Transition SST Research Articles

Investigation of pollutants formation in a diesel engine using numerical simulation

The current study aims at simulating the in-cylinder combustion process in a diesel engine and investigating the engine performance and pollutant formation. The combustion simulation is performed on a 3D sector employing appropriate models for various physical and chemical processes contributing in the combustion phenomenon. The overall model includes Transition SST turbulence model, eddy dissipation model for turbulence chemistry interaction, Moss–Brookes model for soot calculation and Zeldovich mechanism for NO production other than the usual transport equations. The numerical solutions are based on the finite volume discretization of the governing partial differential equations. Engine performance has been studied in terms of pressure, temperature and heat release rate while the pollutants formation has been investigated in terms of soot and thermal NO production. The results show that the ignition delay is quite short and that the injection timing may be successfully employed to control the combustion behavior. The simulation results are quite consistent with the expected behavior of the target variables indicating that the CFD analysis can be successfully employed in the diesel engine design. The results validation may be acknowledged in view of the mesh independence test, literature comparison and justification of the models.

Patient Specific Modelling of Blood Flow in Coronary ‎Artery

The performance of the heart is considerably affected by the blocks formed because of the deposition of ‎plaque inside the coronary artery. The blocks (stenosis) either in coronary artery or elsewhere force the ‎heart to work harder for pumping the oxygenated blood to the heart muscles and blood vessels. This ‎study analyses the flow through the stenosed coronary arteries via numerical modelling by using ANSYS ‎FLUENT software. Three real cases with different asymmetric stenosis levels (i.e., block level 33%, 66% ‎& 85%) are analysed by considering blood as a non-Newtonian fluid, and blood flow as pulsatile in ‎nature. As the flow regime falls in transition to turbulent region, the transition Shear Stress Transport ‎‎(SST) k-ω turbulence model is used to take care of the changeover stage from laminar to turbulent flow ‎and vice versa. The results show large variation both in Wall Shear Stress (WSS) and pressure drop near ‎the stenosis. Pressure drop becomes more significant at severe degrees of stenosis (66% and 85%) ‎compared to the mild case (33%). The study throws light on the critical distribution of shear stress and ‎pressure drop along the artery wall, which are considered as indicators of the commencement of heart ‎disease and further growth of stenosis. An indicator, viz., Fractional Flow Reserve (FFR), which relates ‎the percentage of stenosis to the pressure variations, can be used as an index to diagnose the severity of ‎stenosis. All the three cases with different stenotic levels were analysed under hyperaemic conditions and ‎found that even 45% stenosis case can go near to critical at hyperaemic flow conditions. The effect of ‎severity due to vessel constriction can be estimated by comparing the simulated pressure drop and WSS ‎before and after the stenosis, with the ones for a healthy artery. The present study developed a ‎methodology to calculate FFR value for unknown percentage of stenosis based on the simulated results ‎obtained from 33%, 66% and 85% stenosis. Thus, criticality of a patient with certain percentage stenosis ‎can also be evaluated. This simulation technique can be recommended as a non-invasive diagnostic tool ‎for the early detection of atherosclerosis‎‎.

Three-Bladed Horizontal Axis Water Turbine Simulations with Free Surface Effects

The water level above a hydrokinetic turbine is likely to vary throughout the season and even along the day. In this work, the influence of the free surface on the performance of a three bladed horizontal-axis turbine is explored by means of a three-dimensional, transient, two-phase flow computational model implemented in the commercial CFD software ANSYS Fluent 19.0. The <i>k –</i> ω SST Transition turbulence model coupled with the Volume of Fluid (VOF) method is used to track the air-water interface. The rotor diameter is <i>D</i> = <i>0 8m</i>. Two operating conditions are analyzed: deep tip immersion (<i>0.55D</i>) and shallow tip immersion (<i>0.19D</i>). Three tip speed ratios are evaluated for each immersion. Simulation results show a good agreement with experimental data reported in the literature, although the computed torque and thrust coefficients are slightly underestimated. Details of the free surface dynamics, the flow past the turbine and the wake near the rotor are also discussed.

