Impact of pressure gradient history on smooth and rough wall turbulent boundary layers

Abstract

Turbulent boundary layers often occur over surfaces which are considered rough, often with external conditions which influence the development of the boundary layer. One of the most common influences is external streamwise pressure gradients. The combination of roughness and pressure gradients exists in many common engineering applications, including on a ship's hull, turbine blades and leading edges of aerofoils due to icing. This work experimentally explores the effect of the pressure gradients on smooth and rough wall turbulent boundary layers at high Reynolds numbers. There are three main subsections focusing on the mean flow, the turbulence statistics and the structures within the flow.

Experiments are carried out in the University of Southampton's 12 m boundary layer wind tunnel. A NACA 0012 aerofoil of 1.25 m chord mounted in the freestream to impose pressure gradients onto the developing turbulent boundary layer. Flow measurements are initially taken at a single point, one chord downstream of the trailing edge of the aerofoil, using hot wire anemometry. Combined with independent skin friction measurements, this allows the effect of pressure gradient history on the roughness constant to be examined. The effect of pressure gradient history and the differences between smooth and rough walls are also examined using the pre-multiplied energy spectra.

Particle image velocimetry is used to obtain the flow development under different pressure gradient histories. The types of events and variation in flow structures with different pressure gradient histories are examined, as well as the differences between smooth and rough walls. Throughout this work, an integral pressure gradient parameter, $\Delta\beta$, is defined to quantify the upstream pressure gradient history. Based on this parameter, a predictive model for the skin friction variation in the flow due to the pressure gradient history is developed. This is initially done based on a single point measurement before being further developed for the entire flow field.

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Identifiers

ORCID for Thomas David Preskett: orcid.org/0000-0001-9203-1266

ORCID for Bharath Ganapathisubramani: orcid.org/0000-0001-9817-0486

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