13,556 research outputs found
Methodology for sensitivity analysis, approximate analysis, and design optimization in CFD for multidisciplinary applications
In this study involving advanced fluid flow codes, an incremental iterative formulation (also known as the delta or correction form) together with the well-known spatially-split approximate factorization algorithm, is presented for solving the very large sparse systems of linear equations which are associated with aerodynamic sensitivity analysis. For smaller 2D problems, a direct method can be applied to solve these linear equations in either the standard or the incremental form, in which case the two are equivalent. Iterative methods are needed for larger 2D and future 3D applications, however, because direct methods require much more computer memory than is currently available. Iterative methods for solving these equations in the standard form are generally unsatisfactory due to an ill-conditioning of the coefficient matrix; this problem can be overcome when these equations are cast in the incremental form. These and other benefits are discussed. The methodology is successfully implemented and tested in 2D using an upwind, cell-centered, finite volume formulation applied to the thin-layer Navier-Stokes equations. Results are presented for two sample airfoil problems: (1) subsonic low Reynolds number laminar flow; and (2) transonic high Reynolds number turbulent flow
Methodology for sensitivity analysis, approximate analysis, and design optimization in CFD for multidisciplinary applications
Fundamental equations of aerodynamic sensitivity analysis and approximate analysis for the two dimensional thin layer Navier-Stokes equations are reviewed, and special boundary condition considerations necessary to apply these equations to isolated lifting airfoils on 'C' and 'O' meshes are discussed in detail. An efficient strategy which is based on the finite element method and an elastic membrane representation of the computational domain is successfully tested, which circumvents the costly 'brute force' method of obtaining grid sensitivity derivatives, and is also useful in mesh regeneration. The issue of turbulence modeling is addressed in a preliminary study. Aerodynamic shape sensitivity derivatives are efficiently calculated, and their accuracy is validated on two viscous test problems, including: (1) internal flow through a double throat nozzle, and (2) external flow over a NACA 4-digit airfoil. An automated aerodynamic design optimization strategy is outlined which includes the use of a design optimization program, an aerodynamic flow analysis code, an aerodynamic sensitivity and approximate analysis code, and a mesh regeneration and grid sensitivity analysis code. Application of the optimization methodology to the two test problems in each case resulted in a new design having a significantly improved performance in the aerodynamic response of interest
An investigation of combustion instability in aircraft-engine reheat systems
The principal objective of this study was to examine experimentally
the effects of upstream temperature, velocity, gutter blockage, tailpipe
length, and main and pilot fuel flows, on the form of combustion instability
encountered in aircraft reheat systems which is sometimes referred to as 'buzz'.
Tests were carried out at atmospheric pressure for upstream temperatures of
between 200 and 500°C, and upstream velocities ranging from 140 to 200 ft/sec.
Three values of stabilizer blockage were employed, namely 25, 30 and 35%.
The tailpipe length was varied between 9 and 45 inches. Auto-correlation
techniques were used in the frequency analysis of the buzz waveforms.
It was found that a certain minimum tailpipe length is necessary in
order to produce buzz which is then strengthened as the tailpipe length is
increased. Buzz also becomes more pronounced with an increase in gas velocity
but stabilizer blockage appears to have no discernible effect … [cont.]
Photo-induced two-body loss of ultracold molecules
The lifetime of nonreactive ultracold bialkali gases was conjectured to be
limited by sticky collisions amplifying three-body loss. We show that the
sticking times were previously overestimated and do not support this
hypothesis. We find that electronic excitation of NaK+NaK collision complexes
by the trapping laser leads to the experimentally observed two-body loss. We
calculate the excitation rate with a quasiclassical, statistical model
employing ab initio potentials and transition dipole moments. Using longer
laser wavelengths or repulsive box potentials may suppress the losses
Displacement Data Assimilation
We show that modifying a Bayesian data assimilation scheme by incorporating
kinematically-consistent displacement corrections produces a scheme that is
demonstrably better at estimating partially observed state vectors in a setting
where feature information important. While the displacement transformation is
not tied to any particular assimilation scheme, here we implement it within an
ensemble Kalman Filter and demonstrate its effectiveness in tracking
stochastically perturbed vortices.Comment: 26 Pages, 9 figures, 5 table
Incorporating Inertia Into Multi-Agent Systems
We consider a model that demonstrates the crucial role of inertia and
stickiness in multi-agent systems, based on the Minority Game (MG). The inertia
of an agent is introduced into the game model by allowing agents to apply
hypothesis testing when choosing their best strategies, thereby reducing their
reactivity towards changes in the environment. We find by extensive numerical
simulations that our game shows a remarkable improvement of global cooperation
throughout the whole phase space. In other words, the maladaptation behavior
due to over-reaction of agents is removed. These agents are also shown to be
advantageous over the standard ones, which are sometimes too sensitive to
attain a fair success rate. We also calculate analytically the minimum amount
of inertia needed to achieve the above improvement. Our calculation is
consistent with the numerical simulation results. Finally, we review some
related works in the field that show similar behaviors and compare them to our
work.Comment: extensively revised, 8 pages, 10 figures in revtex
Irrelevance of memory in the minority game
By means of extensive numerical simulations we show that all the distinctive
features of the minority game introduced by Challet and Zhang (1997), are
completely independent from the memory of the agents. The only crucial
requirement is that all the individuals must posses the same information,
irrespective of the fact that this information is true or false.Comment: 4 RevTeX pages, 4 figure
Laboratory guide to early life history stages of northeast Pacific fishes
This laboratory guide presents taxonomic information on eggs
and larvae of fishes of the Northeast Pacific Ocean (north of California) and the eastern Bering Sea. Included are early-life-history series, illustrations, and comparative descriptions of 232 species expected to spawn here, out of a total 627 species known to occur in marine waters of this area. Meristic and general life-history data are included, as well as diagnostic characters to help identify eggs and larvae. Most of this information has been gleaned from literature, with the addition of 200 previously unpublished illustrations. (PDF file contains 654 pages.
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