Reynolds-averaged Navier-Stokes Computations Research Articles

A 3D analytical model for orthogonal blade-vortex interaction noise

A 3D analytical model of an Orthogonal Blade-Vortex Interaction (OBVI) for Counter-Rotating Open Rotor (CROR) tonal noise is investigated. The specific influence of two parameters taking into account the three-dimensionality of both the vortex velocity and the convection velocity within the rotor-rotor volume is addressed. The first step is to extract the vortex parameters from a recent unsteady Reynolds-Averaged Navier-Stokes computation and validate different vortex models. Lamb-Oseen and Scully vortices reproduce the behavior of the tip-vortex tangential velocity fairly well. Regarding the vortex axial velocity modeling, a Gaussian profile fits well with numerical results. On the one hand, the impact of the stream-tube contraction unbalances the lobes of the unsteady pressure with opposite phases produced by the OBVI event. This effect is larger than that of an equivalent blade sweep. On the other hand, adding the axial velocity deficit to the tangential one also unbalances the pressure lobes. Finally, from an acoustic point of view using Curle's acoustic analogy, both the stream-tube contraction and the axial velocity deficit have the same effect: they turn an acoustically-low efficient quadrupole into a strong dipole making these parameters fundamental for future CROR OBVI investigations.

Effect of pitch angle on power performance and aerodynamics of a vertical axis wind turbine

Due to growing interest in wind energy harvesting offshore as well as in the urban environment, vertical axis wind turbines (VAWTs) have recently received renewed interest. Their omni-directional capability makes them a very interesting option for use with the frequently varying wind directions typically encountered in the built environment while their scalability and low installation costs make them highly suitable for offshore wind farms. However, they require further performance optimization to become competitive with horizontal axis wind turbines (HAWTs) as they currently have a lower power coefficient (CP). This can be attributed both to the complexity of the flow around VAWTs and the significantly smaller amount of research they have received. The pitch angle is a potential parameter to enhance the performance of VAWTs. The current study investigates the variations in loads and moments on the turbine as well as the experienced angle of attack, shed vorticity and boundary layer events (leading edge and trailing edge separation, laminar-to-turbulent transition) as a function of pitch angle using Computational Fluid Dynamics (CFD) calculations. Pitch angles of −7° to +3° are investigated using Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations while turbulence is modeled with the 4-equation transition SST model. The results show that a 6.6% increase in CP can be achieved using a pitch angle of −2° at a tip speed ratio of 4. Additionally, it is found that a change in pitch angle shifts instantaneous loads and moments between upwind and downwind halves of the turbine. The shift in instantaneous moment during the revolution for various pitch angles suggests that dynamic pitching might be a very promising approach for further performance optimization.

Dynamic Stall Simulations on a Pitching Finite Wing

Unsteady Reynolds-averaged Navier–Stokes computations were carried out on a pitching finite wing model using the finite volume solver DLR-TAU. The comparison with the experimental data reveals a good agreement, especially in the region of the first occurrence of stall. Discrepancies are observed in the blade tip region, where the flow in the numerical data shows a stronger separation along with larger hysteresis effects in contrast to the experiment. Additionally, the analysis of the flowfield reveals the existence of multiple dynamic stall cells over the span. The simulations with different pitching frequencies demonstrate a change in the topology of dynamic stall vortex formation in the spanwise direction. An investigation using turbulence transition modeling based on a approach demonstrated that small differences can be attributed to transition; nevertheless, these can give further insight into the physics of dynamic stall and improve the comparability with the experiment. A comparison with two-dimensional simulations shows strong similarities of the peak values in the sections where the vortex starts to evolve and, although the three-dimensional flow is strongly driven by spanwise flow, the impact on the aerodynamic loads is relatively low.

