K-omega SST Turbulence Model Research Articles

Hydrodynamic Performance Improvement of a Tirhandil Yacht by Stern Form Modifications

This paper presents a comprehensive investigation to improve the hydrodynamic performance of a Tirhandil hull form by modification efforts on the stern region. The form improvement approach combines computational fluid dynamics (CFD) methods with computer-aided design (CAD) systems. The design process for the reference and modified models was carried out by using CAD systems. The hydrodynamic characteristics of the reference hull form were evaluated by employing CFD methods and it was determined that form improvements should be concentrated on the stern region. The modification process was conducted by considering constraints on the design variables in the stern region and the main dimensions of the reference model. A grid independence study was performed to evaluate various grid structures to determine the optimal mesh configuration for the numerical analyses. The SST k-Omega turbulence model was used for the numerical analyses to simulate turbulence structure around the hull form. Achieving around a 13.4% reduction in the total resistance coefficient, the modified model also exhibited decreased wave amplitudes, smoother wave transitions, and a significant reduction or cancellation of shoulder and stern waves.

Periodic morphing of a NACA6409 aerofoil in ground effect, its wake mechanisms and thrust generation

Abstract The camber morphing of an aerofoil in ground effect was investigated using the FishBAC method and Detached Eddy Simulations with the k-omega SST turbulence model at a Reynolds number of 320,000. The aerofoil was periodically morphed at a start location of 25% chord from the leading edge with a trailing edge deflection range of 0.1% to 3% and morphing frequencies between a Strouhal number of 0.45 to 4 at a constant ground clearance of 10%. Periodically morphing the aerofoil using a sinusoidal function showed that lift and drag increased on the downstroke and decreased on the upstroke in the cycle, resulting in periodic values of lift and drag throughout the cycle. The amplitude of lift and drag increased as the morphing frequency and/or trailing edge deflection increased. It was found that the wake characteristics varied as a function of trailing edge deflection and morphing frequency. For small trailing edge deflections below 0.4% and frequencies below a 2.2 Strouhal number, Kelvin Helmholtz shedding was observed, and above this the wake became chaotic. Large trailing edge deflections showed Von-Karman shedding, where the interaction between the lower counter-clockwise vortex and the ground plane resulted in a jet-like flow that caused forward thrust. For the maximum deflection and morphing frequency tested in this study, reversed Von-Karman shedding was observed, which caused forward thrust from the interaction of the two-shedding counter-rotating vortices. Von-Karman or reversed Von-Karman shedding shows positive thrust generation, however, chaotic shedding should be avoided due to large drag gains. Varying the Reynolds number caused the Strouhal number to change as they depend on the same variables. It was found that the Strouhal number variation had a large effect on the wake, however, the Reynolds number had a minimal effect.

Investigation of varying tip clearance gap and operating conditions on the fulfilment of low-speed axial flow fan

Abstract The present study aims to identify the suitable tip clearance and volumetric flow rates for low-speed axial flow fans. The performance of an axial fan depends on various parameters like fan speed, available tip clearance gap, volumetric flow rate, power consumption, and working fluid. Leakage flow occurring at a rotating component such as a blade and solid casing of a ducted axial fan is typically linked to losses and the potential emergence of a rotating stall. The numerical analysis for this study uses the Reynolds Averaged Navier–Stokes equations with the k-omega SST turbulence model to perform the steady-state simulations by varying tip gap and volumetric flow rate. The results proved that the optimum performance was obtained with a tip clearance gap of 1 mm, and maximum fan efficiency was achieved at a volumetric flow rate of 3.9–4.5 m3/s. The novelty of this proposed work is to enhance the efficiency of axial flow fans with circular arc-cambered aerofoils using optimum tip clearance and volumetric flow rate through steady-state simulations. This method can be used in turbomachinery to improve fan performance.

