Effect of pressure gradient histories on turbulence characteristics of turbulent boundary layers over smooth and rough walls

Abstract

Experiments are conducted over smooth and rough walls to examine the effect of pressure gradient history on turbulence statistics of turbulent boundary layers. A NACA0012 aerofoil of a chord with 1.25 m mounted in the freestream is used to impose different pressure gradients onto the flow. Measurements are taken at a single point, one chord downstream of the trailing edge of the aerofoil, using a single hot-wire probe. The turbulence statistics and pre-multiplied energy spectra non-dimensionalised by directly measured skin friction are presented at the point of measurement. Measurements are taken at a friction Reynolds number, Reτ , that varies between 3000 and 27000depending on the surface and freestream speed, which ensures large-scale separation. The mean streamwise turbulence aligns with previous findings that adverse pressure gradients increase the inner-scaled energy in the outer region, while a favourable pressure gradient suppresses the energy. The individual and combined effects of pressure gradient history and roughness on the pre-multiplied energy spectra are examined. Examination of the difference between the pressure gradient and corresponding zero-pressure gradient spectra shows that adverse pressure gradients have a greater effect on the energy of the rough wall. The peak energy scales with a weighted integral of the pressure gradient history normalised by local quantities. In the outer region, this trend is linear, with the position and wavelength at which it is most affected appearing to be approximately a constant.

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Identifiers

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

ORCID for P. Jaiswal: orcid.org/0000-0002-5240-9911

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

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