomplished using interferometry of a transparent, thin film of oil placed on the surface immediately downstream of the hole and with its leading edge perpendicular to the direction of flow. Through lubrication theory, it is understood that the rate of change of the spacing of the interference fringes is proportional to the skin friction at any instant. For reference, a small disk-shaped protrusion of the type often used to trip the boundary layer in wind model tunnel testing is also shown. Three cases with different puff strengths are included. Using a high-speed commercial camera, frame rates in excess of 1000/sec have been recorded; the video shown here was taken at 24 frames/sec to remain within prescribed file size limits.