5 research outputs found
Methodology for computational fluid dynamics code verification /validation
The issues of verification, calibration, and validation of computational fluid dynamics (CFD) codes has been receiving increasing levels of attention in the research literature and in engineering technology. Both CFD researchers and users of CFD codes are asking more critical and detailed questions concerning the accuracy, range of applicability, reliability and robustness of CFD codes and their predictions. This is a welcomed trend because it demonstrates that CFD is maturing from a research tool to the world of impacting engineering hardware and system design. In this environment, the broad issue of code quality assurance becomes paramount. However, the philosophy and methodology of building confidence in CFD code predictions has proven to be more difficult than many expected. A wide variety of physical modeling errors and discretization errors are discussed. Here, discretization errors refer to all errors caused by conversion of the original partial differential equations to algebraic equations, and their solution. Boundary conditions for both the partial differential equations and the discretized equations will be discussed. Contrasts are drawn between the assumptions and actual use of numerical method consistency and stability. Comments are also made concerning the existence and uniqueness of solutions for both the partial differential equations and the discrete equations. Various techniques are suggested for the detection and estimation of errors caused by physical modeling and discretization of the partial differential equations
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Large-scale structure evolution in axisymmetric, compressible free-shear layers
This paper is a description of work-in-progress. It describes Sandia`s program to study the basic fluid mechanics of large-scale mixing in unbounded, compressible, turbulent flows, specifically, the turbulent mixing of an axisymmetric compressible helium jet in a parallel, coflowing compressible air freestream. Both jet and freestream velocities are variable over a broad range, providing a wide range mixing layer Reynolds number. Although the convective Mach number, M{sub c}, range is currently limited by the present nozzle design to values of 0.6 and below, straightforward nozzle design changes would permit a wide range of convective Mach number, to well in excess of 1.0. The use of helium allows simulation of a hot jet due to the large density difference, and also aids in obtaining optical flow visualization via schlieren due to the large density gradient in the mixing layer. The work comprises a blend of analysis, experiment, and direct numerical simulation (DNS). There the authors discuss only the analytical and experimental efforts to observe and describe the evolution of the large-scale structures. The DNS work, used to compute local two-point velocity correlation data, will be discussed elsewhere
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Use of shear-stress-sensitive, temperature-insensitive liquid crystals for hypersonic boundary-layer transition detection
The use of shear-stress-sensitive, temperature-insensitive (SSS/TI) liquid crystals (LCs) has been evaluated as a boundary-layer transition detection technique for hypersonic flows. Experiments were conducted at Mach 8 in the Sandia National Laboratories Hypersonic Wind Tunnel using a flat plate model at near zero-degree angle of attack over the freestream unit Reynolds number range 1.2-5.8x10{sup 6}/ft. Standard 35mm color photography and Super VHS color video were used to record LC color changes due to varying surface shear stress during the transition process for a range of commercial SSS liquid crystals. Visual transition data were compared to an established method using calorimetric surface heat-transfer measurements to evaluate the LC technique. It is concluded that the use of SSS/TI LCs can be an inexpensive, safe, and easy to use boundary-layer transition detection method for hypersonic flows. However, a valid interpretation of the visual records requires careful attention to illumination intensity levels and uniformity, lighting and viewing angles, some prior understanding of the general character of the flow, and the selection of the appropriate liquid crystal for the particular flow conditions
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Coherent structures in compressible free-shear-layer flows
Large scale coherent structures are intrinsic fluid mechanical characteristics of all free-shear flows, from incompressible to compressible, and laminar to fully turbulent. These quasi-periodic fluid structures, eddies of size comparable to the thickness of the shear layer, dominate the mixing process at the free-shear interface. As a result, large scale coherent structures greatly influence the operation and efficiency of many important commercial and defense technologies. Large scale coherent structures have been studied here in a research program that combines a synergistic blend of experiment, direct numerical simulation, and analysis. This report summarizes the work completed for this Sandia Laboratory-Directed Research and Development (LDRD) project