swirl flow—fluent常见问题.doc

The Single Reference Frame (MRF) ModelSolution Strategies for a Rotating Reference Frame: (Segregated solver only) Consider switching the frame in which velocities are solved by changing the velocity formulation setting in the Solver panel. (See Section 10.2.5: Choosing the Relative or Absolute Velocity Formulation for details.) (Segregated solver only) Use the PRESTO! scheme, which is well-suited for the steep pressure gradients involved in rotating flows. Ensure that the mesh is sufficiently refined to resolve large gradients in pressure and swirl velocity. (Segregated solver only) Reduce the under-relaxation factors for the velocities, perhaps to 0.30.5 or lower, if necessary. Begin the calculations using a low rotational speed, increasing the rotational speed gradually in order to reach the final desired operating condition (see below).The Multiple Reference Frame (MRF) Model(is recommended for steady state)limits: Use of the realizable k-e model with multiple reference frames is not recommended. Strictly speaking, the use of multiple reference frames is meaningful only for steady flow. However, FLUENT will allow you to solve an unsteady flow when multiple reference frames are being used. In this case, You should carefully consider whether this will yield meaningful results for your application. Particle trajectories and pathlines drawn by FLUENT use the velocity relative to the cell zone motion. For massless particles, the resulting pathlines follow the streamlines based on relative velocity and are meaningful. For particles with mass, however, the particle tracks displayed are meaningless. Similarly, coupled discretephase calculations are meaningless. An alternative approach for particle tracking and coupled discrete-phase calculations with multiple reference frames is to track particles based on absolute velocity instead of relative velocity. To make this change, use the define/models/dpm/options/track-in-absolute-frame text command. If you enable the track-in-absolute-frame option, the injection velocities are specified relative to the absolute frame. You cannot accurately model axisymmetric swirl in the presence of multiple reference frames using the relative velocity formulation. This is because the current implementation does not apply the transformation used in Equation 10.3-3 to theswirl velocity derivatives. Translational and rotational velocities are assumed to be constant (time varying w, vt are not allowed).The MRF Formulation:(define-solver velocity formulation )The relative velocity formulation is recommended, due to faster convergence, in cases where the flow in the rotating frame of reference is mostly rotating with the speed of the rotor, while the flow in the remainder of the domain is mostly slow. In general though, the absolute velocity formulation has proven to be efficient, and is recommended for most cases.Grid Setup for Multiple Reference Frames: If the boundary between two zones that are in different reference frames is conformal, you can simply create the grid as usual, with all cell zones contained in the same grid file. A different cell zone should exist for each portion of the domain that is modeled in a different reference frame. Use an interior zone for the boundary between reference frames. If the boundary between two zones that are in different reference frames is nonconformal, follow the non-conformal grid setup procedure described in Section 6.4.3: Using a Non-Conformal Grid in FLUENT.The Mixing Plane Model:If the flow at this boundary is not uniform, the MRF model may not provide a physically meaningful solution. In the mixing plane approach, each fluid zone is treated as a steady-state problem. Flow-field data from adjacent zones are passed as boundary conditions that are spatially averaged or mixed at the mixing plane interface.limitions: The mixing plane model requires the use of the absolute velocity formulation; you cannot use the relative velocity formulation with the mixing plane model. The LES turbulence model cannot be used with the mixing plane model. The models for species transport and combustion cannot be used with the mixing plane model. The general multiphase models (VOF, mixture, and Eulerian) cannot be used with the mixing plane model. The discrete phase model cannot be used with the mixing plane model for coupled flows. Non-coupled computations can be done, but you should note that the particles leave the domain of the mixing plane.FLUENT mxing-plane算法混合面算法的基本步骤如下:（1）进行转子区域与定子区域的流场计算。