RANS Model Research Articles

A Robust Implementation of a Reynolds Stress Model for Turbomachinery Applications in a Coupled Solver Environment

Abstract In the industrial simulation of flow phenomena, turbulence modeling is of prime importance. Due to their low computational cost, Reynolds-averaged methods (RANS) are predominantly used for this purpose. However, eddy viscosity RANS models are often unable to adequately capture important flow physics, specifically when strongly anisotropic turbulence and vortex structures are present. In such cases, the more costly 7-equation Reynolds stress models often lead to significantly better results. Unfortunately, these models are not widely used in the industry. The reason for this is not mainly the increased computational cost, but the stability and convergence issues such models usually exhibit. In this article, we present a robust implementation of a Reynolds stress model that is solved in a coupled manner, increasing stability and convergence speed significantly compared to segregated implementations. In addition, the decoupling of the velocity and Reynolds stress fields is addressed for the coupled equation formulation. A special wall function is presented that conserves the anisotropic properties of the model near the walls on coarser meshes. The presented Reynolds stress model is validated on a series of semi-academic test cases and then applied to two industrially relevant situations, namely, the tip vortex of a NACA0012 profile and the Aachen Radiver radial compressor case.

Reduction of surface friction drag in scramjet engine by boundary layer combustion

This study investigates the effects of boundary layer combustion on skin friction reduction in a model scramjet combustor. The 4-equation RANS model (Transition SST model) is employed as the turbulence model and one experimental case which involves supersonic turbulent boundary layer combustion is used for validating the numerical simulation method. Then a wall jet device which can be used to inject hydrogen into boundary layer is designed and added to the lower wall of the combustor in the model scramjet engine. The numerical results showed that the optimal ratio between the primary fuel, producing thrust, and the wall jet fuel, used for drag reduction, for getting the largest skin-friction drag reduction when installing the wall jet device in the combustor, is 3:1. The study of modifying the location of the wall jet device showed that setting the wall jet position too close to the upstream or downstream could not get the maximum overall drag reduction. The installation position in the flow path depends on the coupling of primary fuel and wall jet fuel combustion. In practical scramjet design, the distribution ratio of primary to wall jet fuel and the location of wall jet device should be weighed to improve skin-friction reduction efficiency and overall engine performance.

Compressibility Modified RANS Simulations for Noise Prediction of Jet Exhausts with Chevron

The impact of compressibility modified RANS turbulence closures is investigated for high subsonic round and chevron jet flows with Mach = 0.9 and Re = 1.03×106, including the predicted acoustic noise generation. The well-documented chevron jet flow and noise cases, namely NASA SMC000 and SMC006 are selected as the simulation configurations. Two compressibility RANS closures are considered, which are based on the k-ε turbulence model. The first type only considers the compressibility dissipation rate, and the second type accounts for three modifications of compressibility dissipation rate, pressure dilation and production limiter. The acoustic noise is calculated employing the SNGR (Stochastic Noise Generation and Radiation) method using the flow prediction of the three-dimensional RANS simulations. The results show that both of the two types of compressibility modified RANS models improve the accuracy of the mean flow and turbulence quantities. This results in more accurate jet noise predictions than with the standard RANS model. The first type modification is found to be moderate and the second type is remarkable. The noise results by the second type model, i.e. Sarkar2 model, agree with the experimental data quite well. For the mean flow field, the compressibility modified model (Sarkar2 model) estimates a shorter potential jet core, and improved predictions of the velocity in the downstream region are observed. The study demonstrates the importance of considering the compressibility modified RANS closure for the noise prediction of high-speed jets via the comparison to experimental data. Hence, the SNGR method is found to be cost effective for jet noise prediction, when compared to other approaches.

