High Velocity Case Research Articles

Development and Validation of an in-House Direct Simulation Monte Carlo Code for Supersonic Rarefied Gas Microflows

An in-house Direct Simulation Monte Carlo (DSMC) code has been developed using an object-oriented interface for the purpose of eventually modelling gas and plasma microflows in micromechanical systems, by coupling it with an existing Particle-In-Cell (PIC) solver. The code is validated with canonical flow problems such as the micro-Couette and micro-Poiseuille flows. Since the emphasis is on supersonic and highly compressible flows, we perform validation studies of our algorithm for high velocity and Mach number cases, as compared to known analytical or semi-analytical profiles via asymptotic Bhatnagar-Gross-Krook (BGK) and Chapman-Enskog theory. Reasonably good agreement is found for all the test cases, which indicates the reliability of the present algorithm for modelling supersonic rarefied gas flows. Our results have implications for future validation studies of DSMC codes, where concrete quantitative theoretical and experimental comparisons across different flow configurations are required.

Large-eddy simulation, convective instability, and modal causality of coaxial supersonic air–water jets considering a swirl effect

An annular liquid sheet sheared by a coaxial supersonic gas stream with a swirling effect is investigated using Large Eddy Simulation. Despite its wide applications in aerospace and medical devices, the instability and spatial characters have been barely investigated due to the high complexity under supersonic condition. Unlike the conventional use of the temporal dynamic mode decomposition (DMD), DMD is applied in the axial direction to evaluate the transient convective instability. The high-velocity cases show significantly stronger instability in the nozzle near-field. However, swirling has only marginal effects on the convective instability. In addition, proper orthogonal decomposition (POD) extracts the essential spatial topology of velocity, momentum, and pressure fields. Pulsatile and flapping instabilities are observed in the gas flow, where liquid flow demonstrates the schrink/expansion as well as the flapping instabilities. In addition, all POD modes of the pressure field take the form of coherent wavepacket structures, and their wavelength and spatial forms of the wavepackets are dependent on the gas flow speed rather than the swirling. Time coefficients of the leading POD modes of momentum and pressure fields show an interesting correlation. Hence, the causal–effect relationship between these leading modes of momentum and pressure field is quantified via transfer entropy from the information theory. The transfer entropy from the pressure field to the momentum field is generally higher than vice versa, and this trend is enhanced by the swirling in the low-velocity condition.

Numerical analysis of nonlinear time‐fractional fluid models for simulating heat transport processes in porous medium

AbstractHybrid nanomaterials offer potential scope for increasing numerous novel applications when engineered to deliver availably functional properties. In the current assessment, a novel fractional constitutive relationship for buoyance‐driven flow and heat transfer has been developed in order to control the flow and heat transfer behavior. The fluid flow is produced by a permeable moving surface and is exposed to thermal radiation along with an applied magnetic field. This study employed the SiO2/MoS2 hybrid nanoparticles with different mass ratios in water as the base fluid. Moreover, the Darcy expression characterizes the porous spaces with variable porosity and permeability. Governing fractional PDEs are nonlinear and solved numerically with finite‐difference discretization and the L1‐ algorithm. The results show that heat transfer rate enhanced 8% by enhancing the thermal fractional parameter. It is also found that the combination of two nanoparticles in water significantly contributes to heat transfer compared to the presence of a single particle. Further, thermal radiation significantly contributes to heat transfer in fluid flow. Not surprisingly, hybrid particles have excellent performance at high loads and diverse velocity cases.

Surgical Window and Treatment Strategy for Patient with Multiple Fractures Escaping Inflammatory Response: A Case Report

Today’s Orthopedic surgeons must be adept at the evaluation and management of complicated Multiple Injured Patients (MIP). Despite dynamic strategies, advance modern medical care, the mortality rate of 50-70% has been reported due to high-velocity accidental cases with femoral shaft fractures associated with pelvic fractures and injuries to other organs. We hereby present an interesting case of MIP with the unstable pelvic acetabulum, ipsilateral femoral shaft fracture, floating elbow, and distal radius fracture. It is essential to follow an optimal protocol for such complex fractures. Therefore, we stated the appropriate surgical window and treatment intervention to fix all the major fractures by overcoming the hyperinflammatory state.

Finite Element Analysis of Air Gun Impact on Post-Keratoplasty Eye.

PurposeDue to the mechanical vulnerability of eyes that have undergone penetrating keratoplasty (PKP), it is clinically important to evaluate the possibility of corneal wound dehiscence by blunt impact. We have previously developed a simulation model resembling a human eye based on information obtained from cadaver eyes and applied three-dimensional finite element analysis (FEA) to determine the physical and mechanical response to an air gun impact at various velocities on the post-PKP eye.MethodsSimulations in a human eye model were performed with a computer using a FEA program created by Nihon, ESI Group. The air gun pellet was set to impact the eye at three-different velocities in straight or 12° up-gaze positions with the addition of variation in keratoplasty suture strength of 30%, 50% and 100% of normal corneal strength.ResultsFurthermore to little damage in the case of 100% strength, in cases of lower strength in a straight-gaze position, wound rupture seemed to occur in the early phase (0.04–0.06 ms) of impact at low velocities, while regional break was observed at 0.14 ms after an impact at high velocity (75 m/s). In contrast, wound damage was observed in the lower quadrant of the suture zone and sclera in 12° up-gaze cases. Wound damage was observed 0.08 ms after an impact threatening corneoscleral laceration, and the involved area being larger in middle impact velocity (60 m/s) simulations than in lower impact velocity simulations, and larger damaged area was observed in high impact velocity cases and leading to corneoscleral laceration.ConclusionThese results suggest that the eye is most susceptible to corneal damage around the suture area especially with a straight-gaze impact by an air gun, and that special precautionary measures should be considered in patients who undergo PKP. FEA using a human eyeball model might be a useful method to analyze and predict the mechanical features of eyes that undergo keratoplasty.

