cfx1308流体瞬态分析

1、Chapter 8CFX流体瞬态分析CFX瞬态分析 MotivationNearly all flows in nature are transient!Steady-state assumption is possible if we:Ignore unsteady fluctuationsEmploy ensemble/time-averaging to remove unsteadiness (this is what is done in modeling turbulence)In CFD, steady-state methods are preferredLower comput

2、ational costEasier to postprocess and analyzeMany applications require resolution of transient flow:Aerodynamics (aircraft, land vehicles,etc.) vortex sheddingRotating Machinery rotor/stator interaction, stall, surgeMultiphase Flows free surfaces, bubble dynamicsDeforming Domains in-cylinder combust

3、ion, store separationUnsteady Heat Transfer transient heating and coolingMany moreOrigins of Transient FlowNatural unsteadinessUnsteady flow due to growth of instabilities within the fluid or a non-equilibrium initial fluid stateExamples: natural convection flows, turbulent eddies of all scales, flu

4、id waves (gravity waves, shock waves)Forced unsteadinessTime-dependent boundary conditions, source terms drive the unsteady flow fieldExamples: pulsing flow in a nozzle, rotor-stator interaction in a turbine stageKelvin-Helmholtz Cloud InstabilityRotor-Stator Interaction in an Axial CompressorTransi

5、ent CFD AnalysisSimulate a transient flow field over a specified time periodSolution may approach:Steady-state solution Flow variables stop changing with timeTime-periodic solution Flow variables fluctuate with repeating patternYour goal may also be simply to analyze the flow over a prescribed time

6、interval.Free surface flowsMoving shock wavesEtc.Extract quantities of interestNatural frequencies (e.g. Strouhal Number)Time-averaged and/or RMS valuesTime-related parameters (e.g. time required to cool a hot solid, residence time of a pollutant)Spectral data fast Fourier transform (FFT)20Timestep

7、= 2 sInitial Time = 0 sTotal Time = 20 sCoefficient Loops = 524681012141618Time (seconds)5 coefficient LoopsTransient simulations are solved by computing a solution for many discrete points in timeAt each time point we must iterate to the solutionHow to Solve a Transient CaseSimilar setup to steady

8、stateThe general workflow isSet the Analysis Type to TransientSpecify the transient time duration to solve and the time step sizeSet up physical models and boundary conditions as usualBoundary conditions may change with timePrescribe initial conditionsBest to use a physically realistic initial condi

9、tion, such as a steady solutionAssign solver settingsConfigure transient results files, transient statistics, monitors pointsRun the solverHow to Solve a Transient Case1. Analysis TypeEdit Analysis Type in the Outline tree and set the option to TransientSet the Time DurationThis controls when the si

10、mulation will endOptions are:Total TimeWhen restarting this time carriers overTime Per RunIgnores any time completed in previous runsMaximum Number of Timesteps The number of timesteps to perform, including any completed in previous runsNumber of Timesteps per RunFor this run only. Ignores previousl

11、y completed timesteps2. Time Duration and Time Step2. Time Duration and Time StepSet the Time Step sizeThis controls the spacing in time between the solutions pointsOptions are:Timesteps / Timesteps for the RunVarious formats accepted, e.g.0.0010.001, 0.002, 0.002, 0.0035*0.001, 10*0.05, 20*0.06Adap

12、tiveTimestep size will change dynamically within specified limits depending on specified convergence criteria or Courant number2. Time Duration and Time StepThe Time Step size is an important parameter in transient simulationsIt must be small enough to resolve time-dependent features True solutionTi

13、meVariable ofinterestDtTimeVariable ofinterestDtTime step too large to resolve transient changes. Note the solution points generally will not lie on the true solution because the true behaviour has not been resolved.A smaller time step can resolve the true solution2. Time Duration and Time Step and

14、it must be small enough to maintain solver stabilityThe quantity of interest may be changing very slowly (e.g. temperature in a solid), but you may not be able to use a large timestep if other quantities (e.g. velocity) have smaller timescalesThe Courant Number is often used to estimate a time step:

15、This gives the number of mesh elements the fluid passes through in one timestepTypical values are 2 10, but in some cases higher values are acceptableThe average and maximum Courant number is reported in the Solver out timestepA smaller timestep will typically improve convergenceIf required, boundar

16、y conditions can be functions of time instead of constant valuesVelocities, Mass flows, pressure conditions, temperatures, etc. can all be expressed as functionsIn CEL expressions use “t” or “Time”Can read in time varying experimental data through User FORTRAN3. Boundary ConditionsPhysically realist

17、ic initial conditions should be usedA converged steady state solution is often used as the starting pointIf a transient simulation is started from an approximate initial guess the initial transient will not be accurateThe first few timesteps may not convergeA smaller time step may be needed initiall

18、y to maintain solver stabilityFor cyclic behavior the first few cycles can be ignored until a repeatable pattern is obtained246810121416Time (seconds)4. InitializationResidualsThe transient scheme defines the numerical algorithm for the transient termTwo implicit time-stepping schemes are available:

19、First Order Backward Euler (more stable)Second Order Backward Euler (more accurate)The default Second Order Backward Euler scheme is generally recommended for most transient runsTimestep Initialisation controls the way the previous timestep is used as the starting point for the next timestepCan use

20、the last solution “as is”Or the solver can extrapolate the previous solution to try to provide a better starting pointNot recommended at high Courant numbersAutomatic (default) switches between the two depending on the Courant number5. Solver ControlThe Min. and Max. Coeff. Loops set limits on the n

21、umber of iterations to use within each timestepShould aim to converge each timestep within about 3 5 loopsComplex physics may need more loopsIf convergence is not achieved in the maximum number of loops, it is generally better to reduce the timestep size rather than increase the number of loopsThe s

22、olution will proceed to the next timestep regardless of whether the convergence criteria was metImportant to monitor the solution5. Solver ControlTransient ResultsBy default only a final res writtenNo information about the transient solutionNeed to define Transient Results under Output ControlTransi

23、ent Results OptionStandardLike a full results fileCan take up a lot of disk spaceSmallestWrites the smallest can still be used for a restart (still quiet large)Selected VariablesPick only the variables of interest to give smaller filesOutput FrequencyControls how often results are written6. Output C

24、ontrolTransient StatisticsUsed to generate running statistics for solution variables Arithmetic Average, RMS, Minimum, Maximum, Standard Deviation and Full (everything) are available optionsPick the variables of interestStart and Stop Iteration List defines when to begin and end collecting the statistics6. Output Control6. Output ControlMonitor Points are gene