http://www.caeda.com.cn/blog/ PJBlog3 2008-11-20T09:37:56+08:00 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-11-20T09:37:56+08:00 2008-11-20T09:37:56+08:00

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http://www.caeda.com.cn/blog/default.asp?id=98 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-11-18T08:58:03+08:00 2008-11-18T08:58:03+08:00

In CFD-GEOM, a convenient meshing tool called "Butterfly Faces" allows the user to quickly and automatically create a structured mesh for circular topologies. The resulting mesh consists of 5 structured faces (4 sides and 1 core) inside a circle. Parameters are available to set the size of the core and the mesh density.



Figure 1a. Circle with four edges                                          Figure 1b. Resulting butterfly mesh


To start, create a circle and split it into 4 arcs. Create edges on the arcs, making sure that opposite sides have the same number of grid points. In this case (Figure 1a), 11 grid points have been placed on each arc.

In the "Structured Face Options" panel, select the "Butterfly Face" tool, shown in figure 2.



Figure 2. butterfly mesh tool

Select each edge followed by a middle mouse click. Figure 3 depicts what you should see after picking all four edges.



Figure 3. Edge selection for the butterfly mesh tool

The user has control over the radius of the Major and Minor axes. Note: this radius is measured from the center of the circle. Figure 4 shows the Butterfly faces setting options. Once the major and minor radii have been defined, click Apply and you will get a Butterfly mesh as shown on top of this page in figure 1b.



Figure 4. Butterfly faces setting options

  

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know
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http://www.caeda.com.cn/blog/default.asp?id=97 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-11-12T08:34:07+08:00 2008-11-12T08:34:07+08:00

Batch processing through scripting appeared in CFD-VisCART v2006 and this feature kept improving since then. You can see a short description in this past user tip. This can be very useful to create meshes without graphical interaction or to set up similar models over and over. In CFD-VisCART v2009, a new feature will make its use even easier and powerful: You will now be able to dump a batch script from an existing VGD file.

After loading a pre-set vgd file in CFD-VisCART graphical interface, or creating one, you can now output a script file automatically with all the commands needed to recreate this same VGD file in batch mode.
The main advantage obviously is the ability to easily create script files for similar models. After setting up one model, you can dump one script, and modify it for all your similar models. Parameters, such as mesh sizes, components, mesh sources…, can be modified and adapted.
Another advantage of this feature is that you don’t need to know the batch script commands to write the scripts you need. You can simply set up a vgd file as you would normally do in the graphical window and then dump a script that you can reuse, without knowing any scripting commands

You can use this feature from the File menu (Figure 1) or the Toolbar (Figure 2).



Figure 1. Dump Script option from the File menu




Figure 2. Dump Script option from the Toolbar


After clicking on the Dump Script icon, a file saving dialog button will appear to specify the dump script file. The Default extension is .vbt (CFD-VisCART batch text file), but you can specify any name and extension you would like.

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http://www.caeda.com.cn/blog/default.asp?id=96 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-11-05T09:56:51+08:00 2008-11-05T09:56:51+08:00

CFD-VisCART is a 3D unstructured adaptive Cartesian mesh generator. It is ideally suited for external aerodynamics and automotive underhood applications involving extremely complex geometries. Models/geometries in SAT, Parasolid, STL, Nastran, 3DS, Plot3d and FAST formats can be imported into CFD-VisCART.

A typical complete assembly/model of a car or an aircraft consists of hundreds of parts and surfaces. While reading in a model into CFD-VisCART, the user is asked if the model should be read in as a single patch or surface (figure 1). This is a recommended option (specially for large models), as it would result in lower memory being used and consequently faster processing.



Figure 1. Option for reading in as a single patch.

While setting up the meshing parameters, some surfaces of the model may need to be assigned as sources. Also, the user may want to blank-out some of the surfaces to be able to see other underlying surfaces. For these and other similar reasons, the user may want to split the model to some extent after reading it in as a single patch. This tip illustrates the various ways in which a model can be split in CFD-VisCART by using some basic tools (see figure 2).



Figure 2. Detection and splitting tools


Split using Outlines

An outline is a curve that links the continued edges of the model if they are only shared by one surface triangle or by more than two triangles. To use this option for splitting the model, use the following steps :

Select the model
Click the Outline Detection button - Outlines appear on the geometry (see figure 3)
Click the Split Selected Geometry button - a pop-up window shows the user the number of surfaces the model will be split into and asks whether or not to proceed. Click Yes. The result is showed in figure 4.



