Meshing User''''s Guide ANSYS phần 4 potx

Defining Pinch Control Automation Based on Shell Thickness This section describes the steps for defining pinch control automation based on shell thickness.. For information Defining Pinc

Only Automatic pinch controls are regenerated That is, if a pinch control has a Scope Method

of Manual (either because it was created manually or because you made a change to an

Auto-matic pinch control), the pinch control will never be regenerated on refresh See Changing Pinch

Controls Locally (p 183) for information about making changes to pinch controls

How to Define Pinch Control Automation (p 96) provides the steps for defining automatic pinch controls

How to Define Pinch Control Automation

The following sections provide the steps for defining pinch control automation Pinch can be automatedbased on either shell thickness or a user-defined tolerance

Note

Use of pinch control automation will delete all existing pinch controls that have a Scope Method

of Automatic before creating the new pinch controls.

Defining Pinch Control Automation Based on Shell Thickness

This section describes the steps for defining pinch control automation based on shell thickness This procedureapplies to sheet (i.e., surface) models only

To define pinch control automation based on shell thickness:

2 Set Use Sheet Thickness for Pinch to Yes

grayed out

A pinch control object is automatically inserted into the Tree for each region containing features thatmeet the criteria established by the pinch control settings To display details about an individual pinchcontrol, highlight it in the Tree and information about it appears in the Details View For information

Defining Pinch Control Automation Based on a Specified Pinch Tolerance

This section describes the steps for defining pinch control automation based on a tolerance that you specify

To define pinch control automation based on pinch tolerance:

A pinch control object is automatically inserted into the Tree for each region containing features thatmeet the criteria established by the pinch control settings To display details about an individual pinchcontrol, highlight it in the Tree and information about it appears in the Details View For information

How to Define or Change Pinch Controls Manually

You can also make changes to pinch controls, regardless of whether they were created automatically or

Usage Information for Pinch Controls

Keep the following information in mind when using the Pinch feature:

table in Pinch (p 90) for restrictions related to entity types

pinch controls, you can specify multiple faces or multiple edges to act as masters, but only one vertexcan act as master

suppor-ted This is true for all types of manual pinch controls: edge-edge, edge-vertex, vertex-vertex, face-edge,and face-vertex When multiple pinch controls use the same master, the aggregate of the pinch controls

is used to determine the pinch Note that this behavior differs from that of other mesh controls whenmultiples are specified With other mesh controls, the control that appears lowest in the Tree is honored

hard size constraints on slaves, only the slaves with the constraints will be skipped In either case, awarning message will be issued

a Named Selections > Problematic Geometry object appears in the Tree Select this object to view

the surfaces that did not mesh correctly

internal edge” due to pinching, the surface mesher may completely ignore the “single” edge (that is,the surface mesher may mesh over the edge) The reason that this may occur is that a pinch controlnever changes the topology of a model When a surface mesher collects all boundary edge meshesbefore performing surface meshing, it considers the newly created “single” edge to be a regular edgerather than a hard edge, which most users would expect As a result, all edge meshes along the “single”edge may be ignored

For more information, see the descriptions of Master Geometry and Slave Geometry pinch controls

in Pinch Control Automation Overview (p 93)

manually To do so, in the Mesh folder, highlight the Pinch object that you want to change As a result, the Details of the pinch appear in the Details View, where you can change its Scope and

Defining Pinch Control Automation Based on a Specified Pinch Tolerance

Definition Making changes to a pinch that was generated automatically causes the value of the

Scope Method control to change from Automatic to Manual For details about defining or changing

help

controls, a warning will be issued The warning will suggest that you either modify the pinch tolerance

or remove any pinch control(s) in close proximity to the Hard size control(s) in question; otherwise,surface meshing may fail

best practice to check the mesh where pinch controls have been defined close to features If a problemexists in the mesh, flipping the master and slave entities will be sufficient to solve the problem in manycases

sharp angles, and short edges within the same pinch tolerance) as well as for models containing isolated

are best used for isolated problems For example, refer to the meshes in the figure below, which showthe results of applying pinch controls in combination with the Advanced Size Function For the mesh

a Pinch Tolerance of 3e-3 and a Min Size of 4.e-3 were specified Neither is acceptable due to the

presence of high aspect ratio triangles in the mesh In such cases, the use of Virtual Topology or turing within the DesignModeler application is recommended as an alternative to pinch

defea-Figure: Pinch Not Recommended for Models with Multiple Complications when the

Advanced Size Function is On

master face You must choose the master and slaves in such a way that the elements on the face whoseedges are defined as slaves will be stretched onto the master face If the edges would be "squashed,"

no pinch will be created

mesher attempts to generate a mapped mesh for the face If the mesher cannot retain the mappedmesh pattern, it will generate a free mesh instead

will be suppressed and the reason (Overridden) will be reported in the Active read-only field in the

