Using Unsteady Reynolds-averaged Navier-Stokes Research Articles

Effect of Model Fidelity on High-Speed Aeroelastic Behavior of a Cantilever Plate

Turbulence, flow separation, and shock dynamics challenge the modeling and analysis of high-speed aeroelastic behavior. Motivated by this, the importance of modeling the fidelity of the flow is explored in the aeroelastic response of a cantilever plate in an Ma=2.0 separating turbulent flow using unsteady Reynolds-averaged Navier–Stokes (URANS) and URANS-enriched local piston theory (LPT). Structural modeling assumptions are also evaluated using both linear and nonlinear representations. Close agreement in the predicted aeroelastic steady state is observed. However, large discrepancies in the dynamic aeroelastic response predictions are found and ultimately linked to the neglect of deformation-induced cavity pressure fluctuations and dynamic flow separation in the LPT model. Interestingly, the dynamic flow separation induces a fluid-driven limit cycle oscillation in the postflutter regime. Furthermore, structural nonlinearity is not found to have a strong impact on the conditions and configurations considered.

Numerical analysis of static and dynamic wind turbine airfoil characteristics in transonic flow

This study performed an aerodynamic characterization of the FFA-W3-211 wind turbine tip airfoil in transonic flow using Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, for both steady and dynamic operational conditions. First, the boundary between subsonic and supersonic flow in static conditions was identified, depending on the angle of attack, the approach flow Mach number, and the Reynolds number. The analysis points out that higher Reynolds numbers promote the occurrence of local supersonic flow. Thereafter, to investigate the dynamic behavior in the transonic flow regime, a sinusoidal pitching motion with representative values was imposed. A hysteresis, similar to but distinct from dynamic stall, was observed for entering and leaving the supersonic and subsonic regions. Elevated reduced frequencies widened the hysteresis loop, resulting in increased normal forces on the airfoil. The study indicated that an increase in reduced frequency leads to an earlier onset of transonic flow. In conclusion, the risk of transonic flow occurring during normal operation of the next generation wind turbines predicted in earlier studies could be corroborated. Moreover, dynamic effects and Reynolds number dependencies can be significant.

Pressure Power Spectra of a ‘Z’ Shaped in Plan Tall Building under Transient Wind Environment

This study foresees the behavior of a ‘Z’ shaped in plan tall building under transient wind storms using unsteady Reynolds-averaged Navier Stokes (URANS) Computational Fluid Dynamics (CFD) technique. Numerical simulations are carried out using ANSYS - CFX, adopting length and velocity scales of 1:300 and 1:5, respectively. Surface streamlines are presented in the article to depict the wind flow characteristics and envisage the regions and nature of flow separations and vortex generations. The flow features also facilitate visualizing the wind pressure contours on building facets. The peak pressure coefficients concerning wind angles of 0° and 90° are presented to enable the cladding design of the building. Moreover, normalized wind pressure power spectra at three different sections along the altitude are offered for 0° to 150° wind at an interval of 30°. It is observed that towards the bottom portion of the building, the peak of the power spectra usually occurs in the low-frequency values. In contrast, at the top and main body of the building, the vortex shedding energy exaggerates at comparatively high reduced frequencies..

Airfoil Tonal Noise Prediction Using Urans

Abstract To examine the feasibility of the laminar boundary layer (LBL), vortex shedding (VS) tonal noise modelling using unsteady Reynolds-averaged Navier–Stokes (URANS) was investigated for the non-symmetric S834 airfoil. A transition SST turbulence model was used to model the laminar-turbulent transition and its vital influence on the laminar bubble and hydrodynamic instabilities generation. The influence of turbulence on the unsteady vortex patterns was investigated. Hence, the hybrid aeroacoustic analysis with Lighthill analogy was conducted to obtain the acoustic pressure field. The approach allowed us to model hydrodynamic instabilities and the resulting VS tonal noise. The frequency of VS matched the experimental data, giving the same 1/3 octave tonal peak only for a limited freestream turbulence regime. The simplification of the present method did not allow us to model the aeroacoustic feedback loop, and resulted in lack of instabilities for higher freestream turbulence.

