Turbulent Transition Research Articles

Исследования на износостойкость конструкции радиального подшипника с учетом реологических свойств микрополярного смазочного материала

The article is devoted to the development and analysis of a model of the motion of a micropolar lubricant in the working gap of a radial plain bearing with a fluoroplastic-containing antifriction composite polymer coating and with a groove on the supporting surface. New models are obtained on the basis of classical equations in the «thin layer» approximation and the continuity equation, which describes the turbulent mode of motion of a lubricant with micropolar rheological properties. The results of the numerical analysis of the obtained models of existing operational characteristics made it possible to obtain a quantitative assessment of the effectiveness of the support profile with a fluoroplastic-containing antifriction composite polymer coating with an axial groove. To complete the complex of studies and verify theoretical developments, experimental studies are carried out. The novelty of the work lies in the concretization of the technique of engineering calculations for the design of an effective radial bearing with an antifriction polymer coating with an axial groove on the bearing surface of the bearing bush, taking into account the dependence of the rheological properties of a micropolar lubricant on pressure and temperature in a turbulent mode, which makes it possible to estimate the value of the main operational characteristics: hydrodynamic pressure, load ability and coefficient of friction, as well as to expand the scope of practical application of models for engineering calculations. Thus, the design of a radial bearing with a polymer-coated support profile, a groove 3 mm wide, ensured a stable ascent of the shaft on a hydrodynamic wedge, which experimentally confirmed the correctness of the results of theoretical studies of a radial bearing with a diameter of 40 mm at a sliding speed of 0,32–3 m/s, load 4,9–24,5 MPa.

The impinging wall effect on flame dynamics and heat transfer in non-premixed jet flames

The impinging jet flame is studied experimentally and numerically accounting for the complex flame-wall interactions in practical combustion devices. Flame dynamics and heat transfer with the effect of impinging wall are analyzed. The 3-D large eddy simulation coupled with detailed chemical reaction mechanism and particle image velocimetry experiment based on cross-correlation measurement principle are performed for verification and further analysis. Results show that vortices are generated due to the Kelvin-Helmholtz instability originated from velocity gradient. The 3-D vortex interactions involving vortex rings and spirals are also indicated by vorticity and the convection of streamwise vorticity is responsible for the effect of vortex spirals associated with turbulent flow transition. In addition, results calculated from four wall thermal conditions are compared and analyzed. Dirichlet condition is inferred to be more suitable for the case of wall materials with higher thermal conductivity. It is indicated that wall thermal condition mainly affects the heat transfer in the near-wall region, but has little effect on the momentum transfer. This study provides references for the adoption of wall conditions in numerical simulation and near-wall treatment in combustion systems.

Neural prediction model for transition onset of a boundary layer in presence of two-dimensional surface defects

Predicting the laminar to turbulent transition is an important aspect of computational fluid dynamics because of its impact on skin friction. Traditional transition prediction methods such as local stability theory or the parabolized stability equation method do not allow for the consideration of strongly non-parallel boundary layer flows, as in the presence of surface defects (bumps, steps, gaps, etc.). A neural network approach, based on an extensive database of two-dimensional incompressible boundary layer stability studies in the presence of gap-like surface defects, is used. These studies consist of linearized Navier–Stokes calculations and provide information on the effect of surface irregularity geometry and aerodynamic conditions on the transition to turbulence. The physical and geometrical parameters characterizing the defect and the flow are then provided to a neural network whose outputs inform about the effect of a given gap on the transition through the ${\rm \Delta} N$ method (where N represents the amplification of the boundary layer instabilities).

Modeling the wall shear stress in large-eddy simulation using graph neural networks

AbstractAs the Reynolds number increases, the large-eddy simulation (LES) of complex flows becomes increasingly intractable because near-wall turbulent structures become increasingly small. Wall modeling reduces the computational requirements of LES by enabling the use of coarser cells at the walls. This paper presents a machine-learning methodology to develop data-driven wall-shear-stress models that can directly operate, a posteriori, on the unstructured grid of the simulation. The model architecture is based on graph neural networks. The model is trained on a database which includes fully developed boundary layers, adverse pressure gradients, separated boundary layers, and laminar–turbulent transition. The relevance of the trained model is verified a posteriori for the simulation of a channel flow, a backward-facing step and a linear blade cascade.

