Amplitude Of Velocity Fluctuations Research Articles

Alfvén waves, alpha particles, and pickup ions in the solar wind

Past studies of the properties of Alfvén waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles often do not participate in the wave motions because they “surf” on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the “observed” wave speed, Vwave=δV Bo/δB, as determined from the ratio of amplitudes of velocity and magnetic fluctuations. The possibility that the modification of Alfvén wave propagation speed is due to pressure anisotropies resulting from pickup ion distributions is investigated.

Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 2. Characteristics of gravity waves

This paper discusses the characteristics of gravity waves in the equatorial region revealed by analyzing radiosonde measurements of wind velocity and temperature fluctuations at 0–35 km, with a height resolution of 150 m, made every 5–7 hours between February 27 and March 22, 1990, in East Java, Indonesia. We conducted hodograph analysis to delineate vertical and horizontal propagation characteristics and found that most gravity waves were generated in the middle of the troposphere and that they propagated upward into the stratosphere. The amplitudes of wind velocity and temperature fluctuations due to gravity waves were larger in the stratosphere than in the troposphere. The direction of horizontal propagation of gravity waves was rather uniformly distributed in the troposphere, but it became eastward in the lower stratosphere, being opposite to that of the mean winds because of quasi‐biennial oscillation. The vertical wavenumber spectra of wind velocity were described fairly well by a saturated model spectrum, although their amplitudes were smaller in the stratosphere. A typical vertical wavelength was 2–2.5 km, while the horizontal phase velocities were 5–7 and 12 m/s in the troposphere and stratosphere, respectively. The amplitudes of small‐scale gravity waves were significantly larger in the equatorial stratosphere than at middle latitudes. Time‐height variation of the wind velocity variance due to gravity waves showed a clear correlation with that for high relative humidity, which implies that cloud convection is an important mechanism of gravity wave generation in the equatorial region.

Effect of vision and stance width on human body motion when standing: implications for afferent control of lateral sway.

1. Measurements of human upright body movements in three dimensions have been made on thirty-five male subjects attempting to stand still with various stance widths and with eyes closed or open. Body motion was inferred from movements of eight markers fixed to specific sites on the body from the shoulders to the ankles. Motion of these markers was recorded together with motion of the point of application of the resultant of the ground reaction forces (centre of pressure). 2. The speed of the body (average from eight sites) was increased by closing the eyes or narrowing the stance width and there was an interaction between these two factors such that vision reduced body speed more effectively when the feet were closer together. Similar relationships were found for components of velocity both in the frontal and sagittal planes although stance width exerted a much greater influence on the lateral velocity component. 3. Fluctuations in position of the body were also increased by eye closure or narrowing of stance width. Again, the effect of stance width was more potent for lateral than for anteroposterior movements. In contrast to the velocity measurements, there was no interaction between vision and stance width. 4. There was a progressive increase in the amplitude of position and velocity fluctuations from markers placed higher on the body. The fluctuations in the position of the centre of pressure were similar in magnitude to those of the markers placed near the hip. The fluctuations in velocity of centre of pressure, however, were greater than of any site on the body. 5. Analysis of the amplitude of angular motion between adjacent straight line segments joining the markers suggests that the inverted pendulum model of body sway is incomplete. Motion about the ankle joint was dominant only for lateral movement in the frontal plane with narrow stance widths (< 8 cm). For all other conditions most angular motion occurred between the trunk and leg. 6. The large reduction in lateral body motion with increasing stance width was mainly due to a disproportionate reduction in the angular motion about the ankles and feet. A mathematical model of the skeletal structure has been constructed which offers some explanation for this specific reduction in joint motion.(ABSTRACT TRUNCATED AT 400 WORDS)

Spin-down to rest in a cylindrical cavity

The nature of the flow during impulsive spin-down to rest in a cylindrical cavity is studied experimentally. Flow visualization using reflective flakes and laser-Doppler velocimetry are the tools of this investigation. The velocimeter is configured to measure simultaneously the azimuthal velocity component at two arbitrarily separated locations within the cylinder. The Ekman number is about 10 −5 and the flow is unstable. The mean angular velocity decreases non-uniformly and monotonically. The velocity fluctuation amplitudes and frequencies decrease steadily. A novel data analysis is used to study the velocity fluctuations, which are neither stationary nor uniform. The assumptions of this analysis are the validity of Taylor's hypothesis of frozen-eddy transport and the ergodicity of the process following that rescaling. The fluctuations are equally dominant during all phases of the spin-down process when scaled with the current mean velocity. The root-mean-squared intensity measurements in the core (r/R r −1 dependence while a uniform value is observed in the buffer region (0.4 r/R k. The flow in the latter phases tends to a single vortex.

