Shear Stress Transport K-omega Research Articles

Aerodynamic characteristics of a tip-jet fan with a large blade pitch angle

This work aims to investigate the availability of a tip-jet on a ducted fan under large blade pitch angle and high solidity conditions. The aerodynamic performance and flow field of a ducted fan in hover are numerically investigated over a range of blade pitch angles at three operating speeds. A numerical experiment is conducted using the shear-stress transport k-omega turbulence model with a refined high-quality structured grid. The maximum thrust, peak efficiency, and stall margin of the ducted fan with tip-jet are the main interests of this investigation. Results indicate a 30% increase in the thrust of the fan with tip-jet near stall margin condition. This aerodynamic improvement seems to increase with the blade pitch angle because the separation flow at the front part of the blade becomes uniform and reattaches to the blade surface due to the entrainment effect of the tip-jet. The nozzle with an angle in downwash direction can improve tip-jet efficiency at a large blade pitch angle. The tip-jet is applicable to a fan with a large pitch angle and high solidity.

Impact of endoscopic craniofacial resection on simulated nasal airflow and heat transport.

Endoscopic craniofacial resections (CFR) are performed for extensive anterior skull base lesions. This surgery involves removal of multiple intranasal structures, potentially leading to empty nose syndrome (ENS). However, many patients remain asymptomatic postoperatively. Our objective was to analyze the impact of CFR on nasal physiology and airflow using computational fluid dynamics (CFD). This is the first CFD analysis of post-CFR patients. Three-dimensional sinonasal models were constructed from 3 postoperative images using MimicsTM . Hybrid computational meshes were created. Steady inspiratory airflow and heat transport were simulated at patient-specific flow rates using shear stress transport k-omega turbulent flow modeling in FluentTM . Simulated average heat flux (HF) and surface area where HF exceeded 50 W/m2 (SAHF50) were compared with laminar simulations in 9 radiographically normal adults. Three adults underwent CFR without developing ENS. Average HF (W/m2 ) were 132.70, 134.84, and 142.60 in the CFR group, ranging from 156.24 to 234.95 in the nonoperative cohort. SAHF50 (m2 ) values were 0.0087, 0.0120, and 0.0110 in the CFR group, ranging from 0.0082 to 0.0114 in the radiographically normal cohort. SAHF50 was distributed throughout the CFR cavities, with increased HF at the roof and walls compared with the nonoperative cohort. Average HF was low in the CFR group compared with the nonoperative group. However, absence of ENS in most CFR patients may be due to large stimulated mucosal surface area, commensurate with the nonoperative cohort. Diffuse distribution of stimulated area may result from turbulent mixing after CFR. To better understand heat transport post-CFR, a larger cohort is necessary.

Circular Free Jets: CFD Simulations with Various Turbulence Models and Their Comparison with Theoretical Solutions

The theory of turbulent free jet is fundamental for the design of comfort ventilation, as free jets frequently occur in mixing and personalized ventilation systems and their characteristics strongly influence the air quality in the breathing zone of an occupant. The aim of this research is to provide recommendations that help researchers and practitioners improve the accuracy and reliability of their computational models of ventilation systems involving circular free jets. To accomplish this, a review of existing theoretical calculation models is performed, and these models are subsequently investigated by computational fluid dynamics. The theoretical solutions of free jets are compared with CFD simulations using various turbulent models such as the standard k-epsilon model, the k-epsilon realizable model, the standard k-omega model, the shear stress transport (SST) k-omega model, and the Reynolds stress model (RSM). The simulated models are represented by profiles of the centreline velocity for a free jet emanating from a round nozzle, because such presentation of the data proved to be particularly helpful for the comparison of the turbulence models. The k-omega SST turbulence closure scheme with standard coefficients produced results of the centreline velocity closest to the average of theoretical solutions investigated, whereas the discrepancy between the simulations and the theoretical models was about 60 % with the k-epsilon standard turbulence model.

