Spike Velocity Research Articles

Retarding viscous Rayleigh-Taylor mixing by an optimized additional mode.

The Rayleigh-Taylor (RT) mixing induced by random interface disturbances between two incompressible viscous fluids is simulated numerically. The ensemble averaged spike velocity is found to be remarkably retarded when the random interface disturbances are superimposed with an optimized additional mode. The mode's wavenumber is selected to be large enough to avoid enhancing the dominance of long-wavelength modes, but not so large that its saturated spike and bubble velocities are too small to stimulate a growing effective density-gradient layer suppressing the long-wavelength modes. Such an optimized suppressing mode is expected to be found in the RT mixing including other diffusion processes, e.g., concentration diffusion and thermal diffusion.

Numerical Study of Pulse Detonation Engine with One-step Overall Reaction Model

<p>This paper presents an insight for the study of transient, compressible, intermittent pulsed detonation engine with one-step overall reaction model to reduce the computational complexity in detonation simulations. Investigations are done on flow field conditions developing inside the tube with the usage of irreversible one-step chemical reactions for detonations. In the present simulations 1-D and 2-D axisymmetric tubes are considered for the investigation. The flow conditions inside the detonation tube are estimated as a function of time and distance. Studies are also performed with different grid sizes which influence the prediction of Von-Neumann spike, CJ Pressure and detonation velocity. The simulation result from the single-cycle reaction model agrees well with the previous published literature of multi-step reaction models. The present studies shows that one-step overall reaction model is sufficient to predict the flow properties with reasonable accuracy. Finally, the results from the present study were compared and validated using NASA CEA.</p><p><strong>Defence Science Journal, Vol. 65, No. 4, July 2015, pp. 265-271, DOI: http://dx.doi.org/10.14429/dsj.65.8730</strong></p>

Factors Correlated with Volleyball Spike Velocity

Background Spike effectiveness represents a determining element in volleyball. To compete at a high level, the player must, in particular, produce a spike characterized by a high ball velocity. Hypothesis Some muscular and physical features could influence ball velocity during the volleyball spike. Study Design Descriptive laboratory study. Methods A total of 19 male volleyball players from the 2 highest Belgian national divisions underwent an isokinetic assessment of the dominant shoulder and elbow. Ball velocity performance (radar gun) during a spike test, morphological feature, and jump capacity (ergo jump) of the player were measured. We tested the relationship between the isokinetic parameters or physical features and field performances represented by spike velocity. We also compared first-division and second-division player data. Results Spike velocity correlated significantly with strength performance of the dominant shoulder (internal rotators) and of the dominant elbow (flexors and extensors) in the concentric mode. Negative correlations were established with the concentric external rotator on internal rotator ratio at 400 deg/s and with the mixed ratio (external rotator at 60 deg/s in the eccentric mode on internal rotator at 240 deg/s in the concentric mode). Positive correlations appeared with both the volleyball players’ jump capacity and body mass index. First-division players differed from second-division players by higher ball velocity and increased jump capacity. Conclusion Some specific strength and physical characteristics correlated significantly with spike performance in high-level volleyball practice. Clinical Relevance Our results could provide useful information for training management and propose some reflections on injury prevention.

Vortex simulations of impulsivelyaccelerated unstable interface

The point vortex method is applied to simulate the motion of an unstable interfacedriven by an impulsive force. The numerical results from the vortex method agree well with previous numerical results and theoretical models. We study various aspects of the dynamics of unstable interfaces for the case of infinite density ratio. It is shown that the spike velocity converges to an asymptotic limit that depends on the initial amplitude and velocity, while the bubble velocity decays to zero. We also find that the growth rates of bubbles and spikes at late times are slower for larger initial amplitude.

