The turbulent penetrative convection into a stable convective boundary layer represents an important phenomenon in environmental engineering and atmospheric science. In the present study, we present a series of numerical simulations performed by two modeling approaches: the high-fidelity Large-Eddy Simulations (LES), and the less computationally demanding transient Reynolds-Averaged Approach (TRANS), but with an advanced sub-scale turbulent heat flux model. By simulating different localized heat sources over the ground, and by performing a direct comparative assessment of results obtained by LES and TRANS, we confirmed an overall good agreement in predicting the time evolution of the horizontally averaged temperature profiles. Similarly, the morphology of instantaneous thermal plumes and large convective structures predicted by TRANS were in reasonable agreement with the referent LES predictions.

We report on the application and comparative assessment of two rational turbulence modelling approaches for the computer simulation of air flow and pollutant dispersion in real urban environment: a stand-alone unsteady Reynolds-averaged Navier-Stokes eddy-viscosity model (URANS), and its blend with Large-eddy simulations (LES) in a hybrid mode (HRL). The elliptic-relaxation (k−ε)ζ−feddy-viscosity model was applied in both methods. The models, verified earlier in a range of engineering flows including heat transfer, were here validated in two environmental benchmark cases, a single building and an idealized urban settlement, both subjected to a steady wind. The velocity field and pollutant concentration are better predicted by the HRL method in both benchmarks. The HRL is then applied to predictions of air flow and spreading of pollutant from road traffic in downtown of a real city (Sarajevo) containing 100 realistic buildings. The simulations were performed with the in-house open-source CFD code T-Flows, specifically optimized for a fast and efficient high-quality unstructured meshing of the terrain orography and building configurations. The benchmark validations demonstrated that, compared with the standard k−ε, in typical urban applications with complex urban shapes and arrangements the accuracy and credibility of the predictions can substantially be improved by applying a physically sounder and yet still relatively simple ζ−f eddy-viscosity model that specifically accounts for the elliptic inviscid wall-blocking effects and near-wall stress anisotropy. The benchmarking also showed that still further improvements are achieved by using the HRL scheme which ensures a rational balance in capturing the important physics and the computational economy.

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.

This paper examines the evolution of closing the Reynolds-averaged Navier-Stokes equations by approximating the Reynolds stresses via the second-moment transport equations themselves. This strategy first proposed by Rotta is markedly in contrast to the more usual approach of computing an effective “turbulent viscosity” to deduce the turbulent stresses as in a Newtonian fluid in laminar motion. This paper covers the main elements in the development of this approach and shows examples of applications in complex shear flows that collectively include the effects of three-dimensional straining, force fields, and time dependence that affect the flow evolution in ways that cannot be readily mimicked with an eddy viscosity model.

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.

We studied cavitating flow over the suction side of a symmetric 2D foil - a scaled-down model of high-pressure hydroturbine guide vanes (GV) - in different cavitation regimes at several attack angles. High-speed imaging was used to analyze spatial patterns and time dynamics of the gas-vapor cavities, as well as for evaluating the characteristic integral parameters. A hydroacoustic pressure transducer was employed to register time-spectra of pressure fluctuations behind the hydrofoil and, thereby, determine dedicated frequencies of unsteady regimes. A PIV technique was applied to measure the velocity fields and its fluctuations, which were compared for the free and forced flow conditions. The active flow control was implemented by means of a continuous liquid supply with different flow rates through a slot channel located in the GV surface at the distaence of 60% of the chord length from the foil leading edge. It was found that the active mass injection does not influence the primary flow upstream of the slot channel position absolutely. At small angles of incidence, the injection flow at velocities in the range between zero to 0.76 of the mean bulk velocity was observed not to practically influence the distributions of turbulent characteristics so that the global difference is only between the free and forced flow conditions. For cavitation-free and cavitation inception cases, the active mass injection was shown to make the flow turbulence structure more developed and the wake past the GV section more intense. However, the active flow control system considered also allows a favorable and efficient flow manipulation, especially at the regimes with developed gas-vapor cavities. Moreover, the active flow management makes it possible to reduce substantially the amplitude or totally suppress the periodic cavity length oscillations and pressure pulsations associated with them.