Details View

Loop Removal

Loops feature to preview the loops that will be removed according to the current settings

The loop removal feature is supported for the following mesh methods:

Uniform Quad method), making changes to a loop after meshing (such as adding a load on

a loop) invalidates the mesh and you will need to re-mesh It is recommended that you apply

loads to the model before meshing when using these controls Refer to the discussion of

protected topology and Patch Independent meshing for related information

The options for defining loop removal are described below

Sheet Loop Removal

The Sheet Loop Removal control determines whether loops will be removed (i.e., meshed over) by the

mesher When Sheet Loop Removal is set to Yes, the mesher removes any loop with a radius less than or equal to the value of Loop Removal Tolerance.

Valid values are Yes and No The default is No.

Loop Removal Tolerance

The Loop Removal Tolerance control sets the tolerance for loop removal Any loop with a radius less than

or equal to the value of Loop Removal Tolerance will be meshed over by the mesher.

Specify a value greater than 0.0.

Automatic Mesh Based Defeaturing

according to the Defeaturing Tolerance you specify here.

Automatic mesh based defeaturing is supported for the following mesh methods:

Automatic Mesh Based Defeaturing

Solid Meshing:

• Patch Conforming Tetrahedron

• Patch Independent Tetrahedron

Defeaturing Tolerance you set here will be populated to the local (scoped) method controls If you

sub-sequently make changes to the local settings, the local settings will override the global Defeaturing Tolerance

set here See the descriptions of the individual methods for more information

The options for defining automatic mesh based defeaturing are described below

Automatic Mesh Based Defeaturing

Turns Automatic Mesh Based Defeaturing on or off When Automatic Mesh Based Defeaturing is On

(default), features smaller than or equal to the value of Defeaturing Tolerance are removed automatically.

Defeaturing Tolerance

Only available when Automatic Mesh Based Defeaturing is On Sets the global tolerance for defeaturing Specify a value greater than 0.0 Specifying a value of 0.0 resets the defeaturing tolerance to default.

sheets and/or solids are present:

Details View is the larger value (i.e., the value being used for sheets)

on the mesh method being used See the descriptions of the individual methods for more information

for everything—sheets and solids

value of Min Size

Defea-turing Tolerance with respect to the specified hard size (essentially, the hard size becomes

the new Min Size).

The Nodes option provides a read-only indication of the number of nodes in the meshed model If the

model contains multiple parts or bodies, you can view the number of nodes in an individual part or body

Elements

The Elements option provides a read-only indication of the number of elements in the meshed model If

the model contains multiple parts or bodies, you can view the number of elements in an individual part or

Mesh Metric

The Mesh Metric option allows you to view mesh metric information and thereby evaluate the mesh quality.

Once you have generated a mesh, you can choose to view information about any of the following meshmetrics:Element Quality (p 106), Aspect Ratio for triangles or quadrilaterals,Jacobian Ratio (p 108),Warping Factor (p 110),Parallel Deviation (p 113),Maximum Corner Angle (p 114),Skewness (p 114), and Orthogonal Quality (p 117) Selecting None turns off mesh metric viewing.

When you select a mesh metric, its Min, Max, Average, and Standard Deviation values are reported in the Details View, and a bar graph is displayed under the Geometry window The graph is labeled with color-

spe-cific mesh statistics of interest

Note

If the model contains multiple parts or bodies, you can view the mesh metric information for an

on the specific part or body of interest In response, the Nodes, Elements, Min, Max, Average,

and Standard Deviation values for the selected metric and part/body are reported in the Details

View (The graph is not available at the part/body level.)

Mesh Metric

Accessing Mesh Metric Information

To access mesh metric information:

By default, the total number of Nodes and Elements in the meshed model is reported in the Details

View

By default, the Min, Max, Average, and Standard Deviation values for the selected metric are reported

Viewing Advanced Mesh Statistics

For this illustration, the Element Quality mesh metric was selected in the Details View, so the bar graph displays the minimum to maximum Element Quality values over the entire mesh.

Figure: Mesh Metrics Bar Graph

In Figure: Mesh Metrics Bar Graph (p 102), the X-axis represents the value of the selected mesh metric Using

represents the number of elements within a particular quality factor range (the default), or the percentage

Metrics Bar Graph (p 102), the axis represents the number of elements The alternative would be for the axis to represent the percentage of the total volume Keep in mind that a model could have a large number

Y-of poorly shaped elements that are confined to a small local area The total volume Y-of these elements might

not be significant compared to the volume of the entire model As a result, the bar corresponding to this

low quality factor may not be significant The Mesh Metric option displays the selected mesh metric without

qualifying the elements for acceptability

Additional characteristics of the bar graph include:

graph

by that bar

view in the Geometry window The geometry becomes transparent and only those elements meeting

After Selecting an Individual Bar (p 103) (The option to click in the column above the bar is helpful if thegraph contains very short bars that are difficult to click.)

