Transition SST Research Articles

CFD simulations of aerodynamic characteristics for the three-blade NREL Phase VI wind turbine model

In order to study the impact of the wind speed and turbulence model on the numerical simulation of the aerodynamic characteristics for the wind turbine, SST k–ω and transition SST turbulence models were used to numerically simulate the aerodynamic characteristics at 7 m/s, 10 m/s and 20 m/s inlet wind speed, and the numerical results were compared with the experimental results. The results show that as the low inlet wind speed, no flow separation occurs over most of the blade surface, and SST k–ω and transition SST turbulence models can accurately predict the aerodynamic characteristics of the blades. As the low inlet wind speed, the flow separation mainly occurs around the root and the middle of blade, and numerical results at the leading edge by SST k-ω turbulence model and near the trailing edge by transition SST turbulence model match the experimental results well. When most of the flow through the blade surface is completely separated, the numerical simulation results by both SST k–ω and transition SST turbulence models have certain differences with experiment results, and the deviations are mainly concentrated on the suction surface and decrease gradually from the blade root to the tip.

The Effects of a Morphed Trailing-Edge Flap on the Aeroacoustic and Aerodynamic Performance of a 30P30N Aerofoil

This paper presents initial results on the aeroacoustic and aerodynamic effects of morphing the trailing-edge flap of the 30P30N aerofoil, over five flap deflections (5–25°), at an 8° angle of attack and a Reynolds number of Re=9.2×105. The Ffowcs-Williams–Hawkings acoustic analogy estimates the far-field noise, whilst the flow field is solved using URANS with the four-equation Transition SST model. Aerodynamic and aeroacoustic simulation data for the 30P30N’s full configuration compare well with experimental results. A Courant number (C) ≤ 1 should be used for resolving tonal noise, whilst a C of up to 4 is sufficient for broadband noise. Sound pressure level results show an average 11% reduction in broadband noise across all flap deflections and frequencies for the morphed configuration compared with the conventional, single-slotted flap. The morphed flap eliminates the multiple tonal peaks observed in the conventional design. Beyond 15° flap deflection, the morphing flap achieves higher stall angles, but with increased drag, leading to a maximum reduction of 17% in Cl/Cd ratio compared with the conventional flap. The methodology reported here for the 30P30N is a quick tool for initial estimates of the far-field noise and aerodynamic performance of a morphing flap at the design stage.

Numerical Investigations of Aerodynamic Characteristics Prediction of High-lift Low Reynolds Number Airfoil

The Micro Aerial Vehicles (MAVs) operate at a critical range of low Reynolds number (Re). The implementation of the low (Re) aerodynamics for MAVs has brought interest into the study of high-lift low Re airfoils. Such investigations may bring new insight into the aerodynamic performance of MAVs flights. The aim of the current investigation is to exam different numerical methods in the aerodynamics prediction of high-lift low Reynolds number S1223 airfoil. For that purpose, the Spallart Allmaras (S-A), two equations SST K-ω and the four equations transition γ-Reθ SST turbulence models were used. Results revealed that the SA turbulence model can predict the pre-stall low angles of attack and provides a good agreement with experimental data and XFOIL results. Whereas the two-equation model SST-enhanced K-ɷ and the transition SST models predict better the unsteadiness of the stall behaviour. XFoil accurately predicts the highest lift coefficient, even if it occurs at a lower angle of attack. These results showed the promising ability of the transition SST selection in predicting the stall behaviour.

A study of the heat transfer of an atomizer gas jet impinging perpendicular on a circular cylinder

This contribution presents the methodology and results from an experimental and numerical study of the heat transfer behavior of the gas-only jet flow from a close-coupled atomizer (CCA) configuration impinging on a heated cylindrical surface. A set of experiments are conducted under different operating conditions (gas pressures 0.6, 0.8 and 1.0 MPa) to measure the local heat transfer coefficient (HTC) on the surface of the cylinder by recording the cooling curve at specific locations along circumferential and axial directions. The numerical study of the heat transfer is executed by employing computational fluid dynamics (CFD) under the same conditions as in the experiments. The Transition SST model is selected for turbulence closure as it obtained results most close to the experimental ones. Predictions of HTC on the cylindrical surface are derived from this model with variable parameters, i.e., nozzle-to-substrate distance (110, 130 and 150 mm), diameter of the cylinder (80, 100, 120 mm) and the temperature difference between the cylinder and ambient temperature (20–730 K). In addition, the gas flows from a circular nozzle and the atomizer configuration with same exit area are compared by the CFD simulation, which shows that the gas jet flow becomes self-similar when the distance to the nozzle is > 80 mm. The experimental and numerical results both show that at each measured position the heat flux q˙ obtains a linear relationship with the temperature difference. The fitted line in the stagnation region has a small negative intercept while that at other positions is close to 0. The local HTC variation in the circumferential and axial directions on the cylinder surface and the parameters affecting the distribution of the HTC are outlined and discussed.

