PhD degree students

 

Madeleine L Combrinck -- PhD Student

email:  [email protected]

Thesis title:  Boundary Layer Response to Arbitrary Accelerating Flow.

Supervisor: Prof Laurent Dala

Co-supervisor: Prof Igor Lipatov (FALT-Moscow of Physics and Technology Institute)

 

Aeroballistic objects in flight are subjected to unsteady accelerating conditions which manifests as fictitious forces in the non-inertial frame of reference.  The standard Navier-Stokes equation cannot capture the relative flow properties involved in accelerated, rotational flow since this form assumes that the flow problem is situated in an inertial reference frame.  In rotational flow problems the symmetry between the inertial frame and the non-inertial frame is broken so that the mathematical model can no longer account for the observed behaviour.  The observed effect can be accounted for by the inclusion of non-inertial acceleration terms that presents itself as fictitious forces on the right hand side of the Navier-Stokes equation.

There are still significant gaps in the body of knowledge with regards to studies in acceleration.  The few studies that investigates these effects, relies mostly on steady acceleration and therefore do not activate all the non-inertial effects.  There are some cases that consider pre-scribed motion and transient effects, but most fail to realise the importance of the boundary layer and makes use of inviscid formulations of the Navier-Stokes equations and slip wall conditions.

The scope of this study includes work:

• Formulation of the non-inertial Navier-Stokes equations for arbitrary accelerations.  A rigorous and formalized approach will be followed and documented.
• Implementation of a non-inertial Navier-Stokes formulation in OpenFOAM.
• Formulation of an asymptotic method for viscous boundary layer calculation.  The boundary layer cannot adequately be described by numerical methods and thus an analytical method must be used.
• Response of the boundary layer to arbitrary acceleration.  Most boundary layer response studies focussed on linear acceleration while aeroballistic airframes require investigation into arbitrary motion.

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Stefan Poprawa (Chief Technical Pilot, Flight Technical & Maintenance Standards) -- PhD Student

email: [email protected]

Thesis title: Statistical Approach to Payload Capability for Civil Aircrafts.

Supervisor: Prof Laurent Dala

Dr. Peter Bidgood / CSIR

 

Large commercial aircraft design requires compromise to contain operating and capital costs whilst providing performance capability that accommodates the requirements of the majority of intended customers. One such compromise is the trade-off between range capability and payload capability: A large commercial aircraft, when uplifting maximum fuel capacity, is unable to carry maximum payload simultaneously and vice versa.

Airlines operating aircraft on routes longer than the design range for maximum payload capability therefore seek to maximise their sellable payload capacity on each flight. Continually varying environmental conditions challenge the performance analysts to provide accurate payload capabilities for such routes. The sales team, however, need to know months in advance how many seats are sellable to potential customers. The risk of flying with empty seats unnecessarily is as taxing to the airline as is the risk of denied boarding and dissatisfied customers.

Traditional approaches to this conundrum look at seasonal average environmental conditions at a predetermined probability level. Such approaches do not minimise the inherent risk adequately, though.

This research aims to develop a more in-depth statistical approach to payload capability forecasting for payload limited routes.

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Daniel Odido – PhD Student

email:    [email protected]

Thesis Title:  Flow control through surface heating

Supervisor: Prof Laurent Dala

 

Surface cooling is known to stabilise boundary layer flow for gases/air. It has hitherto been difficult to implement flow control through wall surface cooling in aircraft due to the diminutive dimensions involved. The boundary layer of a typical aircraft wing with chord length of 1m travelling at 400 km/h is about 0.015 m at the trailing edge. The dimensions at regions before separation are even much smaller; Boundary layer thickness is several orders of magnitude less than longitudinal dimensions of the flow surface. The boundary layer thickness however increases catastrophically with stall. Heating of the wall surface, followed by an unheated region has the potential to stabilise flow in the boundary layer. The net effect on the flow is that of surface cooling, hence stabilisation of the boundary layer.

The investigation considers intermittent heating and cooling of the surface. The boundary layer is acted on before the appearance of the Tollmien-Schlichting waves. The 2D case is considered since the TS instability modes appear earlier than those for 3D flow. Heating causes local changes of density and viscosity. The influence of the heating strip depends on its longitudinal dimensions, t.  An asymptotic analysis is carried out. Three characteristic regions are identified: t <  δ ; t ~ δ and t > δ. The intensity of heating also influences the boundary layer stability. Different mathematical models are developed for the respective cases, and parametric analysis carried out. The stability is analysed through parabolised stability equations. This is validated by numerical analysis. The use of carbon nanostrips is proposed as an effective means of flow control for the dimensions  O(δ). Use of nanomaterials permits the manipulation of the boundary layer at these micro-scales. 

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Marius-Corne Meijer -- PhD Student

email: [email protected]

Thesis title: Aeroelastic prediction for missiles including viscous interactions.

