Best Practices for Aerospace Aerodynamics Peter Ewing
Agenda Pre-processing Ge Geometry Or Origin/Import rt Ge Geometry Pr Prep ep Su Surface Mesh sh Vo Volume Mesh sh Solver Settings Defi fining Fli ligh ght Ph Phys ysics cs Se Setting ng Up So Solvers rs Post-processing Au Automated Data Extract ction on Plotting ng Scen enes es Automated Repo Au porti ting ng
Agenda Pre-processing Ge Geometry Or Origin/Import rt
STAR-CCM+ Parts Geometries ultimately conglomerate in Parts – Laser scans, extracted mesh topology – External CAD modelers, e.g. CATIA, NX – STAR-CCM+ 3D-CAD – Mesh Operation Parts Common Denominator: tessellated surfaces – STL or surface meshes � “dummy” or “flattened” surface meshes – Discrete Mesh Operations � Detached mesh operations are green – 3D-CAD/CAD Parts � Analytic representation, blue or solid grey User should be aware of geometry quality – Especially for “flattened” Parts! STAR-CCM+ requires clean, closed geometry: – To use Boolean operations – To generate a volume mesh Import rt Pr Prep ep Su Surface ce Vo Volume me
CAD is Preferred Aero surfaces & leading edges are complex swept geometries – These features matter! Hierarchy of geometry fidelity : – STAR-CCM+ 3D-CAD – CAD-Clients Direct Link Soli lidWo Works ks Parameter Trans ansfer er – CAD Exchange – X_B /X_T then STP/STEP – DBS, STL, IGES CAD geometry allows several benefits over flattened parts – Project to CAD – CAD-based Mesh Operations – Feature aligned meshing STAR-CCM+ Bi-direc STAR ectional ional li link nk – Parametric design changes � 3D-CAD and CAD-Clients – Persistent Part naming Import rt Pr Prep ep Su Surface ce Vo Volume me
Agenda Pre-processing Ge Geometry Or Origin/Import rt Ge Geometry Pr Prep ep
External Aerodynamics Geometry Preparation Split the body into multiple Part Surfaces: – Inflow/Outflow/Freestream definitions – Allows tracking of physical convergence DPW4 W4 Geo eometry (up uppe per) r) – Trailing Edge for custom controls � Rounded edges DPW4 W4 Geo eometry (lo lower) r) 3D RA RAE28 2822 Airfoil l for 2D si simula ulation on Naming conventions enable filtering and efficient identification, e.g.: – 00 Inlet, 00 Outlet, 00 Freestream, etc. – 01 Wing, 01 Body, 01 Tail, etc. – 02 Symmetry Plane – 03 Interface (Sliding or Overset) Fil ilter selecti tion on box box Import rt Pr Prep ep Su Surface ce Vo Volume me
Low-Speed Far-field Boundary Preparation Pressure e Velocity Inlet et Outf Ou tflow ow Atmospheric flight: – Upstream boundary: � Typically velocity inlet in a round/bullet shape � Distance is 10-20 characteristic lengths – Outflow boundary: � Typically a outflow flat plane cut Exam ample bu bull llet doma domain in � Distance is 20-40 characteristic lengths Wind tunnel configurations should be matched: – Duplicate the geometry – Inlet distances typically set as free stream * – Outlet distance should follow free stream distance – Side walls typically set to symmetry * * If inlet conditions are well measured, duplicate Import rt Pr Prep ep Su Surface ce Vo Volume me
Transonic Far-field Boundary Preparation Freestream settings: – Circular domain will use “Freestream” boundary condition � Upstream position 20-30 characteristic length scales � Downstream position 40-50 characteristic length scales Free eest strea eam m Bou oundary ry Body dy Samp ample tran ansoni nic circ rcular doma domain in Wind tunnel sections can be difficult to reproduce – Transonic wind tunnels typically have slatted configurations – Simulations may contain shock reflections to disrupt upstream flow – Unless specific configuration is well documented, run in Freestream Import rt Pr Prep ep Su Surface ce Vo Volume me
Supersonic and Hypersonic Far-field Boundary Preparation Upstream placed fairly close and aligned with shocks generated by the body – The shock should not interact with the freestream boundary Outlet boundaries can either be Pressure Outlet or Freestream – Hypersonic cases – Outlet can be set to “Pressure” field function to Pressure Ou Outlet et extrapolate Free eest stream eam Body dy Exam ample of of hype personi nic doma domain fo for r Ma Mach 12 12 sphe phere re Axis or Symme mmetry ry Import rt Pr Prep ep Su Surface ce Vo Volume me