An experimental investigation into the role of simultaneous amplitude and phase modulation in the transition of a planar jet

An experimental investigation into the role of simultaneous amplitude and phase modulation of velocity fluctuations characterizing the near-field transition of an initially laminar planar jet at moderate Reynolds number is presented. Very low level periodic acoustic excitation at selected frequencies is applied upstream of the nozzle exit in order to introduce an organized carrier wave into the flow. Instantaneous amplitude and phase modulates are computed directly from hot-wire velocity fluctuation data which allows their respective role in the jet transition process to be quantified. It is found that amplitude modulation serves to provide energy to amplifying sideband fluctuations which multitone phase modulation effectively redistributes to neighboring spectral modes in the nonlinear stages of transition. The phase modulation is found to result from an experimentally verified amplitude dependence in the dispersion relation. The phase angle between amplitude and phase modulation is found to be an important parameter in governing the spectral development of the shear layer by its effect on sideband asymmetry. While the essential physics associated with the modulation phenomenon is found to be quite similar to that previously reported in plane wakes, the presence of the subharmonic resonant interaction in the planar jet shear layer adds an additional dimension to the modulation phenomenon.

LDV measurements of axial drop velocity in evaporating, dense sprays

The axial component of the drop velocity was measured by laser Doppler velocimetry (LDV) within steady hexane-into-nitrogen sprays from a cylindrical nozzle at gas temperatures from 300 to 500 K. Also varied were the injection velocity and the gas-to-liquid density ratio. It was found that the gas temperature had no effect on the mean centerline velocity for distances smaller than 200 and greater than 650 nozzle diameters. At short distances, the core of the spray is not sensitive to the gas temperature; at long distances, the large amount of entrained gas renders inconsequential the degree of vaporization. Beyond 300 nozzle diameters, so much gas has been entrained that the mean axial velocity approaches the fully developed incompressible jet structure. The approach is accelerated by higher gas temperatures and gas-to-liquid density ratios. The axial drop velocity fluctuation amplitude responds to variations in gas temperature similarly to the mean axial drop velocity. Skewness and flatness indicate nearly Gaussian distributions near the axis and rapid increases beyond the half-radius and differ somewhat from those of incompressibl e jets, possibly because small drops vaporize fast and large ones do not follow the flow as closely as the small ones.

Experimental study on the characteristics of the near wake of a blade-like body with an axisymmetric cross-section rotating about one end. 1st report Comparison with two-dimensional flow field.

The characteristics of a velocity fluctuation in the wake of an axisymmetric body were measured from a rotating frame of reference. The results were compared with those in a two-dimensional flow. The following was made clear. In the rotating flow field, the mixing and diffusion of the wake are promoted by both the oncoming flow which is sheared in the spanwise direction and the intense radial outward flow in the dead flow region of the wake. Therefore the dead flow region becomes narrower and the wake width increases more quickly than in the two-dimensional flow. Vortex shedding is less regular and the circulation of the vortex is weakened. The amplitude of velocity fluctuation is smaller and its maximum position, representing vortex shedding, is closer to the body than that in the two-dimensional case. The velocity fluctuation shows a cellular structure in the spanwise direction, the scale of which is about one characteristic length of the body. This mechanism is related to the radial flow in the wake.

Fundamentals concerning wave loading on offshore structures

This article is aimed at relating a certain substantial body of established material concerning wave loading on offshore structures to fundamental principles of mechanics of solids and of fluids and to important results by G. I. Taylor (1928a,b). The object is to make some key parts within a rather specialised field accessible to the general fluid-mechanics reader.The article is concerned primarily to develop the ideas which validate a separation of hydrodynamic loadings into vortex-flow forces and potential-flow forces; and to clarify, as Taylor (1928b) first did, the major role played by components of the potential-flow forces which are of the second order in the amplitude of ambient velocity fluctuations. Recent methods for calculating these forces have proved increasingly important for modes of motion of structures (such as tension-leg platforms) of very low natural frequency.

TRH influences the pterin cofactor- and time-dependent instabilities of rat raphe tryptophan hydroxylase activity assessed under far-from-equilibrium conditions

Systematic effects on the dynamics of rat raphé tryptophan hydroxylase were observed in the presence of thyrotropin-releasing hormone under the far-from-equilibrium conditions of a kinetic scattering paradigm. The peptide reduced the amplitude of velocity fluctuations across fine-grained increases in cofactor or time and induced rare, high-amplitude irregularities suggesting phase transitions at relatively long wavelengths. Respectively, these two effects of the neuropeptide resemble the statistical changes observed in the same enzyme system in the presence of the lithium ion and the tricyclic antidepressant chlorimipramine under comparable assay conditions. Neither the subtleties of control dynamics nor their responses to the peptide were demonstrable under the saturating conditions of Michaelis-Menten or allosteric kinetics. These findings have possible implications for neurotransmitter regulation in view of the most current information about dynamical interactions among proteins, peptides, and ion ligands in aqueous environment.

