SST Model Research Articles

Influence of isolator section on the shock augmented mixing in SCRAMJET engine

The paper aims to numerically study the effect of isolator design on shock-augmented mixing in a generic supersonic Ramjet (SCRAMJET) engine and reduce what would be a very complicated thermodynamics+aerodynamic optimization in the design to a simpler aerodynamic one. Changing the length of the isolator section effectively varies the thickness of the boundary layer that is formed in the isolator, which then enters into the SCRAMJET combustor. The study is conducted on a three-dimensional geometry using the Menter's SST model for turbulence on an in-house unstructured grid RANS solver. In this paper, the authors quantify the effect of the incoming boundary layer thickness on the shock-augment air-fuel mixing in the SCRAMJET combustor. It is observed that for the maximum variation in the isolator length, the normalized mixing volume only improves by 7%, which is only a marginally better mixing of air and fuel obtained as with an increase in the isolator length. The authors conclude that the Prandtl-Meyer expansion fan (PMEF) formed at the flame-holder effectively isolates the combustor from the isolator section to a degree that the isolator dimensions can therefore be chosen entirely on considerations of flow homogenization of inlet air before it enters the combustor section of the SCRAMJET engine. Although, the paper analyses a generic SCRAMJET with specific dimensions, the results are more generally applicable than for the specific design actually considered in the paper. Thus this study is also relevant to SCRAMJET engines that may differ radically from our considered design in form and dimensions.

Validation and Comparisons of Methodologies Implemented in a RANS-VoF Numerical Model for Applications to Coastal Structures

Methodologies to be used in numerical models based on Reynolds-averaged Navier–Stokes (RANS) equations and the volume of fluid (VoF) to deal with waves over coastal structures, which involve wave breaking and overtopping and porous structures, are shown in this manuscript. Two turbulence models, k-ε NLS (non-linear Reynolds stress tensor) and k-ε SCM (stabilized closure model), that are used to avoid the growth of the eddy viscosity, are implemented in the FLUENT® numerical model. Additionally, equations of momentum and turbulence models are adapted to simulate porous media of coastal structures. Comparisons of performance of k-ε NLS, k-ε SCM and standards k-ε and k-ω SST models in several classical cases of regular and random waves on coastal structures are carried out. It was noticed that the standard k-ε turbulence model, and k-ω SST with less intensity, over-predicted eddy viscosity, caused the decay of the free surface elevation and under-estimated wave overtopping discharge. k-ε NLS and k-ε SCM turbulence models have similar performance, with slightly better results of k-ε NLS, showing good agreement with experimental ones.

Evaluation of CFD URANS Turbulence Models for the Building under Environmental Wind Flow with Experimental Validation

Wind flow on and around buildings attains more importance among architectures, builders, urban planners, structural engineers, and wind engineers. Wind tunnel experiments and wind flow assessments of full-scale buildings are expensive and complex in varied terrain conditions. Hence, wind flows are extensively assessed using Computational Fluid Dynamics (CFD). By following the turbulence parameters, CFD turbulence models create the wind tunnel and atmospheric environments. No literature has till elucidated which CFD turbulence model is more suitable for predicting the terrain wind flow on and around high-rise buildings. The efficiency of the CFD models, their performance, and their accuracy must be validated with experimental results, which is indispensable before using the turbulence model in practice. Therefore, this investigation aims to validate the Unsteady Reynolds–Averaged Navier-Stokes (URANS) simulations for a setback tall building under open terrain wind conditions enclosed within the wind tunnel dimensions. The URANS simulation is accompanied with Standard k– ε, Realizable k-ε, RNG k-ε, Standard k–ω, k–ω SST and RSM. The k –ω SST and RSM turbulence models have reproduced the wind pressure coefficients observed from the wind tunnel. However, all turbulence models failed to produce the same velocity profiles at downstream recirculation, as they vary with sampling time. The transient feature, RMS (Root Mean Square), is better reproduced by RSM and k–ω SST models, while the most unsteady features like across wind spectra and eddies were captured by Realizable, RSM and SST using iso-surface. k–ω SST and RSM models predict similar results with the experiment. Where less computational time was required for the SST, it is promising that this model provides both mean pressure and unsteady feature, encouraging more accurate simulation around the buildings.