Split using Critical Features

A critical feature is the curve that links the continued edges of the model if the edge is shared by only two triangles and the angle between these two triangles is smaller than that defined by the user. To use this option for splitting the model, use the following steps:

Select the model
Click the Critical Feature Detection button - a Set Feature Detection Options panel pops up, depicted in figure 5. Here, the user can specify the minimum critical angle. Note: The smaller the angle, the larger the number of critical feature lines.


Figure 5. Critical feature detection

Click Detect - Critical feature lines appear on the model
Click the Split Selected Geometry button

Split using Cutting Planes

Cutting planes, normally used to view the grid on a cross-section, can also be used to split a model. To use this option for splitting the model, use the following steps:

Scroll the cutting plane to the desired location. Figure 6 shows the cutting plane as a light blue line.


Figure 6. Position the cutting plane at the desired location

Select the model
Click the Split Selected Geometry button - make sure no outlines or critical features are present, otherwise the model will be split along these lines.Figure 7illustrates the resulting geometry.



Figure 7. Geometry after splitting with a cutting plane

  

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know
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http://www.caeda.com.cn/blog/default.asp?id=95 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-11-02T09:47:01+08:00 2008-11-02T09:47:01+08:00

ESI Group's CFD Division is proud to announce that the V2009 release is on schedule and ready for BETA evaluation. The fact that this release comes just six months after our last major release proves that ESI Group is working hard to implement the features that the market demands in a timely manner. We welcome and encourage all of our customers to download and use the latest versions which include numerous enhancements.

Included in the V2009 release are significant updates to:

CFD-ACE+ Advanced CFD and Multiphysics Solver
CFD-FASTRAN CFD Physics Solver
CFD-CADalyzer Upfront CFD Design Tool

CFD-GEOM Geometry Modeler and Mesh Generator
CFD-VisCART Cartesian Mesh Generator
CFD-VIEW Solution Analysis and Visualization Tool
Enhancements have been made to increase usability, robustness, and accuracy and we are sure that these new versions will make for a worthwhile upgrade to your CFD toolset.

The remainder of this note gives you some information about the V2009 software release and the BETA program. Please read on to learn more about the updates and how to participate in the BETA program and let us know if you have any comments or questions.

IMPORTANT NOTE: You will require a new license to run V2009.x versions! ESI Group licensing policy requires licensing to the version of the software. This means that if you currently have a V2008.x license file you will need to replace it with a V2009.x license file. These are available upon request to your ESI license or sales representative. If you are not sure who that is, you should contact support.cfd@esi-group-na.com, and please include your existing V2008.x license file for the quickest service.

V2009 Release Highlights

As with previous releases, there are many improvements to features, performance, and robustness. Below is a short bullet list of some of the top reasons why you should consider upgrading to V2009. For a full description of the new and improved features, as well as usage and limitation information, please consult the relevant release note document, which is included in the BETA package download files and can also be found here (login required).

CFD-ACE V2009.0 (view full release notes - login required)
Improved Fast Time Stepping (FTS) performance
Extended Fast Time Stepping (FTS) to VOF and Stress modules
Automated Force and Moment coefficient calculations on specified surfaces

Force and moment information now available for momentum resistance regions
Improved convergence for two-fluid module (may not be available until final release)
Porous media with two-fluid capability (may not be available until final release)
Improved v2f turbulence model
ESI Mobile support (control runs / view residuals remotely on iPhone or iPod-touch)
CFD-FASTRAN V2009.0 (view full release notes - login required)
Spatially varying boundary conditions (not available until final release)
AUSM scheme (may not be available until final release)

CFD-CADalyzer V2009.0 (view full release notes - login required)
Improved animation recording
Autodesk Inventor 64-bit Support

CFD-GEOM V2009.0 (view full release notes - login required)
Remeshing of discrete surfaces
Better handling of discrete shells and domains for use in volume meshing
Boundary layers for discrete surfaces
Advancing front methodology for unstructured triangular surface meshing
Ability to split trimmed surfaces
Ability to combine surfaces
Tool to minimize number of curves

CFD-VisCART V2009.0 (view full release notes - login required)
Smooth / delayed transition of cell sizes
Bounding box surface can be boundary for mesh refinement
Faster mesh generation with less memory usage
Batch scripting improvements

CFD-VIEW V2009.0 (view full release notes - login required)
ESI Mobile support (post-process solutions remotely on your iPhone or iPod-touch)
Improved animation quality
Improved macro user interface
V2009.0 Installation
The V2009.0 BETA software will be installed parallel to your existing V2008.2 (or earlier) RELEASE installation. This means that you will have access to both versions should you desire. However, only V2009.0 will be supported going forward so you should use V2009.0 to get access to future bug fixes and feature improvements. Step-by-step instructions for the installation of V2009.0 can be found here (login required).
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http://www.caeda.com.cn/blog/default.asp?id=94 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-10-28T08:25:11+08:00 2008-10-28T08:25:11+08:00

The mesh has a great influence on the solver convergence and solution of every CFD simulation. The user is strongly advised to check the quality of a numerical mesh. Several parameters that reflect the quality of a mesh are available. In CFD-GEOM, under the tab Mesh/Mesh Query Options, the user can check 2D as well as 3D mesh quality. In figure 1, 3D Mesh Queries for Aspect Ration has been called. See figure 2 for an example for the Centroid to Face Angle.



