Turbo Tools and General Grid Interface

Transcription

1 Turbo Tools and General Grid Interface Theoretical Basis and Implementation Hrvoje Jasak, Wikki United Kingdom and Germany Turbo Tools and General Grid Interface p. 1

2 General Grid Interface Objective Present the working of single and multiple rotating frames of reference solvers Review implementation of the General Grid Interface (GGI) in OpenFOAM and its use in turbomachinery applications Topics Rotating frames of reference Handling of rotor-stator interfaces General Grid Interface: design rationale Numerical considerations: discretising GGI interface GGI interpolation and weight calculation Derived forms: cyclic GGI partial overlap GGI, mixing plane Code components Parallelisation of GGI interfaces Preparing a mesh for GGI Example of use Summary Turbo Tools and General Grid Interface p. 2

3 Introduction Turbomachinery CFD with OpenFOAM Turbomachinery CFD poses special requirements on simulation software Handling rotation of components Treatment of interfaces between rotating and stationary mesh Boundary conditions for total state variables and rotation Data extraction and post-processing for properties of interest Beyond turbo-tools capability, tutorials, validation examples and best practice guidelines are valuable for new users Development in turbomachinery driven by Turbomachinery SIG, led by Chalmers University, Hydro Quebec and Wikki Turbo Tools and General Grid Interface p. 3

4 Rotating Reference Frames Rotating Reference Frame For fast-rotating components, solution in terms of relative velocity is preferable, with effects on the convection term u = u rel + u rot = u rel + ω r Single Rotating Frame of Reference Solution algorithm in terms of u rel is preferred, with the addition of centrifugal and Coriolis force to the momentum equation, implemented in simplesrffoam For easier specification of boundary conditions, use SRFVelocity boundary condition Specify boundary condition either in absolute or relative frame of reference, controlled by the relative switch If relative = true, rotation shall be added to the boundary value SRFModel specifies the mode of rotation, currently rpm Turbo Tools and General Grid Interface p. 4

5 Rotating Reference Frames Multiple Rotating Components Model assumes steady-state simulation of a domain where some components are in rotation and others are stationary Transient effect of rotor-stator interface is lost In stationary regions, solve for absolute velocity u Choice of velocity variable in a rotating region causes complications 1. If solving for u rel in rotating regions, centrifugal and Coriolis force are added, but a transformation of velocity is required at the interface 2. If solving for u, form of the momentum convection term is identical to moving mesh simulations, where all convection terms operate with relative flux (u rel u), with addition of Coriolis force only: this is implemented in MRFSimpleFoam Handling of rotating zones Addition of Coriolis force only in rotating zones Modification of face fluxes to account for (fictitious) rotation of the zone External boundary of the rotating cell zone must be a surface of rotation, otherwise the fluxes after correction for rotation will not be conservative Code supports multiple rotating regions, using MRFZones class Turbo Tools and General Grid Interface p. 5

6 Rotor-Stator Interfaces Handling of Rotor-Stator Interfaces In simple SRF/MRF cases, a steady simulation is assumed and effect or rotor-stator interface is (almost) completely lost... but transient behaviour is sometimes of a primary interest Further complications arise from non-matching number of blade passages between consecutive rotor and stators stages: no easy symmetry conditions Number of options are used regularly Frozen rotor: do nothing. Simulate fixed relative position of rotor and stator Full transient simulation with sliding mesh Options on multiple rotor-stator pairs with circumferential scaling Mixing plane: circumferential averaging on the interface All of the above require special handling in a CFD code Turbo Tools and General Grid Interface p. 6

7 Background on GGI Handling Sliding Mesh Interfaces Turbomachinery applications typically involve components in relative motion: need to handle a set of separate regions as one contiguous mesh Components move relative to each other, but at each time instance create a single contiguous region: attaching and detaching the mesh during simulation This is a subset of topological mesh changes, already implemented in OpenFOAM: do we need anything further? Unfortunately, topological changes do not satisfy all our needs Turbo Tools and General Grid Interface p. 7

