Realizable Turbulence Model Research Articles

Numerical Study on Single and Multi-Element NACA 43018 Wing Airfoil with Leading-Edge Slat and Slotted Flap

An optimal wing configuration is crucial for achieving the best performance during various flight phases, including take-off, cruising, and landing. Such configurations also contribute to maximizing the aircraft's cruising range. This study compares the aerodynamic performance of NACA 43018 wings under different conditions: without high-lift devices, with a slotted flap, and with a combination of a leading-edge slat and slotted flap. Numerical simulations were conducted using the k-ɛ Realizable turbulence model at twelve different angles of attack, with a flow speed of 120 m/s. The results demonstrate that multiple-element wings significantly improve aerodynamic performance, particularly at low angles of attack, by reducing the induced drag coefficient and delaying flow separation.

Physical mechanism for prolonged test duration in shock tubes

The main challenge in the operation of shock tubes is the very short test time. Hence, this work was carried out to optimize the ratio of driver section to driven section length and to investigate its effect on the test time in a shock tube. Three different cases were tested in this paper for driver to driven tube length ratio of 2, 1, and 0.5. The study was conducted numerically using ANSYS Fluent CFD solver and k-epsilon realizable turbulent model. The shock wave strength and speed were monitored at two different locations downstream the driven tube for diaphragm pressure ratio of 10. The numerical model was verified with experimental measurements in a shock tube apparatus at the College of Engineering, Universiti Tenaga Nasional, Malaysia (UNITEN). From the numerical and experimental data, the primary shock strength was very comparable, while using low pressure ratio of 10. Subsequently, higher pressure ratio of 20 and 30 were implemented and the results were compared to select the longest test time. A driver section to driven section ratio of 2 resulted in a longer test time of approximately of 20 ms. In addition, insignificant reduction in the test duration of about 5 % was observed after increasing the driver tube pressure to 30 kPa. In contrast, higher peak pressure was recorded for pressure ratio of 30 for all three cases.

Thermal Hydraulic Analysis of Turbulent Flow of Superheated Steam Through Multiple Configurations of 3D Angular Piping Bend in Power Plant Installations

Abstract One of the most vulnerable parts in high-pressure and temperature pipeline networks in steam turbine power plant installations is pipe bends, due to intense fluctuation of velocity and pressure of the superheated steam flowing through the piping bend. On the other hand, because of unique chemical and physical properties, the transportation of superheated steam can adversely effect on the integrity of the pipe bends in the piping network. Therefore, the potential risk of pipeline failure becomes more probable at bends. Therefore, to ensure the survivability of the piping network, thorough investigations of the superheated steam flowing through pipe bends are of great significance in better understanding its flow behavior. However, to predict flow behavior at piping bend, intensive research by direct experiment at high temperature and pressure might not be possible to some extent. In that case, computer codes and models can help us analyze flow behavior of superheated steam at pipe bends. The current work is a computational fluid dynamics investigation, where numerical examination is carried out by commercial code ansys fluent for thermal hydraulic analysis of flow behavior of superheated steam through multiple configurations of 3D angular piping bend: 90 deg, 120 deg, and 135 deg. Validation of credibility of the computational approach is done against Sudo et al.'s experiment. With relatively good agreement between the experiment and numerical result using air flowing through 90 deg elbow, single-phase turbulent flow of superheated steam is examined by the application of realizable turbulence model (k–ɛ). The final assessment of the computed results has been done using qualitative and quantitative analyses. The study reveals that unlike the pressure profiles, the radial velocity profiles at the bend exit are noticeably different from those at the bend inlet, since they exhibit more rounded peaks and gentler velocity gradients. The study also shows that the radial pressure and velocity profiles do not display any sign of the existence of a pair of vortices.