Figure 1. Mesh Queries tool


Aspect Ratio
If the model contains thin layers, which are mostly used as boundary layer or for the different layers of the MEA inside fuel cells (see figure1), it may require placing several cells normal to the plate (thickness). Due to this, aspect ratios of several thousands can occur (as shown in figure 1). According to our experience, aspect ratios for such models up to 1500 are acceptable, for flow, heat and electrochemistry. If the same geometry is used for a solid stress analysis, this ratio must be reduced down to 15. The reason for this great discrepancy may be in the different solver structure. It is strongly recommended to check this ratio in CFD-GEOM before writing out the DTF file.

Centroid/Face Angle
Especially in unstructured meshes, very small centroid to face angle can occur (see figure 2), mostly inside thin layers but also in arbitrary interfaces. When arbitrary interfaces are called, a common, new interface with additional cells is automatically created the solver. The quality of these cells will be checked by the solver and printed out in the CFD-ACE output file to inform the user about the status of this new interface (see figure 3). Models containing many bad cells as shown in figure 3 will not run successfully. By changing the edges mesh distribution and/or density as well as the number of cells, one can reduce the number of bad cells. As soon as the warning message no longer occurs, the solver will start running satisfactorily.



Figure 2. Centroid/Face Angle quality tool




Figure 3. Arbitrary interface data in output file


Volume
Curved surfaces or splines can create overlapping areas. Due to this, cells with negative volumes can occur. Mesh refinement and/or repairing of geometrical data can resolve this problem. The user can determine the number of such bad cells in CFD-GEOM as well as in the output file.
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http://www.caeda.com.cn/blog/default.asp?id=93 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-10-21T08:21:56+08:00 2008-10-21T08:21:56+08:00

In CFD modeling and analysis, there are times you would like to change the grid and rerun the case. These studies require coarsening and/or refining of the grid on the same model. In complicated 2D or 3D structured grid, changing the edge distribution on individual edges can be time consuming. Matching the same edge distribution on all edges in the group by changing the distribution on the edges one by one can become cumbersome. This work can be made very simple and quick by using the Edge Linking tool in CFD-GEOM.

The Edge Linking tool (Figure 1) allows you to link edges together such that when one edge in the group is updated, the changes are applied to all the edges in that group. By using this option, you can specify the complete grid propagation pattern throughout the model to automatically refine or coarsen the mesh.

Edge linking groups consist of two or more edges. The first edge added to the group is the Master Edge. Subsequent edges added to the group are called Secondary Edges. There are three link options available:

Number of points
Distribution
Both


Figure 1. Edge Linking Tool

Consider the simple example shown in Figure 2; the left edge is the master edge while the remaining three vertical edges are secondary edges. Edge linking groups are colored with the 'Picked Set 3' color, as opposed to the regular dark blue edge color.



Figure 2. Master and Secondary edges in example case

Number of Points: When a secondary edge is linked according to the number of points, it will always have the same number of grid points as the master edge, but its distribution type is independent of that of the Master edge.
Distribution: When linked according to distribution, the secondary edge will always have the same distribution as the Master edge, but the number of points on the secondary edge is independent of that of the Master edge.
Both: When a secondary edge has "Both" link type, it will always have the same number of grid points and distribution as the master. Within an edge linking group, secondary edges can have different link types. An edge can belong to one, and only one, edge linking group.

How to Create an Edge Linking Group:
Click the Edge Linking button in the Edge toolbox. The Edge->Link Edges panel appears
Select a master edge with the left mouse button. It will be highlighted with the "picked" color. When creating a new link group, you should not select a master edge or secondary edge from an existing group.
Select all additional edges to include in the group.
Select one of the Link Options in the Edge->Link Edges panel and click the Apply button. This will create the edge linked group. Please note that all edges in the group will get the currently selected link type.

