Transition SST Research Articles

Numerical Investigation For Convective Heat Transfer and Friction Factor Under Pulsatile Flow Conditions

Under pulsatile flow conditions, the characteristics of convective heat transfer and friction factor for periodic corrugated channel are investigated numerically. The Finite Volume Method (FVM) is used in the numerical study. Three different Reynolds Averaged Numerical Simulation (RANS) based turbulence models, namely the k-ω, the Shear Stress Transport (SST) and the transition SST model are employed and compared with each other. The results are also compared with the previous measurements performed for non-pulsating conditions. Investigations are performed for air flowing through corrugated channel which has sharp corrugation peak with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is varied within the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. Four different sinusoidal pulsatile flow conditions, which are combination of two different frequencies and two different amplitudes, are used. Variations of the Nusselt number and friction factor with the Reynolds number are studied. Effects of the amplitude and period of sinusoidal pulsatile flow conditions are discussed.

A comparative study of turbulence models for non-premixed swirl-stabilized flames

ABSTRACTThe accuracy of turbulent swirl-stabilized flame simulation strongly depends on the choice of turbulence model. In this study, four 3D unsteady turbulence closures, including large eddy simulation, scale-adaptive simulation, and two detached eddy simulation variants, along with four RANS models, including RNG k−ϵ, SST k−ω, transition SST, and RSM, are examined for moderate- and high-swirl case studies. It is observed that the scale-adaptive simulation provides the most accurate results for almost all variables and both swirl conditions in the reactive flow. Only the 3D unsteady models predict the vortex breakdown bubble and flame attachment state correctly. However, based on our error analysis, the flow and composition fields predicted by the RANS models are in acceptable agreement with the experimental fields, especially the ones of transition SST when higher swirl number cases or minor species concentration are of interest. Moreover, it is concluded that the viscosity ratio criterion is a better measure of the local LES quality than the turbulent kinetic energy ratio, and the accuracy of a hybrid simulation may be much more dependent on the ability of the model to operate close to the RANS mode where the grid resolution is not sufficient for a resolving simulation than the fraction of the resolved kinetic energy. Finally, the propriety of the base (RANS) model of a DES for the application of interest is important, such that DES with realizable k−ϵ outperforms the commonly used DES with SST k−ω model.

Numerical investigation of the load reduction potential of trailing edge flap based on closed-loop control

The ability of trailing edge flap (TEF) to mitigate ultimate and fatigue loads of wind turbine blades make the TEF receive many interests. In order to investigate the ability of TEF to mitigate the flapwise force (perpendicular to the wind turbine rotor plane) on the airfoil subject to unsteady inflows, a closed loop control system based on Computational Fluid Dynamics software ANSYS Fluent 17.0 was established using the User Defined Function embedded in the software. The flapwise force of the airfoil was calculated by 2D Reynolds Averaged Navier-Stokes equations with the Transition Shear-Stress Transport SST k-ω (T-SST) turbulence model. The unsteady inflows include step-up inflow, sinusoidal inflow, and random inflow. The influence of control parameters on flap performance was analyzed and the control parameters include control gain, control delay time, and speed limitation of flap deflection. The results showed that compared with the clean airfoil, the fluctuation amplitude of the flapwise force was reduced by 94.5% using TEF under the sinusoidal inflow. Under the random inflow, the standard deviation of the flapwise force was reduced by 99%. Properly increasing the gain and the speed limitation can enhance the TEF performance, but exorbitant speed limitation is not necessary. Moreover, exorbitant gain and delay time can reduce the TEF performance and system stability.

Aerodynamic analysis of different cyclist hill descent positions

Different professional cyclists use very different hill descent positions, which indicates that prior to the present study, there was no consensus on which position is really superior, and that most cyclists did not test different positions, for example in wind tunnels, to find which position would give them the largest advantage. This paper presents an aerodynamic analysis of 15 different hill descent positions. It is assumed that the hill slope is steep enough so pedaling is not required to gain speed and that the descent does not include sharp bends necessitating changes in position. The analysis is performed by Computational Fluid Dynamics (CFD) simulations with the 3D RANS equations and the Transition SST k-ω model. The simulations are validated wind tunnel measurements. The results are analyzed in terms of frontal area, drag area and surface pressure coefficient. It is shown that the infamous “Froome” position during the Peyresourde descent of Stage 8 of the 2016 Tour de France is not aerodynamically superior to several other positions. Other positions are up to 7.2% faster and also safer because they provide more equal distribution of body weight over both wheels. Also several positions that allow larger power generation are aerodynamically superior.