Thermal performance of teardrop pin fins and zig zag ribs in a wedge channel

Abstract Aviation, marine propulsion, and power generation employ gas turbine engines extensively. The combustion process’s high temperatures make gas turbine engine development difficult. For safe turbine blade operation, internal cooling is essential. Ribs, pin-fins, and impinging jets are just examples of the several different types of structures that are typically included in internal cooling systems. The present study investigates the effect of rib placed in between the pin fins in a wedge channel. Nine rib configurations along with base line model without ribs are employed in this study. The SST k-omega turbulence model was employed to simulate the flow field and heat transfer of each case using the commercial software ANSYS Fluent. The investigation demonstrated that the heat transfer distribution and flow field in pin-fin arrays were substantially influenced by the ribs-induced secondary flows, particularly in the spanwise direction, which resulted in a significant heat transfer variation. In comparison to other configurations, the 4H4L rib configuration exhibits the highest thermal performance factor, surpassing the baseline by approximately 16.6 %.

Computational Fluid Dynamics Simulation of Combustion Efficiency for Full-Size Upstream Flare Experiments

Methane emissions from oil and gas production can occur throughout the value chain, but for many producers, one of the most significant sources is flaring. Understanding the influence of the operating conditions and the environmental factors the combustion efficiency and destruction and removal efficiency (CE/DRE) of flares is essential if their role in methane emissions, a potent but short-lived greenhouse gas, is to be better understood and mitigated. An industry-scale experimental study was focused on the emissions of un-assisted flares commonly encountered in upstream oil and gas production. This paper simulates two un-assisted flare tips combustions by using the commercial computational fluid dynamics (CFD) software package Fluent 21R2 to augment the physical experimental testing. Two three-dimensional (3D) flare tips models are built, and the k-omega SST turbulence model and flamelet generated manifold (FGM) combustion model are applied to simulate flaring combustion. The CFD model is first validated against full-scale industry flare tests that use extractive sampling of the combustion plume. CFD results are in good agreement with measured results when the vent gas net heating value (NHV) is greater than 300 BTU/SCF. Greater uncertainty exists for both CFD results and measured data if the NHV is less than 300 BTU/SCF. Then, the CFD model is extended to include high crosswind states up to 50 m/s that cannot be readily or safely examined empirically. The results emphasize the critical role of the vent gas net heating value (NHV) on flare combustion and crosswind in reducing the CE. The comparison helps pave the way for further use of CFD simulation to improve flare designs and modes of operation and supports the use of parametric models to track and report methane losses from flaring.

Improvement of Aerodynamic Performance of NACA 2412 Airfoil using Active and Passive Control Techniques

Improvement of Aerodynamic Performance of NACA 2412 Airfoil using Active and Passive Control Techniques

Ship Flow of the Ryuko-maru Calculated by the Reynolds Stress Model Using the Roughness Function at the Full Scale

The k-omega SST turbulence model is extensively employed in Reynolds-averaged Navier–Stokes (RANS)-based Computational Fluid Dynamics (CFD) calculations. However, the accuracy of the estimation of viscous resistance and companion flow distribution for full-sized vessels is not sufficient. This study conducted a computational analysis of the flow around the Ryuko-maru at model-scale and full-scale Reynolds numbers utilizing the Reynolds stress turbulence model (RSM). The obtained Reynolds stress distribution from the model-scale computation was compared against experimental measurements to assess the capability of the RSM. Furthermore, full-scale computations were performed, incorporating the influence of hull surface roughness, with the resulting wake distributions juxtaposed with the actual ship measurements. The full-scale calculation employed the sand-grain roughness function, and an optimal roughness length scale was determined by aligning the computed wake distribution with Ryuko-maru’s measured data. The results of this study will allow for the direct performance estimation of full-scale ships and contribute to the design technology of performance.