Performance investigations of cross flow hydro turbine with the variation of blade and nozzle entry arc angle

Mistreatment of small hydropower sources requires the use of small turbines that combine efficiency and economy which can suitably provide the power requirements of rural and small societies. In this context, the cross flow hydro turbines are extensively used due to their simpler in design, ease of maintenance, low initial cost. Although the cross flow hydro turbine has moderated efficiency, but it has operated at low head and discharge. Hence, the objective of the present investigation is to increase the efficiency of a cross flow hydro turbine with the geometric modification. In the present study, the numerical investigation of cross flow hydro turbine has been carried out using multi-physics finite volume solver in ANSYS FLUENT. The three dimensional unsteady simulations have been conducted using shear stress transport k-omega turbulence model. The performance of cross flow hydro turbine has been investigated by changing the inlet blade angle from 5° to 40° and nozzle entry arc angle from 65° to 85° with the number of blades being 20. The rotational speed of the turbine has been varied from 200 rpm to 800 rpm. The total pressure and velocity field have been examined to evaluate the performance coefficients. From the unsteady results, the maximum efficiency has been found at the inlet blade angle, 5° and nozzle entry arc angle, 65° with the runner rotational speed of 600 rpm. However, with the consideration of the manufacturing feasibility inlet blade angle and nozzle entry arc angle have been considered as 10° and 65° respectively to be the most promising design configuration. The effect of temperature of the flowing water on performance of the turbine also has been studied. The maximum efficiency at inlet blade angle, 10° has been found to be 93.8% at 600 rpm. By incorporating the nozzle entry arc angle, maximum efficiency has been found to be 97.8% at nozzle entry arc angle, 65° with rotational speed 600 rpm and inlet blade angle, 10°. The efficiency of the current design configuration has been found 20% higher compare to the existing model.

Comparison of the power, lift and drag coefficients of wind turbine blade from aerodynamics characteristics of Naca0012 and Naca2412

In this paper two dimensional models of airfoils (NACA0012, NACA2412) are analyzed for the aerodynamic characteristics at various Reynolds numbers for a range of angles of attack from low lift through stall, based on the chord length of the airfoil. The local characteristics, the lift, drag, pressure coefficients are simulated by using three models the Spalart-Allmaras, the k-epsilon (RNG) and the k-omega shear stress transport (SST). The comparison of predictions and experimental measurements in the wind tunnel of the National Aeronautics Advisory Committee (NACA) for selected aerodynamic airfoils are presented. The blade geometric parameters including chord and twist angle distributions are determined based on aerodynamic parameters results at a specific Reynolds number. This study is carried out by providing an optimal blade design strategy for horizontal-axis wind turbines operating at different Reynolds numbers. As a conclusion the simulation results were compared with the experimental results. In general, good concordances were noted. This approach can be further developed to create the most efficient of horizontal axis wind turbine blade design.

Numerical investigation of a novel contra-rotating vertical axis wind turbine

Darrieus H-rotor has gained much interest in the last few decades as among the reliable devices for wind energy conversion techniques for their relatively simple structure and aerodynamic performance. This study presents a numerical investigation of a novel vertical axis wind turbine (VAWT) with a contra-rotating concept. The aim is to reveal the effectiveness of this concept on enhancing the performance of the turbine as it has yet to be applied to a VAWT system. The simulations were performed using three-dimensional computational fluid dynamics (CFD) models based on K-omega shear stress transport (SST) turbulence model. This computational study covers a wide range of simulation work including a parametric study based on the axial distance between the two rotors and rotor aspect ratio. It was found that the new concept works more efficiently and therefore the performance of the turbine has been improved significantly. The system was capable of producing more than double of torque coefficient compared to a single-rotor system of a similar type. The results also indicate that smaller axial distance tends to enhance the performance output of the system relatively better compared to a larger distance. In terms of rotor aspect ratio, bigger aspect ratio generated the highest amount of power.

Flow and heat transfer behaviors for double-walled-straight-tube heat exchanger of HLM loop

Non-contacted double-walled-straight-tube heat exchanger has been designed for the Lead-Bismuth Eutectic (LBE) loop KYLIN-II, the flow rate and temperature distribution dramatically influence the performance of heat exchanger. The Computational Fluid Dynamics (CFD) and numerical heat transfer method has been used to build the three-dimensional model of the heat exchanger. Adopting the SST (Shear Stress Transport) k-omega model the flow rate and temperature distribution of the heat exchanger model has been calculated. The numerical simulation result shows that the non-uniform LBE flows don’t affect TH performance of HX filled with powder in the gap between two tubes.

