Technology

General Structure

TransAT© is a multiscale, finite-volume code solving the Navier-Stokes fluid-flow equations. Compressibility is pressure based and is used for low Mach-number flows. The code relies on a flexible, multi-block meshing approach used in connection with MPI parallel protocol for HPC systems. Grid generation can be achieved using the Immersed Surfaces method described next, for which the TransAT Suite has a specific grid generator. TransAT can be executed on both Wondows and Linux OS’s. The code can run explicit as well as implicit; both variants have been tested for scaling, for single and multiphase flow. The scaling efficiency studies on 3D turbulent flows conducted on the DOE supercomputer Titan show that the code breaks up the 90-100% scaling on 10,000 processors and an allocation of 14,000 cells per processor.

The Immersed Surfaces Technology (IST) used for meshing is inspired from Interface Tracking techniques for two-phase flows, where free surfaces are described by a hyperbolic equation for the phase function. In the IST the solid is described as the second ‘phase’, with its own thermo-mechanical properties. The solid is defined by its external boundaries using the solid level set function. To help better solve the boundary layer zone when use is made of the IST technique discussed above, more refined sub-blocks (Block Mesh Refinement – BMR) can be automatically generated around solid surfaces. The combined IST/BMR technique has various major advantages over traditional methods, including: Rapid gridding of complex geometries, naturally suitable for rigid body motion and conjugate heat transfer problems, and retains high-order scheme accuracy (Cartesian grids).

Turbulent flow can be treated using RANS as well as scale-resolving methods, including LES, V-LES and DNS, for all class of grids.  RANS models implemented in TransAT are based on the k-E model coded for steady & unsteady flows, using adaptive wall functions, two-layer techniques and low-Re variants. The base RANS model is augmented with important modifications to cope with complex flow scenarios: Kato & Launder, Yap correction, RNG, and swirl correction. In situations where the heat transfer does not scale with the flow motion (non-unity Prandtl number fluids), TransAT adopts a series of high-order Algebraic Heat Flux models for convective heat transfer.

The LES strategy built in TransAT is very stable and highly accurate and follows the guidelines of top literature in terms of wall modelling, gridding, unsteady inflow boundary conditions, numerical schemes, etc.. Various SGS models can be employed in connection to LES, including  incl. the Dynamic and the WALE models. For very high Reynolds number flows, TransAT switches to V-LES which represents an excellent compromise between efficiency and precision as to capturing unsteady turbulence. V-LES has been heavily used for industrial problems for which LES remains computationally expensive.

Multiphase Flow

Multiphase gas-liquid flows can be tackled using either interface tracking methods (ITM) or phase-average models, for both laminar and turbulent flows. Specifically, the Level-Set approach can be employed as ITM’s, with. static and contact-dynamic angle treatment. Particle laden flows rely and the Lagrangian framework, under one-, two- or four-way coupling (granular flows).

In the Mixture (Homogeneous) approach applied to gas-liquid systems, the transport equations are solved for the mixture rather than for the phase-specific quantities. This implies that one mixture momentum equation is solved for the entire flow system, reducing the number of equations to be solved in comparison with the two-fluid model. In many situations however, the model must employed with a prescribed closure law for the interphase slip velocity and associated stresses. In this case, the model is known as Homogeneous ASM. Various slip Pressure-gradient & gravity slip closure models can be used in TransAT, augmented by models for turbulent dispersion mechanism and wall lubrication. The N-Phase approach built-in is an extension of the Homogeneous ASM introduced above, and is invoked in situations involving more than two fluid phases, e.g. methane-water-oil-hydrate, with the oil phase comprising both light and heavy components.

Interface tracking of TransAT is based on the level-set method, consisting in solving a hyperbolic equation to track the interface on a fixed Eulerian grid, using a smooth signed-distance function referring to the shortest distance to the front. The advantage of the method is that it dispenses with interface reconstruction employed in VOF, it can handle merging and fragmentation, and it permits identification of the exact location of the interface. TransAT’s level-set technique works on both Cartesian and BFC grids and uses various re-distancing schemes, including fast marching on narrow bands for BFC grids.

Turbulent multiphase flows can in this product be tackled within both the RANS and Scale-Resolving contexts, including LES and V-LES. Statistical RANS models are used in the context of the Mixture and N-Phase techniques, which could also benefit from the versatility of scale-resolving strategies. The ITM context, however, can only be employed in connection with LES or V-LES (LEIS). The Lagrangian particle module can be used within RANS or LES. In the LES context, a subgrid scale particle-dissipation model based on deconvolution principle is available.

Advanced Physics

TransAT Advanced Physics refer to fluid-flow situations involving multi-components, conductive, convective and radiative heat transfer, reactive flows, microfluidics flow, Non-Newtonian flows, particle sedimentation, slurries, miscible fluids, gas hydrates, multiphase flow through porous media, flows with advanced equation of states (Tait, Peng Robinson), etc.

Most of the advanced-physics models are built on top of the main multiphase-flow modules. For instance, compressibility of multiphase flow systems, dispersed and interfacial phase change, inter-phase mass transfer, shear thinning, visco-elastic free-surface flows (a unique capability to TransAT) belong to the class of advanced physics. Other fluids of complex physics can be treated by TransAT, including slurries, gas hydrates, thixotropic fluids, and binary mixtures.

Various intricate phenomena can also be treated either directly in the UI or via dedicated UDF’s, including particle settling and sedimentation, multistage reaction flocculation subsequent to chemical reaction, ozonation, wall-reactive flows, etc. In the microfluidics sector or surface tension dominated flows, TransAT offers a wide range of models, including wetting using static & dynamic contact angle, sub-grid thin-film and lubrication, nano-particles tracking, electro-wetting, and Marangoni effects.