Numerical Study of Fluid Flow in a Gyroid-Shaped Heat Transfer Element

This paper deals with the design of porous geometry of a heat transfer element. The proposed geometry combines a gyroid triply periodic minimal surface with the recursive principle of geometric body creation. The designed geometry is based on an attempt to increase the heat transfer surface while eliminating negative impacts on the fluid characteristics in the form of pressure loss or increase of the friction coefficient. The proposed geometry of the heat transfer element was compared with a pair of geometries based on the basic gyroid shape but with different channel size parameters. A numerical simulation was performed in Ansys Fluent 2020 R1 using the SST k-omega turbulence model for flow velocities in the range of 0.01 m.s−1 to 0.5 m.s−1, which covered a wide range of the Reynolds number and thus also flow forms in terms of the turbulence intensity. The presented results clearly show lower values of pressure loss and friction coefficient of the proposed geometry compared to the evaluated porous structures. Also, at the same time, they describe the factors positively influencing the mixing process of the liquid in the proposed element, which leads to an increase in the efficiency of the heat transfer process.

Effect of Vortex Generator on Gas Turbine Blade Cooling

The current numerical study suggests using a vortex generator to lessen the consequences of a counter-rotating vortex pair. For this inquiry, the delta winglet pair in the common-flow-down arrangement is selected. A suitable model for turbulence is examined. The x, y, and z vorticity is studied. Additionally, the effect of variations in turbulent kinetic energy is examined. The outcome is contrasted with the reference scenario. Employing the k-omega SST turbulence model and the commercial code FLUENT, numerical analysis is accomplished.

An Investigation on Uncontrolled and Vortex-Generator Controlled Supersonic Jets

The present study is carried out with a motivation to investigate the axisymmetric supersonic jet both experimentally and computationally. An open jet facility was utilized to carry out the experiments, and the results were compared with computational simulations employing the K-omega SST turbulence model using ANSYS software. It is important to note that, the computational validation has been done incorporating the Rayleigh Pitot formula to match the centerline pressure for the uncontrolled jet, which has not been found in any other validation studies according to the authors’ understanding. Besides, the experimental study is extended with a focus on evaluating the impact of Vortex Generators (VGs) on Mach 1.6 supersonic jets. The aim was to enhance jet mixing, a critical factor for improving engine performance. Various nozzle geometry modifications were explored in the past, but VGs emerged as the most effective method for optimizing jet mixing efficiency. The investigation revealed a substantial decrement in the supersonic jet core length when VGs were introduced at the nozzle exit, especially under favorable pressure gradients. This reduction in the supersonic core emphasized the role of VGs in enhancing mixing efficiency. The study also confirmed that VGs significantly distort wave patterns within the supersonic core, crucial for improved jet mixing. This research signifies the importance of VGs in augmenting the mixing of Mach 1.6 jets, offering the potential for improved jet performance and reduced noise emissions in the aerospace industry.

Numerical Inspection of location, density ratio and turbulent kinetic energy of vortex generator in gas turbine blade film cooling application

Abstract Present numerical investigation proposes to mitigate the effects of Counter rotating vortex pair by employing vortex generator. The common-flow-down arrangement of delta winglet pair is preferred for this investigation. The investigation of suitable turbulence model has been studied. Numerical simulation has been carried out to investigate the effect of placement of vortex generator on the characteristics of film cooling effectiveness. Vortex generator located at upstream, downstream and both before, after and both before, after film cooling hole respectively. Result indicates that Vortex generator, located at up-stream of circular film cooling hole will yield better effectiveness. Reynolds number based on free stream velocity and film cooling-hole dimension is kept at 17000. Moreover consequence, in variation of blowing ratio (M = 0.5, 1, 1.5), density ratio (DR = 0.5, 1, 2), secondary flow velocity or jet velocity (uj) and T.K.E on cooling effectiveness has been investigated. Result shows that increment in M, uj and T.K.E responsible for decrement in cooling effectiveness. The results are compared with baseline case. It has been observed that numerical investigation implementing FLUENT commercial code adopting the k-omega SST turbulence model perform better.

