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Lecture Objectives: Advance discretization methods
Review Residual, Stability, Relaxation Simple algorithm
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Advection diffusion equation 1-D, steady-state
W Dx P Dx E Dx w e y Q Example: Equation for temperature of water flowing through hot pipe Assume that diffusion in y direction ins negligible: This is incorrect assumption introduced jut to simplify example to 1-D problem! x Temperature is changing along x Q model Vx T1 T2 T3 T4 … Tn
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Advection diffusion equation 1-D, steady-state
Dx N Dx N+1 Different notation: Dx General equation
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Advection equation 1-D, steady-state
W Dx P Dx E Vx>0 Dx 1) Upwind scheme: Vx<0 2) Central differencing scheme: 3) Hybrid of upwind and central differencing scheme Higher order differencing scheme: Quadratic upwind differencing Scheme (QUICK) N-2 N-1 N N+1 N+2 WW P W E EE We need to find coefficients aP, aW, aE, aWW, aEE,
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Central Difference and Upwind Differencing Schemes
Notation of coefficients in book Stability of the Central Difference Schemes Pe = Diffusion / Advection = (Vx)/(/∆x)<2 It defined by the requirement that coefficient aE>0 Requirement for “boundedness” D=/∆x F=Vx
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Hybrid Differencing Schemes
Notation of coefficients in the book T N W P E S B
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Higher order differencing scheme: Quadratic upwind differencing
Scheme (QUICK) N-2 N-1 N N+1 N+2 WW P W E EE We need to find coefficients aP, aW, aE, aWW, aEE,
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Quadratic upwind differencing
Scheme (QUICK) Coefficients: Advection coefficient: Source: Diffusion coefficients : For advection only:
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Residual calculation for CFD
Residual for the cell RFijk=Fkijk-Fk-1ijk Total residual for the simulation domain RFtotal=S|RFijk| Scaled (normalized) residual RF=S|RFijk|/FF iteration cell position Variable: p,V,T,… For all cells Flux of variable F used for normalization Vary for different CFD software
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Relaxation Relaxation with iterative solvers:
When the equations are nonlinear it can happen that you get divergency in iterative procedure for solving considered time step divergence variable solution convergence Solution is Under-Relaxation: Y*=f·Y(n)+(1-f)·Y(n-1) Y – considered parameter , n –iteration , f – relaxation factor For our example Y*in iteration 101=f·Y(100)+(1-f) ·Y(99) f = [0-1] – under-relaxation -stabilize the iteration f = [1-2] – over-relaxation - speed-up the convergence iteration Value which is should be used for the next iteration Under-Relaxation is often required when you have nonlinear equations!
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Example of relaxation Example: Advection diffusion equation, 1-D, steady-state, 4 nodes 1) Explicit format: 1 2 3 4 2) Guess initial values: 3) Substitute and calculate: 4) Substitute and calculate: Substitute and calculate: ………………………….
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Navier Stokes Equations CFD Specific Solver
Continuity equation This velocities that constitute advection coefficients: F=rV Momentum x Momentum y Momentum z Pressure is in momentum equations which already has one unknown In order to use linear equation solver we need to solve two problems: find velocities that constitute in advection coefficients 2) link pressure field with continuity equation
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Pressure and velocities in NS equations
How to find velocities that constitute advection coefficients? For the first step use Initial guess And for next iterative steps use the values from previous iteration
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Pressure and velocities in NS equations
How to link pressure field with continuity equation? SIMPLE (Semi-Implicit Method for Pressure-Linked Equations ) algorithm W Dx P Dx E Dx Aw Ae Aw=Ae=Aside We have two additional equations for y and x directions The momentum equations can be solved only when the pressure field is given or is somehow estimated. Use * for estimated pressure and the corresponding velocities
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SIMPLE algorithm Guess pressure field: P*W, P*P, P*E, P*N , P*S, P*H, P*L 1) For this pressure field solve system of equations: x: y: ……………….. ……………….. z: Solution is: 2) The pressure and velocity correction P = P* + P’ P’ – pressure correction For all nodes E,W,N,S,… V = V* + V’ V’ – velocity correction Substitute P=P* + P’ into momentum equations (simplify equation) and obtain V’=f(P’) V = V* + f(P’) 3) Substitute V = V* + f(P’) into continuity equation solve P’ and then V 4) Solve T , k , e equations
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SIMPLE algorithm start Guess p* p=p*
Step1: solve V* from momentum equations Step2: introduce correction P’ and express V = V* + f(P’) Step3: substitute V into continuity equation solve P’ and then V Step4: Solve T , k , e equations no Converged (residual check) yes end
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Other methods SIMPLER SIMPLEC variation of SIMPLE PISO
COUPLED - use Jacobeans of nonlinear velocity functions to form linear matrix ( and avoid iteration )
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Newton-Raphson method (example of Jacobean solver)
Faster convergence Used in many professional tools (MathCAD, EES, MatLab, Mathematica, etc) More complex for programming Requires linear solver Based on Taylor-Series Expansion You need first derivative for each function to create the Jacobean matrix Equations in the form where all side are on one side of equality sign Our simple example: X-Y/2= → X-Y/2+1=0 X2-Y= → X2-Y+3=0
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Newton-Raphson method
Section 6.11 of handouts Our simple example: f1 = X-Y/2+1=0 f2 = X2-Y+3=0 Steps: 0) Find derivatives d(f1)/dX = , d(f1)/dY =-1/2 d(f2)/dX =2X , d(f2)/dY =-1 1) Initial guess: Y(0)=2, X(0)=2 2) Find f1(Y(0),X(0))=2-2/2+1=2 f2(Y(0),X(0))=22-2+3=5 3) Using derivatives and guess values find the Jacobean matrix 4) Solve the matrix using linear solver and find DX and DY 5) Find Y(1)=Y(0)+ DY, X(1)=X(0)+ DX, Repeat step (2) with Y(1) and X(1) … Follow the procedure till convergence Unknowns (correction Dxi) Jacobean matrix Function values for guessed variables
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Course Review (so far)
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Conservation Equations Navier Stokes Equations
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Reynolds Averaged Navier Stokes equations
Continuity: 1) Momentum: 2) 3) 4) Similar is for STy and STx 4 equations 5 unknowns → We need to model
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Modeling of Turbulent Viscosity
Fluid property – often called laminar viscosity Flow property – turbulent viscosity MVM: Mean velocity models TKEM: Turbulent kinetic energy equation models Additional models: LES: Large Eddy simulation models RSM: Reynolds stress models
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Discretization and equation solver
SIMPLE algorithm Discretization of RANS Guess p* p=p* Step1: solve V* from momentum equations Step2: introduce correction P’ and express V = V* + f(P’) Step3: substitute V into continuity equation solve P’ and then V Step4: Solve T , k , e equations no Converged (residual check) yes end
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