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Results for “rotational diffusion”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Active control of vaneless diffuser rotating stall

Experiments were carried out to study the feasibility of active stabilization of vaneless diffuser rotating stalls. Pressure fluctuation at the diffuser inlet was monitored and used to control the rotating stall. AC control flow, which is produced by a load speaker, was introduced into the diffuser at the inlet. It is shown that the rotating stall can be suppressed when the phase of the control flow has a certain relation with the phase of the rotating stall. By considering the energy flux due to the AC control flow, it is shown that the rotating stall is suppressed when the control flow has the phase such that the energy is subtracted out from the diffuser flow. Discussions are made on the relations between the energy flux and the amplitude of the pressure fluctuation due to the rotating stalls.

Yoshida, Yoshiki↗
NASA NTRS2004

Parametric Study of a Mach 2.4 Transport Engine with Supersonic Through-Flow Rotor and Supersonic Counter-Rotating Diffuser (SSTR/SSCRD)

A parametric study is conducted to evaluate a mixed-flow turbofan equipped with a supersonic through-flow rotor and a supersonic counter-rotating diffuser (SSTR/SSCRD) for a Mach 2.4 civil transport. Engine cycle, weight, and mission analyses are performed to obtain a minimum takeoff gross weight aircraft. With the presence of SSTR/SSCRD, the inlet can be shortened to provide better pressure recovery. For the same engine airflow, the inlet, nacelle, and pylon weights are estimated to be 73 percent lighter than those of a conventional inlet. The fan weight is 31 percent heavier, but overall the installed engine pod weight is 11 percent lighter than the current high-speed civil transport baseline conventional mixed-flow turbofan. The installed specific fuel consumption of the supersonic fan engine is 2 percent higher than that of the baseline turbofan at supersonic cruise. Finally, the optimum SSTR/SSCRD airplane meets the FAR36 Stage 3 noise limit and is within 7 percent of the baseline turbofan airplane takeoff gross weight over a 5000-n mi mission.

Tran, Donald H.↗
NASA NTRS1972

Effect of slow rotational diffusion on angular correlations.

The theory for perturbed angular correlations of gamma radiation has been extended to include the possibility of adiabatic variation in the interaction Hamiltonian, K, for the intermediate state. The calculation begins from a polycrystalline model. It is shown that adiabatic variation in K introduces a time dependence into the angles which express the orientation of the molecular frame. The relevance of the adiabatic limit to the use of perturbed angular correlations of gamma radiation for study of the motion of radioactive species in viscous media is discussed.

Marshall, A. G.↗
NASA NTRS1982

Comparative study of subsynchronous rotating flow patterns in centrifugal compressors with vaneless diffusers

A comparative experimental investigation of the unstable operating modes of a centrifugal compressor was made. Impeller and/or diffuser rotating stall was observed, depending on the flow conditions. The measured relative rotational speed of this perturbation is cross checked with other experimental data and it is shown that the rotational speed is strongly dependent on the type of rotating stall. The diffuser absolute inlet flow angle at the onset of diffuser rotating stall agrees well with the value predicted by an existing stability criterion.

Frigne, P.↗
NASA NTRS1995

Stellar models with microscopic diffusion and rotational mixing. 1: Application to the Sun

The Yale stellar evolution code has been modified to include the combined effects of diffusion and rotational mixing on H-1, H-4, and the trace elements He, Li-6, Li-7, and Be-9. The interaction between rotational mixing and diffusion is studied by calculating a number of calibrated solar models. The rotational mixing inhibits the diffusion in the outer parts of the models, leading to a decrease in the envelope diffusion by 25%-50%. Conversely, diffusion leads to gradients in mean molecular weight which can inhibit the rotational mixing. The degree to which gradients in mean molecular weight inhibit the rotational mixing is somewhat uncertain. A comparison with the observed solar oblateness suggests that gradients in the mean molecular weight play a smaller role in inhibiting the rotational mixing than previously believed. This is reinforced by the fact that the model with the standard value for the inhibiting effect of mean molecular weight on the rotational mixing depletes no Li on the main sequence. This is clear in contrast to the observations. A reduction in the inhibiting effect of mean molecular weight gradients by a factor of 10 loads to noticeable main-sequence Li depletion.

Chaboyer, Brian↗
NASA NTRS1995

Stellar models with microscopic diffusion and rotational mixing. 2: Application to open clusters

Stellar models with masses ranging from 05.5 to 1.3 solar mass were constructed for comparison with young cluster observations of Li and of rotation velocities. The amount of Li depletion in cool stars is sensitive to the amount of overshoot at the base of the surface convection zone, and the exact metallicity of the models. Even when this is taken into account, the Li observations are a severe constraint for the models and rule out standard models and pure diffusion models. Stellar models which include diffusion and rotational mixing in the radiative regions of stars are able to simultaneously match the Li abundances observed in the Pleiades, the UMa Group, The Hyades, Praesepe, NGC 752, and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to simultaneously explain the presence of the rapidly rotating late G and K stars in the Pleiades and the absence of rapidly rotating late F and early G stars.