Investigation of the thermal performance of a spiral fin-and-tube heat exchanger by numerical methods

Spiral fin-and-tube heat exchangers (SPF HEX) are broadly employed heat transfer (HT) equipment for heating and cooling applications. Therefore, developing a model that predicts the HT characteristics could provide an advantage when selecting the required device considering operation conditions. The efficacy of the HT characteristics of a three-dimensional SPF HEX was examined under various boundary conditions with the help of a commercial computational fluid dynamics (CFD) solver and predicted by employing machine learning (ML) approaches as a rarely studied novel work. Three-dimensional, steady, and incompressible Reynolds-Averaged Navier-Stokes (RANS) formulas, including continuity, momentum and energy were solved to obtain numerical data. k-ε Realizable turbulence model, which has two additional transport equations, was employed. The outlet temperature of water was calculated at various water inlet velocity, air inlet velocity, and air inlet temperature with a constant water inlet temperature. First, the methods of CFD were utilized to obtain sixty cases, then the outlet temperatures of water were obtained at various conditions. The current CFD technique was verified with a work in literature. After collecting a labeled dataset, this data is fed into four recognized ML algorithms: Support Vector Machine (SVM), Linear Regression (LR), Decision Tree Regression (DT), and Random Forest (RF). In the presented results, the SVM algorithm achieved the highest accuracy rate with values of 0.1765 mean squared error (MSE), 0.3294 mean absolute error (MAE), and 0.9940 R-squared (R2). Furthermore, these results indicate that ML algorithms can deliver high prediction performance, potentially reducing the need for many experimental and numerical studies.

A numerical study of cucurbit cultivation in a greenhouse under direct solar radiation and equipped with a direct evaporative cooler in summer season

The performance of direct evaporative cooler to meet cooling demand of a greenhouse in Tehran in the hottest months of the year as well as water and electricity consumption were numerically studied. The suitable values of air change per hour (ACH) was also obtained for cultivation of tomato, cucumber, eggplant and bell peppers. The k-Ԑ Realizable Turbulence and Discrete Ordinates (DO) models was used for simulating the airflow and estimating radiative heat transfer, respectively. The results showed that an ACH of 10 is proper for tomato cultivation. When the ambient temperature is maximal, an ACH of 15 can be used for cucumber cultivation. However, ACH cannot be kept constant, and ACHs of ≤10 should be utilized when the ambient temperature decreases. For eggplants, it is better to use an ACH 20 at hot hours and an ACH of 10 when the temperature declines. For the bell pepper plant, 20 and 10 ACHs are suitable for day and night hours, respectively. Moreover, the maximum daily water consumed in the greenhouse happens in August and equals 222 lit/day for 20 Air Changes per Hour (ACH). The minimum daily water is consumed in July, equaling 106 lit/day for an ACH of 10. In the present study, the electricity rate consumed in the greenhouse equals 127W when ACH = 20, i.e., the consumed electric energy is 3 kWh for the entire system in a 24-h interval.

Numerical simulation of flow coating technology process for lens of automotive lighting lamp

To study the flow pattern of the flow coating technology applied to substrates with a complex three-dimensional surface (the lens of an automotive lighting lamp) and the possible defects (mainly “wrinkles”), numerical simulation was used to simulate the coating implementation process. The complex geometric surface of the substrate was simplified and partitioned such that the simulation resources were significantly reduced. The k–ε realizable turbulence model and the volume of fluid multiphase model were used for the numerical solution. The flow pattern and moving contact line are analyzed by fluid dynamics, as well as the reasons for the formation of “wrinkles” and the “heel” phenomenon. It can be concluded from the comparison of the results and the actual experiments/videos that the method used in this study can effectively simulate the flow field in the implementation of the flow coating technology and predict the main defects of the flow field caused by the three-dimensional geometric characteristics of the substrate.

Numerical Determination Of Cooling Performance On Heat Sink Using Impingement Jet

The use of impingement jet technics drew significant attention of researchers in the recent period. In this method, significant heat transfer rates are achieved. The present study examined the cooling performance of the fin pairs, which are aligned in a consecutively enlarging-contracting pattern and have a rectangular geometry, on a heat sink. The target geometry was optimized in the previous study by using the Taguchi method. The analyses were performed using four different impingement jet velocities (10, 12, 14, and 16 m/sec.), three different nozzle diameters (D=50, 63, and 75 mm), three different heat flux values (q=2222, 3333, and 4444 W/m2), and constant nozzle-to-target distance (h/d=1) were analyzed. These results were simulated numerically by using the ANSYS Fluent software. The k-ε realizable turbulence model was selected as the best model. The numerical results showed that the mean Nusselt number is directly proportional to the increase in the Reynolds number. The Nusselt number also increased with the increasing nozzle diameter value. The results are illustrated graphically (Nu-Re) in the present study. The peak value of the local Nusselt number was found to be at the stagnation point.