Oscillating Turbulent Flow in a Conical Diffuser

Turbulent boundary layers experiencing a time mean adverse pressure gradient and a controllable periodic oscillation are examined experimentally over a tenfold range of dimensionless frequencies, and amplitudes less than ten percent of the lime mean diffuser inlet velocity, for well defined diffuser inlet conditions. Organized velocity fluctuations of the diffuser core and boundary layer flow fields, along with organized wall pressure fluctuations, are characterized using a phase averaging process. Amplitudes of the core velocity fluctuations and wall pressure fluctuations decay in the streamwise direction. Depending on the dimensionless excitation frequency, the amplitude of the organized velocity fluctuations in the boundary layer can exceed the local core flow amplitude by as much as an order of magnitude. At relatively low dimensionless excitation frequency, the organized velocity fluctuation amplitude distribution within the boundary layer grows in an orderly fashion with streamwise distance, and the phase of these fluctuations consistently leads the local core flow fluctuation. At relatively high dimensionless excitation frequency, phase lags of the velocity fluctuations within the boundary layer, relative to the local core flow fluctuation, can extend across the entire boundary layer. Over the range of frequencies examined, the mean flow field of the diffuser remained essentially unaltered.

Flow Near Self-Excited and Forced Vibrating Circular Cylinders

Many studies of flow-bluff body interactions have been undertaken because of the practical need to know the fluid forces acting on a body immersed in a flowing stream. Experiments were performed to investigate the interaction process between the vibrating cylinder and the wake flow for free-stream Reynolds numbers between 550 and 900 in the irregular range. Two circular cylinders of equal diameter were used for experimentation—one was tuned to self-excite under the influence of lift forces and another was a rigid cylinder mounted in a shaker and forced to vibrate sinusoidally perpendicular to the mean flow direction under conditions that duplicated the Reynolds number, Strouhal number, motion amplitude, and relative frequency of the tuned cylinder. Experimental results for the two means of vibration and the mechanics of the wake formation are discussed in terms of the fluid dynamical parameters that influence the vibrations. One finds the wake formation process to be similar for both cylinders under the same conditions of flow, and the amplitude and distribution of near wake velocity fluctuations also to be the same. A phase shift of ninety deg is observed between eighty and one hundred and ten percent of the peak self-excited resonance condition, with the phase angle between the velocity fluctuation and cylinder motion signals being equal for both means of excitation. The critical relative frequency that corresponds to peak resonant conditions is less than unity. The independent parameters that govern the vibrations are not the same for the two means of excitation, and the bounds for the forced and self-excited motions are discussed in terms of the present results and those reported in the literature.

An experimental study of non-linear interaction of velocity fluctuations in the transition region of a two-dimensional wake

As an extension of our previous work (Sato & Kuriki 1961), an experimental study was made on the laminar-turbulent transition of a two-dimensional wake, with emphasis on the non-linear interaction of velocity fluctuations. In the natural transition, a sinusoidal velocity fluctuation first appears as a result of the selective amplification of irregular disturbances. When the amplitude of the fluctuation becomes large, a non-linear interaction takes place: higher harmonics of the sinusoidal fluctuation and a slow, irregular fluctuation are generated. The transition into turbulence is gradual, and there are no abrupt ‘breakdowns’.When a sound with two frequencies (f1 and f2) is introduced into the wake, two velocity fluctuations are induced. They are amplified independently when amplitudes are small. If amplitudes exceed certain values, a mutual interaction takes place, and the growth of each component is suppressed by the presence of the other. At the same time, fluctuations with frequencies of f1 − f2, 2(f1 − f2) … and f1 + f2, 2(f1 + f2) … are produced. The behaviour of the slow, irregular fluctuation in the natural transition closely resembles that of the f1 − f2 component. They seem to be produced by the same mechanism.In the transition region of the wake, a small variation in the amplitude of velocity fluctuation is enhanced by the suppression effect, and a small irregularity in the frequency is amplified by the generation of the f1 − f2 component. These are two elementary processes of randomization of regular fluctuations, which lead to the development of turbulence.

Harmoniques crees par excitation parametrique dans les anemometres a fil chaud a intensite constante

The thermal lag of wires and the large amplitude of velocity fluctuations act together to generate higher harmonics in hot-wire responses. These harmonics are studied by setting on an analog computer the differential equation corresponding to the wire response, a coefficient of which is a random function of time. For various pertubations (gaussian random noise, grid-turbulence, wall-turbulence) the systematic errors affecting the measurements performed by means of constant-current anemometers are evaluated. For example, for a 25 per cent turbulence intensity investigated with a hot-wire having a time constant of 0·4 ms (mean speed of the flow : 10 to 20 m/s), the measured spectra error would be 4 per cent in excess when the slope of the spectral curve is around— 5 3 ; a 60 per cent error in excess is estimated when this slope is around—7. The error on the third moment of the probability density function of the velocity fluctuations would arise to 25 per cent of that due to the non-linearity of the steady state heat-transfer law of the wire, its sign being opposite.