Regardless of whether a grid is unstructured or structured, the initial placement of points in the grid is not necessarily optimal, and therefore, the CFD solution may not very accurately model physical reality. AFRL’s new CFD tool addresses this shortcoming by using the initial CFD solution to determine where grid points should be located, an approach that improves the accuracy of subsequently executed solutions. To minimize the time required to modify the location of grid points, as well as enable the processing of arbitrarily large problems, the AFRL/CRAFT team focused its enhancement efforts on making the adaptation process run in parallel on multiple processors. To demonstrate the new capability, the scientists modeled a gaseous hydrogen valve with the plug set at a 52% open position (see figure on previous page). The pressure drop across the valve was 2600 psi, a drop that produces an underexpanded, annular jet at the nozzle throat, between the plug and the valve seat. The complex flowpath leads to formation of a large secondary flow and highly turbulent region in the exhaust duct downstream of the valve seat. The figure, which depicts a cross section of the CFD mesh and the resulting CFD Mach number contours, illustrates the improvement in jet definition with mesh adaptation. On the original mesh (image a), these structures are diffused and even exhibit some unsteady character; after two adaptations (image b), five distinct shock cells (displayed in red) are observable in the annular jet.Regardless of whether a grid is unstructured or structured, the initial placement of points in the grid is not necessarily optimal, and therefore, the CFD solution may not very accurately model physical reality. AFRL’s new CFD tool addresses this shortcoming by using the initial CFD solution to determine where grid points should be located, an approach that improves the accuracy of subsequently executed solutions. To minimize the time required to modify the location of grid points, as well as enable the processing of arbitrarily large problems, the AFRL/CRAFT team focused its enhancement efforts on making the adaptation process run in parallel on multiple processors. To demonstrate the new capability, the scientists modeled a gaseous hydrogen valve with the plug set at a 52% open position (see figure on previous page). The pressure drop across the valve was 2600 psi, a drop that produces an underexpanded, annular jet at the nozzle throat, between the plug and the valve seat. The complex flowpath leads to formation of a large secondary flow and highly turbulent region in the exhaust duct downstream of the valve seat. The figure, which depicts a cross section of the CFD mesh and the resulting CFD Mach number contours, illustrates the improvement in jet definition with mesh adaptation. On the original mesh (image a), these structures are diffused and even exhibit some unsteady character; after two adaptations (image b), five distinct shock cells (displayed in red) are observable in the annular jet.

As a result of this enhanced capability, scientists can produce more accurate predictions of air vehicle design performance and reduce design options prior to initiating an expensive build and test process. In addition, engineers can use the same technology to assess existing air vehicles, complementing wind tunnel test efforts designed to improve vehicle performance.

Ms. Melissa Withrow (Azimuth Corporation) and Dr. Matt Grismer, of the Air Force Research Laborator y’s Air Vehicles Directorate, wrote this article. For more information, visit http://www.afrl.af.mil/techconn_index.asp. Reference document VA-H-06-04.