CEAS Aerodynamics research conference

Citation

(2004), "CEAS Aerodynamics research conference", Aircraft Engineering and Aerospace Technology, Vol. 76 No. 1. https://doi.org/10.1108/aeat.2004.12776aac.001

CEAS Aerodynamics research conference

CEAS Aerodynamics research conference

Keywords: Aerodynamics, Conference, Military

Held at the Royal Aeronautical Society this conference addressed areas of aerodynamics research across a broad front, but with a particular emphasis on topics having a relevance to military applications. Extending across 3 days and including parallel sessions, the first day's proceedings were largely taken up with military requirements and issues of unmanned combat air vehicles (UCAVs) and weapons as well as Novel Configurations. Many different ideas for UAVs have been proposed and although most propose no unusual aerodynamic stability and control problems, one class – that of combat UAVs, have some novel characteristics. Typical configurations can feature swept and tapered wings, cranked wings, blended wing body, no fin, no tailplane, and novel control surfaces. Thrust vectoring is seen by some to be the ideal yaw-control solution for finless aircraft.

Weapon aerodynamic requirements came in for some personal thoughts including a brief survey of the major weapon sub-systems that interact with overall weapon configurations and in consequence, with aerodynamics, including the carriage platform, propulsion, guidance and control sub-system plus the configuration features. Future systems are thought to be smaller, faster and have more integrated shapes. Particularly interesting was a paper on the impact of naval environment on the performance and operation of UAVs, from the US Naval Air Systems Command. These vehicles require additional refinements compared to land-based vehicles to perform without undue performance penalty, including the vital considerations of launch and recovery in the harsh and unforgiving naval sphere of operations.

Vortical flows were the subject of several papers. With particular reference to UAVs, separated and vortical flows are dominant. The issues include layer instabilities, vortex breakdown and wing stall, vortex interactions, non-slender vortices, multiple vortices, manoeuvring vortices and various interactions. A contribution on the computation of vortical flows noted that the vorticity in the flowfield and the wake of swept wings at an incidence and rotor blades largely determined the distribution of loading. Accurate determination of vortex-dominated flow is important, therefore, in many aeronautical applications. Lift loss due to vortex breakdown was also dealt with and a paper presented a simple physical model of a burst delta-wing leading-edge vortex, giving predictions of both the magnitude and rate of onset of burst-induced losses that match experimental data. Further work is necessary.

Semi-empirical prediction methods for weapons were also considered with date presented for the analysis of several missile configurations. Canard-body-tail configurations were analysed in two ways. Centre of pressure prediction of multi-segment wings was considered – only wings with breaks in the leading-edge sweep, the method transforming a multi-segment planform into an equivalent straight-tapered planform. A new version of the Aeroprediction Code (AP02) was also noted, which has been developed to address the requirements relating to advanced weapon concepts. The new code is an improvement on AP98.

The session devoted to novel configurations consisted of three papers, one from the USA dealing with high aspect ratio unconventional joined wing configurations incorporating laminar flow with tip effects. A joined-wing layout has been considered for high speed design. Interesting features have emerged from the application of direct and inverse design methods. Further work is envisaged in a number of fields including lower speeds and field performance, different aerofoils, pitching moment, fuselage and intake performance, drag prediction, etc.

From Italy came Prandtl box wing layout application to UAV configuration and computational data comparison which focused on the comparison between the experimental and computational data relevant to high subsonic UAV configurations based on the Prandtl box wing layout. Experimental results show that joining the two wings does not reduce the amount of induced drag of the box wing with respect to the relevant biplane configuration. The larger the vertical spacing the larger is the reduction in induced drag.

From QinetiQ UK came the synthesis, design and assessment of air vehicle concepts. In recent years, the performance of air vehicles has been studied, operational requirements have been evolving, increased flexibility has been considered, and the capability to produce accurate performance data for novel air vehicle concepts, including those that are not adequately represented by the existing semi-empirical methods and databases. This capability is now essentially generic in nature and the process provides a firm basis for assessment band conceptual change in the design of future air vehicles.