On computational simulations of dynamic stall and its three-dimensional nature

In this paper, we investigate the three-dimensional nature of dynamic stall. Conducting the investigation, the flow around a harmonically pitching National Advisory Committee for Aeronautics (NACA) 0012 airfoil is numerically simulated using Unsteady-Reynolds-Averaged Navier–Stokes (URANS) and multiple detached eddy simulation (DES) solvers: the Delayed-DES (DDES) and the Improved-DDES (IDDES). Two- and three-dimensional simulations are performed for each solver, and the results are compared against experimental measurements in the literature. The results showed that three-dimensional simulations surpass two-dimensional ones in capturing the stages of dynamic stall and predicting the lift coefficient values, with a distinguished performance of the DES solvers over the URANS ones. For instance, the IDDES simulations, as an inherently three-dimensional solver, predicted the necessary cascaded amalgamation process of vortices to form the adequate strength of the dynamic stall vortex. This vortex size and timing provided accurate and sufficient suction that resulted in identical matching of the numerical and experimental lift coefficients at the peak value. Hence, the hypothesis that dynamic stall has a three-dimensional nature is supported by the superiority of the three-dimensional simulation in all aspects. In conclusion, it is found that dynamic stall is intrinsically a three-dimensional phenomenon.

Aerodynamic Simulation and Optimization of Micro Aerial Vehicle Rotor Airfoil at Low Reynolds Number

This paper describes the aerodynamic simulation and optimization of NACA 0012 airfoil at a low Reynolds number using unsteady Reynolds-averaged Navier-Stokes (URANS) and Spalart–Allmaras turbulence model in Ansys Fluent. The purpose of this paper is to simulate and optimize the airfoil to get better aerodynamic performances at low Reynolds numbers. The Parsec method was selected for the optimization of the NACA 0012 airfoil. Both of these airfoils are simulated using CFD Fluent between 0 to 13-degree angle of attack at a low Reynolds number of 200000. To simulate the airfoil, mesh generation is crucial so an O-grid structured mesh is created. After the simulation, several aerodynamic performances are compared between the airfoils, such as lift coefficient, drag coefficient, pressure coefficient, and lift-to-drag ratio. And the calculated results from Xfoil are taken as references. Between NACA 0012 and optimized NACA 0012, the optimized airfoil showed better aerodynamic performances than the normal one, which was the goal of this paper. Later on, the different flow field variables, such as density, temperature, pressure, and vorticity magnitude were analyzed and compared. Both the airfoils at a different angle of attack were analyzed for these functions, like 7°, 11°, and 20° AOA. During the analytical process, Q-criterion appears to be a very important method of vortex identification in the flow field. With this analysis, we came to know, that as the angle of attack increases the adverse pressure gradient also increases, which creates a big reverse flow.

URANS prediction of ship performance: An analysis of propulsion system failure on turning behaviour of a ship in waves

The loss of ship propulsive power has been recognised as the most frequently occurring incident at sea. Given the fact that the manoeuvrability of a ship is highly reliant on the propulsive power produced by a rotating propeller, propulsion loss may pose potential threats to navigational safety in a real seaway by resulting in further serious incidents, such as collision, contact, and grounding. The key objective of this paper is to analyse the propulsion loss impacts on the turning capability of the KRISO Container Ship (KCS) model in different wave height conditions, using unsteady Reynolds-Averaged Navier-Stokes (URANS) computations. It is expected that this work will be of great interest to Master Mariners and navigating officers who are in charge of ship handling and help enhance navigational safety in real sea states.