Numerical Analysis of Flowfield in Linear Plug Nozzle with Base Bleed

A linear aerospike nozzle is a kind of altitude adaptive nozzle that has optimum performance over the entire operational range. In this work, flow characteristics of a linear plug nozzle and a truncated plug with 40% plug length are studied numerically. Different flow types associated with an increase in nozzle pressure ratio, corresponding wall pressure variation and shock patterns are studied. The effect of base pressure improvement with a secondary flow from the base (base bleed) of the truncated nozzle is also examined. Four base bleed points are considered, and the optimum bleed point is selected based on performance and base pressure improvement. The two-dimensional numerical model is solved using Reynolds-averaged Navier–Stokes equations with the Transition SST turbulence model. Validation shows that the present model is good enough at predicting the flow features associated with a linear plug nozzle. From the three different flow types associated with this nozzle, the transition from one type to other happens at lower nozzle pressure ratios with truncation due to the reduction in length of the plug. Thrust coefficient increased by 3.54% in the case of the bleed 3 configuration with a secondary flow of 2% of inlet mass flow rate when compared with 40% plug length without base bleed.

Numerical Investigation of the Trailing Edge Shape on the Added Damping of a Kaplan Turbine Runner

Hydraulic turbine runners experience high excitation forces in their daily operations, and these excitations may cause resonances to runners, which may induce high vibrations and shorten the runner's lifetimes. Increasing the added damping of runners in water can be helpful to reduce the vibration level during resonances. Some studies have shown that the modification of the trailing edge shape can be helpful to increase added damping of hydrofoils in water. However, the influence of blade trailing edge shape on the added damping of hydraulic turbine runners has been studied in a limited way before. Due to the difficulties to study this problem experimentally, the influence of blade trailing edge shape on a Kaplan turbine runner has been studied numerically in this paper and the one-way FSI method was implemented. The performances of three different turbulence models, including the k − ϵ , k − ω SST , and transition SST models, in the added damping simulation of the NACA 0009 hydrofoil were evaluated by comparing with the available results of the two-way FSI simulation in the references. Results show that, unlike the significantly different performances in the two-way FSI method, the performances of all the turbulence models are very close in the one-way FSI method. Then, the k − ϵ turbulence model was applied to the added damping simulation of a Kaplan turbine runner, and results show that the modification of the blade trailing edge shape can be helpful to increase the added damping to some extent.

Submerged floating tunnel cross-section analysis using a transition turbulence model

Concepts of a submerged floating tunnel (SFT) for novel sea-crossings have been researched in recent years. An SFT tube should be moored afloat by tensioned mooring systems to maintain the tube position under complex hydrodynamic loads. In-line force is amongst the dominant hydrodynamic parameters in the SFT cross-section design and the mooring system reliability evaluation. Selecting a suitable in-line force computation method is crucial to successful and accurate SFT cross-section optimization. The transition SST model is an effective turbulence transition prediction tool in the boundary layer computation subjected to tidal flow at both low and high Reynolds numbers. Two types of parametric Bézier curves applied in airfoil optimization are used to describe the SFT cross-section. We show that an SFT cross-section described by a leading-edge Bézier-PARSEC (BP) curve has better hydrodynamic performance than a trailing-edge BP curve of equal aspect ratio (AR). To avoid large flow separation, an AR not exceeding 0.47 is recommended. An SFT cross-section design should balance hydrodynamic performance and construction cost. The SFT cross-section with AR = 0.47 using the leading-edge BP curve with fixed clearance has a comparatively small in-line force and a minimum material cost.

Validation and comparison of turbulence models for predicting wakes of vertical axis wind turbines

Vertical axis wind turbine (VAWT) array design requires adequate modelling of the turbine wakes to model the flow throughout the array and, therefore, the power output of turbines in the array. This paper investigates how accurately different turbulence models using 2D computational fluid dynamics (CFD) simulations can estimate near and far wakes of VAWTs to determine an approach towards accurate modelling for array design. Three experiments from the literature are chosen as baselines for validation, with these experiments representing the near to far wake of the turbine. Five URANS turbulence models were chosen due to their common and potential usage for VAWT CFD: models k–ω SST, k–ω SST LRN, k–ω SSTI, transition SST, and k–kl–ω. In addition, the lifting line-free vortex wake (LLFWV) model was tested as an alternative to CFD for the far turbine wake where it was appropriate for use. The results for turbulent kinetic energy and vorticity were compared for the first experiment, whilst streamwise and cross-stream velocity were used for the other two experiments. It was found that none of the turbulence models tested or LLFVW produced adequate estimations within the methodology tested, however, transition SST produced the closest estimations. Further adjustments to the methodology are required to improve accuracy due to their large impact on results including use of 3D CFD, adjustment of surface roughness, and inlet flow characteristics.

