Thermal Damping of Weak Magnetosonic Turbulence in the Interstellar Medium

We present a generic mechanism for the thermal damping of compressive waves in the interstellar medium (ISM), occurring due to radiative cooling. We solve for the dispersion relation of magnetosonic waves in a two-fluid (ion-neutral) system in which density- and temperature-dependent heating and cooling mechanisms are present. We use this dispersion relation, in addition to an analytic approximation for the nonlinear turbulent cascade, to model dissipation of weak magnetosonic turbulence. We show that in some ISM conditions, the cutoff wavelength for magnetosonic turbulence becomes tens to hundreds of times larger when the thermal damping is added to the regular ion-neutral damping. We also run numerical simulations, which confirm that this effect has a dramatic impact on cascade of compressive wave modes.

Shock-Wave Properties and Deformation-Induced Structure of Commercial-Purity Titanium

The shock compressive wave profiles of commercial-purity titanium samples under different loading conditions have been measured. The spall strength of titanium as a function of the strain rate and temperature of deformation has been found. High-rate plastic deformation mechanisms have been studied. High-rate plastic deformation under the investigated loading conditions has been shown to occur by slip and twinning. The α → ω transformation has been established to begin at 12.2 GPa.

Shear and compressive wave data acquisition using multicomponent vibrating source and landstreamer

<p>In the vicinity of Ottawa, Ontario, Canada, we have recorded many multicomponent seismic data sets using an in-house multicom­ponent vibrator source named Microvibe and a landstreamer receiver array with 48 3-C 28-Hz geophones at 0.75-m intervals. The receiver spread length was 35.25 m, and the near-offset was 1.50 m. We used one, two or three source and three receiver orientations — vertical (V), inline-horizontal (H1), and transverse-horizontal (H2). We identified several reflection wave modes in the field records — PP, PS, SP, and SS, in addition to refracted waves, and Rayleigh-mode and Love-mode surface waves. We computed the semblance spectra of the selected shot records and ascertained the wave modes based on the semblance peaks. We then performed CMP stacking of each of the 9-C data sets using the PP and SS stacking velocities to compute PP and SS reflection profiles.</p><p>Despite the fact that any source type can generate any combination of wave modes — PP, PS, SP, and SS, partitioning of the source energy depends on the source orientation and VP/VS ratio. Our examples demonstrate that the most prominent PP reflection energy is recorded by the VV source-receiver orientation, whereas the most prominent SS reflection energy is recorded by the H2H2 source-receiver orientation with possibility to obtain decent shear wave near surface data in all other vibrating and receiving directions.</p><p>Pugin, Andre and Yilmaz, Öz, 2019. Optimum source-receiver orientations to capture PP, PS, SP, and SS reflected wave modes. The Leading Edge, vol. 38/1, p. 45-52. https://doi.org/10.1190/tle38010045.1</p>

Effect of longitudinal deformation on wave propagation in truss core sandwich panel

The longitudinal deformation has been ignored by most previous study on vibroacoustic behaviours of truss core sandwich panels. This paper investigates its effect by developing two theoretical models. One, named as full model, considers both flexural and longitudinal deformation of face sheets while the other, named as flexural model, incorporates only flexural deformation. By comparing free characteristic waves obtained from two models, one find that flexural model ignore two waves, that is, a compressive wave and a global flexural one. The ignored waves cause vibration in transverse direction even at high frequencies, and thus result in vibration peaks under forced vibration. Therefore the flexural model would underestimate the vibration response, and the longitudinal deformation of face sheets cannot be ignored during dynamic analysis of the sandwich panels.

Control of the Aero-Engine Nacelle Intake Flow Separation Caused by Crosswind Condition Using Plasma Actuation

To develop active flow control technique which can suppress the nacelle intake flow separations caused by crosswind effectively, microsecond plasma actuation is used to control the flow separations of a typical nacelle intake model. Both experimental and numerical investigations have been implemented to uncover the corresponding flow control effects. The plasma actuation is installed near the inception point of the nacelle intake flow separations. According to the experimental and numerical results, the nacelle intake flow separations caused by crosswind are suppressed by the plasma actuation. The frequency of the plasma actuation as well as the scale of the flow separation are influential to the flow control effects. The compressive wave induced by the plasma actuation will act on the separated flow as well as the interface between the flow separation zone and the mainstream zone. This is the mechanism behind the suppression of nacelle intake flow separations using microsecond plasma actuation.

