cfd presentation final draft
TRANSCRIPT
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CFD RefinementBy: Brian Cowley
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Overview1. Background on CFD2. How it works3. CFD research group on campus for which
problem existso Our current techniques
4. The problem I address5. Possible solutions – testing different solution
schemes6. Test results7. Possible concerns8. Implications of results 9. Further research
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Background: What is CFD? A branch of fluid mechanics that
emphasizes the use of computers
Is able to create simulations of almost every aspect of fluid movement
Used in development of many modern technologies
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How it works – the problem Relies entirely on computers for
calculations. Why? Calculations are based on the Navier-Stokes
equations
oThere is no solution that exists to these equations
• The terms within the equation are partial differentials
So what do we do, since computers cannot do calculus?
𝜌 (𝜕𝑢⇀
𝜕𝑡 +𝑢⇀·𝛻⇀𝑢− 𝑓
⇀ )−𝜇 (𝛻⇀ )2 𝑢+𝛻⇀𝑝=0𝛻⇀·𝑢⇀=0
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How it works – the solution We use discrete math instead of
continuous math The discretized Navier-Stokes equation
o Replacing differentials with finite differences
o Computer algorithms can therefore be used• Make some initial guess (initialize) the flow
field (the inviscid solution)• Iterate until the initial guess converges the
asymptotically steady state solution Can be on the order of millions of iterations
𝜔 𝑖 , 𝑗𝑛+1−𝜔𝑖 , 𝑗
𝑛
𝛥𝑡 + 1𝑠𝑖 , 𝑗
2 [ (𝑠𝑖+1 , 𝑗 ) (𝑉 𝑖+1 , 𝑗 )𝑛 (𝜔𝑖+1 , 𝑗 )
𝑛− (𝑠𝑖− 1, 𝑗 ) (𝑉 𝑖−1 , 𝑗 )𝑛 (𝜔 𝑖−1 , 𝑗 )
𝑛
2 𝛥𝜇 +(𝑠𝑖 , 𝑗+1 ) (𝑉 𝑖 , 𝑗+1 )
𝑛 (𝜔 𝑖 , 𝑗+1)𝑛− (𝑠𝑖 , 𝑗 −1 ) (𝑉 𝑖 , 𝑗− 1)
𝑛 (𝜔𝑖 , 𝑗− 1)𝑛
2 𝛥𝜂 ]= 1𝑅𝑒𝑀 (𝑆𝑖 , 𝑗 )
𝑛 [ (𝜔𝑖− 1 , 𝑗 )𝑛−2 (𝜔𝑖 , 𝑗 )𝑛+(𝜔𝑖+1 , 𝑗 )
𝑛
( 𝛥µ )2+
(𝜔𝑖 , 𝑗−1 )𝑛−2 (𝜔𝑖 , 𝑗 )𝑛+(𝜔𝑖 , 𝑗+1 )
𝑛
( 𝛥𝜂 )2 ]
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CFD lab on campus – the project
Investigates the effects of induced jet flow over airfoils
Can be accomplished in two ways:o Electrodes along the leading edge of a
wing• Effectively ionizes air
o By the use of flexible membranes
Effects:o Causes the air over the wing to be more
attached
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Benefits of research: Reduced air drag over
aircraft bodyo Two forces in x-direction in flight:
thrust and drago If drag is reduced you need less
thrust• Reduces the amount of fuel
needed (monetary return)
Increases operational envelope
o Aircrafts could operate at higher pitch angles and slower speeds without stalling
Angle of attack = 5.5
Angle of attack = 6.5
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The current CFD solver
Forward time, central difference
Uses central difference scheme o Known to be stable
Uses the left and right pointsUses every point in domain
(𝜕𝑢𝜕 𝑥 )𝑖=𝑢𝑖+1−𝑢𝑖−1
2 Δx
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The current CFD solver
Forward time, central difference
Uses central difference scheme o Known to be stable
Uses the left and right pointsUses every point in domain
(𝜕𝑢𝜕 𝑥 )𝑖=𝑢𝑖+1−𝑢𝑖−1
2 Δx
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The problem: Is the current finite difference scheme the optimal choice?
Not been tested for accuracy in predicting flow fields
Current is being used because it worked Other methods could be:oMore accurateoFaster to computeoMore stable
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Test scheme one: The forward difference scheme
Approximates using middle and right (front) point
Eliminates right end of domain
( 𝜕𝑢𝜕 𝑥 )𝑖=𝑢𝑖+1−𝑢𝑖
Δx
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Test scheme one: The forward difference scheme
Approximates using middle and right (front) point
Eliminates right end of domain
( 𝜕𝑢𝜕 𝑥 )𝑖=𝑢𝑖+1−𝑢𝑖
Δx
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Test scheme two: The backward difference scheme
Approximates using middle and Left (back) point
Eliminates right end of domain
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Test scheme two: The backward difference scheme
Approximates using middle and Left (back) point
Eliminates right end of domain
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Testing Criteria
Is the solution more accurate then central difference?
Does the solution converge faster? Is it stable?
o Will it work for a larger variety of conditions?
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Results – The forward difference scheme Appears to be unconditionally unstable
o Numbers within domain diverge o Causes program to fail
All output files have been empty
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Results – The backwards difference scheme Successfully produces solutions and output
files
Has smaller residuals then original versiono LHS-RHS=0 o Indication of accuracy
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Concerns - The backwards difference scheme
Residuals fall alarmingly fast within first few iterationso Original fell from order 101 to 10-6 o Backwards difference fell from order 101 to 10-9
• Can be indication of inaccuracy
Values for the vorticity are alarmingly low o Original had a scale that commonly went from zero to 4o Backward scheme had a scale that went from zero to E-6
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Concerns - The backwards difference scheme Plots of simulated fluid flow are distorted Plots of the vorticity fields shown below:
Original central difference scheme
Modified Backwards difference scheme
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Concerns - The backwards difference scheme
Distortion could be caused by technicality of using new schemeo Both the original and new
schemes use two spacial steps o At left end of the domain there
are no points to its left• Therefore the program cannot
accurately approximate here• This error could propagate
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Implications
The forwards difference scheme appears to be useless o However, resolving the double spacing issue could
fix it
Backwards difference scheme is promisingo It has been shown to have benefits
• Possibly more accurateo Spacing issue still needs to be resolved
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Further research
Work needs to be done on fixing the spacing issue
More data needs to be collected to validate resultso Compared with same lab geometry in many
trialso Compared with different geometry
• Geometry for which a true analytical solution exists