Overset Grids in STAR-CCM+: Methodology, Applications and Future Developments Eberhard Schreck and Milovan Peri ć CD-adapco
Advantages of Overset Grids • Easier to perform and automate parametric studies: – With a single set of grids, many different configurations can be computed; – Grid quality not affected by changing position/orientation of bodies; – Boundary conditions easier to set… • Easier to handle relative motion of bodies: – Arbitrary motion can be handled; – Paths can cross; – Tangential motion at close proximity can be handled…
Overset Grids Method in STAR-CCM+, I Control volumes are automatically labelled as: Active cells, or Passive cells. In active cells, regular discretized equations are solved. In passive cells, no equation is solved – they are temporarily or permanently de-activated. Active cells along interface to passive cells refer to donor cells at another grid instead of the passive neighbours on the same grid... The first layer of passive cells next to active cells are called acceptor cells... The user can visualize the cell status as a scalar field…
Overset Grids Method in STAR-CCM+, II Currently, triangular (2D) or tetrahedral (3D) interpolation elements are used, with either distance-weighted or linear interpolation... Other (higher- order) interpolations will come… Background grid N 1 , N 2 , N 3 – Neighbors from the same grid; N 4 , N 5 , N 6 – Neighbors from the overlapping grid. Overset grid
Overset Grids Method in STAR-CCM+, III No explicit interpolation of solution is performed… Solution is computed on all grids simultaneously – grids are implicitly coupled through the linear equation system matrix...
Convergence of Iterations Residuals history for a laminar flow around an object… Implicit coupling of grids allows convergence to round- off level of residuals…
Overset Grids Method in STAR-CCM+, IV Overset grids usually involve: One background mesh, adapted to environment; One or more overset grids attached to bodies, overlapping the background mesh and/or each other. Each grid represents a separate Region in STAR-CCM+ terminology... Both background and overset mesh(es) can be generated (or imported) in the usual way, region by region… Any grid type can be used for any region… Almost all physics models can be applied…
Overset Grids Method in STAR-CCM+, V Each grid (background and overset) can move according to one of the standard motion models available in STAR- CCM+… Each grid can also deform (e.g. in a coupled fluid- structure interaction simulation) using any available morphing technique… Overset grids can fall out of solution domain (cut-out by boundary surface). Overset grids can overlap each other. Overset grids have boundary regions of type “overset”.
Tips and Tricks… In the overlapping zone, cells should be of comparable size in both meshes (recommendation) : Interpolation errors in the coupling equation should be of the same order as when computing convective and diffusive fluxes (interpolation over half a cell); The coarser of the two coupled meshes determines the error level… Between two body walls, at least 4 cells on both background and overset grid are needed to couple them (requirement) . The overset grid should not move more than one cell per time step in the overlapping zone (recommendation) .
Examples of Application • Parametric studies (varying angle of attack) • Bodies moving relative to each other • Engineering problems that can be solved with overset grids easier than otherwise…
Application to Parametric Studies, I Flow around a car model at different angles of attack A horizontal section through both grids (only active cells are shown). Total number of cells: ca. 1 million Vertical section through the two grids (only active cells are shown). Same grids and boundary conditions – many positions (easy to automate).
Application to Parametric Studies, II -30° -15° 0° 15° 30° Velocity distribution in a section parallel to bottom wall for different angles of attack
Application to Parametric Studies, III Residual history from the computation of flow around a vehicle in a wind tunnel at different angles of attack History of computed forces from the computation of flow around a vehicle in a wind tunnel at different angles of attack
Application to Parametric Studies, IV Simulation of motion of a container ship in Stokes waves propagating from right to left: initial vessel orientation 30° (upper) and -30° (lower) relative to the direction of wave propagation. Single set of grids, same boundary conditions, different vessel orientations – easy to automate…
Motion of Windscreen Wipers Overset grids allow simulation of wiper action on a windscreen (VoF , locally fine grid around wipers, intersecting paths, FSI…)
Flow Around Moving Rudder Overset grid attached to rudder is morphed by projecting vertices of one wall boundary to a “guiding surface” (hull wall) in every time step. This approach has also been applied to real windscreen wipers…
Injector Needle Motion, I Overset grids allow easier simulation of processes in fuel injectors and similar devices (axial motion, vibration, deformation, VoF , cavitation…)
Injector Needle Motion, II Pressure Volume fraction of liquid (three phases involved: liquid, vapor, air), simulation of cavitation…
Simulation of Pouring • Pouring optimization: Reduce misruns; Increase yield (skull reduction); Use STAR-CCM+ to find optimized pouring curve (CD-adapco/ Access); Variation in rotational speed, pouring height and position …
Simulation of Coating by Dipping Overset grids allow simulation of coating by dipping bodies into paint bath (arbitrary body motion, VoF, e-coating model for paint layer growth, forces on body parts, trapped air and liquid pockets…)… Demonstration by CD-adapco (upper) and grid motion from a real application in automotive industry (right)
Simulation of Coating by Spray Overset grids allow simulation of coating by moving spray heads (fast spinning nozzles with arbitrary translation and rotation, electrically charged spray droplets, liquid film on car surface…) Demonstration by CD- adapco
Other Possible Applications • Rotating machinery (instead of sliding interfaces); • Internal combustion engines (moving piston and valves); • Gear pumps, screw pumps/extruders etc.; • Valves, opening/closing doors/windows etc.; • Relative motion of flying or floating bodies (ships passing each other in a river or channel; car overtaking or passing by, missiles, torpedo etc.); • Mixers with moving parts of complex shape and intersecting paths; • … and many others, involving parametric studies or part motion…
Optimization of a Tidal Turbine
Simulation of Lifeboat Launching
Current Developments – Overset Grids Dynamic overset-wall boundary condition: Wall boundary is automatically converted to overset boundary when it comes into overlapping zone. This simplifies simulations when overset grids slide partially along wall and partially inside another grid:
Dynamic Overset-Wall Boundary, I
Dynamic Overset-Wall Boundary, II
Future Developments – Overset Grids The most important future developments include: – Implementation of higher-order interpolation; – Optimization of parallel processing; – Modelling of contact (valves, impact…); – Automatic mesh adaptation to fulfil requirements of overset grids (avoid failures due to inadequate grids in the overlapping zone): • Minimum number of cell layers in gaps; • Similar cell size in overlapping zone; • Refining the background grid ahead and coarsening behind a moving body.
Thank you for your attention!
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