Range Of Standoff Distances Research Articles

Laser interferometer measurement and reconstruction of supersonic projectile acoustic pressure fields

Sensing acoustic fields with a laser interferometer offers several advantages over conventional techniques: the probe beam does not disturb the medium in which it operates, the interferometer optics can be placed far from the source, and interferometry typically has a wider bandwidth than microphones. These advantages are particularly useful in situations where precise measurements of high-amplitude shock-generating sources are required. In this work we use a laser interferometer to study the generation and propagation of shock wave signatures of ballistic projectiles. A laboratory experiment was conducted to collect interferometer data from a variety of supersonic projectiles over a range of standoff distances. Simultaneously-captured microphone data is compared to the interferometer measurements, and techniques for inversion and reconstruction of the sound pressure field are discussed.

Effect of stand-off distance on “counterjet” and high impact pressure by a numerical study of laser-induced cavitation bubble near a wall

In experiments of the collapse and rebound of a laser-produced cavitation bubble near a wall, a so-called “counterjet”, i.e., a jet in the direction away from the bubble and the wall, was typically observed during the rebound phase for a certain range of stand-off distances. It was previously suggested that a mechanism for the formation of the counterjet might be cavitation inception that follows the self-penetration of the jet torus shock wave. Numerical simulations in the current work show that this counterjet may in fact be part of the bubble spiral trailing behind the main part as the bubble is distorted by the ring vortex and becomes toroidal in shape after the initial collapse. Since the spiral is trailing behind the main bubble, it appears to be a jet in the direction away from the bubble and the wall, giving the impression of the formation of a counterjet. However, the simulations show that the velocities in the spiral are also towards the wall. The effects of phase transition on various aspects of bubble collapse dynamics are also examined. It is found that the experimental bubble-wall distance evolution can only be better captured through the inclusion of phase transition modeling. Without phase transition, the bubble-wall distance history in the simulations diverges from the experimental one upon the first collapse. After these validations, the effects of stand-off distance on the impact pressure are studied systematically with results compared with experiments. It is found that the largest impact pressures can be as high as 1–2 GPa as a result of the strong focus of a nearly spherical shock onto the symmetry axis and along the wall that occurs when the stand-off distances are low with Ho/Ro<0.1. The impact pressure of ∼0.1 GPa at Ho/Ro<0.7 on the other hand is found to be the result of the focus of the ring-vortex-focusing shock onto the symmetry axis. Both the magnitude of the impact pressure and the respective stand-off distance range are consistent with previous experimental results.

Modeling the compressibility of Saturn's magnetosphere in response to internal and external influences

AbstractThe location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure DP and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturn's magnetopause standoff distance varies as and measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth (α≈6) and a more easily compressible case appropriate for Jupiter (α≈4). In this study we employ a 2‐D force balance model of Saturn's magnetosphere to investigate magnetospheric compressibility in response to changes in DP and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding DP values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with DP but vary from ∼4.8 for high DP conditions, when the magnetosphere is compressed (≤25 RS), to ∼3.5 for low DP conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed DP, with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk‐like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere.

Jet Flight Patterns of Linear Shaped Charges

A typical jet flight pattern of linear shaped charges (LSCs) is studied, and analytical expressions are built for two important parameters of the angle of liner collapse line φ and jet projection angle ω. In order to simplify the nonlinear nature of the liner collapse behavior of LSCs, important assumptions are made, including (1) a constant jet projection velocity at the given standoff distance and (2) the liner collapse occurs in a steady-state fashion, generating a single straight collapse line during the detonation. Based on the assumptions, analytical expressions are derived and the result is compared with side-view images of LSC jet flight to evaluate accuracy and applicability. The analytical approach delivers reasonable accuracy in a given range and is applicable to a short range of standoff distances.

Defining parametric dependencies for the correct interpretation of speckle dynamics in photon Doppler velocimetry

