SST Model Research Articles

Numerical Investigation on the Pressure Drag of Some Low-Speed Airfoils for UAV Application

Progressive advancements in small to medium-sized fixed-wing UAVs call for prototype designing to be fast, accurate, and economical. This requires the numerical assessment of airfoil performance to be based on high fidelity replication of wind tunnel data. Furthermore, integration of drag reduction techniques is attractive as improvements in endurance, payload capacity and reduction in carbon footprints can be attained. Since a variety of suitable airfoil geometries are currently available for this application, selecting a fitting candidate can be difficult therefore risking potential gain in efficiency. The aim of this paper is to assist in resolving this issue by investigating the variation in pressure drag and its distribution with respect to the type of airfoil geometry, angle of attack, and the contribution of pressure towards the total drag at low Reynolds numbers. The airfoils selected in this study comprises of the NACA 4415, FX 61-184, E420 and S1223 which are preferred for subsonic UAV applications in addition to having the NACA 0012 serving as a standard profile. Performance of the S1223 airfoil was examined at a chord-based Reynolds number of 0.3 x 106 with the remaining airfoils at 1.0 x 106 for a range of angle of attack of around 0-10°. The unsteady 3-equation Intermittency SST model from ANSYS FLUENT 2020 was utilized with gradual reduction of timestep from 0.001s, 0.0005s, 0.00025s and 0.0001s. Experimental lift and drag validation across the airfoils generally suggest that the transitional model regularly outperforms XFOIL. Among the selection, concave airfoils such as the E420 and S1223 excel in delivering high lift at the expense of an increase in drag. Evaluation of the l/d ratio alone may underestimate their potential. Hence, further studies should focus on the implementation of drag reduction techniques on concave airfoils to enhance their performance. At the maximum tested angle of attack, the E420 reaches a cl value of 2.09 and S1223 at 1.98 while the FX 61-184 only at 1.57 and NACA 4415 at 1.36.

Rapid Prediction Method of Fan Broadband Noise Based on A Three-dimensional Sound Source Model

The fan/compressor broadband noise is an important noise source of civil large bypass ratio turbofan engine.. The empirical method requires massive amounts of test data for support. Adopting the computational aeroacoustic (CAA) method to predict the fan/compressor broadband noise will cost high amounts of computing resources or time. Thus, this study combines flow field calculation with analytical model to study the prediction method of fan/compressor broadband noise, this method can be used for quick and efficient preliminary evaluation and parameter studies. On the basis of the simulation and modeling of the stator – rotor interference phenomenon of fans, rotor wake flow field and the corresponding parameters are obtained. On this basis, the three-dimensional sound source model is used to calculate the blade surface sound source and its propagation under different working conditions. The distribution of sound energy in the duct is obtained. Then, the broadband noise prediction of fan is further acquired. The broadband noise of a typical single-stage TA36 fan is predicted using this method. Study results suggest that (1) turbulence model and turbulence kinetic energy of inlet boundary condition will have an important effect on the prediction results of broadband noise. (2) When the inlet turbulence kinetic energy is small, different turbulence models slightly influence the prediction results of broadband noise; when the inlet turbulence kinetic energy is large, the sound pressure level of sound field obtained by SST model is approximately 5dB larger than that obtained by model. This method can be used in the early stage of fan/compressor design, and the characteristics of blade broadband noise can be rapidly obtained.

Computational analysis of heat transfer due to turbulent annular jet impingement

Present paper deals with the numerical study of fluid flow and heat transfer due to an annular turbulent jet impingement on a flat heated plate. The geometrical configurations of annular jet was selected such a way that the mass and momentums efflux at the annular jet exit will be same as a circular jet having same inner diameter at a particular Reynolds number. Three dimensional turbulent flow field was resolved using Transition SST Model by solving the mass, momentum and energy equation with SIMPLE algorithm. A highly refined mesh was used for numerical computation after validation of present model with established results of same geometrical configuration. The flow structure and heat transfer characteristic of the annular jet was compared with the conventional circular jet and also with an established two dimensional study of same configurations and reported in this paper. It was observed that that two dimensional studies on annular jet under predicts heat transfer characteristics.

