Skin friction measurement of turbulent boundary layer at very high Reynolds numbers on the wind tunnel wall using floating element device

Abstract

Measurement of the skin friction coefficient on a flat plate using direct methods has been a subject of study for decades. One promising method for accurately measuring this coefficient involves the use of Floating Element (FE) devices. This study utilizes an FE device with a platform area of 62 mm × 40 mm to assess the non-stationary skin friction drag at very high Reynolds numbers of the incompressible turbulent boundary layer with a zero-pressure gradient on the wall of the T-128 transonic wind tunnel at Central Aero-hydrodynamic Institute (TsAGI). Our measurements of the wall friction coefficient fall within ReƟ ranges between 3.63 × 104 and 1.86 × 105, owing to the high dynamic pressure achieved in the wind tunnel. The values of momentum thickness were obtained from measuring the total pressure inside the boundary layer using a rake with pressure probes. Due to the excitation of the device at its natural frequency, a resonant phenomenon was observed, which increases the uncertainty of the measurements and prevents accurate measurement of the actual friction force. The paper includes a notable method for reducing the random uncertainty of the measurements by 45–82%, utilizing dynamic corrections to account for the actual friction force. The obtained mean friction coefficients exhibit good agreement with other experimental methods (employing anemometers or pressure probes), semi-empirical relations, and Direct Numerical Simulations (DNS).