Supervisor: Prof Laurent Dala

 

Missile aerodynamics involves complex flow interactions between components over a wide range of speeds and missile orientations. Various semi-empirical and analytical methods exist to predict aerodynamic loading on the missile components; these methods typically trade-off accuracy for speed of computation. Such "engineering codes" often involve simplifying analogies to the flow physics that allow dominant effects to be modelled with reduced computational effort. The importance of correctly modelling the flow physics increases when considering the interaction between the structural dynamics and aerodynamics of components -- aeroelasticity. The prediction of the aeroelastic integrity of missile components over the design operating envelope is an important part of the design cycle; however, the computational cost of the analysis associated with coupled computational fluid dynamics (CFD) / computational structural dynamics (CSD) typically results in the analysis being conducted quite late in the design cycle.

The aim of the present work is to develop approximate models for aeroelastic prediction for missiles, accounting for important viscous interactions, such as the effect of the body vortex at small to moderate angles of attack on the flutter boundary of the fins. The developed analytical methods are to be benchmarked against coupled CFD/CSD computations.

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Masters degree students

 

Willie Viljoen – M.Eng student

Email: [email protected]

Title: Parametric Development of a Serrated Leading Edge High-Lift Device

Supervisor: Prof Laurent Dala

Co-supervisor: Dr Sean Tuling

 

High-lift devices (HLDs) are integrated into aircraft wings and deployed during take-off and landing to enhance the lifting capability of wing profiles that are generally designed for efficient cruising. For large commercial jetliners, such systems are typically comprised of trailing edge flaps and leading edge devices such as spanwise slats or Krueger flaps. When deployed, HLDs (in particular spanwise slats) contribute substantially to the effective perceived noise (EPN) emitted by the airframe around airports. The airfield performance improvements (shorter runway requirement, lower landing speeds, greater climb rate, etc.) provided by HLDs are also offset by the resulting increase in weight, cost and complexity of the aircraft.  In keeping with constant efforts to reduce these penalties, as well as international noise reduction goals^{1, 2}, an alternative leading edge HLD design approach is proposed in the form of a simple Krueger flap with a serrated leading edge (SLE). Vortex sheets induced along the leading edges of the serrations are expected to produce additional vortex lift and energise the boundary layer over the main wing to delay the onset of stall. A high-fidelity numerical model of the EuroLift TC12 profile³, modified with the SLE device, will be developed to optimise the serration dimensions and device setting for high-lift performance and minimal noise propagation. The model is to be validated in the low-speed wind tunnel at the CSIR (to be confirmed). It is expected that the optimised SLE HLD will be lighter and less expensive than conventional devices, while providing similar lift enhancement at reduced aeroacoustic noise.

¹ Europe – ACARE 2020 Vision: 10dB (50%) EPN reduction by 2020 compared to aircraft technology in 2000

² USA – NASA: 10dB (50%) and 20dB (75%) EPN reduction by 2007 and 2022 respectively, compared to aircraft technology in 1997

³ Provided courtesy of the High-Lift Department of Airbus Germany GmbH in Bremen

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Atlarelang M.P. Moletsane – MEng student

Email: [email protected]

Dissertation title: Guidance of a Supersonic Projectile using Plasma-actuators

Supervisor: Prof Laurent Dala

                                     Co-supervisor: Dr Johan Heyns

 

Supersonic projectiles are mainly encountered in landward ballistics, where the artillery shells used therein can have masses between 40-50kg and ranges of 50km. In such a configuration of unpowered and uncontrolled flight, the Coriolis force acting on such a projectile becomes significantly large so as to unpredictably change the projectile’s trajectory, thus worsening the artillery’s precision or enlarging the circular error probable (CEP). Current solutions to this control problem include the course corrective fuse (CCF) ^[1] where the closed-loop control system comprises of a GPS system for determining the heading of the projectile, and physical actuators, in the form of a drag-ring brake, canards and fins retro-fitted onto the projectile, for effecting the trajectory correcting measures. More recent, and more effective at this flow regime due to the large aerodynamic forces present at supersonic flight, has been Gnemmi et. al’s^[2] patented plasma actuation systems. The working principle of this actuator is that the discharge of a plasma around the nose-cone of the projectile imbalances the pressure distribution around this axisymmetric body, such that a judicious placement and firing of these plasma jets around the nose-cone can theoretically correct the trajectory of a typical long-range supersonic projectile. Current work by Gnemmi et. al includes wind-tunnel testing of the steady-state case, where the magnitude of force generated by this plasma actuator could not be validated beyond reasonable doubt as being sufficient for changing the trajectory of the generic missile proposed by the French-German Research Institute of Saint-Louis (ISL).

The present work is to computationally simulate the unsteady axisymmetric two dimensional case where the control objective is to generate sufficient force for controlling the projectile’s pitch throughout its flight-path of acceleration/deceleration. Principle outcomes of this undertaking are to develop analytical methods for simplifying the computations, as well as simulate this case of free-flight in OpenFoam®, with requisite computational modules such as one for plasma flows, being developed for the open-source computational fluid dynamics (CFD) community.