External Aerodynamics Research Articles

Extreme shockwave systems in problems of external supersonic aerodynamics

The stationary shockwave systems (the sequences of shocks, isentropic expansion and compression waves), which arise at a planar supersonic flow of perfect inviscid gas around the bodies are investigated theoretically. The domains of the existence of shockwave systems under consideration are found analytically and numerically for the model problems of supersonic aerodynamics (the flow around a single plate, the plate with the frontal shield, polygonal profiles), the parameters of systems are determined, which provide the extrema of the force and thermal loadings as well as of the aerodynamic coefficients of streamlined bodies.

Modelling of natural and bypass transition in aerodynamics

Most of transition models are proposed for modelling of the bypass transition common in the internal aerodynamics especially in turbomachinery where free stream turbulence is the dominant parameter affecting the transition onset. Free-stream turbulence level in the external aerodynamics is usually noticeably lower and so the natural transition often occurs in flows around airfoils. The transition model with the algebraic equation for the intermittency coefficient proposed originally by Straka and P�ihoda (3) for the bypass transition was modified for modelling of the transition at low free-stream turbulence. The modification is carried out using experimental data of Schubauer and Skramstad (18). Further, the three-equation k-kL- model proposed by Walters and Cokljat (10) was used for the modelling of the transition at low free-stream turbulence. Both models were tested by means of the incompressible flow around airfoils at moderate and very low free-stream turbulence.

External Aerodynamics Simulations in a Rotating Frame of Reference

This paper presents the development of a tool integrated in the UNS3D code, proprietary of Alenia Aermacchi, for the simulation of external aerodynamic flow in a rotating reference frame, with the main objective of predicting propeller-aircraft integration effects. The equations in a rotating frame of reference have been formulated in terms of the absolute velocity components; in this way, the artificial dissipation needed for convergence is lessened, as the Coriolis source term is only introduced in the momentum equation. An Explicit Algebraic Reynolds Stress turbulence model is used. The first assessment of effectiveness of this method is made computing stability derivatives of a NACA 0012 airfoil. Finally, steady Navier-Stokes and Euler simulations of a four-blade single-rotating propeller are presented, demonstrating the efficiency of the chosen approach in terms of computational cost.

Numerical experiments modelling turbulent flows

The work aims at investigation of the possibilities of modelling transonic flows mainly in external aerodynamics. New results are presented and compared with reference data and previously achieved results. For the turbulent flow simulations two modifications of the basic k − ω model are employed: SST and TNT. The numerical solution was achieved by using the MacCormack scheme on structured non-ortogonal grids. Artificial dissipation was added to improve the numerical stability.

Interference effects of cooling airflows on a generic car body

A significant contribution to the aerodynamics of road vehicles is caused by the cooling airflow. The cooling air flows through the engine compartment. Internal losses occur through the radiator and through the engine compartment. Additionally interference effects occur between the flow through the engine compartment and the flow field around the car. For most cars, all these effects result in additional aerodynamic drag, which is caused by the cooling airflow.In this study, numerical and experimental investigations were performed to understand the effects of cooling-air flow on the vehicle's aerodynamics. A generic car-model was equipped with a simplified underhood section. The cooling air mass flow could be varied to understand the cooling air mass flow on the cooling drag. A variety of measurements and numerical simulations were performed to explain the phenomenona of cooling air flows. Total pressure and hot-wire measurements in the wake of the car visualise the flow field, which is altered by the cooling air. Force measurements, surface pressure measurements and oil-streak patterns were performed to understand the influence of the cooling air on the bluff body. With the measurements and with the CFD-simulations, this study will explain where the cooling drag originates from and how the cooling air interferes with the external aerodynamics.

The continuous adjoint method as a guide for the design of flow control systems based on jets

PurposeThe purpose of this paper is to propose the use of the continuous adjoint method as a tool to identify the appropriate location and “type” (suction or blowing) of steady jets used in active flow control systems.Design/methodology/approachThe method is based on continuous adjoint and covers both internal and external aerodynamics. The adjoint equations, including the adjoint to the SpalartAllmaras turbulence model and their boundary conditions are formulated. At the cost of solving the flow and adjoint equations just once, the sensitivity derivatives of the objective function with respect to hypothetical (normal) jet velocities at all wall nodes are computed. Comparisons of the computed sensitivities with finite differences and parametric studies to assess the present method are included.FindingsThough the sensitivities are computed for zero jet velocities, they adequately support decision making on: the recommended location of jet(s), at boundary nodes with high absolute valued sensitivities; and the selection between suction or blowing jets, based on the sign of the computed sensitivities. Regarding adjoint methods, two important findings of this work are: the role of the adjoint pressure which proves to be an excellent sensor in flow control problems; and the prediction accuracy of the proposed adjoint method compared to the commonly made assumption of “frozen turbulence”.Originality/valueFirst use of the continuous adjoint method using full differentiation of the turbulence model, in flow control optimization. A low‐cost design tool for recommending some of the most important jet characteristics.

