SSTk-ωturbulence Model Research Articles

Investigation of the effects of volume change on flow structure and acoustic in a silencer

This study numerically examined the propellant flow from gunfire using the kω-SST turbulence model and their sound attenuation using the Ffowcs-Williams and Hawkins equations (FW-H). For simulation, a pressure-based solver and 3D axisymmetric geometry were used. The second-order implicit time approach and the second-order upwind scheme spatial discretization were used in the simulation. The maximum exit pressure was 3.748 MPa for the suppressor with a length of 70 mm and diameter of 20 mm. However, when the diameter suppressor increased by 1/6, the maximum exit pressure was reduced to 3.4961 MPa. When the length increased by 1/6, the maximum pressure became 3.3636 MPa. Lastly, when the diameter and length were increased by 1/6, the maximum exit pressure became 3.177 MPa. For this suppressor, 20.835 dB (12.29%) sound pressure level attenuation was achieved with 16.823 MPa (84.115%) overpressure reduction and 484.86 K or 32.32% temperature reduction. Generally, the attenuation increased with the increase in the suppressor’s internal volume.

Evaluating the Reactions of Bridge Foundations to Combined Wave–Flow Dynamics

As the ongoing development of national infrastructure progresses, we see an increase in the construction of deep-water bridges, specifically cross-sea bridges. This paper uses Stokes’s wave theory to simulate and analyze how a bridge foundation dynamically responds to wave–fluid interactions. Firstly, the governing equations, boundary conditions and initial conditions of fluid motion are derived, expanded and solved via Stokes’s wave theory, and a spectral model is simulated and plotted. Based on the P-M spectrum and equal frequency method, a method of wave height attenuation during wave propagation is proposed. Using an SSTK-ω turbulence model, a numerical wave flume is established considering the fluid model, the selection of element type and the boundary conditions set, and the influencing factors of wave propagation (attenuation) are analyzed. Waves with different wave parameters (period, depth and height) are numerically simulated and compared with the theoretical values. Finally, we perform an analysis of the dynamic response under wave–current coupling conditions. We establish different operational scenarios and obtain the following results: under a load duration of 200 s, the peak transverse displacements for spans 1, 2 and 3 measure at 0.84 m, 0.63 m and 0.62 m, respectively. The peak transverse displacements under operational scenarios 2 and 3 show reductions of 25.0% and 25.7%, respectively, when compared to scenario 1. However, large transverse displacements remain. This suggests that the influence of waves and water flow on the transverse displacement of the main span should not be overlooked.

Heat Flux Prediction in a Kerosene Fuelled Scramjet Combustor Through Cfd

Thermo-structural design of high speed propulsive systems require heat transfer as an input. Nonreacting and reacting flow simulations are carried out to predict the heat transfer characteristics of scramjet combustor using commercial software. The software employs finite volume method to solve 3-D RANS equations along with SST-kω turbulence model and infinitely fast chemistry. The flow through a convergent-divergent nozzle is taken as validation test case and very good match is obtained between computed heat flux and experimental data. It is observed that minimum 10 micron 1st grid spacing is required to predict wall heat flux accurately and grid independence of heat flux data is demonstrated. Heat transfer coefficient is independent of various isothermal walls and computed heat fluxes are higher at combustion intense zone behind fuel injection struts.

Study on the Effect of Oxidative Jet and Vortex Structure in Fluidic Throat Combined with Thrust Vector Control

To examine the impact of oxidative jets on the thrust vector angle, secondary combustion efficiency, and combustion chamber pressure, inert gas (nitrogen) and pure oxygen are injected into the primary flow, which includes combustible components, at various locations in the divergence section and throat using different injection techniques. The simulations utilize Reynolds-averaged Navier-Stokes equations coupled with the SST k - ω turbulence model in two-dimensional numerical simulations and large-eddy simulation in three-dimensional studies. The numerical method is validated through schlieren experiments, and the vortex is identified using the Liutex-Omega method. The vortex structures and flow characteristics are analyzed. The results indicate that, at the same flow rate, the vector control effect of pure oxygen is superior to nitrogen only in the divergence section, but inferior to nitrogen in both the divergence section and throat. However, with improved vector control, the peak of the vector angle is achieved at a lower flow rate in the case of pure oxygen. When the secondary flow is introduced only in the divergence section, the flow ratio corresponding to the peak point in the pure oxygen case is approximately 14.3% earlier than that in the nitrogen case. The introduction of the pure oxygen jet enhances the secondary combustion efficiency of the primary flow, but to a limited extent. Additionally, when the jet is introduced at the throat, the effect of the pure oxygen case on adjusting the combustion chamber pressure is inferior to that of the nitrogen case. Concerning flow details, the trailing lower vortex replaces the trailing major vortex to become the highest magnitude vortex when the momentum flux ratio is small.