CFD modeling and validation of heat transfer inside an autoclave based on a mesh independency study

Autoclave processing is the main technology used in the manufacturing of structural aerospace composite parts. To optimize the autoclave process, the thermal behavior of the part and mold can be investigated through simulations. Computational fluid dynamics (CFD) provide a significant contribution to studies on heat transfer and airflow patterns, which are key points in an optimization applied to achieve a homogeneous temperature distribution inside composite parts. The solution is produced by solving the 3 D unsteady Navier–Stokes equations. This paper describes a systematic numerical study using the CFD approach to significantly improve the modeling efficiency of the heat transfer coefficient (HTC) inside an autoclave and maintain a high level of accuracy. Considering the modeling cost, calculation time, and accuracy of the results, a reasonable hybrid mesh is used based on a mesh independency study. The level of grid independence is examined using the general Richardson extrapolation method. In addition, a more robust autoclave model is presented, which is unaffected by the inlet turbulence. Further, the inlet fluid velocity and turbulence models have been identified as sensitive influencing factors. In this study, the Spalart–Allmaras turbulence model shows the best performance compared with the standard [Formula: see text] and [Formula: see text] SST models. Finally, the results are validated with the experimental data. The mean error of the simulated temperatures in the calorimeter for the front, middle and rear positions are [Formula: see text]C, [Formula: see text]C, and [Formula: see text]C, indicating a good agreement with the experiments. This paper provides guidelines on the use of a CFD simulation to predict the heat transfer during the autoclave curing process with high accuracy and reduced numerical effort.

The impact of urban block typology on pollutant dispersion

Block typology is a central component for urban planning and a key element to represent the urban form, which affects the airflow. Therefore, the aim of this work consists of investigating the impact of different urban block typologies in urban pollutant dispersion. Five typologies derived from real cities were investigated using the computational fluid dynamics technique: single-block, detached buildings, central courtyard, inner courtyards, and row. The numerical simulations were performed using URANS (Unsteady Reynolds-Averaged Navier-Stokes) equations and the κ−ω SST model to represent the turbulence effects. The model was validated using wind tunnel experimental data. Urban air quality was assessed using five parameters: mean age of air, net escape velocity, pollutant concentration at pedestrian level, and pollutant mass fluxes towards the urban canopy and outwards from it. The results showed that the single-block typology revealed an average pollutant concentration at pedestrian height 80% lower than that estimated in the detached building typology, which presented the highest concentration, probably due to the highest aspect ratio associated with permeable facades. Further, the higher the wind velocity, the lower the mean age of air. Finally, the unbuilt areas’ disposition within the block proved to be the more important parameter influencing the pollutant dispersion.

Численное исследование нестационарного течения газа в камере сгорания прямоточного воздушно-реактивного двигателя с учетом процесса теплообмена

Проведено численное исследование нестационарного турбулентного сверхзвукового течения в камере сгорания прямоточного воздушно-реактивного двигателя. Описана методика экспериментального измерения температуры на стенке осесимметричного канала в камере сгорания двигателя. Математическое моделирование обтекания исследуемой модели двигателя проводилось для скоростей набегающего потока M = 5 ... 7. Начальные и граничные условия задачи соответствовали реальному аэродинамическому эксперименту. Проанализированы результаты численного расчета. Рассмотрено изменение распределения температуры вдоль стенки канала с течением времени. Проведена оценка согласованности полученных экспериментальных данных с результатами математического моделирования. Purpose. The aim of this study is a numerical simulation of unsteady supersonic gas flow in a working path of ramjet engine under conditions identical to aerodynamic tests. Free stream velocity corresponding to Mach numbers M=5 ... 7 are considered. Methodology. Presented study addresses the methods of physical and numerical simulation. The probing device for thermometric that allows to recording the temperature values along the wall of internal duct was proposed. To describe the motion of a viscous heat-conducting gas the unsteady Reynolds averaged Navier - Stokes equations are considered. The flow turbulence is accounted by the modified SST model. The problem was solved in ANSYS Fluent using finite-volume method. The initial and boundary conditions for unsteady calculation are set according to conditions of real aerodynamic tests. The coupled heat transfer for supersonic flow and elements of ramjet engine model are realized by setting of thermophysical properties of materials. The reliability testing of numerical simulation has been made to compare the results of calculations and the data of thermometric experimental tests. Findings. Numerical simulation of aerodynamic tests for ramjet engine was carried out. The agreement between the results of numerical calculations and experimental measurements for the velocity in the channel under consideration was obtained; the error was shown to be 2%. The temperature values were obtained in the area of contact of the supersonic flow with the surface of the measuring device for the external incident flow velocities for Mach numbers M = 5 ... 7. The process of heating the material in the channel that simulated the section of the engine combustion chamber was analyzed. The temperature distribution was studied depending on the position of the material layer under consideration relative to the contact zone with the flow. Value. In the course of the work, the fields of flow around the model of a ramjet engine were obtained, including the region of supersonic flow in the inner part of axisymmetric channel. The analysis of the temperature fields showed that to improve the quality of the results, it is necessary to take into account the depth of the calorimetric sensor. The obtained results will be used to estimate the time of interaction of the supersonic flow with the fuel surface required to reach the combustion temperature.