Aerodynamics and structural analysis of wind turbine blade

The ultimate objective of the paper is to increase the reliability of wind turbine blades through the development of the airfoil structure, to calculate an optimum blade shape for the procedure begins with the choice of airfoils characteristics. Then an initial wind blade design is determined using blade element momentum. The blade plays a pivotal role, because it is the most important part of the energy absorption system. Practical horizontal axis wind turbine (HAWT) designs use airfoils to transform the kinetic energy in the wind into useful energy and it has to be designed carefully to enable to absorb energy with its greatest efficiency. There are many factors for selecting a profile. One significant factor is the chord length and twist angle which depend on various values throughout the blade. In this work, the airfoil sections used in horizontal axis wind turbine (HAWT) are S818; S825 and S826 airfoils used in NREL phase 2 and phase 3 wind turbines. They have several advantages in meeting the intrinsic requirements for wind turbines in terms of design point, off-design capabilities and structural properties. The lift and drag coefficients data for these airfoils sections are available and Matlab code were used to obtain the coordinates of a wind turbine blade. Aerodynamic and static structural analyses are presented. The commercially available software FLUENT is employed for calculation of the flow field using the Reynolds-averaged Navier Stokes (RANS) in conjunction with the k-omega shear stress transport (SST), based on finite-element method (FEM). Both grid and time step were optimized to reach independent solutions. The present work represents the basis to develop an accurate three dimensional Horizontal-Axis Wind Turbine (HAWT) model and may be used to support wind tunnel experiments.

Effects of Rib Shapes on the Entropy Generation in a Ribbed Duct

Ducts are often used in reactors and heat exchangers. Using rough surfaces in ducts is recognized as an effective technique to enhance heat transfer. In this research, an entropy generation analysis is carried out for turbulent flow inside a ribbed duct. Numerical simulations are performed to study the effects of rib shapes and flow Reynolds numbers on friction factor, Nusselt number, and different types of entropy generation containing the frictional and thermal ones. All simulations are performed for Reynolds number in the range of 20,000 to 60,000 and six rib shapes (rectangular, square, isosceles trapezoidal, rectangular trapezoidal, equilateral triangle, and nonequilateral triangle). A shear stress transport k-omega turbulence model is used for the numerical simulations. It was found that the ribs with no inclined surfaces such as square and rectangular shapes create more thermal irreversibility. Moreover, it was shown that among different rib shapes, the equilateral triangle rib has the highest frictional entropy generation over the entire range of the Reynolds number, whereas the rectangular rib has the lowest one.

Three dimensional optimization of small-scale axial turbine for low temperature heat source driven organic Rankine cycle

Advances in optimization techniques can be used to enhance the performance of turbines in various applications. However, limited work has been reported on using such optimization techniques to develop small-scale turbines for organic Rankine cycles. This paper investigates the use of multi-objective genetic algorithm to optimize the stage geometry of a small-axial subsonic turbine. This optimization is integrated with organic Rankine cycle analysis using wide range of high density organic working fluids like R123, R134a, R141b, R152a, R245fa and isobutane suitable for low temperature heat sources <100°C such as solar energy to achieve the best turbine design and highest organic Rankine cycle efficiency. The isentropic efficiency of the turbine in most of the reported organic Rankine cycle studies was assumed constant, while the current work allows the turbine isentropic efficiency to change (dynamic value) with both operating conditions and working fluids. Three-dimensional computational fluid dynamics analysis and multi-objective genetic algorithm optimization were performed using three-dimensional Reynolds-averaged Navier-Stokes equations with k-omega shear stress transport turbulence model in ANSYSR17-CFX and design exploration for various working fluids. The optimization was carried out using eight design parameters for the turbine stage geometry optimization including stator and rotor number of blades, rotor leading edge beta angle, trailing edge beta angle, stagger angle, throat width, trailing half wedge angle and shroud tip clearance. Results showed that using working fluid R123 for a turbine with mean diameter of 70mm, the maximum isentropic efficiency was about 88% and power output of 6.3kW leading to cycle thermal efficiency of 10.5% showing an enhancement of 14.08% compared to the baseline design. Such results highlight the potential of the 3D optimization technique to improve the organic Rankine cycle performance.

