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!