Enhancing formula student car performance: Nose shape optimization via adjoint method

For a formula student vehicle, the nose is the first point of flow contact and decides the flow direction to help other aero components work efficiently. Therefore, an optimized nose shape is crucial for the optimum running condition of the vehicle. In recent years, Gradient-based optimization in the automotive industry has emerged with an adjoint approach. The adjoint solver is used to modify the geometry such as the nose of a car to the best optimal shape with respect to other design variables. Hence, the present study encompasses an examination conducted under six distinct Reynolds number conditions, specifically, 8.15 × 105, 9.05 × 105, 9.96 × 105, 10.87 × 105, 11.77 × 105, and 12.68 × 105. The simulation setup is validated using the Ahmed Body's 30° slant angle. The setup uses the k-omega SST turbulence model to capture the airflow phenomenon over the racing car. The simulation setup shows only a 0.07 % variation from the existing experimental findings. The results are represented in terms of different 2D plots, contours and streamline plots. The optimized nose has 27 % less drag and consumes 27.09 % less fuel than the base model at a Reynolds number of 10.8 × 105. To the automotive industry and race community, this optimization technique will be expected to enhance the vehicle's overall performance.

Wind tunnel testing of a Formula Student vehicle for checking CFD simulation trends

The aerodynamic performance analysis of Formula Student racecars has been mostly done by teams with CFD tools, for time and cost savings, that often lack proper validation. To address this, the FST Lisboa team performed a detailed wind tunnel (WT) test campaign, using a one-third scale model, under different configurations, including variable ride heights, bullhorn appendix, and rear wing flap settings, also replicated in CFD. The simulations used RANS with the SST k-omega turbulence model, with a 13.7 million polyhedral mesh for the test chamber region domain. Both experimental and numerical errors were estimated from the instrumentation and mesh convergence analysis, respectively. Comparisons were made between WT and CFD both in terms of local flow, using tufts for flow visualization, and global flow, using lift, drag, and pitching moment coefficients. Overall, the numerical streamlines agreed very well with the orientations of the tufts in experiments, but some discrepancies were found in regions of cross-flow and high-frequency unsteadiness, mainly caused by limitations of the visualization technique. The gamma transition model in CFD was abandoned as it could not replicate the WT observations. In terms of aerodynamic coefficients, a strong correlation was found between WT and CFD. The parametric studies revealed that the simulations captured the experimental sensitivity to each car setting parameter studied but the uncertainties did not enable a full quantitative evaluation of the aerodynamic performance. The drag reduction system significantly impacted the aerodynamic balance of the racecar, while the current bullhorn design proved to be ineffective. The ride height increase led to higher downforce, mostly due to the higher pitch angle of the vehicle, with negligible variation of the aerodynamic balance. This work validated the team CFD studies, building confidence in that trends estimated in numerical parametric studies are likely to be translated to the real prototype performance.

CFD Analysis of Building Cross-Ventilation with Different Angled Gable Roofs and Opening Locations

The geometric shape of the roof and the opening position are important parameters influencing the internal cross-ventilation of buildings. Although there has been extensive research on natural ventilation, most of it has focused on flat or sloping roofs with the same opening positions. There is still limited research on the impact of different opening positions and sloping roofs on natural ventilation. In this study, computational fluid dynamics (CFD) was used to investigate the air exchange efficiency (AEE) in general isolated buildings. These buildings encompassed three distinct opening configurations (top–top, top–bottom, and bottom–top) and six varying slope angles for gable roofs (0°, 9°, 18°, 27°, 36°, and 45°). Computational simulations were carried out using the SST k-omega turbulence model, and validation was performed against experimental data supplied by the Japanese AIJ Wind Tunnel Laboratory. Grid independence validation was also conducted to ensure the reliability of the CFD simulation results. The study revealed that the highest AEE was 48.1%, achieved with the top–bottom opening configuration and a gable roof slope angle of 45°. Conversely, the lowest AEE was 31.4%, attained with the bottom–top opening configuration and a gable roof slope angle of 27°. Furthermore, it was observed that when the opening configuration was set to top–top and bottom–top, the slope angle of the gable roof had minimal influence on AEE, with an average AEE of only around 33%. When the opening configuration was top–bottom, it was found that there was a positive correlation between the gable roof slope angle and AEE.