A Skeletal Mechanism for MILD Combustion of n-Heptane/Air Mixtures

ABSTRACT Moderate, intense or low-oxygen dilution (MILD) Combustion has many attributes such as low emissions, noiseless combustion with high thermal efficiency which are attractive for greener combustion systems. Past studies investigated flow and chemical kinetics effects in establishing this combustion mode for methane-air mixtures. Many of the practical fuels are large hydrocarbons and hence this study aims to develop a skeletal mechanism suitable for turbulent MILD combustion simulations of n-heptane/air. Computer Assisted Reduction Mechanism approach is employed to develop a mechanism involving 36 species and 205 reactions, which is validated for wide range of conditions using measurements or results obtained from a comprehensive mechanism. This mechanism is found to be excellent for predicting both ignition delay times and flame speeds for MILD conditions. The OH* and CH* obtained using quasi-steady state assumptions agree well with those obtained using the comprehensive mechanism with these chemiluminescent species. The performance of the skeletal mechanism for turbulent MILD combustion simulation is tested using Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with a finite-rate chemistry model. The computed statistics of temperature and O H number density agree quite well with measurements for highly diluted combustion cases.

Influence of Vane Trailing Edge Flow on the Formation of Cavity Cells and Rim Sealing

Abstract Hot gas ingestion into the turbine rim seal cavity is an important concern for engine designers. To prevent ingestion, rim seals use high-pressure purge flow; however, the penalty is that excessive use of the purge flow decreases engine thermal efficiency. In this paper, a one-stage turbine operating at engine-representative conditions was used to study the effect of steady and time-resolved under-platform cavity temperatures and pressures across a range of coolant flowrates in the presence of vane trailing edge (VTE) flow. This study correlates time-resolved pressure with time-resolved temperature to identify primary frequencies driving ingestion. At certain flowrates, the time-resolved pressures are out of phase with the temperatures, indicating ingestion. Measurements from high-frequency response pressure sensors in the rim seal and vane platform were also used to determine rotational speed and quantity of large-scale structures (cells). In a parallel effort, a computational model using Unsteady Reynolds-averaged Navier–Stokes (URANS) was applied to determine swirl ratio in the rim seal cavity and time-resolved rim sealing effectiveness. The experimental results confirm that at low purge flowrates, the VTE flow influences the unsteady flow field by decreasing pressure unsteadiness in the rim seal cavity. Results show an increase in purge flow increases the number of unsteady large-scale structures in the rim seal and decreases their rotational speed. However, VTE flow was shown to not significantly change the cell speed and count in the rim seal. Simulations point to the importance of the large-scale cell structures in influencing rim sealing unsteadiness, which is not captured in current rim sealing predictive models.

Numerical investigations of a membrane morphing wind turbine blade under gust conditions

Flexibility during flight using elasto-flexible membrane-material systems is an attractive solution regarding loads reduction under unsteady flow conditions. Based on this idea, a wind turbine blade with flexible surfaces is designed from the NASA-Ames Phase VI configuration and investigated under gust conditions. Two concepts are modelled, a flexible as well as a rigid blade geometry. While the rigid concept is only simulated using Unsteady Reynoldsaveraged Navier Stokes (URANS) method, the flexible blade is computed with Fluid-Structure Interaction simulations coupling URANS and Finite Element Method. In both cases, the fluid is modelled with a rigid-body rotation method. The latest enables to take into account the rotation of the blade and, for the flexible case, to simultaneously capture the strong relationship between the flow and the membrane's deformation. The resulting axial force, pressure distribution, vorticity, and flow field velocity are analyzed for both geometries. While the rigid geometry serves as a baseline, the flexible case is investigated, focusing on the difference between rigid and flexible configurations.

Thermal performance prediction of street trees inside isolated open spaces – evaluations from real scale retrofitting project

Climate change and the urban heat island (UHI) effects are increasing heat stress and adversely impacting outdoor thermal comfort in urban areas. The study demonstrates that thermal comfort conditions can be improved by reducing air temperature and surface temperature with the integration of street trees into the urban environment. In this work, computational fluid dynamics (CFD) simulations using unsteady Reynolds-averaged Navier–Stokes (URANS) equations have been performed to analyze the cooling effect of street trees for heatwave period (18–22 June 2015) in a hot-humid urban environment. The results are then compared in-term-of air/surface temperature, flow-velocity and apparent temperature for the vegetation case, open-space case, and built case. The analysis shows that the vegetation can effectively decrease surrounding temperature (a reduction of 1.2 K), thereby reducing energy consumption and effectively promote thermal comfort conditions. The study findings will encourage city planners and citizens to take action for urban greening.