Test and simulation

The first paper in this section dealt with flight test techniques to evaluate performance benefits during formation flights. The Autonomous Formation Flight research project has been implemented at the NSA Dryden Flight Research Center to demonstrate the benefits of formation flight and develop advanced technologies to facilitate these benefits. Two F/A-18 aircraft have been modified to precisely control and monitor relative position and to determine performance of the trailing aircraft. Flight test manoeuvres and analysis techniques have been developed to determine the performance advantages, including drag and fuel flow reductions and improvements in range. By flying the trailing aircraft through a matrix of lateral, longitudinal and vertical offset positions, a detailed map of the performance benefits has been obtained in two flight conditions. Significant performance benefits have been obtained during this flight test phase. Drag reductions of more than 20 per cent and fuel flow reductions of more than 18 per cent have been measured in flight conditions of Mach 0.56 and an altitude of 25,000 ft. The results show favourable agreement with unpublished theory and generic predictions. An F/A-18 long range mission at Mach 0.8 and an altitude of 409,000 ft has been simulated in the optimum formation pattern and has demonstrated a 14 per cent fuel reduction when compared with the controlled aircraft of similar configuration.

From Canada came a dissertation on “From water tunnel to aircraft post-stall flight simulation” which addressed the deficiencies of an existing F/A-18 flight simulation model in which a post-stall analysis was undertaken supported by non-planar water tunnel experiments. The losses of scaling and the effects of the dynamic test environment were confronted. The water tunnel high-alpha data obtained over a range of side angles including rotary and static forced moments, engine inlet flow effects and flow visualisation data were compared with other sources. The domain of applicability of the water tunnel data was determined. It was shown that even though neither the Reynolds number nor the Mach number was simulated, the dynamic water tunnel data can qualitatively be the representative of full scale F/A-128 high alpha aerodynamics in this flight regime. The introduction of nonplanar parameters allowed the consolidation of the dynamic database including inlet flow effects.

A self-organising hybrid Cartesian grid solution system with multigrid was discussed in a German paper. An automatic adaptive hybrid Cartesian generation and solution system was presented together with application for turbulent flow around complicated three- dimensional geometries. As an example, the Reynolds-average Navier-Stokes equations are solved for the generic F-15 fighter aircraft. This demonstrates the application of the hybrid Cartesian grid approach to highly complex geometries. This configuration includes an inlet and an exhaust of the engine. The simulation was performed with freestream conditions at Mach 0.70, zero angle of attack and Reynolds number of 10 × 10.

Flow control and high lift

Two sessions concerned laminar flow and high lift for civil transports. Laminar flow presented two papers, both concerned with the DLR DoI 228 aircraft, one dealing with HYLTEC laminar flow systems flight tests, system layout and instrumentation on the Do 228. In this contribution, a detailed description is given of the suction system and layout including the jet-pump, ice and contamination protection via foam fluid and competing thermal hot air ice protection combined with a Krueger slat for contamination protection with high lift abilities. For application to commercial aircraft, robust laminar flow systems and structural concepts need to be developed. The controlling and data acquisition processes were distributed to three computer systems due to the complexity of the control system including the large number of sensors for temperatures, pressures and flows values and the high requirements for security and stability. The challenge to the development of the software was to configure the system in spite of the complexity and the large number of involved signals and parameters in a manner that the system works with a high level of reliability and security. Simplified systems for commercial use are currently being developed.

The next paper, also on this aircraft, dealt with flight tests of de-icing and anti- contamination systems. Much effort has gone into hybrid laminar flow (HLF) technology, but relatively little flight experience has been gained with hardware and systems that will be required to protect the wing surface against ice accretion and insect contamination. Hence, a new leading edge box has been developed for the wing of the Do 228, equipped with a laser-drilled suction surface in combination with different test panels for bleed air anti-icing, shield protection fluidic anti-contamination, fluidic anti-icing and a reference area. Suction pressure is provided by a bleed-driven jet pump. The HYLTEC flight test ended in 2002 and clearly demonstrated that protection systems against icing and insect contamination can be successfully integrated into boundary layer suction system.

Propulsion integration and high lift

These parallel sessions were characterised in the first paper of the initial session from Dstl, Farnborough, which dealt with the design of intakes for subsonic UAVS. It showed how new design freedom together with new constraints such as a requirement for low observability have resulted in a trend towards highly integrated air intake designs. Traditional figures of merit for intake performance will not necessarily be adequate for the selection of such designs. Idealised diverterless and pitot intakes are compared with a simple model that reveals some expectations regarding their relative performance. Practical design considerations and some research issues were addressed. His conclusions were that new freedoms are afforded in the design of intakes, for unmanned vehicles but many new design challenges are presented and there is plenty of scope for novelty.