How to Add an Edge to an Edge Linking Groups:
Select the Edge Linking button from the Structured Edge toolbox.
Select an edge in the group to which you want to add edges.
Select any additional edges. These edges should not belong to existing groups.
Select the link type and press the middle mouse button to complete the group. All edges in the group will get the currently selected link type.

How to Remove an Edge from existing Edge Linking Group:
Select the Edge Linking button from the Structured Edge toolbox.
Select the edge you want to remove. If the edge is part of an edge linking group, both it and the master edge will be highlighted.Note that you cannot remove the master edge from the group.
Click the Remove button in the edge linking toolbox. This will remove the edge from the edge linking group. You can repeat this process for other edges in the group.

How to Delete an Edge Linking Group:
Select a member of the group.
Press CTRL-A to select all.
Click the Remove button.

Useful Tips:
For edges that have faces built from them, it is recommended that you only use the link option on them when all edges in the group have the same number of grid points.
When you assign a "Both" or "Distribution" link type to a secondary edge, and the distribution type is hyperbolic tangent, the secondary and Master edges should be of similar length. Otherwise, you may encounter a situation in which the sum of the specified endpoint spacing is larger than the length of the secondary edge. In such situations, CFD-GEOM assigns a uniform power law distribution to the secondary edge. ]]>

http://www.caeda.com.cn/blog/default.asp?id=92 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-10-14T08:23:10+08:00 2008-10-14T08:23:10+08:00

Special approaches are needed when dealing with the modeling and analysis of various complex rotating systems having a stator and a rotor, in single or multistage, with change in the frame of reference from fixed to rotating (namely fans, pumps, compressors, turbines, propellers, impellers, diffusers, stators, vanes etc.). Indeed, all parts attached to the rotating shaft will be rotating with a certain angular velocity with respect to the machine axis, while the stationary parts such as stator, casing, inlet(s), exit(s) etc., do not move. When viewed by a stationary observer, the flow around the moving parts is always unsteady, even if the flow is apparently of steady state. When viewed by an observer stationed on the rotor, such a flow appears steady. For unsteady flow problems such as transients, it does not matter whether the observer is situated on the stationary or moving parts.


So, the important issue is the transfer of information from one stationary frame to the moving reference frame and vice-versa. By changing the frame of reference, quantities like normal, shear and deformation stress tensors are invariant. However, quantities like velocity, acceleration and rotating tensors depend on the frame of reference, e.g. static pressure does not change with change in coordinate frame (Pstatic); but total pressure changes with the coordinate frame. Additional complexities in rotating systems arise from three dimensionality, complex shapes with curvature, accelerated/decelerated flows, rotor-wake-stator/stator-wake-rotor interactions, etc.

Hence, special approaches/models come in handy to convert a very complex transient problem into a steady state problem and thus reduce the modeling and analysis time drastically. Proven approaches are:

Body Force Model (also known as lumped parameter model, mostly applicable for fans),
Multiple Reference Frame model (MRF),
Mixing Plane Model (MPM),
Sliding Mesh method.
All the above models are made available in CFD-ACE+. This tip describes the Multiple Reference Frame (MRF) feature. If you are interested in this tip please read on.



The MRF model accounts for complete blade design and fan details to simulate complex turbo machinery. It is a steady state approximation where the fluid zone in the fan region is modeled as a rotating frame of reference and the surrounding zones are modeled in a stationary frame. Contrary to the Body Force Model, the MRF model includes the geometry of the fan blades. The fan blades are modeled stationary but since the fluid domains surrounding them are in a rotating frame, the pressure jump and the swirl components are given by the presence of blades as wall without the need of experimental data as an input.

The major difference between MRF and Mixing Plane Methods is that the MRF directly translates the properties of the flow at the interfaces between the rotating and stationary zones, whereas the MPM averages the properties of the flow circumferentially. The MPM avoids flow field non-uniformities that arise from the fact that the fan blades are modeled stationary.

The steady state approximation of MRF allows individual cell zones to rotate or translate with different speeds. This is achieved by dividing the whole problem domain (say a rotor/stator interaction for rotating machinery) into separate zones where the flow is solved in stationary or rotating coordinate systems. The MRF approach is good when the flow (say between two rotors moving in counter directions or a rotor and a stator) is nearly uniform.



MRF and Navier-Stokes Equations of Motion:
The MRF transforms the fluid velocities from stationary to rotating frames using the following relation:

Stationary Frame



Rotating Frame



Where: Velocity Relative to Rotating Frame = Absolute Velocity - Whirl Velocity




Solving the equations of motion in the rotating reference frame results in additional acceleration terms in the momentum equation. Also, for a reference frame rotating at a constant angular velocity, the body force per unit mass includes both Coriolis and centrifugal forces. MRF solves the equations of motion in both stationary frame (for absolute velocity) and rotating frame (velocity in the rotating reference frame). Therefore, with the general MRF capability, steady state analysis can be performed on various components of the rotating system using local reference frames, either stationary or rotating as appropriate.

