Defense Tech Briefs - Improved Accuracy of Computational Fluid Dynamics Calculations

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Improved Accuracy of Computational Fluid Dynamics Calculations

Dec 01 2006

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Grid adaptation technique improves the accuracy of engineering predictions regarding air vehicle performance.

Understanding how air flows over the surfaces of an air vehicle can help AFRL designers maximize the vehicle’s performance and minimize its cost. AFRL scientists recently developed a tool that improves the accuracy of airflow simulations that result from computational fluid dynamics (CFD) calculations. As part of a Small Business Innovation Research effort, AFRL collaborated with Combustion Research and Flow Technology (CRAFT), Inc., to develop the tool for use with unstructured CFD programs. The new tool uses the solver’s initial solution to determine where grid points should be added or removed within the CFD mesh, a process which then improves the solver’s solution in a second—and any subsequent—iteration. This enhanced accuracy improves AFRL’s ability to support the warfighter with lower-cost, higher-value designs.

A comparison of an initial CFD grid and corresponding Mach number predictions, along with the predictions from a subsequent mesh adaptation

CFD programs solve mathematical equations to predict what will happen when a fluid, such as air, flows over or through a structure. These programs solve the equations at each point of a three-dimensional grid of data points— the CFD mesh—located on and around the structure. This grid can be either structured or unstructured. To visualize a structured grid, imagine the grid formed by the lines on a sheet of (Cartesian) graph paper, molded along one edge to conform to the structure, and extending into the third dimension. Incorporating additional sheets of graph paper in this manner, at the same time joining the grid lines between those sheets, produces multiple blocks that cover the entire structure and the space surrounding it. To envision an unstructured grid, first imagine points placed on the structure and throughout the space around it, and then picture lines drawn between these points to form various geometric shapes (e.g., tetrahedrons or prisms). The points at which the CFD program solves the equations can be at the line intersections (for the structured grid) or at the corners of geometric shapes (for the unstructured grid). Alternative solution points may be at the centroids—either of the geometric shapes formed in an unstructured grid or of the hexahedrons formed by a structured grid’s lines. While the regularity of a structured grid’s pattern simplifies the process of solving the mathematical equations to a predetermined level of accuracy, the corresponding CFD-based mesh development requires an immense amount of time and expertise to design a complex model’s optimal block structure. Conversely, the lack of geometric restrictions imposed upon an unstructured grid enables scientists to automate their creation more easily than for structured grids. For example, scientists can generate an unstructured grid for a complex vehicle geometry in days or weeks, versus the weeks or months needed for a comparable structured grid.

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