5,519 research outputs found
Reduction method for thermal analysis of complex aerospace structures
A reduction method which combines classical Rayleigh-Ritz modal superposition techniques with contemporary finite-element methods is applied to transient nonlinear thermal analysis of aerospace structures. The essence of the method is the use of a few thermal modes from eigenvalue analyses as basis vectors to represent the temperature response in the structure. The method is used to obtain approximate temperature histories for a portion of the Shuttle orbiter wing subject to reentry heating and for a large space antenna reflector subject to heating associated with a low Earth orbit. The reduction method has excellent potential for significant size reduction for radiation-dominated problems such as the antenna reflector. However, for conduction-dominated problems such as the Shuttle wing, especially those with complex spatial and temporal variations in the applied heating, additional work appears necessary to find alternate sources of basis vectors which will permit significant problem size reductions
Development of a reduced basis technique for transient thermal analysis
A technique to reduce the degrees of freedom in static and dynamic problems, the reduced basis method, is described. The method combines the classical Rayleigh-Ritz approximation with contemporary finite element methods to retain modeling versatility as the degrees of freedom are reduced. Applications to a nonlinear dynamic response problem are discussed efforts to apply the method to nonlinear transient thermal response problems are summarized. The selection of basis vectors for reducing the system of equations is addressed
Flutter analysis of two parallel elastically coupled flat plates
Flutter of two parallel elastically coupled flat plates was investigated analytically. A closed-form solution including both aerodynamic and structural damping is presented for flutter of flat orthotropic plates coupled by an elastic medium. Both plates are simply supported along the side edges but are supported by deflectional, rotational, and torsional springs of arbitrary stiffness at the leading and trailing edges. Two-dimensional quasi-steady aerodynamics was utilized in the solution. Since the large number of variables present in the problem precludes extensive parametric studies, results are presented to indicate the basic flutter characteristics of coupled two-plate systems and to assess the validity of previously published modal solutions for similar problems
Effects of substrate deformation and sip thickness on tile/sip interface stresses for shuttle thermal protection
A nonlinear analysis was used to study the effects of substrate deformation characteristics and strain isolator pad (SIP) thickness on TILE/SIP interface stresses for the space shuttle thermal protection system. The configuration analyzed consisted of a 5.08 cm thick, 15.24 cm square tile with a 12.7 cm square SIP footprint bordered by a 1.27 cm wide filler bar and was subjected to forces and moments representative of a 20.7 kPa aerodynamic shock passing over the tile. The SIP stress deflection curves were obtained after a 69 kPa proof load and 100 cycles conditioning at 55 kPa. The TILE/SIP interface stresses increase over flat substrate values for zero to peak substrate deformation amplitudes up to 0.191 cm by up to a factor of nearly five depending on deformation amplitude, half wave length, and location. Stresses for a 0.23 cm thick SIP found to be up to 60 percent greater than for a 0.41 cm thick SIP for identical loads and substrate deformation characteristics. A simplified method was developed for approximating the substrate location which produces maximum TILE/SIP interface stresses
Approximation methods for combined thermal/structural design
Two approximation concepts for combined thermal/structural design are evaluated. The first concept is an approximate thermal analysis based on the first derivatives of structural temperatures with respect to design variables. Two commonly used first-order Taylor series expansions are examined. The direct and reciprocal expansions are special members of a general family of approximations, and for some conditions other members of that family of approximations are more accurate. Several examples are used to compare the accuracy of the different expansions. The second approximation concept is the use of critical time points for combined thermal and stress analyses of structures with transient loading conditions. Significant time savings are realized by identifying critical time points and performing the stress analysis for those points only. The design of an insulated panel which is exposed to transient heating conditions is discussed
The effects of an extra U(1) axial condensate on the radiative decay eta' --> gamma gamma at finite temperature
Supported by recent lattice results, we consider a scenario in which a
U(1)-breaking condensate survives across the chiral transition in QCD. This
scenario has important consequences on the pseudoscalar-meson sector, which can
be studied using an effective Lagrangian model. In particular, generalizing the
results obtained in a previous paper (where the zero-temperature case was
considered), we study the effects of this U(1) chiral condensate on the
radiative decay eta' --> gamma gamma at finite temperature.Comment: 15 pages, LaTeX fil
Preparation of nondegenerate coherent superpositions in a three-state ladder system assisted by Stark Shifts
We propose a technique to prepare coherent superpositions of two
nondegenerate quantum states in a three-state ladder system, driven by two
simultaneous fields near resonance with an intermediate state. The technique,
of potential application to enhancement of nonlinear processes, uses adiabatic
passage assisted by dynamic Stark shifts induced by a third laser field. The
method offers significant advantages over alternative techniques: (\i) it does
not require laser pulses of specific shape and duration and (\ii) it requires
less intense fields than schemes based on two-photon excitation with
non-resonant intermediate states. We discuss possible experimental
implementation for enhancement of frequency conversion in mercury atoms.Comment: 22 pages, 8 figures, 1 table, submitted to PHys. Rev.
