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.@en

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.@en

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=fe/f0=0:5, 1, 1.5, 2, 2.5, 3, 4, 5 and three forcing amplitudes Δe=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.@en

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.@en

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.@en