The paper provides a brief overview of recent computational studies of flow and heat transfer control by rotary oscillations of an infinite circular cylinder at a relatively broad set of imposed frequencies and amplitudes [1, 2]. A study for a previously unreachable high subcritical Reynolds number Re = 1.4 × 105 showed that the efficiency of this control method increases with Re concerning the issue of drag and lift reduction. High-frequency oscillations even lead to around 90 % reduction of the drag. However, the benefits for heat transfer enhancement is not that obvious as the bulk Nusselt number shows only small variations. At the same time its angular distribution around the cylinder becomes much more homogeneous due to oscillations which practically can prevent local overheats.

We studied numerically the heat transfer in flow over a rotationally oscillating cylinder at a subcritical Reynolds number (Re=1.4×10⁵) that is an order of magnitude higher than previously reported in the literature. This paper is a follow-up of the earlier study of hydrodynamics and drag force in a range of forcing frequencies and amplitudes (Palkin et al., 2018). This time we focus on heat transfer and its correlation with the observed flow field and vortical patterns. Four forcing frequencies f=f_e/f₀=0,1,2.5,4 for two forcing amplitudes Ω=Ω_eD/2U_∞=1 and 2 are considered, where f₀ is the natural vortex-shedding frequency, U_∞ the free-stream velocity and D the cylinder diameter. The parametric study was performed by solving three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed by a wall-integrated second-moment (Re-stress) model, verified earlier by Large-eddy simulations and experiments in several reference cases including flows over a stagnant, as well as rotary oscillating cylinders at the same Re number. The thermal field, treated as a passive scalar, was obtained from the simultaneous solution of the energy equation, closed by the standard (GGDH) anisotropic eddy-diffusivity model. The computations showed that for the unforced cylinder heat transfer is characterized by very high local rates due to a strong thinning of the thermal boundary layer as a result of the impact and interactions of large coherent structures with the wall. The overall average Nusselt number does not change much for the forced cylinder but its time-averaged, phase-averaged and instantaneous circumferential profiles show some profound differences compared to the stationary cylinder. The distribution of Nu on the back surface becomes more uniform with less frequent occurrence of high values, especially for the higher frequencies f=2.5 and f=4. This is attributed to diminishing of the mean-recirculation zone as well as to the overall suppression of turbulent fluctuations. The rotary oscillation of the cylinder appears potentially efficient in achieving a more uniform circumferential distribution of Nu and avoiding local overheats and hot spots.

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=1:4×10⁵. Considered are eight forcing frequencies f=f_e/f₀=0:5, 1, 1.5, 2, 2.5, 3, 4, 5 and three forcing amplitudes Δ_e=ΔD/2U_∞ =1, 2, 3, non-dimensionalized with f₀, which is the natural vortex-shedding frequency without forcing, U_∞ 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,Δ = 2 and f = 4, Δ = 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 Δ = 2. However, a significant reduction of 78% appears with the increase of f already for f = 2.5 and Δ = 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>10⁴, since the calculated power expenditure for cylinder oscillation under realistic scenarios is several times smaller than the power saved by the drag reduction.

We study the flow over a cylinder placed between two parallel rigid walls using Large-eddy simulations and Particle Image Velocimetry. The Reynolds number based on the inflow velocity and diameter of the cylinder is 3750 corresponding to the subcritical regime with laminar separation. Three-dimensional visualization shows the presence of the horseshoe vortex system prior to the cylinder. The comparison of time-averaged velocity fields and fluctuations shows good agreement between simulations and experiments. Spectral analysis suggests the presence of low-frequency modulations of the recirculating bubble.

We study the flow over an infinite cylinder at high sub-critical Reynolds numbers of Re = 1.4 × 10⁵ using the second-moment closure Reynolds Stress Model. The model of Jakirlić and Hanjalić [1] has been previously validated against large-eddy simulations (LES) and experimental data [2]. The focus is on the control of drag and lift forces acting on the cylinder by rotary oscillations of the bluff body with a set amplitude and frequency. We show that, although at low frequencies the drag force fluctuates around the same value as for the non-rotating case, the recirculating bubble behind the cylinder shrinks for sufficiently large amplitudes of oscillations. The results agree with the data from the literature for lower Reynolds numbers suggesting the investigation of the effect of drag reduction at higher frequencies of rotation.

