Reynolds-Averaged Navier-Stokes CFD Research Articles

The broad study of blade cascade under controlled torsional flutter: Dynamics of the flow and stability analysis

The experimental and numerical investigation of the flow instabilities acting on rigid blades and vice versa was conducted for both compressor and turbine configuration. The blade cascade consisted of five rectangular NACA 0010 blades, with three middle blades capable of performing harmonic motion with one degree of freedom (pitching) using force excitation. The base case (all blades fixed) and excited regime were examined. The influence of various angles of attack, harmonic frequency values, amplitude values, inter-blade phase angles and Reynolds numbers (Re) were tested. The mean flow properties as well as the fluid - structure interaction (FSI) were studied using Particle Image Velocimetry (PIV), Reynolds-averaged Navier-Stokes (RANS) CFD methods and using force measurement. Additionally, two different approaches, namely traveling wave mode (TWM) and aerodynamic influence coefficient (AIC), were adopted to estimate the aeroelastic stability of the blade cascade, and the results were compared. The results show significant aeroelastic coupling between the blades in both compressor and turbine configuration. However, the aerodynamic coupling effect for torsional flutter is more prominent in turbine configuration.

The contribution of mesh opening angle to the drag of netting panels

To ensure the economic and environmental sustainability of the fisheries and aquaculture industries, it is necessary to address issues related to fuel consumption, environmental degradation, and fish welfare. Hence, we need a thorough understanding of the filtration efficiency and the hydrodynamic forces acting on towed fishing gears and netting structures.Here we apply a Reynolds-averaged Navier-Stokes (RANS) CFD method to model the flow through netting panels, where we vary the operational and design parameters of flow speed, netting solidity, twine diameter, mesh opening angle and the incidence angle of the flow to the panel. Thus, we create a simulated data set which we analyze to provide a fundamental understanding of the functional relationships for the pressure drop and tangential drag coefficients, and the flow deflection in terms of these parameters. We pay particular attention to the effect of mesh opening angle, a parameter that has not received much attention in the literature. We demonstrate that it has a large influence on the drag and lift coefficients and consequently on the hydrodynamic efficiency of netting panels.These results will be particularly useful for reducing the hydrodynamic forces on netting structures and improving the fuel efficiency of towed fishing gear operations.

Estimating flow fields with reduced order models

The estimation of fluid flows inside a centrifugal pump in realtime is a challenging task that cannot be achieved with long-established methods like CFD due to their computational demands. We use a projection-based reduced order model (ROM) instead. Based on this ROM, a realtime observer can be devised that estimates the temporally and spatially resolved velocity and pressure fields inside the pump. The entire fluid-solid domain is treated as a fluid in order to be able to consider moving rigid bodies in the reduction method. A greedy algorithm is introduced for finding suitable and as few measurement locations as possible. Robust observability is ensured with an extended Kalman filter, which is based on a time-variant observability matrix obtained from the nonlinear velocity ROM. We present the results of the velocity and pressure ROMs based on a unsteady Reynolds-averaged Navier-Stokes CFD simulation of a 2D centrifugal pump, as well as the results for the extended Kalman filter.

Analysis of discrepancies between 3-D coupled and uncoupled schemes based on CFD in full engine simulation

Full engine simulations based on three-dimensional computational fluid dynamics (3-D CFD) allow the visualisation of flow details accompanied by total performances for aeroengine design. In this study, 3-D coupled and uncoupled schemes are performed to simulate the full engine performance of a micro turbojet engine. For the 3-D uncoupled scheme, the general performance map of each core component based on CFD simulation is used as a component model for solving the equations of thermodynamic equilibrium without aerodynamic data crossing the boundary of each component. For the 3-D coupled scheme, a full engine simulation is performed using a Reynolds-averaged Navier-Stokes CFD solver coupled with a power balance iteration, and all components are assembled in one computational domain. The calculation processes of these two schemes are introduced, and the feasibility of using the Regula-Falsi method in the coupled scheme for power balance iteration is verified. Despite some discrepancies, the overall performance results yielded by these two schemes are consistent with the experimental results. A detailed comparison of these two schemes is presented to elucidate the reasons contributing to the discrepancies. The results show that the uncertainty of the boundary conditions limited by the 3-D uncoupled simulation method and the low-dimensional forced mixing at the component's interfaces can result in significant calculation errors. Interactions among core components are demonstrated methodically. Finally, based on a comparative analysis with the 3-D coupled scheme, correction strategies for the component data-based model are proposed to improve the accuracy of uncoupled simulations. With these coupling corrections, the maximum relative error of the thrust between these two schemes reduces by 50% and the result is more close to the experiment.

