Maximum Oscillation Amplitude Research Articles

Aerodynamics of a covered pedestrian bridge of a trapezoidal section

Neighbouring offshore platforms are often connected by bridges, which are exposed to high wind forces. Many of these bridges are trapezoidal in section, a shape which has not attracted sufficient research in the past. An experimental programme investigated the pressure distribution from which lift and drag forces are evaluated. The effect of free-stream turbulence, roundness of the edges, and Reynolds number are examined. Finally, an aeroelastic model was built and the dynamic response was investigated in the range of reduced velocities from 2.5 to 8.7. The maximum amplitude of oscillation occurred at the highest reduced velocity tested.

マイクロアクチュエータによる柱状物体後流の周波数制御

The "lock-in" phenomenon has been investigated experimentally by vibrating cylindrical bluff body surfaces locally using piezoelectric actuators. The maximum amplitude of oscillation is of the order of 20 pm (nearly 1.0 X 10-3 times the body thickness), and the excitation frequency covers below and above a natural vortex shedding frequency from the body. The Reynolds number renges from 5.25 X 103 to 2.31 x 104. Substantial entrainment of vortex shedding frequency has been observed by exciting the upstream surface of sidewalls, where a short laminar separation bubble exists. Vortex shedding becomes more periodical and the total pressure in the wake becomes high by the local excitation of body surfaces. Possibilities for application to the active control of vibration and noise due to vortex shedding, as well as to drag reduction are discussed.

Decremental Oscillation Curve Fitting of Heart Rate Response Following Face Immersion into Cold Water.

Using 18 healthy volunteers, we attempted to fit the heart rate response to a decremental oscillation curve by the least-squares method following facial immersion into cold water. The decremental oscillation equation was as follows: HR = alpha.e(-betat) sin omegat + gamma, where HR is the instantaneous heart rate at t s, alpha the maximum amplitude of the decremental oscillation, beta the decremental rate, omega the angular velocity and gamma the basal heart rate. Each subject immersed his/her face fully in a basin filled with ice-cold water (4 degrees C) for 30 s. The heart rate response to facial immersion was significantly fitted to the decremental oscillation curve with a correlation coefficient > 0.75 in all the trials. Double trials with a 15-min intertrial interval did not show any significant change in the coefficients. Atropine sulfate decreased the value of coefficient omega and increased that of gamma but did not change alpha or beta, indicating that omega is determined mainly by parasympathetic tone. There were no differences for any coefficient between men and women. The coefficients beta and gamma increased in association with age. Given the significant fitting to the decremental oscillation curve of the heart rate response, we propose that the corresponding electrical circuit consists of a resistance and an inductor connected in series, with a condenser in a parallel circuit with a direct current impulse. This method of fitting with the corresponding electric circuit may be useful for analysis of the heart rate response to physiological stimulus.

バネで支持された球の一様流中での振動

The dynamic instability of an elastic supported sphere in the uniform flow is studied experimentally by using the low speed open wind tunnel. If the oscillation was made by an initial deformation due to the outer disturbance, the oscillation was damped when the wind speed was below the some critical wind speed. However, if the wind speed increased over that critical wind speed, the limitcycle oscillation appeared. This oscillation disappeared and the small amplitude randam oscillation appeared, when the wind speed increased over the another higher critical wind speed, and the oscillation of the sphere damped rapidly even if the oscillation was made by the initial deformation due to outer disturbance. In the present study, eight series of the wind tunnel tests were done i. e. the two spheres and the four pairs of the spring were used. The limitcycle dynamic instability were observed in the Reynolds number range 3×103<Re<4×104. The ratio of the limitcycle dynamic instability disappearing critical wind speed to the one appearing wind speed was between 4.8 to 2.2 in the each wind tunnel test. The maximum vibration amplitude of the sphere oscillation was about 0.5d in the each test.