Mechanism of frequency lock-in in transonic buffeting flow

Frequency lock-in can occur on a spring suspended airfoil in transonic buffeting flow, in which the coupling frequency does not follow the buffet frequency but locks onto the natural frequency of the elastic airfoil. Most researchers have attributed this abnormal phenomenon to resonance. However, this interpretation failed to reveal the root cause. In this paper, the physical mechanism of frequency lock-in is studied by a linear dynamic model, combined with the coupled computational fluid dynamics/computational structural dynamics (CFD/CSD) simulation. We build a reduced-order model of the flow using the identification method and unsteady Reynolds-averaged Navier–Stokes computations in a post-buffet state. A linear aeroelastic model is then obtained by coupling this model with a degree-of-freedom equation for the pitching motion. Results from the complex eigenvalue analysis indicate that the coupling between the structural mode and the fluid mode leads to the instability of the structural mode. The instability range coincides with the lock-in region obtained by the coupled CFD/CSD simulation. Therefore, the physical mechanism underlying frequency lock-in is caused by the linear coupled-mode flutter – the coupling between one structural mode and one fluid mode. This is different from the classical single-degree-of-freedom flutter (e.g. transonic buzz), which occurs in stable flows; the present flutter is in the unstable buffet flow. The response of the airfoil system undergoes a conversion from forced vibration to self-sustained flutter. The coupling frequency certainly should lock onto the natural frequency of the elastic airfoil.

Assessment of high-order finite volume methods on unstructured meshes for RANS solutions of aeronautical configurations

This paper is concerned with the application of k-exact finite volume methods for compressible Reynolds-Averaged Navier-Stokes computations of flows around aeronautical configurations including the NACA0012, RAE2822, MDA30P30N, ONERA-M6, CRM and DLR-F11. High-order spatial discretisation is obtained with the Weighted Essentially Non-Oscillatory and the Monotone-Upstream Central Scheme for Conservation Laws methods on hybrid unstructured grids in two- and three- dimensions. Schemes of fifth, third and second order comprise the foundation of the analysis, with main findings suggesting that enhanced accuracy can be obtained with at least a third-order scheme. Steady state solutions are achieved with the implicit approximately factored Lower-Upper Symmetric Gauss-Seidel time advancing technique, convergence properties of each scheme are discussed. The Spalart-Allmaras turbulence model is employed where its discretisation with respect to the high-order framework is assessed. A low-Mach number treatment technique is studied, where recovery of accuracy in low speed regions is exemplified. Results are compared with referenced data and discussed in terms of accuracy, grid dependence and computational budget.

Investigation of isothermal convective heat transfer in an optical combustor with a low-emissions swirl fuel nozzle

Modern combustor design optimization is contingent on the accurate characterization of the combustor flame side heat loads. The present work focuses on the experimental measurement of the isothermal (non-reacting) convective heat transfer along a fused silica optical can combustor liner for Reynolds numbers ranging between 11,500 and 138,000. The model combustor was equipped with the SoLoNOx swirl fuel nozzle from Solar Turbines Incorporated, subjecting the liner walls to realistic isothermal flow and turbulence fields. Infrared (IR) imaging through fused silica was demonstrated, and a novel estimation of the three-dimensional conduction heat losses for steady state constant heat flux experiments was developed. A maximum heat transfer augmentation of ∼18 was observed with respect to fully developed turbulent pipe flow correlations. Contrary to other investigations, the augmentation magnitude and distribution are shown to be approximately constant with Reynolds number (particularly away from the impingement location). Particle Image Velocimetry (PIV) was included to support the heat transfer measurements, suggesting that peak heat transfer occurred 0.12 nozzle diameters upstream of the jet reattachment point along the liner. Reynolds-Averaged Navier Stokes (RANS) computations are shown to yield peak heat transfer predictions within 17.4% of the experimental results when using the realizable k-ε turbulence model and enhanced wall treatment. The measurements were further analyzed in the context of results from other heat transfer studies on gas turbine combustors.