Effect of 2D ice accretion on turbulent boundary layer and trailing-edge noise

Trailing-edge noise is known to be sensitive to airfoil shapes, and ice accretion is one cause of an airfoil shape deformation. This paper investigates how trailing-edge noise is affected by the airfoil shape deformation due to ice accretion. The formation of ice-induced flow separation and the development of a turbulent boundary layer are analyzed to understand the correlation between the altered flow physics due to ice accretion inside the boundary layer and trailing-edge noise. The near-wall flow behind the leading-edge ice accretion is analyzed by using Reynolds-Averaged Navier Stokes CFD in OpenFOAM, and trailing-edge noise is investigated using an empirical wall pressure spectrum model in conjunction with Amiet’s trailing-edge noise theory. Validations of tools against measurement data are presented. Liquid water content, freestream velocity, and ambient temperature are varied to investigate the impact of flow conditions on the ice accretion shape and the resulting boundary layer flow characteristics at the trailing edge. It is found that a more significant leading edge deformation due to ice accretion generates larger ice-induced flow separation bubbles, which increases the trailing-edge boundary layer thickness. As a result, an increase in low- and mid-frequency noise is observed. The purpose of this paper is not only to understand the effect of ice accretion on trailing-edge noise but also to comprehensively analyze how flow physics inside the turbulent boundary layer is altered by the presence of various ice accretion shapes.

Towards CFD-based optimization of urban wind conditions: Comparison of Genetic algorithm, Particle Swarm Optimization, and a hybrid algorithm

• Genetic algorithm, particle swarm optimization and a hybrid algorithm are compared. • CFD simulation and evolutionary algorithms are coupled to optimize wind conditions. • Wind speed is improved at street scale for a generic urban area. • The use of different evolutionary algorithms can impact wind speed by up to 425%. • A hybrid algorithm of PSO and GA is superior over PSO and GA. Urban morphology can significantly impact urban wind conditions. Finding an optimum morphology to improve the wind conditions for a given urban area can be very challenging as it depends on a wide range of parameters. In this perspective, meta-heuristic algorithms can be useful to reach/approximate optimum solutions. While the satisfactory performance of meta-heuristic algorithms has been shown for different complex engineering problems, a detailed evaluation of these algorithms has not yet been performed for urban wind conditions. Therefore, this study aims to systematically evaluate the performance of meta-heuristic algorithms for CFD-based optimization of urban wind conditions at street scale. Three algorithms are considered: (i) Genetic algorithm (GA), (ii) Particle Swarm Optimization (PSO), and (iii) a hybrid algorithm of PSO and GA. The focus is on a compact generic urban area, while the height of the involved buildings is considered as the optimization variable. In total, 714 high-resolution 3D steady Reynolds-averaged Navier-Stokes (RANS) CFD simulations are performed in combination with the standard k-ε turbulence model. The results show that the hybrid algorithm is superior as it can improve the wind conditions by about 425% and 100%, compared with GA and PSO, respectively.