Nonuniform Approach to Terminal Velocity for Single Mode Rayleigh-Taylor Instability

The temporal development of a single mode Rayleigh-Taylor instability consists of three stages: the linear, free fall and terminal velocity regimens. The purpose of this paper is to report on new phenomena observed in the approach to terminal velocity. Our numerical study shows an unexpected nonuniform approach to terminal velocity. The nonuniformity applies especially to the spikes, which are fingers of heavy fluid falling into the light fluid, but it also applies to the rising bubblesof light fluid. For spikes especially, our results call into question the meaningfulness of a terminal velocity for moderate values of the Atwood number A. After a short time period of pseudo-terminal plateau, the spike velocity increases to a significantly higher maximum, followed by a decrease. This phenomena appears to be due to a slow evolution in the shape of the spike and bubble. We find a relation between the spike (bubble) acceleartion and the tip curvature. In correlation with an increase in the spike velocity, the main body of the spike becomes narrower and the tip curvature increases. Our numerical results are by the Front Tracking method. The very late time simulations considered here required stabilization by a small value for the viscosity, so that the compressible Navier-Stokes equations govern the dynamics.

The structural variety of chemical spike-electric field interactions

The effect of an imposed field on chemical spikes propagation in a reaction diffusion medium is investigated. A scaling model developed earlier is adopted wherein the electric field term is added. The system of equation is solved numerically for a given set of model parameters, varying either one of two key parameters: a rate coefficient q and a field factor μ. Over different domains of the field factor μ (at fixed q), one or three “electro-spike” solutionsare obtained. The spike velocity and thickness are studied as the parameters q and μ are varied. The various wave profiles are plotted. Using a linear stability analysis, the electro-spikes are shown to be stable. Concluding reflections are made in connection with the observed spike propagation in cardiac fibers.

Numerical Analysis of Turbulent Structure in a Square-Sectioned Bend with Short Inlet Length Duct. Influence of Boundary Layer Thickness at Bend Entry on Developing Flow in a Square-Sectioned Bend.

A numerical analysis has been performed on developing turbulent flow in a square-sectioned 180 deg bend. The ratio of bend mean radius of curvature to hydraulic diameter is 3.375 and a straight duct 6 hydraulic diameters long is attached to the inlet plane of the bend. In calculation, an algebraic stress model was adopted in order to predict Reynolds stresses precisely, and a boundary-fitted coordinate system was introduced as the method of coordinate transformation. Calculated results were compared with the experimental data available. Moreover, the influence of boundary layer thickness at the bend entry is examined using the calculated results. As a result of this study, we conclude that the present method can predict well the mean flow velocity as well as its a minimum and the measured spike velocity in streamwise velocity fluctuations near the symmetry at 90 deg. Furthermore the calculated results confirm that thinner entry boundary layers in the turbulent flow give rise to lower cross-stream velocity. As a characteristic feature, it is pointed out that the secondary flow breaks down into several eddies in the place of the classical single vortex structure when the boundary layers are thin. Therefore, distorted contour lines of streamwise flow appear at angles greater than 90 deg where the secondary flow begins to break down.

Propagated subthreshold oscillatory response and classical electrotonic response of squid giant axon.

The small-signal subthreshold response of the excised squid giant axon to an internal point source of current is oscillatory and propagates with decrement at a velocity close to the spike velocity.

Observation of Antiferromagnetic Phase Transitions by Ultrasonic Techniques

The usefulness of ultrasonic techniques in determining the phase boundaries of antiferromagnets (in the H-T plane) is discussed. The various magnetic phase transitions which occur in antiferromagnets are described with particular emphasis on transitions in uniaxial antiferromagnets of the easy-axis type. The phase boundaries between the antiferromagnetic, spin-flop, and paramagnetic phases are discussed in the molecular field approximation. Some results of more sophisticated treatments are also included. The main ultrasonic phenomena which occur near the various phase transitions of uniaxial antiferromagnets are described. They are (a) λ anomalies in the attenuation and velocity near the second-order transitions, (b) an attenuation spike (velocity dip) at the spin-flop transition, and (c) an abrupt attenuation increase (velocity decrease) at the spin-flop transition. Examples of phase boundaries determined ultrasonically in MnF2, FeF2, CoF2, Cr2O3, α-Fe2O3, Cr, and EuTe are summarized.