Figure: Geometry View After Selecting an Individual Bar

value associated with the bar, along with either a number of elements or the percent of total volume

and Holding on an Individual Bar (p 104), 0.176 is the mid-point of the range of metric values covered

by the selected bar, and there are 10 elements with values that fall within that range The 10 elements

are displayed in the Geometry window.

Viewing Advanced Mesh Statistics

Figure: Clicking and Holding on an Individual Bar

selected bars are displayed in the Geometry window.

click on empty white space on the graph Empty white space does not include the column of white space

above a bar, as clicking in this area selects the bar and displays only those elements associated with it

is the transparent geometry

information

a body and then click an individual bar to view the elements corresponding to the selected bar, elements

in the hidden body are not displayed in the Geometry window, even if they meet the criteria that the

bar represents

click on the graph and drag the mouse downward and to the right to define the area to zoom; then

release the mouse button) To reset the graph to its initial view, hold the ALT key, click on the graph

and drag the mouse downward and to the left; then release the mouse button

global values For example, the value 198 in Figure: Clicking and Holding on an Individual Bar (p 104) isthe maximum end of the range for the Y-axis, based on the current content of the graph If you zoom

the values of the X-axis and Y-axis labels change accordingly along with the content of the graph

Using the Bar Graph Controls

When you click the Controls button on the graph, the graph is replaced by the controls page as shown in

Figure: Bar Graph Controls Page (p 105) Clicking the X button applies any changes on the controls page and

returns you to the graph

Figure: Bar Graph Controls Page

• Y-Axis Option - Determines what the heights of the bars represent Options include Number of Elements and Percent of Volume/Area The default is Number of Elements.

• Number of Bars - Determines the number of bars to include in the graph You can enter any whole

number greater than or equal to 0 The default is 10 When you click Update Y-Axis, the Min and/or

page reflect the new number of bars

• Range - Defines a range for the selected metric to display only those elements that fall within the

ing on metric) and click Update Y-Axis (Determining the distribution and location of all the bad

elements at one time is helpful in cases where you may need to re-import your model into the

DesignModeler application to remove the corresponding problematic geometry.) Click Reset to return

to the X-Axis defaults (Note: Negative values are acceptable.)

– Y-Axis - Specify a Min and/or Max value By lowering the Max value, you can clip the Y-axis for easier visualization of small bars, especially as they relate to different element types Click Reset to return to the Y-Axis defaults.

not appear in the mesh are read-only on the controls page Select the element types that you want to

include in the graph, or click Select All to include all available element types in the graph By default,

all available element types are selected

Calculation Details

For information about the calculations that are performed for each metric, refer to:

Element Quality

Aspect Ratio Calculation for Triangles

Aspect Ratio Calculation for Quadrilaterals

A quality factor is computed for each element of a model (excluding line and point elements) The Element

Quality option provides a composite quality metric that ranges between 0 and 1 This metric is based onthe ratio of the volume to the edge length for a given element A value of 1 indicates a perfect cube orsquare while a value of 0 indicates that the element has a zero or negative volume

Aspect Ratio Calculation for Triangles

The aspect ratio for a triangle is computed in the following manner, using only the corner nodes of theelement (Figure: Triangle Aspect Ratio Calculation (p 107)):

Figure: Triangle Aspect Ratio Calculation

I

J

K Basic

Rectangle

Triangle

Midpoint

Midpoint 0

I

J K

through the midpoints of the other 2 edges In general, these lines are not perpendicular to eachother or to any of the element edges

ele-ment edge midpoints and the triangle apex

6 rectangles is most stretched, divided by the square root of 3

The best possible triangle aspect ratio, for an equilateral triangle, is 1 A triangle having an aspect ratio of

20 is shown in Figure: Aspect Ratios for Triangles (p 107)

Figure: Aspect Ratios for Triangles

Aspect Ratio Calculation for Quadrilaterals

The aspect ratio for a quadrilateral is computed by the following steps, using only the corner nodes of theelement (Figure: Quadrilateral Aspect Ratio Calculation (p 108)):

Calculation Details

Figure: Quadrilateral Aspect Ratio Calculation

L

K

J I

0

L

K

J I

corner locations and perpendicular to the average of the corner normals The remaining steps are

performed on these projected locations

element center In general, these lines are not perpendicular to each other or to any of the elementedges

edge midpoints The aspect ratio of the quadrilateral is the ratio of a longer side to a shorter side ofwhichever rectangle is most stretched

of 20 is shown in Figure: Aspect Ratios for Quadrilaterals (p 108)

