E0_Ch05_PrismV12

Chapter 5 Prism Meshingg Introduction to ANSYS ICEM CFD 5-1 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualPrism Meshing • Inflation layers – To better simulate boundary layer effects – Mesh orthogonal to surface with facesMesh orthogonal to surface with faces perpendicular to boundary layer flow direction • Procedure – Set Global Prism ParametersSet Global Prism Parameters – Select Parts to grow layers from • Typically wall boundaries – Set Local Parameters for each partp • Local overrides global • Zero or blank entries will defer to global settings – Run mesher • From existing mesh – Extrude into tetra/hexa mesh – Extrude from surface tri mesh, then fill volumes • Run automatically during Volume Mesh creation 5-2 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Run automatically during Volume Mesh creation Prism Meshing Training ManualPrism - Global Parameters • Global Prism Parameters – Growth law • exponential: height = h(r)(n-1) [n is layer #]• exponential: height = h(r)(n 1) [n is layer #] • linear: height = h(1+(n-1)(r-1)) – Initial height of first layer – h in formulae above • Auto calculated if not specified – Based on factor of edge length of base triangle/quad – Height determined so that top layer volume is slightly less than that of tetra/hex just above it Number of layers n– Number of layers n – Height ratio r – Total height of all layers – Usually specify 3 of the above 4 parameters C ill l l t th i i Other global parameters explained later • Compute params will calculate the remaining parameter (total height usually left blank) – Or specify only Height ratio and Number of layers for auto calculation of initial height These are the defaults Individual surface/curve Total height Height ratio (r) 5-3 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 – These are the defaults. Individual surface/curve height/ratio/layers will override these defaults if set Initial height (h) Prism Meshing Training ManualSmooth Tetra/Prism Transition • Leave initial height as “0” – This causes the initial height to float in order to reduce the volume change between the lastreduce the volume change between the last prism and adjacent tetra. Initial InitialInitial height specified Initial height = 0 5-4 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualSetting Prism Parameters on Parts • Prism extrusion areas defined by the parts – Mesh > Part Mesh Setup – Toggle on Prism for parts where inflation layers are desired • Surface mesh (tri/quad) gets extruded into prisms – Set Height, Height Ratio, Num Layers f f• Will use global defaults if not set or zero Applying these settings causes these parameters to be applied to eachbe applied to each individual surface within each part If Apply inflationIf Apply inflation parameters to curves is toggled on, they will also be set on each curve 5-5 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 set on each curve within each part Prism Meshing Training ManualSetting Prism Parameters on Volume Parts • Normally toggle prism on only for parts that contain surfaces (becomes surface mesh) Only one volume part selected• Can also toggle on prism for parts that contain material points (becomes volume mesh) selected – For interior surface mesh, this defines the allowable volumes for extrusion – Selecting no volume parts has the same result as selecting all volume partsresult as selecting all volume parts Interior surface 5-6 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Both or no volume parts selected Prism Meshing Training ManualSetting Prism Parameters on Surfaces • Mesh > Surface Mesh Setup • You can specify different local height and ratio on any selected surface without y moving the surface to a new part • Usually set height and/or ratio smaller on specific surfaces to avoid collisionp Height = 0.2Height on part = 0.4 5-7 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Collisions occurred when the height was 0.4 on all surfaces No collisions after Prism Meshing Training ManualSetting Prism Parameters on Curves • Mesh > Curve Mesh Setup • You can get Prism to transition linearly across a surface by not setting a height on the surface,a surface by not setting a height on the surface, but instead set a different height on each curve on the opposite sides of the prism surface • Height ratio and Num. of layers have no affectHeight ratio and Num. of layers have no affect on prism for curve settings Height = 0.01 Height = 0 on 5-8 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Height = 0.003 g surface