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