Drawbacks of phase change models in simulating flashing of steam with a subsaturation downstream boundary condition

AbstractThe development of phase change models applicable to a wide range of temperatures, pressures, and mass flow rates is primarily limited by the metastable or partially stable behaviour of the fluid. Due to this, the fluid does not change phase even after crossing saturation conditions. Most liquid–vapour phase change models have been developed primarily for the cavitation process where the phase change is not sustained, occurs in a very narrow region of space, or occurs under equilibrium conditions. In this paper, the mechanism of phase change is discussed along with the review of three different computational fluid dynamics (CFD)‐based phase change methods available in OpenFOAM, which are used to simulate flashing of steam, in applications related to steam assisted gravity drainage (SAGD) systems. The first method is based on barotropic compressibility (BC) used with the realizable turbulence model, the second combination is of mass transfer model (MTM) with the realizable turbulence model, and the third one is based on two fluid (TF) method along with a family of turbulence models. These methods are tested on a converging–diverging nozzle with pressure driven phase change. It is demonstrated that these methods are not able to adjust their physics to different depressurization rates, do not account for liquid to be in superheated conditions, and have significant discrepancies with experimental results. In the end, better approaches to model this category of phase change are discussed.

Mechanism of multiphase coupling transport evolution of free sink vortex

In the evolution of confluence sink vortex with a free surface, there exists some physical processes , such as multiphase coupling, mass transfer, and intensive energy exchange. Here, the transport mechanism of multiphase coupling is a complex dynamic problem with highly nonlinear characteristics. The mechanical modeling and numerical solution of multiphase viscous coupled transport are facing a significant challenge. To address the above problem, a method of modeling and solving multiphase coupling transport of the free sink vortex is proposed. Based on the coupled level set and volume-of-fluid (CLSVOF) method, a multiphase coupling transport model of the free sink vortex is set up with a continuous surface tension model and a realizable (<i>k</i>-<i>ε</i>) turbulence model. By using an effective volumetric correction scheme, the high-speed rotating flow is calculated, and the mass conservation of flow field and the velocity field without divergence are ensured. Then, an interphase coupling solution approach accurately traces the multiphase fluid distribution and multiphase interface. The multiphase coupling interface and cross-scale vortex cluster transport laws are obtained according to the multi-characteristic physical variables. The interaction mechanism between the multiphase coupling transport process and the pressure pulsation characteristics is revealed. The results show that the multiphase coupling transport is the critical state of the fluid medium transition. The vortex microclusters are subjected to different spatiotemporal disturbance modes and form the layered threaded waveforms at the interface. With the increase of the nozzle sizes, the multiphase coupling process is strengthened, and the coupling energy shock causes nonlinear pressure pulsation. This study can offer valuable references to the researches of the vortex transport mechanism, cross-scale solution of vortex cluster, and flow pattern tracking.

Numerical Simulation about the Characteristics of the Store Released from the Internal Bay in Supersonic Flow

To understand the influence of the initial release conditions on the separation characteristics of the store and improve it under high Mach number (Ma = 4) flight conditions, the overset grid method and the Realizable turbulence model coupled with an equation with six degrees of freedom are used to simulate the store released from the internal bay. The motion trajectory and the attitude angle of the store separation under the conditions of different centroid, velocity, height and control measures are given by the calculated result. Through analysis, the position of the centroid will affect the separation of the store, which needs to be considered in the design. Increasing the launching height is conducive to the separation of the store. If the store has an initial velocity, it can leave the internal bay more quickly and reduce the probability of collision with the wall. Cylindrical rod and slanted aft wall control measures can improve the attitude of the store and make the store fall more smoothly.