Comparative Study on the Prediction of Aerodynamic Characteristics of Mini - Unmanned Aerial Vehicle with Turbulence Models

When dealing with CFD simulations the turbulent nature is seen on most of the engineering flows and these flows need to be solved. Reliable and applicable CFD outputs can be obtained from the accurate modelling of the turbulence as it is one of the most vital elements of CFD modelling. The RANS equations are extensively employed to analyse the complex flows over aircraft and for this purpose, a turbulence model is needed for turbulent flow analyses. Compatible turbulence must be chosen for the exact predictions of aircraft aerodynamic characteristics. In this report, numerical analyses of Mini-UAV are done to compare the outputs of different types of turbulence models for the prediction of aircraft aerodynamic characteristics for various flight conditions i.e., -4° to 40° at Mach number 0.04. ANSYS software is employed for this aerodynamic analysis. The six turbulence models compared are the Spalart-Allmaras, k-ɛ standard, k-ɛ RNG, k-ɛ Realizable, k-ω standard, and k-ω SST model. The impact of several turbulence models on the Mini-UAV aerodynamic performance is also studied numerically and validated with existing experimental data from reliable sources. The lift and drag coefficients for both numerical and experimental results exhibit an excellent correlation at Mach number 0.04. The recorded maximum variation of the coefficient of lift and the lift-to-drag ratio is determined to be 9.43% at a stall angle of attack (38°) and 7.01% at a 4˚ angle of attack among the six various turbulence models. The summation of deviation of coefficients of lift and drag values i.e., 0.0166 for the Spalart-Allmaras turbulence model is found to be less compared with all the other turbulence models so it can be said that the Spalart-Allmaras turbulence model is the best fit in terms of coefficients of lift and drag for Mini-UAV applications for subsonic flow.

Numerical Study of Secondary Flow Characteristics on the Use of the Winglets

The upper surface of the wing is constantly observed by experts on the wing. In this area, a variety of phenomena cause how much aerodynamic performance is demonstrated. Changing or adding geometry to the wing changes the flow characteristics of the upper surface. The use of winglets will certainly change the flow characteristics around the wingtip which will have a large effect on the area behind and next to the winglet itself. This research was carried out using Ansys 19.1 and the turbulent k-ω SST model. The airfoil used is Eppler 562 which is commonly used in unmanned aerial vehicles. Freestream flow velocity that will be used is 10 m/s (Re = 2.34 x 104) with an angle of attack (α) = 12° and 15°. From this study, it was found that the pressure contour in the y-z plane showed a reduction in area and intensity through the use of winglets. Secondary flow formed by using the forward wingtip fence results in a smaller area around the wingtip area than other configurations. Besides, the value of pressure on the upper surface which is equipped with a forward wingtip fence shows that pressure is still quite high when compared to the others.

Computational investigation on heat transfer augmentation of a circular tube with novel hybrid ribs

Present study reports a computational investigation on heat transfer and pressure drop characteristics for flow through a heat exchanger tube fitted with novel hybrid ribs by using magnetic nanofluid (Fe3O4). Effects of different rib geometry on heat transfer and pressure drop characteristics have been investigated for Reynolds number ranging from 3 000 to 22 000. Until now, there is little information available in the literature on the method of quantifying the effect of forced convection on the heat transfer and pressure drop of hybrid rib (HR) inserts by using magnetic nanofluid (MNF). The transition SST models along with governing equations (continuity, momentum, and energy equations) are numerically solved with ANSYS Fluent 19.2. The simulation results are validated with established correlations and excellent agreement was found. Heat transfer coefficient is more in combined arrangement (HR and MNF) compared to acting alone arrangement (only MNF).