東京湾における近慢性内部振動

Internal near-inertial oscillations were observed by moored measurements in summer of 1979 in Tokyo Bay. The characteristics of observed oscillations are as follows; 1) The maximum amplitude of oscillations reached to about 8 cm s-1 2) The observed period ranged from 16 to 20 hours, which are shorter than the inertia period in Tokyo Bay; 3) The phase difference of oscillations between the surface and bottom layers is about 180°; 4) The major axis of the oscillations in the northernmost area is directed along the coast. The observed oscillations are explained as the transverse internal seiche in a stratified flat-bottomed channel with straight walls. The characteristics of the observed oscillations are also confirmed by the numerical experiments using the multi-level model with realistic basin and with the time change of wind.

On the Semiannual Variation in the Northern Hemisphere Extratropical Height Field

Abstract A qualitative examination of the spatial distribution of the maximum semiannual oscillation (SAO) amplitudes in the Northern Hemisphere (NH) extratropical 500-hPa height field suggests that the SAO has a very dominant zonally asymmetric east-west structure. A comparison between the NH stationary eddies and the NH SAO shows that the NH midlatitude SAO can be explained almost entirely as being a result of the spatial and temporal asymmetries in the annual variation of the stationary eddies. It is also shown that the polar sea SAO maxima over Siberia and Alaska are related to the east-west stationary eddy distribution. It is suggested that, ultimately, the mechanism for the SAO in the NH extratropics is simply a result of land-sea contrasts, similar to the mechanism proposed by van Loon for the Southern Hemisphere (SH) extratropical SAO. The chief difference, however, is that the NH extratropics are dominated by east-west land-sea contrasts due to the larger continental land masses in the NH, while ...

Electron acceleration from the breaking of relativistic plasma waves

ELECTRONS in a plasma undergo collective wave-like oscillations near the plasma frequency. These plasma waves can have a range of wavelengths and hence a range of phase velocities1. Of particular note are relativistic plasma waves2,3, for which the phase velocity approaches the speed of light; the longitudinal electric field associated with such waves can be extremely large, and can be used to accelerate electrons (either injected externally or supplied by the plasma) to high energies over very short distances2a¤-4. The maximum electric field, and hence maximum acceleration rate, that can be obtained in this way is determined by the maximum amplitude of oscillation that can be supported by the plasma5a¤-8. When this limit is reached, the plasma wave is said to a¤˜breaka¤™. Here we report observations of relativistic plasma waves driven to breaking point by the Raman forward-scattering instability9,10 induced by short, high-intensity laser pulses. The onset of wave-breaking is indicated by a sudden increase in both the number and maximum energy (up to 44 MeV) of accelerated plasma electrons, as well as by the loss of coherence of laser light scattered from the plasma wave.

Stochastic resonance and nonlinear response in double-quantum-well structures.

Coherent motion of a wave packet in a double-quantum-well structure can be induced by irradiating it with a weak resonant laser field. The amplitude of this motion exhibits a ``stochastic resonance'' as a function of the relaxation rate in the double well and of the field amplitude. The maximum oscillation amplitude at each temperature does not depend on the field amplitude, demonstrating the failure of linear-response theory to describe the quantum stochastic resonance.

Intensive ion beams in the transverse magnetic field of an electromagnetic separator

Based on state-of-the-art knowledge, the processes of the formation of intense ion beams in the transverse magnetic field of a commercial electromagnetic separator are analyzed. It is shown that, given the conditions of closed electron drift, the potentials will surely equalize along the fast ion trajectories all the way through the beam length; hence, no improvement in dynamic decompensation is caused by the low-frequency noise of the source. It is established that increasing the full beam current forces the maximum amplitude of the current density oscillations out to the beam periphery crosswise. For the first time, it is shown that suppressing noise oscillations in the arc discharge of an ion source for the formation of a highly stable ion beam with a minimum inner potential drop is possible. At low pressures, with collective interaction out of play, the drop is determined by Coulomb's collisions between the beam particles and compensating electrons. The analysis of collective phenomena likely to occur in the ion-beam plasma of a separator gives evidence of an experimental space potential buildup with increasing residual gas pressure in the calutron, leading to a high-frequency ion-electron beam instability. It is pointed out that this instability combined with an additional decompensation results in a noticeable broadening of the beam energy spectrum, which in turn makes for a lower quality of separation.