Effect of Leading-Edge Gap Size on Multiple-Element Wing NACA 43018

Generally, large airplanes are added with more than one high lift device to improve aerodynamic performance. Each configuration of the addition of high lift devices will result in different aerodynamic performances. The added high-lift devices include flaps, leading-edge slats, vortex generators, fences, and others. This study shows the aerodynamic performances of the NACA 43018 wing airfoil with the addition of leading-edge slats and plain flaps in cruise conditions with variations of leading-edge gap size. The angles of attack used include 0°, 2°, 4°, 6°, 8°, 10°, 12°, 15°, 16°, 17°, 19°, and 20°. Numerical simulations have been performed using Ansys Fluent 19.1 with a k-ε Realizable turbulent model. The aerodynamic performance of the configuration that uses the leading-edge slat shows a higher value than the rectangular wing even though the lift and drag coefficients produce lower values. The use of leading-edge slats produces fluctuations, which are indicated by different values of the pressure coefficient, that show the interaction of adding momentum from the main flow from the direction of the leading edge. The increase in the angle of attack causes bubble separation in the leading-edge gap size, which is like a pseudo-body transforming from convergent to smaller.

Cost-effective numerical analysis of the DrivAer fastback model aerodynamics

This paper investigates cost-efficient ways of accurately numerically simulating the aerodynamic phenomena around the DrivAer model, a well-established conventional car reference model created by the Technical University of Münich (TUM). After processing the geometry of the Fastback configuration, a high quality ANSYS Poly-Hexcore mesh was constructed in ANSYS Fluent Meshing, reducing the number of cells needed for a highly accurate simulation. The grid independence study was conducted with the GCI method where convergence was achieved by a 10.5-million-cell mesh. The simulations were performed in ANSYS Fluent, where modeling using k-ε Realizable turbulence model, with Scalable wall functions and the SimpleC algorithm was chosen. The results showed good prediction of the drag and pressure coefficients, proving the accuracy and efficiency of the approach, while the flow and vortices were visualized and presented in the corresponding diagrams, being indicative of the aerodynamic structures around automotive vehicles.

Numerical Prediction of Long-Term Droplet Erosion and Washing Efficiency of Axial Compressors Through the Use of a Discrete Mesh Morphing Approach

Abstract Online water washing represents an operation strategy commonly used to reduce compressor performance deterioration due to blade fouling. Since this kind of washing is applied when the machine operates close to full load conditions, injected droplets are strongly accelerated and consequently impact the rotor blades at high velocity, thus inducing undesirable phenomena like erosion. Here, we present a novel technique to study long-term water droplets erosion by also considering the geometry modification caused by droplets impacts. Two-phase unsteady numerical simulations were carried out, considering the injection of water droplets and their transport across the fluid flow in the first part of a real compressor, which is modeled in the region extending from the inlet to the rotor blades of the first stage. Simulations are performed on the whole machine to account for the asymmetric distribution of the spray injectors, the machine struts, inlet guide vanes (IGVs), and rotor blades. The k−ɛ realizable turbulence model with standard wall functions was coupled with the discrete phase model to track injected droplets motion. Droplets-wall interaction is modeled following the Stanton–Rutland approach aiming at detecting the effect of droplet impact (deposit, rebound, and splashing) depending on the impact conditions. Moreover, a semi-empirical erosion model developed by the authors was used to evaluate the erosion induced by the droplets injection. Material removal due to erosion is converted into nodal mesh displacement that is used by a secondary routine to implement the mesh morphing scheme. The mesh modification is applied at discrete steps to reduce the computational load. This technique is adopted to account for the blades geometry modification due to water droplet erosion leading to performance losses. Moreover, an estimation of the compressor operating life before maintenance operations is given and the water washing efficiency during the whole life of the machine is evaluated by means of proper indices. At the end of the simulation workflow, erosion phenomena are observed in all the compressor regions, especially in the rotor where erosion peaks are reached at the hub of the blades leading edge. The rotor blades wet surface was found to remain almost constant at around 50% during compressor water washing. Erosive phenomena were proved to evolve nonlinearly with time indicating the need to account for the geometry modification for obtaining an accurate prediction of the long-time process.