Chaboyer, B.↗
NASA NTRS1990

Beryllium in the Galactic halo - Surface abundances from standard, diffusive, and rotational stellar evolution, and implications

The recently observed upper limits to the beryllium abundances in population II stars are much lower than population I detections. This difference reflects an intrinsic difference in the initial abundances and is not caused by different degrees of depletion driven by stellar evolution processes from similar initial abundances. Evolutionary sequences of models from the early premain sequence to beyond the turnoff that correspond to halo dwarfs with Fe/H abundances of -1.3, -2.3, and -3.3 are constructed, and standard, diffusive, and rotational mechanisms are used to estimate a maximal possible beryllium depletion. Halo star models in the T(eff) range 6000 to 5000 K might be rotationally depleted by a factor of 1.5-2, and the total depletion should be no more than (conservatively) a factor of 3. Implications for cosmology, cosmic-ray theory, and Galactic chemical evolution are discussed.

Deliyannis, Constantine P.↗
NASA NTRS1972

Effects of vane-induced rotation on diffuser flow distortion in an axisymmetric mixed-compression inlet

An investigation of vane-induced flow rotation to modify distorted steady-state total-pressure patterns in the subsonic diffuser of a supersonic mixed-compression inlet was conducted. Radial static-pressure gradients generated by the rotation was the mechanism used to modify the total-pressure distributions. Significant redistribution of circumferential distortion patterns into more compatible radial patterns was realized, but flow problems near the duct walls reduced the general effectiveness of the technique. Total-pressure losses associated with the swirl vanes were slight. Limited turbulence data indicated that vane istallation resulted in reduced turbulence levels.

Meleason, E. T.↗
NASA NTRS1972

Effect of molecular shape and flexibility on gamma-ray directional correlations.

Huntress' formalism (1968) is used to calculate the angular correlation behavior for an ensemble of radioactive nuclei, each of which is bound to an asymmetric-top molecule. The calculation is carried out both in an extreme narrowing limit (rapid motion) and in an adiabatic limit (slow motion). The two limiting cases span nearly the whole range of rates of rotational reorientation. It is shown that the problem of internal rotation is formally identical to that of rotation of a symmetric-top molecule. The experimentally observable time-integrated anisotropy is plotted for several rates of internal rotational diffusion.

Marshall, A. G.↗
NASA NTRS1977

ESR studies of the slow tumbling of vanadyl spin probes in nematic liquid crystals

ESR line shapes that are appropriate for slowly tumbling vanadyl spin probes in viscous nematic liquid crystals were calculated by the stochastic Liouville method. Because of the symmetry possessed by vanadyl, the analysis and interpretation of these line shapes was simplified considerably. Spectral line shapes agreed well with experimental spectra of VOAcAc in the nematic liquid crystal Phase V and BEPC. Deviations from Brownian rotational diffusion were noted. A slowly fluctuating torque analysis yielded good agreement with the experimental spectra.

Eastman, M. P.↗
NASA NTRS1977

On the nature of photospheric magnetic fields beneath large coronal holes

Proposed mechanisms for the formation of coronal holes are considered; the crucial issue appears to be whether the holes are permeated by rigidly rotating fields. It is suggested that the interaction between such a field and the differentially rotating, diffusive solar envelope will produce a fore aft asymmetry in the distribution of fields which emerge to the photosphere. An initial study is carried out in the context of an illustrative example, and the results indicate that the asymmetry may be observed for a certain range of parameters involving the properties of the solar envelope and the characteristic size of the emerging field pattern.

Frankenthal, S.↗
NASA NTRS1978

Molecular rotation and reorientation - Microscopic and hydrodynamic contributions

The relative roles of microscopic and hydrodynamic contributions to molecular rotation and reorientation are examined within the framework of the microscopic boundary layer theory recently proposed by the authors. The theory is applied to rough spheres, for which computer simulation data are available and to experimental results on spherical top molecules. Attention is focused on rotational diffusion constants, the kappa parameter introduced by Kivelson et al., and orientational relaxation times. It is shown that, while collective effects are present and often nonnegligible, the motion of small molecules is dominated by its microscopic aspects. Experimental trends which can incorrectly suggest dominance by hydrodynamic contributions are discussed in some detail. Finally, the transition to the regime where collective effects are dominant is considered.