Aerodynamic Performance of Wind Turbine Airfoil DU 91-W2-250 under Dynamic Stall

Airfoils are subjected to the ‘dynamic stall’ phenomenon in significant pitch oscillations during the actual operation process of wind turbines. Dynamic stall will result in aerodynamic fatigue loads and further cause a discrepancy in the aerodynamic performance between design and operation. In this paper, a typical wind turbine airfoil, DU 91-W2-250, is examined numerically using the transition shear stress transport (SST) model under a Reynolds number of 3×105. The influence of a reduced frequency on the unsteady dynamic performance of the airfoil model is examined by analyzing aerodynamic coefficients, pressure contours and separation point positions. It is concluded that an increasingly-reduced frequency leads to lower aerodynamic efficiency during the upstroke process of pitching motions. The results show the movement of the separation point and the variation of flow structures in a hysteresis loop. Additionally, the spectrum of pressure signals on the suction surface is analyzed, exploring the level of dependence of pressure fluctuation on the shedding vortex and oscillation process. It provides a theoretical basis for the understanding of the dynamic stall of the wind turbine airfoil.

Numerical investigations of the effect of rotating and non-rotating shaft on aerodynamic performance of small scale urban vertical axis wind turbines

The shaft is a very important part of small scale vertical axis wind turbines (VAWTs) being used in urban environments. The shedding vortices will be formed as wind passes around the shaft, the shedding vortices released by the shaft have a negative effect on the aerodynamic performance of the blades passing the wake of the wind shaft, and this will lead to lower power output of the wind turbine. The objective of this study is to explore the differences of the effect of the rotating and non-rotating shafts of VAWTs on the aerodynamic performance of small scale urban VAWTs. In addition, the effect of surface roughness on the performance of the rotating and non-rotating shaft wind turbines is also investigated. The dynamic analysis of the VAWT is carried out by the method of Computational Fluid Dynamics. The aerodynamic loads in the study are obtained by solving the two-dimensional (2D) unsteady Navier-Stokes equations with the Transition SST turbulence model. The results are validated with experimental results. The results show that when the ratio of the shaft diameter to the wind turbine diameter (α) is 3.9%, the power coefficient (Cp) of the rotating shaft wind turbine is 2.2% higher than that of the non-rotating shaft wind turbine. Increasing the shaft diameter-based Reynolds number (Res) will increase the Cp of rotating and non-rotating shaft wind turbines, when Res is 2.6 × 104 and the Cp of the non-rotating shaft wind turbine is 5.7% higher than that of the rotating shaft wind turbine. The Cp of the non-rotating and rotating shaft wind turbines is reduced by 6.9% and 6.3%, respectively, by increasing the turbulence intensity from 4% to 16%. The optimal power output occurs under the condition of the non-rotating shaft wind turbine, and the relative surface roughness (ks/ds) is 0.005, where it is observed that Cp increased by 2.1% in comparison to that of the smooth shaft wind turbine.

Computational study of the laminar to turbulent transition over the SD7003 airfoil in ground effect

In this work, we present a numerical study of the laminar-turbulence transition flow around a symmetrical air-foil at a low Reynolds number in free flow and near the ground surface at different angles of attack. Finite volume method is employed to solve the unsteady Reynolds-averaged Navier–Stokes (RANS) equation. In this way, the Transition SST turbulence model is used for modeling the flow turbulence. Flow around the symmetrical airfoil SD7003 is numerically simulated in free stream and near the ground surface. Our numerical method can detect different aspects of flow such as adverse pressure gradient, laminar separation bubble and laminar to turbulent transition onset and the numerical results are in good agreement with the experimental data.