Experimental and Numerical Analyses of a Novel Wing-In-Ground Vehicle

The AeroCity is a new form of transportation concept that has been developed to provide high-speed ground transportation at a much lower cost than the existing high-speed railway. Utilizing the Wing-in-Ground (WIG) effect, the AeroCity vehicle does not require complex infrastructures like other contemporary concepts, such as the Hyperloop or Maglev trains. In the current work, the aerodynamic characteristics of the AeroCity vehicle are examined through a Computational Fluid Dynamics (CFD) analysis. The results from the CFD analysis qualitatively match with the findings of wind tunnel experiments. Surface streamlines and boundary layer measurements correspond well with the numerical data. However, the force measurements show a discrepancy. It is found that the separation bubble over the side plates is not captured by the CFD, and this is responsible for an under-prediction of the drag at higher free-stream velocities. The Transition SST model improved the matching between the experiments and numerical simulations. The influence of the moving ground is numerically investigated, and the effect of non-moving ground on the vehicle aerodynamics was found not to be significant. Finally, the inclusion of the track wall is examined. It is found that the merging of the wingtip vortices is responsible for a significant drag increase and, therefore, an alternative track geometry should be investigated.

Effects of Reynolds Number and Protuberance Amplitude on Twin-Protuberance Airfoil Performance

A stall occurs in the flow around an airfoil when the angle of attack reaches a critical value, leading to a dramatic drop in airfoil performance. Recently, a passive leading-edge protuberance control method, inspired by the fin of a humpback whale, has demonstrated obvious advantages in improving airfoil stall. Hence, this study experimentally and numerically investigates the aerodynamic characteristics of an airfoil with twin protuberances at the leading edge. First, the aerodynamic characteristics were investigated for six Reynolds numbers via wind-tunnel experiments, obtaining the hysteresis caused by the test sequence. Then, numerical simulations were performed using a transition shear stress transport turbulence model, and the numerical accuracy was verified. Finally, the influence of structural parameters on stall behavior was revealed by changing the amplitude of the protuberance. The results indicate that the twin-protuberance airfoil produces step-by-step and one-side stall phenomena. The influence of vortices on the movement of separation bubbles on the lower surface is presented. The critical angle of attack for stall increases with increasing Reynolds number; its value for deep stall increases by more than 2 deg. As the protuberance amplitude is reduced, the influence on the airfoil flow is weakened, and the first stall angle increases.

Optimization and Design of a Flexible Droop-Nose Leading-Edge Morphing Wing Based on a Novel Black Widow Optimization Algorithm—Part I

An aerodynamic optimization for a Droop-Nose Leading-Edge (DNLE) morphing of a well-known UAV, the UAS-S45, is proposed, using a novel Black Widow Optimization (BWO) algorithm. This approach integrates the optimization algorithm with a modified Class-Shape Transformation (CST) parameterization method to enhance aerodynamic performance by minimizing drag and maximizing aerodynamic endurance at the cruise flight condition. The CST parameterization technique is used to parameterize the reference airfoil by introducing local shape changes and provide skin flexibility to obtain various optimized morphing airfoil configurations. The optimization framework uses an in-house MATLAB algorithm, while the aerodynamic calculations use the XFoil solver with flow transition estimation criteria. These results are validated with a CFD solver utilizing the Transition (γ−Reθ) Shear Stress Transport (SST) turbulence model. Numerical studies verified the effectiveness of the optimization strategy, and the optimized airfoils have shown a significant improvement in overall aerodynamic performance by up to 12.18% drag reduction compared to the reference airfoil, and an increase in aerodynamic endurance of up to 10% for the UAS-S45 optimized airfoil configurations over its reference airfoil. These results indicate the importance of leading-edge morphing in enhancing the aerodynamic efficiency of the UAS-S45 airfoil.