CFD Simulation of Two Tandem Floating Offshore Wind Turbines in Surge Motion

High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulation is employed to investigate the variations in the power performance of two tandem in-line floating offshore horizontal axis wind turbines for the scenario in which the upstream rotor is oscillating in surge motion and the downstream rotor is positioned in a distance of 3D (D: turbine diameter) and is stationary. The rotors are the NREL-5MW reference turbine. The platform surge period and wave amplitude are 9 s and 1.02 m, respectively. The results are presented for 100 full surge periods. It is found that the surge motion of the upstream rotor results in: (i) sinusoidal fluctuations in the power and thrust coefficients (CP and C T) of the upstream rotor with a standard deviation (std) of 9.7% and 5.5%, respectively; (ii) such fluctuations in CP and CT are less regular with a std of 4.2% and 2.8% for the downstream rotor, respectively. A low-frequency oscillating mode with a period nearly 10 times the surge period is also observed for the downstream rotor. The mean Cp and Ct of the downstream rotor are 28.9% and 38.5% of the upstream one.

Numerical Study about Aerodynamic Interaction for Coaxial Rotor Blades

In the present study, the interaction effect by inter-rotor spacing of coaxial rotor blades in hovering and forward flight is studied by using a Reynolds-averaged Navier–Stokes CFD flow solver based on mixed meshes. The meshes were composed of unstructured near-body region and Cartesian off-body region. The flux-difference splitting scheme of Roe was used for computing inviscid fluxes on the near-body region, whereas the 7th order WENO scheme was used for the computation of the inviscid fluxes on the off-body region to capture vortex with high resolution. The predicted results of the baseline inter-rotor spacing were compared with the reference results of other researchers for validation. The computed results of three different inter-rotor spacings were compared in terms of the thrust and torque coefficients for each flight condition. The cause of the different interaction effect by inter-rotor spacing between hovering and forward flight was also investigated.

Impact of trim on added resistance of KRISO container ship (KCS) in head waves: An experimental and numerical study

In this study, added resistance and motion responses of KRISO Container Ship (KCS) were evaluated experimentally and numerically in six different trim angles. A series of towing tank experiments were performed for six different trim angles at design speed in calm water and regular head waves. The ship motions and added resistance were measured for several wavelength conditions considering short and long wave ranges with wave steepness of 1/60. Next, computations of the towed model in calm water and waves were performed using Unsteady Reynolds-Averaged Navier-Stokes (URANS) CFD and 3-D potential methods. Effects of trim angles on added resistance were analysed and results concerning the performance of the vessel at different trim angles were plotted. Experimental and numerical results for the heave and pitch motions and the added resistance were compared and URANS CFD simulation results showed good agreement with the experimental data for the ship in head waves. Also, the results were compared to those from potential theory and range of trim and wave conditions were identified for the application of rapid linear potential flow method.

Low wind speed aerodynamics of asymmetric blade H-Darrieus wind turbine-its desired blade pitch for performance improvement in the built environment

An asymmetric blade vertical-axis wind turbine (VAWT) is one of the emerging technologies for harvesting power in the built environment, which has low wind speed. Although asymmetric blade improves VAWT’s performance, the effect of blade pitch angle on its design is hardly ascertained. In this paper, unsteady 2D Reynolds-averaged Navier–Stokes CFD simulations are carried out to investigate the effect of blade pitch angle on the aerodynamic performance of a NACA 63-415 asymmetric blade H-Darrieus VAWT at a low wind speed of 6.0 m/s. Its detailed flow physics at different operating and pitch angle conditions is investigated, and important performance insights are obtained to elucidate its desired blade pitch for performance improvement. The present study shows that positive pitch angle (+ 5°) improves the turbine performance in upwind position, whereas negative pitch angle (− 5°) augments the turbine performance in downwind position as well as causes less wake effect than positive pitch angle. Further, optimal pitch angle (+ 5°) is found out at which the maximum power coefficient of 0.271 is obtained for an operating tip speed ratio 2.4. The present study delineates how desired blade pitch improves the performance of asymmetric blade VAWT for sustainable power generation in the built environment.

