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