A Numerical Investigation on the Equivalence of Shock−Wave/ Boundary-Layer Interactions using a Two Equations RANS Model

A detailed numerical investigation of two different modes of shock wave-turbulent boundary layer interaction (SWBLI) is presented. Equivalence of ramp induced SWBLI (R-SWBLI), and impingement shock based SWBLI (I-SWBLI) is explored from the computational study using an in-house developed compressible flow solver. Multiple flow deflection angles and ramp angles are employed for this study. For all the investigated cases, a freestream Mach number of 2.96 and Reynolds number of 3.47×107m−1 are considered. The k−ε model with the improved wall function of present solver predicted wall pressure distributions and separation bubble sizes very close to the experimental measurements. However, the separation bubble size is slightly over overpredicted by the k−ω model in most of the cases. The effect of overall flow deflection angle and upstream boundary layer thickness on the SWBLI phenomenon is also studied. A nearly linear variation in separation bubble size is observed with changes in overall flow deflection angle and upstream boundary layer thickness. However, the equivalence of SWBLI is noted to be independent of these two parameters. The undisturbed boundary thickness at the beginning of the interaction is identified as the most adequate scaling parameter for the length of the separated region.

Prediction of gas cavities size and structure and their effect on the power consumption in a gas-liquid stirred tank by means of a two-fluid RANS model

Aerated cavities behind the impeller blades in stirred tanks affect the power transferred to the liquid that in turns affects heat and mass transfer, thus the development of fully predictive simulation methods to detect the formation of cavities, their size and structures is of paramount importance for an effective simulation of aerated reactors and bioreactors. In this work, operating conditions corresponding to different cavity structures are investigated by means of a Reynolds averaged two-fluid model without adjustable parameters. Based on the comparison with previous experiments and correlations, the method proved to be reliable in the prediction of the transition between vortex-clinging and small '3-3′ cavities, cavity size and power drawn reduction. For the first time, small '3-3′ cavities with volume fractions close to unity are obtained with a steady approach. The power reduction mechanism is observed and a novel interpretation of the formation of the asymmetrical cavities is proposed.

Numerical Study of Three-Dimensional Surface Jets Emerging from a Fishway Entrance Slot

The efficiency of a fishway is determined by the ability of immigrating fish to follow its attraction flow (i.e., its jet) to locate and enter the fishway entrance. The hydraulic characteristics of fishway entrance jets can be simplified using findings from widely investigated surface jets produced by shaped nozzles. However, the effect of the different boundary conditions of fishway entrance jets (characterized by vertical entrance slots) compared to nozzle jets must be considered. We investigate the downstream propagation of attraction jets from the vertical slot of a fishway entrance into a quiescent tailrace, considering the following boundary conditions not considered for nozzle jets: (1) slot geometry, (2) turbulence characteristics of the approach flow to the slot, and (3) presence of a lateral wall downstream of the slot. We quantify the effect of these boundary conditions using three-dimensional hydrodynamic-numeric flow simulations with DES and RANS turbulence models and a volume-of-fluid method (VoF) to simulate the free water surface. In addition, we compare jet propagation with existing analytical methods for describing jet propagations from nozzles. We show that a turbulent and inhomogeneous approach flow towards a vertical slot reduces the propagation length of the slot jet in the tailrace due to increased lateral spreading compared to that of a jet produced by a shaped nozzle. An additional lateral wall in the tailrace reduces lateral spreading and significantly increases the propagation length. For highly turbulent flows at fishway entrances, the RANS model tends to overestimate the jet propagation compared to the transient DES model.

Effect of building cross-section shape on air pollutant dispersion around buildings

This research investigates the effect of building cross-section shape on the air pollutant dispersion around an isolated building. A Computational Fluid Dynamic (CFD) model was developed and validated based on detailed wind tunnel experimental data. Both Large Eddy Simulation (LES) and Realizable k-ε models were used for turbulence closure and LES was found to provide a better agreement with experimental results than the RANS model, particularly in replicating pollutant dispersion characteristics associated with wind-structure interaction. The LES model was then utilized to study the effect of building cross-section shape. Four building cross-section shapes, square, chamfered, curved and circular, along with different air pollutant emission locations were studied. It has been found that the area around an isolated building can be categorized into three emission regions, namely the windward critical emission region, the leeward critical emission region and the side non-critical emission region. The air pollutant emissions from the leeward critical emission region affect the building the most. Furthermore, the size of this region depends on the building cross-section shape, with the circular and curved buildings having a smaller leeward critical emission region than those for the chamfered and square buildings. Evidently, air pollutant dispersion around a building is dominated by wind-structure interaction which in turn is dependent on the building cross-sectional shape. These characteristics can be utilized to mitigate air pollutant dispersion in urban environments in conjunction with careful consideration of the air pollutant emission location relative to a building.