Numerical Simulation Studies on Stall Suppression of a NACA0015 Airfoil

This study aims to achieve an improved airfoil performance at low Reynolds number, and to determine the optimum position and size of rectangular cross-section burst control plate (BCP) to suppress stall in airfoil. The type of airfoil used in the present study is NACA0015 (National Advisory Committee for Aeronautics) airfoil with 200 mm of chord (c) length. Here, rectangular cross-section burst control plates with different sizes and at different locations are investigated numerically at the low Reynolds number of 1.6×105. Total of three positions (0.05c, 0.1c and 0.2c from the leading edge of airfoil), and four sizes (with heights 0.3 mm, 0.7mm, 1mm and 1.5 mm, and constant width 4 mm) of rectangular BCPs are simulated in ANSYS Fluent software using Transition SST model. The results indicate that the rectangular cross-section burst control plate is an effective device in the suppression of airfoil stall. For 0.7 mm and 1 mm height BCPs, the stall angle is postponed by 2° for all positions, while for 0.3 mm and 1.5 mm height BCPs, the reduction in sudden fall of lift can be observed but at the cost of reduction in maximum lift coefficient. Among various configurations, the 1mm height BCP located at 0.2c position is found to be most effective in the suppression of stall.

Application of Computational Fluid Dynamics Method for Cross-flow Turbine in Pico Scale

Crisis electricity was a crucial issue in the rural area. Crossflow turbine (CFT) in pico in pico scale is the best option for electricity provider for rural areas. Due to its usefulness and development of computer technology, computational fluid dynamics method application for CFT study becomes increasingly frequent. This paper compiles the implementation of the computational fluid dynamic (CFD) approach for CFT on a pico scale. Based on the literature, the Renormalization Group (RNG) turbulence model is recommended to predict the flow field that occurs in CFT because its error is lower than others turbulence models, the RNG error of 3.08%, standard of 3.19%, and transitional SST of 3.10%. Furthermore, six-degrees of freedom (6-DoF) is recommended because it has an error of 3.1% than a moving mesh of 9.5% for the unsteady approach. Thus, based on the review, the RNG turbulence model and 6-DoF are recommended for the CFT on the pico scale.

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.

Effects of helical obstacle on heat transfer and flow in a tube

New configurations of helical obstacle were numerically investigated in this paper to reveal their effects on both thermal and hydrodynamics fields of flow in a straight circular tube. The obstacle height ratio (hr) of 0.125, 0.25 and 0.5 and helix pitch ratio (y) of 3, 5 and 10 were tested through the present effort. The fluid was air and it flowed with range of Reynolds number of 4000 ≤ Re ≤ 10,000 and the tube was under constant heat flux. ANSYS 17.1 was used and transition SST model was chosen to solve the problem. There is a significant increase in Nusselt number, friction factor and thermal performance factor was noticed as height ratio increased or helix pitch ratio decreased. All the obtained thermal performance factor from the studied cases were more than 1 and the higher one was 2.12 which obtained at hr and y of 0.5 and 3 respectively.

Numerical Simulation of Flow and Heat Transfer Characteristics of a Liquid Jet Impinging on a Cylindrical Cavity Heat Sink

In this study, the flow and heat transfer characteristics of a liquid jet impinging on cylindrical cavity heat sinks with a local body heat source were numerically investigated. The Transition SST turbulence model was validated and adopted. The parameters of structure and flow, including d/D = 2, 3, and 4, h/D = 0.05, 0.10, 0.15, and 0.20, and Re = 5,000, 10,000, and 23,000, were investigated. The results revealed that the adoption of a cylindrical cavity structure can improve the heat transfer capacity of the heat sink. A horseshoe vortex introduced by an inclined jet near the cavity edge region improved the heat transfer performance. The maximum enhancement of the cylindrical cavity heat sink was 11.8% compared with the flat plate heat sink when d/D = 3, h/D = 0.15, and Re = 23,000.