Wrapping What does it do? – Enables fast turn-around of broken geometry – Standard use case is for unification of assemblies of “broken” (i.e. not clean and closed) Parts How do I know if I should wrap? – Inefficient control over the CAD or Parts are flattened – Extensive* Surface Repair work is required: � Inefficient (or no) control of CAD workflow � Many CAD based-errors (e.g. too many pieces) to fix efficiently in CAD � Too many tessellation errors to efficiently fix in Surface Repair – Simulation fidelity is independent of intricate details affected by Wrapper Features worth investigating: – Works well in the PBM structure � Maintains Part Surface naming convention � Operation can be “Detached” to create new Part – Partial Wrapping � Speeds up the wrapping process – Project to CAD Used by permission: Sikorsky / American Helicopter Society Import rt Pr Prep ep Su Surface ce Vo Volume me
STAR-CCM+ Surface Repair Comments What does it do? – Checks triangulations for valid clean/closed geometry – Manipulate underlying triangulations (tessellations) How do I know if I should Surface Repair? – The underlying Part is not clean/closed manifold – There is no control of the CAD to fix within CAD If a Part Requires Repair: – Don’t panic! � Undo/Forward-do buttons – Surface Repair can repair the parts: � Up-to-date guide flags remaining fixes � Create new Part Surfaces where needed � Create new Part Curves where needed Keep in Mind: – It’s like sewing up a bundle of triangles: Connect dots, zip edges – Goal is to create a manifold, air-tight surface Import rt Pr Prep ep Su Surface ce Vo Volume me
Agenda Pre-processing Ge Geometry Or Origin/Import rt Ge Geometry Pr Prep ep Su Surface Mesh sh
Automated Surface Mesher Settings Defau efault set ettings gs Automatic Surface Repair Model: ‘Off’ Surface Remesher Settings: Curva vature=76 76 – Increase minimum face quality to 0.20 Surface mesher settings: – Base Size to Characteristic Length/10, e.g.: � Chord length/10 Chord/100 00 � Characteristic Body length/10 – Surface Curvature: 36 - 54 – Surface Growth Rate: 1.05 - 1.20 Growth = 1.05 Gr 05 Custom Surface Controls: – Edge proximity on bodies to 3 – Lifting Surfaces: � Basic Curvature to 76 Proxi oximity ty � Growth rate to 1.05 - 1.10 � Target Size: Chord/100 – Trailing Edges: Minimum Target Size to ¼ of t.e. thickness NACA0010 NACA 010 – Inlet/Outlet/Freestream/Symmetry Boundaries: � Target Surface Size to be at least characteristic Import rt Pr Prep ep Surface Su ce Volume Vo me length
Agenda Pre-processing Ge Geometry Or Origin/Import rt Ge Geometry Pr Prep ep Su Surface Mesh sh Vo Volume Mesh sh
Quasi-2D Core Volume Mesh Models 2D Automated Meshing (PBM): – Requires an initial 3D body � 2D section lies on z-axis � Does not need to be CAD – Applications: � Airfoil analyses � Test mesh settings � Testing of physics settings � Supersonic 2D/Axisymmetric NLF-0 -0416 16 Directed Mesher (PBM): – Ordered style grids – High quality grids for supersonic flows – Best practice topology for hypersonic cases – Requires an initial 3D CAD body – Workflow tip: � Split patches in the CAD-Client or in 3D-CAD � On Geometry transfer, choose “All CAD Edges” option � Choose to “Initialize Patches by CAD Edge” � Allows for macro automation 2D Axisy isymmetr tric c Hyp yperson sonic bi-conic conic Import rt Pr Prep ep Su Surface ce Vo Volume me
Core Volume Mesh Models Trimmer or Polyhedral are both acceptable topologies – Refinement in flow regions of interest are key to capturing flow features in the simulation Polyhedral mesh: – Aerospace cases mesh in serial – Pseudo-random orientation of faces reduces numerical dissipation – Smooth growth away from bodies – Optimizer can increase mesh quality Lockheed Martin Public Release: ORL201102002 – Prefer to control mesh based solely on remeshed surface � Volume controls to catch the hard spots Trimmer mesh model: – Massively parallel – Faster, requires less memory – Aligning the trimmer mesh model to the main flow directions can reduce numerical dissipation – Mesh refinement/coarsening in factors of 2 � Use of volume control to control location of transitions Import rt Pr Prep ep Su Surface ce Vo Volume me
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