Cheng, W. and Pullin, D. I. and Samtaney, R.
(2018)
*Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers.*
Journal of Fluid Mechanics, 835
.
pp. 327-362.
ISSN 0022-1120.
http://resolver.caltech.edu/CaltechAUTHORS:20171214-160643790

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## Abstract

We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height ε, invariant in the spanwise direction. Based on the two parameters, ε/D and the Reynolds number Re_D = U_∞D/ν where U_∞ is the free-stream velocity, D the diameter of the cylinder and ν the kinematic viscosity, two main sets of simulations are described. The first set varies from 0 to 1/32 while fixing Re_D = 3.9 x 10^3. We study the flow deviation from the smooth-cylinder case, with emphasis on several important statistics such as the length of the mean-flow recirculation bubble L_B, the pressure coefficient C_P, the skin-friction coefficient C_fθ and the non-dimensional pressure gradient parameter β. It is found that, with increasing ε/D at fixed Re_D , some properties of the mean flow behave somewhat similarly to changes in the smooth-cylinder flow when Re_D is increased. This includes shrinking L_B and nearly constant minimum pressure coefficient. In contrast, while the non-dimensional pressure gradient parameter β remains nearly constant for the front part of the smooth cylinder flow, β shows an oscillatory variation for the grooved-cylinder case. The second main set of LES varies Re_D from 3.9 x 10^3 to 6 x 10^4 with fixed ε/D = 1/32. It is found that this Re_D range spans the subcritical and supercritical regimes and reaches the beginning of the transcritical flow regime. Mean-flow properties are diagnosed and compared with available experimental data including C_P and the drag coefficient C_D. The timewise variation of the lift and drag coefficients are also studied to elucidate the transition among three regimes. Instantaneous images of the surface, skin-friction vector field and also of the three-dimensional Q-criterion field are utilized to further understand the dynamics of the near-surface flow structures and vortex shedding. Comparison of the grooved-cylinder flow with the equivalent flow over a smooth-wall cylinder shows structural similarities but significant differences. Both flows exhibit a clear common signature, which is the formation of mean-flow secondary separation bubbles that transform to other local flow features upstream of the main separation region (prior separation bubbles) as is increased through the respective drag crises. Based on these similarities it is hypothesized that the drag crises known to occur for flow past a cylinder with different surface topographies is the result of a change in the global flow state generated by an interaction of primary flow separation with secondary flow recirculating motions that manifest as a mean-flow secondary bubble. For the smooth-wall flow this is accompanied by local boundary-layer flow transition to turbulence and a strong drag crisis, while for the grooved-cylinder case the flow remains laminar but unsteady through its drag crisis and into the early transcritical flow range.

Item Type: | Article | ||||||
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Additional Information: | © 2017 Cambridge University Press. (Received 5 July 2017; revised 12 September 2017; accepted 18 October 2017) Published online: 27 November 2017. This work was partially supported by the KAUST baseline research funds of R.S. The Cray XC40, Shaheen, at KAUST was utilized for all the reported LES. | ||||||

Group: | GALCIT | ||||||

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Subject Keywords: | boundary layer separation, turbulence simulation, turbulent flows | ||||||

Record Number: | CaltechAUTHORS:20171214-160643790 | ||||||

Persistent URL: | http://resolver.caltech.edu/CaltechAUTHORS:20171214-160643790 | ||||||

Official Citation: | Cheng, W., Pullin, D., & Samtaney, R. (2018). Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers. Journal of Fluid Mechanics, 835, 327-362. doi:10.1017/jfm.2017.767 | ||||||

Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | ||||||

ID Code: | 83933 | ||||||

Collection: | CaltechAUTHORS | ||||||

Deposited By: | George Porter | ||||||

Deposited On: | 15 Dec 2017 03:20 | ||||||

Last Modified: | 02 Feb 2018 21:51 |

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