Towards accurate numerical fluid dynamics of pump-turbine runners in deep partial load operation

The operation of pumped storage power plants at off-design conditions requires an understanding of the flow field and the resulting damage to ensure a safe and economic operation. Within this study, CFD simulations are carried out for an existing pump-turbine at deep part load operation to enable calculating the damage in later structural analyses. Furthermore, this study includes the comparison between the SST and SBES turbulence model at different mesh sizes. The simulation results are compared with model test data regarding head, torque and pressure fluctuations at a bottom ring and draft tube cone sensor. Whereas all simulation setups are within a reasonable range to the available model test data, the pressure fluctuations on the runner blades as well as the resolved vortices within the runner blade channels show a qualitative difference between both models. The SBES turbulence model resolves more vortices and thus different pressure fluctuations. Subsequent structural analyses show higher accuracy of the SBES turbulence model compared with the SST model. A mesh refinement from 25 million nodes to 80 million nodes with the SBES model reveals a better resolution of vortices, but only local differences in pressure fluctuations.

Turbulence Model Comparative Study for Complex Phenomena in Supersonic Steam Ejectors with Double Choking Mode.

Scholars usually ignore the non-equilibrium condensing effects in turbulence-model comparative studies on supersonic steam ejectors. In this study, a non-equilibrium condensation model considering real physical properties was coupled respectively with seven turbulence models. They are the k-ε Standard, k-ε RNG, k-ε Realizable, k-ω Standard, k-ω SST, Transition SST, and Linear Reynolds Stress Model. Simulation results were compared with the experiment results globally and locally. The complex flow phenomena in the steam ejector captured by different models, including shock waves, choking, non-equilibrium condensation, boundary layer separation, and vortices were discussed. The reasons for the differences in simulation results were explained and compared. The relationship between ejector performance and local flow phenomena was illustrated. The novelty lies in the conclusions that consider the non-equilibrium condensing effects. Results show that the number and type of shock waves predicted by different turbulence models are different. Non-equilibrium condensation and boundary layer separation regions obtained by various turbulence models are different. Comparing the ejector performance and the complex flow phenomena with the experimental results, the k-ω SST model is proposed to simulate supersonic steam ejectors.

Heat transfer in pipes with twisted tapes: CFD simulations and validation

Inserts are placed inside heat exchangers to promote turbulence and maximize the heat transferred. Twisted tapes enhance heat transfer with minimal pressure drop increase for double pipe heat exchangers. Their design typically relied on experimental correlations, but nowadays CFD software is gaining interest. The choice of the turbulent model is of paramount importance and not addressed in the literature. This research aims to compare the combinations of k-ε, k-ω and RSM as well as their different wall treatments available in Ansys Fluent® and literature experimental data. Different twist ratios and Reynolds numbers are tested. Currently, no research is found in literature comparing different CFD methods for this type of units. The main objective of this research is to find the combination of RANS turbulent model and wall treatment that will most accurately reproduce the global values needed (Nusselt number and friction factor) when designing a heat exchanger with twisted tape inserts. Results show that the selection of the wall treatment is far more relevant than the turbulence model. Simulations have less discrepancy between themselves than the empirical correlations. Best performing models were k-ε Standard with ML wall treatment, which provided an average deviation from correlations ranging between 15 and 18%. K-ω SST models also provided accurate performance when estimating friction factor values with 17 to 20% deviation. Results provide clues for choosing a suitable turbulent model and are useful to minimize the error provided by the models.

Assessment of Turbulence Models over a Curved Hill Flow with Passive Scalar Transport

An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses (<u′v′> > 0) inside the recirculating zone. Very strong APG induced outer secondary peaks in <u′v′> and turbulence production as well as an evident negative slope on the constant shear layer.

Experimental and numerical studies on the use of a needle for variable capacity single-phase ejectors

Vapor ejectors raise interest for cooling applications given their low electrical consumption, however, a single ejector suffers from a lack of capacity flexibility. The current study explores the use of an adjustable needle to control the primary nozzle throat area (At) in view to allow variable ejector capacity. Tests were done using a large scale test bench working with the refrigerant R245fa. CFD simulations using k-ω SST and, for the first time, Scale-Adaptive Simulation (SAS), were also done. A comparison of the simulation results using the two different turbulence models (ω SST and SAS) showed negligible difference. Also, the simulation results with the ω SST model showed good agreement with the experimental values. For fixed operating conditions, the simulation showed that the reduction of the throat area from 1.00At to 0.61At increased the entrainment ratio from 24% to 56%, but reduced the critical outlet pressure from 440 kPa to 315 kPa. For all conditions, the transiting exergy efficiency ηtr,ex first increased with the increase of the outlet pressure (Pout), reached a maximum value when Pout is close to the critical outlet pressure (Pcrit), and then decreased sharply due to the decrease of the entrainment ratio. Finally, CFD modelings with a blunt tip needle to see the effect of the needle geometry and in order to have a lower throat area value of 0.39At. This novel study shows that the ejector equipped with a needle is able to maintain ejector performance with changes in operating conditions.