The application of porous media to simulate the upstream effects of gas turbine injector swirl vanes

Numerical methods are invaluable in the initial phase of a project for evaluating potential design solutions. The design of aircraft gas turbine combustor external aerodynamics is one example where such methodology is applied. The system is geometrically complex and requires a high fidelity mesh to resolve small geometric features, such as the injector swirl vanes. Typically the injector alone accounts for approximately 20% of the mesh cells of the entire domain. This makes it expensive to model in terms of both requisite computational resource and run time. This paper presents a numerical method for replacing the injector swirl vane passages with mathematically porous volumes which replicate the required pressure drop. This is preferential to modelling the fully featured injector as it allows a significant reduction in the cell count. This in turn allows significant time savings that serve to expedite the initial design of the combustor external aerodynamics. It also makes the geometry easier to change, scale and re-mesh during development. This method has been rigorously tested through a range of approach conditions to ensure that it is robust enough for use in the design process. The loss in accuracy owing to the simplification has been demonstrated to be less than 4.4%, for all tested flowfields. This error is dependent on the flow conditions and is generally much less for passages fed with representative levels of upstream distortion.

Deterministic unsteady and aeroelastic flow simulations with high-order FVM schemes

During the last few years, unsteady and aeroelastic simulations have become a standard part of CFD computations in aeronautical applications. They are usually used with various types of turbulent models the most of which are derived from system of the Reynolds-Averaged Navier–Stokes equations (RANS). However, the RANS system itself is derived using time-averaging and hence it neglects certain type of unsteadiness in its own nature. Therefore it is necessary to investigate what is the ability of the contemporary numerical methods (i.e. combinations of numerical schemes and turbulence models) to simulate various types of unsteady flows. The authors carried out a series of numerical tests of two types of unsteady flows in external aerodynamics. Namely the transonic compressible flow over harmonically oscillating NACA 0012 aerofoil and subsonic compressible flow over the same aerofoil with oscillations induced by the flow itself (i.e. considering the aeroelastic interaction). Three different high-order finite volume schemes with various contemporary turbulence models were chosen for the testing. In the first researched case the computations were carried out using: the Modified Causon’s scheme “as reported by Furmanek and Kozel (Kybernetika 45(4):567–579, 2009)” (derived from TVD form of the classical MacCormack scheme) and the solver edge developed by the Swedish defence and research agency FOI (n-stage Runge–Kutta scheme, central differences with Jameson’s artificial dissipation, geometric multigrid, implicit residual smoothing). The tested models of turbulence were: the one-equation Spalart–Allmaras model, the two-equation Kok’s TNT model and the EARSM model. Unfortunately, none of these models produced results able to serve as a reliable numerical simulation of the first researched case. The main cause is probably the interaction between the turbulent boundary layer and shock-wave formed on the top side of the aerofoil. For the purpose of further investigation the Modified Causon’s scheme was adapted for modelling aeroelastic interaction between aerofoil and flow with one degree of freedom. Results of this simulation correspond better to the real flow, but it is necessary to carry out another tests to find out what is the exact cause of the observed problems.

Numerical experiments for turbulent flows

The aim of the work is to explore the possibilities of modelling transonic flows in the internal and external aerodynamics. Several configurations were analyzed and calculations were performed using both inviscid and viscous models of flow. Viscous turbulent flows have been simulated using either zero equation algebraic Baldwin-Lomax model and two equation k—ω model in its basic version and improved TNT variant. The numerical solution was obtained using Lax-Wendroff scheme in the MacCormack form on structured non-ortogonal grids. Artificial dissipation was added to improve the numerical stability. Achieved results are compared with experimental data.

