2,612 research outputs found
A brief review of some mechanisms causing boundary layer transition at high speeds
In high speed flight, the state of the boundary layer can strongly influence the design of vehicles through its effect on skin friction drag and aerodynamic heating. The major mechanisms causing boundary layer transition on high speed vehicles are briefly reviewed and some empirical relations from the unclassified literature are given for the transition Reynolds numbers
Trajectory characteristics and heating of hypervelocity projectiles having large ballistic coefficients
A simple, approximate equation describing the velocity-density relationship (or velocity-altitude) has been derived from the flight of large ballistic coefficient projectiles launched at high speeds. The calculations obtained by using the approximate equation compared well with results for numerical integrations of the exact equations of motion. The flightpath equation was used to parametrically calculate maximum body decelerations and stagnation pressures for initial velocities from 2 to 6 km/s. Expressions were derived for the stagnation-point convective heating rates and total heat loads. The stagnation-point heating was parametrically calculated for a nonablating wall and an ablating carbon surface. Although the heating rates were very high, the pulse decayed quickly. The total nose-region heat shield weight was conservatively estimated to be only about 1 percent of the body mass
Non-equilibrium behavior at a liquid-gas critical point
Second-order phase transitions in a non-equilibrium liquid-gas model with
reversible mode couplings, i.e., model H for binary-fluid critical dynamics,
are studied using dynamic field theory and the renormalization group. The
system is driven out of equilibrium either by considering different values for
the noise strengths in the Langevin equations describing the evolution of the
dynamic variables (effectively placing these at different temperatures), or
more generally by allowing for anisotropic noise strengths, i.e., by
constraining the dynamics to be at different temperatures in d_par- and
d_perp-dimensional subspaces, respectively. In the first, case, we find one
infrared-stable and one unstable renormalization group fixed point. At the
stable fixed point, detailed balance is dynamically restored, with the two
noise strengths becoming asymptotically equal. The ensuing critical behavior is
that of the standard equilibrium model H. At the novel unstable fixed point,
the temperature ratio for the dynamic variables is renormalized to infinity,
resulting in an effective decoupling between the two modes. We compute the
critical exponents at this new fixed point to one-loop order. For model H with
spatially anisotropic noise, we observe a critical softening only in the
d_perp-dimensional sector in wave vector space with lower noise temperature.
The ensuing effective two-temperature model H does not have any stable fixed
point in any physical dimension, at least to one-loop order. We obtain formal
expressions for the novel critical exponents in a double expansion about the
upper critical dimension d_c = 4 - d_par and with respect to d_par, i.e., about
the equilibrium theory.Comment: 17 pages, revtex, one figure and EPJB style files include
Trajectory module of the NASA Ames Research Center aircraft synthesis program ACSYNT
A program was developed to calculate trajectories for both military and commercial aircraft for use in the aircraft synthesis program, ACSYNT. The function of the trajectory module was to calculate the changes in the vehicle's flight conditions and weight, as fuel is consumed, during the flying of one or more missions. The trajectory calculations started with a takeoff, followed by up to 12 phases chosen from among the following: climb, cruise, acceleration, combat, loiter, descent, and paths. In addition, a balanced field length was computed. The emphasis was on relatively simple formulations and analytic expressions suitable for rapid computation since a prescribed trajectory had to be calculated many times in the process of converging an aircraft design, or finding an optimum configuration. The trajectory module consists of about 2500 cards and operational on a CDC 7600 computer
Transonic rotor tip design using numerical optimization
The aerodynamic design procedure for a new blade tip suitable for operation at transonic speeds is illustrated. For the first time, 3 dimensional numerical optimization was applied to rotor tip design, using the recent derivative of the ROT22 code, program R22OPT. Program R22OPT utilized an efficient quasi-Newton optimization algorithm. Multiple design objectives were specified. The delocalization of the shock wave was to be eliminated in forward flight for an advance ratio of 0.41 and a tip Mach number of 0.92 at psi = 90 deg. Simultaneously, it was sought to reduce torque requirements while maintaining effective restoring pitching moments. Only the outer 10 percent of the blade span was modified; the blade area was not to be reduced by more than 3 percent. The goal was to combine the advantages of both sweptback and sweptforward blade tips. A planform that featured inboard sweepback was combined with a sweptforward tip and a taper ratio of 0.5. Initially, the ROT22 code was used to find by trial and error a planform geometry which met the design goals. This configuration had an inboard section with a leading edge sweep of 20 deg and a tip section swept forward at 25 deg; in addition, the airfoils were modified
A program for calculating turbofan-driven lift-fan propulsion system performance
The performance of a turbofan-powered lift fan propulsion system for vertical takeoff and landing (VTOL) aircraft is calculated. The program formulation consists of taking bleed air from a turbofan engine, heating the bleed air in an interburner, and passing it through a tip turbine to drive a lift fan. Two options are available: bleed air from the engine exhaust, or bleed air that has passed through the engine fan only. This computer program will benefit persons unfamiliar with the thermodynamics of engine cycle analysis
The effects of violating detailed balance on critical dynamics
We present an overview of the effects of detailed-balance violating
perturbations on the universal static and dynamic scaling behavior near a
critical point. It is demonstrated that the standard critical dynamics
universality classes are generally quite robust: In systems with non-conserved
order parameter, detailed balance is effectively restored at criticality. This
also holds for models with conserved order parameter, and isotropic
non-equilibrium perturbations. Genuinely novel features are found only for
models with conserved order parameter and spatially anisotropic noise
correlations.Comment: 4 pages, revtex, no figure
Urinary cholesterol in follow-up of patients with urologic carcinomas or benign prostatic hyperplasia
Sizing Up DDT
A lot of people have now heard about “DDT.” Part of this hearsay is true; part isn’t; much is misleading. Perhaps we can satisfy some curiosity and correct some erroneous impressions about this much publicized material. Is it really proving to be as good for killing insects as its first reports indicated? We’ll see
A review of high-speed, convective, heat-transfer computation methods
The objective is to provide useful engineering formulations and to instill a modest degree of physical understanding of the phenomena governing convective aerodynamic heating at high flight speeds. Some physical insight is not only essential to the application of the information presented here, but also to the effective use of computer codes which may be available to the reader. Given first is a discussion of cold-wall, laminar boundary layer heating. A brief presentation of the complex boundary layer transition phenomenon follows. Next, cold-wall turbulent boundary layer heating is discussed. This topic is followed by a brief coverage of separated flow-region and shock-interaction heating. A review of heat protection methods follows, including the influence of mass addition on laminar and turbulent boundary layers. Next is a discussion of finite-difference computer codes and a comparison of some results from these codes. An extensive list of references is also provided from sources such as the various AIAA journals and NASA reports which are available in the open literature
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