The numerical simulation and analysis of turbulent flow behavior in 5 × 5 fuel rod bundle with split-type mixing vane

In this research, the numerical simulation and analysis of flow behavior in 5 × 5 fuel rod bundle with consideration of spacer grid has been conducted. ANSYS FLUENT 19.0 with the consideration of the transition SST (Shear-Stress Transport) with SAS (Scale-Adaptive Simulation) turbulence model is used for the numerical simulation. The obtained simulation results are reasonable compared with the experimental data based on the comparison in the aspect of velocity distribution. The effect of the spacer grid on the flow behavior in the fuel rod bundle has also been investigated. The mixing vanes in the spacer grid can cause various strong flow branches; these flow branches develop and cause the complex flow behavior in the fuel rod bundle. Additionally, the secondary flow, the swirling factor and the mixing factor were investigated for the quantitative analysis of the spacer grid effect on the flow mixing behavior in fuel rod bundle.

Simulation of Swirling Flow with a Vortex Breakdown Using Modified Shear Stress Transport Model

The well-known Shear Stress Transport (SST k–ω) turbulence model was modified and examined. Two industrially relevant problems with curved and rotating channels have been selected to assess the modification potential: a rotating lid in a confined cylinder and swirling flow through a three-dimensional abrupt expansion pipe. The postulated amendment simplified the rotation and curvature correction term that was suggested earlier by Smirnov and Menter [J. Turbomach. 2009, 131 (4) 041010]. The new formulation avoids the calculation of the complex Lagrangian derivatives by implementing the Richardson number (Ri) in the applied rotation function. The numerical computations were performed using OpenFOAM-2.4.x. The results show the expected capability of the Shear Stress Transport model with Curvature Correction Modification (SSTCCM) to handle the curvature effects and system rotation. The paper compares the SSTCCM model with the conventional eddy viscosity models (EVMs): standard k–ϵ; Re-Normalization Group (RNG) k–ϵ, and the original SST k–ω.

Analysis of the Magnus Moment Aerodynamic Characteristics of Rotating Missiles at High Altitudes

The Magnus moment characteristics of rotating missiles with Mach numbers of 1.3 and 1.5 at different altitudes and angles of attack were numerically simulated based on the transition SST model. It was found that the Magnus moment direction of the missiles changed with the increase of the angle of attack. At a low altitude, with the increase of the angle of attack, the Magnus moment direction changed from positive to negative; however, at high altitudes, with the increase of the angle of attack, the Magnus moment direction changed from positive to negative and then again to positive. The Magnus force direction did not change with the change of the altitude and the angle of attack at low angles of attack; however, it changed with altitude at an angle of attack of 30°. When the angle of attack was 20°, the interference of the tail fin to the lateral force of the missile body was different from that for other angles of attack, leading to an increase of the lateral force of the rear part of the missile body. With the increasing altitude, the position of the boundary layer with a larger thickness of the missile body moved forward, making the lateral force distribution of the missile body even. Consequently, Magnus moments generated by different boundary layer thicknesses at the front and rear of the missile body decreased and the Magnus moment generated by the tail fin became larger. As lateral force directions of the missile body and the tail were opposite, the Magnus moment direction changed noticeably. Under a high angle of attack, the Magnus moment direction of the missile body changed with the increasing altitude. The absolute value of the pitch moment coefficient of the missile body decreased with the increasing altitude.