Minimum momentum flux ratio required to prevent air curtain breakthrough in case of cross-curtain pressure gradients: CFD versus analytical equation

This paper presents a numerical study on the required momentum flux ratio to prevent air curtain breakthrough in case of cross-curtain (i.e. cross-jet) pressure gradients. 2D steady Reynolds-averaged Navier-Stokes (RANS) CFD simulations with the RNG k-ε turbulence model are employed for jet Reynolds numbers ranging from 5,000 to 30,000. First, the computational model is validated based on particle image velocimetry (PIV) measurements. Second, the influence of several jet parameters on the separation efficiency is evaluated for a moderate cross-jet pressure difference of 10 Pa. These are the ratio of the jet discharge momentum flux to the jet cross-flow momentum flux (momentum flux ratio), the jet height-to-width ratio and the jet discharge angle. Finally, the minimum deflection modulus to prevent jet breakthrough and the corresponding momentum flux ratio by an analytical equation and by CFD are compared. The results show that, for the configuration under study: (1) jets with the smallest height-to-width ratios (β = 18) provide the highest separation efficiency; (2) inclined jets with discharge angles α0 = 5° and 10° provide slightly higher separation efficiency than straight jets (α0 = 0°) and jets with α0 = 20°; (3) the maximum modified separation efficiency is reached at lower momentum flux ratios for jets with smaller height-to-width ratios and for inclined jets; (4) the analytical and CFD values of the optimal momentum flux ratio differ with up to 31.2%. This study shows how the separation efficiency of air curtains can be improved by adjusting certain jet parameters.

Numerical evaluation of the severity of consequences of external fire and explosion incident at a nuclear power plant

The International Atomic Energy Agency emphasizes on the safety assessment of fires involved by the crash of a large commercial aircraft into a nuclear power plant. In the guideline, it requires that a total load of aviation fuel that causes damage to exterior plants’ components should be included when performing the initial stages of the nuclear plant site evaluation process and over the entire lifetime of the plant operation. In this study, the effects of jet fuel and hydrogen-induced external explosion on a nuclear power plant was investigated and analyzed. A turbulence model based on Reynolds-averaged Navier-Stokes CFD solver, called FLACS, was used to determine the explosion parameters within the plant vicinity. The simulation results of key explosion parameters for butane explosion show a deflagrative overpressure of 0.27 bar and impulse load of 0.015 bar·s at some preselected position. An elevated local temperature of about 2030 K is recorded for this fuel. Hydrogen explosion causes a maximum overpressure of 0.37 bar, and a maximum positive pressure impulse load of 0.022 bar·s at the exterior walls of building structures. From the findings, it showed that building obstacles have a substantial influence on the evolution of fireball and overpressure propagation. The computed overpressure and impulsive loadings observed are capable of causing substantial structural damages and vulnerabilities. A significantly elevated flame temperature recorded would have a harmful effect on the safety function of structures, systems and components (SSCs) that are needed to execute reactor shutdown. The findings of this study may be used to revisit the safety evaluation of a nuclear power plant (NPP) site with regards to the risks and consequences associated with external explosion due to aircraft impact. It is also useful in designing the layout of the NPP and site placement of relevant items important to safety.

Shear stress generated by radial flow impellers at bioreactor integrated membranes

The study reveals the hydrodynamics at the surface of a submerged tubular membrane module integrated in a stirred membrane bioreactor. The reactor is equipped with a conventional six flat-blade impeller imposing radial circulation across the membrane interface. Simulation and computer visualization of “real” flow using a Reynolds-averaged Navier-Stokes model and CFD methodology are employed. A variety of model solutions at various mixing intensity are obtained and the mixing conditions are assessed by delineation of the near-wall zones and identification of the zones' shear rate and shear stress values. Shear rate non-uniformity along the surface of the tubular module is visualized. Shear stress values as high as 160 Pa at the membrane module lower section and as low as 0.6 Pa at the module upper section has been determined. Referring to reported data for shear stress near flat plate stirred filtration cells and external narrow-channel cross-flow systems, the mixing conditions are expected to allow enhanced access of the retentate fluid to the membrane surface, as well as possible low membrane fouling potential related to microfiltration practice.