Numerical Investigation on Oscillation Behavior of a Non-isothermal Self-excited Jet in a Cavity: The Effects of Reynolds Number and Temperature Differences

A self-excited oscillating jet can be naturally produced by discharging a plane jet into a rectangular cavity due to pressure effects and without a need for external aid. In recent years, the self-oscillatory jet in non-isothermal conditions has attracted research interests because of its wide range of industrial applications. Therefore, the current study aimed to compare the oscillatory behavior of downward vertical self-excited jet with Reynolds number (Re) 1000 and 3000 under various temperature differences (0, 100, and 300 K) between inletflow and cavity’s wall. Computational solutions were obtained using unsteady Reynolds averaged Navier-Stokes (URANS) and energy equations for an incompressible flow. The numerical simulation was carried out by the finite-volume based tool OpenFOAM code. The results showed that depending on the value of temperature difference, oscillatory and non-oscillatory flows were observed. Also, at Re=3000, the temperature differences can change oscillation frequency up to 10% compared to isothermal conditions. This value reaches 58% at Re=1000. The results indicated that where the Archimedes number is less than 0.1, the effects of temperature differences between jet and cavity walls on the oscillating behavior are negligible.

URANS PREDICTION OF THE SLAMMING COEFFICIENTS FOR PERFORATED PLATES DURING WATER ENTRY

The slamming coefficients for perforated plates of various perforation ratios and layout configurations were predicted using Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver STAR-CCM+. The numerical model was validated by comparing with experimental measurements of slamming coefficient for a circular cylinder. The slamming coefficients and free surface profiles of perforated plates were then predicted at full-scale. It was found the air compressibility plays an important role by studying flat plate water entry phenomena. For perforated plates with small gap length/width ratios, the ability of the trapped air to evacuate through the space between the bottom of the plate and free surface is similar. For perforated plates with different gap number at a fixed perforation ratio, the slamming coefficient is increased with the increase in gap length/width ratio. However, a further increase in length/width ratio may impose a negative impact on the escape of trapped air due to the increase of gap number.

Computational study on self-oscillatory flow induced by vertical and horizontal jets in partially heated and cooled cavities

In recent years, non-isothermal jets have received much attention due to their wide range of applications in the industry. The present work concentrates on comparing the buoyancy mediating effects on the performance and oscillating behavior of horizontal and vertical self-excited jets. To this aim, the oscillating behavior and performance characteristics of self-excited jets in horizontal and vertical directions have been simulated and compared under different thermal boundary conditions (e.g., heated, cooled, and adiabatic wall cavities). Computational solutions were obtained using unsteady Reynolds averaged Navier-Stokes (URANS) and energy equations for an incompressible flow. Investigations have been performed for different nozzle thicknesses/widths (0.5–2 cm) at a constant flow rate and inlet temperature of 300 K. The results showed that non-isothermal conditions did not significantly affect the oscillating behavior of horizontal jets, while this could be much more dramatic in vertical jets. In vertical jets with a nozzle thickness of 2 cm, under non-isothermal conditions, the oscillation frequency and the average Nusselt number are about 17% and 40% different from those of the isothermal jet, respectively. The results also indicated that when the Archimedes number exceeds 0.1, the effect of the jet orientation on its performance and oscillating behavior will be more pronounced.