Aerodynamic characteristic of wind turbine with the leading edge slat and Microtab

The leading edge slat can increase the flow energy of airfoil suction surface, and Microtab can hinder the flow of pressure surface. In order to combine the advantages of two passive flow control methods, this paper creatively proposed to combine the leading edge slat with Microtab to simulate the S809 airfoil, which slat had the contour characteristics of S809 airfoil. The transition SST turbulence model was applied to estimate the aerodynamic characteristics of the S809 airfoil. According to research, the flow control approach of the leading edge slat combined with Microtab could effectively restrain the flow separation and ameliorate the aerodynamic characteristics of the S809 airfoil. For Case-12, when the airfoil angle of attack was 16.22°, the boundary layer flow separation was restrained completely, Cl was increased to 3.06 from 1.79 and increased by 171% compared with clear airfoil. Research shown that the position of slat, the position and height of the Microtab were significant factors for the aerodynamic characteristics of the S809 airfoil. Microtab position at 0.95c and height at 0.03c performed the best effect to inhibit flow separation and improve lift coefficients.

Heat Transfer and Pressure Loss Simulations of Matrix Cooling Channels for Gas Turbine Airfoils

The paper selected 10 geometrical parameter combinations to numerically investigate the dependencies of heat transfer enhancement, flow resistances and overall thermal performance on three geometrical factors including rib inclination angle, channel blockage ratio and subchannel number. The investigated Reynolds number starts at 5,000 and can be as high as 90,000. The turbulence model selected was Transition SST model. The results show that the matrix cooling channels of 30deg rib angle have greater heat transfer enhancement and much higher pressure loss than that of 60deg rib angle. Moreover, matrix cooling channels of higher channel blockage ratio have greater heat transfer enhancement and higher flow friction, while the relationship between the overall thermal performance and the channel blockage ratio depends on the Reynolds number. It was also concluded that the heat transfer performance factor of matrix cooling channel does not change linearly with the subchannel number and the effect of subchannel number on heat transfer performance is less significant than the other two factors. Besides, matrix cooling channels of higher subchannel number have higher pressure loss and poorer overall thermal performance.

Numerical investigation on NACA0012 airfoil with tubercular structure

Humpback whales have very special fins, which reduce the friction force by the projections on the fins and enhance the lift force with increasing angle of attack. Thus, it has opportunity to move with less energy. In this study, the tubular structure was formed and adapted to the NACA0012 airfoil profile. Numerical modelling of tubercle wing was performed in ANSYS FLUENT. In numerical modelling, k-ω SST and Transition SST turbulence models were used to investigate tubercle wing performance. The tubercle structure on the wing provided to canalize the flow in the tubercle cavities, thus flow separation decreased. Consequently, it was observed that the aerodynamic performance of the wing increased when the sinusoidal protrusions known as tubercle structure were applied to the leading edge of the wing.

CFD simulation for the modelling of the powder flow on continuous coaxial nozzles in the laser DED process

Laser Directed Energy Deposition (L-DED) is becoming an alternative for the repair and coating of surfaces. Moreover, world-leading machine-tool builders are implementing the L-DED process into hybrid machine tools, to provide the ability to add and subtract material in the same platform.However, the L-DED process is still very complex to set up, since it involves a high number of parameters and physical phenomena. One of the challenges that machine-tool manufacturers face is the characterization of the powder flow through coaxial nozzles. The control of the powder distribution at the focal plane is key to improving the process effiency and to ensuring the quality of the clads. Simulation can be used to increase the understanding of the fluid-dynamic behaviour of complex flows, but it is still far from being considered a robust and closed solution. This work presents a CFD model for the simulation of the powder flow in continuous coaxial nozzles, typically used in the L-DED process. Firstly, a critical comparison of different turbulence algorithms is presented, focusing on their field of application, and supported by a literature review. Secondly, a CFD model for the powder flow simulation of a commercial coaxial nozzle is built and validated with experimental testing. Based on the obtained results, the k-ω transition SST turbulence model is considered the most appropriate for this kind of simulation. Moreover, a study of the influence of the restitution coefficient is presented, being 0.90 the most suitable for the MetcoClad 6 powder.

NUMERICAL STUDY OF LAMINARIZATION OF LOW REYNOLDS NUMBER STRONGLY HEATED TURBULENT FLOW IN HEATED CORRUGATED VERTICAL TUBE WITH SINUSOIDAL WALLS

Transitional k-ω SST transition model was used to numerically investigate the turbulent low Reynolds number flow of Helium in a heated vertical corrugated tube with sinusoidal walls. The investigation was conducted for a Reynolds number range of 4000 to 8000, a non-dimensional wall heat flux value range of 0.003 to 0.008, and a ratio of amplitude to the wavelength of the corrugation range from 0.02 to 0.10. The results show that an increase in the amplitude to wavelength ratio suppresses flow laminarization due to increased turbulence kinetic energy production. A non-dimensional wall heat value of as much as 0.008 was found to be inadequate to laminarize the flow in corrugated tubes with an amplitude to wavelength ratio of 0.1. A laminarization map is proposed that predicts the threshold value of wall heat flux value above which the flow would laminarize for a given value of inlet Reynolds number and amplitude to wavelength ratio.