Estimation of annual energy production using dynamic wake meandering in combination with ambient CFD solutions

In the present study, combination of the standalone dynamic wake meandering (DWM) model with Reynolds-averaged Navier-Stokes (RANS) CFD solutions for ambient ABL flows is introduced, and its predictive performance for annual energy production (AEP) is evaluated against Vestas’ SCADA data for six operating wind farms over semi-complex terrains under neutral conditions. The performances of conventional linear and quadratic wake superposition techniques are also compared, together with the in-house implemention of successive hierarchical merging approaches. As compared to our standard procedure based on the Jensen model in WindPRO, the overall results are promising, leading to a significant improvement in AEP accuracy for four of the six sites. While the conventional linear superposition shows the best performance for the improved four sites, the hierarchical square superposition shows the least deteriorated result for the other two sites.

Aerodynamics of an aerofoil in transonic ground effect: Methods for blowdown windtunnel scale testing

AbstractExperimental aerodynamic testing of objects in close ground proximity at high subsonic Mach numbers is difficult due to the construction of a transonic moving ground being largely unfeasible. Two simple, passive methods have been evaluated for their suitability for such testing in a small blowdown wind tunnel: an elevated ground plane, and a symmetry (or mirror-image) approach. The methods were examined using an unswept wing of RAE2822 section, with experiments and Reynolds-Averaged Navier Stokes CFD used synergistically to determine the relative merits of the techniques. The symmetry method was found to be a superior approximation of a moving ground in all cases, with mild discrepancies observed only at the lowest ground clearance. The elevated ground plane was generally found to influence the oncoming flow and distort the flowfield between the wing and ground, such that the method provided a less-satisfactory match to moving ground simulations compared to the symmetry technique.

Detached-Eddy Simulation of High-Reynolds-Number Beveled-Trailing-Edge Boundary Layers and Wakes

Flow over three different trailing-edge geometries is studied using incompressible detached-eddy simulation and unsteady Reynolds-averaged Navier Stokes CFD methods. Of interest is the ability of DES, coupled, with localized overset-grid refinement, to resolve the proper physics of separated flows from trailing edges—trailing-edge turbulence and vortex shedding, in particular. The DES model is shown to provide a good qualitative description of the trailing-edge flow. However, the modeled separations are overly energetic due to premature separation related to artificially low turbulence levels from upstream. The transition from RANS to DES is isolated as an issue. The simulated physics of the wake are shown to be in agreement with other LES studies: the model produces the “rib/roller” structures representing the first instability modes, horseshoe vortices are observed, and in regions of high resolution, small scales are formed, as expected. The turbulence statistics are qualitatively similar to benchmark data near the trailing edge and in the near wake, however, quantitative comparisons of urms show an over prediction in magnitude of 50%–100%. Despite this, the results are promising, and future modeling efforts have been motivated and identified.

Nonaxisymmetric Turbine End Wall Design: Part I— Three-Dimensional Linear Design System

A linear design system, already in use for the forward and inverse design of three-dimensional turbine aerofoils, has been extended for the design of their end walls. This paper shows how this method has been applied to the design of a nonaxisymmetric end wall for a turbine rotor blade in linear cascade. The calculations show that nonaxisymmetric end wall profiling is a powerful tool for reducing secondary flows, in particular the secondary kinetic energy and exit angle deviations. Simple end wall profiling is shown to be at least as beneficial aerodynamically as the now standard techniques of differentially skewing aerofoil sections up the span, and (compound) leaning of the aerofoil. A design is presented that combines a number of end wall features aimed at reducing secondary loss and flow deviation. The experimental study of this geometry, aimed at validating the design method, is the subject of the second part of this paper. The effects of end wall perturbations on the flow field are calculated using a three-dimensional pressure correction based Reynolds-averaged Navier–Stokes CFD code. These calculations are normally performed overnight on a cluster of work stations. The design system then calculates the relationships between perturbations in the end wall and resulting changes in the flow field. With these available, linear superposition theory is used to enable the designer to investigate quickly the effect on the flow field of many combinations of end wall shapes (a matter of minutes for each shape). [S0889-504X(00)00902-8]