As shown in figure 2, CFD-ACE+ offers three options under MRF. They are:




Figure 2: CFD-ACE+ Multiple Reference Frame Options




Frame (Global): Select this option when you would like to solve the system as a Rotating Frame Reference to yield relative velocity solutions. Inputs are: Point on axis of rotation and Angular velocity vector.
Multiple Frame (VC Based): Select this option for more than one rotating frame of reference, like for fans and impellers. Inputs are: Point on axis of rotation and Angular velocity vector.
Multiple Frame (Geometry based, see figure 3): Use this option when the rotating domains are not defined as separate volume conditions. Here, a virtual cylinder will be created to define the rotating region. Two levels adjacent to this region are used as the rotating frame boundaries. Inputs are: center location, normal direction, thickness, radius of virtual cylinder geometry and angular rotation speed.



Figure 3: CFD-ACE+ Multiple Reference Frame - Geometry Based Option


To work with the first two options of MRF, select ‘Rotation’ under ‘MO’ tab. Select ‘Rotation’ under ‘VC’ tab, activate ‘Rotating Frame’, choose Fluid zone to be rotated from Explorer and specify the values for input fields.

Select Multiple frame (Geometry based) option from ‘Rotation’ under ‘MO’ tab. Select ‘MRF’ tab adjacent to ‘BC’ tab in ACE-GUI control panel, hit on ‘Add’ button in the Explorer window to add a multiple reference frame, choose virtual cylinder zone ‘MRF’ from Explorer and specify the values for input fields.

Convention followed in CFD-ACE+ for Angular rotation speed; angle is positive for counter-clockwise rotation and negative for clockwise rotation.

All the boundary conditions values must be specified in absolute frame of reference. Walls that rotate and appear in absolute frame of reference, e.g. blade surfaces must be specified as rotating walls. The ACE-SOLVER will automatically convert all velocities specified from the absolute frame to the rotating frame using the frame information provided in the Problem Type setting.


Try using the MRF feature next time. If you have any questions about this feature or would like us to discuss other topics in the future please let us know.
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http://www.caeda.com.cn/blog/default.asp?id=91 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-10-07T08:16:47+08:00 2008-10-07T08:16:47+08:00

While working with CFD-VIEW, the user has the option of saving an mdl file. This file stores all the entities created in CFD-VIEW, from the point the DTF file (or other data file) was imported, to their current state at the time of saving. The user can thus reopen this file anytime later to resume work.

By default, the ‘Save All Data with Model’ option under the File menu is checked (see figure 1), making the mdl file store all parts of the model, including the data that was read from the DTF file. Although this makes the file independently portable, the file size can be large.



Figure 1. ‘Save All Data with Model’ option

If the user does not plan to move the DTF file from its current location, the above option can be unchecked before saving the mdl file. The mdl file will then store only a link to the DTF file instead of the entire DTF data, thus significantly reducing its size.
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http://www.caeda.com.cn/blog/default.asp?id=90 admin http://www.caeda.com.cn/blog/ caeda@vip.sina.com 2008-10-01T08:45:00+08:00 2008-10-01T08:45:00+08:00

Carpet Plots are used for visualization of computed data on 2D surfaces. A carpet plot is created by displacing the 2D surface vertically based on one of the variables computed over that surface. The carpet plot can also be colored by the same or different variable.

To create a carpet plot (see figure 1):

Create a plane cut (Z_cut shown in figure 1).
Click on the Carpet Plot icon - the control panel will switch to carpet plot controls. This panel asks for two inputs - Carpet Plot Elevation and Carpet Plot Color.



Figure 1. Carpet plot icon and controls


Using this tool, two figures (see figure 2) showing temperature and velocity magnitude distribution on a Z-cut can be replaced with one carpet plot as depicted in figure 3. In this example, the carpet plot was created over the Z-cut (a 2D surface). The inputs are also shown.



Figure 2. Velocity magnitude (left) and temperature (right) distribution





Figure 3. Carpet plot using temperature for elevation and velocity magnitude for color

With this type of plot, the regions of high and low temperatures are readily visible, as well as the strong gradient identified by the steep regions of the surface.


The Carpet plot menu (see figure 4) allows the user to change axes labels, the surface properties and take a snapshot of the plot.



Figure 4. Carpet plot menu


If you have any questions about this feature or would like us to discuss some other topic in the future, please let us know.
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