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1、Chapter 15Review and Tips,Introduction to CFX,Domain Interfaces,Domain Interfaces can be used as part of a meshing strategy as well as for connecting different domains or reference frames togetherBoundary conditions are created in each domain when a Domain Interface is created;generally you should n
2、ot edit these directlyWhen the mesh is different on each side of the interface a GGI(General Grid Interface)is usedThis will use more memory in the Solver than a continuous meshAccuracy across a GGI interface is usually not a concern as long as the mesh length scales on each side are similarAutomati
3、c Domain Interfaces are created by CFX-Pre in some casesAlways check these and dont assume that all the required Domain Interfaces have been created,Sources,Sources are used to account for physics or processes that have not been directly resolved in the simulationMomentum sources can be used to crea
4、te a pressure drop(e.g.a screen,a porous material)or a pressure rise(e.g.a fan)Energy sources can account for heat added/removed from the simulationWhen sources are functions of the solved variable(e.g.momentum sources that are functions of velocity,energy sources that are functions of temperature)t
5、he Source Coefficient should be setThe Source Coefficient must be negative otherwise the solver will divergeMay need to re-write the Source so that is has a negative derivative,Transient Simulations,In a transient analysis the timestep should be small enough to capture the transient behaviour of int
6、erestBoundary conditions can be functions of timeConvergence should be monitored so that each timestep is convergedIt is generally better to reduce the overall timestep size to improve convergence rather than increasing the number of coefficient loopsRemember to create the Transient Results object b
7、efore running,Turbulence,Estimate the flow Reynolds Number to determine if the flow is laminar or turbulentCheck y+values to make sure the near-wall mesh is suitabley+300 for a Wall Function solutiony+=2 with the SST model for a low-Re solutionThe SST model is a good choice for a general turbulence
8、modelBe aware of the limitations of the turbulence model chosenRANS models resolve the mean flow field,therefore a lot of transient turbulent structures are not capturedThese may be important when simulating noise and vibrationThe k-e model can give inaccurate separation predictions,Heat Transfer,Hi
9、gh speed flows(Mach 0.2)should use the Total Energy modelThe double precision setting for the Solver is recommended for CHT simulations(i.e.when a solid domain is included)Always make sure energy imbalances have reached acceptable levels in CHT casesEnable Viscous Work or Viscous Dissipation if heat
10、ing due to viscous effects is importantIf thermal radiation is modeled choose an appropriate model depending on the optical thicknessThin Wall modeling and thermal contact resistances can be set at domain interfaces,Moving Zones,Moving boundaries can be simulated in several different waysFor rotatin
11、g walls,a wall velocity can simply be imposed if the motion is purely tangential(e.g.a rotating hub or a solid brake disk)When the rotating walls have a normal component of velocity they must be placed inside a rotating domain(e.g.blades,vented brake disk)Stationary walls then become counter-rotatin
12、g in the rotating domain and must form surfaces of revolution(i.e.no normal component of velocity)Although a Mesh Motion approach is possible,it is much more computationally expensiveMesh motion is usually used to simulate deforming boundaries or linear/cyclic motion,Moving Zones,At a change in refe
13、rence frame a frame change model is usedFrom low fidelity/cost to high fidelity/cost the choices are Frozen Rotor,Stage or Transient Rotor StatorOther approaches for moving regions are:Rigid Body MotionA 6-DOF solves calculates the solid body motionUsed in conjunction with Mesh MotionImmersed SolidU
14、sed to simulate moving solids that cannot be accommodated with Mesh Motion,Why Does My Case Fail in the Solver?,First carefully read the error messageThe error message may recommend setting an Expert ParameterThis may be an appropriate fix,or it may mask an underlying problemExample:+-+|Checking for
15、 Isolated Fluid Regions|+-+2 isolated fluid regions were found in domain R1 turn off this check by setting the expert parameter check isolated regions=f.This error usually means domain interfaces are missing,so setting the expert parameter would not usually be appropriate,Why Does My Case Fail in th