Tidal power extraction on a streamwise bed slope

Three-dimensional Reynolds-averaged Navier–Stokes computations are presented for an actuator disc representation of an ideal tidal stream energy extracting device, operating within a channel that slopes in the streamwise direction. Downwards facing, horizontal and upwards facing slopes are considered at the same mass flow rate and depth at the device position, for both high and low channel blockage ratios. The downwards facing slope is shown to present a greater available kinetic power to the disc than the horizontal and upwards facing slopes. This is due to the velocity profile being more strongly sheared as a result of the adverse pressure gradient, which results in greater thrust variation over the disc area. Conversely, during uphill flows devices experience reduced thrust variation, as the velocity profile is more uniform due to the favourable pressure gradient. Although downhill flows can deliver greater overall power due to increased flow shear, uphill flows are more efficient at converting the power presented to the disc into power removed and this is shown to be due to the downstream flow constriction and resulting increased pressure coefficient drop through the bypass flow.

Aerothermal characteristics of a transonic tip flow in a turbine cascade with tip clearance variations

A significant portion of flows over a modern high-pressure turbine blade tip is transonic, and the transonic tip leakage flows lead to significant aerodynamic losses and high heat loads onto the blade tips. This paper aims to develop a deeper understanding of the transonic tip leakage flow physics and its influence on the loss mechanism and blade-tip heat transfer. Three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) calculations were performed using the ANSYS CFX 14.5 numerical prediction code, adopting the SST k-ω turbulence model to investigate the sensitivity of aerothermal performance of transonic tip flows to tip clearances in a RT27a turbine cascade. The transonic tip leakage flow pattern within the tip gap, the flowfield downstream of the cascade, and the blade tip heat transfer distribution are studied. The numerical results give a reasonable agreement with the experimental data. The tip aerodynamics and surface heat transfer variations with tip clearances are opposite between leading and trailing edge regions of blade tips, and tip clearance effects on the former are relatively small. As the tip clearance increases, the shock wave reflections are delayed but more evident, and it therefore leads to reduced leakage massflow density and decreased heat loads on the rear part of blade tips. Despite this, since the leakage flow near the leading edge of blade tips remains subsonic resulting in increased leakage mass flowrate and tip heat transfer, the leakage losses and overtip heat loads are increased with the increasing tip clearance.

Flow and windage due to bolts on a rotating disc

The cooling air in a rotating machine is subject to windage as it passes over the rotor surface, particularly for cases where nonaxisymmetric features such as boltheads are encountered. The ability to accurately predict windage can help reduce the quantity of cooling air required, resulting in increased efficiency. Previous work has shown that the steady computational fluid dynamics solutions can give reasonable predictions for the effects of bolts on disc moment for a rotor–stator cavity with throughflow but flow velocities and disc temperature are not well predicted. Large fluctuations in velocities have been observed experimentally in some cases. Time-dependent computational fluid dynamics simulations reported here bring to light the unsteady nature of the flow. Unsteady Reynolds-averaged Navier–Stokes calculations for 120° and 360° models of the rotor–stator cavity with 9 and 18 bolts were performed in order to better understand the flow physics. Although the rotor–stator cavity with bolts is geometrically steady in the rotating frame of reference, it was found that the bolts generate unsteadiness which creates time-dependent rotating flow features within the cavity. At low throughflow conditions, the unsteady flow significantly increases the average disc temperature.

Performance optimizations for scalable implicit RANS calculations with SU2

In this paper, we present single- and multi-node optimizations of SU2, a widely-used, open-source Computational Fluid Dynamics application, aimed at improving performance and scalability for implicit Reynolds-averaged Navier–Stokes calculations on unstructured grids. Typical industry-standard implementations are currently limited by unstructured accesses, variable degrees of parallelism, as well as the global synchronizations inherent in traditionally used Krylov linear solvers. Therefore, we rely on aggressive single-node optimizations, such as hierarchical parallelism, dynamic threading, compacted memory layout, and vectorization, along with a communication-friendly agglomeration (geometric) linear multigrid solver. Based on results with the well-known ONERA M6 geometry, our single core and shared memory optimizations result in a speedup of 2.6X on the latest 14-core Intel® Xeon™ 11Intel, Xeon, and Intel Xeon Phi are trademarks of Intel Corporation in the U.S. and/or other countries. Software and workloads used in performance tests may have been optimized for performance only on Intel microprocessors. Performance tests, such as SYSmark and MobileMark, are measured using specific computer systems, components, software, operations and functions. Any change to any of those factors may cause the results to vary. You should consult other information and performance tests to assist you in fully evaluating your contemplated purchases, including the performance of that product when combined with other products. For more information go to http://www.intel.com/performance E5-2697v3 processor when compared to the baseline SU2 implementation with 14 MPI ranks. In multi-node settings, the hybrid OpenMP+MPI multigrid implementation achieves 2X higher parallel efficiency on 256 nodes over conventional Krylov-based (GMRES) methods.