Valves with flow control by synthetic jets

Synthetic jets are currently in the focus of attention as means for flow control in external aerodynamics. Less known and studied is their use in internal aerodynamics, where several existing papers discussed separation control in diffusers. The subject of this paper, however, is a completely new idea — using the synthetic jet in flow control valves having no moving components. The valve considered would operate as a bistable diverter: by the action of the synthetic jet, the controlled fluid is deflected into one of available two flowpaths. A preliminary experiment performed with an axisymmetric valve version demonstrated viability of the idea.

Recent improvements in the Zonal Detached Eddy Simulation (ZDES) formulation

An efficient generalized Zonal Detached Eddy Simulation method (ZDES) is presented, which aims at performing hybrid Reynolds Averaged Numerical Simulation (RANS)/Large Eddy Simulation (LES) calculations for both internal and external aerodynamics problems. It is based on a zonal formulation of the hybrid length scale that allows to combine the zonal approach with the best features of Delayed Detached Eddy Simulation (DDES) (Spalart et al. Theor Comput Fluid Dyn 20:181–195, 2006). In other words, the presumed weak point of a zonal approach, namely that the location of separation has to be known in advance, is now overcome. What is more, the problem of slow LES content development in mixing layers when they are treated neither in RANS nor in LES mode is investigated. It is argued that the subgrid length scale Δmax = max(Δx, Δy, Δz) entering DDES is physically justified to shield the boundary layer but is definitely not a good subgrid length scale in LES mode. Remedies are proposed based on new zonal subgrid length scales that depend not only on the grid spacing but also on the flow solution and especially on the local vorticity vector. The method is validated on a spatially developing mixing layer as well as in a backward facing step flow and then applied to a three-element airfoil. It is argued in this latter case that a precise control of the RANS mode thanks to a zonal approach is essential. More generally, in all simulated cases in this study, ZDES has proven to be very efficient as regards the behavior in LES mode while retaining the strongest asset of DDES, namely the treatment of the attached boundary layer in RANS mode. The issue of zonal or non-zonal treatment of turbulent flows is also briefly discussed.

Validating the URANS shear stress transport γ −Reθ model for low‐Reynolds‐number external aerodynamics

SUMMARYA numerical investigation on low‐Reynolds‐number external aerodynamics was conducted using the transitional unsteady Reynolds‐averaged Navier–Stokes shear stress transport γ −Reθ model and the ANSYS‐CFX computational fluid dynamics suite. The NACA 0012 airfoil was exposed to chord‐based Reynolds numbers of 5.0 ×104, 1.0 ×105 and 2.5 ×105 at 0°, 4°and 8°angles of attack. Time‐averaged and instantaneous flow features were extracted and compared with fully turbulent shear stress transport results, XFLR5 panel e N method results, and published higher order numerical and experimental studies. The current model was shown to reproduce the complex flow phenomena, including the laminar separation bubble dynamics and aerodynamic performance, with a very good degree of accuracy. The sensitivity of the model to domain size, grid resolution and quality, timestepping scheme, and free‐stream turbulence intensity was also presented. In view of the results obtained, the proposed model is deemed appropriate for modelling low‐Reynolds‐number external aerodynamics and provides a framework for future studies for the better understanding of this complex flow regime. Copyright © 2011 John Wiley & Sons, Ltd.

Application of strand meshes to complex aerodynamic flow fields

We explore a new approach for viscous computational fluid dynamics calculations for external aerodynamics around geometrically complex bodies that incorporates nearly automatic mesh generation and efficient flow solution methods. A prismatic-like grid using “strands” is grown a short distance from the body surface to capture the viscous boundary layer, and adaptive Cartesian grids are used throughout the rest of the domain. The approach presents several advantages over established methods: nearly automatic grid generation from triangular or quadrilateral surface tessellations, very low memory overhead, automatic mesh adaptivity for time-dependent problems, and fast and efficient solvers from structured data in both the strand and Cartesian grids.The approach is evaluated for complex geometries and flow fields. We investigate the effects of strand length and strand vector smoothing to understand the effects on computed solutions. Results of three applications using the strand-adaptive Cartesian approach are given, including a NACA wing, isolated V-22 (TRAM) rotor in hover, and the DLR-F6 wing-body transport. The results from these cases show that the strand approach can successfully resolve near-body and off-body features as well as or better than established methods.

