A numerical investigation of the asymmetric wake mode of a squareback Ahmed body - effect of a base cavity

Lucas, J-M, Cadot, O ORCID: 0000-0001-7323-6418, Herbert, V, Parpais, S and Delery, J (2017) A numerical investigation of the asymmetric wake mode of a squareback Ahmed body - effect of a base cavity. JOURNAL OF FLUID MECHANICS, 831. pp. 675-697.

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Abstract

<jats:p>Numerical simulations of the turbulent flow over the flat backed Ahmed model at Reynolds number <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline1" /><jats:tex-math>$Re\simeq 4\times 10^{5}$</jats:tex-math></jats:alternatives></jats:inline-formula> are conducted using a lattice Boltzmann solver to clarify the mean topology of the static symmetry-breaking mode of the wake. It is shown that the recirculation region is occupied by a skewed low pressure torus, whose part closest to the body is responsible for an extra low pressure imprint on the base. Shedding of one-sided vortex loops is also reported, indicating global quasi-periodic dynamics in conformity with the seminal work of Grandemange <jats:italic>et al.</jats:italic> (<jats:italic>J. Fluid Mech.</jats:italic>, vol. 722, 2013, pp. 51–84). Despite the limited low frequency resolution of the simulation, power spectra of the lateral velocity fluctuations at different locations corroborate the presence of this quasi-periodic mode at a Strouhal number of <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline2" /><jats:tex-math>$St=0.16\pm 0.03$</jats:tex-math></jats:alternatives></jats:inline-formula>. A shallow base cavity of <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline3" /><jats:tex-math>$5\,\%$</jats:tex-math></jats:alternatives></jats:inline-formula> of the body height reduces the drag coefficient by <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline4" /><jats:tex-math>$3\,\%$</jats:tex-math></jats:alternatives></jats:inline-formula> but keeps the recirculating torus and its interaction with the base mostly unchanged. The drag reduction lies in a global constant positive shift of the base pressure distribution. For a deep base cavity of <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline5" /><jats:tex-math>$33\,\%$</jats:tex-math></jats:alternatives></jats:inline-formula> of the body height, a drag reduction of <jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112017006541_inline6" /><jats:tex-math>$9.5\,\%$</jats:tex-math></jats:alternatives></jats:inline-formula> is obtained. It is accompanied by a large elongation of the recirculation inside the cavity that considerably attenuates the low pressure sources therein together with a symmetrization of the low pressure torus. The global quasi-periodic mode is found to be inhibited by the cavity.</jats:p>

Item Type:	Article
Uncontrolled Keywords:	instability control, separated flows, wakes
Depositing User:	Symplectic Admin
Date Deposited:	21 Nov 2017 14:26
Last Modified:	19 Jan 2023 06:50
DOI:	10.1017/jfm.2017.654
Related URLs:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3012636