Optimization of the surface heating for a stationary cascade turbine blade in wet steam flow

The purpose of this study is to obtain the optimal surface heating for a stationary cascade turbine blade in wet steam flow by a genetic algorithm. The numerical method is conducted by employing two-dimensional Navier–Stokes equations coupled with a SSTk-ω turbulence model. Nucleation and droplet growth equations are solved using the Eulerian-Eulerian approach. The numerical results show good agreement with well-established experiments. Nozzle efficiency (NE), integral of local entropy changes at the outlet (ILE), mean wetness at the outlet (MWO), mean momentum at the outlet (MMO), and cost price (CP) are objective functions. The ultimate purpose is to minimize the (ILE), (MWO), and (CP) and maximize the (NE) and (MMO) together. Since higher surface heating rates decrease MWO and MMO, while increasing ILE, CP, and NE based on optimization results, there is an optimum for the surface heating rate to gain the best performance of steam turbines. According to the numerical results, the optimum – is equal to 0.04467 (kWcm2). In the optimal case compared to the non-heat case, NE and MWO are improved 0.26% and 19.94%, respectively. In addition, the ILE and MMO are degraded 0.9%, 0.32%, respectively, and CP is estimated 0.0027 ($cm2.h).

Shock Boundary Layer Interaction and Aero-Optical Effects in a Transonic Flow over Hemisphere-on-Cylinder Turrets

Hemisphere-on-cylinder turrets are the main airborne optical platform structure. However, an unsteady shock boundary layer interaction (SBLI) would act on flow separation and turbulent wake, which causes serious aero-optical effects with high spatial and temporal frequency characteristics. In this paper, the SBLI phenomenon of a hemisphere-on-cylinder turret is recorded in a wind tunnel at Ma = 0.7 using shadowing and Mach-Zehnder interferometer measurements. Its wavefront distortion is measured using the Shack-Hartmann measurement. The detached eddy simulation (DES) based on SST k - ω turbulence model and ray-tracing methods are used to reproduce the transonic flow and optical aberration. Experiments and simulations suggest that the SBLI causes the flow to separate earlier relative to a subsonic flow over the turret. The time-averaged root-mean-square of optical path difference (OPD) over the beam aperture is 0.56 λ∼0.59 λ with λ as the wavelength, while the root-mean-square of the time-averaged OPD is about 0.45 λ. The local shock and wavefront distortion have dual peak frequencies at S t D = f D / U ∞ = 0.24 and 0.34, different from the single-peak-frequency phenomenon of a subsonic flow over turrets. Fast model decomposition of wavefront can be performed by proper orthogonal decomposition (POD) of its Zernike coefficients. The first two modes contain the shock’s reciprocating motion.

Effect of multiple cavities and tip injection on the aerothermal characteristics of the squealer tip in turbine stage

For modern gas turbines, the blades of the first-stage high-pressure turbines are subject to significant aerodynamic and thermal loads. As the region where the aerothermal environment is most severe, the aerothermal performance of the blade tip is crucial. Either the tip injection or squealer tip with ribs can effectively improve the tip leakage flow distribution, and thus reduce the tip thermal loads, while the effect of the combination of the two remains to be explored, especially when the number of ribs increases. To further understand the combined effects of the rib number and the tip injections on the aerothermal characteristics, steady-state numerical simulations of the first stage turbine aerothermal field are carried out with varied rib numbers and tip injection configurations in the current study. The kω SST turbulence model was used to solve the 3D Reynolds-Averaged Navier-Stokes (RANS) equations. The heat transfer coefficient distributions and the leakage flow rate distributions are compared with those of the conventional squealer tip case. The results show that adding a rib inside the tip cavity would reduce the size of the reflux vortex near the leading edge of the blade (LE), and effectively reduce the averaged heat transfer coefficient on the tip. In addition, an increase in the rib number would make the coolant accumulate in the cavities, thus increasing the cooling performance. The averaged heat transfer coefficient of the coolant injected squealer tip with three ribs (Rib3C) decreases by 19.55%, compared with the performance of the conventional squealer tip, due to the combined action of adding the multiple ribs and the coolant injection. Comprehensively considering both improving the aerodynamic performance and reducing the blade tip heat load using TOPSIS analysis, the coolant injected squealer tip with two ribs (Rib2C) achieves the best overall performance. In general, ribs arranged in the upstream region of the blade would improve the coolant coverage by blocking the separation region at the cavity bottom. To improve the harsh tip thermal environment and reduce the tip aerodynamic loss, optimizing the rib layout inside the cavity and the coolant injection configurations is a practical solution.

