Rate Of Momentum Research Articles

The Wake of a Kestrel (Falco Tinnunculus) in Flapping Flight

ABSTRACT The structure of the wake behind a kestrel in medium-speed flight down a 36 m length of corridor was analysed qualitatively and quantitatively by stereophoto grammetry of multiple flash photographs of the motion of small soap-covered helium bubbles. The wake consists of a pair of continuous, undulating trailing vortices. The upstroke is therefore aerodynamically active and the circulation appears to remain constant along the wing whose geometry is altered during the course of the wing stroke. It is argued that the flight kinematics, and so the wake structure, of the kestrel may be typical of flapping flight at medium speeds and a flight model based on this wake geometry is presented. Rough estimates of the rate of momentum generated in the wake balance the weight almost exactly and a direct estimate of the induced power requirement from the wake measurements is obtained. The significance of these results for the various alternative aerodynamic descriptions and energetic predictions of models of flapping animal flight is briefly assessed.

Nonlinear stability of two-phase jets

The effect of solid particles on the flow stability and secondary regime branching in plane submerged jets is studied. The presence of the particles has an important influence on the macrostructure and microstructure of the jet flows, modifying the rate of turbulent momentum, heat and mass transfer [1,2].

Initialization with the data assimilation method

The initial conditions generated by a data assimilation technique with a general circulation model were evaluated. Two types of dynamic initialization, i.e., “the forward adjustment” and “the forward-backward adjustment”, were tested using the real data produced by the NMC objective analysis. The degree of balance in the initial condition was examined in terms of the smoothness in the development of the rate of precipitation, angular momentum, and kinetic energy during the starting period of the prediction. The quality of the initialization was, however, appraised by the performance of the subsequent prediction. For the forward adjustment, various types of schemes of data insertion were studied with regard to the amount of shock produced and the degree of faithfulness of the injected data. In dealing with forward-backward processes, one difficult problem is how to design a reversible algorithm. An approximate method is presented for maintaining the reversibility in a system which includes kinetic energy dissipation, radiation, and the moist heating. In addition, the question is discussed as to which physical processes of the model must be included for the forward-backward initialization. It is found that the dynamic initialization using the forward-backward data assimilation produces predictions that are, in some ways, better than those from the static or conventional initialization. However, for the forward data assimilation the predictions are consistently worse than those from the static initialization. DOI: 10.1111/j.2153-3490.1978.tb00816.x

Nuclear viscosity

The decay rate of momentum in a nuclear reaction is given by an exact formula expressed in terms of the T matrix. A special case, where a viscosity coefficient can be estimated, is considered.Received 29 July 1974DOI:https://doi.org/10.1103/PhysRevC.11.82©1975 American Physical Society

Plasma losses through an adiabatic cusp

Previous calculations of β = 1 cusp losses are extended to the case where there exists a third ‘adiabatic’ invariant of the particle motion, related to the magnetic moment, and additional to the particle energy and angular momentum. Simple analytic results are obtained for the particle and energy loss rate from a Maxwellian plasma in both plane and axisymmetric geometry, electric fields being assumed zero everywhere. The results are valid for a plasma radius large relative to the ion gyro-radius, in which case the adiabaticity in the sense used reduces the loss rate by a factor ⅔λ⅔ to give an overall scaling as λ–⅔ with external mirror ratio λ. The absolute value is about a factor two less than the previously accepted result (for λ = 1 only) for an axisymmetric cusp. cusp. The enhanced scaling with A arises from the absence of a certain class of particles in the sheath, which must be everywhere adiabatic; its applicability to a simple theta pinch is queried, however. The end loss rate of angular momentum is also obtained, and is non-zero at a point cusp. This is shown to lead to rapid rotational instability in a collisionless theta pinch.

A field study of the development of wind-waves

This paper presents the results of observation on the development of wind-waves which were generated in a lake water about 420 cm deep with a fetch 12 km long. Measurements of surface elevation were carried out at the end of an observational pier where the water depth was 80 cm. The wave momentum flux, i.e., the growth rate of the wave momentum, was estimated from both significant waves and power spectral densities for the wave records. The values obtained by the two ways accorded fairly well and they were 5∼7 % as large as the wind stress measured simultaneously. The exponential growth rate of spectral densities for a frequency component was in good accord with that observed bySnyder andCox (1966) and by others. If these growth rates are applied to all the components of the spectrum, the wave momentum flux must exceed the wind stress. This cannot explain the experimental results nor can be physically accepted. The difference of spectral densities between the two successive runs showed that the increase of spectral densities was. limited in several bands of frequency. The phenomena are discussed in relation with the ‘overshoot-undershoot effects’ studied byBarnett andSutherland (1968).