A numerical study of asymmetric mixer- ejector noised suppression nozzles, from Russia, concerned the development of an exhaust system for future transport aircraft. This is closely connected with solving two main problems: nozzle thrust losing and jet noise reduction. One of the two ways to solve the problem is using ejector nozzles in which the noise suppression effect is partly achieved by high speed jet fragmentation. Experimental investigation of thrust and acoustic characteristics of noise suppression nozzles is rather difficult because some results could hardly be trusted due to insufficient information about local flow parameters in the mixing zone. Experimental data are reasonably comparable with CFD data and maximum discrepancy between the numerical and tunnel data is about 1 per cent of the nozzle isotropic gross thrust. Further optimisation of nozzle configurations is possible.

From Bombardier Aerospace-Shorts came a paper on optimisation of thrust reverser cascade performance using aerodynamic and structural integration. It focused on the design of a cascade with a cold stream thrust reverser. Methodologies for a weight reduction process are established using CFD and finite element analysis (FEA). Aerodynamic and structural simulations were carried out using realistic operational conditions for three different design configurations with a view to minimise the aerodynamics and structural performance. At last it was significantly improved for both of the reduced weight designs with reductions in both maximum vandisplacement and mid-vane stress loads. Optimum structural performance was achieved with 10 per cent weight reduction.

High lift (civil transports)

This session consisted of three papers, the first from the University of Manchester containing a review of current leading edge device technology and of options for flow control. It notes that an innovative step change is required to improve or replace the current mechanically deployed leading edge high lift systems for civil transport aircraft. The paper provides a review of such technologies and possible future ideas. There is an emphasis on those flow control technologies that will be sufficiently mature enough for implementation within the next decade. The techniques are with regard to the practical implications of a design. After significant landmarks, the Boeing 777 high lift aerodynamic design is outlined as a notable development, with the use of CFD codes prominent. Flow control techniques are described. Blowing is given extensive treatment. Air vortex generation and acoustic generation are described.

From ONERA came a paper on CFD methods for transport aircraft high lift systems in which major results and findings of the numerical work package of the European high lift programme EUROLIFT are outlined. The main objective is to validate and test numerical methods for the prediction of high lift flows for transport aircraft configurations. The activities comprise the assessment of current CFD methods for 3D flows, evaluation of the means for code improvements and transient predictions. A general capability to predict maximum high lift on a simplified wing/fuselage high lift configuration is demonstrated by a variety of numerical approaches. Major shortcomings are in general: the reliable and exact simulation of large separation areas and the turn-around time to computer 3D lift polars. Advanced tunnel modelling and features such as the preconditioning technique show a potential to overcome these deficiencies. Promising results with respect to transition prediction are obtained on a swept, high lift wing using a database method. The results obtained in the numerical activities represent major ingredients for a consistent numerical approach for the flight simulation of transport aircraft high lift configurations including all maximum lift determining effects.

Airbus Germany, used advanced measuring techniques in high lift validation experiments, and described industrial measurement methods used in two European research projects, EUROLIFT and EUROPIV. At first the standard tools of force, moment, pressure distribution measurements, lift visualisation and oil flow visualisation, are described. The focus of the paper is an overview of the advanced measurement methods; hot film technique, particle image velocimetry (PIV), boundary layer rake measurement and infrared measurements. These advanced techniques are necessary to get a detailed view of the flow field around the aircraft, to understand high lift configurations and help to improve them.

Computational fluid mechanics 1 and micro-air-vehicles

The first of these parallel sessions was from the USA on high resolution computational unsteady aerodynamic techniques applied to manoeuvring UCAVs. Also, a method of high- resolution simulation is proposed for UCAVs undergoing manoeuvres at high angles of attack at transonic speed. Motivation for this is the need to develop such a method is first presented to show payoff in the design cycle, followed by the results of using the method on a current manned fighter aircraft. Finally, a notional UCA shape from Boeing Military Aircraft is presented to show the ability of the method to accurately capture the relevant performance in these difficult flight regimes.

Glasgow University presented papers on the numerical study of the flow field around the tip of a square wing and a CFD-study of cavity, follows, as well as a CFD investigation of wind tunnel interferences effects on delta wing aerodynamics.

Micro air vehicles consisted of three papers from UK Universities, one from bath dealing with noting that low aspect ratio wings used for fixed wing micro air vehicles are subject to strong stratospheric turbulence relative to low speed flight. In this investigation, the unsteady aerodynamics of four generic low aspect ratio wings have been studied in a water tunnel, using a grid generator with oscillating flaps. Flow visualisation and forcer measurements have been performed. The conclusions exhibit a variety of separate and vortex dominated flow fields, some of the latter showing unusual features at low Reynolds numbers. The main object was to study the unsteady aerodynamics of such wings.