Heat transfer characteristics and novel optimization of a supercritical N-decane-Cooling tube under gravity condition

To study the flow and heat transfer characteristics of supercritical n-decane in the actual regenerative cooling tube, the heat transfer characteristics in a horizontal square tube under a single-side heated condition with considering and ignoring gravity were numerically investigated. The k-ωSST turbulence model was used and tested against the experimental data. The heat transfer characteristics on circumferential walls were presented and the radial flow pattern in the main flow zone and the boundary layer zone was analyzed to explore the mechanism of the heat transfer under gravity conditions. It was found that the heat transfer was enhanced at the bottom wall and weakened at the remaining three walls because of the secondary flow caused by gravity effect. Furthermore, the heat transfer would be enhanced when the velocity component of the boundary layer along the normal direction of the wall is toward the wall. Besides, a brand-new structure constructed by ribs section and smooth section was proposed. The additional pressure drop caused by the enhanced heat transfer structure can be reduced by up to 95.4% with the same heat transfer performance compared with the traditional ribs-tube structure which ribs spread all over a tube.

Effects of vortex generator on the performance of a flush-intake duct in oblique navigation

Abstract Effects of vortex generator (VG) on the flow field in a flush-intake duct for waterjet propulsion are investigated numerically under various navigation conditions. Flow’s Reynolds-averaged Navier-Stokes equations (RANS) are solved with the SST-Kω turbulence model. Four VG blades are installed at the ship bottom ahead of the intake entrance. The VG blade is rectangular shaped with a height of 0.1D, a length of 0.3D, and an angle of attack of 20°. Results show that under the forward moving condition, VG device effectively restrains the flow separation on the duct ramp side and improves the section uniformity at intake exit under low inflow velocity ratio (IVR) condition. The duct efficiency gets improved over a wide IVR range. Under the oblique moving condition, there is no separation region on the duct ramp side at low IVR, but significant vortex appears on the lip side. VG device does not introduce any obvious extra negative effects on the hydraulic performance.

Study and Simulation of the Thrust Vectoring in Supersonic Nozzles

The present study aims to numerically simulate the thrust vectoring by secondary injection. The analysis of flow for this study was done using ANSYS Fluent. The kω-sst turbulence model was used. The baseline solver was selected as a double-precision Density-based coupled solve with implicit time integration. Least squares cell-based spatial discretization in which the solution was assumed to vary linearly was used and a second-order upwind scheme is used for interpolating the values of pressure, momentum, turbulent kinetic energy, specific dissipation rate, and energy. The computational analysis was conducted under study conditions. The initialization for the steady-state problem was done using full multigrid (FMG) initialization to get the initial solution and the boundary was provided to get the reference value. The results of the study with and without fluid injection are presented in terms of pressure, angle of deflection, and efficiency. These results are then compared with those obtained numerically or experimentally by other authors. This comparison was very interesting and the results were very close, the error was only 5%.

Calibration of the k-ω SST Turbulence Model for Free Surface Flows on Mountain Slopes Using an Experiment

We calibrate the k-ωSST turbulence model for free surface flows in the channel or on the slope using machine learning techniques. To calibrate the turbulence model, an experiment is carried out in an inclined rectangular research chute. In the experiment, the pressure values in the flow are measured at different distances from the bottom; after transforming data, the flow velocity profile is obtained. The k-ωSST turbulence model is calibrated based on experimental data using the Nelder-Mead optimization algorithm. The calibrated turbulence model is then used to calculate the glacial lake Maliy Azau outburst flood on the Elbrus (Central Caucasus).

