Overview and Recent Developments of Dynamic Mesh Capabilities

Transcription

1 Overview and Recent Developments of Dynamic Mesh Capabilities Henrik Rusche and Hrvoje Jasak and Wikki Gmbh, Germany Wikki Ltd, United Kingdom 6th OpenFOAM Workshop, State College, USA, June 2011 Overview and Recent Developments of Dynamic Mesh Capabilities p. 1

2 Introduction Objective Review existing dynamic mesh techniques and new developments as implemented in OpenFOAM Topics 1. Dynamic Mesh Class & Topological Changes 2. Traditional Techniques: Automatic Mesh Motion and Topological Changes 3. Radial Basis Function in Mesh Motion and Morphing 4. Alternatives to Dynamic Mesh 5. Fluid-Structure Interaction Examples 6. Summary Overview and Recent Developments of Dynamic Mesh Capabilities p. 2

3 Background: Dynamic Mesh Class Dynamic Mesh Handling in OpenFOAM Dynamic mesh handling is well established in OpenFOAM: moving boundaries with dynamic mesh deformation, 6-DOF solid body motion, cell layering and sliding, adaptive mesh refinement Techniques are powerful, validated and well established In-cylinder flows in internal combustion engines: moving piston and valves Turbomachinery simulations: rotor-stator interaction Naval hydrodynamics: floating bodies with 6-DOF solvers Fluid-structure interaction: deforming fluid mesh Dynamic Mesh Class OpenFOAM is an excellent platform for complex physical modelling: dynamic mesh extends the scope to complex engineering geometries Common interface to all dynamically changing meshes In discretisation, all mesh changes reduce to point motion: cells and faces introduced and removed at zero area/volume with no data mapping or conservation errors Concerns for the future: Extreme cases of domain deformation and ease-of-use of topological mesh change engine and adaptivity Overview and Recent Developments of Dynamic Mesh Capabilities p. 3

4 Traditional Techniques Automatic Mesh Motion Motion will be obtained by solving a mesh motion equation, where prescribed boundary motion acts as a boundary condition Choices for a simplified mesh motion equation: Laplace equation with constant and variable diffusivity (k u) = 0 Linear pseudo-solid equation for small deformations [µ( u+( u) T )+λi u] = 0 Works well, but fails in extreme rotation or deformation (eg. squish) Overview and Recent Developments of Dynamic Mesh Capabilities p. 4

5 Traditional Techniques Topological Mesh Changes Primitive mesh operations: add/modify/remove a point, a face or cell Topology modifier: package primitive operations under easy interfaces Attach-detach boundary Cell layer additional-removal interface Sliding interface Error-driven adaptive mesh refinement Dynamic mesh: combine topology modifiers and user-friendly mesh definition to create a dynamic mesh for a class of problems Examples: mixer mesh, 6-DOF motion, IC engine mesh (valves + piston), solution-dependent crack propagation in solid mechanics Overview and Recent Developments of Dynamic Mesh Capabilities p. 5

6 Cross Flow Heat Exchanger Meshing each channel is not practical! Model as a three phase system with 2 sets Navier-Stokes equations with porous media terms 2 turbulence models 2 fluid enthalpy equations Solid heat conduction But where is the dynamic mesh? Overview and Recent Developments of Dynamic Mesh Capabilities p. 6

7 Cross Flow Heat Exchanger We utilise single domain approach to avoid mapping heat sources In this approach, the mesh goes through three states during segregated solution fluid stream 1 solved fluid stream 2 solved solid solved Implementation uses Attach/Detach topological modifiers Single domain block-solution of energy equations is an option Overview and Recent Developments of Dynamic Mesh Capabilities p. 7

8 Cross Flow Heat Exchanger Overview and Recent Developments of Dynamic Mesh Capabilities p. 8

9 Cross Flow Heat Exchanger Overview and Recent Developments of Dynamic Mesh Capabilities p. 9

10 Radial Basis Function Radial Basis Function Automatic Mesh Motion Mathematical tool which allows data interpolation from a small set of control points to space with smoothness criteria built into the derivation Used for mesh motion in cases of large deformation: no inverted faces or cells Control points chosen on a moving surface, with extinguishing function used to control far-field mesh motion Implemented by Frank Bos, TU Delft and Dubravko Matijašević, FSB Zagreb Overview and Recent Developments of Dynamic Mesh Capabilities p. 10

11 Radial Basis Function RBF Mesh Morphing RBF morphing object defines the parametrisation of geometry (space): 1. Control points in space, where the parametrised control motion is defined 2. Static points in space, whose motion is blocked 3. Range of motion at each control point: (d 0,d 1 ) 4. Set of scalar parameters δ for control points, defining current motion as d(δ) = d 0 +δ(d 1 d 0 ), where 0 δ 1 For each set of δ parameters, mesh deformation is achieved by interpolating motion of control points d over all vertices of the mesh: new deformed state of the geometry Mesh in motion remains valid since RBF satisfies smoothness criteria Using RBF in Optimisation Control points may be moved individually or share δ values: further reduction in dimension of parametrisation of space Mesh morphing state is defined in terms of δ parameters: to be controlled by the optimisation algorithm Overview and Recent Developments of Dynamic Mesh Capabilities p. 11

12 Geometric Shape Optimisation HVAC 90 deg Bend: Flow Uniformity at Outlet Flow solver: incompressible steady-turbulent flow, RANS k ǫ model; coarse mesh: cells; 87 evaluations of objective with CFD restart RBF morphing: 3 control points in motion, symmetry constraints; 34 in total Objective: flow uniformity at outlet plane iter = 0 pos = ( ) v = size = iter = 5 pos = ( ) v = size = iter = 61 pos = (( ) v = size = Overview and Recent Developments of Dynamic Mesh Capabilities p. 12

13 ... even more alternatives Tetrahedral Edge Swapping (UMass Amherst) Tetrahedral cell quality examined while moving the mesh: bad cells trigger local remeshing answers dynamicmesh interface Overset Grid (with Penn State) Multiple objects meshed body-fitted with overlap Hole cutting algorithm to remove excess overlap cells Mesh-to-mesh interpolation with implicit updates built into patch field updates and linear solver out-of-core operations Immersed Boundary Method (with U Zagreb and UC Dublin) (Static) background mesh is intersected with the surface representation of the objects Three cases are identified: fluid, solid and interpolation cells Solid cells and interpolation cells require special numerical treatment Overview and Recent Developments of Dynamic Mesh Capabilities p. 13

14 Fluid-Structure Interaction Fluid-Structure Interaction Solver and Dynamic Meshes Current incarnation of the FSI solver is substantially more advanced from previous versions. Substantial new developments Large deformation formulation in absolute Lagrangian formulation Independent parallelisation in the fluid and solid domain Parallelised data transfer in FSI coupling Crack propagation with topological changes and self-contact New mesh motion and dynamic boundary handling techniques available Working on Immersed Boundary Method on both sides of FSI interface Overview and Recent Developments of Dynamic Mesh Capabilities p. 14

15 Summary Progress in Dynamic Mesh Handling in OpenFOAM There is no one right way to perform dynamic mesh simulations in OpenFOAM: choose the best method for the problem Standard techniques are well established, robust and parallelised Continuing work on efficiency: motion equation technique is expensive! Community contributions expand the scope: new developers New Techniques Radial Basis Function in mesh motion and mesh morphing Automatic re-meshing: tetrahedral edge swapping Overset grid technology Immersed boundary method Overview and Recent Developments of Dynamic Mesh Capabilities p. 15