Simulation of Micro-Chip Heat Sink to Investigate the Thermo-Fluid Behaviour and Temperature Distribution

The efficient cooling from heat sinks is important for the proper functioning and longevity of a central processing unit (CPU). In this Project, ANSYS simulation of a modified micro-fin heat sink was done and the results were analyzed to get an idea of the approximate rate of Heat transfer, Temperature distribution and Thermo fluid behavior of the heat sink mounted on the central processing unit. Air cooling methods by free convection and conduction are used for heat extraction. The simulation was based on the effects on inlet fluid velocity, design of fins and location of source on the performance of the Heat Sink. The Turbulent SST model is used. The Heat transfer rate, temperature and pressure distribution were obtained for different designs of fin. After complete analysis, an increased air flow velocity at inlet and thin sheet fins with source at the bottom of heat sink provides the most effective heat transfer rate.

Numerical Simulation of Irregular Breaking Waves Using a Coupled Artificial Compressibility Method

Wave breaking is widely recognized as a very challenging phenomenon to emulate using numerical/computational methods. On that condition, the transition from modelling regular to irregular breaking waves is not trivial. Even though some issues are surpassed in CFD simulations, there still are two substantial problems to account for. The first one entails the proper generation of irregular waves in a numerical wave tank, while the second is the introduction of the turbulent regime of breaking in the solver. The present work addresses these two problems by employing the Stabilized k−ω SST model for turbulence closure and by proposing an efficient and accurate method for irregular wave generation. Apart from that, an artificial compressibility method is used for coupling the system of equations, which solves these equations in a non-segregated manner and overcomes problems pertaining to the existence of the interface in free-surface flows. The methodology is validated through the test case of irregular wave propagation over a submerged breaker bar and a piecewise sloped bottom, indicating the ability of the method to capture irregular breaking wave phenomena. Simulations are in fair agreement with experimental data regarding energy spectra and free surface time-series, while results suggest that the known over-prediction of turbulent kinetic energy (TKE) is significantly constrained by the stabilized k−ω SST model.

Numerical investigation of a near-wake flowfield with base bleed

A computational study of a supersonic flow in the base region and the nearest wake of a cylindrical body moving at a supersonic speed has been carried out. In this case, the “prehistory” of the flow was taken into account, i.e. the configuration of the computational domain was as close as possible to the real one. The use of various RANS turbulence models for calculating flow in the base region and the nearest wake was analyzed. The following turbulence models were considered: 1) SST model; 2) Standard k–ε model; 3) k–ε RNG model; 4) Standard Reynolds Stress (RS) model; 5) RS BSL model. Based on a comparison of the calculation results with experimental data, it is shown that: 1) when calculating the flow in the base region and in the wake of the vehicle, it is very important to take into account the “prehistory” of the flow, i.e. to calculate the flow around the entire vehicle; 2) the best match was obtained using the the k–ε RNG model. The effect of base bleed on the near-wake flowfield of a cylindrical afterbody aligned with a Mach 2.5 flow has been investigated.

Digital Analysis of Common Turbulence Patterns in Centrifugal Pump Flow Simulation Based on COMSOL Multiphysics Software

The present paper tackles the convergence and performance of three numerical turbulence models in the flow simulation. The benchmark analysis was performed using the COMSOL Multiphysics code, and the turbulence on a centrifugal water pump was generated numerically using the k−ε,k−ω, and k−ω SST models. However, the geometry was conducted on SolidWorks due to its complexity. First, the flow modeling was driven by solving the stationary Navier-Stokes equations. Then, the effects of the tested models, on the numerical CFD simulation, were examined. The analyzed results demonstrated that the best calculation precision was obtained using the k−ω model, whereas the lowest was provided by the k−ω SST model. A remarkable pumping performance was also recorded.