Numerical Investigation on Effect of Turbulence Model Selection for Aerodynamic Prediction of Axial Flow Fan

Axial fans are one of the major power consuming cooling systems in most of the devices being used in daily basis. So, it is very important to predict the aerodynamic performance of the fan design. One way of estimating the fan performance in full 3D scale is by using Computational fluid dynamics. Turbulence models plays an important role in application of CFD for Axial flow fans. It is important to choose proper turbulence model in order to get the most accurate results. In this study, a baseline model experiment was chosen and validated. Both steady and transient studies have been carried out. Steady state simulation was performed using Reynolds Averaged Navier Stokes turbulence models such as Spalart Allmaras, RNG k-ε with swirl dominant flow, k-ω SST model and Transition SST model. Transient simulation is performed using RANS model such as Spalart Allmaras, Standard k-ε with standard wall function, RNG k-ε with swirl dominant flow, k-ω SST model, Detached Eddy Solver and Standard k-ω with embedded Large Eddy Solver. Both studies reveal that with coarse mesh URANS model can predict more accurate than Hybrid URANS models. Out of all the models DES showed a very good prediction in terms of pressure drop.

Сравнение моделей турбулентности для закрученных течений

The paper considers a turbulent fluid flow in a rotating pipe, known as the Taylor --- Couette --- Poiseuille flow. Linear RANS models are not suitable for simulating this type of problems, since the turbulence in these flows is strongly anisotropic, which means that solving these problems requires models accounting for turbulence anisotropy. Modified linear models featuring corrections for flow rotations, such as the SARC model, make it possible to obtain satisfactory solutions. A new approach to turbulence problems has appeared recently. It allowed a novel two-fluid turbulence model to be created. What makes this model different is that it can describe strongly anisotropic turbulent flows; moreover, it is easy to implement numerically while not being computationally expensive. We compared the results of solving the Taylor --- Couette --- Poiseuille flow problem using the novel two-fluid model and the SARC model. The numerical investigation results obtained from the novel two-fluid model show a better agreement with the experimental data than the results provided by the SARC model

Direct Numerical Simulation of Turbulent Heat Transfer at Low Prandtl Numbers in Planar Impinging Jets

DNS simulations of planar impinging jets are performed to provide reference data to validate RANS models for low-Prandtl turbulent heat transfer. In that configuration, a hot plane jet is blown from a slot at the top wall of a channel and impinges on the cooler bottom wall. The present DNS simulations are carried out at Re=4000 and 5700, based on the jet width and velocity, and for Pr down to 0.01. Two cases are investigated: the case of a laminar uniform jet profile and the case of a fully turbulent inlet profile coming from an auxiliary channel flow simulation running in parallel. The DNS results show how the heat transfer in this configuration evolves from the turbulence-dominated case at Pr=1 to the molecular diffusion-dominate case at Pr=0.01. The temperature field at Pr=0.01 is much smoother than at higher Prandtl number because the much larger heat diffusivity quickly diffuses the temperature fluctuations. The comparison between the laminar and the fully-developed turbulent inflows also show different heat transfer characteristics in the vicinity of the stagnation point, depending on the presence of velocity fluctuations in the bottom-wall boundary layers.

Physical and numerical modeling of focused wave interactions with a low mound breakwater

Las ondas enfocadas, fenómeno que ocurre en el océano, es responsable del fallo de estructuras localizadas costa afuera. Existen pocos trabajos enfocados a su estudio en zonas cercanas a la costa. En el presente trabajo se estudió la estabilidad y funcionalidad de un dique vertical con banqueta baja en presencia de ondas enfocadas. Para ello, se realizaron ensayos en un canal de oleaje. Mediciones de superficie libre, presiones, subpresiones, y rebase fueron utilizadas para validar un modelo numérico que resuelve las ecuaciones RANS. Las predicciones de rebase y presiones/subpresiones máximas del modelo numérico presentaron diferencias promedio con respecto a las mediciones de 10% y 8%, respectivamente. Posteriormente, el modelo numérico se utilizó para evaluar el papel de la posición relativa de la estructura. Finalmente, se compararon los resultados del modelo físico con formulaciones semi-empíricas, encontrando diferencias significativas en rebase (≈100%) y en presiones y subpresiones (88%). Este estudio demuestra que el uso de modelos tipo RANS es una alternativa a las clásicas formulaciones semi-empíricas, pues se comparan mejor con las mediciones durante este tipo de fenómenos.

