Effects of fetch length on turbulent boundary layer recovery past a step-change in surface roughness
Fuente:
arXiv
Enregistré dans:
| Auteurs principaux: | , , , |
|---|---|
| Format: | Preprint |
| Publié: |
2024
|
| Sujets: | |
| Accès en ligne: | |
| Tags: |
Ajouter un tag
Pas de tags, Soyez le premier à ajouter un tag!
|
| _version_ | 1866929484568985600 |
|---|---|
| author | Formichetti, Martina Wangsawijaya, Dea D. Symon, Sean Ganapathisubramani, Bharathram |
| author_facet | Formichetti, Martina Wangsawijaya, Dea D. Symon, Sean Ganapathisubramani, Bharathram |
| contents | Recent studies focusing on the response of turbulent boundary layers (TBL) to a step-change in roughness have provided insight into the scaling and characterisation of TBLs and the development of the internal layer. Although various step-change combinations have been investigated, ranging from smooth-to-rough to rough-to-smooth, the "minimum" required roughness fetch length over which the TBL returns to its homogeneously rough behaviour remains unclear. Moreover, the relationship between a finite- and infinite-fetch roughness function (and the equivalent sandgrain roughness) is also unknown. In this study, we determine the minimum "equilibrium fetch length" for TBL developing over a smooth-to-rough step-change as well as the expected error in local skin friction if the fetch length is under this minimum threshold. An experimental study is carried out where the flow is initially developed over a smooth wall, and then a step-change is introduced using patches of P24 sandpaper. 12 roughness fetch lengths are tested in this study, systematically increasing from $L = 1δ_2$ up to $L = 39δ_2$ (where \textit{L} is the roughness fetch length and $δ_2$ is the TBL thickness of the longest fetch case), measured over a range of Reynolds numbers ($4\cdot10^2 \leq Re_τ\leq 2\cdot10^5$). Results show that the minimum fetch length needed to achieve full equilibrium recovery is around $20δ_2$. Furthermore, we observe that $C_f$ recovers to within 10\% of its recovered value for fetch lengths $\geq 5δ_2$. This information allows us to incorporate the effects of roughness fetch length on the skin friction and roughness function. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2409_02082 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Effects of fetch length on turbulent boundary layer recovery past a step-change in surface roughness Formichetti, Martina Wangsawijaya, Dea D. Symon, Sean Ganapathisubramani, Bharathram Fluid Dynamics Recent studies focusing on the response of turbulent boundary layers (TBL) to a step-change in roughness have provided insight into the scaling and characterisation of TBLs and the development of the internal layer. Although various step-change combinations have been investigated, ranging from smooth-to-rough to rough-to-smooth, the "minimum" required roughness fetch length over which the TBL returns to its homogeneously rough behaviour remains unclear. Moreover, the relationship between a finite- and infinite-fetch roughness function (and the equivalent sandgrain roughness) is also unknown. In this study, we determine the minimum "equilibrium fetch length" for TBL developing over a smooth-to-rough step-change as well as the expected error in local skin friction if the fetch length is under this minimum threshold. An experimental study is carried out where the flow is initially developed over a smooth wall, and then a step-change is introduced using patches of P24 sandpaper. 12 roughness fetch lengths are tested in this study, systematically increasing from $L = 1δ_2$ up to $L = 39δ_2$ (where \textit{L} is the roughness fetch length and $δ_2$ is the TBL thickness of the longest fetch case), measured over a range of Reynolds numbers ($4\cdot10^2 \leq Re_τ\leq 2\cdot10^5$). Results show that the minimum fetch length needed to achieve full equilibrium recovery is around $20δ_2$. Furthermore, we observe that $C_f$ recovers to within 10\% of its recovered value for fetch lengths $\geq 5δ_2$. This information allows us to incorporate the effects of roughness fetch length on the skin friction and roughness function. |
| title | Effects of fetch length on turbulent boundary layer recovery past a step-change in surface roughness |
| topic | Fluid Dynamics |
| url | https://arxiv.org/abs/2409.02082 |