Coupling CFD and Thermal Analyses with Pointwise

Engineers from W.R. Davis Limited and Pointwise, Inc. collaborated on a project to couple the ShipIR thermal analysis software with ANSYS Fluent® computational fluid dynamics (CFD) software for more accurate prediction of infrared signatures of naval vessels. ShipIR includes advanced models for infrared sky radiance, atmospheric propagation, and sea reflectance. Coupling it with a CFD code allows more accurate prediction of the effects of flow phenomena, like the exhaust plume from the ship's engines, on its thermal signature.

The coupling between ShipIR and Fluent is bidirectional: thermal radiation predictions from ShipIR are transferred to Fluent and wall and fluid temperatures are transferred back to ShipIR. Typically the temperatures predicted by the two analysis tools converge to within an RMS difference of 0.3 degrees C within five or six iterations.

This new coupling was tested by applying it to a portion of the Canadian Forces Auxiliary Vessel Quest, shown below and for which shipboard measurements are available.

ShipIR model of the Canadian Forces Auxiliary Vessel Quest
Figure 1 - ShipIR model of CFAV Quest

For these initial tests, only the funnel and mast region of the model, highlighted in yellow above, was used because it is an area of active interaction between the fluid and thermal analyses. The funnel contains seven exhaust outlets: two for the main propulsion diesel engines, two for ship service diesel generators, one for the gas turbine propulsor, one for the ship emergency diesel generator, and one T-shaped boiler exhaust chimney. It also has four air inlet vents: two for the main machinery compartment, a gas turbine air intake, and a gas turbine silencer air intake. In addition there are two air outlet vents: one for the main machinery compartment and one for the gas turbine silencer.

ShipIR and ANSYS Fluent have different grid requirements. ShipIR only needs a surface mesh. It uses a much coarser grid than Fluent, but it prefers to use predominantly quadrilateral cells with a minimum of skewing. The ShipIR mesh of the funnel and mast is shown below.

ShipIR mesh of the Quest's funnel and mast
Figure 2 - ShipIR mesh for funnel and mast

Fluent needs a complete volume mesh with finer resolution than ShipIR, but it can handle general cell types. For this study, a triangular surface mesh with tetrahedral volume mesh was used. To ensure consistency between the thermal and fluid analyses, the surface grids for each were made as similar as possible. The ShipIR mesh was used as a geometric database in Pointwise for generating the Fluent mesh. In this way, the exact same discrete shape is being modeled in both the thermal and fluid analysis. The resulting Fluent surface mesh is shown below.

CFD surface mesh generated in Pointwise
Figure 3 - Fluent surface mesh created in Pointwise

In addition to using the same discrete geometry, the ShipIR mesh element edges were preserved in the Fluent mesh by treating each ShipIR mesh element as a separate surface in Pointwise. This allowed for simple transfer of temperature and fluid conditions and exact matching of boundary condition regions between ShipIR and Fluent. In the close up view of the funnel below, the underlying quadrilateral nature of the grid is apparent even though triangular surface elements are being used for the Fluent grid.

Matching of the ShipIR and CFD meshes
Figure 4 - Close view shows matching between the ShpIR and Fluent meshes.

This surface grid was used as the boundary for creating a tetrahedral volume mesh for Fluent. The volume was created in two blocks: one near the funnel and one in the far field to make it easier to make localized changes to the funnel grid. A cut through the near-funnel block is shown below.

Cut through the CFD volume mesh
Figure 5 - A cut through the tetrahedral volume grid colored by cell volume

The meshes were used to successfully couple ShipIR and Fluent and perform an analysis of the Quest funnel region. Flowfield temperature contours are shown on a cut through the fluid volume in the image below.

Temperature of the fluid
Figure 6 - Temperature contours in the fluid volume

The results show a significant change in temperature and thermal signature between the initial ShipIR and final, coupled ShipIR-Fluent solutions. The next steps are to construct a full-ship model of the CFAV Quest, perform a sensitivity analysis on the Fluent modeling options, and conduct a full-ship measurement to validate the new approach.

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