Investigation of Inclined Turbulators for Heat Transfer Enhancement in a Solar Air Heater

ABSTRACTIn the present study, the effect of forward inclined turbulators on the heat transfer enhancement in a duct is investigated, for forced convection. Turbulator configurations with three different pitch ratios and three different inclination angles are investigated for seven Reynolds numbers within the range 500–50,000. Investigations are performed experimentally as well as computationally, within a computational fluid dynamics framework. A distinguishing feature of the latter has been the employment of a turbulence model, the transitional shear stress transport model that is applicable throughout the presently considered range of Reynolds numbers containing laminar, transitional, and turbulent regions. At the beginning of the study, measurements and predictions are validated against analytical and empirical expressions known for a plain duct. The results obtained for turbulators configurations indicate that Nusselt number increases with the inclination angle but decreases with the pitch ratio. The influence of the inclination angle on the Nusselt number and thermal enhancement factor is found to be stronger than that of the pitch ratio. For all Reynolds numbers and for all configurations, the thermohydraulic performance is observed to increase, leading to thermal enhancement factors within the range 2–5. In all cases, a quite good agreement of the predictions and experiments is observed, which increases the confidence in the accuracy of both approaches.

Detached Eddy Simulation Model for the DU-91-W2-250 Airfoil

This paper presents aerodynamic investigations of the DU-91-W2-250 airfoil at Reynolds number of 3 · 106 employing 2D Reynolds-averaged Navier–Stokes (RANS) solver and 3D detached eddy simulation (DES) technique. RANS simulations are performed in the angle of attack range between -20° and +20° whereas DES results are given only for the angle of attack of 7.08°.Measurements have been done at the LM Wind Power Low Speed Wind Tunnel. The lift and drag are obtained from airfoil pressure and wake rake respectively.The obtained numerical results, lift and drag coefficients as well as static pressure distributions are in a good agreement with the experimental results in the linear part of the lift coefficient curve. The Transition SST turbulence model gives much more appropriate results in comparison with the k-ω SST model, especially for the drag at low angles of attack. The DES approach allows to obtain 3D flow characteristics near the S-shaped airfoil tall.

The Development of an Optimal Aerodynamic Design for a Human Powered Vehicle

With growing concerns about air pollution, Human Powered Vehicle (HPV) offers a possible solution. With limited power available from human muscle, HPVs must be as efficient as possible because aerodynamic drag is the dominant retarding force at high speed. Two fairings for the use cases of pure speed and everyday use are conceived, designed, implemented and operated where the key parameters of frontal area, surface area and shape are evaluated and compared with a benchmark model. The fairings are designed using the NACA 65-415, 66-018 and 66-021 airfoils. Computational fluid dynamics (CFD) simulations were done using the realizable k-ɛ model with scalable wall functions. In addition, low-speed comparative wind tunnel testing was conducted using the Taylor’s Wind Tunnel with 1/10 scale models. It was found that the pure speed fairing having the lowest values of frontal and surface area produced the lowest drag, followed by the benchmark model and lastly the everyday use model which has the highest values of the respective parameters. The same trends were found in the wind tunnel. Due to the CFD limitation of not simulating boundary layer transition, the shape parameter could not be evaluated and thus future simulations using the transition SST turbulence model is recommended.

CFD analyses of the wind drags on Khaya Senegalensis and Eugenia Grandis

The drag coefficients of one mature Khaya Senegalensis tree and one Eugenia Grandis tree in Singapore were analysed using computational fluid dynamics (CFD) modelling and validated with field measurements. For the numerical method, an innovative laser scanning approach was used to generate the tree geometries and to calculate the three-dimensional (3D) leaf area density distribution within the canopies. The canopies were represented by multiple porous domains and the turbulent effect of leaves was simulated by source and sink terms as a function of the calculated leaf area density. Computational fluid dynamics analyses using ANSYS 17.2 software were carried out on both leafless and leafed tree models. Three turbulent models, the Realizable k-ε model, Transition Shear Stress Transport model and Reynolds Stress Model were compared, and it was found that the differences in the drag forces among the turbulent models were negligible when they were meshed appropriately based on the grid independence study. The computed drag coefficient of the Khaya Senegalensis tree and the Eugenia Grandis tree from CFD modelling were similar within the range between 0.6 and 0.7. From field monitoring including the wind velocity and stem strains, the drag coefficients of both trees were calculated to be 0.51 and 0.40. Possible reasons causing the difference and limitations of the numerical method are discussed.

