Fluids & Structural Mechanics > CM Projects > Overset Grid Methods (OGM)

Images | Animations (Below)

Overset grid techniques enable the extension of traditional structured and unstructured solution methods in computational fluid dynamics (CFD) to problems of greater geometric complexity, including better resolution of geometric details and the simulation of bodies in relative motion. The CM division employs overset techniques in all of its in-house analysis codes (NPHASE, UNCLE-M, OVER-REL, CFD-SHIP, UNCLE-TF) and applies overset methods to the analysis of complex underwater vehicles, Figure 1, to predict the motion of underwater vehicles in close proximity, Figure 2, to facilitate parametric design studies by allowing parts of the grid assembly to be added, removed, or modified independently from the rest, Figure 3, and to resolve local flow features, such as jets, vortices, and turbulent wakes, Figure 4. Other overset applications appear on this site in the Turbomachinery and Vehicle Analysis section and Drag Reduction & Flow Control section. Overset methods are also critical for biological system flows whose geometries are inherently complex, and related applications can be found in the Biological/Biomedical Flows section (see the TABs above).

In addition, the CM division plays an active leadership role in the overset community. In 2008, the ARL will host the 9th Overset Grid Symposium at University Park. ARL also develops and maintains several generalized overset tools that allow users to assemble overset grids (SUGGAR), add overset capabilities to their existing flow solvers (DiRT), and conduct accurate surface integrations when surface grids overlap (USURP).

SUGGAR is a structured, unstructured, and generalized overset grid assembler, which performs the hole-cutting and determination of interpolation stencils required in overset grid assembly. In addition to handling both cell-centered and vertex-based data structures, SUGGAR is designed for efficiency in handling dynamic grids in relative motion, functioning both as a stand-alone code and a callable procedure.

The donor-receptor transaction library (DiRT) is a library of routines for handling the donor/receptor transactions necessary when modifying an existing flow solver to incorporate the interpolation stencils generated by either PEGASUS or SUGGAR. DiRT has been used by researchers across the country to add overset capability to more than a dozen two- and three-dimensional, structured and unstructured flow solvers for flow regimes extending from low speed incompressible to hypersonic non-equilibrium flow.

Another requirement for the use of overset grids is the ability to conduct integration over surfaces composed of overlapping grids. To avoid double counting in the overlapping regions, a method is needed to eliminate the overlap and construct a unique surface representation in its place, Figure 5. USURP applies the polygon Boolean difference operator to overlapping surface elements in order to calculate a weighting coefficient for each surface element that is used to multiply the differential force and moment contributions of each element. USURP is implemented in a manner that makes it applicable to both structured and unstructured grids as well as cell-centered and vertex-based solution methods and has been distributed by Penn State under an open source license to nearly two dozen agencies, universities, and businesses across the United States and in France.

Images
Figure 1
View Image (184kb)

Figure 1:

Overset grid methods are used to extend the advantages of structured grids to more complex geometry
Figure 2
View Image (240kb)

Figure 2:

Overset techniques allow individual vehicle grids to be superimposed in order to calculate the effects of relative motion in close proximity
Figure 4
View Image (228kb)

Figure 3:

The original analysis of this underwater vehicle (left) was modified using overset methods to add a sail (center) and later to shift the sail further aft (right) without disturbing the rest of the original grid.
Figure 4
View Image (232kb)

Figure 4:

Overset grid methods are used here for local flow resolution of a turbulent flow nozzle (left) and circulation flow control jet (right).
Figure 4
View Image (134kb)

Figure 5:

Overlapping surface grids have been modified to allow accurate surface integration in these examples using the full space shuttle launch vehicle assembly (left) with detail (right)

Animations
image
View Animation (306kb)

Animation 1:

A demonstration of the use of overset grids to handle the flow solution of multiple bodies in proximity to one another. The overset interpolations and hole cutting required to do this are handled by SUGGAR, which is invoked through a procedure call by the flow solver at each time step.

image
View Animation (744kb)

Animation 2:

Like Animation 1, this is a demonstration of the use of overset grids to handle the flow solution of multiple bodies in proximity to one another. The overset interpolations and hole cutting required to do this are handled by SUGGAR, which is invoked through a procedure call by the flow solver at each time step.

The red cells are "active" cells where the equations of motion are solved directly, while the blue cells are cells in which the flow properties are transferred between grids using overset interpolation methods.

image
View Animation (457kb)

Animation 3:

This shows the corresponding flow solution obtained for the case shown in suggar-slice-2.avi, colored by contours of static pressure.  The smooth variation of flow properties across grid boundaries is evident, as is the influence that each vehicle has on the pressure field surrounding the other.