Numerical study of film cooling effectiveness of a gas turbine blade under variable blowing ratio and turbulence intensity

AbstractThe present paper investigates a three‐dimensional simulation of film cooling on a C3X turbine blade with a single hole at a suction surface. The Reynolds averaged Navier–Stokes approach with k–ε realizable turbulence model and enhanced wall function are used for the numerical simulation. To simulate the jet flows, the length of the jet input approximately 4.5 times the diameter of the hole is added to the geometry so that the jet outlet flow is closer to the actual condition. The density ratio of the cooling flow to the mainstream flow is assumed about 2. The numerical results in four blowing ratios of 0.5, 0.7, 1.0, and 1.4, and at the low turbulence intensity (0.02%), and high turbulence intensity (12%) are extracted and compared for the turbine blade with a single hole. The results show that the turbulence intensity has a dual effect on the film cooling effectiveness and a higher blowing ratio increases the strength of the jet against the cross‐flow. Moreover, it is illustrated that the distribution of the film cooling effectiveness in higher blowing ratios and high turbulence intensity is more uniform than the low blowing ratios and low turbulence intensity.

Experimental and numerical study of novel Coanda-based unmanned aerial vehicle

Conventional Coanda-based unmanned aerial vehicles (UAV) experience thrust losses in the radial direction. To address these losses, a rectangular, linear arrangement of the Coanda surface was adopted in the proposed novel design. This arrangement minimizes the area change in the radial direction to recover such thrust losses. A prototype of the proposed UAV structure was 3D printed and assembled with a single 9-inch propeller. Performance characteristics of the UAV were evaluated through static testing on a dynamometer under different loading conditions. Experimental results were validated through computational fluid dynamics (CFD) simulations, using the k-ε realizable turbulence model, while the multiple reference frame (MRF) approach was applied in steady state. CFD simulations provided good overall agreement with experimental results having errors less than 8%. Numerical comparison between the novel Coanda design and a conventional Coanda design, having similar radial dimensions, showed that the novel design offered an overall 17% improvement in thrust per the side surface area, thus demonstrating an effective reduction of thrust losses in the radial direction.

Numerical Analysis of Porpoising Stability Limit of a Blended-Wing–Body Aircraft During Ditching

The ditching processes of a blended-wing–body (BWB) aircraft under different initial speeds and pitch angles are simulated by numerically solving the unsteady Reynolds-averaged Navier–Stokes equations and the realizable turbulence model using the finite volume method. The volume-of-fluid model is adopted to capture the water–air interface. The six-degree-of-freedom model is employed to couple fluid dynamics and aircraft rigid-body kinematics. The global moving mesh is used to deal with the relative motion between the aircraft and the water. It is found that the plane composed of initial speed and pitch angle can be divided into two regions by a stability limit line, that is, the porpoising motion region (the aircraft takes coupled oscillatory motion between heaving and pitching) with large initial speeds and pitch angles and the stable motion region with low initial speeds and pitch angles. When the initial speed is large, the aircraft’s pitch-up moment peak and heaving amplitude are enhanced. Hence the aircraft carries out the porpoising motion. For the large initial pitch angle, the water entry depth of the aircraft increases and the waterline moves forward, which produces a more significant pitch-up moment peak and overload peak. As a result, the aircraft conducts the porpoising motion. The porpoising stability of the BWB configuration is obviously worse than the conventional and flying wing configurations. When the BWB aircraft ditches on water, the pilots should reduce the initial speed and pitch angle as much as possible to avoid the dangerous porpoising motion.

Pressure Drop in Seven-Pin Wire-Wrapped Rod Bundle for the Sodium Cartridge Loop in Versatile Test Reactor

This work studies the hydrodynamics of the seven-pin wire-wrapped rod bundle in the sodium cartridge loop for the Versatile Test Reactor (VTR) through scaled water experiments and computational fluid dynamics (CFD) simulations. The scaling analysis is first performed to demonstrate the hydrodynamic similarity between water and sodium flows at the same Reynolds number . A separate-effects test facility is designed and constructed based on the scaling analysis. Detailed experimental data on the pressure drop covering a wide range of values (1165 to 27 689) are obtained, which are used to evaluate existing correlations for friction factor and to benchmark CFD simulations. The experimentally determined friction factors agree well with the Upgraded Cheng and Todreas Detailed Correlation and Pacio-Chen-Todreas Detailed Model within but are significantly underpredicted by Rehme’s correlation by 25%. Various CFD near-wall treatment methods are tested using ANSYS Fluent and evaluated by experimental data. It is found that when the recommended wall values are met, most of the near-wall treatment methods can give accurate friction factor predictions. The resolved near-wall method () with the Shear Stress Transport turbulence model and the scalable wall functions () with the realizable turbulence model can predict within . The standard wall functions () and nonequilibrium wall functions () with the realizable model can predict within ± 10%.