Interference Effects of Cooling Air-Flows with External Aerodynamics

Interference Effects of Cooling Air-Flows with External Aerodynamics

Evaluation of Aeroelastic Uncertainty Analysis Methods

DOI: 10.2514/1.47118 Flutter is a destructive and potentially explosive phenomenon that is the result of the simultaneous interaction of aerodynamic, elastic, and inertial forces. The nature of flutter mandates that flutter flight testing be cautious and conservative. Because of this, further investigation of uncertainty analysis with respect to the flutter problem is desiredandwarranted.Predictionof flutterinthetransonicregimerequirescomputationallyexpensivehigh-fidelity simulation models. Because of the computational demands, traditional uncertainty analysis is not often applied to transonic flutter prediction, resulting in reduced confidence in the results. The work described herein is aimed at exploring various methods to reduce the existing computational time limitations of traditional uncertainty analysis. Specifically,thecouplingofdesignofexperimentandresponsesurfacemethodsandtheapplicationofanalysisare appliedtoavalidatedaeroelasticmodeloftheAGARD445.6wing.Fromahigh-fidelitynonlinearaeroelasticmodel, a linear reduced-order model is produced that captures the essential dynamic characteristics. Using reduced-order models, the design of experiment, response surface methods, and -analysis approaches are compared with traditional Monte Carlo-based stochastic simulation. All of these approaches to uncertainty analysis have advantages and drawbacks. Results from these methods and their robustness are compared and evaluated. HE potentially explosive nature of flutter mandates that flutter flight testing be cautious and conservative. The development of a flutterstabilityparameterwasundertakenfourdecadesagotogivea more reliable technique to predict the onset of flutter. Other methods of predicting flutter such as damping versus velocity are known to have a number of shortcomings, one of which can be the sudden degradationindampingasillustratedintheexplosive flutterexample shown in Zimmerman and Weissenburger [1]. In this classic paper, the flutter stability parameter was developed using the classical two-dimensional, two-degree-of-freedom (pitch/plunge) aeroelastic wing model, and the technique was shown to be applicable for higher-degree-of-freedom analysis. The flutter stability parameter (flutter margin) is calculated using the decay rate and damped frequency. Traditionally, computational aeroelasticity for loads and flutter prediction combines a linear finite element formulation for the structure with linear aerodynamic methods. At the same time, the prediction of aerodynamic performance and control surface effectivenessaccountsfortheeffectsofthestructuralelasticdeformations on the external aerodynamics by means of correction factors applied to results obtained when the aircraft is assumed to be rigid. Both practices are well-established in aircraft design and give accurate, reliable, and rather inexpensive predictions for static and dynamic

Characterization of Aerospace Vehicle Performance and Mission Analysis Using Thermodynamic Availability

The fundamental relationship between entropy and aerospace vehicle and mission performance is analyzed in terms of the general availability rate balance between force-based vehicle performance, available energy associated with expended propellant, and the overall loss rate of availability, including the vehicle wake. The availability relationship for a vehicle is analytically combined with the vehicle equations of motion; this combination yields the balance between on-board energy rate usage and rates of changes in kinetic and potential energies of the vehicle and overall rate of entropy production. This result is then integrated over time for a general aerospace mission; as examples, simplified single-stage-to-orbit rocket-powered and air-breathing missions are analyzed. Examination of rate of availability loss for the general case of an accelerating, climbing aerospace vehicle provides a powerful loss superposition principle in terms of the separate evaluation and combination of loss rates for the same vehicle in cruise, acceleration, and climb. Rate of availability losses is also examined in terms of separable losses associated with the propulsion system and external aerodynamics. These loss terms are cast in terms of conventional parameters such as drag coefficient and engine specific impulse. Finally, rate losses in availability for classes of vehicles are described.

Experimental Study of Shock Oscillation over a Transonic Supercritical Profile

Shock wave/turbulent boundary-layer interaction and flow separation may induce self-sustained large-scale oscillations on a profile at transonic Mach number. This phenomenon, known as transonic buffet, is at the origin of intense pressure fluctuations which can have detrimental effects, both in external and internal aerodynamics. The present paper describes a new experiment executed in the ONERA S3Ch transonic wind tunnel on shock oscillations over the OAT15A supercritical profile. These experiments have allowed the precise definition of the conditions for buffet onset and the characterization of the properties of the periodic motion from unsteady surface pressure measurements. The flowfield behavior has been described in great detail thanks to high-speed schlieren cinematography and surveys with a two-component laser Doppler velocimetry along with a conditional sampling technique. The first aim of this study was to provide the computational fluid dynamics community with well-documented test cases to validate advanced computing methods. Concerning the physics of the phenomenon, it is suggested that it is mediated by acoustic waves which are produced at the trailing edge and which travel on the two sides of the airfoil. Also, the experimental results strongly suggest that the phenomenon is essentially two-dimensional, even if three-dimensional effects are also detected.

