me 408 lecture2
TRANSCRIPT
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ME 408 LECTURE-2
MODELLING
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Modeling Build the model
Modeling considerations
Element Type
Real Constants
Material Properties
Sections
Geometry/Modeling WorkPlane& Coordinate systems
Keypoints
Lines Areas
Volumes
Meshing
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As you begin your model generation, you will (consciously orunconsciously) make a number of decisions that determine howyou will mathematically simulate the physical system: What are the objectives of your analysis?
Will you need to vary/modify model data?
Will you need to change the geometric topology of the model, e.g. add
holes to the model? Will you model all, or just a portion, of the physical system?
How much detail will you include in your model?
What kinds of elements will you use? How dense should your finiteelement mesh be?
In general, you will attempt to balance computational expense(CPU time, etc.) against precision of results as you answerthese questions.
The decisions you make in the planning stage of your analysiswill largely govern the success or failure of your analysis efforts.
Modeling considerations
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Linear or Higher Order Elements
Take Advantage of Symmetry The axis of symmetry mustcoincide with the global Cartesian Y-axis.
Negative nodal X-coordinates are not permitted.
The global Cartesian Y-direction represents the axial direction, the globalCartesian X-direction represents the radial direction, and the globalCartesian Z-direction corresponds to the circumferential direction.
Your model should be assembled using appropriate element types: For axisymmetric models, use applicable 2-D solids with KEYOPT(3) = 1,
and/or axisymmetric shells. In addition, various link, contact, combination,and surface elements can be included in a model that also contains
axisymmetric solids or shells. (The program will not realize that these "other"elements are axisymmetric unless axisymmetric solids or shells are present.)
How Much Detail to Include
Appropriate Mesh Density
Modeling considerations
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Modeling considerations
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Modeling considerations
Characterization of problem
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Modeling considerations
The ANSYS program does not assume a
system of units for your analysis.
Units must however be consistent for all
input data.
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BEAM MESH
CIRCUit Multi-Point Constraint
COMBINation PIPE
CONTACt PLANE
FLUID PRETS (Pretension)HF (High SHELL
Frequency) SOLID
HYPERelastic SOURCe
INFINite SURFaceINTERface TARGEt
LINK TRANSducer
MASS MATRIX
VISCOelastic (or viscoplastic)
Element Type
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Main Menu> Preprocessor> Element Type> Add/Edit/Delete
Element Type
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The ANSYS element library contains
more than 150 different element types
Each element type has a unique number and
a prefix that identifies the element category ET,1,BEAM4
ET,2,SHELL63
Element Type
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Many element types have additional options,
known as KEYOPTs, and are referred to as
KEYOPT(1), KEYOPT(2), etc. e.g.:
KEYOPT(9) forBEAM4 allows you to chooseresults to be calculated at intermediate locations
on each element
KEYOPT(3) forSHELL63 allows you to suppressextra displacement shapes
Element Type
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Real Constants
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Real Constants
Element real constants are properties thatdepend on the element type, such as cross-sectional properties of a beam element e.g. real constants forBEAM3, the 2-D beam element,
are area (AREA), moment of inertia (IZZ), height(HEIGHT), shear deflection constant (SHEARZ), initialstrain (ISTRN), and added mass per unit length(ADDMAS).
Not all element types require real constants, anddifferent elements of the same type may havedifferent real constant values.
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Real Constants
For line and area elements that require geometry data(cross-sectional area, thickness, diameter, etc.) to bespecified as real constants, you can verify the inputgraphically by using the following commands in the order
shown:Utility Menu> PlotCtrls> Style> Size and ShapeUtility Menu> Plot> Elements
ANSYS displays the elements as solid elements, using a
rectangular cross-section for link and shell elements and acircular cross-section for pipe elements. The cross-sectionproportions are determined from the real constant values.
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Sections
Building a model using BEAM44, BEAM188, or
BEAM189, you can use the section commands
(SECTYPE, SECDATA, etc.) or their GUI path
equivalents to define and use cross sections in your
models.
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Sections
A cross section defines the geometry of the
beam in a plane perpendicular to the beam axial
direction. ANSYS supplies a library of eleven
commonly-used beam cross section shapes,and permits user-defined cross section shapes.
When a cross section is defined, ANSYS builds
a numeric model using a nine node cell for
determining the properties (Iyy, Izz, etc.) of thesection and for the solution to the Poisson's
equation for torsional behaviour.
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Sections
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Geometry/Modeling
Creating a solid model within ANSYS.
Using direct generation.
Importing a model created in a computeraided design (CAD) system.
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Coordinate Systems
Globaland localcoordinate systems are used to locategeometry items (nodes, keypoints, etc.) in space.
The displaycoordinate system determines the system inwhich geometry items are listed or displayed.
The nodalcoordinate system defines the degree of freedom
directions at each node and the orientation of nodal resultsdata.
The elementcoordinate system determines the orientation ofmaterial properties and element results data.
