COURSE INTRODUCTION Tom Hynes

COURSE INTRODUCTION Tom Hynes

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COURSE STRUCTURE


Introduction, meshing and geometry handling




Turbulence




Basic CFD concepts & FORTRAN




Advanced numerical concepts




Ad-hoc mini-lectures during coursework 2

KEY COURSE ELEMENTS Write a basic CFD program
Explore basic performance
Explore performances of different numerical algorithms
2 Reports – also need to discuss taught material
https://www-diva.eng.cam.ac.uk/lecturenotes/part-iib-lecture-notes/4a2computational-fluid-dynamics


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CFD BACKGROUND


Why bother with experiments – Chapman (1975) ρ

Du = −∇p + µ∇ 2 u Dt

[N-S eqn remarkably exact representation of turbulence]

MEA says we really solve in CFD with µ=µ+µnum




CFD = cheats, frauds and deceivers; colourful fluid dynamics. Why ? RANS v LES

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CHAPMAN (1975) – THE DREAM


1014 flops N = 109 -> Road Runner (2008) 1015 flops




Chow and Moin (2012) confirmed Chapman’s estimates




GPUs provide cheap computing

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Available Tools


Making measurements




Use of analytical solutions




Use of Computer simulations (CFDaerospace – 1960s)

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ANALYTICAL SOLUTIONS


Important for gaining ball-park figures for design calculation




Essentially consist of exact solutions to very simplified flow governing equations with very simplified boundary conditions




Can be used as part of a CFD analysis (FWH equation)




Analytical solutions can be useful for verifying the accuracy of CFD programs and also the skill of the person using the computer program

Hopf fibration

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MEASUREMENTS ρ MU M LM ρ AU A LA = µM µA




Wind tunnels




Job to avoid high Ma/compressibility




For most model tests it is virtually impossible to get a model Reynolds number as high as that for the actual system




For ships the Froude number must also be matched – not just Re difficult

ReM = ReA ,

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Airbus model in ARA wind tunnel & US NFAC tunnel

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Wind tunnels are very expensive to maintain and run and they need very expert staff.




Water, mercury and pressurised tunnels can be used




Half wing model will cost around £ 250,000, 25,000 N, +/- 50 micron finish – Price 10 years ago 10




Engine simulator costs about £ 40,000




Test programme for a particular model can cost in excess of 1 million pounds




Wind tunnel blockage effect can be extremely high and significantly disrupt results

IMPROVED SHOCK WAVE – reduced shock noise

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CFD Advantages


Can provide a significant amount of detail about a flow situation




Can provide an effective means for the rapid evaluation of what-if design scenarios




Geometry changes easy




Safe for dangerous experiments




Aircraft and engines sold based on ‘paper OO’ CFD to get range etc O rigs too slow




Can bring to increasing customer confidence and enhancing the image or an organisation in the market place should not be underestimated




Used for weather prediction and is used ‘real time’ for nuclear disasters etc




Computer flow viz and RAVEs for exploring complex flows

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F15 DES Forsythe et al. 2002 AIAA Paper

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USAF – F/18 Tail Buffet, 2007

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Large Scale Problems Fire in buildings

CFD study of flow over a raised runway

30 St Mary Axe - CFD

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CFD used in Most Aeroengine Zones

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MOVING DOMAIN PROBLEMS

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AEROELASTICITY

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Flexible inflatable decelerator

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CFD Problems


Turbulence modelling a big problem




Geometry handling and meshing can be time consuming




Needs careful validation

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Comparison of computational and experimental techniques Area

Computational Methods

Experimental Methods

Capability

Software used for all flow types. Turbulence rarely resolved except through use of simpler models. Enables physical situations to be modelled where experiments would be unsafe. Allows geometry variation to be achieved quickly.

Experiments seen as being the real thing. Exact simulation if full-scale situation can be used. Scale effects can lead to experimental situation also being a model of the desired flow situation.

Accuracy

Depends on algorithms used and computer resources

Should be correct within the limits of experimental error if geometry and scale effects are realistic and equipment is appropriately designed and calibrated.

Detail

All variables calculated at every mesh point or cell. Variables can be integrated to find overall properties.

Easy to find overall properties such as pressure drops and forces and moments. Difficult and expensive to instrument so that anything more than a crude sample of the data is produced.

Cost

Requires relatively cheap hardware but expensive software. Time and care needed to get good results. Specialists required to achieve good results.

Instrumentation expensive in many cases. Raw experiment cheap to carry out but data achieved is very limited. Time and care needed to get good results.

Time

Solutions can take a long time. This depends on the problem being solved and the speed of the computer being used.

Time needed for set-up and calibration. Results usually quick to gather once this is done.

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TRIAD

PFD

CFD

MEASUREMENTS

RANS VALIDATION

THEORY

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NUMERICAL METHODS


Finite difference – good for high order




Finite volume – we will focus on here




Finite element – not popular CFD (mathematically abstract)

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CONCLUSIONS


Simulation is important in the future of engineering




Will be increasingly multi-disciplinary: fluid, structure, driver/pilot interactions




Still methods developments needed and hence careful validation 24