A study of four previously published computational criteria for identifying vortices in high-pressure flows has led to the selection of one of them as the best. This development can be expected to contribute to understanding of high-pressure flows, which occur in diverse settings, including diesel, gas turbine, and rocket engines and the atmospheres of Jupiter and other large gaseous planets.
The quantities involved in the four criteria considered in the study are the following:
- The discriminant of the deformation tensor;
- The second invariant of the deformation tensor;
- The intermediate eigenvalue of the symmetric tensor representing the sum of the square power of the strainrate tensor and the square power of the rotation tensor; and
- The magnitude of the vorticity vector.
The criteria associated with the first three quantities are those inside a vortex core, the discriminant is positive, the second invariant is positive, and the intermediate eigenvalue is negative, respectively. The fourth criterion — taking magnitude of the vorticity as an indication of vortical activity — might intuitively seem to be a good choice, but it is subjective rather than objective because it entails subjective selection of a threshold magnitude value for isolating flow structures of interest in high-vorticity regions.
These criteria were tested by use of a database generated in direct numerical simulations of high-pressure, binary-speciesmixing flows undergoing transitions to turbulence. The quantities involved in the criteria were computed from the database, isosurfaces of these quantities were plotted, and plots were assessed with respect to utility in demarcating flow structures. Of the four criteria, that based on the second invariant was found to yield the most realistic plots of flow structures and to capture structures in all regions of the flow.
The figure presents plots of the second invariant isosurfaces showing vortical features from four of the simulations. The diversity of the features is noticeable and has been interpreted as boding well for the extraction of vortical features from visual data and enabling appropriate comparisons between experimental and computationally simulated flows.
This work was done by Josette Bellan and Nora Okong'o of Caltech 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.. NPO-41932.