Icem_CFD_CFX流體學習資料 Icem CFD CFX流體學習資料 CFX_ICEMCFD_11_培訓教程_2008_04 CFX110 CFX11_04_BC

1、CFX-PreBoundary Conditions,Lecture 4,Boundary Condition Types,Required on all regions at the outer extremities of the domain to be simulated (bound the problem) There are 5 basic flow boundary conditions,Inlet,Opening,Outlet,Wall,Symmetry,as shown earlier,How to Create a Boundary Condition,Defining

2、a Boundary ConditionBasic Settings,Boundary Type Inlet, Outlet, Opening, Wall, Symmetry Location select from composite regions in drop down menu or from all 2D primitives by clicking Coord Frame if more than one exists, select the appropriate frame Frame Type available only in a rotating domain. All

3、ows you to specify quantities based on a rotating or stationary (absolute) frame of reference. Profile Boundary Conditions Discussed in upcoming slides,Inlet Velocity Components-Static Temperature (Heat Transfer) Normal Speed-Total Temperature (Heat Transfer) Mass Flow Rate-Total Enthalpy (Heat Tran

4、sfer) Total Pressure (stable)-Relative Static Pressure (Supersonic) Static Pressure-Inlet Turbulent conditions Outlet Average Static Pressure-Normal Speed Velocity Components-Mass Flow Rate Static Pressure Opening Opening Pressure and Direction-Opening Temperature (Heat Transfer) Opening Pressure (E

5、ntrainment)-Opening Static Temperature (Heat Transfer) Static Pressure and Direction-Inflow Turbulent conditions Static Pressure (Entrainment) Velocity Components Wall No Slip / Free Slip-Adiabatic (Heat Transfer) Roughness Parameters-Fixed Temperature (Heat Transfer) Heat Flux (Heat Transfer)-Heat

6、Transfer Coefficient (Heat Transfer) Symmetry No details (only specify region which corresponds to the symmetry plane,Covered in Theory Section at end of lecture,Defining a Boundary ConditionBoundary Details,Boundary ConditionsSpecifying Well Posed Boundary Conditions,The boundary settings have a st

7、rong effect on the convergence rate and on the accuracy of the results,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Inflow (Inlet) Inlet normal velocity 50 m/s Outflow (Outlet) static pressure 0 Pa,Mass flow,Mass flow,Inflow (Inlet) Inlet normal velocity 50 m/s Outflow (Opening) Aver

8、age pressure 0 Pa,Example 1,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Mass flow diverges,Outlet,Mass flow,Fast convergence of mass flow,restart,Inflow (Inlet) Inlet normal velocity 25 m/s Outflow (Outlet) Static Pressure,Inflow (Inlet) Static Pressure Outflow (Opening) Static Pres

9、sure,Example 2,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Do not set boundary conditions that over-specify or under specify the problem Over Specifying Not giving the solver the freedom to approach the correct answer Difficulty converging Under Specifying Not giving the solver enou

10、gh information to calculate values Non physical solution,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Example,Governing Equations 1) Mass conservation 2) X-momentum conservation,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Example,Case 1: Incompressible Fluid 1) Mass

11、conservation IMPOSSIBLE CRASH 2) X-momentum conservation,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Example,Case 2: Compressible Fluid 1) Mass conservation OVER-SPECIFIED 2) X-momentum conservation UNDER-SPECIFIED,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Example

12、,Case 3: Most Robust Setting 1) Mass conservation WELL DEFINED 2) X-momentum conservation WELL DEFINED,or,5 kg/s,Pstatic = 150 kPa,5 m/s,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Recommended Configurations Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet.

13、The Inlet total pressure is an implicit result of the prediction. Robust: Total Pressure at an Inlet; Velocity/Mass Flow at an Outlet. The static pressure at the Outlet and the velocity at the Inlet are part of the solution. Sensitive to Initial Guess: Total Pressure at an Inlet; Static Pressure at

14、an Outlet. The system mass flow is part of the solution Very Unreliable: Static Pressure at an Inlet; Static Pressure at an Outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combinati

15、on is a very weak constraint on the system,Boundary ConditionsSpecifying Well Posed Boundary Conditions,From the numerical point of view, it is often very useful to extend the location of inlets and/or outlets,Use care; symmetric geometry does not necessarily mean symmetric flow Example: The coanda

16、effect. A jet entering at the center of a symmetrical duct will tend to flow along one side above a certain certain Reynolds number,No Symmetry Plane,Symmetry Plane,Boundary ConditionsSpecifying Well Posed Boundary Conditions,Boundary ConditionsKey Points,Specify boundary conditions that define but

17、do not over-specify the problem Choose the domain bounds which allow for the most accurate boundary conditions,Boundary Conditions Theory,Inlet Turbulence Conditions,Unless turbulence is being simulated, it is accounted for by modeling the transport of turbulence properties for example k and (Domain

18、s Theory) Similar to momentum and mass, turbulence variables require boundary condition specifications,Inlet Turbulence Conditions,Several options exist for the specification of turbulence quantities at inlets (details on next slide) Unless you have absolutely no idea of the turbulence levels in you

19、r simulation (in which case, you can use the Medium (Intensity = 5%) option), you should use well chosen values of turbulence intensities and length scales Nominal turbulence intensities range from 1% to 5% but will depend on your specific application The default turbulence intensity value of 0.037

20、(that is, 3.7%) is sufficient for nominal turbulence through a circular inlet, and is a good estimate in the absence of experimental data,Inlet Turbulence Conditions,Default Intensity and Autocompute Length Scale The default turbulence intensity of 0.037 (3.7%) is used together with a computed lengt

21、h scale to approximate inlet values of k and . The length scale is calculated to take into account varying levels of turbulence. In general, the autocomputed length scale is not suitable for external flows. Intensity and Autocompute Length Scale This option allows you to specify a value of turbulenc

22、e intensity but the length scale is still automatically computed. The allowable range of turbulence intensities is restricted to 0.1%-10.0% to correspond to very low and very high levels of turbulence accordingly. In general, the autocomputed length scale is not suitable for external flows. Intensity and