（2）计算转子出口、定子入口流场变量的平均值，并将结果分别作为下个迭代步中对方的边界条件，从而完成两个区域间的数据信息传递。（3）将平均值设定为边界条件。（4）重复（1）至（3）步的计算过程，直到计算收敛。setting parameters on mixing-planes panel:在三维计算中，再在Mixing Plane Geometry（混合面几何特征）中指定混合面的方向。选项Radial（径向）表示所计算的问题与图10-7 中的算例类似，所有变量是半径r 的函数，这个选项适用于轴向流动机械。选项Axial（轴向）表示所计算的问题与图10-8 中的算例在三维计算中，设定插值点（Interpolation Points）数量，这些插值点就是用来做周向平均的插值点。为了提高周向平均值的计算精度，这个点数应当接近于表面网格的节点数。另一个需要设置的参数是Globle Parameters（全局参数）下面的Under-relaxation（亚松弛因子），在开始计算的时候将这个参数设置为小于1 的数，可以避免计算在初始流场的剧烈调整过程中发散，其原理与其他计算中的亚松弛因子相同，这里不再赘述。混合壁面的选项:混合面模型有两个选项，一个是应用于不可压流计算的固定压强选项，一个是前面介绍的扭矩守恒选项。激活该选项的文字命令如下：define - mixing-planes - set - fix-pressure-level激活扭矩守恒选项的文字命令如下：define - mixing-planes - set - conserve-swirl然后键入enable?，并回答yes，就可以激活扭矩守恒选项。激活总焓守恒选项的文字命令如下：define - mixing-planes - set - conserve-total-enthalpy然后键入enable?，并回答yes，就可以激活扭矩守恒选项。混合面模型求解策略如果交界面附近没有回流，则利用混合面模型可以取得理想的结果。但是在刚开始计算的时候，原本没有回流的区域也可能出现回流，此时应用混合面模型可能无法获得收敛解。解决办法有两个，一个是在刚开始计算时，在交界面附近采用固定的边界条件，以便获得一个接近真实情况的初始流场；另一个是减小松弛因子，在计算稳定后，再逐步增大亚松弛因子。sliding mesh(When a time-accurate solution for rotor-stator interaction (rather than a time-averaged solution) is desired, you must use the sliding mesh model to compute the unsteady flow field.)Set boundary conditions for the sliding action:(a) Change the zone type of the interface zones of adjacent cell zones to interface in the Boundary Conditions panel.(b) In the Fluid panel or Solid panel for each moving fluid or solid zone, select Moving Mesh in the Motion Type drop-down list and set the translational and/or rotational velocity. (Note that a solid zone cannot move at a different speedthan an adjacent fluid zone.)Note that simultaneous translation and rotation can be modeled only if the rotation axis and the translation direction are the same .By default, the velocity of a wall is set to zero relative to the adjacent meshs motion. For walls bounding a moving mesh this results in a no-slip condition in the reference frame of the mesh. Therefore, you need not modify the wall velocityboundary conditions unless the wall is stationary in the absolute frame, and therefore moving in the relative frame. 必须Define the grid interfaces in the Grid Interfaces panelsetting interface(1) Specify the two interface zones that comprise the grid interface by selecting one in the Interface Zone 1 list and one in the Interface Zone 2 list.If one of your interface zones is much smaller than the other, you should specify the smaller zone as Interface Zone 1 to improve the accuracy of the intersection calculation.(2)Set the Interface Type, if appropriate. There are two options: Enable Periodic for periodic problems. Enable Coupled if the interface lies between a solid zone and a fluid zone, or if you would like to model a (thermally) coupled wall between two fluid zones using non-conformal interfaces.preview the sliding mesh: solve-moving meshAs the mesh preview option advances the time step, thereby updating the mesh, it is essential to reread the case file to ensure that the mesh is at the proper position before the calculation is started. If the case file is not read again and the solution is initialized after the mesh preview, the solution time is initialized to zero but the mesh does not go back to its original position.Dynamic Meshes:Specify the pressure-velocity coupling scheme. For transient flow calculations, the PISO algorithm is recommended, as it is the most efficient for such cases (see Section 26.9.2: PISO for details).动网格的计算方法有3 种，即弹性光顺法（spring-base smoothing）、动态层技术（dynamic layering）和局部网格重划法（local remeshing）。弹性光顺法将网格系统看作由节点之间用弹簧相互连接的网络系统，初始网格就是系统保持平衡的弹簧网络系统。任意一个网格节点的位移都会导致与之相连接的弹簧中产生弹性力，进而导致临近网格点上的力平衡被打破。由此波及出去，经过反复迭代最终整个网格系统达到新的平衡时，就可以得到一个变形后的、新的网格系统。