Reactive Atom Plasma (RAP) figuring machine for meter class optical surfaces
A new surface figuring machine called Helios 1200 is presented in this paper. It is designed for the figuring of meter sized optical surfaces with form accuracy correction capability better than 20 nm rms within a reduced number of iterations. Unlike other large figuring facilities using energy beams, Helios 1200 operates a plasma torch at atmospheric pressure, offers a high material removal rate, and a relatively low running cost. This facility is ideal to process large optical components, lightweight optics, silicon based and difficult to machine materials, aspheric, and free form surfaces. Also, the surfaces processed by the reactive atom plasma (RAP) are easy to fine polish through hand conventional sub-aperture polishing techniques. These unique combined features lead to a new capability for the fabrication of optical components opening up novel design possibilities for optical engineers. The key technical features of this large RAP machine are fast figuring capabilities, non-contact material removal tool, the use of a near Gaussian footprint energy beam, and a proven tool path strategy for the management of the heat transfer. Helios 1200 complies with the European machine safety standard and can be used with different types of reactive gases using either fluorine or chlorine compounds. In this paper, first the need for large optical component is discussed. Then, the RAP facility is described: radio frequency R.F generator, plasma torch, and 3 axis computer numerically controlled motion system. Both the machine design and the performance of the RAP tool is assessed under specific production conditions and in the context of meter class mirror and lens fabrication
Photoionization Suppression by Continuum Coherence: Experiment and Theory
We present experimental and theoretical results of a detailed study of
laser-induced continuum structures (LICS) in the photoionization continuum of
helium out of the metastable state 2s . The continuum dressing with a
1064 nm laser, couples the same region of the continuum to the {4s }
state. The experimental data, presented for a range of intensities, show
pronounced ionization suppression (by as much as 70% with respect to the
far-from-resonance value) as well as enhancement, in a Beutler-Fano resonance
profile. This ionization suppression is a clear indication of population
trapping mediated by coupling to a contiuum. We present experimental results
demonstrating the effect of pulse delay upon the LICS, and for the behavior of
LICS for both weak and strong probe pulses. Simulations based upon numerical
solution of the Schr\"{o}dinger equation model the experimental results. The
atomic parameters (Rabi frequencies and Stark shifts) are calculated using a
simple model-potential method for the computation of the needed wavefunctions.
The simulations of the LICS profiles are in excellent agreement with
experiment. We also present an analytic formulation of pulsed LICS. We show
that in the case of a probe pulse shorter than the dressing one the LICS
profile is the convolution of the power spectra of the probe pulse with the
usual Fano profile of stationary LICS. We discuss some consequences of
deviation from steady-state theory.Comment: 29 pages, 17 figures, accepted to PR
An optimality criterion for sizing members of heated structures with temperature constraints
A thermal optimality criterion is presented for sizing members of heated structures with multiple temperature constraints. The optimality criterion is similar to an existing optimality criterion for design of mechanically loaded structures with displacement constraints. Effectiveness of the thermal optimality criterion is assessed by applying it to one- and two-dimensional thermal problems where temperatures can be controlled by varying the material distribution in the structure. Results obtained from the optimality criterion agree within 2 percent with results from a closed-form solution and with results from a mathematical programming technique. The thermal optimality criterion augments existing optimality criteria for strength and stiffness related constraints and offers the possibility of extension of optimality techniques to sizing structures with combined thermal and mechanical loading
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