Lithium‐induced decrease in spontaneous Ca2+ oscillations in single GH3 rat pituitary cells

1. Measurement of [Ca2+]i in single rat pituitary GH3 cells by dynamic single cell imaging techniques demonstrated that under basal conditions there is a large variation in the temporal pattern of [Ca2+]i signalling between individual cells ranging from high frequency asynchronous oscillations to quiescence. 2. We have reported previously that treatment of GH3 cells with 1 mM Li+ (a concentration used therapeutically in the treatment of manic depression) for 7 days reduces basal and thyrotrophin-releasing hormone (TRH)-stimulated levels of mass inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In the present study, we show that this is associated with a reduction in the number of cells exhibiting basal Ca2+ oscillations over a sampling period of 60 s, whereas the maximum amplitude of oscillations is unaffected. 3. The pattern of [Ca2+]i responses to the agonist TRH varied considerably between individual cells, making quantitation of the responses difficult; however, data obtained from measurements made on a population of cells showed that increases in peak [Ca2+]i induced by high concentrations of TRH were reduced in cells treated with 1 mM Li+ for 7 days relative to control cells. 4. The sensitivity of the phosphoinositide pathway to [Ca2+]i was investigated by loading GH3 cells with BAPTA/AM at a concentration sufficient to lower 'basal' [Ca2+]i in a population of cells and to inhibit agonist-stimulated increases in [Ca2+]i. Under these conditions, basal and TRH-stimulated mass Ins(1,4,5)P3 levels were unaffected. 5. These results demonstrate that a 7-day Li+ treatment leads to an alteration in Ca2+ signalling, in particular by reducing the number of cells exhibiting high frequency Ca2+ oscillations under basal conditions. The significance of these results to the clinical effectiveness of Li+ in the treatment of manic depression is discussed.

Theory of the oscillometric maximum and the systolic and diastolic detection ratios.

It is proposed that the maximum in cuff pressure oscillations during oscillometry is due to the buckling of the brachial artery under a cuff. This theory is investigated by means of a mathematical model of oscillatometry that includes the mechanics of the occlusive arm cuff, the arterial pressure pulse waveform, and the mechanics of the brachial artery. A numerical solution is provided for the oscillations in cuff pressure for one cycle of cuff inflation and deflation. The buckling pressure is determined from actual arterial data and the von Mises buckling criteria. The buckling of an artery under a cuff occurs near -2 to 0 mm Hg transmural pressure. This effect corresponds with a maximum arterial compliance and maximum cuff pressure oscillations when cuff pressure is nearly equal to mean arterial pressure (MAP), in support of the suggested theory. The model was also found to demonstrate the basic characteristics of experimental oscillometry, such as an increasing and decreasing amplitude in oscillations as cuff pressure decreases, the oscillations that occur when cuff pressure is above systolic pressure, maximum oscillation amplitudes in the range of 1 to 4 mm Hg, and an oscillatory maximum at cuff pressure equal to MAP. These findings support the case that the model is representative of oscillometry. Finally, the model predicted values for the systolic and diastolic detection ratios of 0.593 and 0.717, respectively, similar to those found empirically. These ratios alter with blood pressure, but the tightness of the cuff wrap did not change their value.

Effects of turbulent flows on vortex-induced oscillation of bridge girders with basic sections

This paper investigates the effects of turbulence characteristics such as turbulence intensity and scale on vortex-induced oscillation of bridge girders with basic sections. The basic sections are selected taking into account of various effects of turbulences on vortex-induced oscillation such as ; the maximum amplitude of vortex-induced oscillation is 1) reduced, 2) enlarged in some cases or 3) not changed by the existence of wind velocity fluctuations, which are referred from literature review. The vortex-induced responses of models are examined not only in grid-generated turbulent flows but also in actively simulated turbulences with large scale through an active gust generator. The effects of turbulence properties on vortex-induced response are evaluated in terms of the ratio of the maximum amplitude in turbulent flows to that in a smooth flow.