Prism Meshing Training ManualRun Prism • Separately – Mesh > Compute Mesh > Prism Mesh – The Select Parts for Prism Layer button pops up theThe Select Parts for Prism Layer button pops up the same menu as the Part Mesh Setup, except non- prism related columns aren’t displayed – Input • Existing Mesh• Existing Mesh • From File (saves memory by not loading mesh) • Automatically linked into volume meshAutomatically linked into volume mesh – Toggle on Create Prism Layers when tetra meshing – Not advisable if this is the first mesh for a particular geometry – Must be confident about setup parameters and sizing – Running prism separately allows you to smooth and error-check the tri or tetra mesh first. 5-9 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualInput as Surface or Volume Mesh Input can be a surface mesh or volume mesh – Surface mesh – Must be a closed boundary mesh M t if l t– Must specify a volume part – Use tetra fill methods after: – Delaunay – Advancing FrontAdvancing Front – T-Grid – Volume mesh – Moves and reconnects tetras Delaunay fillfill 5-10 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism extrudes into tetras Prism Meshing Training ManualPrism – Quality Control Options • Fix marching direction – Maintains normal from surface – Can cause intersections with other mesh • Min prism quality – Either re-smooth directionally or cap/replace with pyramids if quality not met O th i ht• Ortho weight – Weighting factor for node movement from 0 - improving triangle quality, to 1 - improving prism orthogonalityg y • Fillet ratio • Max prism angle • Max height over base See next slides • Max height over base • Prism height limit factor • Ratio multiplier (m) F i ti l th 5-11 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 – For varying exponential growth: height = h(r)(n-1) (m)(n-1) Prism Meshing Training ManualPrism Options – Fillet Ratio – Blends prism grid lines around sharp corners • 0 = no fillet • 1 = fillet ratio equals last prism height – Improves angles further away from the corner – Orients prisms more in direction of flowOrients prisms more in direction of flow – If meshing tight spaces with tight curves (less than 60º), may not have space for a fillet ratio Fillet Ratio = r/h r h Fillet Ratio = r/h 5-12 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Fillet Ratio = 0.0 Fillet Ratio = 0.5 Fillet Ratio = 1.0 Prism Meshing Training ManualPrism options – Max Prism Angle – Controls prism layer growth around bends or adhering to adjacent surfaces – If the Max (internal) Prism Angle is not met, the prism layers will ( ) g , p y end and be capped off with pyramids in those locations – Usually set in the 120º to 179º range Experience pays off here If extruding from one part and not its– Experience pays off here. If extruding from one part and not its neighbor, and the angle between the two surfaces is greater than the Max Prism Angle, the prisms will detach and be capped off with pyramids. This prevents bending the prisms that mightwith pyramids. This prevents bending the prisms that might create lower-quality internal angles. However, the pyramids are usually of lower quality, too. 160 o Pyramids 5-13 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Original mesh Max prism angle = 180o Max prism angle = 140o. Prism Meshing Training ManualPrism Options – Max Prism Angle - Continued • A high (up to 180o) Max Prism Angle keeps the prism layers connected around tight bends. – Set this at 180 to prevent pyramidsp py Max Prism Angle = 140 Max Prism Angle = 180 5-14 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualPrism Options – Max Height Over Base – Restricts prism aspect ratio – Prism layers stop growing in regions where prism aspect ratio would exceed specified value Height (h)prism aspect ratio would exceed specified value • Number of prism layers would not be preserved locally Mesh is made conformal with pyramids at prism (h) Base (b) – Mesh is made conformal with pyramids at prism boundaries – Acceptable values vary widely (typically 0.5 – 8) h/b Largest height over smallest base length Pyramids Max Height Over Base = 1.0Max Height Over Base not set 5-15 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualPrism Options – Prism Height Limit Factor – Restricts prism aspect ratio • Prism height will not expand once this factor is met – Uses the same height over base factor as the Height (h)Uses the same height over base factor as the previous metric except prism layers are not capped off with pyramids – Preserves the specified number of prism