Study on the Applicability of URANS, Large Eddy Simulations, and Hybrid Large Eddy Simulations/RANS Models for Prediction of Hydrodynamics of Cyclone Separator

Abstract Cyclone separators are an integral part of many industrial processes. A good understanding of the flow features is paramount to efficiently use them. The turbulent fluid flow characteristics are modeled using unsteady Reynolds-averaged Navier–Stokes (URANS), large eddy simulations (LES), and hybrid LES/Reynolds–averaged Navier–Stokes (RANS) turbulent models. The hybrid LES/RANS approaches, namely, detached eddy simulation (DES), delayed detached eddy simulation (DDES), and improved delayed detached eddy simulation (IDDES) based on the k−ω SST RANS approaches are explored. The study is carried out for three different inlet velocities (v = 8, 16.1, and 32 m/s). The results from hybrid LES/RANS models are shown to be in good agreement with the experimental data available in the literature. Reduction in computational time and mesh size are the two main benefits of using hybrid LES/RANS models over the traditional LES methods. The Reynolds stresses are observed in order to understand the redistribution of turbulent energy in the flow field. The velocity profiles and vorticity quantities are explored to obtain a better understanding of the behavior of fluid flow in cyclone separators.

Performance analysis of vertical axis water turbines under single-phase water and two-phase open channel flow conditions

We conduct performance analyses of a family of vertical axis water turbines (VAWTs) under single-phase water and two-phase open channel flow conditions using unsteady Reynolds-averaged Navier-Stokes (URANS) numerical simulations. The performance, including self-starting capability and energy harvesting efficiency, of a recently developed hybrid Darrieus-Modified-Savonius (HDMS) design is examined under single-phase water flow conditions. With a two-way coupled fluid-structure interaction (FSI) approach, we demonstrate that the water turbine has excellent self-starting capability. After that, the effects of the Reynolds number, tip speed ratio (TSR), and geometric design of the turbine, including turbine size, solidity, and number of blades, are systematically studied. Since the main function of the modified Savonius rotor is to assist self-start, and this part has a negligible effect on energy harvesting performance when the turbine is under stable operation, only the Darrieus rotor is studied under two-phase open channel flow conditions to save computational cost. A key finding is that larger blockage ratios result in better energy harvesting performance of the water turbines. When the blockage ratio is 60% and the TSR of the turbine is 5.0, the power coefficient of the water turbine can reach up to 73%, which exceeds Betz's limit.

Numerical investigation of combustion characteristics of upstream crescent cavities in a scramjet combustor

This study investigates the combustion performance of a number of novel, crescent shaped cavities placed directly in front of the fuel injection point in a scramjet combustor. Cavities are commonly used for flameholding in scramjet combustors, however they are almost exclusively placed downstream of the fuel injection point and their effect on mixing performance is limited. The crescent cavities in this study were previously found to enhance mixing in chemically frozen flow and the purpose of this study is to investigate whether this performance enhancement translates to chemically reacting flow. The performance of the cavities was numerically studied using unsteady Reynolds-Averaged Navier–Stokes (URANS) modelling with hydrogen as the fuel and an isothermal 1800 K combustor wall, and performance was compared to a no-cavity baseline. The cavities were found to enhance combustion by up to 114% and heat release by up to 143% compared to the baseline when hybrid fuelling is employed and by 47.4% and 54.7%, respectively, when no hybrid fuelling is used. Total pressure loss in the cavity cases was similar to the baseline case and viscous drag was lower in the cavity cases due to the presence of combustion and fuel near the wall. Flame penetration was lower in the cavity cases than in the baseline however as a result of the increased fuel concentration on the wall, while mean heat flux is lower in the cavity cases than in the baseline. Ignition is also found to occur more rapidly in the cavity cases and the flow inside the cavity is oscillatory in nature, as was the case in chemically frozen flow. Given that the crescent cavities in this study significantly enhance combustion performance with limited drawbacks, especially when hybrid fuelling is used, they are an attractive option for operational scramjet combustor design.