We describe the low Mach variable-density Navier-Stokes numerical iterative solution procedure implemented in the finite-volume unstructured T-FlowS code. As the test cases we use a number of analytic manufactured solutions and Rayleigh-Taylor instability problem from the literature for algorithm verification purposes. The tests show that the code is second-order accurate in agreement with the spatial discretization scheme. We outline the recent combustion ADEF model implemented in the program.

To unravel the widespread perception that the RANS (Reynolds-averaged Navier-Stokes) concept is unreliable in predicting the dynamics of separated flows, we assessed the performance of two RANS closure levels, the linear eddy-viscosity (LEVM) and the second-moment (Reynolds stress, RSM) approaches in a massively separated generic flow over a bluff body. Considered is the canonical, zero-turbulence, cross-flow over an infinite cylinder with reference to our LES and the available DNS and experiments at two Reynolds numbers, Re = 3.9 × 10³ and 1.4 × 10⁵, both within the sub-critical regime with laminar separation. Both models capture successfully the vortex shedding frequency, but the low frequency modulations are detected only by the RSM. At high Reynolds numbers the RSM is markedly superior to the LEVM showing very good agreement with the LES and experimental data. The RSM, accounting naturally for the stress anisotropy and phase lag between the stress and strain eigenvectors, is especially successful in reproducing the growth rate of the turbulent kinetic energy in the initial shear layer which proved to be crucial for accurate prediction of the separation-induced transition. A scrutiny of the unsteady RANS (URANS) stress terms based on the conditional phase-averaged LES data shows a remarkable similarity of the normalized coherent and stochastic (modeled) stress components for the two Reynolds numbers considered. The mixed (cross) correlations, while non-negligible at the low Re number, diminish fast relative to the stochastic ones with increasing Reynolds number and, in the whole, are not significant to undermine the URANS concept and its applicability to high Re flows of industrial relevance.

Relevance. Characteristics of separated turbulent streams are of great importance when designing effective hydroand thermal power plants equipment. In such streams the flow regimes with harmful quasi-periodic high-amplitude oscillations of velocity and pressure behind the body are implemented. Thus, the knowledge on the ways of controlling turbulent streams can not only reduce the wear of working parts of equipment but prevent their destruction. Besides, to study the occurring optimization problems with the parameters, which change in a wide range, one needs the validated turbulence models which save significantly the computing time compared to Large-eddy simulations and direct numerical simulations. The aim of the research is to apply a promising method to control the flow using the rotary oscillations of cylinder around the axis of symmetry. The authors have carried out the investigations using high Reynolds numbers Re-1,4×10⁵, and the validated numerical methods, to demonstrate the capabilities of the chosen control strategy to decrease the drag coefficient and fluctuating lift force effecting the cylinder. Methods. The authors used T-FlowS code which is based on finite-volume method and unstructured grids and solve unsteady Reynoldsaveraged Navier-Stokes equations with second-moments closure. Results. The paper demonstrates the possibility to control the flow - decrease of trace width behind the cylinder, suppression of recirculating zone, increase of vortex shedding frequency, reduction of drag and lift forces. It is shown that at certain oscillating parameters of cylinder the resistance factor may be decreased by 78 % in comparison with non-rotating case.

We perform a series of URANS simulations of the flow over a rotary oscillating cylinder at Re = 1.4 10⁵ to study the possible reduction of the drag and lift forces acting on the body when the rigid wall is rotary oscillating around the axis of symmetry. Two parameters of the external forcing are varied, i.e. the amplitude of rotation and the frequency. We find that the high-frequency and relatively high-amplitude forcing leads to the drag reduction of 78% compared to the non-rotating case. The oscillations (rms) of drag and lift coefficients are also significantly reduced.