Laser speckle dynamics manifest themselves in photon Doppler velocimetry (PDV) data as low-frequency amplitude fluctuations, and analysis of these fluctuations provides insight into the transverse speed of the surface under observation. We previously demonstrated that a single measurement probe is capable of simultaneously measuring (1)axial motion, through frequency analysis of Doppler shifts, and (2)transverse speed, through analysis of the speckle's coherence time. However, the performance of this technique hinges on a correct understanding of the speckle pattern's response to surface motion. In this paper, we model the origination of the speckle pattern, and we describe a methodology for calculating the speckle's coherence time from the autocorrelation of a noisy signal. We then test a suite of optical probes over a range of standoff distances, demonstrating a significant reduction in the speckle's coherence time, which correlates to the increase in speckle boiling when the target surface is located near a probe's focal length. We show that spatial regions of decreased coherence time may be predicted a priori by a probe's parameters, since they stem from boiling dominance. We analyze this result as a function of probe parameters for a surface-scattering target and a volume-scattering target. Although the coherence time's behavior in the focal plane makes velocity extraction difficult, far from the probe's focal lengths, we are able to measure rigid body transverse speeds exceeding 20 m/s with an absolute accuracy of ±15% using the speckle dynamics measured by a PDV setup.

Effect of stand-off distance for cold gas spraying of fine ceramic particles (< 5 μm) under low vacuum and room temperature using nano-particle deposition system (NPDS)

The nano-particle deposition system (NPDS) is a novel fabrication method for metallic and ceramic coatings at room temperature that sprays dry nano-sized powder or micro-sized powder without binders or additives. NPDS has been used to coat metal, ceramic, and polymer substrates with various metal and ceramic powders such as TiO 2, Al 2O 3, Ni, and Sn. Deposition results have been reported, but the process parameters have not been evaluated in detail. The effect of the stand-off distance (SoD), which is an important process parameter and is easily controlled during fabrication, was evaluated in this study to better understand the process. Numerical analysis of the impact velocity of particles using the commercially available ANSYS CFX software package and experiments examining Al 2O 3 deposition on sapphire wafers (Al 2O 3) using NPDS was carried out. In NPDS, the impact velocity of particles is one of the most important deposition parameters because the sole energy source for bonding is the kinetic energy of the particles. The SoD range was set from 1 to 7 mm in 2-mm increments. Numerical analysis showed that the impact velocity of particles increased with increasing SoD. The impact velocity of particles correlated with the mechanical properties of the deposited layers. High impact velocity resulted in high mechanical properties for the deposited layer within the range of SoD tested. SoDs longer than 7 mm were not evaluated because the width and thickness of deposited layer were not suitable for patterning. Additionally, numerical analysis also helped explain the deposition geometry, especially the width of the deposited pattern under the 1-mm SoD condition. The numerical analysis used in this study can be applied to research on the effects of other process parameters.

Comparable Study on Water Cavitation Peening and Traditional Shot Peening of Almen Strips

Comparing with conventional mechanical shot peening (SP) technique, water cavitation peening (WCP) experiments of Almen strips were carried out on a self-manufactured equipment. The results show that WCP demonstrates a wide range of standoff distance (SD) that from the nozzle to the surface of the object. By measuring the colour changes of the Fuji pressure sensing film, over 110 MPa impacting pressure was detected, which is resulted from the bubbles blasting on the sample surface when the SD is from 65 to 100 mm under 40 MPa of operating pressure. 600 MPa compressive residual stress achieved on the suface of the Almen strips after WCPed for 32 min. The depth of the zone affected by the compressive residual stress is about 100 µm. The highest residual stress appears in the top surface layer, while in case of SP it appears in the subsurface. Compared to SP, WCP is capable to get rather smoother surface and cause less deformation of the testing sheet, simultaneously.

Numerical simulations of non-spherical bubble collapse.

A high-order accurate shock- and interface-capturing scheme is used to simulate the collapse of a gas bubble in water. In order to better understand the damage caused by collapsing bubbles, the dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed. Collapse times, bubble displacements, interfacial velocities and surface pressures are quantified as a function of the pressure ratio driving the collapse and of the initial bubble stand-off distance from the wall; these quantities are compared to the available theory and experiments and show good agreement with the data for both the bubble dynamics and the propagation of the shock emitted upon the collapse. Non-spherical collapse involves the formation of a re-entrant jet directed towards the wall or in the direction of propagation of the incoming shock. In shock-induced collapse, very high jet velocities can be achieved, and the finite time for shock propagation through the bubble may be non-negligible compared to the collapse time for the pressure ratios of interest. Several types of shock waves are generated during the collapse, including precursor and water-hammer shocks that arise from the re-entrant jet formation and its impact upon the distal side of the bubble, respectively. The water-hammer shock can generate very high pressures on the wall, far exceeding those from the incident shock. The potential damage to the neighbouring surface is quantified by measuring the wall pressure. The range of stand-off distances and the surface area for which amplification of the incident shock due to bubble collapse occurs is determined.