Large-Eddy Simulation of a Wing–Body Junction Flow

Large-eddy simulations of a wing-body junction flow experimentally studied are presented. Wall junction flows are common in engineering applications, but the flow physics at the corners of the junction is not well understood. Moreover, in these types of flows, the performance of the subgrid-scale models frequently used for large-eddy simulations is not well characterized. To address these issues, large-eddy simulations of the wing–body junction are performed, with multiple levels of grid resolution. Two-dimensional turbulent profiles are generated every time step and introduced to the inlet plane of the computation domain to mimic the unsteady turbulent boundary layer. A Reynolds-averaged Navier–Stokes calculation is used as a precursor to initialize the flowfield of the large-eddy simulations. Then, large-eddy simulations with the Vreman model is performed and the simulation results are analyzed with respect to three specific goals: compare the computed and measured turbulence statistics in the junction region, investigate the flow physics in the corner region, and discuss the potential reasons for the inaccuracies of the subgrid-scale model. The sensitivity of the predicted results to inflow conditions and to subgrid-scale modeling is also investigated.

Investigation of the Unsteady Flow and Noise Generation in a Slat Cove

This study presents hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations of the unsteady flow and noise-generation phenomena in the slat cove of a high-lift wing profile. These computations are part of a joint numerical/experimental aeroacoustics collaborative program dedicated to slat-flow analysis. A dedicated two-element wing profile (slat plus main body) has been designed to isolate slat noise from other possible sources (e.g., the flap), while minimizing mean flow deflection effects, to improve the fidelity of open-jet wind-tunnel measurements. The design of this two-element airfoil has been performed numerically, using an optimization process based on steady Reynolds-averaged Navier–Stokes calculations. This airfoil has been investigated experimentally at the École Centrale de Lyon open jet facility. Unsteady zonal hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations have been performed to provide a comprehensive description of the unsteady flow inside the slat cove, focusing on the noise-generation processes. A detailed analysis of the physics of the unsteady flow inside the slat cove is presented as well as a comparison of numerical results with available experimental data. The pressure spectra associated with the slat-cove flow are characterized by several tonal peaks emerging from the underlying broadband content. The existence of such peaks is attributed to a feedback loop involving the main shear layer inside the slat cove. A theoretical law is proposed to predict the associated tonal frequencies and assessed at the end of the paper.

Entropy-based detached-eddy simulation of the airwake over a simple frigate shape

The wind past the ship superstructure produces an unsteady turbulent airwake which has a significant effect on aircraft performance and consequently pilot workload during ship landing process. Computational fluid dynamics simulations of a generic simple frigate shape ship airwake have been performed using the entropy-based detached-eddy simulation method. The results were compared with the steady-state Reynolds-averaged Navier–Stokes calculations and the wind tunnel data, indicating the capability of entropy-based detached-eddy simulation to resolve the unsteady large-scale turbulent features. An analysis of the airwake flow topology at headwind condition highlights the vortex pairing process which dominates the flowfield above the deck. Additionally, the influence of hangar-door state (open, closed or half-open) on the airwake was also investigated in detail. Determination of this complex flow can assist in the definition of safe ship–helicopter operating limits and future ship design.