Localized heating of the Martian topside ionosphere through the combined effects of magnetic pumping by large scale magnetosonic waves and pitch angle diffusion by whistler waves

<p>We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of periodic (~ 25 s) large scale (100s km) magnetosonic waves propagating into the Martian dayside upper ionosphere. These waves adiabatically modulate the superthermal electron distribution function, and the induced electron temperature anisotropies drive the generation of observed electromagnetic whistler waves. The localized (in altitude) minimum in the ratio f<sub>pe</sub> / f<sub>ce</sub> provides conditions favorable for the local enhancement of efficient wave-particle interactions, so that the induced whistlers act back on the superthermal electron population to isotropize the plasma through pitch angle scattering. These wave-particle interactions break the adiabaticity of the large scale magnetosonic wave compressions, leading to local heating of the superthermal electrons during compressive wave `troughs'. Further evidence of this heating is observed as the subsequent phase shift between the observed perpendicular-to-parallel superthermal electron temperatures and compressive wave fronts. Such a heating mechanism may be important at other unmagnetized bodies such as Venus and comets.</p>

Helium in the Earth's foreshock: a global Vlasiator survey

<p>The foreshock is a region of space in front of the Earth's bow shock, extending along the interplanetary magnetic field. It is permeated by ions and electrons reflected at the shock, low-frequency waves, and various plasma transients. The ion foreshock is dominated by a number of proton populations such as field-aligned beams, gyrating distributions and diffuse ions, as well as proton-excited waves. As the solar wind can contain a significant fraction of helium, it is of great interest to investigate how alpha-particles (He<sup>2+</sup>) are reflected into forming their own foreshock. We investigate the extent of the helium foreshock in relation to foreshock ultra-low frequency waves and protons using Vlasiator, a global hybrid-Vlasov simulation. We confirm a number of historical spacecraft observations at the foreshock regions associated with field-aligned beams, gyrating ion distributions, and specularly reflected particles, performing the first numerical global survey of the helium foreshock. We present wavelet analysis at multiple positions within the foreshock and evaluate the dynamics of gyrating ion populations in response to the transverse and compressive wave components. We also present Magnetosphere Multiscale (MMS) spacecraft crossings of the foreshock edge and compare Hot Plasma Composition Analyzer (HPCA) measurements of energetic ions with our simulation data, showing the variability of the foreshock edge suprathermal ion profiles.</p>

Multiple Degrees of Freedom in the Fish Skull and Their Relation to Hydraulic Transport of Prey in Channel Catfish

Synopsis Fish perform many complex manipulation behaviors without hands or flexible muscular tongues, instead relying on more than 20 movable skeletal elements in their highly kinetic skulls. How fish use their skulls to accomplish these behaviors, however, remains unclear. Most previous mechanical models have represented the fish skull using one or more planar four-bar linkages, which have just a single degree of freedom (DoF). In contrast, truncated-cone hydrodynamic models have assumed up to five DoFs. In this study, we introduce and validate a 3D mechanical linkage model of a fish skull that incorporates the pectoral girdle and mandibular and hyoid arches. We validate this model using an in vivo motion dataset of suction feeding in channel catfish and then use this model to quantify the DoFs in the fish skull, to categorize the motion patterns of the cranial linkage during feeding, and to evaluate the association between these patterns and food motion. We find that the channel catfish skull functions as a 17-link, five-loop parallel mechanism. Despite having 19 potential DoFs, we find that seven DoFs are sufficient to describe most of the motion of the cranial linkage, consistent with the fish skull functioning as a multi-DoF, manipulation system. Channel catfish use this linkage to generate three different motion patterns (rostrocaudal wave, caudorostral wave, and compressive wave), each with its own associated food velocity profile. These results suggest that biomechanical manipulation systems must have a minimum number of DoFs to effectively control objects, whether in water or air.

Compressive Wave Propagation in Highly Ordered Granular Media Based on DEM

In this study, we investigated the propagation of the compressive waves in twp-dimensional highly ordered particle assemblies with different material properties and confining stresses. We focused on the attenuation of velocity amplitude, wave speed and energy dispersion behaviors. The mechanisms of wave propagation through specimens containing inclusions that show different horizontal angles were simulated. The peak velocities change exponentially as a function of time along the wave propagation direction. Energy dispersion exhibits an increasing trend as the angle increases. Then the effects of material parameter on wave propagation speed and the confining stress with the dispersion relations are determined. Finally, the shape of wavefront is studied to show the propagation of stress wave. It is linked to the distribution of equivalent strain. The outer contours of wavefront and strain distribution have the same shape.