2 results
Control of flow around a cylinder by rotary oscillations at a high subcritical Reynolds number
- E. Palkin, M. Hadžiabdić, R. Mullyadzhanov, K. Hanjalić
-
- Journal:
- Journal of Fluid Mechanics / Volume 855 / 25 November 2018
- Published online by Cambridge University Press:
- 18 September 2018, pp. 236-266
-
- Article
- Export citation
-
We report on a numerical study of the vortex structure modifications and drag reduction in a flow over a rotationally oscillating circular cylinder at a high subcritical Reynolds number, $Re=1.4\times 10^{5}$. Considered are eight forcing frequencies $f=f_{e}/f_{0}=0.5$, $1$, $1.5$, $2$, $2.5$, $3$, $4$, $5$ and three forcing amplitudes $\unicode[STIX]{x1D6FA}=\unicode[STIX]{x1D6FA}_{e}D/2U_{\infty }=1$, $2$, $3$, non-dimensionalized with $f_{0}$, which is the natural vortex-shedding frequency without forcing, $U_{\infty }$ the free-stream velocity, $D$ the diameter of the cylinder. In order to perform a parametric study of a large number of cases ($24$ in total) with affordable computational resources, the three-dimensional unsteady computations were performed using a wall-integrated (WIN) second-moment (Reynolds-stress) Reynolds-averaged Navier–Stokes (RANS) turbulence closure, verified and validated by a dynamic large-eddy simulations (LES) for selected cases ($f=2.5$, $\unicode[STIX]{x1D6FA}=2$ and $f=4$, $\unicode[STIX]{x1D6FA}=2$), as well as by the earlier LES and experiments of the flow over a stagnant cylinder at the same $Re$ number described in Palkin et al. (Flow Turbul. Combust., vol. 97 (4), 2016, pp. 1017–1046). The drag reduction was detected at frequencies equal to and larger than $f=2.5$, while no reduction was observed for the cylinder subjected to oscillations with the natural frequency, even with very different values of the rotation amplitude. The maximum reduction of the drag coefficient is 88 % for the highest tested frequency $f=5$ and amplitude $\unicode[STIX]{x1D6FA}=2$. However, a significant reduction of 78 % appears with the increase of $f$ already for $f=2.5$ and $\unicode[STIX]{x1D6FA}=2$. Such a dramatic reduction in the drag coefficient is the consequence of restructuring of the vortex-shedding topology and a markedly different pressure field featured by a shrinking of the low pressure region behind the cylinder, all dictated by the rotary oscillation. Despite the need to expend energy to force cylinder oscillations, the considered drag reduction mechanism seems a feasible practical option for drag control in some applications for $Re>10^{4}$, since the calculated power expenditure for cylinder oscillation under realistic scenarios is several times smaller than the power saved by the drag reduction.
Vortical structures and heat transfer in a round impinging jet
- M. HADŽIABDIĆ, K. HANJALIĆ
-
- Journal:
- Journal of Fluid Mechanics / Volume 596 / 25 January 2008
- Published online by Cambridge University Press:
- 17 January 2008, pp. 221-260
-
- Article
- Export citation
-
In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.