Compression-expansion Waves Research Articles

Excitation of Soliton-Type Waves in Crystals of the A3B Stoichiometry

Using the method of molecular dynamics and taking Ni3Al and Pt3Al as examples, crystals of the A3B composition are considered for the possibility of excitation of soliton-type waves in them. The potentials obtained by the embedded-atom method were used to describe interatomic interactions. It is shown that the harmonic external stimulus can excite waves of the soliton type in a Pt3Al crystal, but not in Ni3Al. Such compression–expansion waves are generated because of excitation of discrete breathers with soft nonlinearity that cannot exist in a Ni3Al crystal near the affected region. The detected waves are capable of propagating to thousands of nanometers along the Pt3Al crystal without losses of integrity and speed. The shape of the obtained wave corresponds to the kink solution of the sine-Gordon equation. The aggregate amount of energy transferred by a wave is determined by the number of rows of atoms involved in fluctuations; this may involve dozens and hundreds of electron volts.

Oscillatory behavior of the bed bulk and the bubbles in a vertically vibrated pseudo-2D bed in bubbling regime

The effect of the bed vessel vibration on the oscillatory behavior of the bed bulk and the bubbles is experimentally studied in the present work by means of Digital Image Analysis (DIA) in a pseudo-2D bed. The bed material was three different powders of Geldart A, B and A/B classifications and was operated in bubbling regime for different superficial gas velocities and vibration amplitudes and frequencies. A tracking methodology was developed in order to follow the oscillatory motion of the bed bulk and each individual bubble in the system. This allowed the analysis of the interaction of the dense phase of the bed with the oscillations of the bubble diameter, position and velocity. The results indicate that both the center of mass of the bed and the bubble characteristics follow the oscillation of the bed vessel with a similar frequency but with a phase delay. The amplitude and phase delay of the oscillation of the center of mass of the bed are more sensitive to variations of the vibration frequency than to variations of the vibration amplitude of the bed vessel. Both the amplitude and the frequency of the bed vessel vibration have a stronger impact on the bubble behavior of beds filled with small particles. The existence of a phase delay between the oscillations of bubble characteristics in the lower and upper sections of the bed indicates the existence of compression-expansion waves in the dense phase that modify the bubble behavior along the bed despite bubbles are interacting with each other. The presence of compression-expansion waves may shed light onto the different behaviors encountered for the mean bubble behavior in vibrated fluidized beds.

Compressible-gas two-fluid modeling of isolated bubbles in a vertically vibrated fluidized bed and comparison with experiments

In this work the size and motion of isolated bubbles in a vertically vibrated fluidized bed are numerically investigated by means of two-fluid model simulations. The oscillations of the bed bulk and the bubble diameter and velocity are compared with experimental results of a pseudo-2D bed using an averaging of cycles method to account for the intrinsic unsteadiness caused by vibration. The effects of gas compressibility and the air plenum of the vibrated bed are also numerically investigated. The results show that the two-fluid model simulations resorting to a compressible gas model are able to reproduce both the cyclic compression and expansion of the bed bulk and the bubble oscillations observed in the experiments. In contrast, the simulations with the incompressible gas model fail to reproduce these effects. The presence of the air plenum in the numerical model diminishes the amplitude of the bed and bubble oscillations and improves their resemblance to the experiments. In the simulations with compressible gas, a phase delay is found between the bed displacement and the oscillation of bubble characteristics. In harmony with experiments, the phase delay is smaller in the lower half of the bed (i.e. close to the distributor) than in the upper half. This effect is not reproduced by the simulations with incompressible gas-phase. These results suggest that the phase delay in vibrated beds is caused by the compression of the gas phase, which leads to compression-expansion waves traveling through the bed. The simulations also confirm that the amplitude of vibration influences the magnitude of the bubble diameter and velocity oscillations, whereas the delay of the bubble characteristics is mainly affected by the bed vibration frequency.

A model for Nanosecond Pulsed Dielectric Barrier Discharge (NSDBD) actuator and its investigation on the mechanisms of separation control over an airfoil

In order to simulate the flow control problem by using Nanosecond Pulsed Dielectric Barrier Discharge (NSDBD), a one-zone inhomogeneous phenomenological model is constructed based on the experimental and theoretical results. The model is coupled with the unsteady Navier-Stokes equations, which can well predict the compression-expansion wave structures and wave speed compared with experimental results and can be applied to the simulation of the flow control by using NSDBD. The model is adopted to investigate the separation control over NACA0015 airfoil using the NSDBD plasma actuator. The separation-control mechanisms are revealed that the spanwise vortices produced by the plasma actuation play the key role. Each plasma actuation can produce a spanwise vortex around the separation point near the leading edge. The spanwise vortices make the separated free-shear layer unstable and shed away, move downstream along the upper wall, control the flow near the wall, and bring outer flow with high kinetic energy into the near wall region to realize the effective separation control over the upper surface of the airfoil.