Unsteady Flow Evolution Through a Turning Midturbine Frame Part 1: Time-Resolved Flow

This paper identifies and analyzes the propagation of aerodynamic deterministic stresses through a two-spool counter-rotating transonic facility representative of modern and future turbine aeroengine sections. The test setup consists of a high-pressure stage, a diffusing turning midturbine frame with turning struts, and a counter-rotating low-pressure rotor. The flowfield downstream of the high-pressure stage is strongly influenced by the stator–rotor interaction. Such a mechanism interacts again with the downstream turning midturbine frame leading to a vane–rotor–vane interaction, which affects the behavior of the low-pressure stage. The results presented were obtained using a fast-response aerodynamic pressure probe for unsteady measurements as well as three-dimensional unsteady Reynolds-averaged Navier–Stokes calculations. The work is presented in two parts. This first part focuses on the explanation of the flow physics that governs the convection of unsteady three-dimensional flow through the midturbine duct. Viscous and inviscid mechanisms are discussed as main drivers for the convection of wakes, secondary vortices, and shocks. The flowfield in the duct is characterized by three superimposed effects: 1) duct diffusion and radial pressure gradient together with turning strut potential field, 2) rotor unsteady work source, and 3) vane/blade interaction phenomena. The understanding of these mechanisms will eventually help to control the unsteadiness content in future architectures where reduced engine component length will enhance the interaction effects.

A numerical study on ship-ship interaction in shallow and restricted waterway

In the present study, a numerical prediction method on the hydrodynamic interaction force and moment between two ships in shallow and restricted waterway is presented. Especially, the present study proposes a methodology to overcome the limitation of the two dimensional perturbation method which is related to the moored-passing ship interaction. The validation study was performed and compared with the experiment, firstly. Afterward, in order to propose a methodology in terms with the moored-passing ship interaction, further studies were performed for the moored-passing ship case with a Reynolds Averaged Navier-Stokes (RANS) calculation which is using OpenFOAM with Arbitrary Coupled Mesh Interface (ACMI) technique and compared with the experiment result. Finally, the present study proposes a guide to apply the two dimensional perturbation method to the moored-passing ship interaction. In addition, it presents a possibility that the RANS calculation with ACMI can applied to the ship-ship interaction without using a overset moving grid technique.

Entropy Minimization Design Approach of Supersonic Internal Passages

Fluid machinery operating in the supersonic regime unveil avenues towards more compact technology. However, internal supersonic flows are associated with high aerodynamic and thermal penalties, which usually prevent their practical implementation. Indeed, both shock losses and the limited operational range represent particular challenges to aerodynamic designers that should be taken into account at the initial phase of the design process. This paper presents a design methodology for supersonic passages based on direct evaluations of the velocity field using the method of characteristics and computation of entropy generation across shock waves. This meshless function evaluation tool is then coupled to an optimization scheme, based on evolutionary algorithms that minimize the entropy generation across the supersonic passage. Finally, we assessed the results with 3D Reynolds Averaged Navier Stokes calculations.

Acoustic Sources and Far-Field Noise of Chevron and Round Jets

This paper investigates numerically the acoustic sources and far-field noise of chevron and round jets. The acoustic sources are described by the fourth-order space–time velocity cross correlations, which are calculated based on a large-eddy simulation flowfield. Gaussian functions are found to fit the axial, radial, and azimuthal cross correlations reasonably well. The axial length scales are three to four times the radial and azimuthal length scales. For the chevron jet, the cross-correlation scales vary with azimuthal angle up to six jet diameters downstream; beyond that, they become axisymmetric like those for a round jet. The fourth-order space–time cross correlation of the axial velocity is the dominant source component, and there are considerable contributions from other source components such as , , , , and cross correlations where 1, 2, and 3 represent axial, radial, and azimuthal directions, respectively. For the chevron jet, these cross correlations decay rapidly with axial distance whereas for the round jet, they remain roughly constant over the first 10 jet diameters. The chevron jet intensifies both the and cross correlations within two jet diameters of the jet exit. The amplitude, length, and time scales of the cross-correlations of a large-eddy simulation velocity field are investigated as functions of position and are found to be proportional to the turbulence amplitude, length, and time scales that are determined from a Reynolds-averaged Navier–Stokes calculation. The constants of proportionality are found to be independent of position within the jet, and they are quite close for chevron and round jets. The scales derived from Reynolds-averaged Navier–Stokes are used for source description, and an acoustic analogy is used for sound propagation. There is an excellent agreement between the far-field noise predictions and measurements. At low frequencies, the chevron nozzle significantly reduces the far-field noise by 5–6 dB at 30 deg and 2–3 dB at 90 deg to the jet axis. However, the chevron nozzle slightly increases high-frequency noise. It was found that and cross correlations have the largest contribution to the jet noise at 30 deg to the jet axis, whereas the cross correlation has the largest contribution to the jet noise at 90 deg to the jet axis. The Reynolds-averaged Navier–Stokes calculations are repeated with different turbulence models, and the noise prediction is found to be almost insensitive to the turbulence model. The results indicate that the modeling approach is capable of assessing advanced noise-reduction concepts.