Seasonal variation of air quality in a city with a large river was investigated by means of numerical simulations of air movement and pollutant dispersion over inversion-capped diurnal cycles using a Reynolds-averaged Navier-Stokes (RANS) approach with algebraic turbulent flux model. The study accounts for the effects of urban heat island (UHI), terrain orography and high thermal inertia of the river body. The case mimics the real environment of the Krasnoyarsk region with the river Yenisei (Russia). Two scenarios were considered typical of the winter and summer seasons. The study is focused on the dynamics of dispersion of CO emanating mainly from road traffic, which remains fairly uniform throughout the year. The simulation starts from a mild low-altitude inversion with penetrative convection gradually developing over the daytime and attenuating during the night. The main difference between the two cases is in the temperature of the river surface relative to the ambient air. In winter, the non-freezing river acts as a source of positive thermal buoyancy, while in summer the cool river at the daytime acts in the opposite way, as a heat sink. The effect of the river-induced air circulation appears significant enough to account for the observed winter accumulation of the pollutant in the city center.

We report on numerical analysis of the effects of inflow conditions on sub-optimal combustion in a coal-dust utility boiler of 230 MW_e, believed to be the cause of the observed high-temperature corrosion on fireside membrane walls in the furnace diffuser. To this purpose, a precursor simulation of coal-air mixture in a real double-swirl burner in its full complexity was carried out, and the results extrapolated to 24 burner exits for the simulation of combustion in the boiler. The simulations were performed using Ansys Fluent CFD software for solving RANS equations for turbulent flow, combustion of dispersed coal-dust particles, species transport and reactions with the standard chemical kinetics and radiation models. The comparison of combustion results obtained with precursor burner simulations and those with the commonly imposed uniform burner-exit properties revealed considerable differences in the furnace diffuser. Especially notable are different distributions of CO and O₂ concentrations in the burner-exits near field, which are suspected to be the precursors of the membrane wall corrosion. The simulation of the burner reveals also a sub-optimal and incomplete coal - air mixing with a consequent non-uniform and asymmetric particles distribution in the mixture entering the furnace.

We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air.

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 the experimental study of turbulent transport in the near field of swirling jets at Re = 5000 by using simultaneously the stereoscopic particle image velocimetry and planar laser-induced fluorescence methods. The present focus is on the analysis of terms of the mean (axial) momentum and scalar (mass) transport equations, estimated from the mean velocity and concentration, the Reynolds stress and turbulent flux components, measured in the central axial plane of the jets. A proper orthogonal decomposition has also been applied to the velocity data sets to analyse contribution of the coherent velocity and scalar fluctuations to the turbulent momentum and mass fluxes for different swirl rates.

The paper presents some results of numerical simulations of the effects of reburning mechanically-activated micronized coal on reduction of NO_x and unburnt carbon residual in a tangentially fired boiler of 500 t/h steam production. The conventional RANS approach was used to compute two-phase (reactive dispersed particles in gaseous medium) multi-component system, with some modifications related to particle heat transfer and their reactions. The comprehensive model was verified in simulation of the same boiler with conventional firing, as well as in an experimental pilot-scale combustor using micronized, non-activated and mechanically-activated coal. The comparison with the standard dust-coal firing showed that reburning reduces NO_x by 37.5%, but leads to an increase in heat loss due to unburnt carbon residual by 61%, primarily due to the imposed suboptimal stoichiometric ratio. Switching to reburning micronized coal reduced NO_x by 49% while heat loss was still substantial, 42% higher than in the conventional firing. Using mechanically-activated micronized coal of the same granulation brought only marginal further improvement in NO_x reduction (to 50%), but to a remarkable decrease in heat loss, only 3.3% higher than in the case without reburning. The simulations were performed for a set of plausible operating parameters (stoichiometric ratios, flow rates of reburn coal, flue gas recirculation, overfire air), which all need to be optimized to achieve maximum effects. Nevertheless, the simulations demonstrated that reburning of activated micronized coal is a feasible option to achieve significant NO_x reduction without being penalized with excessive unburnt fuel/heat residual.