8 Sliding Interface Topological Mesh Changes: Sliding Interface Sliding interface topology modifier Defined by a master and slave surfaces As surfaces move relative to each other, perform mesh cutting operations and replace original faces with facets Re-assemble mesh connectivity on all cells and faces touching the sliding surface: fully connected 3-D mesh Polyhedral mesh support in OpenFOAM facilitates topological changes Once the mesh is complete, there is no further impact in the code! Connectivity across interface changes with relative motion rotor side stator side rotor side stator side Turbo Tools and General Grid Interface p. 8

9 GGI Design Rationale GGI Interface in Turbomachinery Apart from fully overlapped cases, turbomachinery meshes contain similar features that should employ identical methodology, but are not quite the same Non-matching cyclics for a single rotor passage Partial overlap for different rotor-stator pitch Mixing plane: perform averaging instead of coupling directly Component coupling requires data manipulation (copy, transform, average) In such cases, the behaviour is closer to a coupled boundary condition, but the numerics is similar to sliding interface Objective: mimic behaviour of sliding interface without changing the mesh Turbo Tools and General Grid Interface p. 9

10 GGI Discretisation FVM Discretisation on a GGI Interface Review discretisation of convection and diffusion when faces are replaced; volumetric integral terms are not affected Convection operator splits into a sum of face flux integrals Z V (φu) dv = I S φ(n u)ds = X f φ f (s f u f ) = X f φ f F where φ f is the face value of φ and F = s f u f is the face flux Diffusion operator captures the gradient transport I S γ(n φ)ds = X f Z S f γ(n φ) ds = X f γ f s f ( φ) f Face terms: interpolated value and face gradient φ f = f x φ P + (1 f x )φ N, s f ( φ) f = s f φ N φ P d f Turbo Tools and General Grid Interface p. 10

11 GGI Discretisation FVM Discretisation on a GGI Interface When cutting is performed, total face area is replaced by facets Discretisation on the interface can be rewritten as a sum of facet operations. Inverting the loop, we can is introduce shadow neighbour values φ s N values for the in front of the face, creating the effect as if the interface is integrally matched φ s N = X t w t φ t where t denotes a selection of cell/face values on the other side Consistency conditions: simple averaging is not flux-conservative Area of original face must be equal to sum of facet areas replacing it X w t = 1 for all faces on both sides t If face A touches face B, perceived facet area must be the same w A B S A = w B A S B Turbo Tools and General Grid Interface p. 11

12 GGI Interpolation GGI Interpolation Role of GGI interpolation is to calculate shadow interpolation weights Idea 1: form a matrix equation for weights and solve: does not work (HJ) Idea 2: use geometrical cutting as in sliding interface and calculate weights as per original definition. Also provides the addressing GGI Intersection Algorithm w t = S facet S Developed and implemented by Martin Beaudoin, Hydro Quebec Components 1. Quick reject in 3-D: Axis-Aligned Bounding Box 2. Projection into common plane 3. Quick reject in 2-D: Separating Axis Theorem 4. Point in polygon detection: Horman-Agathos algorithm 5. Polygon intersection: Sutherland-Hodgman clipping algorithm Result: facet area, GGI addressing and weights Turbo Tools and General Grid Interface p. 12

13 Derived Forms of GGI Extending Basic GGI Algorithm GGI operates as a coupled patch field condition: interpolate shadow and update Cyclic GGI Create transformed surface of the shadow patch and calculate weights Transform scalar/vector/tensor data according to rank Use GGI interpolation on transformed shadow data and update as usual Partial Overlap GGI Create transformed surface of the shadow patch by copying the geometry multiple times to achieve full overlap and calculate weights Transform scalar/vector/tensor data according to rank and expand over number of copies Use GGI interpolation on transformed shadow data and update as usual GGI interpolation is useful beyond GGI: provides flux conservative and function-monotonic interpolation Conjugate heat transfer with non-matching solid-fluid boundaries Fluid-structure interaction: force-conservative interpolation Turbo Tools and General Grid Interface p. 13

14 Mixing Plane Interface Mixing Plane Interface Turbomachinery components include multiple identical channels: modelling a single channel simplifies the simulation Banded circumferential averaging at rotor-stator interface allows the transient problem, with substantial reduction in simulation cost Circumferential averaging introduces a one-time mixing loss, but without other adverse effects: blade wakes and passage-to-passage variation averaged out Consistency in interpolation is essential for mass conservation and accuracy! Turbo Tools and General Grid Interface p. 14