Large-eddy simulation of flow turbulence in clarification systems

Prediction of turbulent flow is required for design and assessment of clarifier systems that have been implemented throughout history to treat water in the urban water cycle through physical clarification. Yet, turbulent flow modeling is a relatively new tool that has not existed until the last half-century and can be, and often is, a tenuous component in a computational fluid dynamics simulation of unit operations and processes. Common Reynolds-averaged Navier–Stokes equation (RANS) approaches can be inadequate to obtain consistent and accurate flow solutions. In contrast, this study presents an application of large-eddy simulations (LES) for a clarification system with a high-order spectral element method employing 48 million degrees of freedom. Turbulent and unsteady flow characteristics are investigated, and statistics are examined for such a system. Simulation results are compared with laser Doppler anemometry measurements for mean flow velocity, turbulence kinetic energy, and Reynolds shear stress. LES results agree well with measurements, and the differences between LES and measurements are generally less than the reported measurement error. LES results capture the transition behavior from a jet-like flow at the near-inlet region to an open-channel flow at the downstream end of the system. Furthermore, LES results reveal that the widely adopted log-law of a classical turbulent boundary layer is not established in the system even at the most downstream location. Preliminary examination of commonly used RANS models identifies the challenges in application of RANS to such systems. The results from this study provide a benchmark for turbulence modeling of common water clarification systems.

Effect of the Inclusion of Photovoltaic Solar Panels in the Autonomy of UAV Time of Flight

Photovoltaic technology and unmanned aerial vehicles are both alluring areas with a lot of potential to explore. Consequently, they have an ability to adapt and progress when faced with new challenges, hence their wide range of applications. An auspicious combination between the two is born from the Unmanned Aerial Vehicles’ (UAVs) inability to to overcome some of its problems, namely the autonomy one. This article springs from the need to vanquish the problem, finding a more permanent solution. Its aim consists in the installation of solar photovoltaic panels in the structure of a UAV, with the objective of studying being its influence on the vehicle’s time of flight. To accomplish this, a theoretical study will be made, encompassing all the potential variables together with its influence. In order to verify the credibility of these claims, a prototype, based on the original aerial vehicle structure form and material, is constructed, using a finite element tool. Later, the prototype is used to evaluate possible harsh circumambient air to structure interactions, modeled by the fluid motion describer Navier–Stokes equations. For a smooth approach involving lighter computational power, a RANS model is used to asses the equations. Based on its results the chosen solar technology credibility is evaluated. A simulation of solar cells will also be carried out, accepting as input previously studied parameters which will modify its performance. Bearing in mind the produced results, it is concluded that the solar panels can only significantly augment the time of flight in very specific conditions.

Development of Experimental and Numerical Methods for the Analysis of Active Clearance Control Systems1

Abstract The ever increasing performance requirements of modern aeroengines necessitate the development of effective ways to improve efficiency and reduce losses. Casing temperature control is particularly critical from this point of view, since thermal expansion directly affects the blade tip clearance and thus the associated leakages. To limit the turbine tip flows, active clearance control (ACC) systems have been implemented over the last decades. These systems are usually based upon impingement cooling, generated by a series of perforated manifolds enclosing the turbine casing. When dealing with aeroengine low pressure turbines, the current trend in increasing the engine bypass ratio, so as to enhance the system propulsive efficiency, pushes the limits of ACC traditional design performance. The reduction of the pressure head at the ACC system inlet requires lower nozzle-to-target distances as well as denser impingement arrays to compensate the reduction of the jets' Reynolds number. Literature correlations for the impingement heat transfer coefficient estimation are then out of their confidence range and also RANS numerical approaches appear not suitable for future ACC designs. In this work, methodologies for the development of accurate and reliable tools to determine the heat transfer characteristics of low pressure ACC systems are presented. More precisely, this paper describes a custom designed finite difference procedure capable of solving the inverse conduction problem on the target plate of a test sample. The methodology was successfully applied to an experimental setup for the measurement of the thermal loads on a target plate of a representative low pressure ACC impinging system. The experimental outcomes are then used to validate a suitable numerical approach. Results show that RANS model is not able to mimic the experimental trends, while scale-resolving turbulence models provide a good reconstruction of the experimental evidences, thus allowing to obtain a correct interpretation of flow and thermal phenomena for ACC systems.

