Unsteady two dimensional airloads acting on oscillating thin airfoils in subsonic ventilated wind tunnels

The numerical calculation of unsteady two dimensional airloads which act upon thin airfoils in subsonic ventilated wind tunnels was studied. Neglecting certain quadrature errors, Bland's collocation method is rigorously proved to converge to the mathematically exact solution of Bland's integral equation, and a three way equivalence was established between collocation, Galerkin's method and least squares whenever the collocation points are chosen to be the nodes of the quadrature rule used for Galerkin's method. A computer program displayed convergence with respect to the number of pressure basis functions employed, and agreement with known special cases was demonstrated. Results are obtained for the combined effects of wind tunnel wall ventilation and wind tunnel depth to airfoil chord ratio, and for acoustic resonance between the airfoil and wind tunnel walls. A boundary condition is proposed for permeable walls through which mass flow rate is proportional to pressure jump

Spectral methods for CFD

One of the objectives of these notes is to provide a basic introduction to spectral methods with a particular emphasis on applications to computational fluid dynamics. Another objective is to summarize some of the most important developments in spectral methods in the last two years. The fundamentals of spectral methods for simple problems will be covered in depth, and the essential elements of several fluid dynamical applications will be sketched

[Research activities in applied mathematics, fluid mechanics, and computer science]

This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, fluid mechanics, and computer science during the period April 1, 1995 through September 30, 1995

Coupled Continuum and Molecular Model of Flow through Fibrous Filter

A coupled approach combining the continuum boundary singularity method (BSM) and the molecular direct simulation Monte Carlo (DSMC) is developed and validated using Taylor-Couette flow and the flow about a single fiber confined between two parallel walls. In the proposed approach, the DSMC is applied to an annular region enclosing the fiber and the BSM is employed in the entire flow domain. The parameters used in the DSMC and the coupling procedure, such as the number of simulated particles, the cell size, and the size of the coupling zone are determined by inspecting the accuracy of pressure drop obtained for the range of Knudsen numbers between zero and unity. The developed approach is used to study flowfield of fibrous filtration flows. It is observed that in the partial-slip flow regime, Kn ⩽ 0.25, the results obtained by the proposed coupled BSM-DSMC method match the solution by BSM combined with the heuristic partial-slip boundary conditions. For transition molecular-to-continuum Knudsen numbers, 0.25 \u3c Kn ⩽ 1, the difference in pressure drop and velocity between these two approaches is significant. This difference increases with the Knudsen number that confirms the usefulness of coupled continuum and molecular methods in numerical modeling of transition low Reynolds number flows in fibrous filters

Performance Modeling and Prediction for the Scalable Solution of Partial Differential Equations on Unstructured Grids

This dissertation studies the sources of poor performance in scientific computing codes based on partial differential equations (PDEs), which typically perform at a computational rate well below other scientific simulations (e.g., those with dense linear algebra or N-body kernels) on modern architectures with deep memory hierarchies. We identify that the primary factors responsible for this relatively poor performance are: insufficient available memory bandwidth, low ratio of work to data size (good algorithmic efficiency), and nonscaling cost of synchronization and gather/scatter operations (for a fixed problem size scaling). This dissertation also illustrates how to reuse the legacy scientific and engineering software within a library framework. Specifically, a three-dimensional unstructured grid incompressible Euler code from NASA has been parallelized with the Portable Extensible Toolkit for Scientific Computing (PETSc) library for distributed memory architectures. Using this newly instrumented code (called PETSc-FUN3D) as an example of a typical PDE solver, we demonstrate some strategies that are effective in tolerating the latencies arising from the hierarchical memory system and the network. Even on a single processor from each of the major contemporary architectural families, the PETSc-FUN3D code runs from 2.5 to 7.5 times faster than the legacy code on a medium-sized data set (with approximately 105 degrees of freedom). The major source of performance improvement is the increased locality in data reference patterns achieved through blocking, interlacing, and edge reordering. To explain these performance gains, we provide simple performance models based on memory bandwidth and instruction issue rates. Experimental evidence, in terms of translation lookaside buffer (TLB) and data cache miss rates, achieved memory bandwidth, and graduated floating point instructions per memory reference, is provided through accurate measurements with hardware counters. The performance models and experimental results motivate algorithmic and software practices that lead to improvements in both parallel scalability and per-node performance. We identify the bottlenecks to scalability (algorithmic as well as implementation) for a fixed-size problem when the number of processors grows to several thousands (the expected level of concurrency on terascale architectures). We also evaluate the hybrid programming model (mixed distributed/shared) from a performance standpoint