Numerical Investigation of the Streamlining of Nozzle Arrays Equipped with Grid Valves

The purpose of one of the stages of this research is to develop the technique for the numerical investigation of the flow of the nozzle array equipped with the grid valve for its further use for the solution of the problem relating to the optimal design of the flow passage in cogeneration turbines. The development of this technique was divided into two parts, in particular the simulation of the two-dimensional or flat flow and the simulation of the three-dimensional flow. This research paper gives some data obtained for the first part of the developed technique, i. e. the verification data obtained by the comparison of the results of numerical and experimental investigations of the two-dimensional flow of the nozzle array with the grid valve. The Fluent software package was used for the simulation and computation of the flow of operating fluid. Computational domains restricted by one interblade channel were constructed for a different degree of the opening of the nozzle closure diaphragm. The grids were constructed for computational domains. The computations were performed to define the effect of the used turbulence model on the computation results. The best coincidence with experimental data was obtained when using the k–ω SST model. The computations done allowed us to obtain the flow patterns in the interblade channel and behind it and the distribution of the coefficients of the loss of kinetic energy over the grid front for a different level of the opening of closure diaphragms at the inlet to the nozzle ring. The obtained research data allow us to draw the following conclusions: the selected computation parameters enable a qualitative reflection of physical phenomena in the flow both for subsonic and supersonic flows of the cascade; the computation coefficients of the loss of kinetic energy showed a satisfactory coincidence with experimental data and it is indicative of that the development of the technique for the numerical investigation of the flow of nozzle arrays with grid valves is possible in principle when using the Fluent software package. The data obtained through this research will be used for the development of the technique for the numerical investigation of the three-dimensional flow of nozzle arrays with grid valves.

Effect of geometrical features on the contra-rotating propeller hydrodynamic performances

New contra-rotating four-bladed DTMB propeller configurations operating in open water are numerically studied to determine their hydrodynamic performances. The unsteady turbulent flow around propellers is modelled by RANS equations with Kω SST model then solved by a CFD code. The computational domain is divided in two blocks linked with a rotating interface. The obtained results show that thrust and efficiency of the contra-rotating (CRP) increase compared to the single propeller, leading to a significant reduction of the propeller diameter. The variation in axial spacing and angular displacement seems to have little effect on the CRP efficiency. The results also show that the thrust can be further improved by adopting a moderate negative twist angle of the rear propeller.
 Keywords: CRP contra-rotating propeller, axial spacing, angular spacing, twist angle, CFD.

Modeling Bubble Entrainment and Transport for Ship Wakes: Progress Using Hybrid RANS/LES Methods

This article presents progress on modeling bubble entrainment and transport around ships using hybrid Reynolds-averaged Navier-Stokes/large eddy simulation (RANS/ LES) methods. Previous results using a Boltzmann-based polydisperse bubbly flow model show that LES perform better than RANS in predicting transport of bubbles to depth, a very important process to predict bubbly wakes. However, standard DES-type models fail to predict proper turbulent kinetic energy (TKE) and dissipation, needed by bubble entrainment, breakup, and coalescence models. We propose different approaches to obtain TKE and dissipation in LES regions and evaluate them for cases of increasing complexity, including decay of isotropic turbulence, a flat plate boundary layer, and the flow in the wake of the research vessel Athena. An exponential weighted average is used to estimate statistics and obtain the averaged quantities in regions with resolved turbulence. The TKE is satisfactorily predicted in the cases tested. A modified ω equation in the SST model is proposed to implicitly compute the dissipation, showing superior results than the standard DES models, although further improvements are necessary. A hybrid RANS/LES approach is proposed, which focused at conserving total TKE as the flow crosses RANS/LES interfaces, as previously performed for zonal approaches but attempting a DES-like detection of regions suitable for LES, critical for large-scale computations of bubbly flows involving complex geometries. A general form of a dynamic forcing term is derived to transfer the modeled TKE to resolved TKE with a controller to guarantee proper conservation of the energy transferred. It was verified that the model is not sensitive to grid size or time step. Improvements to DDES and the proposed TKE-conserving hybrid RANS/ LES method show encouraging results, although remaining challenges are discussed.

