Computation Of Turbulent Flows Research Articles

Computational aeroacoustic modeling using hybrid Reynolds averaged Navier–Stokes/large‐eddy simulations methods with modified acoustic analogies

AbstractThis study employs a numerical approach to investigate noise mechanisms in tandem cylinders. The tandem cylinder configuration is an excellent model for aircraft landing gear. The study is therefore primarily motivated by the need to understand the aircraft landing gear as a primary contributor to airframe noise during approach and landing. Fluctuations in the flow properties induced by turbulent flow around the tandem cylinders are computed and analyzed. A hybrid improved delayed detached eddy simulation turbulence model is employed to compute the boundary layer flow as well as to compute the fluctuations in the flow properties. The numerical methodologies for the turbulent flow computations are implemented on the OpenFOAM software platform. The study also considers the impact of neglecting the volume sources and employs two modified versions of Curle's and Ffowcs‐Williams and Hawkings analogies. The results are in good agreement with published numerical and experimental data in the existing literature.

Application of approximate dispersion-diffusion analyses to under-resolved Burgers turbulence using high resolution WENO and UWC schemes

This paper presents a space-time approximate diffusion-dispersion analysis of high-order, finite volume Upwind Central (UWC) and Weighted Essentially Non-Oscillatory (WENO) schemes. We perform a thorough study of the numerical errors to find a-priori guidelines for the computation of under-resolved turbulent flows. In particular, we study the 3-rd, 5-th and 7-th order UWC and WENO reconstructions in space, and 3-rd and 4-th order Runge-Kutta time integrators. To do so, we use the approximate von Neumann analysis for non-linear schemes introduced by Pirozzoli. Moreover, we apply the “1% rule” for the dispersion-diffusion curves proposed by Moura et al. [41] to determine the range of wavenumbers that are accurately resolved by each scheme. The dispersion-diffusion errors estimated from these analyses agree with the numerical results for the forced Burgers' turbulence problem, which we use as a benchmark. The cut-off wavenumbers defined by the “1% rule” are evidenced to serve as a good estimator of the beginning of the dissipation region of the energy cascade and they are shown to be associated to a similar level of dissipation, with independence of the scheme.Finally, we show that WENO schemes are more diffusive than UWC schemes, leading to stable simulations at the price of more dissipative results. It is concluded both UWC and WENO schemes may be suitable schemes for iLES turbulence modeling, given their numerical dissipation level acting at the appropriate wavenumbers.

Implicit Time-Spectral Method for Unsteady Reynolds-Averaged Navier–Stokes Computations of Turbulent Flows

An efficient implicit Fourier time-spectral method is developed for the unsteady Reynolds-averaged Navier–Stokes (URANS) computations of turbulent flows. In addition to the time-spectral URANS equations, the time-spectral Spalart–Allmaras turbulence model equation is also solved by the space–time lower-upper symmetric Gauss–Seidel (LU-SGS) implicit scheme. The efficient LU-SGS sweeps are directly extended to the time domain without introducing additional approximation error, delayed implicit temporal coupling, and additional algorithmic steps. The ability of the algorithm is verified through the test cases of the pitching NACA0012 airfoil and pitching ONERA M6 wing. Dominant Fourier modes of the drag coefficient and its components are identified. Special attention is paid to the nonlinear behavior of the surface pressure and skin friction coefficients in the shock-free area, especially the region downstream the shock where the influence of turbulence is significant. For the test case of a pitching ONERA M6 wing, difference in local flow unsteadiness for the two branches of the -type shock is observed. The phenomenon is further discussed through study of the shock motion range and the upstream Mach number for each shock branch.

A scaling law for the required transition zone depth in hybrid LES-DNS

There is a substantial need to better understand multi-physics interactions in realistic laboratory experiments, yet it is currently unfeasible to perform direct numerical simulations (DNS) of most such problems. As a result, reduced-order models such as those used to represent unclosed subgrid-scale stresses in large-eddy simulations (LES) and advanced numerical techniques, including adaptive mesh refinement and nested grid structures, are required for the study of multi-physics interactions in practical situations. In particular, it is now possible to model many real-world flows with LES fidelity, and to selectively refine the computational mesh in a small region of the domain to resolve multi-physics interactions with DNS fidelity. However, rules and guidance for the valid implementation of this multi-fidelity ‘hybrid LES-DNS’ approach have yet to be fully formulated. Within the embedded DNS region, multi-physics interactions will be influenced by (i) macroscale features (e.g. large-scale shear gradients or body forces that produce turbulence), (ii) LES-regulated mesoscale features (e.g. advecting turbulent eddies), and (iii) microscale features that are subgrid-scale to the LES (e.g. layers of fast-reacting intermediate chemical species). In this study, we address the second of these effects, which poses particular challenges due to the transfer and overlapping nature of advecting mesoscale eddies between the purely LES (i.e. macroscale) and purely DNS (i.e. microscale) regions. By combining dimensional analysis and computational results, we formulate a scaling law for the required depth of the transition zone surrounding the embedded DNS region within hybrid LES-DNS computations of realistic turbulent flows.