The Response of a Vertical Cylinder in Waves

The response of a flexible cylinder as a function of the ratio of its natural frequency to the wave frequency has been investigated over a range of Keulegan-Carpenter numbers, showing that the peak response in the transverse direction occurs at a ratio of about 2 and that this response is not influenced by motion in the in-line direction. The maximum in-line response occurs at a frequency ratio of approximately 1, but in this case it is influenced by motion in the transverse direction. A method to predict the maximum amplitude of oscillation of the cylinder was reasonably successful. This method requires a knowledge of the forces or moments on a fixed cylinder and the structural characteristics of the flexible cylinder.

Hydraulic jump on chutes with roll waves

1. A comparison of laboratory and on-site data on a determination of the maximum range of oscillations at the end of a direct hydraulic jump when waves enter it from a chute with the results of calculations by theoretical formulas (1), (2), and (3) confirms the applicability of one of these formulas (2) for superrapid flow and flow transitional from superrapid to rapid. 2. The stilling basin generates secondary waves, reaching half of the depth of the basin d with respect to its height. With submergence of the basin from the lower pool, the range of variations of the level increases additionally by 2.0–2.5 times. 3. On the apron behind the stilling basin, the drop of waves is insignificant, since the wave transformation coefficient at distance (40–90)hn, where hn is the natural depth, remains equal to\(2\bar a/h'_W = 1.5 - 3.3\). 4. The periods and lengths of the waves transformed in the stilling basin decrease with increase of discharge and Froude number Fr0 and approach in value the wave periods. 5. Formulas (6) and (10) were obtained for calculating the maximum amplitude of oscillations of the free surface and maximum depth at the crest of oblique waves on the narrowing sections of the wave chutes and they were checked experimentally, which proved the applicability of these formulas for calculating a nonstationary oblique hydraulic jump. 6. The proposed empirical formulas (12)–(17) can be recommended for an approximate evaluation of the parameters of the largest first waves on the narrowing stretch. 7. Under these conditions, the use of a stilling basin as an energy dissipator of a superrapid flow is not rational, since not dissipation but generation of secondary waves is observed in it. When designing narrowing sections of chutes, it is necessary to take into account an increase of depth of the oblique jump with passage of roll waves. 8. For dissipation of excess kinetic energy of a high-velocity unsteady flow, the authors developed and described earlier [4] special energy dissipators of the flow of water, which underwent checking and showed sufficient effectiveness.

Response characteristics of a vortex-excited cylinder at low reynolds numbers

An experimental investigation has been conducted into vortex-induced, cross-flow oscillations of a circular cylinder mounted elastically in a water channel. The Reynolds number ranged between 90 and 150, a regime where the vortex street is fully laminar. The frequency and amplitude of cylinder oscillation and the vortex-shedding frequency were measured in and around the lock-in region. Fluid velocities in the cylinder wake and cylinder response were recorded simultaneously at Re = 115 in the lock-in region, and traces of cylinder motion were taken in and around resonant conditions. The maximum oscillation amplitude occurs near the lower limit of the lock-in region, while no substantial differences in final response are detectable between the cylinder amplitude developing from rest and the free stream velocity being varied with the cylinder already oscillating. The cylinder oscillation frequency was found to be slightly higher than its natural frequency in air, in the greatest part of the lock-in region.

Observation of an antenna‐plasma instability

During a rocket flight to high altitude (585 km) a narrow band signal on an electric antenna was sometimes observed, whose frequency varied as the rocket turned. Such a signal cannot be natural, but apparently must be generated by the interaction of the rocket‐antenna system with the ambient plasma. Conditions for development of the instability are investigated. Maximum oscillation amplitude occurs when the antenna is aligned with the Earth's magnetic field. Similar observations have been reported by Gurnett and Mosier [1969]. Several attempts are made to understand the nature of this interaction, but without success. The instability treated by Fiala, due to the interaction of an inductive antenna impedance with stray capacitance to a phase shifted point in the preamplifier, can be ruled out. It appears that a negative antenna resistance due to interaction with waves Doppler‐shifted through zero is an unlikely explanation. The rocket velocity seems too small to give such an anomalous Doppler shift, and even if the observations of plasma density and temperature are stretched, the positive sheath resistance is larger than calculated negative resistances. Ion transit time instability in the sheath would only work at 10 times higher frequency. Interaction of the flowing plasma with sheath waves around the antenna is suggested and appears promising but the theory is not sufficiently developed for meaningful comparison.