Blade thickness effects on viscous flutter in a radial turbocharger turbine

Transient blade loading limits the lifetime of turbocharger turbine blades. This study investigates the flutter instability of a radial turbocharger turbine blade under pulsating inlet conditions. The viscous Navier-Stokes equations with the SST-kω turbulence model and curvature correction were solved. A time-marching 3D finite volume method was used in the CFX17 CFD solver to model the vibrating blade in a traveling wave mode applying Fourier Transformation. For flutter calculations, moving boundaries with specified modal displacements were used. An area of instability was recognized on the suction-side of the rotor blade. FSI steady-state analysis was then performed to assess the effects of shock position and blade profile on the blade stability in the recognized vulnerable region. The results show that a higher trailing edge radius increases the stability and leads to a significant reduction of flutter risk whereas the maximum thickness and leading-edge radius do not notably affect the flutter occurrences.

COMPUTATIONAL STUDIES OF AEROELASTICITY OF AIRCRAFT ENGINE TURBINE BLADE

Transient blade loading limits the lifetime of aircraft engine turbine blades. Thus, accurate prediction of the unsteady aerodynamic loading and coupled fluid-structure interactions would improve the life the lifetime of the turbine blades. This study investigates the flutter instability of an axial turbine blade under unsteady aerodynamic loading. The viscous Navier-Stokes equations with the SST-kω turbulence model are employed. The results show that the flutter phenomenon causes unsteady oscillations of the aerodynamic coefficients lift and drag.

Numerical Modeling of Compound Channels for Determining Kinetic Energy and Momentum Correction Coefficients Using the OpenFOAM Software

Abstract The non-uniformity of the flow velocity distribution in each section of compound channels and in the main channel-floodplain interface area causes errors in estimating water surface profile, flood routing, pollution transfer, and so on. To reduce the impacts of non-uniformity on the exact calculation of kinetic energy and momentum, α and β correction coefficients are used, respectively. However, the determination method of these coefficients is a challenging issue in river engineering. This study used the OpenFOAM Software to determine these coefficients numerically for two laboratory models of compound open channels of which the data are available, using the single-phase pimpleFoam solver to do modeling in the mentioned software and the k-ωSST turbulence model to calculate the flow characteristics. Based on the results, the highest difference (13%) between the results estimated by the software and those obtained from the lab experiments was seen in the low flow depth where the flow left the main channel and entered the floodplain of a very shallow depth, possibly due to the grid generation of this area. This difference decreased as the flow depth increased, and its average was 6.65% for α coefficient and 2.32% for β coefficient in all cases, which means the results of numerical modeling and the experimental data conformed well, and the OpenFOAM software can be successfully used in flow modeling and analyzing flow characteristics in compound channels.

Numerical Investigation of The Effect of Airfoil Trailing Edge Inflation at low Reynolds Number

To investigate the effect of airfoil shape design on aerodynamics of the airfoil, a new modified airfoil shape design is introduced in this paper. Thick symmetrical NACA 0021 airfoil is used as base model in this study that the trailing edge end curvature was altered in design shape to be inflated to have 25%, 50% and 100% scaled of the leading-edge curvature at the same cord length and same maximum thickness data. Numerical investigation, using shear-stress transport kω-SST turbulence model, is performed to study the effect of airfoil trailing edge inflation on the aerodynamic characteristics at low Reynolds Number. The base airfoil model, NACA0021, is validated numerically and shows a good agreement with previous experimental results. The results show that the new modified airfoil with 100% inflation delays the flow separation to higher angle of attack and increases the maximum lift coefficient about 40% compared with the base NACA0021 airfoil.

층류-난류 천이 모델을 적용한 프로펠러 단독 성능 해석에 관한 CFD 시뮬레이션

In the present study, the model-scale Propeller Open Water (POW) tests for the propeller of 176K bulk carrier and 8600TEU container ship were conducted through Computational Fluid Dynamics (CFD) simulation. In order to solve the incompressible viscous flow field, the Reynolds-averaged Navier-Stokes (RaNS) equations were employed as the governing equations. The γ-Reθ(gamma-Re-theta) transition model combined with the SST k-ωturbulence model was introduced to describe the laminar-turbulence transition considering the low Reynolds number of model-scale. Firstly, the flow simulation developing over a flat plate was performed to verify the transition modeling, in which the wall shear stresses were compared with experiments and other numerical results. Then, to investigate the effect of the model, the CFD simulation for the POW test was performed and the simulated propeller performance was validated through comparison with the experiment conducted at Korea Research Institute of Ships & Ocean Engineering (KRISO).