Turbulent flow in a square cross-sectioned bubble column computed by a scale-resolving Reynolds-stress model

The present study focuses on the computations of turbulent flow in a square cross-sectioned bubble column by utilizing the two-fluid model (TFM) in conjunction with advanced Reynolds-stress models (RSMs) within the Unsteady Reynolds-averaged Navier–Stokes (URANS) framework. The use of such an advanced modeling approach in combination with the TFM, rarely employed for two-phase flow computations, is motivated by its inherent capability of resolving both Reynolds-stress anisotropy and stress-dissipation anisotropy, also of the corresponding residual turbulence. The presently adopted RSMs are based on the formulation proposed initially by Jakirlić and Hanjalić (2002) for incompressible single-phase flows. Two different RSM versions (Jakirlić and Maduta (2015)), both based on the homogeneous dissipation (εh) concept that employs the specific dissipation rate (ωh=εh/k) as the length-scale-determining variable, are applied in the present work. The baseline model version is formulated within the conventional RANS framework, whereas the advanced model represents an instability-sensitized, eddy-resolving RSM variant, capable of adequately capturing the fluctuating turbulent motion in accordance with the SAS methodology (Scale-Adaptive Simulation) proposed by Menter and Egorov (2010). The results obtained by both RSMs are discussed along with the corresponding experimental database made available by Deen and co-workers (Deen et al., 2000, 2001; Deen, 2001). Additionally, the most-widely used modeling approach for two-phase flows is followed by utilizing the Standard high-Reynolds number k-ε model for the purpose of a comparative assessment. Furthermore, both RANS models are extended by two different proposals for a model term accounting for the so-called bubble-induced turbulence (BIT). The model equations are implemented into the open source software OpenFOAM® based on the finite-volume method on unstructured meshes. The results obtained exhibit high level of agreement with the experimental reference demonstrating high potential of both Reynolds-stress models in computing the bubbly flows. This relates in particular to the correspondingly captured resolved turbulence intensity in this bubble-plume-induced unstable flow event, leading subsequently to a correctly returned velocity field. The mean flow asymmetry, characterizing the baseline Eddy-Viscosity Model (EVM) performance was remedied only after introducing the BIT-related source term contributing appropriately to intensified turbulence production.

Embedded large eddy simulation of transitional flow over NACA0012 aerofoil

An accurate computation of near-field unsteady turbulent flow around aerofoil is of outstanding importance for aerofoil trailing edge noise source prediction, which is a representative of main contributor to airframe noise and fan noise in modern commercial aircraft. In this study, an embedded large eddy simulation (ELES) is fully implemented in a separation-induced transitional flow over NACA0012 aerofoil at a moderate Reynolds number. It aims to evaluate the performance of the ELES method in aerodynamics simulation for wall-bounded aerospace flow in terms of accuracy, computational cost and complexity of implementation. Some good practice is presented including the special treatments at RANS-LES interface to provide more realistic turbulence generation in LES inflow. A comprehensive validation of the ELES results is performed by comparing with the experimental data and the wall-resolved large eddy simulation results. It is concluded that the ELES method could provide sufficient accuracy in the transitional flow simulations around aerofoil. It is proved to be a promising alternative to the pure LES for industrial flow applications involving wall boundary layer due to its significant computational efficiency.

A modified STRUCT model for efficient engineering computations of turbulent flows in hydro-energy machinery

A modified STRUCT (MST) turbulence model for efficient engineering computations of turbulent flows in hydro-energy machinery is proposed in this paper. The MST model switches between URANS and LES-like modes using a new damping function to adjust the turbulent viscosity. Compared with the original STRUCT method, the modifications are as follows: (1) the BSL k-ω model with the Spalart-Shur correction is chosen as the new baseline to improve the sensitivity to rotation and curvature; (2) a new adaptive time-scale ratio is proposed to avoid the arbitrariness of geometric averaging operation in the original method; (3) the normalized helicity is introduced into the new damping function to detect the energy backscatter phenomenon. Five classical high Reynolds number flow cases are tested. The results show that the turbulent viscosity of the MST model is reasonably reduced in the massively separated regions and LES-like mode is activated, which captures more turbulent vortices and fluctuations on the URANS grids. With high efficiency and robustness, the MST model inherits the advantages of the original STRUCT method and improves the prediction accuracy of the turbulence with rotation and curvature, which enables efficient engineering computations of turbulent flows in hydro-energy machinery.

