cfd and mdo state of the art and the future, 16-17 october 2017…€¦ · cfd and mdo –state of...
TRANSCRIPT
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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; …
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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?
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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?
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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?
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
Modelling tools and modelling challenges
• 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?
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
Modelling tools and modelling challenges
• 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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
Modelling tools and modelling challenges
• 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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
Modelling tools and modelling challenges
• 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
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
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!
CFD and MDO – State of the Art and the Future, 16-17 October 2017, RAeS, London
Questions?