Large eddy simulation of natural convection heat transfer and fluid flow around a horizontal cylinder

Natural convection around a single horizontal cylinder has been extensively studied for heat transfer characteristics, but when coupled with fluid flow, the laminar-to-turbulent transition in buoyant plume poses severe challenge on modelling fidelity and physical interpretability. To address this challenge, this paper conducts detailed verification, validation and analysis of high-fidelity large eddy simulation (LES) for an unconfined horizontal cylinder in water with a Rayleigh number of 8 × 107, complemented by Reynolds-Averaged Navier-Stokes (RANS) computations. To the best of the authors' knowledge, this is the first LES study with acceptable accuracy as validate by experimental data for buoyant plume above a single horizontal cylinder. It is found that LES is far more sensitive to mesh resolution and boundary condition setting than RANS. An alarm is set for the use of periodic condition in LES on lateral boundaries of computational domain due to its inferior accuracy than RANS with transition SST model. The finely-tuned LES with pressure condition on lateral boundaries shows satisfactory agreement with experimental data in terms of heat transfer on cylinder surface and buoyant plume velocity, based on which new physical insight on transitional behavior of thermal plume is obtained. It is found that after leaving the cylinder, buoyant plume is laminar and accelerates, subject to work input from buoyancy force, while its temperature keep decreasing due to heat loss to atmosphere. Flow instability appears first in upward velocity at a streamwise Grashof number of 1.5 × 108, where transition to turbulence onsets. Thermal plume continues to accelerate until it begins to sway horizontally, where energy dissipation into turbulence becomes the major loss mechanism of mean flow energy. Thus, cross-stream diffusivity is augmented notably due to turbulent stresses, leading to smoothing of transverse velocity distribution and reduction of transversely-averaged mean kinetic energy. Transversely-averaged turbulent kinetic energy keeps increasing in transitional regime until the streamwise Grashof number reaches 7 × 109 and then declines to approach an asymptotic value, signifying the end of transition. Overall speaking, buoyancy work dominates the change in mean flow energy, while mean shear outweighs buoyancy in producing turbulent kinetic energy.

Aerodynamic drag analysis based on SST model bicycle helmet surface depression effect

Aerodynamic drag analysis based on SST model bicycle helmet surface depression effect

Modelling the Wake Flow Behind a Model of Horizontal Axis Wind Turbine using RANS Approach: A Comparative Study

Steady CFD simulations, which is based on Reynolds Average Navier-Stokes (RANS) approach, were conducted for a model of Horizontal Axis Wind Turbine (HAWT) and compared with experimental data in order to assess the accuracy of these models in simulating the wake flow of HAWTs. The models can be categorized into two categories: fully developed models (Standard k-e model, Realizable k-e model, and SST k-ω model) and transitional models (K-kl-ω transition model and Transition SST Model). In this paper, ANSYS FLUENT 19.2 was being used to execute the simulations of the model turbine standing on a closed-loop wind tunnel at Norwegian University of Sciences and Technology (NTNU). Experiments were operating on a high turbulence intensity uniform inflow. Velocity, turbulence intensity, and turbulent kinetic energy profiles are illustrated at two downstream cross sections. Furthermore, contours of these parameters are set to investigate the developing of wake flow behind the turbine. It is demonstrated that models reasonably predict the velocity profile at the wake region. Transitional models are more accurate in predicting the power of the turbine. Turbulent kinetic energy and turbulence intensity were underestimated for all models.

Numerical study of ADS windowless spallation target based on diffuse interface method

The study of spallation targets is an important part of ADS system research. Liquid windowless spallation target is the current research hotspot. The working fluid forms a turbulent flow containing gas and liquid in the windowless target, and an accurate prediction of the working conditions is an important part of the study of spallation targets. First, briefly introduce the numerical method of the diffuse interface method, and use water as the working fluid, based on thise method, the four types turbulence model of k-ε model, k-ω model, SST model and Spalart-Allmaras model are used to perform the hydraulic numerical calculation of the ADS liquid windowless spallation target. Through experimental comparison and calculation cost comparison, it is found that the k-ε model is most suitable for the coupled diffuse interface method to predict the working conditions of windowless spallation targets. At the same time, turbulence analyses are performed on the calculation results of the four turbulence models. Then, continue to use water as the working medium, and use the diffuse interface method and k-ε model to perform detailed numerical calculation and analysis of the liquid windowless spallation target. The effects of outlet pressure and inlet velocity on the length of the free interface and the recirculation zone are studied. Finally, using liquid Lead-Bismuth Eutectic as the working fluid, using the diffuse interface method and k-ε model, the effects of outlet back pressure and inlet velocity on the height of the free interface and the pressure along the way were studied.

