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=fe/f0=0,1,2.5,4 for two forcing amplitudes Ω=ΩeD/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.@en

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.@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

Turbulent jets are known to support large-scale vortical wave packets traveling downstream. We show that a propagating helical wave represents a common form of the "optimal" eigenfunction tracking these structures from the near to the far field of a round jet issuing from a pipe. Two first mirror-symmetric modes containing around 5% of the total turbulent kinetic energy capture all significant large-scale events and accurately replicate the full shear-layer dynamics of the azimuthal wave number m=1. A family of the most energy-containing traveling waves represents low wave numbers and is described in terms of "empirical" dispersion laws.@en

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.@en

Large-eddy simulations of a wall-bounded turbulent slot-jet have been performed to analyze the dynamics of quasi-two-dimensional large-scale meandering vortical structures and their interaction with small-scale stochastic turbulence. Despite a wide scale separation, LES indicate that there is an energy exchange between the two spectral ranges in both directions. The phenomenon is of relevance to fluid/pollutant discharge into shallow rivers or water basins.@en