Simulations of Wall Bounded Turbulent Flows Using General Pressure Equation

The general pressure equation (GPE) based method is fully explicit, and the method does not require either solving the pressure Poisson equation nor executing sub-iteration for incompressible flow simulation. However, few numerical validations of GPE method are available, especially under complex flows like turbulence. In this work, GPE is used to conduct direct numerical simulations of the turbulent lid-driven cavity (LDC) flow at $${\text {Re}}=3200$$ and fully developed turbulent flow through a square duct at $${\text {Re}}_{\tau }=360.$$ Predicted turbulence statistics are compared with existing numerical and experimental data, providing an excellent quantitative agreement. The intricate flow patterns such as the Taylor–Gortler-like vortices in LDC flow and the mean secondary flow at the cross-section in the square duct are captured, showing both qualitative and quantitative agreements with measurements. Results from the present study indicate the capability of the GPE method for accurate incompressible turbulent flow calculation.

CFD Analysis of Mechanisms Underlying the Porosity-reducing Effect of Atomized Flows in High-pressure Die Cast Products

In high-pressure die casting, attention has been paid to the J factor, which is defined by the speed of the metal injected at the gate and the shape of the gate. In casting experiments using a piston die, the porosity of the product can be reduced by increasing the J factor such that the metal flow passing through the gate forms an atomized flow. To clarify the underlying mechanisms, we developed a system for simulating a two-phase flow of air and aluminum by large-scale calculations of turbulent flow. During the development of the system, we injected metal into an open space and performed imaging to confirm the state of the atomized flow. The system was then verified by reproducing the atomized flow. The analysis results visualized the many small turbulent vortices generated in the thick part far from the gate. We demonstrated that the change from small to longitudinal vortices promoted entrainment of air into the aluminum and increased the efficiency of air expulsion outside the die through an increase in the J factor.

Wavelet-Based Adaptive Unsteady Reynolds-Averaged Navier–Stokes Simulations of Wall-Bounded Compressible Turbulent Flows

A wavelet-based adaptive approach, called wavelet-based adaptive unsteady Reynolds-averaged Navier–Stokes (WA-URANS), is proposed to solve the URANS equations for the computation of wall-bounded internal and external compressible turbulent flows. The new approach uses anisotropic wavelet-based mesh refinement, and its effectiveness is demonstrated for flow simulations with two different turbulence models, namely, the Spalart–Allmaras and models for a variety of two- and three-dimensional flow configurations with arbitrary geometries, different speed regimes, and various boundary conditions. Supersonic plane channel flow, a weakly compressible channel flow with periodic hill constrictions, a subsonic zero-pressure-gradient flat-plate boundary-layer flow, the separated flow over NASA wall-mounted hump, the Bachalo–Johnson flow (axisymmetric transonic bump flow), and a flow past a circular cylinder at a subcritical Reynolds number are tested as benchmark flows. The effectiveness and efficiency of the new wavelet-based approach are demonstrated by comparing the results of the WA-URANS simulations with literature data. This comprehensive validation of the WA-URANS approach encourages its application to practical problems of industrial interest, and demonstrates the feasibility of extending this approach to wavelet-based wall-modeled large-eddy simulation (LES) and hybrid URANS/LES. Finally, the current study serves as a baseline for the development of a unified hierarchical framework for eddy-resolving turbulence modeling, capable of performing computations of different fidelity, ranging from adaptive direct numerical simulations to adaptive URANS computations.

Numerical Simulation of Turbulent Flow and Pollutant Dispersion in Urban Street Canyons

In this study, we have developed a numerical model based on an open source Computational Fluid Dynamics (CFD) package OpenFOAM, in order to investigate the flow pattern and pollutant dispersion in urban street canyons with different geometry configurations. In the new model, the pollutant transport driven by airflow is modeled by the scalar transport equation coupling with the momentum equations for airflow, which are deduced from the Reynolds Averaged Navier-Stokes (RANS) equations. The turbulent flow calculation has been calibrated by various two-equation turbulence closure models to select a practical and efficient turbulence model to reasonably capture the flow pattern. Particularly, an appropriate value of the turbulent Schmidt number has been selected for the pollutant dispersion in urban street canyons, based upon previous studies and careful calibrations against experimental measurements. Eventually, the numerical model has been validated against different well-known laboratory experiments in regard to various aspect ratios (a relationship between the building height and the width of the street canyon), and different building roof shapes (flat, shed, gable and round). The comparisons between the numerical simulations and experimental measurements show a good agreement on the flow pattern and pollutant distribution. This indicates the ability of the new numerical model, which can be applied to investigate the wind flow and pollutant dispersion in urban street canyons.