Figure: Aspect Ratios for Quadrilaterals

Jacobian Ratio

Jacobian ratio is computed and tested for all elements except triangles and tetrahedra that (a) are linear(have no midside nodes) or (b) have perfectly centered midside nodes A high ratio indicates that the mappingbetween element space and real space is becoming computationally unreliable

Jacobian Ratio Calculation

An element's Jacobian ratio is computed by the following steps, using the full set of nodes for the element:

natural coordinates and real space In an ideally-shaped element, RJ is relatively constant over theelement, and does not change sign.

R J Sampling Locations Element Shape

Integration points10-node tetrahedra

having all edges the same length will produce a Jacobian ratio of 1)

If the maximum and minimum have opposite signs, the Jacobian ratio is arbitrarily assigned to be

-100 (and the element is clearly unacceptable)

ex-tremely rare occurrence), the Jacobian ratio is arbitrarily assigned to be -100

mapping is from one natural coordinate to 2-D or 3-D space) and has no determinant For this case,

a vector calculation is used to compute a number which behaves like a Jacobian ratio This calculationhas the effect of limiting the arc spanned by a single element to about 106°

A triangle or tetrahedron has a Jacobian ratio of 1 if each midside node, if any, is positioned at the average

of the corresponding corner node locations This is true no matter how otherwise distorted the elementmay be Hence, this calculation is skipped entirely for such elements Moving a midside node away from theedge midpoint position will increase the Jacobian ratio Eventually, even very slight further movement will

because it suddenly changes from acceptable to unacceptable- “broken”

Figure: Jacobian Ratios for Triangles

Any rectangle or rectangular parallelepiped having no midside nodes, or having midside nodes at the points of its edges, has a Jacobian ratio of 1 Moving midside nodes toward or away from each other can

Jac-obian Ratios for Quadrilaterals (p 110))

Jacobian Ratio Calculation

Figure: Jacobian Ratios for Quadrilaterals

Warping Factor Calculation for Quadrilateral Shell Elements

A quadrilateral element's warping factor is computed from its corner node positions and other availabledata by the following steps:

Note

When computing the warping factor for a quadrilateral shell element, the Meshing application

assumes 0 thickness for the shell In contrast, the defined shell thickness is included when the FEModeler application computes the warping factor Because of this difference in computation, the

warping factor value reported by the Mesh Metric control in the Meshing application and the

warping factor value reported by the FE Modeler Mesh Metrics tool will differ

Average Normal Calculation (p 111))

Figure: Shell Average Normal Calculation

outline on Figure 4-19: Shell Element Projected onto a Plane)

In Figure: Shell Element Projected onto a Plane (p 111), this distance is 2h Because of the way the averagenormal is constructed, h is the same at all four corners For a flat quadrilateral, the distance is zero

Figure: Shell Element Projected onto a Plane

“thickness warping factor” is computed as the edge height difference divided by the average elementthickness This could be substantially higher than the area warping factor computed in 4 (above)

is the larger of the area factor and, if available, the thickness factor

Warping Factor Calculation for Quadrilateral Shell Elements

Figure: Quadrilateral Shell Having Warping Factor (p 112) shows a “warped” element plotted on top of a flatone Only the right-hand node of the upper element is moved The element is a unit square, with a realconstant thickness of 0.1.

When the upper element is warped by a factor of 0.01, it cannot be visibly distinguished from the underlyingflat one

When the upper element is warped by a factor of 0.04, it just begins to visibly separate from the flat one

Figure: Quadrilateral Shell Having Warping Factor

Warping of 0.1 is visible given the flat reference, but seems trivial; however, it is well beyond the error limitfor a membrane shell Warping of 1.0 is visually unappealing This is the error limit for most shells

much distortion Furthermore, the warping factor calculation seems to peak at about 7.0 Moving the nodefurther off the original plane, even by much larger distances than shown here, does not further increase thewarping factor for this geometry Users are cautioned that manually increasing the error limit beyond itsdefault of 5.0 for these elements could mean no real limit on element distortion

Warping Factor Calculation for 3-D Solid Elements

The warping factor for a 3-D solid element face is computed as though the 4 nodes make up a quadrilateralshell element with no real constant thickness available, using the square root of the projected area of theface as described in 4 (above)

The warping factor for the element is the largest of the warping factors computed for the 6 quadrilateralfaces of a brick, 3 quadrilateral faces of a wedge, or 1 quadrilateral face of a pyramid Any brick elementhaving all flat faces has a warping factor of zero (Figure: Warping Factor for Bricks (p 113))