Nonstationary vibration of a rotating shaft with nonlinear spring characteristics during acceleration through a critical speed. A critical speed of a 1/2-order subharmonic oscillation.

This paper deals with the nonstationary oscillations of a flexible rotating shaft with nonlinear spring characteristics. In particular, we investigate a phenomenon during constant acceleration and deceleration passing through a critical speed of a 1/2-order subharmonic oscillation of forward precession. In numerical simulations, we examined the influence of the angular acceleration λ and the initial angular position ψ0 of a rotor unbalance on the maximum amplitude of the subharmonic oscillation. As a result, the following points are clarified: (1) the maximum amplitude depends markedly on λ and ψ0; (2) in order to always pass through this critical speed with finite amplitude during acceleration, an angular acceleration greater than a certain value λ0 is necessary; and (3) when the angular acceleration is less than this critical value λ0 (0<λ<λ0), the shaft's ability to pass through this critical speed depends on ψ0. We ascertained the validity of these theoretical results by experiments.

Vibration of a rotating shaft with nonlinear spring characteristics during acceleration through a critical speed. 2nd report. A critical speed of a 1/2-order subharmonic oscillation.

This paper deals with a nonstationary vibration of a flexible rotating shaft with nonlinear spring characteristics. Especially, we investigate a phenomenon during constant acceleration and deceleration passing through a critical speed of a 1/2-order subharmonic oscillation of forward precession. In numerical simulation, we examined the influence of the angular acceleration λ and the initial angular position ψ0 of a rotor unbalance on the maximum amplitude of the subharmonic oscillation. As a result, the followings are clarified : (1) in order to always pass through this critical speed with a finite amplitude during acceleration, an angular acceleration greater than a certain value λ0 is necessary, (2) the maximum amplitude depends on ψ0 markedly, and (3) whether the shaft can pass through this critical speed or not depends on ψ0 when 0 < λ < λ0. We ascertained the validity of these theoretical results by experiments.

Wave-breaking amplitude of relativistic oscillations in a thermal plasma.

The maximum amplitude of relativistic plasma oscillations (${\ensuremath{\upsilon}}_{\mathrm{ph}}$ near $c$) is obtained with a combined one-dimensional waterbag and warm-fluid model. The waterbag description is used to obtain expressions for the pressure and internal energy as functions of the proper density. A relativistic Euler's equation that is valid for arbitrarily large amplitudes and an analytic expression for the wave-breaking amplitude in the limit ${\ensuremath{\upsilon}}_{\mathrm{ph}}\ensuremath{\cong}c$ are obtained. Even a small amount of thermal energy can significantly reduce the maximum plasma-wave amplitude relative to the cold wave-breaking value. The significance of the results for recent accelerator schemes is discussed.

Self‐Sustained Pressure Oscillations in Two‐Dimensional Cavity

The pressure oscillations within a two-dimensional, rectangular cavity have been explored. The flow was subsonic and the chord-to-depth ratio of the cavity ranged between 0.25 and 1.25. The experimental setup, comprised a plexiglass channel-cavity model, was inserted within the test section of a wind tunnel. The moveable walls of the cavity were adjusted for various cavity geometries. A pressure transducer was flush mounted along the upstream wall of the cavity and was used to monitor, with the help of an oscilloscope, the pressure fluctuations within the cavity. Within a Strouhal number range of 0.4 - 2.4, periodic pressure fluctuations as high as 50% of the free-stream dynamic pressure were observed. The modulation of the external flow is influenced by the cavity which acts as a simple harmonic oscillator with a certain natural frequency. As the velocity of the external flow is increased gradually, the frequency of the pressure fluctuations experiences step changes, shifting to higher or lower frequency levels. Of the two parameters describing the cavity geometry and their effects on pressure oscillations, the cavity length appears to influence the frequency, and the cavity depth effects the amplitude. The maximum amplitude of the pressure oscillations within the cavity were observed to occur under mode 2 and for a cavity geometry corresponding to a chord-to-depth ratio of about 0.5.