Effects of porous substrates on the structure of turbulent boundary layers
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arXiv
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| Format: | Preprint |
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2023
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| _version_ | 1866909091559899136 |
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| author | Jaiswal, Prateek Ganapathisubramani, Bharathram |
| author_facet | Jaiswal, Prateek Ganapathisubramani, Bharathram |
| contents | 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< Retau < 15000) and Permeability based Reynolds number (1<ReK< 50) are attained. For substrates with ReK=1, the flow behaviour remains similar to flow over impermeable smooth walls and as such Townsend's hypothesis remains valid. In contrast, a substantial reduction in velocity disturbances and associated length scales is achieved for permeable (ReK>1) and dense (relative to viscous scales) foam at the thick substrate limit (h/s>10), which leads to the breakdown of outer-layer similarity. As porosity is increased, a thin substrate limit is reached (h/s), and the foam becomes sparse relative to viscous scales (s+ > 100). For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Such substrates are unable to attenuate velocity disturbances and the dependence of substrate thickness (h/s) on spectral energy content of turbulent fluctuations ceases to exist. The present study shows that transition from thick to thin substrate flow behavior depends not only on thickness-to-pore ratio (h/s) but also on substrate density relative to viscous scales of the flow. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2301_04102 |
| institution | arXiv |
| publishDate | 2023 |
| record_format | arxiv |
| spellingShingle | Effects of porous substrates on the structure of turbulent boundary layers Jaiswal, Prateek Ganapathisubramani, Bharathram Fluid Dynamics 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< Retau < 15000) and Permeability based Reynolds number (1<ReK< 50) are attained. For substrates with ReK=1, the flow behaviour remains similar to flow over impermeable smooth walls and as such Townsend's hypothesis remains valid. In contrast, a substantial reduction in velocity disturbances and associated length scales is achieved for permeable (ReK>1) and dense (relative to viscous scales) foam at the thick substrate limit (h/s>10), which leads to the breakdown of outer-layer similarity. As porosity is increased, a thin substrate limit is reached (h/s), and the foam becomes sparse relative to viscous scales (s+ > 100). For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Such substrates are unable to attenuate velocity disturbances and the dependence of substrate thickness (h/s) on spectral energy content of turbulent fluctuations ceases to exist. The present study shows that transition from thick to thin substrate flow behavior depends not only on thickness-to-pore ratio (h/s) but also on substrate density relative to viscous scales of the flow. |
| title | Effects of porous substrates on the structure of turbulent boundary layers |
| topic | Fluid Dynamics |
| url | https://arxiv.org/abs/2301.04102 |