Numerical and Experimental Study of a Jet-and-Recirculation Stabilized Low Calorific Combustor for a Hybrid Power Plant

An atmospheric prototype burner is studied with numerical and experimental tools. The burner system is designed for operation in a hybrid power plant for decentralized energy conversion. In order to realize such a coupled system, a reliable combustion system has to be established. Numerical and experimental findings in the presented study demonstrate the capabilities of the novel burner system in suitable operation conditions. In this system, a solid oxide fuel cell (SOFC) is mounted upstream of the burner in the gas turbine system. The combination of both realizes a large operational flexibility with comparably high overall efficiency. Since the combustor is operated with SOFC off-gas, several challenges arise. Low calorific combustion needs careful burner design and numerical modeling, since the heat-loss mechanisms occur to be in the order of magnitude of thermal power output. Thus, different modeling strategies are discussed in the paper. The numerical studies are compared with experimental results and high-quality simulation results complement limited measured findings with easy-to-use low fidelity RANS models. A priori measurements are employed for the selection of investigation points. It is shown that the presented combustor system is able to cover low-calorific combustion over a large range of operation conditions, despite major heat-loss effects, which are characterized by means of numerical CFD (Computational Fluid Dynamics) modeling.

The effect of turbulence on mass transfer in solid fuel combustion: RANS model

In this paper, a kinetic-diffusion surface combustion model is examined. The model is modified such that two effects of turbulence are included: 1) enhancement of the mass transfer due to relative velocity between particles and fluid and 2) reduction of the mass transfer due to turbulence-induced particle clustering. Details of the implementation are discussed and the influence of parameters such as air-fuel ratio, particle number density, particle diameter, turbulence intensity and characteristic length scales are studied theoretically. A simplified numerical model of a combustion chamber is created to explore the effects of the combustion model predictions. Finally, the model is incorporated into simulations of an industrial-scale boiler to investigate the effect of turbulence on the net surface reaction rate in a real system. The study shows that although on average this effect is rather minor, there exist regions in which the carbon conversion rate is either decreased or increased by turbulence.

Análisis numérico del comportamiento del flujo en la sección de la garganta de una tobera cónica experimental

El patrón de flujo en toberas supersónicas lo definen los perfiles aerodinámicos de la geometría de las paredes internas, entre otros parámetros, donde la garganta es una sección crítica. En el presente trabajo, el objetivo es analizar el comportamiento del flujo en la sección recta de la garganta de una tobera cónica experimental de un motor de cohete sonda de combustible sólido. El flujo sobreexpandido se simuló con el código ANSYS-Fluent en un dominio computacional 2D, empleando el modelo RANS y el modelo de turbulencia de Menter, y la ecuación de Sutherland para la viscosidad en función de la temperatura. Se llevaron a cabo cinco casos de estudio para la longitud de garganta en el rango de 1-10 mm. Se obtuvo fluctuaciones de número de Mach, presión y temperatura, ondas de choque oblicuas en la sección de la garganta para la longitud de 10 mm; para longitudes menores disminuyó la intensidad de la magnitud del choque. Se concluye que para la longitud de garganta de 1 mm el flujo es transónico sin la presencia de choques oblicuos. En la sección divergente, las ondas de choque varían en su intensidad y cambian de posición.

An immersed boundary method for practical simulations of high-Reynolds number flows by <i>k-ε</i> RANS models

An immersed boundary method for practical simulations of high-Reynolds number flows by <i>k-ε</i> RANS models

Development of machine learning techniques to enhance turbulence models

The implementation of the machine learning methods of convolutional neural networks to enhance RANS closure models is presented. The RANS models are not universal and accurate, however they are computationally affordable. Finding a way to improve the model predictability will be an advantage. For this, machine learning algorithms based on available high-fidelity data sets for canonical flow cases obtained from DNS and measurements can be helpful. The application of these algorithms for a fully-developed turbulent channel flows with periodic hills, in a square duct and for other cases is considered.

Improvement of stall prediction accuracy on NACA airfoils in RANS model

Improvement of stall prediction accuracy on NACA airfoils in RANS model