CFD and MDO State of the Art and the Future, 16-17 October 2017…€¦ · CFD and MDO –State of...

CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London


Optimizing Tribological Flow Including Fluid-Structure Interaction

Alison McMillan

Wrexham Glyndwr University


Context: What’s a “solids” person doing here?

• Aerospace + CFD probably implies– Flight aerodynamics,– Aero-engine stage efficiency, or perhaps– Combustion system modelling

• From the “solids” side of the fence:CFD people seem only to be interested in the spaces between the metalwork – the “free stuff” rather than the “metal bits” sold to customers.

• As multi-physics modelling capability develops, we have to stop thinking “solids” and “fluids”, and start thinking

Problem Solving


Why CFD people should care about “Solids”

• Solid parts found in aerospace machines almost certainly were “fluid” at some point during their creation– Metallics:

• Raw material; alloying processes; casting – controlled solidification for crystal structure; lubrication during machining operations; …

– Composites:• High viscosity non-Newtonian polymer fluid forced between fibres

at high pressure; exothermic cure kinetics; heat transfer; …


Why CFD people should care about “Solids”

• Machines frequently have – Moving parts which must be lubricated

• Bearings; gears; contacting surfaces which must be protected

– A need for a cooling system• Frequently the act of managing contact generates heat which must be

dissipated;

– A need to remove debris• An oil system picks up products of wear and removes them from the contact

site to protect the contacting surfaces

– Aerospace – a need to maintain a suitable environment for humans• Pressure; temperature; containing suitable levels of O2; not containing poisons

or nasty smells


But the fluid dynamics for tribology is simple …

• Flow in pipes or between narrow spaces – Low Re– Simple geometries

• Poiseuille flow: • Hand calcs, job done!

• It is because this approach has been so successful for so long, it has escaped the interest to today’s fluid dynamicists

Pressure

Velocity profile


Challenge: Bringing Solids and Fluids together

• These problems matter to “Solids” people– They design the bearing

systems, etc– But they don’t have

confidence and know-how to use CFD

• CFD people have the skills– But they don’t appreciate

why these problems are important

– Nor that they offer some wonderful new challenges

Simple hydrostatic bearing

• Pressure in the recess volume: 𝑝𝑟 =6𝜇 ln ൗ

𝑟𝑜𝑟𝑖

𝜋ℎ3𝑄

• Pressure drop through the land: 𝑝 𝑟 = 𝑝𝑟ln Τ𝑟𝑜

𝑟

ln ൗ𝑟𝑜

𝑟𝑖


Challenge: Bringing Solids and Fluids together

• These problems matter to “Solids” people– They design the bearing

systems, etc– But they don’t have

confidence and know-how to use CFD

• CFD people have the skills– But they don’t appreciate

why these problems are important

– Nor that they offer some wonderful new challenges

Simple hydrostatic bearing What’s the best shape for the recess volume?


Research opportunities: Surface perturbations

Tribology questions, for which CFD is needed• If we have something like Poiseuille Flow, but the surfaces

are “rough”, how does that affect the flow?

Does roughness cause pressure points or waves on the structure?


Research opportunities: Fluid-Structure Interaction

Tribology questions, for which CFD is needed

• How does the elasticity of the solid respond to the pressure applied by the fluid?

Fluid velocity

Fluid pressure

Membrane von Mises stress

Membrane downward deflection

Co-simulation• CFD analysis • Structural analysis• Surprisingly, even for quite a

stiff structure, the fluid dynamics becomes VERY lively! – Start stiff– Expect a large STEP time

Are trumpeters right to be choosy about the alloy choice for their instrument?


Significance when things go wrong…

• Consider a gas turbine engine– Consider the LP shaft system

• Transfers torque from the turbine through the fan

• Rotates very fast

– Consider the thrust bearing• Transmits the full motive force of the

engine through to the engine structure and hence to the aircraft

• What if the oil does not reach the bearing?

Engine must shut down before shaft can fail

• Focus of a “fluids” person:– Efficiency

• Focus of a “solids” person– Safety

Both points of view are important


Modelling tools and modelling challenges

• The problem with tribology is that there are easy “hand calc” problems, and then there are very difficult problems that require multiphysics simulation.

• There’s not much in between.

• Applying CFD skills to problems is a major challenge of familiarity gap.

Real world

Model world



• How to simulate “dirty” fluid? • What does surface roughness

really do to the flow?• To what extent does

component elasticity influence flow?

• What is the real hydrodynamic pressure distribution between tribological surfaces?

• How do “real” fluid properties vary with temperature and pressure?

• If we were to create an optical system for detecting particulates in fluid, could CFD help in the design and validation of that system?

• When designing a lubricant or cooling fluid delivery system, how can we be sure that it not only works when it is new, but that it is robust under degradation?



• The Mathematics toolset: Navier-Stokes equations– Mathematical analysis: which terms to keep and which

are “negligible” – have CFD users become desensitized to this question?

• CFD toolset: The current CFD packages and code– CFD questions of robustness of solution, convergence,

modelling appropriateness – lots of pit-falls• Grid size• Boundary layer • Turbulence model choice



• The Physics toolset: Physics criticality– What are the really significant questions?

• Is there good maths for it• Is there computer code for it

• The Engineering toolset: Teams– Engineering leadership

• Make sure that there is someone on the team who can answer aspects of each of those questions

• Facilitate those team members to work together and communicate effectively with each other



• The “Next Generation” of CFD tools– Continuum Mechanics (well established capability)

• N-S based with all the hard bits chopped out or approximated• Conventional computing, algorithms can be parallel, maximum problem size

~10M degrees of freedom

– Molecular Dynamics (established but not mainstream)• Models at the molecular level• There is a limit to the number of molecules in a model, and a huge gap

between modelling molecules and a continuum

– Lattice Boltzmann methods (beginning to be established)• Fills the scale gap between Molecular Dynamics and N-S CFD• Potentially the perfect tool for multiphysics aspects of tribology problems


Conclusions:• Tribology problems are too Easy and too Difficult

– Typically, aerospace CFD people don’t have much exposure to these issues.– Typically, aerospace tribology people don’t have much exposure to CFD

• Navier-Stokes based CFD could be usefully applied: using many of the more recently developed capabilities– Multiphysics simulation– Co-simulation– Optimization

• Lattice Boltzmann capability is just around the corner– Using it now would help drive its future development

These Tribology problems, and potential methods of solution, present some new research avenues for eager academics!


Questions?

CFD and MDO State of the Art and the Future, 16-17 October 2017…€¦ · CFD and MDO –State of...

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