This software computes two-phase turbulent flows with phase change, and the solution is independent of the grid spacing on which it is computed and on the discretization order used for the differential equations. Grid spacing and discretization-order independence can be achieved by reformulating the large-eddy simulation (LES) equations. Previous software followed the conventional LES equations, whereas the present one follows the explicitly filtered formulation (EFLES).

Predictions from conventional LES are known to be grid-spacing and spatial discretization-order-dependent. In the preceding article, LES was reformulated for compressible single-phase flow by explicitly filtering the nonlinear terms in the governing equations so as to render the solution grid-spacing and discretization-order-independent. Having shown in that article that the reformulated LES (EFLES) yields a grid-spacing and discretization-order independent solution for compressible single-phase flow, the potential of EFLES for evaporating two-phase flow where the small scales have an additional origin compared to single-phase flow is investigated here.

Thus, a database was created through direct numerical simulation (DNS), which when filtered, serves as a template for comparisons with both conventional LES and EFLES. Both conventional LES and EFLES are conducted with two gas-phase SGS models; the drop-field SGS model is the same in all these simulations. For EFLES, simulations performed with the same SGS model for the gas phase, but two different drop-field SGS models, were compared. Moreover, to elucidate the influence of explicit filtering versus gas-phase SGS modeling, EFLES with two drop-field SGS models, but devoid of gas-phase SGS models, was conducted.

The results from all these simulations were compared to those from DNS and from the filtered DNS (FDNS). Similar to the single-phase flow findings, the conventional LES method yields solutions that are both grid-spacing and spatial-discretization-order-dependent. The EFLES solutions are found to be grid-spacing-independent for sufficiently large filter-width to grid-spacing ratio, although for the highest discretization order, this ratio is larger in the two-phase flow compared to the single-phase flow. For a sufficiently fine grid, the results are also discretization-order-independent. Absence of a gas-phase SGS model leads to build-up of energy near the filter cut-off, indicating that while explicit filtering removes energy above the filter width, it does not provide the correct dissipation at the scales smaller than this width. A wider viewpoint leads to the conclusion that although the minimum filter-width to grid-spacing ratio necessary to obtain the grid-independent solution might be different for various discretization-order schemes, the grid-independent solution thus obtained is also discretization-order-independent.

This work was done by Josette Bellan of Caltech and Senthilkumaran Radhakrishnan for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to NPO-49209.


NASA Tech Briefs Magazine

This article first appeared in the February, 2015 issue of NASA Tech Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.