On the Investigation of the Effect of Tower and Hub Exclusion on the Numerical Results of a Horizontal Axis Wind Turbine

In order to construct a 3D rotor model, periodicity and Multiple Reference Frame (MRF) strategies were applied to address full Computational Fluid Dynamics (CFD) that incorporate the Reynolds-averaged Navier–Stokes (RANS) equations. A simulation model was run to examine the power of the wind turbines, both including and excluding the hub and tower. The purpose of this was to identify an approach that would decrease the computational time cost of wind turbine simulation. Firstly, a full-scale horizontal axis wind turbine simulation was carried out, and subsequently, comparisons were made with the results of the wind tunnel experiment utilizing the K-omega SST and Sparlat Allmaras viscosity models. The results were compared to determine the optimum model with the lowest simulation time and highest accuracy to the experimental results. Following this, the complete models’ simulation was compared to the model excluding the hub and tower in order to check the disparity in the results of the two. Additionally, to ascertain the computational cost saving, the ratio of the two simulation times was established. The findings show that the two models produce results that correspond well to the experimental results, and that the power coefficient had a greater value for the complete model than the simple model. Furthermore, it was found that the power coefficient values of the full model have significantly higher levels of similarity to the experimental values. The Sparlat Allmaras and K-omega SST viscosity models returned error percentages of 0.52% and 12.6% respectively. For the partial model utilizing the same computer device, a 5.7% error rate was recorded, with a computational time saving of approximately 34%.

하이브리드 난류 모델을 이용한 전류고정덕트 후류의 고정도 수치 해석

A hybrid turbulence model has developed by combining a sub-grid scale model using dynamic k equation in LES with k-ω SST model of RANS equation. To ascertain potential applicability of the hybrid turbulence model, fully developed turbulent channel flows at ReSUBτ/SUB=180 have been simulated of which computational domain has a top wall with coarse cells and a bottom wall with fine cells. The streamwise mean velocity and turbulent intensity profiles showed a good agreement with DNS data when using the hybrid model rather than using a single model in k-ω SST or dynamic k equation models. Computational simulations of turbulent flows around KVLCC2 with a pre-swirl duct have been mainly performed using the hybrid turbulence model. Compared to the results obtained from RANS simulation with k-ω SST model as well as LES with dynamic k equation SGS model, turbulent wakes of the duct in the present simulation using the hybrid turbulence model were very similar to that of LES. Also, the resistances acting on hull, rudder and duct in hybrid turbulence model were similar to those in RANS simulation whereas the viscous forces acting on the hull in LES had a significant error due to coarse cells inappropriate to the sub-grid scale model.

Maxillary sinus aeration analysis using computational fluid dynamics

The maxillary sinus aeration using the computational fluid dynamics (CFD) method based on individual adult patients’ computed tomography (CT) scans were analyzed. The analysis was based on CT images of 4 patients: one with normal nose anatomy and three with nasal septal deviation (NSD) and concha bullosa (CB). The CFD simulation was performed using the Reynolds-Average Simulation approach and turbulence closure based on linear eddy viscosity supplemented with the two-equation k-omega SST model. As a result, it was found that the lower part of NSD has the most significant impact on the airflow change within the maxillary sinuses compared to CB and the upper part of NSD. In a healthy nose, the airflow in the sinuses is continuous, while NSD and CB change this flow into pulsatile. Multiple changes in the direction of flow during one respiratory phase were observed. The flow intensity within the maxillary sinus opening is lower on the NSD side. The concept of vorticity measure is introduced to evaluate and compare various patients qualitatively. Typically, the lowest values of such measures are obtained for healthy airways and the highest for pathological changes in the nasal cavity.

Response of Internal Flowfield to Downstream Pressure Oscillation in a Scramjet Engine

ABSTRACT The response of the internal flowfield to back pressure oscillation in a scramjet engine is studied numerically. The physical domain of interest includes both the isolator and combustor. The theoretical formulation is based on the full conservation equations in three dimensions. Turbulence closure is achieved using the SST model to improve the prediction of shear stress under adverse pressure gradient. As part of the downstream boundary condition, a high pressure is imposed to simulate the pressure condition in the situation with combustion. Periodic pressure oscillations in the frequency range of 376–1128 Hz are also imposed to mimic flow oscillations caused by combustion instability. The downstream disturbance is found to influence the supersonic core flow in the isolator through separated boundary layers. A shock train is initiated from corner separation shocks, and X-type shocks take place to adjust the pressure between the core flow and the separation bubble on the wall. When the frequency of back pressure oscillation is lower, the shock front fluctuates over a larger displacement. The shock front acceleration is correlated to the rate of change of pressure in the separation bubble on the wall, which increases with increasing back pressure frequency. However, when the oscillation increases to 1128 Hz, the rate of change of pressure in the separation bubble decreases, resulting in a more stable shock front. The shock train acts as a low-pass filter and damps flow oscillation in the isolator-combustor.