layers (h) Base (b)Preserves the specified number of prism layers – Will fail if sizes of adjacent elements differ by more than a factor of 2 A t bl l id l (t i ll 0 5 8) h/b Largest height over smallest base length – Acceptable values vary widely (typically 0.5 – 8) 5-16 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Limit factor = 0.5Limit factor not set Prism Meshing Training ManualPrism Options-Part Control • New volume part – Can specify new Part for prism elements Leaving these parts blank will i h it th• Must specify if extruding from surface- only mesh • If extruding into volume mesh, prism will inherit tetra volume Part if not inherit the names from the current meshwill inherit tetra volume Part if not specified • Side part – For quad faces on side boundaryq y • Top part – For tri faces capping off top of last prism layery • Extrude into orphan region – Extrude prisms away from existing volume, not into it 5-17 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 – Must specify new volume, side and top part, or they’ll be in ORFN Prism Meshing Training ManualPrism Options - Smoothing P t i/t t f b t i lit• Prepares tri/tetra for best prism quality – Set surface/volume steps to 0 if only extruding one layer • Otherwise defaults adequate • Value depends on model/user experience – Set surface smoothing steps to zero for a tri/tetra mesh that is already smoothed – Triangle quality typeTriangle quality type • Laplace typically best for eventual prism quality • Other types recommended only for trouble shooting – comes with experience – Max directional smoothing stepsMax directional smoothing steps • Redefines extrusion direction based on initial prism quality • internally calculated for each layer • Other Advanced Prism Meshing Parameters – Detailed in Help menu (usually left default) • Read a Prism Parameters File 5-18 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 – To set all values from a previously saved file, perhaps from a previous model (.prism_params file, written when prism is run) Prism Meshing Training ManualSmoothing a Tetra/Prism Mesh After generating prisms: Edit Mesh > Smooth Mesh Globally – Prisms are smoothed during prism generation – If input mesh was a tetra mesh, the tetras adjacent to the last prism layer will be messed up – First smooth only the tetras and tris S t PENTA 6 t F• Set PENTA_6 to Freeze • Don’t want to modify the prism layers at this point – Once tetra and tri elements are as smooth as possible smooth all elements 1st step possible, smooth all elements – Set PENTA_6 to Smooth – Decrease the Up to quality value so as not to distort prism elements too much 2nd step The prisms get compromised a bit step 5-19 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 compromised a bit when everything is on smooth Prism Meshing Training ManualSplitting Prism Layers – If many prism layers are desired, it is faster, but less robust – to create “fat” layers and then split them with mesh editing – Edit Mesh > Split Mesh > Split Prisms– Edit Mesh > Split Mesh > Split Prisms – Fix ratio: The first layer is split such that its resulting layers employ the given growth ratio Fix initial height: The first layer is split such– Fix initial height: The first layer is split such that its first sub-layer is of the given height – Specify the number of layers to result from each existing layerg y – Can split specified or all existing layers 5-20 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualRedistributing Prism Layers – Edit Mesh > Move Nodes > Redistribute Prism Redistribute prism layers after splitting Edge – Fix ratio: The initial height and subsequent layer heights will be adjusted to achieve this growth ratio – Fix initial height: The growth ratio is the variable that will be adjusted to achieve this initial height 5-21 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualDemo – 3D Pipe Junction • Setting prism parameters • Extrude prism into tetra mesh • Initial height as zerog • Smoothing • Cut plane 5-22 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. Release 12.0 January 2010 Prism Meshing Training ManualWorkshops • Workshop 1 – Waterjacket Section – Setting prism parameters – Prism into tetra mesh – Inspecting pyramids – Setting local heights – Remesh to get rid of pyramids • Workshop 2 – WingbodyWorkshop 2 – Wingbody – Prism from surface mesh – Create a density region – Delaunay filly – Smoothing – Hexa core fill 5-23 ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved. 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