Effects of porous substrates on the structure of turbulent boundary layers

Abstract

Three different porous substrates (with different pore sizes, 𝑠 and permeabilities, 𝐾) are used to examine their effect on the structure of boundary layer flow over them. The flow is characterised with single-point hot-wire measurements as well as planar Particle Image Velocimetry. In order to elucidate differences in shallow and deep flows past porous substrate,foams with two different thickness (ℎ) are used (for all three substrates). A wide range of Friction Reynolds number (2000 < 𝑅𝑒_𝜏 < 13500) and Permeability based Reynolds number (1 < 𝑅𝑒_𝐾 < 50) are attained. For substrates with 𝑅𝑒_𝐾 ∼ 1, the flow behaviour remains similar to flow over impermeable smooth walls and as such Townsend’s hypothesis remains valid. Very-large-scale motions are observed over permeable foams even when the 𝑅𝑒_𝐾 > 1. In contrast, a substantial reduction in velocity disturbances and associated length scales is achieved for permeable foams with intermediate values of pore density and relative foam thickness (ℎ/𝑠), which affects outer-layer similarity. As permeability is increased by increasing pore size, the foam becomes sparse relative to viscous scales at high Reynolds number. For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Permeability attenuates the wavelengths associated with the outer-layer peak.

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

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