Energy and Momentum Loss Rates for Hot Electrons in Silicon

AbstractUsing of time‐dependent perturbation and deformation potential theories for electronphonon interaction, the rate of change of crystal momentum and energy of electron along the field direction has been estimated in silicon. It is found that the loss rate of crystal momentum of the electron along field direction, due to acoustic phonon collisions is greater than that due to optical phonon collisions, whereas the rate of loss of energy of electrons due to optical phonon interactions is greater than that due to acoustic phonon collisions. The dominant acoustic phonon contribution to the loss rate of the crystal momentum of the electron along field direction is confirmed by the observed and estimated temperature dependence of the drift velocity of electrons in silicon for high electric fields.

Studies of combustion efficiency and flame shape for opposed-jet flameholders.

An investigation was undertaken in order to determine the flame-spreading characteristics of an opposed-jet stabilized flame and their relation to chamber combustion efficiency. Tests were conducted in a standard combustion tunnel at atmospheric pressure, using premixed propane and air as reactants. The approach velocities in the test section were subsonic, ranging from 65 to 140 fps. The equivalence ratio of the mainstream ranged from 0.8 to 1.47. A chamber combustion efficiency equation based on the increase in the rate of momentum was used to study the variation of chamber combustion efficiency in the flame spreading region. This efficiency was shown to correlate with the fraction of the duct cross-sectional area occupied by the flame, as well as the fraction of the inlet mass flow rate which was burning. The entrainment characteristics of the opposed-jet stabilized flame were examined downstream from the tip of the opposed-jet tube. The increase of the burning mass with respect to the burning area was found to be dependent on the percentage of the duct area that was occupied by the flame. The theory of Miesse, which relates combustion efficiency and flame shape, was shown to correlate with the experimental data for certain values of the order of reaction and Damkohler's first ratio.

Vortex gas accelerator

The exhaust velocity of a propellant gas from an isothermal cavity can be significantly increased by a factor of approximately 1.7 above that corresponding to the usual isentropic expansion by utilizing multiple reheat by blackbody radiation from the walls during expansion. In order to reduce the necessary volume and the friclional drag with the walls during this reheat and expansion process, the gas is wrapped up in a rotational flow pattern in which angular momentum per unit mass is constant along all streamlines. As a consequence, the expansion in the nozzle corresponds to the conversion of rotational to linear momentum, rather than the usual thermal to linear. In order to absorb the radiant energy from the walls, the gas must have a sufficient optical opacity (achieved by a small contaminant addition to hydrogen) such that the gas in the cavity is approximately one radiation mean free path thick. R OTATIONAL gas flow can be used to enhance the exhaust velocity from a constant-temperature source provided radiation-transport heating can be made greater than the vortex flow frictional decay time. In simplest terms, a partially isothermal expansion of the working fluid can be achieved as opposed to the usual adiabatic expansion. The additional enthalpy supplied to the fluid during isothermal expansion results in a higher specific impulse for rocket applications. Since this enthalpy must be added to the fluid by radiation flow, the opacity, density, frictional decay rate, and radiation intensity become the determining quantities. There are two general forms of rotational gas flow: one is at constant angular velocity (frequently referred to as rotation) and describes the lowest order state of a gas in equilibrium inside a rotating cylinder; the second form occurs at constant angular momentum per unit mass and, to exist, requires that the rate of angular momentum supplied to the flow pattern must be large compared to the frictional drag, both with the walls and internally. The state of constant angular momentum, therefore, exists either transiently or as the result of a continuous flow of injected angular momentum and partially degraded exhaust. In this paper we are primarily concerned with the second form of rotational gas flow in which the gas is injected tangentially at the periphery of a cylindrical cavity and expands radially toward the axis. The gas leaves the cavity by axial flow through a hole of smaller radius at one end of the cavity (Fig. 1). Both axial and radial velocities are considered as small perturbations to the primary flow, which is circular. The energy transferred in the isothermal expansion from the state of the gas at the outer wall to that at the smaller radius of exhaust is stored in kinetic energy of rotational velocity. This rotational velocity can then be converted to axial velocity in a standard nozzle (Appendix A). It is useful to describe the density and/or pressure distribution of the wheel rotation flow in order to gain a qualitative understanding of the flow pattern near injection. In particular, one would wish to substantiate the possibility of injecting the gas tangentially at the periphery, in a thin layer, at near-constant angular momentum and pressure equilibrium with the surrounding gas. The subsequent radial and rotational flow is assumed to occur at constant