The Influence of Dump Gap on External Combustor Aerodynamics at High Fuel Injector Flow Rates

The increasing demand to reduce fuel burn, hence CO2 emissions, from the gas turbine requires efficient diffusion to reduce the system pressure loss in the combustor. However, interactions between prediffuser and combustor can have a significant effect on diffuser performance. For example, the consequence of increased fuel injector flow at a dump gap set using conventional design guidelines has been shown (Walker, A. D., Carrotte, J. F., and McGuirk, J. J., 2007. “Compressor∕Diffuser∕Combustor Aerodynamic Interactions in Lean Module Combustors,” ASME Turbo Expo 2007—Power for Land Sea and Air, Paper No. GT2007-27872) to introduce a destabilizing interaction between fuel injector and upstream components. The present paper concentrates on examining the effects of increased dump gap. Dump gap ratios of 0.8, 1.2, and 1.6 were employed, with each test utilizing the same inlet guide vane, compressor rotor, integrated outlet guide vane (OGV)∕prediffuser, and dump geometry. The flow fraction of compressor efflux entering the combustor cowl was set to be representative of lean combustors (50–70%). Measurements were made on a fully annular rig using a generic flame tube with metered cowl and inner∕outer annulus flows. The results demonstrate that, with fixed cowl flow, as the dump gap increases, component interactions decrease. At a dump gap ratio of 0.8, the proximity of the flame tube influences the prediffuser providing a beneficial blockage effect. However, if increased to 1.2, this beneficial effect is weakened and the prediffuser flow deteriorates. With further increase to 1.6, the prediffuser shows strong evidence of separation. Hence, at the dump gaps probably required for lean module injectors, it is unlikely the prediffuser will be influenced beneficially by the flame tube blockage; this must be taken into account in the design. Furthermore, with small dump gaps and high cowl flow fraction, the circumferential variation in cowl flow can feed upstream and cause OGV∕rotor forcing. At larger dump gaps, the circumferential variation does not penetrate upstream to the OGV, and the rotor is unaffected. The optimum dump gap and prediffuser design for best overall aerodynamic system performance from rotor through to feed annuli is a compromise between taking maximum advantage of upstream blockage effects and minimizing any 3D upstream forcing.

Vorticity Dynamics in Axial Compressor Flow Diagnosis and Design

It is well recognized that vorticity and vortical structures appear inevitably in viscous compressor flows and have strong influence on the compressor performance. However, conventional analysis and design procedure cannot pinpoint the quantitative contribution of each individual vortical structure to the integrated performance of a compressor, such as the stagnation-pressure ratio and efficiency. We fill this gap by using the so-called derivative-moment transformation, which has been successfully applied to external aerodynamics. We show that the compressor performance is mainly controlled by the radial distribution of azimuthal vorticity, of which an optimization in the through-flow design stage leads to a simple Abel equation of the second kind. The satisfaction of the equation yields desired circulation distribution that optimizes the blade geometry. The advantage of this new procedure is demonstrated by numerical examples, including the posterior performance check by 3D Navier–Stokes simulation.

Combination of body‐fitted and embedded grids for external vehicle aerodynamics

PurposeThis paper seeks to reduce the time it takes to perform external aerodynamic simulations without compromising accuracy. At present, cleaning up CAD data sets, in particular for undercarriage parts, takes several man‐weeks.Design/methodology/approachBody‐fitted and embedded mesh techniques are combined to obtain accurate external aerodynamic solutions for realistic car geometries with minimal user intervention. The key idea is to mesh with typical body‐fitted RANS grids the external shape of the vehicle, which is smooth and requires detailed physical modeling. The underhood and undercarriage are treated as embedded surfaces. The flow in this region is massively separated, requiring LES runs and isotropic grids. This makes it a suitable candidate for embedded grids.FindingsComparisons with body‐fitted and experimental data for a typical car show that this approach can yield drag predictions with an error less than 5 percent.Practical implicationsThe present approach reduces turnaround times for complete car geometries to one to two days, without compromising accuracy.Originality/valueTo the authors' knowledge, this is the first time such an approach has been tried and validated for external aerodynamics.