AEATECHNOLoGY ENGINEERING SOFTWAR Lecture 8 CFX-4& Output File CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -1 CFX-4.3 V4.3.1H Lecture 8 CFX-4 & Output File
AEATECHNOLoGY ENGINEERING SOFTWAR CFX4 Basic Concepts General form of equations solved in CFX-4 (Uφ) (rVφ)=S Integrating the equation over the control volume ∫分0m+m0,成-JrV=JS, a transient convection diffusion source CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -2 CFX-4.3 V4.3.1H CFX-4 Basic Concepts General form of equations solved in CFX-4 Integrating the equation over the control volume transient convection diffusion source U S t + ( ) - () = t dV + U ndA - ndA = SdV
AEATECHNOLoGY ENGINEERING SOFTWAR CFX4 Basic Concepts All terms are discretized in time and space linear equations are obtained -∑ALm P=1,2,…, NCELL Discretization Transient term backward difference crank-Nicolson Convection term Hybrid, HUW, QUICK, CCCT, etc Diffusion term: 2nd order central differencing The linear equation is spatially coupled Direct matrix inversion is too expensive, so iterative linear equation solvers are used >ICCG. AMG. Stone. line relaxation etc CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -3 CFX-4.3 V4.3.1H CFX-4 Basic Concepts All terms are discretized in time and space, linear equations are obtained - Discretization > Transient term: backward difference, Crank-Nicolson > Convection term: Hybrid, HUW, QUICK, CCCT, etc. > Diffusion term: 2nd order central differencing. - The linear equation is spatially coupled. Direct matrix inversion is too expensive, so iterative linear equation solvers are used > ICCG, AMG, Stone, line relaxation, etc. Ap p Anb S nb - nb = u p = 1, 2, , NCELL
AEATECHNOLoGY ENGINEERING SOFTWAR CFX4 Basic Concepts CFX-4 uses a segregated nonlinear solver (SIMPLEC). Each nonlinear equation is solved independently of others initial guess U momentum equation (=)sweeps Innel nonlinear V momentum equation iterations) outer iterations W momentum equation pressure(mass equation Convergence. final solution CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -4 CFX-4.3 V4.3.1H CFX-4 Basic Concepts CFX-4 uses a segregated nonlinear solver (SIMPLEC). Each nonlinear equation is solved independently of others. initial guess final solution U momentum equation V momentum equation W momentum equation pressure (mass) equation … ... sweeps (inner iterations) nonlinear outer iterations convergence
AEATECHNOLoGY ENGINEERING SOFTWAR About Convergence Convergence the solution has reached an unchanging state in CFx command file, the convergence criteria is set with the mass source tolerance the mass source tolerance is set as a percentage of total mass flow rate the solver defines convergence when the mass source residual is less than the mass source tolerance the mass source residual is not dimensionless it is the sum of the absolute values of the net mass flow rate into or out of ever ry cell. ve/ CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -5 CFX-4.3 V4.3.1H About Convergence Convergence: - the solution has reached an unchanging state. - in CFX command file, the convergence criteria is set with the mass source tolerance. - the mass source tolerance is set as a percentage of total mass flow rate. - the solver defines convergence when the mass source residual is less than the mass source tolerance. - the mass source residual is NOT dimensionless, it is the sum of the absolute values of the net mass flow rate into or out of every cell. |r| i i ncell = 1
AEATECHNOLoGY ENGINEERING SOFTWAR About Convergence a mass source tolerance of 10e-3 x total mass flow rate corresponds to <0. 1% mass imbalance in the system for convergence, residuals for enthalpy and other scalars should also decrease by a reasonable amount, and the monitoring point values not change the flow field cannot be expected to be physically meaningful if convergence has not been achieved CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -6 CFX-4.3 V4.3.1H About Convergence - a mass source tolerance of 1.0E-3 x total mass flow rate corresponds to < 0.1% mass imbalance in the system. - for convergence, residuals for enthalpy and other scalars should also decrease by a reasonable amount, and the monitoring point values not change. - the flow field CANNOT be expected to be physically meaningful if convergence has not been achieved
AEATECHNOLoGY ENGINEERING SOFTWAR About Convergence Good convergence Poor convergence I RESIDAL PICYI 1 RESIDUAL PLOT S.A-ECRELEUA3 131 1。i 1 1,。三40 L.3匚c≤ L 二.E+0 CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -7 CFX-4.3 V4.3.1H About Convergence Good convergence Poor convergence
AEATECHNOLoGY ENGINEERING SOFTWAR About Convergence For transient problems, a converged solution should be obtained for each time step 1 RESIDUAL PLOT c山 mmmm CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -8 CFX-4.3 V4.3.1H About Convergence - For transient problems, a converged solution should be obtained for each time step
AEATECHNOLoGY ENGINEERING SOFTWAR Output File The CFX output file, m# fo, is an asci file which provides numerical data on the calculation The user can browse the output file for important information CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -9 CFX-4.3 V4.3.1H Output File The CFX output file, m##.fo, is an ascii file which provides numerical data on the calculation. The user can browse the output file for important information
AEATECHNOLoGY ENGINEERING SOFTWAR Output File The output file by default contains physical models numerics ). Front-end printing model topology real boundary conditions 2) residual and monitoring point values 3). flow field variables at the end of the calculation /time step 4). wall information at the end of the calculation /time step 5)flow boundary flux information 6). summary of convergence, workspace and CPU time used CFX-4 CFX4.3V4.3.1H Introductory Training(Dec 1999)
CFX-4 Introductory Training (Dec 1999) 8 -10 CFX-4.3 V4.3.1H Output File The output file by default contains: 1). Front-end printing physical models & numerics model topology real boundary conditions 2). residual and monitoring point values 3). flow field variables at the end of the calculation / time step 4). wall information at the end of the calculation / time step 5). flow boundary flux information 6). summary of convergence, workspace and CPU time used * *