Performance and near-wake characteristics of a bidirectional horizontal-axis tidal turbine

Horizontal-axis tidal turbines (HATTs) have the potential to produce considerable amount of clean renewable energy from ocean tides. To reduce costs and improve reliability, a full-scale bidirectional HATT (BHATT) with a centrosymmetric hydrofoil was designed to operate in two opposite flow directions. To better understand the performance and the near-wake characteristics of the BHATT, a three-dimensional (3D) numerical model with a k-ω SST model and a sliding mesh method was applied. The numerical framework was validated using two different experiments, and the mesh convergence was tested. The result shows that the BHATT can achieve a power coefficient of 43.5% at design tip speed ratio (TSR), and cause a remarkable circumferential flow in the near-wake from it. In addition, the presence of a support structure causes a decrease and a fluctuation in both power and thrust coefficient, especially when the rotor is downstream from the support structure. Similarly, a misalignment also leads to a decrease in both power and thrust coefficient. However, it causes almost no fluctuation. Finally, the influence of different support structures on the performance of the BHATT in different misalignment cases was studied.

Numerical Investigation of a Supersonic Flow in the Near Wake Region of a Cylindrical Afterbody

A computational study of a supersonic flow in the base region and the nearest wake of a cylindrical body moving at a supersonic speed have been carried out. A mathematical model of high-enthalpy flows is presented. In this case, the "prehistory" of the flow was taken into account, i.e., the configuration of the computational domain was as close as possible to the real one. The use of various turbulence models for calculating flow in the base region and the nearest wake was analyzed. The following turbulence models were considered: 1) the Spalart --- Allmaras model; 2) SST model; 3) standard k--ε model; 4) k--ε model with compressibility correction; 5) k--ε RNG (renormalized group) model; 6) k--ε Realizable model; 7) standard Reynolds Stress (RS) model; 8) RS BSL (Reynolds stress baseline) model. Based on a comparison of the calculation results with experimental data, it is shown that: 1) when calculating the flow in the base region and in the wake of the vehicle, it is very important to take into account the "prehistory" of the flow, i.e., to calculate the flow around the entire vehicle; 2) the best match was obtained using Reynolds Stress models and the k--ε RNG model

The Accuracy of the Numerical Solution in Predicting Ahmed Body Components Drag Coefficients

In recent years, the aerodynamic drag became a major interest of automotive industry as it is one of the main components affecting the fuel consumption. To reduce aerodynamic drag force and improve the energetic efficiency, a complete understanding of the flow around ground vehicles is highly required. This study focused on the accuracy of a three different turbulence models in predicting Ahmed body components drag coefficients. Ahmed Body is a simplified model which mimics the bluff bodies in automotive aerodynamics. This study uses ANSYS Fluent to simulate the flow around Ahmed Body. Three different turbulence models (K-epsilon, K-omega SST and SST) were used in the study. The drag coefficient components for the different sections of the model were validated with the experimental published data. The results have shown that the SST model predicted accurately the total drag coefficient but failed to provide good agreement for the components drag coefficients.

Performance of a bidirectional horizontal-axis tidal turbine with passive flow control devices

Horizontal-axis tidal turbines (HATTs) have the acknowledged potential to extract a large amount of green renewable energy from ocean tides. Among these, bidirectional HATTs (BHATTs) with centrosymmetric hydrofoils have advantages in terms of reliability and maintenance cost. To improve the performance of BHATTs, this paper investigates the influence of different passive flow control devices (PFCDs), such as wing fences, winglets, and squealers, on the performance of the BHATT. To the end, a three-dimensional (3D) numerical model with a k-ω SST model and a sliding mesh method was applied to simulate a 18 m diameter BHATT. The numerical framework was validated using two experiments. The mesh convergence was tested. The results show that the wing fences can effectively suppress the spanwise flow above the suction surface of blades, although they cannot improve the BHATT performance. To inhibit the wingtip vortices and enhance the torque of the rotor, a series of centrosymmetric winglets are designed and optimized. The optimal BHATT can produce up to 2.3% more energy at design tip speed ratio (TSR).