Numerical study on the influence of airfoil geometry and profile on the rapidity of high-speed planing trimaran

The planing trimaran has unique rapidity and stability. In this paper, wings are added to the planing trimaran to provide additional aerodynamic lift in high-speed range (volume based Froude number ranging from 4.37 to 8.74), achieving the purpose of reducing the resistance of the planing trimaran. By solving Reynold-Averaged Navier-Stokes(RANS) equations and combining SST k-omega turbulence model with Volume of Fraction(VOF) method, a numerical simulation method is established to analyze the motion characteristics of the planing trimaran. The geometry and airfoil profiles are mainly studied in this paper. The geometry of wings includes rectangular wings, trapezoidal wings, and swept wings. The profile shape of the wing includes three series of NACA wings and two special wings. And the impact of different wings on the planing trimaran rapidity by comparing its hydrodynamic characteristics and aerodynamic characteristics are analyzed. The comparison shows that adding wings to the planing trimaran can significantly reduce the resistance at high speed, with trapezoidal wings having the best aerodynamic performance. All airfoils mentioned in the article can reduce resistance by about 20% at the highest speed, and the airfoil named 1# can reduce the resistance by up to 33.2%.

Influence of wall roughness on pressure distribution and performance of centrifugal dredge pumps

Wall roughness is an important factor that affects pump head and efficiency. Based on the open source program OpenFOAM, the wall function method and the equivalent sand roughness model were used to study the influence of wall roughness on the performance of centrifugal dredge pumps. Then, the steady state of the dredge pump under 10 different types of wall roughness was calculated using the k-omega SST turbulence model. The pressure distribution under different flow rates and rotation speeds was also analyzed. The results reveal that the relationship between the increase of wall roughness and the performance of the dredge pump can be roughly divided into three stages, which may be related to the link between roughness and the viscous layer. In particular, the pump performance decreases significantly, and the shaft power increases linearly when it is in the third stage (complete rough region). Finally, different degrees of roughness have minimal effects on the pressure distribution of the impeller.

Numerical study on the effect of airfoil attack angle and height on the resistance reduction of the planing trimaran

The planing trimaran has unique rapidity and stability. In this paper, airfoils are added to the planing trimaran to provide additional aerodynamic and hydrodynamic lift, achieving the purpose of reducing the resistance of the planing trimaran. By solving Reynold-Averaged Navier-Stokes (RANS) equations and combining SST k-Omega turbulence model with Volume of Fraction (VOF) method, a numerical simulation method is established to analyze the motion characteristics of the planing trimaran. The attack angle and the height of airfoils are mainly studied in this paper. NACA4404 and a special airfoil named 2# airfoil are selected as the research objects. The range of attack angle is 0–8°. The drag reduction effect of the airfoil does not change linearly with the increase of the attack angle in the high speed range. NACA4404 and 2# airfoil have the best drag reduction effect at 2°and 4°respectively. The range of airfoil height is 0.8h–1h (“h” is the wing height from baseline). For NACA4404, lower wing height is beneficial to reduce the resistance, especially in the low speed range. The 2# airfoil has poor performance in the low speed range after reducing the height, and can increase the resistance by up to 16.3%.