An a posteriori-implicit turbulent model with automatic dissipation adjustment for Large Eddy Simulation of compressible flows

In this work we present an a posteriori high-order finite volume scheme for the computation of compressible turbulent flows. An automatic dissipation adjustment (ADA) method is combined with the a posteriori paradigm, in order to obtain an implicit subgrid scale model and preserve the stability of the numerical method. Thus, the numerical scheme is designed to increase the dissipation in the control volumes where the flow is under-resolved, and to decrease the dissipation in those cells where there is excessive dissipation. This is achieved by adding a multiplicative factor to the dissipative part of the numerical flux. In order to keep the stability of the numerical scheme, the a posteriori approach is used. It allows to increase the dissipation quickly in cells near shocks if required, ensuring the stability of the scheme. Some numerical tests are performed to highlight the accuracy and robustness of the proposed numerical scheme.

A technique of flux reconstruction at the interfaces of nonconforming grids for aeroacoustic simulations

SummaryA flux reconstruction technique is presented to perform aeroacoustic computations using implicit high‐order spatial schemes on multiblock structured grids with nonconforming interfaces. The use of such grids, with mesh spacing discontinuities across the block interfaces, eases local mesh refinements, simplifies the mesh generation process, and thus facilitates the computation of turbulent flows. In this work, the spatial discretization consists of sixth‐order finite‐volume implicit schemes with low‐dispersion and low‐dissipation properties. The flux reconstruction is based on the combination of noncentered schemes with local interpolations to define ghost cells and compute flux values at the grid interfaces. The flow variables in the ghost cells are calculated from the flow field in the grid cells using a meshless interpolation with radial basis functions. In this study, the flux reconstruction is applied to both plane and curved nonconforming interfaces. The performance of the method is first evaluated by performing two‐dimensional simulations of the propagation of an acoustic pulse and of the convection of a vortex on Cartesian and wavy grids. No significant spurious noise is produced at the grid interfaces. The applicability of the flux reconstruction to a three‐dimensional computation is then demonstrated by simulating a jet at a Mach number of 0.9 and a diameter‐based Reynolds number of 4×105 on a Cartesian grid. The nonconforming grid interface located downstream of the jet potential core does not appreciably affect the flow development and the jet sound field, while reducing the number of mesh points by a factor of approximately two.

Important properties of turbulent near-wall flows which are not accounted by modern rans models

Hypothesis based on experiments of various authors which explains the origin of many difficulties arising in the development of RANS models was proposed in the paper. A model design method based on this hypothesis is proposed. Models based on this method are much simpler than traditional ones. In addition, these models demonstrate the possibilities that are inaccessible to traditional models - they reproduce the regularities of cascade energy transfer, results of dissipative scales calculations correspond to the experiments. At present the method is verified by calculations of forced turbulent flows in boundary layers and pipes and channels.

Numerical Simulation of Gas-Solid Two-Phase Flow and Deposition in a Novel Carbon Nanotubes Particles Trap

The effects of gas-solid two-phase flow, flow velocity, model structure and temperature on deposition characteristics in Carbon nanotubes (CNTs) particles trap were simulated by Euler-Lagrangian model, adopt Realizable k–ε Model calculation of turbulent flow in trap, tracking of particle trajectories using stochastic orbital model. The simulated results are in good agreement with the experimental results, deposition of particles at orifice edge of baffle; The adhesion of particles to the wall surface is consistent with the phenomena presented by previous simulations. With the increase of temperature, the deposition rate of particles increases. The more the number of holes in baffle, the more particles are captured. At high wall temperature, the particle aggregation positions of concentric and heterocentric baffle trap are different. The deposition of the four wall particles in the concentric hole trap is obvious at the exit location, the particle deposition of heterocentric hole trap on baffle is obvious. For the vertical flow field, particles are preferentially deposited on the bottom baffle, and the heterocentric trap has a higher deposition rate than that of the concentric hole. The simulation reveals the difficult flow characteristics of particles in the trap, and the conclusions obtained are of great significance to improve the reliability of the design and amplification of the trap.