15 Mixing Plane Interface Implementation of a Mixing Plane Interface Basic GGI interpolation tool already performs the correct operation Area-weighted interpolation: can be done in ribbons Strict mass conservation: banded patches to achieve circumferential averaging in a prescribed manner Implementation: mixing plane = two GGI interfaces used back-to-back (The more we work on this, the easier it becomes: no interpolation bias) Mixing plane work by Martin Beaudoin, Hydro Quebec Turbo Tools and General Grid Interface p. 15

16 GGI Code Components Implementation of GGI in OpenFOAM Interpolation and geometry Basic algorithms: Horman-Agathos, Sutherland-Hodgman Templated GGI interpolation, abstracting patch type Instantiated interpolation for stand-alone patch and polypatch GGI patch and discretisation (identical for cyclic GGI and partial overlap) Mesh patch with interpolation: ggipolypatch, ggipointpatch Matrix support: ggilduinterface, ggilduinterfacefield Coupled FV patch with discretisation support ggifvpatch and ggifvpatchfield: constrained patch Special support for AMG coarsening, to be done consistently on all levels: processorgamginterface and processorgamginterfacefield Turbo Tools and General Grid Interface p. 16

17 Parallelisation of GGI Parallelisation of GGI In parallel, sliding GGI changes processor-to-processor connectivity Trouble in weights calculation and in scheduling of processor-to-processor communications: dangerous or inefficient Solution: Global sync of GGI data Complete sliding surface must be present on all CPUs: decomposition In each evaluation, gather-scatter of shadow data for complete interface Evaluate GGI as usual: local patch only addresses a part of sliding surface Turbo Tools and General Grid Interface p. 17

18 Prepare for GGI Definition of a GGI Patch and Field Build the mesh in the usual way, with disconnected components Prepare face zone for master and slave surface wooster*685-> setset faceset insidezone new patchtoface insideslider faceset outsidezone new patchtoface outsideslider quit wooster*685-> setstozones -noflipmap Boundary file definition: constant/polymesh/boundary insideslider outsideslider { { type ggi; type ggi; nfaces 36; nfaces 36; startface 1192; startface 1228; shadowpatch outsideslider; shadowpatch insideslider; zone insidezone; zone outsidezone; bridgeoverlap false; bridgeoverlap false; } } Turbo Tools and General Grid Interface p. 18

19 Prepare for GGI Field Definition GGI is a constrained condition: forces patch field type boundaryfield { insideslider { type }... } ggi; Parallel decomposition: GGI patch surface must be present on all CPUs in its entirety Decomposition dictionary: new entry for global face zones globalfacezones ( insidezone outsidezone ); Some care is required in choice of linear equation solvers Turbo Tools and General Grid Interface p. 19

20 Simple Examples Simple Examples of GGI Interfaces in Use Turbo Tools and General Grid Interface p. 20

21 Simple Examples Capsizing Body with Topological Changes or GGI Full capsize of a floating body cannot be handled without topology change Mesh motion is decomposed into translational and rotational component External mesh performs only translational motion Rotation on capsize accommodated by a GGI interface Automatic motion solver handles the decomposition, based on 6-DOF solution Mesh inside of the sphere is preserved: boundary layer resolution Precise handling of GGI interface is essential: boundedness and mass conservation for the VOF variable must be preserved Turbo Tools and General Grid Interface p. 21

22 Summary Summary GGI interface allows coupling of mesh components without the need for topological mesh changes GGI discretisation is identical to sliding interface with mesh cutting Interpolation weights are calculated using polygon clipping Derived forms: cyclic GGI and partial overlap based on GGI interpolation Mixing plane implemented as a pair of back-to-back GGIs Parallelisation: complete GGI surface present on all CPUs. Added option of preserving faces in decomposition without attached cells Recent updates for communication scheduling and improved parallel scaling The code is complete, validated and ready for use Turbo Tools and General Grid Interface p. 22