Laminar and turbulent flow development study in a rectangular duct with 180° sharp bend by using stereo particle image velocimetry and liquid crystal thermography measurements

This work presents a detailed insight into the flow progression and surface heat transfer distribution across the sharp 180° bend of a two-pass rectangular duct for laminar (Re = 800) and turbulent (Re = 6500) in-flow conditions. Stereoscopic particle image velocimetry (stereo PIV) as well as two-dimensional and two-component PIV measurements and liquid crystal thermography techniques are appropriately used for flow and heat transfer characterization across the complete sharp 180° bend. The centrifugal instabilities arise due to the sharp bend, which induces the secondary flows in the form of counter-rotating vortex pairs commonly known as Dean vortices. These secondary vortices play a significant role in the localized laminar–turbulent transition and turbulence augmentations for laminar and turbulent inflow conditions. Subsequently, quantitative analysis shows that complete 180° turning of flow resulted in intense augmentation of spatially averaged turbulence quantities. Root mean square (RMS) fluctuations in the transverse direction V¯T|rms increase by 298% and 186% for respective flow conditions. Augmentation of ∼ 287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations (V¯N|rms) are observed. These augments in transverse and wall-normal velocity fluctuations result in a very sharp amplification of spatially averaged turbulent kinetic energy (k¯), that is, 1825% for inlet laminar and 928% for inlet turbulent flow regimes.

Two-dimensional stable lattice Boltzmann simulation of turbulent flow in wavy walled channel

In recent years, the lattice Boltzmann equation has developed into a promising technique for computational fluid dynamics (CFD). The lattice Boltzmann model (LBM) approach is derived from the Boltzmann equation and kinetic theory, as opposed to the standard CFD methods that are based on direct discretization of the Navier–Stokes equations. In this paper, Newtonian flow passing through a wavy walled channel has been examined for laminar to turbulent transition by using the LBM. The simple LBM for this problem becomes unstable as the Reynolds number increases and the laminar to turbulent transition begins. When Ehrenfest’s limiters are introduced in the LBM, the simulation becomes stable for higher Reynolds numbers. Two types of channel geometries are studied here, the channel walls of relatively small amplitude and channel walls of large amplitude. Our findings are that for large amplitude channel walls, flow becomes unsteady for lower Reynolds numbers as compared to that for small amplitude channel walls. For large amplitude walls, the vortices formed exhibit periodic shedding inside the channel furrows and remain there. For small amplitude walls, the vortex shedding starts downstream of the channel. The present LBM variant is able to simulate small amplitude channel flow for a Reynolds number Re = 800 and large amplitude channel flow for a Reynolds number R = 570.

A model of random pulsations of radiant heat flow from a flammable liquid fire

The object of the study is the process of liquid combustion in a tank or in a spill. Unlike the standard approach that assumes the shape of the flame is constant, random pulsations of the flame due to the turbulent mode of liquid combustion are considered. The pulsations lead to the random nature of the mutual radiation coefficient and the temperature of the radiating surface of the flame. This leads to a random value of the radiant heat flux density from the fire. Using the central limit theorem allows justifying the assumption about the normal law of the distribution of the radiant heat flux density, the mutual radiation coefficient and the temperature of the radiating surface of the flame. The assumption of a normal distribution law allows calculating the mathematical expectation of the heat flux density. It is shown that the average value of the heat flux density increases with an increase in the dispersion of the temperature of the radiating surface and the coefficient of mutual radiation, as well as with an increase in the correlation coefficient between them. It means that neglecting random flame pulsations can lead to underestimates of the average heat flux density from a fire. The dispersion of the radiant heat flux density was found and it was shown that it increases with the growth of the dispersion of the flame temperature and the mutual radiation coefficient. The standard deviation of the heat flux density can be more than 40 % of its average value if the standard deviations of the flame temperature and the mutual radiation coefficient are up to 10 % of their average values. The obtained results can be used to clarify the thermal effect of a liquid fire on neighboring objects.