Hynes, J. T.↗
NASA NTRS1983

Coherent anti-Stokes Raman spectroscopic modeling for combustion diagnostics

The status of modelling the coherent anti-Stokes Raman spectroscopy (CARS) spectra of molecules important in combustion, such as N2, H2O, and CO2, is reviewed. It is shown that accurate modelling generally requires highly precise knowledge of line positions and reasonable estimates of Raman linewidths, and the sources of these data are discussed. CARS technique and theory is reviewed, and the status of modelling the phenomenon of collisional narrowing at pressures well above atmospheric for N2, H2O, and CO2 is described. It is shown that good agreement with experiment can be achieved using either the Gordon rotational diffusion model or phenomenological models for inelastic energy transfer rates.

Hall, R. J.↗
NASA NTRS1992

Flow-induced gelation of living (micellar) polymers

The effect of shear velocity gradients on the size (L) of rodlike micelles in dilute and semidilute solution is considered. A kinetic equation is introduced for the time-dependent concentration of aggregates of length L, consisting of 'bimolecular' combination processes L + L-prime yield (L + L-prime) and unimolecular fragmentations L yield L + (L - L-prime). The former are described by a generalization (from spheres to rods) of the Smoluchowski mechanism for shear-induced coalesence of emulsions, and the latter by incorporating the tension-deformation effects due to flow. Steady-state solutions to the kinetic equation are obtained, with the corresponding mean micellar size evaluated as a function of the Peclet number P (i.e., the dimensionless ratio of the flow rate and the rotational diffusion coefficient). For sufficiently dilute solutions, only a weak dependence of the micellar size on P is found. In the semidilute regime, however, an apparent divergence in the micellar size at P of about 1 suggests a flow-induced first-order gelation phenomenon.

Bruinsma, Robijn↗
NASA NTRS1986

Fluctuation and rotation in diffusion-influenced monomolecular reactions

Models of partially absorbing boundaries for calculating diffusion-influenced chemical reactions are examined analytically. The limitations of diffusion-equation and random-walk models are indicated; the presence of an ambiguous length parameter in a standard radiation-boundary-condition (RBC) model is demonstrated; the RBC model for a boundary with oscillation (at a rate much faster than the rate of thermal collisions) between absorbing and nonabsorbing states is considered; an alternative stochastically switching absorber (SSA) model is introduced and shown to be physically consistent; the equivalence of one-dimensional RBC and SSA models under some conditions is shown; one time passage is calculated for an axially symmetric three-dimensional SSA model with anisotropic absorption; and the analogy between rotation and stochastic switching is explored.

Bashford, Donald↗
NASA NTRS1989

A theoretical study of fluid forces on a centrifugal impeller rotating and whirling in a vaned diffuser

The fluid forces on a centrifugal impeller rotating and whirling in a vaned diffuser are analyzed on the assumption that the number of impeller and diffuser vanes is so large that the flows are perfectly guided by the vanes. The flow is taken to be two dimensional, inviscid, and incompressible, but the effects of impeller and diffuser losses are taken into account. It is shown that the interaction with the vaned diffuser may cause destabilizing fluid forces. From these discussions, it is found that the whirling forces are closely related to the steady head-capacity characteristics of the impeller. This physical understanding of the whirling forces can be applied also to the cases with volute casings. At partial capacities, it is shown that the impeller forces change greatly when the flow rate and whirl velocity are near to the impeller or vaned diffuser attributed rotating stall onset capacity, and the stall propagation velocity, respectively. In such cases the impeller forces may become destabilizing for impeller whirl.

Tsujimoto, Yoshinobu↗
NASA NTRS2012

Scientific Verification of Faraday Rotation Modulators: Detection of Diffuse Polarized Galactic Emission

The design and performance of a wide bandwidth linear polarization-modulator based on the Faraday effect is described. Faraday Rotation Modulators (FRMs) are solid-state polarization switches that are capable of modulation up to approx 10 kHz. Six FRMs were utilized during the 2006 observing season in the Background Imaging of Cosmic Extragalactic Polarization (BICEP) experiment; three FRMs were used at each of BICEP fs 100 and 150 GHz frequency bands. The technology was verified through high signal-to-noise detection of Galactic polarization using two of the six FRMs during four observing runs in 2006. The features exhibit strong agreement with BICEP fs measurements of the Galaxy using non-FRM pixels and with the Galactic polarization models. This marks the first detection of high signal-to-noise mm-wave celestial polarization using fast, active optical modulation. The performance of the FRMs during periods when they were not modulated was also analyzed and compared to results from BICEP fs 43 pixels without FRMs.

Moyerman, S.↗