Numerical and One-Dimensional Studies of Supersonic Ejectors for Refrigeration Application: The Significance of Wall Pressure Variation in the Converging Mixing Section

This paper studies the pressure variation that exists on the converging mixing section wall of a supersonic ejector for refrigeration application. The objective is to show that the ejector one-dimensional model can be improved by considering this wall’s pressure variation which is typically assumed constant. Computational Fluid Dynamics (CFD) simulations were used to obtain the pressure variation on the aforementioned wall. Four different ejectors were simulated. An ejector was obtained from a published experimental work and used to validate the CFD simulations. The other three ejectors were a modification of the first ejector and used for the parametric study. The secondary mass flow rate, m˙s, was the main parameter to compare. The CFD validation results indicate that the transition SST turbulence model is better than the k-omega SST model in predicting the m˙s. The results of the ejector one-dimensional model were compared before and after incorporating the wall pressure variation. The comparison shows that the effect of the pressure variation is significant at certain operating conditions. Even around 2% change in the average pressure can give around 32% difference in the prediction of m˙s. For the least sensitive case, around 2% change in the average pressure can give around 7% difference in the prediction.

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.

Laminar Flow Control and Drag Reduction using Biomimetically Inspired Forward Facing Steps

This paper explores the use of shark-skin inspired two-dimensional forward facing steps to attain laminar flow control, delay boundary layer transition and to reduce drag. Computation Fluid Dynamics (CFD) simulations are carried out on strategically placed forward facing steps within the laminar boundary layer using the Transition SST model in FLUENT after comprehensive benchmarking and validation of the simulation setup. Results presented in this paper indicate that the boundary layer thickness to step height ratio (d/h), as well as the location of the step within the laminar boundary layer (x/L), greatly influence transition onset. The presence of a strategically placed forward facing step within the laminar boundary layer might damp disturbances within the laminar boundary layer, reduce wall shear stress and energize the boundary layer leading to transition onset delay and drag reduction as compared to a conventional flat plate. Results presented in this paper indicate that a transition delay of 20% and a drag reduction of 6% is achievable, thereby demonstrating the veracity of biomimicry as a potential avenue to attain improved aerodynamic performance.

Local variations of air velocity in the vicinity of filter pleats in transitional airflow regime – Experimental and numerical approaches

This study combines experimental velocity measurements and numerical simulations to characterize the airflow in the vicinity of pleated filters used in ventilation systems. Experimental velocity and turbulence intensity profiles were measured using Hot Wire Anemometry (HWA), upstream and downstream of the filter. Significant local velocity gradients were detected downstream of the filter. The maximum local velocity detected was as much as three times the inlet velocity. The gradient was constant up to 2.5 cm from the pleat entrance. However, only slight velocity variations were observed upstream of the filter. Computational Fluid Dynamics (CFD) simulations were performed using the Ansys Fluent code. Since the flow regime of the airflow approaching filters was transitional, the three following flow models were tested: laminar, standard k -ε, and transition SST. The numerical velocity data were in fairly close agreement with the experimental velocity profiles upstream and downstream of the filter, especially in close proximity to the filter medium. No significant turbulence intensities were detected close to the medium surface for the filtration velocity tested.

Performance Evaluation of a Vortex Induced Piezoelectric Energy Converter (VIPEC) with CFD Approach

A novel vortex induced piezoelectric energy converter (VIPEC) was present in this paper to harvest flow kinetic energy from the ambient environment through a piezoelectric beam. The converter consists of a circular cylinder, a pivoted beam attached to the tail of the cylinder and several piezoelectric patches. Vortex induced pressure difference acts on the beam and drives the beam to squeeze piezo patches to convert fluid dynamic energy into electric energy. Transition Shear Stress Transport (SST) combined with Scale Adaptive Simulation (SAS) model was employed to predict the turbulent flow and flow separation around the cylinder with various beam lengths at high Reynolds number of 8 × 104 based on the computational fluid dynamics (CFD) approach. The accuracy of SST-SAS model was investigated through verification and validation studies. The output voltage equation was derived from the piezoelectric constitutive equation. It was revealed that the beam length influences the flow wake pattern, the separation angle and shedding frequency greatly through changing the adverse pressure gradient around the cylinder. The wake pattern becomes symmetrical about the beam when the beam length is longer than a critical value. The length of the beam has little influence on the separation angle. When the beam length is about 1.3 times the diameter of the cylinder, the shedding frequency and output voltage achieves its maximum, and the separation angle is minimal. Maximal output voltage reaches 20 mV.