Aerodynamic modelling of some planforms for insect-like flapping wings was from Cranfield and dealt with the problem of modelling insect-like flapping wings with a unique set of challenges due to the severe manoeuvres and unconventional aerodynamics at low speeds and the presence of leading edge vortex, among other factors. This has been modelled using a number of methods.

AIAA invited session 2

Several papers were presented here on micro air vehicles in the USA, the first on active flow control application on a mini ducted fan UAV which concerned a new kind of UAV investigation based on the flying ducted fan concept. This vehicle uses a mechanically simplified control approach employing synthetic jet active flow control in lieu of moving control surfaces or articulated rotor blades. As a result the UAVs propulsion and control systems are simplified to a single moving part; a fixed pitch propeller. The active control of flow separation over a surface as a means of discrete static aerodynamic control is briefly approached and then explored from the theoretical standpoint.

A paper on analysis and design of rotors concerned design tools developed for ultra low Reynolds numbers rotors, which combine enhanced actuatorring/blade theory with airfoil section data based on two-dimensional Navier-Stokes calculations. This performance prediction method is combined with an optimisation for both design and analysis applications. Performance predictions from these tools have been compared with three- dimensional Navier-Stokes analyses and experimental data for a 2.5 cm diameter rotor with chord Reynolds numbers below 10,000. Comparisons among these analyses and experimental data show reasonable agreement both in the global thrust and power required, but the spanwise distribution of these quantities exhibits significant deviations. It also reveals that three-dimensiomal and rotational effects significantly change local airfoil section performance. These may limit detail design, but are still useful.

A contribution on flexible-wing-based micro air vehicles documents the development and evaluation of such a vehicle technology that reduces adverse effects of gusty wind conditions and unsteady aerodynamics, exhibits desirable flight stability and embraces structural durability. Salient features of the flexible-wing- based UAV include the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment and flight data analysis. The potential includes enhanced agility and reconfiguration for storage.

Store carriage and release and flow control

These parallel sessions began with a paper from Canada on store separation simulation and its application to CTS system design. This presents a graphical programming approach to simulate the release of weapons and stores. The methods at MATLAB and Simulink to calculate store separation trajectories of a range of store/ carriage characteristics are described. The simulation uses ejection, aerodynamic and gravitational forces and moments acting on the store to solve the six degree of freedom Euler equations of motion and predict store trajectories. This technique has been incorporated via the analysis of store separation wind tunnel tests on which grid and/or load measurements provide aerodynamic coefficients of the released store subjected to the interference flow field of the parent aircraft. Two modes of operation of the proposed CTS design are evaluated to permit evaluation.

The second paper came from Europe and concerned separation aspects of new generation stores with autopilots. It dealt with new bombs and missiles capable of hitting their targets with an accuracy never reached before. These advanced weapons include some families of actively controlled stores, the four smart GPS-guided bombs and stand-off missiles. These kind of stores perform real controlled flights before reaching their targets. Complex aerodynamic configurations and behaviour offer significant problems of integration not encountered earlier and extensive work is required.

From the USA, prediction of an aerodynamically dominated store separation was detailed which dealt with the challenge in the future when good analogies do not exist for smaller, lighter bombs. A typical weapon is notable not only for its light weight, but also for its rack of a strong ejection force. Using a torsion spring rather than the rack's built-in ejector piston further enhanced the relative importance of aerodynamic forces, as well as emphasising the accuracy of CFD data more than has been necessary with separation dominated by weight and ejector forces.

Flow Control began with a paper from Manchester University on PIV measurement and visualisation of synthetic jet flow in quiescent conditions. In this paper, the characteristics of synthetic jets in quiescent conditions produced by circular, square and rectangular orifices of different aspect ratios, are investigated. Flow visualisation and PIV measurement provide a quantitative and qualitative insight into jet properties with a variety of orifice shape and diaphragm vibration amplitude. The results show that the jet structure varies significantly with the Reynolds number.

Passive control of jet oscillation by a bump, from Queen's University, Belfast, dealt with the numerical investigation of the Contour Bump concept in transonic periodic flow over a 18 per cent thick biconvex aerofoil and a NASA0012 aerofoil. For both aerofoils, the bump which was located in the vicinity of mean shock position, had a positive influence on drag reduction and buffeting alleviation. For the biconvex aerofoil, the bump is located on the lower surface, whereas for NASA 0012 the bump is positioned on the upper surface underneath the shock. In both the cases, there is also an effort on the frequency of periodic vibration.