Development of micro-scale axial and radial turbines for low-temperature heat source driven organic Rankine cycle

Most studies on the organic Rankine cycle (ORC) focused on parametric studies and selection working fluids to maximize the performance of organic Rankine cycle but without attention for turbine design features which are crucial to achieving them. The rotational speed, expansion ratio, mass flow rate and turbine size have markedly effect on turbine performance. For this purpose organic Rankine cycle modeling, mean-line design and three-dimensional computational fluid dynamics analysis were integrated for both micro axial and radial-inflow turbines with five organic fluids (R141b, R1234yf, R245fa, n-butane and n-pentane) for realistic low-temperature heat source <100°C like solar and geothermal energy. Three-dimensional simulation is performed using ANSYSR17-CFX where three-dimensional Reynolds-averaged Navier-Stokes equations are solved with k-omega shear stress transport turbulence model. Both configurations of turbines are designed at wide range of mass flow rate (0.1–0.5)kg/s for each working fluid. The results showed that n-pentane has the highest performance at all design conditions where the maximum total-to-total efficiency and power output of radial-inflow turbine are 83.85% and 8.893kW respectively. The performance of the axial turbine was 83.48% total-to-total efficiency and 8.507kW power output. The maximum overall size of axial turbine was 64.685mm compared with 70.97mm for radial-inflow turbine. R245fa has the lowest overall size for all cases. The organic Rankine cycle thermal efficiency was about 10.60% with radial-inflow turbine and 10.14% with axial turbine. Such results are better than other studies in the literature and highlight the potential of the integrated approach for accurate prediction of the organic Rankine cycle performance based on micro-scale axial and radial-inflow turbines.

Thermodynamic analysis of shark skin texture surfaces for microchannel flow

The studies of shark skin textured surfaces in flow drag reduction provide inspiration to researchers overcoming technical challenges from actual production application. In this paper, three kinds of infinite parallel plate flow models with microstructure inspired by shark skin were established, namely blade model, wedge model and the smooth model, according to cross-sectional shape of microstructure. Simulation was carried out by using FLUENT, which simplified the computation process associated with direct numeric simulations. To get the best performance from simulation results, shear-stress transport k-omega turbulence model was chosen during the simulation. Since drag reduction mechanism is generally discussed from kinetics point of view, which cannot interpret the cause of these losses directly, a drag reduction rate was established based on the second law of thermodynamics. Considering abrasion and fabrication precision in practical applications, three kinds of abraded geometry models were constructed and tested, and the ideal microstructure was found to achieve best performance suited to manufacturing production on the basis of drag reduction rate. It was also believed that bionic shark skin surfaces with mechanical abrasion may draw more attention from industrial designers and gain wide applications with drag-reducing characteristics.

A numerical simulation of wheel spray for simplified vehicle model based on discrete phase method

Road spray greatly affects vehicle body soiling and driving safety. The study of road spray has attracted increasing attention. In this article, computational fluid dynamics software with widely used finite volume method code was employed to investigate the numerical simulation of spray induced by a simplified wheel model and a modified square-back model proposed by the Motor Industry Research Association. Shear stress transport k-omega turbulence model, discrete phase model, and Eulerian wall-film model were selected. In the simulation process, the phenomenon of breakup and coalescence of drops were considered, and the continuous and discrete phases were treated as two-way coupled in momentum and turbulent motion. The relationship between the vehicle external flow structure and body soiling was also discussed.

Numerical Simulation of Floating Airboat: Estimation of Hydrodynamic Forces

Airboat motion is always a specific maneuver operation. The flow around the airboat and the forces acting on it are quite different from those for a ship in normal act. By solving the unsteady Reynolds- Averaged Navier-Stokes (RANS) equations, the transient flow field around an airboat undergoing unsteady motion is simulated and the varying hydrodynamic force in effect of the different currents acting on the hull is evaluated in this article. OpenFoam 2.1.0 and extended toolbox will be used to do simulation. The numerical results obtained with K-omega shear stress transport turbulence models and the volume of fluid method as the suitable turbulence model are analyzed and compared with experimental results.

Numerical prediction of cavitating flow around a hydrofoil using pans and improved shear stress transport k-omega model

The prediction accuracies of partially-averaged Navier-Stokes model and improved shear stress transport k-? turbulence model for simulating the unsteady cavitating flow around the hydrofoil were discussed in this paper. Numerical results show that the two turbulence models can effectively reproduce the cavitation evolution process. The numerical prediction for the cycle time of cavitation inception, development, detachment, and collapse agrees well with the experimental data. It is found that the vortex pair induced by the interaction between the re-entrant jet and mainstream is responsible for the instability of the cavitation shedding flow.

Scalability of the parallel CFD simulations of flow past a fluttering airfoil in OpenFOAM

The paper is devoted to investigation of unsteady subsonic airflow past an elastically supported airfoil during onset of the flutter instability. Based on the geometry, boundary conditions and airfoil motion data identified from wind-tunnel measurements, a 3D CFD model has been set up in OpenFOAM. The model is based on incompressible Navier-Stokes equations. The turbulence is modelled by the Menter’s k-omega shear stress transport turbulence model. The computational mesh was generated in GridPro, a mesh generator capable of producing highly orthogonal structured C-type meshes. The mesh totals 3.1 million elements. Parallel scalability was measured on a small shared-memory SGI Altix UV 100 supercomputer.