ITB formation in gyrokinetic flux-driven ITG/TEM turbulence

The formation mechanism of internal transport barriers (ITBs) in flux-driven turbulence is studied by means of the full-f gyrokinetic code GKNET. In the adiabatic electron case with a weak magnetic shear configuration, toroidal momentum injection can change the radial mean electric field through radial force balance, leading to a kind of driven ITB formation in which the ion thermal diffusivity by ion temperature gradient (ITG) turbulence decreases to the neoclassical transport level. Only cocurrent toroidal rotation in the outer core region can benefit the ITB formation, and this mechanism is identified to originate from a positive feedback loop between the radial shear and resultant momentum flux. On the other hand, in the kinetic electron case with a reversed magnetic shear configuration, robust co-intrinsic rotation is driven near the surface in ITG turbulence and sustains the shear through the radial force balance, leading to the spontaneous reduction of ion turbulent thermal diffusivity, while this is not observed in the adiabatic electron case. In the presence of electron heating, counter-intrinsic rotation by trapped electron mode turbulence is selectively driven in the negative magnetic shear region, which provides steeper shear formation and a resultant larger reduction of ion turbulent thermal diffusivity. This indicates that the co-existence of different modes can trigger the ‘discontinuity’ of mode structure, intrinsic rotation, and resultant mean near , leading to spontaneous ITB formation.

Numerical simulation of the transition of a Newtonian fluid to a viscoplastic state in a turbulent flow

The non-isothermal turbulent flow and transition to a viscoplastic state of high-viscosity oil with physical and chemical properties depending on fluid temperature is numerically studied. The turbulence of fluid flow in a pipe is described using the Reynolds stress, Reynolds algebraic stress, and two-parameter isotropic models. The simulations show the boundaries of the region of the manifestation of Newtonian behavior and fluid transition to a viscoplastic state. The Reynolds stress model and the algebraic Reynolds stress model show significant anisotropy in the velocity fluctuation profiles of Newtonian and non-Newtonian fluids. The shift of the locus of maximal magnitudes of turbulent pulsations, Reynolds stress, and turbulent kinetic energy towards a flow core region is observed. The height of the zone with a completely stopped fluid is predicted and determined numerically. The appearance of a stagnation zone near the wall causes a significant decrease in wall friction and heat transfer due to a decrease in the environment temperature.

Бесполостная дрена составного профиля, усиленная дренажной трубой

Purpose: Calculated substantiation of cavityless drain structure that provides for effective draining of poorly permeable soils. Hydraulic calculation methodology of composite profile cavityless drain, laid with a slope but without inclusion thitherwards of a drainage pipe, was developed earlier. Drainage pipe reinforces drainage capability of cavityless drain with the preservation of all its other advantages. Methods: Known differential equation of liquid filtration in a soil is used. It’s of special importance that the calculation method may be used at all modes of water movement in a drainage: laminar, transient or turbulent. The dependencies substantiated with experiments, held by Northern Scientific-Research Institute of Hydrotechnique and Melioration, were applied for transient and turbulent modes. The complex form of drainage transection is taken into account with the help of special coefficient which leads it to equivalent by area rectangular section. It’s proved that such transfer doesn’t lead to significant inaccuracy of results obtained. Results: Dependencies have been obtained allowing to calculate consumptions, flowing along drain cavityless part and along drain pipe. If drain is long, it’s feasible to divert a water periodically from it in lateral direction by collectors. Dependencies are derived which according to, the distance between intermediary collectors, whereto a water comes from drains that’re laid with a slope, and the water depth in drain cavityless part in characteristic sections are defined. Practical significance: Cavityless drainage is applied in agriculture, in the systems of engineering protection from environmental sinking and pollution. Cavityless drains of composite profile have attracted railwaymen’s attention providing effective draining of subgrade, they simultaneously increase its bearing capacity. Its application is especially effective on poorly permeable soils when laying in the zone of seasonable freezing of soils.

