Capabilities Of Turbulence Models Research Articles

Four-dimensional particle tracking velocimetry measurements of unsteady three-dimensional vortex onset and progression for 5415 straight ahead, static drift, and pure sway

Large-scale towed system four-dimensional particle tracking velocimetry (4DPTV) implementation and measurements are presented for towing tank tests for a naval combatant 5415 ship model for straight ahead, static drift β = 10°, and pure sway βmax = 10° conditions. The results, for the first time, provide instantaneous volumetric flow field data around the ship model including near the hull surface, and a complete description of the sonar dome 3D unsteady vortex separation onset and progression due to the 4DPTV larger measurement volume and higher data rates in comparison with previous results using tomographic particle image velocimetry (TPIV); thereby, providing the essential data required for the assessment of current hybrid Reynolds-averaged Navier–Stokes/large eddy simulation turbulence modeling capabilities and guidance for the necessary modeling improvements. The 4DPTV system is summarized and compared with the previous TPIV, including the camera calibration procedures, trigger systems, and synchronization with the sway motion. Analysis procedures, including data reduction procedures and the 3D vortex core detection and voxel labeling techniques, are described. The identification of the complex vortex separations and breakdown is aided using complementary detached eddy simulations. The vortex–vortex interaction process is visualized from instantaneous volumetric datasets. The pros and cons of the 4DPTV vs TPIV, the comparison between the second- and third-generation vortex visualization technique, and the statistical convergence error analysis of sonar dome port vortex for β = 10° are discussed. Plans for increased spatial resolution of the 4DPTV system and additional data reduction techniques for detailed turbulence analysis are also discussed.

Permodelan Numerik Aliran Angin Pada Jalan Antara Bangunan Tinggi Dengan Modifikasi Sudut Datang

In fluid dynamics analysis, one of the things to do is to perform numerical modeling validated on the resultsof experimentation. In numerical modeling of wind flow there are several forms of modeling used includingRANS, LES, DNS, etc. where the modeling has its own advantages and disadvantages. Among these models,RANS is a model that has the cheapest computer expense compared to other models so that it has the highestworkability. Therefore, rans method testing (Reynolds Averaged Navier Stokes) was conducted to determinethe capability of turbulence models in checking wind speed contours on the road between 4 simplesymmetrical tall buildings with 0o, 30o, and 45o attack an gles validated with the results ofexperimentation. This research was conducted using RANS modeling (Reynolds Averaged Navier Stokes) andstandard turbulence model k-ε and validated using Low Speed Wind Tunnel and PIV (Particle ImageVelocimetry). According to the results of the test, U/Uo wind speed conditions obtained in wind modelingwith RANS and k-ε standards have errors that are still acceptable.

New Concept for Design in Turbomachinery Applications Using Full RANS Gradient Methodology

Abstract Centrifugal compressors are mainly designed with the best efficiency point in mind and the focus oriented toward the impeller. The vaned diffuser geometry is usually developed using simple shapes, such as prismatic blades or wedges, which are unable to account for the flow distortion at the impeller outlet. Investigations of diffuser modifications using computational fluid dynamics (CFD) during the design phase can help to extend the operating range. In order to assess the effect of such modifications, simulations in off-design conditions need to be performed. Accurate predictions of local flow structures at such conditions require a robust flow solver with mixing-plane interfaces and advanced turbulence modeling capabilities. In recent years diffuser modifications were performed mainly based on manual trial-and-error approaches and show considerable potential for improvements. To accelerate the process and allow automation, which is crucial in industrial applications, it is suggested to use an adjoint-based optimization algorithm for diffuser shape design. In this work, a discrete adjoint solver implemented into a robust pressure-based CFD framework was used. While many optimization studies follow the frozen-turbulence approach, which essentially ignores the dependency of the sensitivity on the turbulence quantities, the present adjoint solver fully includes such effects. Automatic differentiation even allows for flexibility with respect to the choice of turbulence model. Any model available in the primal solver can also be used in the adjoint solver. In this particular case, the k–ɛ enhanced wall treatment model was used, since it shows good convergence behavior and is capable of predicting the machine characteristic even at off-design conditions. Mesh morphing was realized via the free-form deformation method, which provides stable mesh deformation and offers a simple and fast setup procedure. Several optimizations were conducted for the RWTH Aachen “Radiver” centrifugal compressor case with vaned diffuser. It was shown that an optimization strategy using a multi-operating point approach, considering part- and overload conditions, is capable of improving the peak efficiency as well as the overall shape of the machine characteristic.

