5 results
Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers
- T. Medjnoun, C. Vanderwel, B. Ganapathisubramani
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- Journal:
- Journal of Fluid Mechanics / Volume 886 / 10 March 2020
- Published online by Cambridge University Press:
- 17 January 2020, A31
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The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness ($S/\unicode[STIX]{x1D6FF}\approx O(1)$). Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter ($\unicode[STIX]{x1D709}$), which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to $55\,\%$ of the total shear stress when strong secondary motions occur.
The instantaneous structure of secondary flows in turbulent boundary layers
- C. Vanderwel, A. Stroh, J. Kriegseis, B. Frohnapfel, B. Ganapathisubramani
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- Journal:
- Journal of Fluid Mechanics / Volume 862 / 10 March 2019
- Published online by Cambridge University Press:
- 16 January 2019, pp. 845-870
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Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is not very sensitive to the Reynolds number range investigated, and direct numerical simulations and experiments produce similar results in the mean flow as well as the dispersive and turbulent stress distributions. The numerical methods capture time-resolved features of the instantaneous flow and provide insight into the near-wall flow structures, that were previously obscured in the experimental measurements. Proper orthogonal decomposition was shown to capture the essence of the secondary flows in relatively few modes and to be useful as a filter to analyse the instantaneous flow patterns. The secondary flows are found to create extended regions of high Reynolds stress away from the wall that comprise predominantly sweeps similar to what one would expect to see near the wall and which are comparable in magnitude to the near-wall stress. Analysis of the instantaneous flow patterns reveals that the secondary flows are the result of a non-homogeneous distribution of mid-size vortices.
Characteristics of turbulent boundary layers over smooth surfaces with spanwise heterogeneities
- T. Medjnoun, C. Vanderwel, B. Ganapathisubramani
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- Journal:
- Journal of Fluid Mechanics / Volume 838 / 10 March 2018
- Published online by Cambridge University Press:
- 18 January 2018, pp. 516-543
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An experimental investigation of a turbulent boundary-layer flow over a heterogeneous surface is carried out to examine the mean flow and turbulence characteristics, and to document the variation of skin friction that might affect the applicability of traditional scaling and similarity laws. The heterogeneity is imposed along the spanwise direction and consists of streamwise-aligned smooth raised strips whose spanwise spacing $S$ is comparable to the boundary-layer thickness ($S/\unicode[STIX]{x1D6FF}=O(1)$). Single-point velocity measurements alongside direct skin-friction measurements are used to examine the validity of Townsend’s similarity hypothesis. The skin-friction coefficients reveal that the drag of the heterogeneous surface increased up to 35 % compared to a smooth wall, while velocity measurements reveal the existence of a log layer but with a zero-plane displacement and a roughness function that vary across the spanwise direction. Lack of collapse in the outer region of the mean velocity and variance profiles is attributed to the secondary flows induced by the heterogeneous surfaces. Additionally, the lack of similarity also extends to the spectra across all scales in the near-wall region with a gradual collapse at small wavelengths for increasing $S$. This suggests that the effect of surface heterogeneity is not necessarily felt at the smaller scales other than to reorganise their presence through turbulent transport.
Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise
- P. Chaitanya, P. Joseph, S. Narayanan, C. Vanderwel, J. Turner, J. W. Kim, B. Ganapathisubramani
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- Journal:
- Journal of Fluid Mechanics / Volume 818 / 10 May 2017
- Published online by Cambridge University Press:
- 04 April 2017, pp. 435-464
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This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number $St_{h}=fh/U$, where $f$, $h$ and $U$ are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.
Chapter Seven - Traits, states and rates: understanding coexistence in forests
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- By Drew W. Purves, Microsoft Research Cambridge, Mark C. Vanderwel, Microsoft Research Cambridge
- Edited by David A. Coomes, University of Cambridge, David F. R. P. Burslem, University of Aberdeen, William D. Simonson, University of Cambridge
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- Book:
- Forests and Global Change
- Published online:
- 05 June 2014
- Print publication:
- 20 February 2014, pp 161-194
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Summary
Introduction: why do tree species coexist?
The question of why there is more than one plant species on Earth is probably not one for ecology. Rather, it would appear to us at least that it is up to systems biology and evolutionary biology to explain why the enormous variation in structure and function exhibited by individual plants – a variation that makes sense given the huge range of physical environments that they occupy – occurs primarily as species-to-species variation, rather than as variation among ecotypes via local adaptation, or variation among individuals via phenotypic plasticity. However, given that plant species are so very different, the question of why we appear to observe the long-term co-occurrence of multiple species in the same region certainly is a question for ecology, so much so that the paradox of coexistence has remained central to community ecology for decades (e.g. Gause 1934; Grubb 1977; Hutchinson 1961; MacArthur 1970).
An important recent development has been the realisation, thanks to neutral theory, that the long-term co-occurrence of multiple taxonomic species is not, by itself, a paradox at all (Chave 2004; Hubbell 2001). We now know that it could take an enormous amount of time for a mixed community to drift to monodominance in any one region, if species were indistinguishable in terms of their traits. But this still leaves the challenge of explaining why we observe the long-term co-occurrence of species that are measurably different in traits that obviously affect fitness, such as growth, mortality and reproductive rates (see Purves & Turnbull 2010). Theoretical ecology has provided one kind of answer to this question, by identifying a suite of fundamental mechanisms that can maintain the coexistence of multiple species (Chesson 2000a). Although it is likely that there are new mechanisms still to be discovered, theoretical ecologists are almost entirely agreed that coexistence requires some form of negative feedback: if one species becomes too dominant, its performance declines, which in turn reduces its abundance; the opposite occurs for species that drift to abundances that are too low (Chesson 2000a; and for forests see Dislich, Johst & Huth 2010). In the presence of such negative feedbacks, communities can exhibit stable coexistence of multiple species, where the community exhibits a typical mixture of species (or mixture of traits) that it tends to return to after perturbations.