Spectral transition of multiscale turbulence in the tokamak pedestal

The transition in the turbulence spectrum from ion-scale dominated regimes to multiscale transport regimes that couple ion and electron scales is studied with gyrokinetic simulations of turbulent transport. The simulations are based on DIII-D high-confinement mode (H-mode) plasma parameters in the tokamak pedestal. The transition is initiated by varying the ion temperature gradient. To our knowledge, no full multiscale simulations of pedestal-like transport have been done previously. The experimental parameters lie in a bifurcation region between the two regimes. At long wavelengths, a complex, ion-direction hybrid mode is the dominant linearly unstable drift wave, while an electron temperature gradient-driven mode is unstable at short wavelengths. In the transition from the multiscale branch to the ion-scale branch, the magnitude of the ion-scale poloidal wavenumber spectrum of the nonlinear turbulent energy flux increases and the magnitude of the high-wavenumber spectrum decreases. The decrease in the electron-scale transport is due to nonlinear mixing with ion-scale fluctuations and the ion-scale-driven zonal flows. A shift in the total energy associated with the fluctuating electrostatic potential intensity from dominantly drift kinetic energy in the multiscale regime to dominantly potential intensity in the ion-scale regime is well-correlated with the trend in the total energy flux.

High-Order Accurate Numerical Simulation of Supersonic Flow Using RANS and LES Guided by Turbulence Anisotropy

This paper discusses accuracy improvements to Reynolds-Averaged Navier–Stokes (RANS) modeling of supersonic flow by assessing a wide range of factors for physics capture. Numerical simulations reveal complex flow behavior resulting from shock and expansion waves and so, a supersonic jet emanating from rectangular nozzle is considered. PIV based experimental data for the jet is available from literature and is used for validation purposes. Effect of various boundary conditions and turbulence modeling approaches is assessed qualitatively and quantitatively. Of particular interest are the inlet conditions considering the turbulence intensity and the effect of upstream air supply duct, the effect of nozzle wall surface roughness on nozzle internal flow and downstream, wall y+ sensitivity for boundary layer resolution and laminar to turbulent transition modeling. In addition to mesh sensitivity, domain dependency is conducted to evaluate the appropriate domain size to capture the kinetic energy dissipation downstream of the nozzle. To further improve the flow characteristics, accounting for the anisotropy of Reynolds stresses is also one of the focuses. Therefore, non-linear eddy viscosity-based two-equation model and Reynolds stress transport model are also investigated. Additionally, the results of baseline linear (Boussinesq) RANS are compared. Corresponding comparisons with high-fidelity LES are presented. Jet self-similar behavior resulting from all simulation fidelities is assessed and it appears that turbulent flow in LES becomes self-similar, but not in RANS. Finally, various factors such as the nozzle geometry and numerical modeling choices influencing the anisotropy in jet turbulence are discussed.

Quantification of wind turbine energy loss due to leading‐edge erosion through infrared‐camera imaging, numerical simulations, and assessment against SCADA and meteorological data

AbstractQuantification of the performance degradation on the annual energy production (AEP) of a wind farm due to leading‐edge (LE) erosion of wind turbine blades is important to design cost‐effective maintenance plans and timely blade retrofit. In this work, the effects of LE erosion on horizontal axis wind turbines are quantified using infrared (IR) thermographic imaging of turbine blades, as well as meteorological and SCADA data. The average AEP loss of turbines with LE erosion is estimated from SCADA and meteorological data to be between 3% and 8% of the expected power capture. The impact of LE erosion on the average power capture of the turbines is found to be higher at lower hub‐height wind speeds (peak around 50% of the turbine rated wind speed) and at lower turbulence intensity of the incoming wind associated with stable atmospheric conditions. The effect of LE erosion is investigated with IR thermography to identify the laminar to turbulent transition (LTT) position over the airfoils of the turbine blades. Reduction in the laminar flow region of about 85% and 87% on average in the suction and pressure sides, respectively, is observed for the airfoils of the investigated turbines with LE erosion. Using the observed LTT locations over the airfoils and the geometry of the blade, an average AEP loss of about 3.7% is calculated with blade element momentum simulations, which is found to be comparable with the magnitude of AEP loss estimated through the SCADA data.