The results coordinate system is used to transform nodal or
element results data to a particular coordinate system forlistings, displays, or general postprocessingoperations(POST1).
The working plane, which is separate from the coordinatesystems discussed in this chapter, is used to locategeometric primitives during the modeling process.
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Coordinate Systems
(a) Cartesian (X, Y, Z components)
coordinate system 0 (C.S.0)
(b) Cylindrical(R, , Z components)
coordinate system 1 (C.S.1)
(c) Spherical(R, , components)
coordinate system 2 (C.S.2)
(d) Cylindrical(R, , Y components)
coordinate system 5 (C.S.5)
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Geometry/Modeling
Creategeometrical entities
Operateperform Boolean operations
Move / Modifymove or modify geometrical entities
Copycopygeometrical entities
Deletegeometrical entities
Update Geomupdate the geometry in
relation to for example buckling analysis
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Modeling - Create
The hierarchy of modeling entities is as listed
below: Elements (and Element Loads)
Nodes (and Nodal Loads)
Volumes (and Solid-Model Body Loads)
Areas (and Solid-Model Surface Loads)
Lines (and Solid-Model Line Loads) Keypoints (and Solid-Model Point Loads)
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It is a good idea to use keypoints as
reference points in the modeling phase
Create Keypoints (In Active CS)
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Mesh Attributes
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Meshing Size Controls
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MP,EX,1,2E11 ! Young's modulus for material ref. no. 1 is 2E11
MP,NUXY,1,0.3 !Poissons ratio for material ref. no. 1 is 0.3
MP,DENS,1,7800 ! Density for material ref. no. 1 is 7800
Material Properties
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Types of Loads
Structural: displacements, forces, pressures,temperatures (for thermal strain), gravity
Thermal: temperatures, heat flow rates, convections,internal heat generation, infinite surface
Magnetic: magnetic potentials, magnetic flux, magneticcurrent segments, source current density, infinite surface
Electric: electric potentials (voltage), electric current,electric charges, charge densities, infinite surface
Fluid: velocities, pressures
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Types of Loads
Loads are divided into six categories:
DOF constraints
forces (concentrated loads)
surface loads
body loads
inertia loads
coupled field loads
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Load Step
A load step is simply a configuration of loads for
which a solution is obtained. In a linear static or
steady-state analysis, you can use different load
steps to apply different sets of loads -wind loadin the first load step, gravity load in the second
load step, both loads and a different support
condition in the third load step, and so on. In a
transient analysis, multiple load steps applydifferent segments of the load history curve.
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Substep
Substeps are points within a load step at which solutions arecalculated. You use them for different reasons:
In a nonlinear static or steady-state analysis, use substepsto apply the loads gradually so that an accurate solution canbe obtained.
In a linear or nonlinear transient analysis, use substeps tosatisfy transient time integration rules (which usually dictatea minimum integration time step for an accurate solution).
In a harmonic response analysis, use substepsto obtainsolutions at several frequencies within the harmonicfrequency range.
Equilibrium iterations are additional solutions calculated at agiven substepfor convergence purposes. They are iterativecorrections used only in nonlinear analyses (static ortransient), where convergence plays an important role.
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Application of Loads
Most loads are applied either
on the solid model (on keypoints, lines, and
areas) or
on the finite element model (on nodes andelements)
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DOF Constraints
A DOF constraintfixes a degree of freedom (DOF) toa known value. Examples of constraints are specified
displacements and symmetry boundary conditions in a
structural analysis, prescribed temperatures in a
thermal analysis, and flux-parallel boundary conditions
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Forces (Concentrated Loads)
A force is a concentrated load applied at a node in themodel. Examples are forces and moments in a structuralanalysis, heat flow rates in a thermal analysis, andcurrent segments in a magnetic field analysis
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Forces (Surface Loads)
A surface loadis a distributed load applied over asurface. Examples are pressures in a structural analysisand convections and heat fluxes in a thermal analysis
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Body Loads
A body loadis a volumetric or field load. Examples aretemperatures and fluences in a structural analysis, heatgeneration rates in a thermal analysis, and currentdensities in a magnetic field analysis
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Misc. Loads Inertia Loads
Inertia loads are those attributable to the inertia (mass matrix) of a body,such as gravitational acceleration, angular velocity, and angularacceleration. You use them mainly in a structural analysis
Coupled-Field Loads Coupled-field loads are simply a special case of one of the above loads,
where results from one analysis are used as loads in another analysis.For example, you can apply magnetic forces calculated in a magneticfield analysis as force loads in a structural analysis
Axisymmetric Loads and Reactions
Loads to Which the DOF Offers No Resistance If an applied load acts on a DOF which offers no resistance to it (i.e.
perfectly zero stiffness), the ANSYS program ignores the load.
Initial Stress Loading Initial stress loading is only allowed in a static or full transient analysis(the analysis can be linear or nonlinear). Initial stresses can be appliedonly in the first load step of an analysis.
Applying Loads Using TABLE Type Array Parameters
Graphing or Listing the Boundary Condition Functions