《CFX5.5培训资料》（英文版）Solver Control

CFX! Solver control CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -1 Solver Control

CFX! Solver control Settings for the behavior of the CFX-5 solver Maximum number of timesteps Timestep selection Transient timestep control for results output Convergence criteria Order of differencing scheme Advanced Options >Mesh Adaption CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -2 Solver Control Settings for the behavior of the CFX-5 solver > Maximum number of timesteps > Timestep selection > Transient timestep control for results output > Convergence criteria > Order of differencing scheme > Advanced Options > Mesh Adaption

CEX! Fl low Solver parameters form Convergence Control Solver Parameters Convergence criteria Convergence Control Advection scheme Convergence Criteria Options Advection Scheme Mesh Adaption Also Timestep results selection for Options transient simulations L·Ay」 CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -3 Flow Solver Parameters Form - Convergence Control - Convergence criteria - Advection scheme - Options - Mesh Adaption - Also Timestep results selection for transient simulations

CFX! Transient Timesteps Action Create Specify creation of transient results files Object: Transient Res File Set time to every for intervals or at for specific Transient Resuts Files time values AT1s Set Minimal Results File with specific variables to reduce file sizes (if not full results files get written at each timestep) Time of Resutts File Every Only accessible if transient was selected in the 5 Domain Options of the domains form V Minimal Results File Variables CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -4 Transient Timesteps Specify creation of transient results files Set time to every for intervals or at for specific time values Set Minimal Results File with specific variables to reduce file sizes (if not full results files get written at each timestep) Only accessible if transient was selected in the Domain Options of the Domains form

CFX! Flow Solver parameters form Convergence Control 口区 > Max Number of Timesteps 〓 Convergence Control Convergence Control Sets the maximum timesteps Max, No of Timesteps for the current run Equation Class Scope Equation Class Scope A‖ Equation Classes Timestep Selection All Equation Classes Auto Timestep Set Physical Timestep s Physical Timestep Physical Timestep Auto Timestep User Max Local Timestep Factor Close Expression Editor CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -5 Flow Solver Parameters Form - Convergence Control > Max. Number of Timesteps * Sets the maximum timesteps for the current run > Equation Class Scope > Timestep Selection * Auto Timestep * Physical Timestep * Auto Timestep User Max * Local Timestep Factor

CFX! Solver parameters Auto timestep Default setting for timestep control Use internally calculated fixed physical timestep size based on boundary conditions, initial guess and geometry of the domain Auto timestep: user maximum Use an internally calculated timestep, but the value is capped by a user specified maximum If the internally calculated timestep is less than the specified maximum the solver uses this value for the first five timesteps After five timesteps, the timestep is doubled for each successive timestep and checked against the specified maximum value. When this value is reached, it is used for the remaining timesteps If the internally calculated value is greater than or equal to the specified maximum the maximum value is used for all timesteps 8-6 CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -6 Solver Parameters Auto timestep. - Default setting for timestep control. - Use internally calculated fixed physical timestep size based on boundary conditions, initial guess and geometry of the domain. Auto timestep: user maximum. - Use an internally calculated timestep, but the value is capped by a user specified maximum. - If the internally calculated timestep is less than the specified maximum, the solver uses this value for the first five timesteps. After five timesteps, the timestep is doubled for each successive timestep and checked against the specified maximum value. When this value is reached, it is used for the remaining timesteps. - If the internally calculated value is greater than or equal to the specified maximum, the maximum value is used for all timesteps

CFX! Solver parameters Physical timestep Permits true transient simulation Value near an L/v scale usually gives good convergence rate Not obvious sometimes CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -7 Solver Parameters Physical timestep - Permits true transient simulation - Value near an L/V scale usually gives good convergence rate - Not obvious sometimes

CFX制 Solver parameters Local timestep factor, E. Different time scales are used in different regions of the domain Smaller time steps for regions of fast flow and larger time steps for regions of slow flow Can produce poor asymptotic convergence if mesh has large range in element size Similar concept to false time stepping in CFX-4 or etfact in TASCflow Useful on uniform mesh when L/v scale not obvious or vastly different local velocity scales are present in problem Only suitable for starting a run, not accurate for a fully converged solution. Large physical timestep needed 8-8 CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -8 Solver Parameters Local timestep factor, E. - Different time scales are used in different regions of the domain. - Smaller time steps for regions of fast flow and larger time steps for regions of slow flow. - Can produce poor asymptotic convergence if mesh has large range in element size. - Similar concept to false time stepping in CFX-4 or etfact in TASCflow. - Useful on uniform mesh when L/V scale not obvious or vastly different local velocity scales are present in problem. - Only suitable for starting a run, not accurate for a fully converged solution. Large physical timestep needed

CFX! Timestep selection Too large -bouncy convergence or divergence Too small -slow convergence and not fully accurate Initial timesteps may need to be very small(transonic flows, large regions of separated flows, openings with simultaneous inflow and outflow .. a physical timestep of 1/3 the smallest local L/V scale is usually optima Once the solver has got going(e.g. 5-10 iterations) then the asymptotically optimal physical timestep can be use d Typically, a local timestep factor of 5.0 is small 89 CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -9 Timestep Selection - Too large - bouncy convergence or divergence - Too small - slow convergence and not fully accurate - Initial timesteps may need to be very small (transonic flows, large regions of separated flows, openings with simultaneous inflow and outflow …) - A physical timestep of 1/3 the smallest local L/V scale is usually optimal - Once the solver has got going (e.g. 5-10 iterations) then the asymptotically optimal physical timestep can be used - Typically, a local timestep factor of 5.0 is small

CFX! Timestep selection For advection dominated flows. residence time for the flow is a good approximation for the timestep This is the time taken for a fluid particle to make its way through the flow domain For rotating components, timestep 1/o(o in rad/s) For buoyancy dominated flows, the following timestep Is more appropriate 8g△T 8-10 CFX5.5V5.5.1H 2001AEA Technology

CFX 5.5 V 5.5.1H © 2001AEA Technology 8 -10 Timestep Selection - For advection dominated flows, residence time for the flow is a good approximation for the timestep. This is the time taken for a fluid particle to make its way through the flow domain - For rotating components, timestep ~ 1/ ( in rad/s) - For buoyancy dominated flows, the following timestep is more appropriate: g T l t   =  max