Three-dimensional vortex dynamics and transitional flow induced by a circular cylinder placed near a plane wall with small gap ratios

The flow past a cylinder in proximity to a plane wall is investigated numerically for small gap ratios. Three vortex dynamic processes associated with different hairpin vortex generation mechanisms are identified for the first time, and the wake-induced turbulent transition is analysed. The vortex shedding is suppressed at $G/D = 0.1$ , while the spanwise vortex is generated via a Kelvin–Helmholtz instability and evolves into hairpin vortices. For $G/D= 0.3$ , the upper and lower rollers alternatively shedding from the cylinder, interact with the secondary vortex. The split secondary vortex merges with the upper roller and results in a new vortex downstream, which develops into hairpin vortices. When $G/D = 0.9$ , the secondary vortex interacts with the lower roller and then evolves into hairpin vortices. A tertiary vortex induced by the secondary vortex is observed, rotating in the opposite direction to the secondary vortex the wake-induced transitions share the same route. The velocity fluctuations deviate from the optimal growth theory in the pre-transitional region. In the transitional region low-frequency disturbances penetrate the sheltering edge to generate streaks where the disturbance energy declines. In the turbulent region the logarithmic layer is formed, indicating that the turbulent equilibrium is established.

Variations of Subgrid-Scale Turbulent Fluxes in the Dry Convective Boundary Layer at Gray Zone Resolutions

Abstract In the numerical gray zone of the convective boundary layer (CBL), the horizontal resolution is comparable to the size of organized convective circulation. As turbulence becomes partially resolved, gridscale variations of the subgrid-scale (SGS) turbulent fluxes become significant compared to the mean. Previously, such variations have often been ignored in scale-adaptive planetary boundary layer (PBL) schemes developed for the gray zone. This study investigates these variations with respect to height and resolution based on large-eddy simulations. It is found that SGS fluxes exhibit maximum variability at the center of the gray zone, where the resolved and the SGS mean fluxes are approximately equal. A simple analytical model is used to associate such characteristic variations to the nonlinear interactions of the dominant energy-containing mode of CBL turbulence. Examination of the horizontal distribution of the SGS fluxes reveals their preferential location over the updraft edges surrounding the core. A priori analysis further suggests the ability of a scale-similarity closure to reproduce the unique spatial patterns of the SGS fluxes at gray zone resolutions. Four scale-adaptive PBL schemes are evaluated focusing on their representations of the modeled SGS flux variability. Their shared shortcomings as a result of their gradient diffusion–based formulation are exposed. This study suggests that a mixed model consisting of a scale-adaptive PBL scheme to represent the mean, and a scale-similarity component to account for gridscale variability to be advantageous for the gray zone. Significance Statement As Gresho and Lee’s famous quote on numerical schemes goes, “Don’t suppress the wiggles. They’re telling you something!” In the numerical gray zone, where the grid spacing is comparable to the characteristic length scale of the flow, turbulence is partially resolved. The “wiggles” (or gridscale variability) reflecting the resolved heterogeneity of turbulent fluxes is an outstanding feature of the gray zone. However, they are often overlooked as error bars to the mean. This study uncovers the significance of these error bars, and characterizes and explains their variations with height and grid spacing. A suitable model that captures such variations is investigated to help build better planetary boundary layer schemes.

Structure of the most unstable modes in anisotropic turbulence

Dynamics of most unstable modes from isotropic state is the topic of this paper considered. Initial conditions for these modes are given to be agreed with the well-known energy spectra of fully developed turbulence. The simplified equation for the small-scale velocity of incompressible fluid is applied. The simple shear turbulent flow is considered. The final expressions for Reynolds stresses, dissipation transfer should be averaged over the time of the energetic eddies turnover.

Simulation of the mixing process in a slot channel with jet injection

The article presents the results of modeling in a two-dimensional formulation of the hydrodynamic process of mixing two streams of Newtonian fluids in a channel with jet injection. The mixing process takes place in a turbulent flow mode, without chemical and thermal interaction. The article considers the flow of a mixture in a flat slit channel when mixing two liquids. The first component of the mixture flows through the main wide channel. At a distance of 50 mm from the point of entry into the channel perpendicular to the main channel, there is a radial jet nozzle with a width of 1 mm, through which the second component enters the mixing channel. The article discusses several problems with different initial mixing rates of components. The characteristics corresponding to water and ethyl alcohol were taken as test components for mixing liquids. The simulation was carried out in the ANSYS-Fluent software package. The purpose of this work is to study the process of mixing two liquids in a slit channel. The article presents the results of modelling the mixing process.