Enhancing turbulence: a potential strategy for drag reduction based on ground transportation systems (GTS) model

PurposeThe purpose of this paper is to investigate the mechanism and performance of a potential strategy, which is to enhance turbulence to carry out drag reduction for heavy trucks.Design/methodology/approachEnhancing turbulence deflector (ETD) was placed on the roof surface of an ground transportation system (GTS) to investigate the drag reduction mechanism of enhancing turbulence. Transition shear-stress transport improved delay detach eddy simulation model was adopted to simulate the unsteady small-scale flow around the ETD.FindingsBy optimizing the three influencing factors, diameter, streamwise length and streamwise position, the optimized ETD has achieved a maximum drag reduction of 7.04%. The analysis of flow field results shows that enhancing turbulence can effectively suppress flow separation and reduce the negative pressure intensity in the wake region of GTS.Originality/valueThe present work provides another potential possibility for the improvement of the aerodynamic performance of heavy trucks.

Numerical Investigation of the Performance, Hydrodynamics, and Free-Surface Effects in Unsteady Flow of a Horizontal Axis Hydrokinetic Turbine

This paper presents computational fluid dynamics (CFD) simulations of the flow around a horizontal axis hydrokinetic turbine (HAHT) found in the literature. The volume of fluid (VOF) model implemented in a commercial CFD package (ANSYS-Fluent) is used to track the air-water interface. The URANS SST k-ω and the four-equation Transition SST turbulence models are employed to compute the unsteady three-dimensional flow field. The sliding mesh technique is used to rotate the subdomain that includes the turbine rotor. The effect of grid resolution, time-step size, and turbulence model on the computed performance coefficients is analyzed in detail, and the results are compared against experimental data at various tip speed ratios (TSRs). Simulation results at the analyzed rotor immersions confirm that the power and thrust coefficients decrease when the rotor is closer to the free surface. The combined effect of rotor and support structure on the free surface evolution and downstream velocities is also studied. The results show that a maximum velocity deficit is found in the near wake region above the rotor centerline. A slow wake recovery is also observed at the shallow rotor immersion due to the free-surface proximity, which in turn reduces the power extraction.

Effect of spanwise groove on the dynamic stall characteristics of an airfoil

Numerical simulations are conducted to investigate the effect of triangular groove on the dynamic stall characteristics of a NACA 0012 airfoil at a Reynolds number of 135,000. The right-angled triangular grooves are placed at either 10%, 25%, or 50% chord locations on the suction and have depths of 0.025c and 0.05c, measured normal to the surface of the airfoil. The solutions that are second order accurate in time and space are obtained using pressure-based finite volume solver and the 4-equation transition SST turbulence model viz. γ- Re θt is used to predict transition and viscous stresses accurately. The airfoil is in harmonic pitch motion about its quarter-chord with a maximum circular frequency of 18.67 rad/s. The results suggest that the presence of a groove, except for the deeper grove at 0.5c, quickens the dynamic stall, but with smaller rise in C l,max and a less severe fall in lift at the stall. The mean C l value during the downstroke is improved by up to 8% for the deeper groove at 0.25c, reducing the hysteresis in lift significantly. The grooves at 0.1c, 0.25c, and 0.5c also reduce the drag by 4%, 7%, and 9% during a complete cycle, with subsequent improvements of 54%, 69%, and 63% in the l/d ratio. The current finding can be thus used to enhance the performance of flapping wing MAVs, helicopter rotors, and wind turbine blades as these applications encounter the dynamic stall phenomena frequently.

Accuracy of the Gamma Re-Theta Transition Model for Simulating the DU-91-W2-250 Airfoil at High Reynolds Numbers

Accurate computation of the performance of a horizontal-axis wind turbine (HAWT) using Blade Element Momentum (BEM) based codes requires good quality aerodynamic characteristics of airfoils. This paper shows a numerical investigation of transitional flow over the DU 91-W2-250 airfoil with chord-based Reynolds number ranging from 3 × 106 to 6 × 106. The primary goal of the present paper is to validate the unsteady Reynolds averaged Navier-Stokes (URANS) approach together with the four-equation transition SST turbulence model with experimental data from a wind tunnel. The main computational fluid dynamics (CFD) code used in this work was ANSYS Fluent. For comparison, two more CFD codes with the Transition SST model were used: FLOWer and STAR-CCM +. The obtained airfoil characteristics were also compared with the results of fully turbulent models published in other works. The XFOIL approach was also used in this work for comparison. The aerodynamic force coefficients obtained with the Transition SST model implemented in different CFD codes do not differ significantly from each other despite the different mesh distributions used. The drag coefficients obtained with fully turbulent models are too high. With the lowest Reynolds numbers analyzed in this work, the error in estimating the location of the transition was significant. This error decreases as the Reynolds number increases. The applicability of the uncalibrated transition SST approach for a two-dimensional thick airfoil is up to the critical angle of attack.