Afterburning Effects on Thermal Environment of Liquid Oxygen/Kerosene Rocket Under Low Altitudes

Heavy launch vehicles use liquid oxygen (LOX) and kerosene propellants for their high specific impulse and controlling precision. However, incomplete burnt exhaust gas reacts easily with air, which will influence the thermal environment. To investigate the afterburning effect on the plume flowfield at low altitudes, we established a supersonic exhaust plume model by using the three-dimensional Reynolds-averaged Navier–Stokes (RANS) methods and realizable turbulence model. Also, the nine-species and fourth-step chemical reactions are adopted to simulate the afterburning process. The validity of our numerical model is confirmed by the good agreement between calculation results and experiment data. On this basis, the thermal environment of the LOX/kerosene rocket between frozen and reaction flow at five typical altitudes are calculated and compared. The numerical results show that the secondary combustion reactions mainly occur in the mixing layer, with about an 11.9–18.6% increase in the peak temperature after considering the afterburning effects. The increase in range of the maximum temperature of the engine exhaust plume gradually reduces as the flight height increases. At the same altitude, the afterburning has more significant effect on flowfield for a larger distance from the nozzle exit. The results provide a vital foundation of analysis and calculation for the design of the LOX/kerosene rocket thermal protection.

Numerical simulation of reynolds number effect of a free incompressible isothermal turbulent coaxial jet

This paper studies the effect of Reynolds number on a two-dimensional free incompressible isothermal coaxial turbulent jet over a range of high Reynolds numbers. This is necessary because of its application in noise control and mixing. The Reynolds numbers at the nozzle exit were 9824, 19648, 29472, 39296 and 49120. The models were designed in ANSYS Design Modeler and the numerical simulation was done using a finite volume based Computational Fluid Dynamics (CFD) in ANSYS FLUENT using the two-dimensional Realizable turbulence model. The Governing equations were discretized using the finite volume method with the solution based on the PISO algorithm. The decay of centerline velocity, turbulent kinetic energy profile, the radial profile of axial velocity and similarity profile were investigated along the flow direction. Contour plot indicates that the velocity is high at the jet exit and decreases downstream due to the rapid mixing of the inner and outer jet and the surrounding fluid. It is found generally that Reynolds number plays significant role especially before self-similarity region. The result shows that increasing the Reynolds number give rise to more turbulence which in turn decreases the potential core length, turbulent kinetic energy and enhances the mixing of the fluid. However, at the jet exit, the flow with the lowest Reynolds number has the highest turbulent kinetic energy because it suffers the greater shear. The spreading of the jet was more or less independent of the Reynolds number beyond the self-similarity region. It is also found that the velocity profile is brought to congruence at about z/D=25 for the Reynolds numbers considered

Effect of Waveform Channel on the Cooling Performance of Hybrid Microchannel

To improve comprehensive hydrothermal performance of microfluidic cooling, a new type of hybrid microchannel is proposed. In this paper, turbulent flow and heat transfer characteristics of rectangular cross section straight hybrid microchannel, trapezoidal cross section straight hybrid microchannel, and trapezoidal cross section waveform hybrid microchannel (TWHM) are compared numerically by a realizable turbulence model under different pump power and heat flux. The results show that TWHM has a greater pressure drop at the same Reynolds number but has the best heat dissipation performance at the same pump power, in which temperature difference can be reduced up to 55.01%. The effect of arc height and chord length on the cooling performance of TWHM is also studied. As increases or decreases under the same pump power, the friction factor increases, while the total thermal resistance and bottom surface temperature difference decrease gradually. The cooling performance of channels becomes closer as the pump power increases. In addition, with the same cross-sectional area, the cooling performance of the waveform hybrid microchannel can be further improved by optimizing cross-sectional shape . The best cooling performance can be obtained at .