For non-tetrahedral cell zones (non-triangular in 2D), the spring-based method is recommended when the following conditions are met: The boundary of the cell zone moves predominantly in one direction (i.e., no excessive anisotropic stretching or compression of the cell zone). The motion is predominantly normal to the boundary zone.By default, spring-based smoothing on non-triangular or non-tetrahedral cell zones are turned off. If you want to use spring-based smoothing on all cell shapes, you can turn on the model for these zones using the spring-on-all-shapes? text-interface command: define-models-dynamic-mesh-controls-spring-on-all-shapes? Spring-Based SmoothingYou can control the spring stiffness by adjusting the value of the Spring Constant Factor between 0 and 1. A value of 0 indicates that there is no damping on the springs, and boundary node displacements have more influence on the motion of the interior nodes. A value of 1 imposes the default level of damping on the interior node displacements as determined by solving Equation 10.6-8. 从图中可以发现1时壁面发生变形，壁面附近网格因为过度加密而质量下降，0时壁面没有发生变形。因此在实际计算中应该在0 到1 之间选择一个适当的值。the Boundary Node Relaxation. A value of 0 prevents deforming boundary nodes from moving (equivalent to turning o smoothing on deforming boundary zones) and a value of 1 indicates no under-relaxation.这个值应该取为0 到1 之间的一个值，以保证边界节点以合适的移动量发生移动。局部网格重划法是对弹性光顺法的补充。在网格系统用三角形或四面体网格组成时，如果边界的移动和变形过大，可能导致局部网格发生严重畸变，甚至出现体积为负的情况。在这种情况下，一个简单的处理方法就是去掉由原来网格系统经过弹性光顺得到的新网格，在原来的位置上重新划分网格，这就是局部网格重划法的基本思路。Local Remeshing Method:1)Face Region Remeshing Method:For face region remeshing, FLUENT marks the region of faces on the deforming boundaries at the moving boundary based on minimum and maximum length scales. zone, the faces on the deforming face zone can be remeshed only if the following conditions are met: The faces are triangular (or linear in 2D). The faces to be remeshed are all adjacent to moving loops (i.e., moving nodes). Note that you cannot use the face region remeshing method in conjunction with hanging node adaption. Local Face Remeshing Method: The local face remeshing method only applies to 3D geometries. Using this method, FLUENT marks the faces (and the adjacent cells) on the deforming boundaries based on the face skewness. zone, the faces on the deforming face zone can be remeshed only if the following conditions are met: The faces are triangular. The faces do not exists across zones or features. Note that you cannot use the local face remeshing method in conjunction with hanging node adaption. For more information on hanging node adaption.Local Remeshing Based on Size Functions:Instead of marking cells based on minimum and maximum length scales, FLUENT also marks cells based on the size distribution generated by the sizing function if the Sizing Function option under Options is enabled.和就是需要设置的Size Function Variation（尺寸函数增量）和Size Function Rate（尺寸函数变化率）。以边界网格尺寸为基准控制内部网格的大小，等于0.5 表示内部网格的尺寸至少是边界网格的1.5 倍，等于-0.5 表示内部网格的尺寸等于边界网格的0.5 倍，如果等于0 则表示内部网格与边界网格大小相同。的取值在1 和1 之间，用于控制网格从边界到内部区域的变化速率。取正值表示网格变化速率较慢，取负值则表示变化速率较快，取0 则表示网格从边界到内部区域呈线性变化。You can also control the resolution of the sizing function with Size Function Resolution. The resolution determines the size of the background bins used to evaluate the size distribution with respect to the shortest feature length of the current mesh. By default, the Size Function Resolution is 3 in 2D problems, and 1 in 3D problems.动态层技术是根据边界的移动量动态地增加或减少边界上网格层的技术，因此动态层技术适用于六面体网格、楔形网格等可以在边界上分层的网格系统。动态层技术在边界上假定一个优化的网格层高度，在边界发生移动、变形时，如果临近边界的一层网格的高度同优化高度相比大到一定程度时，就在边界面与相邻网格层之间增加一层网格。相反，如果边界向内移动，临近网格被压缩到一定程度时，临近一层网格又会被删除。用这种办法保持边界上的网格保持一定的密度。Applicability of the Dynamic Layering MethodYou can use the dynamic layering method to split or merge cells adjacent to any moving boundary provided the following conditions are met: All cells adjacent to the moving face zone are either wedges or hexahedra (quadrilaterals in 2D) even though the cell zone may contain mixed cell shapes. The cell layers must be completely bounded by one-sided face zones, except when sliding interfaces are used (see Section 10.6.3: Applicability of the Face Region Remeshing Method). If the bounding face zones are two-sided walls, you must split the wall and wallshadow pair and use the coupled sliding interface option to couple the two adjacent cell zones. Note that you cannot use the dynamic layering method in conjunction with hanging node adaption. 如果网格层j 扩大，单元高度的变化有一临界值：hmin(1+s)h0 式中hmin 为单元的最小高度，h0 为理想单元高度，s 为层的分割因子。在满足上述条件的情况下，就可以对网格单元进行分割，分割网格层可以用常值高度法或常值比例法。在使用常值高度法时，单元分割的结果是产生相同高度的网格。在采用常值比例法时，网格单元分割的结果是产生是比例为s 的网格。在Layering（动态层）标签下，可以设置与动态层模型相关的参数。通过设定Constant Height（常值高度）与Constant Ratio（常值比例）可以确定分解网格的两种方法。Split Factor（分割因子）和Collapse Factor（合并因子）则分别为s 和c 。sett