Predicting turbulent flows in butterfly valves with the nonlinear eddy viscosity and explicit algebraic Reynolds stress models

The development of turbulence modeling is crucial for the numerical prediction of the flow behavior, especially for separation, stagnation, reattachment, recirculation, and streamline curvature of the flow through complex structures. In this study, the capability of turbulence models was estimated for predicting the flow in a butterfly valve. The explicit algebraic Reynolds stress model (EARSM) and nonlinear eddy viscosity model (NLEVM) were evaluated in terms of the velocity profile, turbulence intensity, and Reynolds stress, and their results were compared with those of the standard k–ε and renormalization group (RNG) models. A numerical validation was conducted with the flow past a backward-facing step as the benchmark test. Comparison with the validation test showed that the NLEVM accurately predicted the reattachment length. For the flow in a butterfly valve, the NLEVM and EARSM indicated a smaller velocity increase than the standard k–ε and RNG models in the recirculation area near the valve region. The NLEVM and EARSM demonstrated an ability to predict anisotropic stresses with a dominant stress value near the valve region.

Investigation of conical shock wave/boundary layer interaction in axisymmetric internal flow

A RANS (Reynolds-averaged Navier-Stokes simulation) study is performed on an internal duct at a free-stream Mach number 4.0, to investigate the internal conical shock wave/boundary layer interaction (CSBLI) developing from an axisymmetric wall surface. An effort is devoted on the analysis of the flow features of the internal CSBLI phenomena through a simple physical model. The work aims to extend the investigation of Zuo et al. [1] on external CSBLI. The numerical strategy is first described and the capability of turbulence models to capture SBLI physics is then assessed by comparison with experiments. In the interacted region a small recirculation bubble is identified, and the foot of the conical shock forms a λ system. Unlike the external CSBLI case, a horseshoe pattern of separation whose strength and size are reduced away from the symmetry plane is observed. The topology of flow separation is similar to the one of two-dimensional closed separation. The pressure in the pipe increases rapidly by the effect of multi-reflected conical shock waves. The wall-surface pressure in the separation bubble continues to increase due to three-dimensional area compression effect.

Numerical study on drag and lift coefficients of a marine riser at high Reynolds number using COMSOL multiphysics

Flow around a marine riser in water at the drag crisis regime was investigated using numerical modelling. In this regime, the drag coefficient drops off at a certain Reynolds number due to a change from laminar to turbulent flow. The aim is to investigate the capability of turbulence model to predict drag coefficient through COMSOL Multiphysics, a computational fluid dynamic (CFD) transient solver and compared against existing numerical models and experiment by Maritime Research Institute Netherlands (MARIN). Numerically, drag and lift forces depend on the point of separation from the cylinder in which different turbulence modelling will result in varying separation point and will lead to different vortex formation and the drag force. Reynolds Average Navier-Stokes (RANS) was employed using the k-ε and Menter’s Shear Stress Transport (SST) turbulence model in two-dimensional CFD simulation. Six Reynolds numbers, similar to the test case, were considered. It can be concluded that the standard k-ε turbulence model, can only provide a good approximation at high turbulence regime, which is Reynolds number of 3.15 × 105 and higher. While, SST turbulence model can provide a good approximation at subcritical regime which before the sudden drop of drag force regimes.