Experimental and Computational Investigation of Flow Structure in Buoyancy-Dominated Rotating Cavities

Abstract The flow and heat transfers inside high-pressure (HP) compressor rotating cavities are buoyancy driven and are known to be extremely difficult to predict. The experimental data of laser-Doppler anemometry (LDA) measurements inside an engine representative cavity rig are presented in this paper. Traverses using a two component LDA system have been carried out in the shaft bore and the cavity regions in order to map the axial and tangential velocity components. The velocity data are collected for a range of Rossby, Rotational, and Axial Reynolds numbers, Ro, Reθ, and Rez, 0.08<Ro<0.64, 7×105< Reθ<2.83×106, and 1.2×104< Rez<4.8×104, respectively, and for values of the buoyancy parameter βΔT, 0.284<βΔT<0.55. Numerical study using unsteady Reynolds-averaged-Navier–Stokes (URANS) simulations has been carried out to elucidate flow details for a few selected cases. The experimental results revealed that the Swirl number (Xk) varies from a value < 1 near the bore to near solid body rotation at increased radii within the cavity. The analysis of frequency spectrum of the tangential velocity inside the cavities has also shown the existence of pairs of rotating and contra-rotating vortices. There is generally satisfactory agreement between measurements and computational fluid dynamics (CFD) simulations. There is also convincing evidence of two or more separate regions in the flow dominated by the bore flow and rotation.

Volumetric Velocimetry Measurements of Purge–Mainstream Interaction in a One-Stage Turbine

Abstract Air-cooled gas turbines employ bleed air from the compressor to cool vulnerable components in the turbine. The cooling flow, commonly known as purge air, is introduced at low radius, before exiting through the rim-seal at the periphery of the turbine discs. The purge flow interacts with the mainstream gas path, creating an unsteady and complex flowfield. Of particular interest to the designer is the effect of purge on the secondary-flow structures within the blade passage, the extent of which directly affects the aerodynamic loss in the stage. This paper presents a combined experimental and computational fluid dynamics (CFD) investigation into the effect of purge flow on the secondary flows in the blade passage of an optically accessible one-stage turbine rig. The experimental campaign was conducted using volumetric velocimetry (VV) measurements to assess the three-dimensional inter-blade velocity field; the complementary CFD campaign was carried out using unsteady Reynolds-averaged Navier–Stokes (URANS) computations. The implementation of VV within a rotating environment is a world first and offers an unparalleled level of experimental detail. The baseline flow-field, in the absence of purge flow, demonstrated a classical secondary flow-field: the rollup of a horseshoe vortex, with subsequent downstream convection of a pressure-side and suction-side leg, the former transitioning in to the passage vortex. The introduction of purge, at 1.7% of the mainstream flowrate, was shown to modify the secondary flow-field by enhancing the passage vortex, in both strength and span-wise migration. The computational predictions were in agreement with the enhancement revealed by the experiments.

Aerodynamic performance improvements of a vertical axis wind turbine by leading-edge protuberance

A detailed numerical study has been carried out to investigate the effects of leadingedge protuberance as a novel flow-separation control-technique for vertical-axis-windturbine (VAWT). The aerodynamic performance of a stationary protuberanced blade and an associated H-type Darrieus VAWT made of three protuberanced blades were investigated using unsteady Reynolds-Averaged-Navier-Stokes (URANS) and Implicit Large Eddy Simulation (ILES) methods. The current study involves a comprehensive set of five different types of sinusoidal leading-edge protuberances with three different wavelengths and amplitudes. It is found that this passive flow-control-method can favourably change lift and drag at stall and post-stall regions, which is of high importance for VAWTs. Static stall variation can be changed from leading-edge-stall type to trailing-edge-stall type. This is attributed to a “bi-periodic” phenomenon over the blade suction side and a reduced turbulence kinetic-energy production. Protuberance amplitude was found to be more important than the wavelength in improving aerodynamic performance. Of all the tested cases, the protuberance with amplitude of 1% chord-length (c) and wavelength of 2.5%c behaves the best. The leading-edge protuberance can significantly increase VAWT power-coefficient at low tip-speed-ratios, which are dominated by post-stall conditions. The dynamic stall was delayed and the coherent spanwise wavy flow-structures on the blade suction-side were observed.