16、e Solver?,“Insufficient Memory Allocated”type errorsFirst check the.out file to see which process was running(Solver,Partitioner or Interpolator)Increase the Memory Alloc Factor in the Solver Manager(Define Run enable Show Advanced Controls Solver/Partitioner/Interpolator tab)“Not enough free memory
17、 is currently available on the system”A system limitation has been reached!“Memory”refers to RAMPossible solutions:Run in parallel or increase the number of partitions to distribute the memory loadReduce the memory requirements for the caseSmaller meshFewer or smaller GGI interfaces,Why Does My Case
18、 Fail in the Solver?,“Floating point exception:Overflow”The solver has divergedOften some of the equations willshow“F”instead of“OK”beforethe error messageWhen this error occurs in the firstfew iterations perform some basicchecks:Are the boundary conditions physical?Whats the Reference Pressure?What
19、 pressure is set at the boundaries?Whats the initial pressure?What direction would you expect the flow to go given the specified pressures?Reduce the timescale,particularly if the solver fails later in the run,Why Does My Case Fail in the Solver?,“Floating point exception:Overflow”Write out backup f
20、iles before thefailure and examine the solutionfields(Pressure,Velocity,)Look for the max/min values,theywill usually be very high/lowCan set the expert parameter“backup file at zero”to write out afile before the first iteration,showing the initial guessLook for the first“F”if U,V,W or P failed in t
21、he 10th iteration,but Turbulence failed in the 9th iteration,then check the turbulence field,Why Does My Case Not Converge?,Walls placed at outletsIf the warning message shown to the right appears during the solution it means that flow is trying to come back in through an outlet boundaryNot a proble
22、m if the message then goes awayOtherwise the outlet may be located in a recirculation zone,-COEFFICIENT LOOP ITERATION=6 CPU SECONDS=5.754E+05-|Equation|Rate|RMS Res|Max Res|Linear Solution|+-+-+-+-+-+|U-Mom|0.82|3.3E-06|3.3E-04|4.1E-02 OK|V-Mom|0.82|2.2E-06|5.6E-04|6.4E-02 OK|W-Mom|0.64|2.3E-06|9.2
23、E-05|1.6E-02 OK|P-Mass|0.66|2.3E-07|6.9E-06|21.6 1.7E-01 ok|+-+-+-+-+-+-+|*Notice*|A wall has been placed at portion(s)of an OUTLET|boundary condition(at 83.8%of the faces,89.9%of the area)|to prevent fluid from flowing into the domain.|The boundary condition name is:PV33.|The fluid name is:D2O.|If
24、this situation persists,consider switching|to an Opening type boundary condition instead.|+-+|K-TurbKE|0.45|1.4E-05|5.9E-04|5.9 2.7E-07 OK|E-Diss.K|0.45|4.5E-05|2.8E-03|7.3 6.5E-06 OK|+-+-+-+-+-+,Move the outlet or use an Opening boundaryOr,if the area fraction that has been“walled off”is 100%,then
25、the local fluid pressure is likely less than the specified boundary pressure,Why Does My Case Not Converge?,Changing the timescale can help convergenceSlow steady convergence may be accelerated through a larger timescaleBouncy convergence or solver failure may be fixed with a smaller timescaleSometi
26、mes simulations which are run in steady state mode will not converge even with good mesh quality and a well selected timescaleIf a steady state run shows oscillatory behavior of the residual plots,the flow may be transientRun the case in transient mode and observe if the residuals reduceIf convergen
27、ce has stalled try running in double-precisionWrite out the residual fields(Output Control Results Output Equation Residuals)and use Isosurfaces to look for the locations with high residuals,Setting Expert Parameters,Expert Parameter can be set in CFX-Pre,or by editing the CCLIn CFX-Pre:Inset Solver
28、 Expert ParameterMost,but not all Expert Parameters are shown in CFX-Pre,Setting Expert Parameters,In CCL add the EXPERT PARAMETER:object under the FLOW:object and type in the parameterYou can use the Command Editor in CFX-Pre(Tools Command Editor)to type in CCL,Mesh Refinement Studies,Errors in a c
29、onverged solution arise from:Numerical ErrorsE.g.round-off errors,convergence(lack-of)errorsModel ErrorsE.g.accuracy of boundary conditions,physical modelsDiscretization ErrorsErrors arising from converting the continuous governing equations into a discrete form that can be solved on a computerDiscr
30、etization errors reduce with mesh spacingMesh refinement studies are used to estimate the significance of discretization errors on your solutionMesh refinement studies are recommended for each new type of simulation you perform,Mesh Refinement Studies,A mesh refinement study consist of solving the s
31、ame case on progressively finer meshesEach mesh should be significantly finer than the previous,e.g.100k nodes,200k nodes,400k nodesThe quantities of interest should be evaluated and compared for each meshWhen the quantity reaches a steady value discretization errors are no longer significant,Quantity of Interest,#of Elements,Appropriate mesh,