A comparison study on the capabilities of turbulence models and density models of CFD on realistic terrain

A comparison study on the capabilities of turbulence models and density models of CFD on realistic terrain

Computational Analysis of CompressorBlade

This computational work aims at analyzing the flow behavior through a linear cascade of compressor blades with the help of Computational Fluid Dynamics using the FLUENT software. An attempt has been made to study the boundary layer behavior on the blades at various flow incidence angles and Reynolds numbers, based on flow inlet velocity and blade chord. A Computational Fluid Dynamics based computational model is created with the reference to an experimental work already carried out. This study will bring out the methodology for solving the problem using Computational Fluid Dynamics tools. Particularly, the capability of turbulence model to predict laminar, turbulent and transition in the boundary layer flows has been made. The geometric modeling and meshing of the flow domain has been carried out using the software’s CATIA and GAMBIT respectively.Development of boundary layer, point of flow separation, flow reattachment and pressure coefficient over the blade surfaces have been found and compared with the experimental results.

Numerical Study of Flow Past a Circular Cylinder Using Hybrid Turbulence Formulations

Two multiscale-type turbulence models are implemented in the PAB3D solver. The models are based on modifying Reynolds-averaged Navier―Stokes equations. The first scheme is a hybrid Reynolds-averaged Navier―Stokes and large eddy simulation model using the two-equation ke model with a Reynolds-averaged Navier―Stokes and large eddy simulation transition function dependent on grid spacing and the computed turbulence length scale. The second scheme is a modified version of the partially averaged Navier―Stokes model, where the unresolved kinetic energy parameter f k is allowed to vary as a function of grid spacing and the turbulence length scale. Solutions from these models are compared to Reynolds-averaged Navier―Stokes results and experimental data for a stationary and rotating cylinder. The parameter f k varies between zero and one and has the characteristic to be equal to one in the viscous sublayer, and when the Reynolds-averaged Navier―Stokes turbulent viscosity becomes smaller than the large eddy simulation viscosity. The formulation, usage methodology, and validation example are presented to demonstrate the enhancement of PAB3D's time-accurate and turbulence modeling capabilities. The models are compared to Reynolds-averaged Navier―Stokes results and experimental data for turbulent separated flows and laminar separated flows around stationary and rotating cylinders. For a stationary cylinder, the turbulent separated case is accurately simulated using the general two-equation ke turbulence model (eddy-viscosity model). PAB3D accurately predicts the drag coefficient C D , lift coefficient C L , and the Strouhal number St. The laminar separated case was a challenge for the Reynolds-averaged Navier―Stokes computation with an eddy-viscosity turbulence model. The Reynolds-averaged Navier―Stokes with large eddy simulation and partially averaged Navier―Stokes performed well and showed marked improvements over the Reynolds-averaged Navier―Stokes solution. The modified partially averaged Navier―Stokes model was the most accurate. For the rotating cylinder, the spin ratio varied from zero to one, and the partially averaged Navier―Stokes results were in good agreement with published experimental data. Reynolds-averaged Navier―Stokes with large eddy simulation and partially averaged Navier― Stokes capture both temporal and spatial fluctuations and produce large-scale structures that do not occur in the Reynolds-averaged Navier―Stokes simulation. The current results show promise for the capability of partially averaged Navier―Stokes in simulating unsteady and complex flow phenomena.

