overset grids in star-ccm+: methodology, applications and...
TRANSCRIPT
Overset Grids in STAR-CCM+: Methodology,
Applications and Future Developments
Eberhard Schreck and Milovan Perić
CD-adapco
• 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…
Advantages of Overset Grids
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+, I
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
Overset
grid
N1, N2, N3 –
Neighbors from
the same grid;
N4, N5, N6 –
Neighbors from
the overlapping
grid.
Overset Grids Method in STAR-CCM+, II
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 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+, IV
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”.
Overset Grids Method in STAR-CCM+, V
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).
Tips and Tricks…
• Parametric studies (varying angle of attack)
• Bodies moving relative to each other
• Engineering problems that can be solved with overset
grids easier than otherwise…
Examples of Application
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, I
Velocity distribution in a section
parallel to bottom wall for different
angles of attack
30°
-30° -15°
0° 15°
Application to Parametric Studies, II
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, III
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…
Application to Parametric Studies, IV
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
Demonstration by CD-adapco
(upper) and grid motion from a
real application in automotive
industry (right)
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…)…
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
• 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…
Other Possible Applications
Optimization of a Tidal Turbine
Simulation of Lifeboat Launching
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:
Current Developments – Overset Grids
Dynamic Overset-Wall Boundary, I
Dynamic Overset-Wall Boundary, II
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.
Future Developments – Overset Grids
Thank you for your attention!