Overset Grids in STAR-CCM+:

Methodology, Applications and

Future Developments

Eberhard Schreck and Milovan Perić

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-ε 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 Mueller’s 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!