Rans Predictions of Free Surface Effects on Axisymmetric Underwater Body

The aim of the current investigation is an assessment of the influence of free surface effects on hydrodynamic coefficients of an axisymmetric underwater body using Reynolds averaged Navier-Stokes (RANS) solver. This paper mainly focuses on the capabilities of turbulence models (two high-Re models, k-ε Re-Normalised Group (RNG) and k-ε Realizable and one low-Re model, k-ε Abe-Kondoh-Nagano (AKN) together with volume of fluid (VOF) method for hydrodynamic investigation of an axisymmetric underwater body (Afterbody1) near the free surface. The current numerical investigations were carried out to simulate measured hydrodynamic coefficients for several experimental parameters, including velocity variations from 0.4 m/s (Reynolds number Rev = 2.12x 105) to m/s (Reynolds number Rev = 7.42 x 105), depth of submergence from 0.75D to 4.0D, and angle of attack 0° to 15° on a vertical plane for a model scale of 1:2. Numerical simulations were conducted at a grid density of 0.46 million cells based on grid independence study using a commercial flow solver, Fluent. Free surface effects on hydrodynamic coefficients (drag, lift and pitching moment) are studied from the calculated hydrodynamic forces and moments. Validation of numerical results is done by comparing with measured values. The present investigation found that the k-ε Realizable model together with VOF method consistently provided superior performance in predicting the free surface wave effects on hydrodynamic coefficients for Afterbody1 model. This study is expected to aid the selection of apposite RANS model for simulation of hydrodynamic forces over underwater bodies near the free surface.

Calculations of High-Temperature Jet Flow Using Hybrid Reynolds-Averaged Navier-Stokes Formulations

Two multiscale-type turbulence models are implemented in the PAB3D solver. The models are based on modifying the Reynolds-averaged Navier Stokes equations. The first scheme is a hybrid Reynolds-averaged- Navier Stokes/large-eddy-simulation model using the two-equation k(epsilon) model with a Reynolds-averaged-Navier Stokes/large-eddy-simulation transition function dependent on grid spacing and the computed turbulence length scale. The second scheme is a modified version of the partially averaged Navier Stokes model in which the unresolved kinetic energy parameter f(sub k) is allowed to vary as a function of grid spacing and the turbulence length scale. This parameter is estimated based on a novel two-stage procedure to efficiently estimate the level of scale resolution possible for a given flow on a given grid for partially averaged Navier Stokes. It has been found that the prescribed scale resolution can play a major role in obtaining accurate flow solutions. The parameter f(sub k) varies between zero and one and is equal to one in the viscous sublayer and when the Reynolds-averaged Navier Stokes turbulent viscosity becomes smaller than the large-eddy-simulation viscosity. The formulation, usage methodology, and validation examples are presented to demonstrate the enhancement of PAB3D's time-accurate turbulence modeling capabilities. The accurate simulations of flow and turbulent quantities will provide a valuable tool for accurate jet noise predictions. Solutions from these models are compared with Reynolds-averaged Navier Stokes results and experimental data for high-temperature jet flows. The current results show promise for the capability of hybrid Reynolds-averaged Navier Stokes and large eddy simulation and partially averaged Navier Stokes in simulating such flow phenomena.

Numerical prediction of shock induced oscillations over a 2D airfoil: Influence of turbulence modelling and test section walls

The present study deals with recent numerical results from on-going research conducted at ONERA/DMAE regarding the prediction of transonic flows, for which shock wave/boundary layer interaction is important. When this interaction is strong enough ( M ⩾ 1.3), shock induced oscillations (SIO) appear at the suction side of the airfoil and lead to the formation of unsteady separated areas. The main issue is then to perform unsteady computations applying appropriate turbulence modelling and relevant boundary conditions with respect to the test case. Computations were performed with the ONERA elsA software and the URANS-type approach, closure relationships being achieved from transport-equation models. Applications are provided for the OAT15A airfoil data base, well documented for unsteady CFD validation (mean and r.m.s. pressure, phase-averaged LDA data, …). In this paper, the capabilities of turbulence models are evaluated with two 2D URANS strategies, under free-stream or confined conditions. The latter takes into account the adaptive upper and lower wind-tunnel walls. A complete 3D URANS simulation was then performed to demonstrate the real impact of all lateral wind-tunnel walls on such a flow.

Predictive capabilities of turbulence models for a confined swirling flow

The predictive capability of the Reynolds stress transport (RST) turbulence model is investigated for a confined swirling flow with the presence of a swirl-induced flow reversal at the centerline.