Spatially uniform microflows induced by thermoviscous expansion along a traveling temperature wave: Analogies with electro-osmotic transport

We discover that thermoviscous expansion along a traveling wave in a microfluidic channel may be capable of generating a spatially uniform flow profile in a time-averaged sense. We further delineate that the resultant complex flow characteristics, realized by virtue of an intricate interplay between thermal compression-expansion waves and temperature-dependent viscosity variations and controlled by an external heating, may be remarkably characterized by a unique thermal penetration depth scale (analogous to Debye length in electro-osmosis) and a velocity scale (analogous to the Helmholtz Smulochowski velocity in electro-osmosis) that in turn depends on the considerations of "thin" and "thick" microchannel limits, as dictated by the thermal penetration depth as compared to the lateral extent of the microfluidic channel. We show that, when the thermal penetration depth is small as compared to the channel height, a uniform velocity profile is generated in the channel in a time-averaged sense. The velocity scale characterizing this uniform flow may be represented by a function of the thermal diffusivity, volumetric expansion coefficient and thermal viscosity coefficient of the fluid, characteristic amplitude and speed of the thermal wave, as well as the channel height. Results from the present study are expected to provide valuable insights towards arresting hydrodynamic dispersion in microchannels by nonelectrochemical means, following a pH-independent route.

Three-Dimensional Dynamic Contact Problem for an Elliptic Crack Interacting with a Normally Incident Harmonic Compression–Expansion Wave

The paper deals with the three-dimensional dynamic problem for an elliptic crack interacting with a normally incident harmonic compression–expansion wave, considering the contact interaction of the crack faces. An asymmetric solution is obtained using an iteration algorithm developed earlier. Numerical results are presented

Dynamics of vertically vibrated two-dimensional granular layers

Vibrational motion and dynamics of two-dimensional layers composed of identical inelastic solid disks are investigated experimentally and characterized in terms of the dimensionless acceleration. Several vibrational regimes with different degrees of vibrofluidization are studied by means of the layers' videorecordings and tracking the motion of one larger disk immersed into each bed of smaller particles. It is shown that depending on the vibrational acceleration, the larger disk either ultimately rises on top of the layer or vigorously moves throughout it, thereby indicating possibilities for efficient mixing. In a certain narrow range of the vibrational acceleration the layer is observed to repack and move as a single block. This acceleration range is well described by the model of an absolutely plastic body moving above a vibrated plate. Small deviations from this acceleration range lead to a significant layer expansion and distortion of its upper surface due to transverse waves. The vibrofluidization regimes are also characterized by measuring the force acting on the vessel's bottom and the time of its contact with the layer. The propagation speed of the compression-expansion waves is estimated and found consistent with the predictions of our earlier semiempirical and analytical models.

Numerical study on the interaction between unsteady compression and unsteady expansion wave

A new control method to alleviate the impulsive noise at the exit of high-speed railway tunnel was applied to the compression wave at the entrance of the tunnel. This method uses the interaction phenomenon of unsteady expansion wave and unsteady compression wave. Unsteady expansion wave was assumed to be made instantaneously by the simple theory of shock tube. Total Variation Diminishing method was employed to solve the axisymmetric unsteady compressible flow field with a specified compression wave. Numerical results show that the maximum pressure gradient of the propagating compression wave decreases with increase of the wave length of the unsteady expansion wave. It is found that the impulsive noise reduction can be obtained when the unsteady expansion wave with a large wave length is emitted just before the train enters the tunnel. The present results give the possibility to reduce the impulsive noise at the exit of tunnel.

Fluctuons—III. Gravity

The fluctuon principle provides a model of gravity. Mass and field are interpreted in terms of density depression in the full sea of vacuum fermions. Gravitons are interpreted as self-perpetuating chains of transient fermion-antifermion pairs (propagating gravitational fluctuons) and gravitational waves as compression-expansion waves of vacuum density. The direct gravitational interaction between absorbing particles is repulsive (whether the absorbers are permanent positive energy particles or trapped fluctuons). The attractive gravitational interaction arises from the ‘pushing’ effect of propagating gravitational fluctuons originating in the low mass regions of space, where the density of trapped fluctuons is highest. The model implies the principle of equivalence and through this the general relativistic equations of gravitation.

Fluctuons—II. Electromagnetism

The fluctuon principle is used to formulate the electromagnetic interaction. Virtual and real photons are interpreted as chains of transient electron-positron pairs. Electromagnetic waves are interpreted as compression-expansion waves of vacuum density. The fine structure constant is interpreted in terms of the probability of an interaction between a transient electron-positron pair and a charged absorber. The model eliminates the problem of infinite self-energies.

The radiatively driven discrete acoustic wave

A complete and detailed study of a radiatively driven plane acoustic wave in a non-grey radiating and absorbing gas is carried out on the assumption of local molecular equilibrium. Specifically, the response of the gas in a semi-infinite space to a step input of radiation from a stationary black wall is investigated. The problem is physically interesting because radiative heat addition is the only driving mechanism, and this mechanism is unique and fundamental to the field of radiative gas dynamics. The solution shows that the heat addition gives rise initially to a compression-expansion wave in the gas, with the wave front controlled by radiation. This wave-front disturbance, though caused initially by the direct effect of radiative transfer, eventually outruns the region of appreciable heating near the wall and becomes a modified-classical disturbance that propagates away from the wall at the isentropic speed of sound. The radiative heat addition continues directly to affect the gas near the wall and in this manner drives the modified-classical wave indirectly by causing the formation of an ‘effective gas piston’. The solution thus exhibits a linearized phenomenology corresponding to that observed in the non-linear leading wave associated with the nuclear fireball.