Liquid sloshing in a two-dimensional rectangular tank: A numerical investigation with a T-shaped baffle

In this study, the liquid sloshing in a closed, partially filled, T-shaped baffled and unbaffled two-dimensional rectangular tank was numerically investigated. The tank was rotated with two different rotation angles of 4° and 8°, and a fixed angular velocity of 3.3 rad/s, which was determined by taking the natural frequency of the tank into consideration, for the filling depths of 50% and 75%. The baffle heights as well as the rotation angles and filling depths were systematically varied and the effect of these parameters on the hydrodynamic loads on the tank wall and free surface elevations was examined. The calculations were carried out in two different ways, by means of laminar and turbulent viscous flow solvers, which use the finite differences and finite volume discretization techniques in conjunction with the “volume of fluid” technique, respectively. The general flow topology, free surface deformations, and vorticity distributions are provided and discussed in detail. It was found that the baffle was fully effective in pressure and wave damping when its height was greater than 80% of the liquid level. • Laminar and turbulent flow computations were performed for the simulation of liquid sloshing in a two-dimensional rectangular tank with a T-shaped baffle. • The effect of the baffle height, liquid depth and rotational angles on the pressure levels acting on the side wall were demonstrated. • The topological features of the flow, free surface deformations and vorticity distributions at constant sloshing phases were presented.

Near-Wall Determination of the Turbulent Prandtl Number Based on Experiments, Numerical Simulation and Analytical Models

The Reynolds-averaged computation of turbulent flow with heat transfer most commonly models the turbulent heat flux as directly related to the turbulent flux of momentum through the turbulent Prandtl number. Its significant deviation from a uniform bulk flow value for high molecular Prandtl numbers needs to be adequately described to predict accurately the heat transfer. The present study derives a model for the near-wall variation of this important parameter, used as input into an analytical solution of heated turbulent pipe flow. The basic functional form of the profile of the turbulent Prandtl number is determined from direct numerical simulations (DNS), and experimental data are used for model calibration. The analytically predicted Nusselt numbers agree very well with experimental measurements, proving the reliability of the proposed model for the turbulent Prandtl number also for Reynolds numbers well beyond the scope of DNS. The validation against experiments further highlights the significant effect of the temperature-dependent material properties of the considered high Prandtl number liquids. Numerical simulations often discard this aspect to reduce the computational effort. The present combination of DNS, analytical solution, and experiments appears as a convenient approach for modeling turbulent key quantities such as the turbulent Prandtl number, which is well applicable to other convective flow conditions and Prandtl number regimes, as well.

Heating modes and design optimization of cogeneration steam turbines of powerful units of combined heat and power plant

An important scientific and technical problem of increasing the efficiency of CHP steam turbine units through the optimization of their operation modes and the creation of new highly efficient flow parts of cogenerating turbines is solved. Solutions to the problem of rational distribution of heat loads between the network heaters of cogeneration turbines during the heating period are presented. The calculations were performed using the software package SCAT which was developed in IPMach NAS of Ukraine. The carrying out of calculations of three-dimensional turbulent flows in flow parts of turbines using modern software systems is an effective direction of increased efficiency of power equipment. For the numerical research of three-dimensional currents, steam in the flow part of the steam turbine software package IPMFlow which is developed in IPMach NAS of Ukraine is used. With the use of software package IPMFlow, the researches of three-dimensional currents steam in the flow part of the medium pressure cylinder of the steam turbine of series T-100-130 are carried, which showed the feasibility of optimizing the geometry of the flow part in order to improve gas-dynamic characteristics of blades apparatuses.

Two-dimensional calculations of stratified turbulent flow in a pipe

Two-dimensional calculations of stratified turbulent flow in a pipe

HIFiRE-5b Flow Computations and Attitude Determination via Comparison with Flight Data

Laminar and turbulent flow computations are carried out for a range of conditions bounding the flight conditions of the Hypersonic International Flight Research and Experimentation (HIFiRE) 5b laminar–turbulent transition research flight. The resulting computational database is used to create an interpolant for heat transfer and pressure in the following variables: angle of attack, yaw angle, Mach number, Reynolds number, circumferential station, and streamwise station. Flight pressure is compared to angle-of-attack- and yaw-dependent computational results for pressure distribution at selected time points in the descent trajectory of the HIFiRE-5b flight. Computed pressures, normalized by freestream pressure, are interpolated to the flight Mach numbers at each time point throughout the descent trajectory; and the angle of attack and yaw are estimated from measured pressure by determining the combination minimizing the difference between the measured and computed pressures. The resulting vehicle attitude is compared to the vehicle attitude derived from inertial measurement unit and Global Positioning System results from the flight. The two methods showed excellent agreement for the entirety of the analyzed reentry portion of the trajectory.