Bar Amplitude Research Articles

Growth of alternate bars in unsteady flow

A theoretical model is formulated to investigate the development of the amplitude of alternate bars in unsteady flows. The problem is tackled by means of a weakly nonlinear analysis developed in a neighborhood of the threshold conditions for bar formation. Bar response to unsteady flow is found to depend on a parameter that is a measure of the ratio between the time scale of the basic flow and the time scale of bar growth. The present theory shows that if is O(1), as often occurs in nature, flow unsteadiness affects the instantaneous growth rate and phase of bar perturbations and controls the final amplitude reached by the bed configuration. A procedure for determining the final amplitude for a given flood event is proposed. Flume experiments were performed to test the main theoretical results. The bed response to unsteady flow was measured for different values of the period of the flood. The observed temporal behavior of the bar amplitude proves to be strongly affected by the unsteady character of the flow for of O(1), as predicted by the theory.

A new high-pressure cell for differential pressure-jump experiments using optical detection

Abstract A differential pressure-jump autoclave with optical detection has been developed, which is capable to perform pressure-jumps of 150 bar amplitude to either. lower or higher pressures within the absolute pressure range up to 2.5 kbar. The principle of a rupture diaphragm is applied and the mechanical relaxation time of the apparatus is about 0.1 millisecond. Optical detection of light either transmitted or scattered under an angle of 90° can be utilized to follow a proceeding reaction. The apparatus has been used to study the rate of demixing of binary liquid mixtures after performing a pressure-jump into the miscibility gap. The results of the obtained light intensity/time curves indicate no distinct cross-over behaviour between two different phase separation kinetics.

Temporal covariance model of human motion perception.

We propose a model of direction-sensitive units in human vision. It is a modified and elaborated version of a model by Reichardt [Z. Naturforsch . Teil B 12, 447 (1957)]. The model is applied to threshold experiments in which subjects view adjacent vertical bars with independently (typically sinusoidally), temporally modulated luminances. The subject must report whether the patterns moved to the left or to the right. According to the model, a basic motion-detecting unit consists of two subunits tuned to opposite directions. Each performs a spatial and temporal linear filtering of its input; outputs of the filters are multiplied, and the multiplied output is integrated (for a time that is long relative to the modulation period). The model's output consists of the difference between the subunit outputs. Direction of movement is indicated by the sign of the model output. Mathematical analysis of the model yielded several predictions that were confirmed experimentally. Specifically, we found that (1) performance with complex patterns can be predicted by spatiotemporal Fourier analysis that results in the segregation and linear addition in the output for different temporal frequencies; (2) under special conditions, performance depends on the product of adjacent bar amplitudes, offering strong support for the multiplication principle; (3) performance is unaffected by addition of stationary patterns; and (4) addition of homogeneous flicker normally produces no effect but under special conditions reverses perceived direction. These and other results confirm our model and reject several other models, including Reichardt 's original model.

Matter tensor for a freely falling oscillating bar in a gravitational wave

Matter tensor and motion of a test body is obtained. The test body is an oscillating bar; the background metric is a Bondi plane wave. Amplitude and frequency of bar and wave are arbitrary and not necessarily constant.

The genealogy of periodic orbits in a plane rotating galaxy

We study the various families of periodic orbits in a dynamical system representing a plane rotating barred galaxy. One can have a general view of the main resonant types of orbits by considering the axisymmetric background. The introduction of a bar perturbation produces infinite gaps along the central familyx1 (the family of circular orbits in the axisymmetric case). It produces also higher order bifurcations, unstable regions along the familyx1, and long period orbits aroundL4 andL5. The evolution of the various types of orbits is described, as the Jacobi constanth, and the bar amplitude, increase. Of special importance are the infinities of period doubling pitchfork bifurcations. The genealogy of the long and short period orbits is described in detail. There are infinite gaps along the long period orbits producing an infinity of families. All of them bifurcate from the short period family. The rules followed by these families are described. Also an infinity of higher order bridges join the short and long period families. The analogies with the restricted three body problem are stressed.