Numerical investigation of jet impingement cooling of a low thermal conductivity plate by supercritical pressure carbon dioxide

Jet impingement cooling is widely used due to its high heat transfer rates, especially within the stagnation region. Jet impingement cooling by supercritical pressure fluids has even higher heat transfer coefficients with the proper working conditions than regular fluids with no burnout. This paper presents a numerical investigation of supercritical pressure carbon dioxide jet impingement cooling of a low thermal conductivity plate. Predictions of 11 RANS turbulence models were first validated against the published experimental data done by our research group. The effects of the inlet temperatures and pressures on the heat transfer were studied numerically for a wide range of heat fluxes with the flow phenomena observed in the experiment further explained by the numerical results. The SST k-ω and Transition SST models gave more accurate predictions of the average heat transfer coefficient with differences of less than 15% for the validated case. The maximum average heat transfer coefficient occurred when the inlet temperature was lower than and the surface temperature was slightly higher than the pseudocritical temperature. The maximum average heat transfer coefficient increased with increasing inlet temperature when the inlet temperature was lower than the pseudocritical temperature and decreased with increasing pressure.

Numerical investigation of three turbulence simulation models for S809 wind turbine airfoil

The contributions in terms of energy production and reliability benefits of horizontal axis wind turbines are growing on a yearly basis throughout the world, which has made them the predominant technology for wind energy harvesting. When numerically simulating wind turbines, the selection of turbulence modelling approach can be of pivotal importance since it defines the numerical costs and gains of the simulation. In this paper, 2D computational fluid analysis of S809 Don Somners airfoil is targeted to evaluate the merits, drawbacks, and capabilities of three different common turbulence-modeling approaches, namely Spalart-Allmaras, k-w SST, and Transition SST models. The results are validated through comparison against experimental data on pressure coefficient distribution; a significant agreement between experimental data and Transition SST model is observed, which makes this model the reference model in this study. The other two approaches are compared with the results of the aforementioned model in terms of pressure distribution and flow separation predictions. The results indicate that k-w and Spalart-Allmaras models are as reliable as Transition SST when the Reynolds number and angle of attack are high enough (i.e. higher than 10 to 12°). At low Reynolds numbers, which means under 5 to 6°, the reference model is observed to be the most reliable among the three. The results of this study are of great importance when full rotor simulation is the task in hand, since they define the most appropriate approach to use with each flow condition to achieve an optimized numerical simulation procedure.

Numerical Study of the Effect of Winglets on the Performance of a Straight Blade Darrieus Water Turbine

This study deals with the three-dimensional unsteady numerical simulation of the flow around a cross-flow vertical-axis water turbine (CFWT) of the Darrieus type. The influence of turbine design on its hydrodynamic characteristics and performance is investigated by means of a time-accurate Reynolds Averaged Navier Stokes (RANS) commercial solver. The flow unsteadiness is described using a transient rotor-stator model in connection with a sliding interface. A classical Darrieus straight blade turbine, based on the NACA0025 airfoil, has been modified adding winglets (symmetric and asymmetric designs) to the blades’ tips with the objective of reducing the strength of the detached trailing vortices. The turbulent features of the flow have been modelled by using different turbulence models (k-ε Renormalization Group, standard Shear Stress Transport, transition Shear Stress Transport and Reynolds Stress Model). As a result, the predicted hydrodynamic performance of the turbine including winglets increases, independently of the employed turbulence model, being the improvement higher when a symmetric winglet design is considered. Moreover, visualization of skin friction lines pattern and their connection with vorticity isosurfaces, illustrating the flow detachment in the three blade configurations, has been carried out. Finally, a short discussion about the intermittency behavior along a turbine revolution is presented.

Steady and unsteady analysis of NACA 0018 airfoil in vertical-axis wind turbine

Numerical results are presented for aerodynamic unsteady and steady airfoil characteristics of the NACA 0018 airfoil of a two-dimensional vertical-axis wind turbine. A geometrical model of the Darrieus-type wind turbine and the rotor operating parameters used for numerical simulation are taken from the literature. Airfoil characteristics are investigated using the same mesh distribution around the airfoil edges and two turbulence models: the RNG k- and the SST Transition. Computed results for the SST Transition model are in good agreement with the experiment, especially for static airfoil characteristics.

