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Introduction to Unstructured Solver

The unstructured solver within LAVA solves the Navier-Stokes equations on arbitrary polyhedral unstructured meshes. The solver supports various numerical schemes and models for Reynolds-Averaged Navier-Stokes (RANS) equations or Large-Eddy Simulations (LES). A high-level overview of its capabilities compared to other LAVA solvers is provided in LAVA Introduction.

We will briefly go over the available numerical schemes and capabilities here. For more details, please see:

Grids

The unstructured solver is designed to work with unstructured grids of arbitrary cell types. This includes standard types such as tetrahedra, hexahedra, pyramids and prisms. However, the cell-centered formulation works best with cell types with 6 or more faces as this improves the stencil width, leading to better accuracy and robustness.

LAVA release includes the LAVA Voronoi Mesher, a parallel and automated mesher that can generate high-quality Voronoi meshes that are particularly suited for scale-resolving applications which demand low dissipation fluxes. The Voronoi mesher also has early support for anisotropic wall layers that can support wall resolved RANS applications, although this feature is still under development. More details about the Voronoi mesher can be found here:

The solver also supports externally generated grids as long as they are provided in arbitrary polyhedral cell type CGNS format with a single volume zone.

Numerical Scheme

LAVA unstructured uses a finite-volume discretization with a density-based compressible formulation. Primitive variables \(Q=[p,u,v,w,T,Y_i]\) are stored at cell centers.

Up to \(2^\textrm{nd}\) order spatial accuracy is supported for both the convective and diffusive terms. Explicit time integration (\(2^\textrm{nd}\), \(3^\textrm{rd}\) or \(4^\textrm{th}\) order accurate) is typically used for LES and WMLES applications while implicit time integration (\(2^\textrm{nd}\) order accurate) for RANS and URANS applications with dual time-stepping. For implicit simulations, an ILU preconditioned GMRES linear solver is utilized.

Turbulence Models

  • Spalart-Allmaras turbulence model for RANS closure
  • Constant coefficient Vreman subgrid scale model for LES closure
  • Musker's and blended-log wall models
  • A momentum/energy penalty based flow tripping

Multi-species

LAVA Unstructured supports multi-species mixtures. However, the species are inert, i.e. chemical reactions are not supported. Multi-species diffusion is handled via effective mixture viscosity, without multi-component diffusion.

Conjugate Heat Transfer

Conjugate heat transfer simulations where solid heat conduction is solved tightly coupled with fluid flow. Solid heat conduction is modeled in 1D only, in the wall normal direction. This disregards solid heat conduction in transverse directions and only recommended where the normal direction heat conduction is expected to be dominant. A separate mesh for the solid is not required.

In-situ Post Processing

Several in-situ post-processing options are available:

  • Loads: Output integrated force and moments over user defined groups of boundary surfaces
  • Point probes: Probe flow variables at given set of point coordinates
  • Line probes: Probe flow variables along user defined lines
  • Surface probes: Sample flow variables on to user supplied triangulated surfaces
  • Iso-surfaces: Compute iso-surfaces of primitive and derived flow quantities (e.g. Q-criterion, density gradient magnitude...) and write as triangulated surfaces
  • Cut-planes: Sample flow variables on to user defined cut planes
  • Sub-volume: Output volume solution for user defined subsets of the domain

Warning

Iso-surfaces and cut-planes are only supported for GPU runs.

Hardware

LAVA unstructured is compatible with Intel, AMD and ARM based CPU systems as well as NVIDIA GPU systems. Implicit solutions (e.g. RANS) is currently only supported on CPU systems.