Nearshore Bars Along United States Gulf Coast: ABSTRACT

The microtidal, low-wave energy, United States coast of the Gulf of Mexico is characterized by an abundance of shoreface bars. Four distinct component bars can be identified, though interference patterns exist. Vertical air photos since 1945 and numerous overflights and field measurements during the last two years have shown that the equilibrium bar configuration at any given site is time-invariant. During storms, however, the bars go through cycles of change in a manner similar to those identified by Short. Based on plan geometry the following four bar types are identified. Multiple (10 or more) longshore bars: Multiple longshore bars, without interference from other bar types, occur on the low-tide platform in front of a steep beach face, along shoreline segments of convex plan form and low wave energy. The bars have a sinusoidal cross-profile, constant height, and constant spacing suggesting that they form in response to a standing wave pattern established through interference between incident wind waves and waves reflected from the steep beach face. Transverse bars: Bars oriented at steep angles relative to the shoreline dominate in low energy environments (Mississippi Sound and other sheltered embayments) along the central Gulf coast. Intersecting transverse bars, forming an overall rhombic pattern, dominate the lagoonal platform behind the Mississippi-Alabama barrier chain. The mainland shoreface of Mississippi Sound, however, is dominated by intersecting transverse and multiple longshore bars. Generally, transverse bar spacing appears to be proportional to bar amplitude. The transverse bar morphology reflects a complex interaction between incident, high-frequency waves and the resultant longshore currents. Crescentic bars: The Gulf beaches of barriers and spits of the Florida panhandle, Alabama, and Mississippi are characterized by crescentic bars or single longshore bars. The crescentic form is best developed adjacent to, or between, shore-normal structures which might be jetties, groins, or natural headlands. This suggests that the crescentic bar morphology forms in response to a wave motion transverse to that of the incident waves, i.e., edge waves which are expectedly best developed trapped between reflecting shore-normal structures. Longshore bars: These bars characterize the entire Texas coast and many Louisiana barriers. This coastline has the highest wave energy in the study area. The constant crest-to-trough water depth ratio, the asymmetric cross-profile, and the landward decrease in size and spacing suggest that these are breakpoint bars. These four component bars, all resulting from different mechanisms of generation, occur either individually or in different patterns of superposition creating six commonly observable bar morphologies along the Gulf Coast barriers and mainland shores. Specific coastal subenvironments are characterized by time-invariant equilibrium bar configurations. End_of_Article - Last_Page 1565------------

On Transverse Vibration of an Elastic Circular Bar which is immersed in a Water Region

We consider the free vibration (of infinitesimally small amplitude) of an elastic circular bar, both ends of which are in state of fixed ends. This circular bar is supposed to be immersed in a water region which is made up by two parallel plane (rigid) walls. Starting from the differential equation for the transverse vibration of a straight elastic bar, wherein the effect of surrounding water is taken into account, the value of natural frequency of vibration is shown in a form of determinantal equation (of infinite order). Therefrom, an approximate formula for the natural frequency is also given.

Accelerometer Calibration Technique

A technique has been developed for the dynamic calibration of accelerometers or other vibration measuring instruments. It is believed that the technique is more convenient than the reciprocity method for making rapid calibrations over a fairly large frequency range. For each calibration frequency the accelerometer is attached to a free-free bar driven electromagnetically at a known sinusoidal amplitude. The accelerometer output is compared in magnitude and phase with the amplitude of the bar. This is done by means of strain gauges on the bar which are calibrated periodically by direct optical measurement of bar amplitude. The importance of various design factors is discussed and also the accuracy of the technique with relation to amplitude and frequency. The error in the phase measurements is about 2 degrees and the double amplitude can be measured to about 0.003 mm. The error in measuring a 200-g accelerometer at 1000 c.p.s. is about 3 percent.