Enhancement in heat transfer from heat generating core debris using a passive approach in fast reactors: 3-D CFD analysis

The article presents the results of three-dimensional numerical analysis to assess the natural convection heat transfer in pool-type sodium-cooled fast reactors during post-accident heat removal. To improve the coolability of the destroyed core, suitable modifications in core catcher design are suggested. The equations for conservation of mass, momentum and energy are solved in their transient form using finite volume based discretization in (r,θ,z) system. Turbulence is modeled using the k−ω SST model based on sensitivity analysis of different turbulence models and validation exercises. A comparative analysis of 3-D and 2-D simulations is carried out. Various feasible geometrical arrangements for core collection trays are analyzed in detail for better coolability of the core debris by natural convection. Multiple debris collection trays with and without cooling pipes are studied towards achieving coolability of core debris in case of core disruptive accident. Results show that better safety margins are achieved by using 3-D as compared to 2-D simulation. The incorporation of cooling pipes in distributed heat sources on multiple trays leads to enhanced coolability of the debris within the vessel. Maximum temperatures on the core catcher/collection trays and in the core debris (with the selected core catcher design) are 860 K and 932 K respectively. These are substantially below the acceptable thermal design limits. Furthermore, detailed insight of the flow field confirms the turbulence in the lower plenum, partly due to impingement of passive jets from multiple cooling pipes on core debris and partly due to flow bypass through the slots at upper tray. It is established that the use of passive pipes and multi-tray concept for core catcher can successfully accommodate the whole core debris arising from core disruptive accidents within the main vessel without exceeding the safe thermal design limits.

Application of Counter Rotating Rotors for Improving Performance of Savonius Turbines

In this paper, a new configuration is proposed for using Savonius rotors by installing two Counter-Rotating Savonius Rotors (CRSR) together. In order to study the aerodynamic performance of the CRSR turbine, 2D unsteady turbulent flow around the CRSR turbine is simulated using commercial CFD software ANSYS-Fluent16.0. Turbulence modeling is done using $$k - \omega$$ SST model. Numerical results demonstrate a considerable improvement in aerodynamic performance of the CRSR turbine in comparison to a single conventional rotor. Effect of the distance between two rotors and different angular positions of two rotors on the CRSR turbine performance is studied. It is shown that the most effective parameter influencing the performance of the CRSR turbine is the distance between the rotors. Numerical results reveal that, locating two rotors in an optimal phase angle (θ = 90) and distance(S = 1.0D0), average power coefficient of the CRSR turbine can be increased to 21%, which indicates a rise of 38% compared to conventional Savonius rotors.

Airflow over low-sloped gable roof buildings: Wind tunnel experiment and CFD simulations

In this study, the impact of roof slope on the flow characteristics over low-sloped gable roofs was investigated using steady computational fluid dynamics (CFD) simulations based on a k-ω SST turbulence model. A measurement database of the flow field over a scaled model of 15° was created using particle image velocimetry (PIV). Sensitivity analyses for the grid resolutions and turbulence models were performed. Among the three common Reynolds-averaged Navier-Stokes equations (RANS) models, the k-ω SST model exhibited a better performance, followed by the RNG model and then the realizable k- model. Next, the flow properties over the differently sloped (0° to 25°) building models were determined. It was found that the effect of roof slope on the flow characteristics was identified by changing the position and size of the separation bubbles, 15° was found to be approximately the sensitive slope at which the distribution of the separation bubbles changed significantly. Additionally, it is suggested additional attention focused on the distributions of the negative pressure on the windward surfaces (especially 5° and 10° roofs) and the possible snow redistribution on the leeward surfaces.

Quantifying structural uncertainties in Reynolds-averaged Navier–Stokes simulations of wind turbine wakes

Reynolds-averaged Navier Stokes (RANS) based modeling is considered the mainstream computational fluid dynamics (CFD) approach for wind energy applications. Considering the inherent shortcomings associated with RANS models, quantification of uncertainties is of obvious importance if these models are to be used for design and optimization of wind farms. In the present work, structural uncertainties of RANS closure are quantified for simulations of wake flow behind a stand-alone wind turbine. The uncertainty is modeled by introducing perturbations to the Reynolds stress tensor. We specifically focus on perturbations in the eigenvalues of the tensor. The k-ω SST model and large-eddy simulation (LES) data are used as the baseline RANS model and reference data, respectively. Initially we compare the unperturbed RANS simulation against LES data, and show that the k-ω SST model generally tends to predict higher levels of isotropy in the turbulent wake. A comparison between LES data and differently perturbed k-ω SST simulations is made for the evolution of velocity deficit and turbulence intensity behind the turbine. A satisfactory coverage of LES profiles is observed when the amount of introduced perturbation is adjusted based on a priori comparison of the baseline RANS and LES results with the exception of the turbulent intensity immediately behind the turbine.