Identification of probabilistic size of breathing zone during single inhalation phase in semi-outdoor environmental scenarios

This study investigates outdoor public health by predicting the airflow fields and probabilistic size of breathing zones. Computational fluid dynamics (CFD) simulations were performed in a simplified semi-outdoor domain, utilizing a validated computer-simulated person (CSP) with an integrated nasal cavity. The simulations were conducted for eight wind orientations (0°, 90°, 180°, 270°, 45°, 135°, 225°, and 315°), four wind velocities (Uref = 0.25, 0.5, 0.75, and 1.0 m/s), and one inhalation flow rate (18.7 L/min), considering both steady and transient conditions. The RANS-based equations were solved using the SST k-omega turbulence model. Breathing zones were computed and visualized using the scale for ventilation efficiency 5 (SVE5) and reverse time-traced vector techniques. The results indicated that wind orientation influenced the air velocity, temperature, and breathing zone distribution. The steady-state condition tended to overestimate breathing zones, whereas, under transient conditions, they assumed a semi-cylindrical form that extended horizontally, with a slight slope from the nostrils towards the direction of the wind source. The horizontal extension of the breathing regions increased at high wind speeds and with a smaller cylinder radius compared to calm conditions. Eventually, this study proposed new definitions of the breathing zone in the semi-outdoor environment in different SVE5 values. These findings can contribute to air quality management and aid in assessing the probability of airborne transmission in public spaces.

Passive Control of Boundary Layer on Wing: Numerical and Experimental Study of Two Configurations of Wing Surface Modification in Cruise and Landing Speed

Minimizing the carbon footprint of the aviation industry is of critical importance for the forthcoming years, allowing the mitigation of climate change through fossil fuel economy. Significant progress toward this goal can be achieved through the aerodynamic optimization of wing surfaces. In a previous study, a custom-designed wing equipped with an Eppler 420 airfoil, including an appendant custom-designed blended winglet, was developed and studied in flight conditions. The present paper researches potential improvements to the aerodynamic behavior of this wing by attempting to regenerate the boundary layer. The main goal was to achieve passive control of the boundary layer, which would be approached by means of two different configurations. In the first case, dimples were added at the points where the separation of the boundary layer was expected, for the majority of the wing surface; in the second case, bumps of the same diameter were added at the same points. Both wings were studied in two different Reynolds (Re) numbers and five angles of attack (AoA). The computational fluid dynamics (CFD) simulations were implemented using a pressure-based solver, the spatial discretization was conducted with a second-order upwind scheme, and the k-omega SST (k-ω SST) turbulence model was applied by utilizing the pseudo-transient method. The experimental procedure was conducted in an open-type subsonic flow wind tunnel, for Reynolds 86,000, with 3D-printed models of the wings having undergone suitable surface treatment. The numerical and experimental results converged, showing a degradation in the wing’s aerodynamic performance when bumps were implemented, as well as a slight improvement for the configuration with dimples.

On the hydrodynamic response and slamming impact of a cylindrical FPSO in combined wave-current flows

Owing to the lower construction and transportation costs, cylindrical floating production storage offloading (CFPSO) has received extensive attention in the industry. Currently, attention has primarily been focused on the damping performance of heave plate while few studies have touched upon hydrodynamic characteristics under complex combined wave-current condition. In this paper, we numerically investigate the hydrodynamic response and slamming impact of a CFPSO under combined wave-current flows. The active wave generating-absorbing boundary condition (GABC) along with the buoyancy-modified k-omega SST turbulence model are utilized to generate high-precision waves and current based on the open source computational fluid dynamics (CFD) framework OpenFOAM. A self-developed six degree of freedom (6DOF) motion module is used for solving rigid body motion and updating mesh motion. The numerical results of motion response and impact pressure are compared with the experimental data to verify the accuracy of present simulations. The correlation between impact pressure and relative wave elevation and wave velocity is analyzed and three types of slamming events have been identified. The flow fields such as vorticity, pressure and streamlines in the vicinity of the CFPSO are also presented and discussed, which provides a reference for structural design for the CFPSO under complex sea conditions.