Swept and normal shock wave/turbulent boundary layer interaction control with “smart” flap actuators, from Australia, looks at active control of the swept/shock turbulent boundary layer interaction using smart flap actuators. The actuators are manufactured by bonding the piezoelectric material to an inert substrate to control the bleed suction rate through a plenum chamber. Communication of signals is provided across the shock, allowing rapid thickening of the boundary layer approaching the shock, which splits into a series of weaker shocks forming a lambda foot, thus reducing wave drag. Active control allows optimum control of the interaction, as it would be capable of positioning the control region around the original shock position and hence the rate of mass transfer.

Computational fluid dynamics 2 and experimental techniques

These parallel sessions concluded the presentation of papers before the conference overview and podium discussion. The first led off with a paper from Imperial College on the prediction of drag and lift of wings from velocity and vorticity fields. This continues the work outlined earlier and has closed-form expressions for further evaluation of forces on a body in the flows derived in the earlier paper, which have been extended to different forms. The expressions require only a knowledge of the velocity field (and its derivatives) in a finite and arbitrarily chosen region enclosing the body. Implementations are made on three- dimensional inviscid flows over wings and wingbody combinations. Further implementation on three-dimensional viscous flow over wings has also been investigated.

From the University of Limerick, Ireland, came a contribution which presented comparisons between hot wire measurements within the boundary layer on the suction surface of a high pressure turbine blade in a subsonic liner cascade and data obtained from a commercial aerodynamic code. Various transition modelling strategies were investigated. These involved a laminar region and a turbulent region and another, where the turbulent model was applied over the entire flow field. Systematic variations of key blade geometry parameters including camber, thickness, flow angles, etc., were carried out on the blade profile which produced a minimum entropy profile loss per unit work output. Boundary layer work was performed at discrete locations on the surface and comparisons made with CFD.

From various authors came a paper on the forced response analysis of a vertical take-off lift fan. The aim of the paper was to describe a series of multi bladerow forced response analyses carried out on a shaft driven lift fan. This fan, used for vertical landing and take-off, is situated behind the cockpit and contains seven bladerows, two of which are on counter- rotating rotors. The aim of the analysis is to evaluate the vibration amplitude of the two rotor bladerows for a range of configurations and speeds. The situation of the lift fan creates vcreate4s several low order harmonics. The forced response analysis methodology is based on an integrated aeroelasticity model. Also, a number of forced analyses of increasing complexity were undertaken over a period of years. The conclusions include the fact that given the various uncertainties associated with aerodynamic boundary conditions, damping and blade mistuning, the agreement between the predicted and measured rotor blade vibration is remarkably good. The relative rate of response in various modes was noted in all cases and there were less discrepancies when the response levels were high.

Experimental techniques began with a paper from Manchester University on flow measurement and visualisation with thermochromic liquid crystals in which the use of these crystals in supplementing aerodynamic studies is illustrated with results obtained from flow visualisation of turbulent events occurring during laminar-turbulent transition. In addition, flow separation and reattachment over a backwards facing model and a delta wing is also shown. It is illustrated that this technique is not only capable of revealing the characteristics of surface flow that are consistent to those obtained by conventional methods, but also in improving the qualitative information over an entire surface. This paper demonstrates that thermochromic crystals can be a very effective flow measurement and visualisation tool for studying flow phenomena in aerodynamic applications.

Molecular image sensing for pressure and temperature surface mapping of aerodynamic flows, also from Manchester, addresses the two technical challenges of PSP technology – temperature sensitivity and aerodynamic intrusiveness. A lifetime imaging system to measure surface pressure from the luminescent delay of anodised aluminium dye with a temperature correction capability has been developed. The analysis showed that the paint exhibited fast response characteristics with a transient behaviour after pulsed excitation. Utilizing this characteristic by taking three images with different gated times, the pressure and temperature were determined. The performance of the system was evaluated over a wide range of pressures and temperatures using an AA-PSP coated substrate inside a calibration chamber. PSP data were in good qualitative agreement with conventional pressure transducers even at the regions where a strong interaction was observed. Schlieren visualisation studies have assisted the interpretation of the measured surface data.

From Cranfield University (RMCS Shrivenham) came an experimental and numerical study of an open transonic cavity in which velocity measurements are presented from an open flow rectangular cavity at M¼0.82. Two-dimensional particle image velocimetry (PIV) results are compared with unsteady and steady RANS CFD models using a realisable model along the cavity centreline. Velocity components are compared for three depths within the cavity and good agreement was reached between the PIV and steady CFD model, although the PIV results show some discrepancies at the downstream end of the cavity.