Numerical Investigation on Hydrodynamic Performance of a Ducted Propeller for Vectored Underwater Robot

This paper investigated hydrodynamic performance of the Ka4-70+No.19A ducted propeller astern of a vectored underwater robot at diverse deflection angles. Employing SSTk-ω turbulence model combined with moving reference frame technique, numerical computation of the ducted propeller in a fully developed turbulence behind hull was carried out. The validity of the model was verified by comparing the numerical results of open water performance and the experimental values. The hydrodynamic performance of the ducted propeller was worked out and discussed in detail. The wake flow and thrust deduction fraction corresponding to different deflection angles were analyzed. Results show that the ducted propeller generates more thrust and requires more torque at lager deflection angle. In addition, the thrust deduction fraction increases with the increase of the deflection angle.

Evaluation of low fidelity and CFD methods for the aerodynamic performance of a small propeller

The increasing use of unmanned aerial vehicles and micro air vehicles creates a strong demand for the accurate aerodynamic performance of small-diameters fixed-pitch propellers. The techniques range from low-fidelity to high-fidelity methods. Among the low-fidelity techniques, there is the blade element momentum theory (BEMT). Computational fluid dynamics (CFD) is categorized as a high-fidelity method. This study aims to employ these methods for the analysis of an APC propeller 14 × 7e. Geometric modeling was performed in a 3D scanner and simulations via BEMT and CFD. Analyses using the BEMT model were conducted with two formulations. The first one considered a linear lift coefficient up to the stall limits, without specifying the post-stall lift coefficients. The second one incorporates three-dimensional flow equilibrium effects, and best represents the post-stall conditions. In the CFD models, two approaches for the compatibility of the rotating and stationary domains were evaluated, the stationary frozen rotor (FR) and the transient arbitrary mesh interface (AMI). The kω SST turbulence model was employed. To investigate the influence of the transition laminar-turbulent boundary layer, the Gamma Theta transitional model was also considered. In the comparison of the results with the experimental tests, it was observed that the appropriate choice of analysis method had a strong relationship with the Reynolds number. For low advanced ratio velocities, the best results were obtained with the BEMT models. BEMT linear model overpredicted the power and efficiency for high advance ratio, while BEMT with tridimensional flow equilibrium presented consistent results with a low computational cost. On the other hand, the CFD-AMI model shows better results for higher advanced ratio velocities but is computationally more expensive. The CFD-FR model maintains a constant pattern of error that, in the analysis of efficiency, did not compromise the results.

Study of underexpanded sonic jets from square convergent micro nozzles

Numerical simulations using the RANS with the SSTk-ω turbulence model are performed to understand the flow feature of an underexpanded sonic jet from a convergent nozzle with a square cross-section of 1 mm × 1 mm at the nozzle exit. The nozzle pressure ratio, which is the ratio of the plenum pressure upstream of the nozzle to back pressure, is held constant at 4.0 and the Reynolds number based upon the flow properties and the height at the nozzle exit is 5.94 × 104. The near-field shock structure in shock-containing square jets is demonstrated with the three-dimensional representations of the magnitudes of the density and pressure gradient vectors.

The capability of Ansys CFX to predict the mixing phenomena in ROCOM test facility

This work consists of a Computational Fluid Dynamics (CFD) modeling of a reference experiment on boron dilution in the Rossendorf coolant mixing Model (ROCOM) as part of a coordinated research project of the International Atomic Energy Agency, namely, “Application of numerical codes of fluid dynamics to the design of nuclear power plants”. This coordinated project aims to address the application of CFD codes to the process of optimizing the design of nuclear power plants related to pressurized water reactors and to evaluate the performance and predictive capabilities of these codes and to contribute to their validation. In this context, a three-dimensional numerical simulation study was carried out using CFD code ANSYS CFX v14.5, to study the boron mixing phenomenon at the core inlet and the downcomer of the ROCOM test facility. The phenomenon of experimental mixing occurs by the injection of a tracer (sodium chloride) into one of the loops of the ROCOM installation mainly containing demineralized water in its primary circuit. The concentration field of the tracer is measured and simulated at the entrance of the heart and in the lowering. The SST-kω turbulence model used in this study could reasonably predict the distribution of the injected tracer in measurement locations within the test facility. The results of this numerical simulation were compared to the Benchmark data provided by the ROCOM experimental facility of the Helmholtz-Zentrum Dresden-Rossendorf Institute.