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A turbulent circular jet with variable density caused by the mixing of air with gases of different densities (air, helium, and carbon dioxide) is studied. The Reynolds number is fixed for all cases: Re = 5300. Using proper orthogonal decomposition and direct numerical simulation data, a comparative analysis of the three gases under consideration is carried out. It is shown that with a decrease in the ambient gas density, the frequency of coherent structures formation drops, and the number of proper orthogonal decomposition (POD) modes necessary for constructing a low-dimensional flow model decreases.

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 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.

Flows between two closely spaced bounding surfaces are frequently appear in engineering applications and natural flows. In current paper the flow over a cylinder in a narrow rectangular duct was investigated by numerical computations of Navier-Stokes equations using Large eddy simulations (LES) at ReD = 3 750 based on cylinder diameter and the bulk velocity at inflow boundary. The influence of the bounding walls was demonstrated by comparing mean flow streamlines with the flow over an infinite cylinder at close Reynolds numbers. A comparison between the time-averaged velocity field in front and past the cylinder with experimental from the literature data showed good agreement although the characteristic horseshoe vortex structures are highly sensitive to Reynolds number and turbulence level at inflow boundary. Most energetic modes in recirculating region were revealed by spectral analysis. These low-frequency modulations were characterized by the pair of dominating vortices which are expected to have high influence on the heat transfer in near wake of the cylinder.

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.

We apply deep reinforcement learning to active closed-loop control of a two-dimensional flow over a cylinder oscillating around its axis with a time-dependent angular velocity representing the only control parameter. Experimenting with the angular velocity, the neural network is able to devise a control strategy based on low frequency harmonic oscillations with some additional modulations to stabilize the Kármán vortex street at a low Reynolds number Re=100. We examine the convergence issue for two reward functions showing that later epoch number does not always guarantee a better result. The performance of the controller provide the drag reduction of 14% or 16% depending on the employed reward function. The additional efforts are very low as the maximum amplitude of the angular velocity is equal to 8% of the incoming flow in the first case while the latter reward function returns an impressive 0.8% rotation amplitude which is comparable with the state-of-the-art adjoint optimization results. A detailed comparison with a flow controlled by harmonic oscillations with fixed amplitude and frequency is presented, highlighting the benefits of a feedback loop.

Large-eddy simulations of the swirling annular jet flow are performed in a model of a combustion chamber at two different flow regimes with and without the central jet at Re = 5000. The mesh convergence study provides well-described time-averaged statistics even for a coarse mesh, while small-scale features are only resolved after a proper refinement. The central jet is shown to significantly modify the recirculation bubble and partially suppress the precessing vortex core.

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 report on Large-eddy simulations (LES) of a flow around a cylinder in a narrow duct channel (slot) at the Reynolds number 3750 based on the bulk velocity and the diameter of the cylinder. Secondary streamwise vortical structures in the far wake are investigated appearing near the mixing layer at the midplane and near the duct channel surface. We investigate the correlation between the heat flux and friction velocity fluctuations and evaluate the propagation speed of these disturbances, which appears to be different.

We perform Direct numerical simulations of the flow over a cylinder to study the influence of the thermal boundary conditions on the Nusselt number distribution. The results show very good agreement with some data from the literature. The total Nusselt number for the constant heat flux (CHF) boundary condition is higher for some 15% than for the constant wall temperature (CWT) condition for both Re < 200 and 1000.

We studied numerically the heat transfer in flow over a rotationally oscillating cylinder at a subcritical Reynolds number ( R e = 1.4 × 10 5 ) 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 / f 0 = 0 , 1 , 2.5 , 4 for two forcing amplitudes Ω = Ω e D / 2 U ∞ = 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 Large-eddy simulations (LES) of flow around a short cylinder mounted in a narrow plane channel in a range of Reynolds numbers 1000, 2000, 3750 based on the bulk velocity of the flow and diameter of the cylinder supplemented with Particle image velocimetry (PIV) measurements for the highest considered Re. First two cases appear to be steady, however, for Re=3750 the flow becomes unsteady with the wake dominated by periodic vortex shedding. In front of the cylinder typical horseshoe vortices are identified intensifying the skin friction and heat transfer on the wall, while in the near wake we observe a quasiperiodic low-frequency secondary motion in the form of a pair of counterrotating eddies developing in the transverse direction. The Karman vortex street remains the dominant pattern, but further downstream from the cylinder the transport across the channel is associated with the secondary streamwise vortices, as also previously observed in slot jets. We observe their impact on heat transfer and skin friction on the wall of the channel.

cited By 0; Conference of All-Russian Conference with the School for Young Scientists Thermophysics and Physical Hydrodynamics 2016, TPH 2016 ; Conference Date: 19 September 2016 Through 25 September 2016; Conference Code:125046; International audience; 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. © Published under licence by IOP Publishing Ltd.

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.

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 Jakirlic and Hanjalic [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 study the flow over a cylinder placed between two parallel rigid walls using Large - eddy simulations. The Reynolds number based on the inflow velocity and diameter of the cylinder is 3750 corresponding to the subcritical regime with laminar separation.The comparison of the time - averaged flow patterns shows very good agreement with experiments.The unsteadiness of the recirculating bubble features the low - frequency modulation observed for the first time.