We report on experimental study of flow and pressure pulsations in draft-tube of a laboratory air model of Francis-99 hydro-turbine operating over a broad range of regimes employing a rapid prototyping of the swirl generators and computerized measurements. In total, 867 operating regimes were examined corresponding to different combinations of the runner rotation speed and flowrates. The velocity measured by a computer-automated laser-Doppler anemometer and the pressure recording by wall-mounted acoustic sensors for a selection of operating conditions reveal a variety of patterns of the flow and vortex structures, with clearly identifiable regimes with the maximum coherent flow pulsations at non-optimal operating conditions. The regimes with distinct precessing vortex cores show notable rearrangement of the velocity and vorticity fields, accompanied by a sharp increase in the amplitude of the coherent pressure pulsations, as also confirmed by the peak cross-correlation of the spectra of pressures signals from two diametrally placed wall-mounted microphones. The paper closes with a scrutiny of the swirl number, its distribution and the relation of zero-swirl with the isogonal best-efficiency loci in a broad range of operating conditions. For the nominal best efficiency regime, the integral swirl number appears to have a small positive value of about S = 0.11, but the tangential velocity distribution reveals two concentric counter-rotating cylindrical rod-like streams with a negative swirl (relative to the runner rotation direction) in the inner region and positive in the outer. In the regimes with the PVC formation, as illustrated for the maximum pressure pulsations regime at S > 0.5, the axial velocity appears to be almost stagnant in the core (though positive apart from the cowl wake), the flow being pushed into the peripheral annular region.

We report on the experimental investigation of cavitating flow control over a 2D model of guide vanes of a Francis turbine by means of a continuous tangential injection of liquid along the foil surface. The generated wall jet, providing supplementary mass and momentum, issues from a nozzle chamber inside the hydrofoil through a spanwise slot channel on its upper surface. High-speed imaging was used to distinguish cavity flow regimes, study the spatial patterns and time dynamics of partial cavities, as well as to evaluate the characteristic integral parameters of cavitation. Time-resolved LIF visualization of the jet discharging from the nozzle was employed to check if the generated wall jet is stable and spanwise uniform. Hydroacoustic measurements were performed by a hydrophone to estimate how the amplitudes and frequencies of pressure pulsations associated with cavity oscillations change with the injection rate. A PIV technique was utilized to measure the mean velocity, its fluctuations and the dominant turbulent shear stress component, which were all compared for different flow conditions and with the results for the unmodified (standard) foil. The effect of injection rate on cavitation and flow dynamics was examined for three attack angles, 0, 3 and 9°, and a range of cavitation numbers corresponding to different regimes. The low-speed injection was shown to lead to an intensification of turbulent fluctuations in the boundary layer and shrinking of the attached cavity length by up to 25% compared to the case without injection. The injection with a high velocity, in turn, causes a rise of the local flow velocity and a reduction of turbulent fluctuations near the wall, which, consequently, increases the foil hydrodynamic quality at a relatively low energy consumption for generation of the wall jet. However, in this case the vapor cavity becomes longer. Thus, the low-speed injection turns out to be effective to mitigate cavitation but the injection at a high velocity is more preferable from the standpoint of the flow hydrodynamics. In the whole, the implemented control method showed to be quite an efficient tool to manipulate cavitation and hydrodynamic structure of the flow and, thereby, under certain conditions, to suppress the cavitation-caused instabilities.

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.

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.