Оценки равновесной температуры на поверхности сферического затупления спускаемого орбитального аппарата

The descent of a stabilized capsule from a low near-earth orbit along a ballistic trajectory is considered. The descent of a stabilized capsule from a low Earth orbit along a ballistic trajectory is considered. A reliable preliminary estimate of the intensity of the thermal impact of the air flow, which enters the surface of the capsule, is the equilibrium temperature of this surface. The value of the equilibrium temperature follows from the condition of equality of the density of the total heat flux brought to the surface and the density of the heat flux, removed due to the own radiation of this surface. Based on the peculiarities of the heat exchange process on the surface of the spherical blunt capsule three sections of the trajectory are distinguished. At free-molecular flowing of the blunt, at the section of descent the intensity of heat exchange is assumed to be proportional to the energy of the molecule flow. At the section corresponding to the continuum flow regime, the intensity of convective and radiative heat transfer is estimated by empirical dependences obtained by processing the calculated and experimental results. When combining laminar and turbulent flow modes on the attenuation surface, the highest of two local values of convective heat flux density is taken. At the intermediate section of the descent, the Knudsen number is taken as an argument determining the intensity of convective heat transfer, and empirical dependences are used to estimate the radiation component. A quantitative analysis of the change in the time of descent of the maximum value of the equilibrium temperature on the blunted surface for a ball capsule and a capsule with a ball segment was carried out. The results of such analysis are the purpose of the present work and allow to identify the most heat-stressed areas of the descent trajectory on the blunt surface at the preliminary stage of the design and to choose the acceptable materials of the thermal protection coating

Boundary layer transition induced by low-speed synthetic jets

The effect of low-speed, circular synthetic jets (SJ) on the turbulent transition of a laminar boundary layer is studied through direct numerical simulations. The SJ capability in fixating the streamwise location of transition onset is analyzed in terms of its operation parameters (reduced frequency F+ and momentum coefficient Cμ). The effect of free-stream turbulence (FST) on the near-wall vortical structures generated by the synthetic jet is analyzed as well, to mimic the actual operation of the control system. Velocity spectra, phase portraits, and dynamic mode decomposition allow us to investigate flow unsteadiness and transition to a chaotic state. In most of the investigated cases, SJs successfully promote transition, as the result of varicose-symmetric hairpin-like vortices generated at the jet exits. In particular, it is found that increasing the momentum coefficient always reduces the size of the laminar region; a non-monotonic behavior of the laminar fetch is noted as the reduced frequency is increased, suggesting the existence of an optimal frequency value. Combination of FST and SJ actuation results in spanwise-asymmetric vortical structures, with little difference in the location of the transition onset as compared to the previous case. The present analysis can be used to gather information on the practical implementation of low-speed SJ actuators as active turbulators.

Existence of finite and anisotropic heavy ion parallel compressibility pinch in the gyrokinetic turbulence

Turbulence-driven heavy ion transport in hot magnetized plasma is investigated by means of the gyrokinetic theory and simulations. A finite heavy ion parallel compressibility pinch (${\mathrm{\ensuremath{\Gamma}}}_{s,\ensuremath{\parallel}}$) is found in the gyrokinetic framework, in contrast to the conventional understanding that ${\mathrm{\ensuremath{\Gamma}}}_{s,\ensuremath{\parallel}}$ is negligible. A perturbation theory clarifies the turbulence frequency dependence of ${\mathrm{\ensuremath{\Gamma}}}_{s,\ensuremath{\parallel}}$, resolving the discrepancy with experimental observations. It is also predicted by a nonlocal approach of the parallel advection term that ${\mathrm{\ensuremath{\Gamma}}}_{s,\ensuremath{\parallel}}$ is strongly anisotropic on a magnetic surface. The parameter dependence shows that decreasing the heavy ion mass ${m}_{s}$ strongly enhances ${\mathrm{\ensuremath{\Gamma}}}_{s,\ensuremath{\parallel}}$ through kinetic effects, leading to a deviation from the $1/{m}_{s}$ scaling. Moreover, the pinch direction can be reversed in nonlinear trapped electron mode turbulence through the inverse cascade.