Optimal Turbulent Schmidt Number for RANS Modeling of Trailing Edge Slot Film Cooling

It has been previously demonstrated that Reynolds-averaged Navier–Stokes (RANS) simulations do not accurately capture the mixing between the coolant flow and the main flow in trailing edge slot film cooling configurations. Most RANS simulations use a fixed turbulent Schmidt number of either 0.7 or 0.85 to determine the turbulent scalar flux, based on the values for canonical flows. This paper explores the extent to which RANS predictions can be improved by modifying the value of the turbulent Schmidt number. Experimental mean 3D velocity and coolant concentration data obtained using magnetic resonance imaging techniques are used to evaluate the accuracy of RANS simulations. A range of turbulent Schmidt numbers from 0.05 to 1.05 is evaluated and the optimal turbulent Schmidt number for each case is determined using an integral error metric which accounts for the difference between RANS and experiment throughout a three-dimensional region of interest (ROI). The resulting concentration distribution is compared in detail with the experimentally measured coolant concentration distribution to reveal where the fixed turbulent Schmidt number assumption fails. It is shown that the commonly used turbulent Schmidt number of 0.85 overpredicts the surface effectiveness in all cases, particularly when the k-omega shear stress transport (SST) model is employed, and that a lower value of the turbulent Schmidt number can improve predictions.

Experimental and numerical study of turbulent flow and heat transfer inside hexagonal duct

Flow and heat transfer characteristics in transition and turbulent regions are studied experimentally and numerically in a horizontal smooth regular hexagonal duct under constant wall temperature boundary condition covering a range of Reynolds number from 2.3 × 103 to 52 × 103. Two types of k-omega (standard and shear stress transport (SST)) and three types of k-e (standard, renormalization (RNG), and realizable) turbulence model are employed for transition and turbulent regions, respectively. Both average and fully developed Darcy friction factor and Nusselt number are presented as a function of Reynolds number. It is seen that k-omega SST and k-e realizable turbulence models gave the best agreement with the experimental data in transition and turbulent regions, respectively. All the experimental results are correlated within an accuracy of ±13 % and ±7 % for Nusselt number and Darcy friction factor, respectively. Results obtained in this study are compared with circular duct results using hydraulic diameter.

Study on Surface Vortices in Pump Sump

One of commonly physical phenomena encountered in pump sump systems in which its significant influence to the hydraulic performance of pump system plays an important role in the field of fluid engineering, is the appearance of free surface and submerged vortices. In this paper, a study of the vortices behavior and their formative mechanism of asymmetry is considered in this paper by using numerical approach. The Reynolds-Averaged Navier-Stokes (RANS) equations and k-omega Shear Stress Transport turbulence model used to describe the properties of turbulent flows, in company with VOF multiphase model, are implemented by Fluent code with multi-block structured grid system. In the numerical simulation, the calculated elevation of air-water interface and vortex core contours are used to classify visually surface vortices as well as submerged vortices. It is shown that the free surface vortex is identified by the concavity of liquid region from the free surface and swirling flow at that own plane. To investigate the distinctive behavior of these vortices corresponding to each given flow rate at the same water level, some numerical testing of them are considered here in such a manner that the flow pattern of surface vortex are obtained similarly to the obtained results from experiment. Furthermore, the influence due to the change of grid refinement and the variation of depth of the concavity are also considered in this paper. From that, these influential factors will be implemented to design a good pump sump with higher performance in the future.

Comparing turbulence models for flow through a rigid glottal model

Flow through a rigid model of the human vocal tract featuring a divergent glottis was numerically modeled using the Reynolds-averaged Navier-Stokes approach. A number of different turbulence models, available in a widely used commercial computational fluid dynamics code, were tested to determine their ability to capture various flow features recently observed in laboratory experiments and large eddy simulation studies. The study reveals that results from unsteady simulations employing the k-omega shear stress transport model were in much better agreement with previous measurements and predictions with regard to the ability to predict glottal jet skewing due to the Coanda effect and the intraglottal pressure distribution or related skin friction coefficient, than either steady or unsteady simulations using the Spalart-Allmaras model or any other two-equation turbulence model investigated in this study.