Geometry optimisation of vertical axis wind turbine with Gurney flap for performance enhancement at low, medium and high ranges of tip speed ratios

Optimisation of Gurney flap (GF) geometries mounted on a three-straight-bladed vertical axis wind turbine (VAWT) is evaluated with computational fluid dynamics (CFD) solution and Taguchi method, considering the blade rotating effect at three tip speed ratios (TSRs) of low, medium and high ranges. A hybrid RANS-LES model applying stress-blended eddy simulation with transition shear-stress transport turbulence model is adopted. Results analysis confirms that GF geometry optimisation needs to consider multiple rotational blades rather than a single stationary one, owing to different optimum GF geometries and their performances at a different range of TSRs. It is found that VAWT with GF can significantly improve the power coefficient at low range TSRs (up to 233.19%), but it decreases with the increase of TSRs ranges (up to 69.94% and 41.36% at medium and high ranges of TSRs, respectively).

Natural convection heat transfer and fluid flow in a thermal chimney with multiple horizontally-aligned cylinders

To address the scarcity of natural convection research for the tube array with three or more columns of horizontally-aligned cylinders, this paper conducts RANS/URANS simulations for multiple cylinder columns in one or two rows, validated by particle-image velocimetry and heat transfer measurements. Parametric study with varying horizontal and vertical pitches is implemented by RANS with transition SST model. Key findings are then substantiated and examined by URANS. It is found that the horizontal pitch influences the balance between the chimney effect and the blockage effect, thus has an optimum with respect to the natural draft velocity. For all the vertical distances computed in the two-row thermal chimney model, the flow pattern stays in a plume-dominant mode at the large horizontal pitch and a chimney-dominant mode at the small horizontal pitch. Interestingly, at the intermediate horizontal pitch (pitch/diameter≈2.5), the flow pattern switches from plume-dominant to chimney-dominant over a minor increase of the vertical pitch (from 4.5 to 6 diameters), leading to significant augmentation in natural draft velocity and heat transfer on cylinder surfaces. This distinct thermo-flow behavior, identified for the first time to the authors’ knowledge, may be harnessed to improve the performance of passive heat exchangers.

Influence of the Curvature Correction on Turbulence Models for the Prediction of the Aerodynamic Coefficients of the S809 Airfoil

Using the appropriate procedure, Computational Fluid Dynamics allows predicting many things in several fields, and especially in the field of renewable energies, which has become a promising research axis. The present study aims at highlighting the influence of the curvature correction on turbulence models for the prediction of the aerodynamic coefficients of the S809 airfoil using the Computational Fluid Dynamics code ANSYS Fluent 17.2. Three turbulence models are used: Spalart-Allmaras, Shear Stress Transport k-ω and Transition SST. Experimental results of the 1.8 m × 1.25 m low-turbulence wind tunnel at the Delft University of Technology are used in this work for comparison with the numerical results for a Reynolds number of 106. The results show that the use of the curvature correction improves the prediction of the aerodynamic coefficients for all the turbulence models used. A comparison of the three models is also made using curvature correction since it gave better results. The Transition SST model is the one that gives the best results for the lift coefficient, followed by the Shear Stress Transport kω model, and finally the Spalart-Allmaras model. For the drag coefficient, Transition SST model is the best, followed by the Spalart-Allmaras model, and finally the Shear Stress Transport kω model.

Investigation of Convective Heat Transfer and Friction Factor in Corrugated Channels with Different Inclination Angles Using Computational Fluid Dynamics

Convective heat transfer and friction factor properties for periodic undulating channels are numerically investigated. The finite volume method (FVM) was used in the numerical study. Three different Reynolds averaged numerical simulation (RANS) based turbulence models, shear stress transport SST k-ω and transition SST model were used and compared with each other. Two different channels were used; the first geometry is a sharp corrugated channel with a 30° inclination angle, and the second geometry is a sharp corrugated channel with a 45° inclination angle. The Reynolds number varies in the range of 2000-7500. The Prandtl number was kept constant at 0.7. Nusselt number, friction factor, Reynolds number, and variations of the good factor were investigated. The effect of inclination angle and pitch are discussed.