Methodology, Applications and Future Developments Eberhard Schreck - - PowerPoint PPT Presentation

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

• f 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

• verset 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-

• ff 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

• utside 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

• rientation 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

• f various devices

(lifeboats, missiles etc.).

Simulation of Store Separation

Simulation of Missile Launching

Mach Number / Surface Temperature Temperature

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

• n 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

• f 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

• verset 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!