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Spatial organisation of velocity structures for large passive scalar gradients

Laskari, Angeliki and Saxton-Fox, T. and McKeon, B. J. (2020) Spatial organisation of velocity structures for large passive scalar gradients. Journal of Fluid Mechanics, 885 . Art. No. A33. ISSN 0022-1120. https://resolver.caltech.edu/CaltechAUTHORS:20200116-083832946

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Abstract

Velocity structures associated with large streamwise density gradients in an incompressible turbulent boundary layer (with air as the working fluid, Pr = 0.71) are analysed experimentally using planar image velocimetry and aero-optic measurements. The resulting flow topologies for the velocity fluctuations associated with large negative and positive density gradients are in excellent agreement with results for coolings and heatings in time, respectively (Antonia & Fulachier, J. Fluid Mech., vol. 198, 1989, pp. 429–451). The current results are complimentary to those from Saxton-Fox et al. (AIAA J., vol. 57 (7), 2019, pp. 2828–2839), on the signature of the vertical velocity structures associated with large density gradients. In the present work, these structures are shown to exhibit a sign change, consistent with the scalar gradient, and are localised in the wall-normal direction with an average height of approximately 0.1δ, almost constant for increasing distance from the wall. The corresponding small-scale streamwise fluctuations also exhibit a consistent sign change, which is found to originate, on average, from upstream leaning structures. The emerging picture for the velocity field is then that of a multiscale phenomenon, where small-scale structures, responsible for large optical aberrations, are superimposed on the back of large-scale bulge-like structures that are known to populate the outer layers. The proposed conceptual model is consistent with early ideas of ‘typical’ eddies (Falco, Phys. Fluids, vol. 20 (10), 1977, pp. S124–S132), the hairpin vortex model and associated shear layers (Adrian et al., J. Fluid Mech., vol. 422, 2000, pp.1–54), as well as with notions of multiscale velocity organisation in shear layers (Klewicki & Hirschi, Phys. Fluids, vol. 16 (11), 2004, pp. 4163–4176; Saxton-Fox et al. 2019), and it provides new insight into the geometry of the small-scale velocity structures.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1017/jfm.2019.977DOIArticle
ORCID:
AuthorORCID
Laskari, Angeliki0000-0002-1051-174X
Saxton-Fox, T.0000-0003-1328-4148
McKeon, B. J.0000-0003-4220-1583
Additional Information:© 2020 Cambridge University Press. Received 24 July 2019; revised 11 October 2019; accepted 18 November 2019. Published online by Cambridge University Press: 06 January 2020. The support of the Office of Naval Research under award N00014-17-1-3022 is gratefully acknowledged. The authors report no conflict of interest.
Group:GALCIT
Funders:
Funding AgencyGrant Number
Office of Naval Research (ONR)N00014-17-1-3022
Subject Keywords:boundary layer structure, turbulent boundary layers
Record Number:CaltechAUTHORS:20200116-083832946
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200116-083832946
Official Citation:Laskari, A., Saxton-Fox, T., & McKeon, B. (2020). Spatial organisation of velocity structures for large passive scalar gradients. Journal of Fluid Mechanics, 885, A33. doi:10.1017/jfm.2019.977
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100751
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:16 Jan 2020 17:31
Last Modified:16 Jan 2020 17:31

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