The development and breakaway of a compressible air jet with streamline curvature and its application to the coanda flare

This study concerns an underexpanded jet, issuing from a convergent slot into quiescent air, as it is deflected by a convex surface of constant radius. Emphasis Is placed on the mechanism of breakaway, a phenomenon whereby the jet leaves the surface tangentially. An optical system based on the standard Z-type Schlieren configuration and capable of interferometric, Schlieren and shadowgraph techniques has been designed. The techniques are interchanged simply, a laser source being employed for Interferometry and a Xenon spark source for Schlieren and shadowgraph. Vibrations limit the interferometry and improvements are discussed. Shadowgraph and both spark and continuous Schlieren techniques gave good results. Total pressure traverses and surface oil flow visualization show that the influence of secondary flows on breakaway is small. Measurements of the coefficient of discharge show an increase both as the stagnation pressure Is Increased and as the slot width is reduced. The existence of a separation bubble has been established from surface static pressure measurements and shadowgraph and Schlieren photographs. Surface oil flow visualization shows a region of reversed flow withhin the bubble. The bubble grows as the stagnation pressure Is increased and eventually causes breakaway. A potential flow calculation method using the method of characteristics has been developed. Calculation of a fully attached Jet is inaccurate because the separation bubble is ignored. A calculation using the measured surface static pressures accurately predicts the main features of the first shock cell. Reattachment occurs further downstream of the jet and its breakaway should involve a coupling of the solutions of the outer shear layer, potential core and separated boundary layer, the latter including reversed flow

Aeronautical engineering: A special bibliography, supplement 29, March 1973

This special bibliography lists 410 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1972

Investigation on interactions of unsteady wakes and film cooling on an annular endwall

In recent decades, greater interest in the effect of rotational wakes on gas turbine film cooling applications has produced increasing numbers of studies on these unsteady phenomena. Wakes are primarily shed from upstream components such as transition duct walls, stator vanes, and rotors. Studies have shown that in areas of unsteady flow, the best performing parameters in conventional steady investigations may not be the best for unsteady applications. One common method of modeling the unsteady wake interaction in subsonic flows is the use of spoke wheel type wake generators using cylindrical rods to produce the velocity detriment and local increase in turbulence intensity. Though the impact of wakes have been studied for decades on airfoil losses and boundary layer transition, only recently has the film cooling and wake interaction been investigated. The existing work is primarily on leading edge models and airfoil cascades. The primary parameter characterizing the unsteady wakes is the dimensionless or reduced frequency known as the Strouhal number. The film cooling jet itself has dominant frequencies resulting from the shear and the jet trailing wake shedding, depending on the injectant flow rate. There exist great deficiencies in the fundamental understanding of the interaction of these two frequencies. Heat transfer considerations are also relatively recent being studied only since the early 1990\u27s. Heat transfer coefficients and film cooling effectiveness have been reported for leading edge and linear airfoil cascades. In the case of the linear cascade, no data can be taken near the endwall region due to the varying tangential velocity of wake generating rod. The current work expands on this initiative incorporating a sector annular duct as the test setting for the rotating wakes focusing on this endwall region.; Studies in to the effect of the rods in this alternate orientation include film cooling effectiveness using temperature sensitive paint, impact of wake rod to film cooling hole diameter ratio, and time accurate numerical predictions and comparisons with experimental work. Data are shown for a range of momentum flux ratios and Strouhal numbers. The result of this work sets the stage for the complete understanding of the unsteady wake and inclined jet interaction

Aeronautical Engineering: A continuing bibliography with indexes, supplement 107

Reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1979 are listed in this bibliography