Evaluation of Vortex Generators for Separation Control in a Transcritical Cylinder Flow

A numerical study is presented, aimed at evaluating the capability of vortex generators to produce a significant delay of separation and drag reduction in flows past smooth bluff bodies in the transcritical flow regime, with turbulent boundary layers ahead of separation. A fairly large series of steady Reynolds-averaged Navier–Stokes computations is carried out of the incompressible flow past a circular cylinder, with a simple and generic geometry, with and without vortex generators. Their effect is emulated with a body force, applied in a refined-grid region, instead of gridding the actual geometry. The flow is treated as steady, for reasons that are discussed. The parametric study includes 10 different vortex generator designs, as well as varying positions, spacings, and heights. Drag reductions of up to 60% are obtained, including those with sub-boundary-layer designs, with a height equal to half the boundary-layer thickness. Design recommendations are given, with the tolerance to vortex shedding and other variations in the flow conditions taken into account.

Improving cooling effectiveness by use of chamfers on the top of electronic components

A Computational Fluid Dynamic (CFD) study based on Reynolds Averaged Navier–Stokes (RANS) approach is carried out to predict the mean velocity field and the heat transfer rate of an impinging jet in cross-flow configuration on a heated wall-mounted cube. Targeting an electronic cooling configuration, the aim is to investigate the effect of geometrical modification of the component on the cooling effectiveness. For the same cross flow Reynolds number ReH=3410, three levels of impinging jets are computed as well as a case without impinging jet that will serve as baseline case for comparison. The results from the RANS computation are compared to experimental data from published scientific literature. The validation shows qualitatively good agreement and almost all flow structures are well reproduced by the computation. In an attempt to optimize the wall heat flux over the cube surface, a new geometry is proposed without sharp corners on the top cube face. Numerical results show that with minor geometrical modification (chamfer), the fluid flow structure around the electronic component is radically transformed and the heat transfer rate can be improved. The highest cooling effectiveness improvement is realize for the highest Reynolds number ratio Rej/ReH=1.5 and for the chamfer height of 4mm.

Full-scale unsteady RANS CFD simulations of ship behaviour and performance in head seas due to slow steaming

It is critical to be able to estimate a ship׳s response to waves, since the resulting added resistance and loss of speed may cause delays or course alterations, with consequent financial repercussions. Slow steaming has recently become a popular approach for commercial vessels, as a way of reducing fuel consumption, and therefore operating costs, in the current economic and regulatory climate. Traditional methods for the study of ship motions are based on potential flow theory and cannot incorporate viscous effects. Fortunately, unsteady Reynolds-Averaged Navier–Stokes computations are capable of incorporating both viscous and rotational effects in the flow and free surface waves. The key objective of this study is to perform a fully nonlinear unsteady RANS simulation to predict the ship motions and added resistance of a full scale KRISO Container Ship model, and to estimate the increase in effective power and fuel consumption due to its operation in waves. The analyses are performed at design and slow steaming speeds, covering a range of regular head waves, using a commercial RANS solver. The results are validated against available experimental data and are found to be in good agreement with the experiments. Also, the results are compared to those from potential theory.