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Overset Grids in STAR-CCM+:
Methodology, Applications and
Future Developments
Eberhard Schreck and Milovan Peri CD-adapco
Overset grids:
History
Advantages and challenges
Overset grids in STAR-CCM+:
Methodology
User interface
Tips and tricks
Examples of application
Future developments
Introduction
Overset grids were used already 30 years ago
The main motivation has been to use multiple regular grid blocks to handle complex geometry
In October 2012, the 11th Overset Grid Symposium was held in Dayton (most presentations available at Symp. Website):
The multi-block approach still dominating (tens to hundreds
of grid blocks, up to 10% of grid points involved in
interpolation);
Unstructured grids being used by few groups, resulting in a
smaller number of grid blocks;
Different numerics, grid coupling (interpolation) and hole-
cutting algorithms same problems (orphan nodes)
Overset Grids History
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
Complex logic and coding is required for an automatic handling of arbitrary body motion and multiple overset
grids
Situations can arise where coupling of predefined overset grids is not possible (orphan cells)
Parallelization and load balancing are challenging
Challenges with Overset Grids
Control volumes are 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...
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...
Different interpolation functions can be used to express values at acceptor cells via values at donor cells (different interpolation elements)
Donor cells must be active cells.
The change of cell status is controlled by the solver and happens automatically.
The user can visualize the cell status as a scalar field (this can help in case of problems mostly due to inadequate grids)...
Interpolation elements are
not unique when grids move, continuity is important
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
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 Method in STAR-CCM+, VI
Working with Overset Grids, I
STAR-CCM+ infrastructure for interfaces has been extended overset grids are another type of volume interface
New intersector-module was added to STAR-CCM+ to handle:
Cell status (hole-cutting algorithms);
Searching for donors to each acceptor cell;
Definition of interpolation factors, etc
The solver is almost unaffected almost all models can be used (coupled and segregated solver, VOF, Lagrangian and Eulerian multiphase flows etc.)
Working with Overset Grids, II
No compromises on usability:
Any grid type can be used;
Most physics models can be applied;
All motion models can be used;
Processing pipeline (meshing, solving, analysing) is unaffected;
Only two additional set-up steps:
New region interface (with interface options)
New boundary condition
Working with Overset Grids, III
Background
region
Overset
region
Overset interface
for regions Background and Over
Set-up of overset grid computation of flow around
a pitching foil in a channel: one background grid for
the channel and one overset grid for the region
around foil.
Background
region
Overset
region
Overset
boundary
Overset grid surface has boundary type
OversetMesh
Front and back planes are symmetry planes.
The overset region has one boundary that is fully
submerged within background region...
Working with Overset Grids, IV
Volume Mesh Representation includes active cells used to plot results...
Working with Overset Grids, V
Active cells in overset grid Active cells in background grid
Acceptor cells (value -2)
Active cells (value 0)
Passive cells (value 1)
Checking Overlap Cell Status (scalar field): acceptor cells must separate active and passive cells direct contact is not allowed...
Background
Working with Overset Grids, VI
Acceptor cells (value -2)
Active cells (value 0)
Checking Overlap Cell Status (scalar field): the overset grid here contains only active and acceptor cells...
Working with Overset Grids, VII
Over
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
Visualization - Isolines
Pressure contours with lines: small imperfections (two lines visible
within overlap zone) visible only at few locations most contours are almost perfectly continuous (grid from previous slides)
Convergence of Iterations
Residuals history for a laminar flow around an object
Implicit coupling of grids allows convergence to round-
off level of residuals
Overlap of Overset Regions
Example of overset grids overlapping each other.
Overset + Morphing, FSI
Example of combination of overset grids and morphing when
simulating large deformation of structures.
Overset-Lagrangian
Example of overset grids
in combination with
Lagrangian multiphase
flow model (overset grids
move and fall partly
outside solution domain;
particles are not affected
by internal grid motion).
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 body 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: time step 1000 s, rotation 15 per time
step, standard k- turbulence model, under-relaxation 0.9/0.1/0.9 for velocities/pressure/turbulence, wind speed 40 m/s
Application to Parametric Studies, III
History of computed forces from the computation of flow around
a vehicle in a wind tunnel at different angles of attack (since the
time step is very large, steady-state solutions are obtained).
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
Application to Parametric Studies, V
Simulation of Lifeboat Launching
Wave propagates from
left to right
Wave propagates from
right to left
Overset grids allow
simulation of launching
of various devices
(lifeboats, missiles
etc.).
Simulation of Store Separation
Simulation of Missile Launching
Ma
ch
Nu
mb
er
/ S
urf
ac
e T
em
pera
ture
Tem
pera
ture
Simulation of missile launch using DFBI (1 DoF)
and overset grids (small gaps)
Vessels With Crossing Paths
Overset grids allow simulation of relative motion of bodies whose
paths are crossing (neither sliding nor morphing are applicable)
Overtaking Cars
Overset grids allow simulation of passing by, overtake, tunnel entry
and other interaction problems with any vehicle type
Overturning Car
Overset grids allow simulation of vehicle dynamics during motion
on a curved path (virtual elk-test)
Windscreen Wipers
Overset grids allow simulation of wiper action on a windscreen
(VoF, locally fine grid around wipers, intersecting paths, FSI)
Simulation of Flow in a Mixer, I
Overset grids allow simulation of mixing processes in arbitrarily
shaped vessels with any shape and motion of mixing parts
Simulation of Flow in a Mixer, II
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
Fluid-Structure Interaction: Ball Valve
Coupled simulation of flow (STAR-CCM+) and motion of a ball valve
(ABAQUS) using overset grids (for details see Alan Muellers pres.)
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 hight and position
Coating by Dipping
Simulation by CD-adapco
Overset grids allow simulation of coating by dipping bodies into
paint bath (arbitrary body motion, VoF, e-coat model for paint layer
growth, forces on body parts, trapped air and liquid pockets)
Coating by Spray
Simulation by CD-adapco
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)
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
Simulation of Pouring, II
Thank you for your attention!