Prediction and measurement of the aerodynamic performance of a wind turbine

Aerodynamic behavior of a small wind turbine is analyzed, both experimentally and numerically. Mainly, an unsteady three-dimensional formulation is adopted, where the flow turbulence is modelled by an Improved Delayed Detached Eddy Simulation framework, using the four-equation transitional Shear Stress Transport model, as the turbulence model. A quite good agreement between the measurements and calculations is observed.

Numerical investigation for convective heat transfer and friction factor under pulsating flow conditions

For pulsating flow, the behaviours of the convective heat transfer and friction factor for a periodic corrugated channel are investigated numerically. The finite volume method is used in the numerical study. Three different Reynolds Averaged Numerical Simulation based turbulence models, namely the k-ω model, the Shear Stress Transport (SST) model and the transition SST model are used and compared. The results are also compared with the previous experiments for non-pulsating flow. Analyses are conducted for air flow through a corrugated channel which has sharp corrugation peaks with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is changed in the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. A sinusoidal pulsatile flow condition which is F=400 and uA*=0.5 is used. Variations of the Nusselt number and the friction factor with the Reynolds number are studied. Non-pulsating flow results and pulsating flow results are compared with each other.

Computational investigation of the velocity and temperature fields in corrugated heat exchanger channels using RANS based turbulence models with experimental validation

The characteristics of convective heat transfer and friction factor for periodic corrugated channels are investigated numerically. In the numerical study, the finite volume method (FVM) is used. Three different Reynolds averaged numerical simulation (RANS) based turbulent models, namely the k-ω, the shear stress transport (SST) model and the transition SST model are employed and compared with each other. Experimental results obtained from a previous study are used for the assessment of the numerical results. Investigations are performed for air flowing through corrugated channels with an inclination angle of 30°. The Reynolds number is varied within the range 2,000 to 11,000, while keeping the Prandtl number constant at 0.70. Variations of the Nusselt number, Colburn factor, friction factor and goodness factor with the Reynolds number are studied. Effects of the corrugation geometry and channel height are discussed. The overall performances of the considered turbulence model are observed to be quite similar. The SST model is observed to show a slightly better overall performance.

Numerical simulations of the aerodynamic response of circular segments with different corner angles by means of 2D URANS. Impact of turbulence modeling approaches

ABSTRACTThis paper presents the results of numerical and experimental investigations on the force coefficients and Strouhal numbers of circular segments considering different corner angles or chord to sagitta ratios. The research is motivated because these geometries are becoming increasingly popular in several engineering disciplines. The so-called D-section (semi-circular cylinder with a corner angle of ) has been experimentally studied in the past, since it is a galloping prone geometry. However, there is a lack of research for cases with different corner angles, and the numerical investigations related to this topic are particularly scarce. In this work, a 2D Unsteady Reynolds Averaged Navier–Stokes approach has been adopted aiming to study the circular segments at the sub-critical regime, considering corner angles from to , and the flow parallel to the rectilinear side. These sections were found to be particularly challenging since they present massive flow separation on the rectilinear side, alongside the inherent difficulties related to modeling the flow along curved surfaces at high Reynolds numbers. The impact of introducing low-Reynolds-number and curvature corrections in the SST turbulence model and the performance of the Transition SST model have been extensively studied.

Heat transfer and flow field in a circular twisted channel

In the present paper, along with experimental study, computational fluid dynamics analysis is performed, using the transition SST model which can predict the change of flow regime from laminar through transition to turbulent. The differential governing equations are discretized by the finite volume method. The investigations are conducted for Reynolds numbers ranging from 100 to 50,000 covering laminar, transitional and turbulent regimes, and for three length and three pitch ratios. The predictions are observed to show a good agreement with the measurements and published correlations of other authors. The analysis indicates that the large length ratio and small pitch ratio yields a higher heat transfer rate with relatively low performance penalty. The transition from laminar to turbulent regime is observed between Reynolds numbers of 2,500 to 3,500 for all cases. For almost all investigated cases the performance factors are greater than unity.