345 research outputs found
Aerodynamic properties of turbulent combustion fields
Flow fields involving turbulent flames in premixed gases under a variety of conditions are modeled by the use of a numerical technique based on the random vortex method to solve the Navier-Stokes equations and a flame propagation algorithm to trace the motion of the front and implement the Huygens principle, both due to Chorin. A successive over-relaxation hybrid method is applied to solve the Euler equation for flows in an arbitrarily shaped domain. The method of images, conformal transformation, and the integral-equation technique are also used to treat flows in special cases, according to their particular requirements. Salient features of turbulent flame propagation in premixed gases are interpreted by relating them to the aerodynamic properties of the flow field. Included among them is the well-known cellular structure of flames stabilized by bluff bodies, as well as the formation of the characteristic tulip shape of flames propagating in ducts. In its rudimentary form, the mechanism of propagation of a turbulent flame is shown to consist of: (1) rotary motion of eddies at the flame front, (2) self-advancement of the front at an appropriate normal burning speed, and (3) dynamic effects of expansion due to exothermicity of the combustion reaction. An idealized model is used to illustrate these fundamental mechanisms and to investigate basic aerodynamic features of flames in premixed gases. The case of a confined flame stabilized behind a rearward-facing step is given particular care and attention. Solutions are shown to be in satisfactory agreement with experimental results, especially with respect to global properties such as the average velocity profiles and reattachment length
The Principle of Least Action and Clustering in Cosmology
A scheme is developed which enables one to trace backwards in time the cosmic
density and velocity fields, and to determine accurately the current-epoch
velocity field from the current-epoch density field, or vice versa. The scheme
implements the idea of Giavalisco \etal\ (1993) that the principle of least
action should be used to formulate gravitational instability as a two-point
boundary-value problem. We argue that the Eulerian formulation of the problem
is to be preferred to the Lagrangian one, on grounds of computational
simplicity, of ease of interfacing with observational data, and of internal
consistency at early times. The scheme is successfully tested on an exact
solution in one dimension, and on currently Gaussian fields in one and two
dimensions. The application of the scheme to real observational data appears to
be eminently feasible, though computationally costly.Comment: 10 pages, uuencoded compressed postcript file, Oxford Astrophysics
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Semiparametric Estimation of Long-Memory Models
This chapter reviews semiparametric methods of inference on different aspects of long memory
time series. The main focus is on estimation of the memory parameter of linear models, analyzing
bandwidth choice, bias reduction techniques and robustness properties of different estimates, with
sorne emphasis on nonstationarity and trending behaviors. These techniques extend naturally to multivariate series, where the important issues are the estimation of the long-run relationship and
testing for fractional cointegration. Specific techniques for the estimation of the degree of persistence
of volatility for nonlinear time series are also considered
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