Investigation on positioning control strategy and switching optimization of an equal coded digital valve system

This article concerns high accuracy positioning control with switching optimization for an equal coded digital valve system. Typically, pulse number modulation control cannot realize micro-positioning due to the characteristics of step-wise flow variation, therefore, a new position controller consisting of a model-based pulse number modulation and a differential pulse width modulation strategy is proposed to control the position of a hydraulic cylinder at high and low velocity cases, respectively. In addition, in order to solve several problems caused by the pulse number modulation and differential pulse width modulation, such as increased number of switchings and large difference among number of switchings of valves, a switching optimization consisting of a switching cost function, a circular buffer and a circular switching method is proposed. An adaptive weight of the switching cost function is proposed for the first time to reduce the total number of switchings under different pressure differences and its design criterion is presented. A circular buffer and a new circular switching method are used to improve the degree of equal distribution of switchings when the pulse number modulation and differential pulse width modulation are used, respectively. Comparative experimental results indicated that the average and the minimum positioning error for the proposed controller are only 10 and 1 μm, respectively. The number of switchings and the degree of equal distribution of switchings are significantly optimized. Moreover, the pressure fluctuations caused by the proposed controller remain acceptable.

BeiDou/GPS relative kinematic positioning in challenging environments including poor satellite visibility and high receiver velocity

Global Positioning System (GPS) satellite tracking loss always occurs under challenging observation conditions such as poor satellite visibility, high receiver velocity and so on. The announcement of China’s BeiDou Navigation Satellite System (BDS) regional service in 2012 has, on one hand, provided a new standalone positioning system and, on the other hand, augmented GPS under challenging observation conditions. This paper carries out BDS/GPS relative kinematic positioning with poor satellite visibility and high receiver velocity including an under-foliage pedestrian positioning at the speed of ∼5 km h−1, a city urban vehicle positioning at the speed of ∼30 km h−1, and an open-sky air vehicle positioning at the speed of ∼130 km h−1. Numerical results reveal that the standalone system cannot always provide reliable kinematic positioning solution in challenging observation environments. Instead, BDS/GPS combined system can provide better kinematic performance, i.e. 94.2% solution with 0.251 m precision (1-sigma) for the under-foliage pedestrian scenario and 97.0% solution with 0.151 m precision (1-sigma) for the city-urban vehicle scenario. As for the high receiver velocity case, both BDS and GPS can provide cm-level kinematic positioning precisions from the perspective of aerial photogrammetry. The BDS/GPS combined system cannot further improve the positioning availability and precision too much.

MHD heat flux mitigation in hypersonic flow around a blunt body with ablating surface

One of the possible applications of magnetohydrodynamic flow control is considered. Namely, the surface heat flux mitigation by means of magnetohydrodynamic (MHD) interaction in hypersonic flow around a blunt body. The 2D computational model realizes a coupled solution of chemically non-equilibrium ionized airflow in magnetic field. Heat- and mass-transfer due to the ablation of materials from the body surface is taken into account.Two cases of free-stream flow conditions are considered: moderate free-stream velocity (7500 m s−1) case and high free-stream velocity (11 000 m s−1) case. It is shown that the first flow case results in moderate ionization in the shock layer, while the second flow case results in high ionization. In the first case, the Hall effect is significant, and effective electrical conductivity in the shock layer is rather low. In the second case, the Hall effect reduces, and effective conductivity is high. Even if the Hall effect is strong, as in the first case, intensive MHD deceleration of the flow behind the shock is provided due to the presence of insulating boundaries, the bow shock front and non-conductive wall of the blunt body. In the second case, high effective conductivity provides a high intensity of MHD flow deceleration. In both cases, a strong effect of MHD interaction on the flow structure is observed. As a consequence, a noticeable reduction of the surface heat flux is revealed for reasonable values of magnetic induction. The new treatment of mechanism for the surface heat flux reduction is proposed, which is different from commonly used one assuming that MHD interaction increases the bow shock stand-off distance, and, consequently results in a decrease of the mean temperature drop across the shock layer. The new effect of ‘saturation of heat flux’ is discussed.

Velocity and stretch characteristics at the leading edge of an aerodynamically stabilized flame

High swirl flows with vortex breakdown are frequently used to create recirculating flow regions for flame stabilization. This paper experimentally characterizes flame stabilization in such flows, motivated by the fact that conditions under which they enable (or do not enable) aerodynamically stabilized flames are not well understood. Such flames are substantially more difficult to experimentally characterize than shear layer stabilized flames, as the flame leading edge moves around significantly and may or may not coincide with a diagnostic laser sheet, in contrast to the near-fixed leading edge of shear layer stabilized flames. This paper characterizes the leading edge conditions, based upon simultaneous stereo Particle Image Velocimetry (sPIV), OH Planar Laser-Induced Fluorescence (PLIF) and OH* chemiluminescence data. Frames are conditionally analyzed when the flame's most upstream point was captured in the laser sheet. These conditioned frames were used to determine the coordinates, local flow velocity and full, three-dimensional, hydrodynamic strain component of flame stretch at the dynamically evolving flame leading edge. Results are taken at two conditions, 70 m/s and 45 m/s axial velocity, referred to as case 1 and 2, respectively. These data show that the leading edge of the flame precesses off-axis, with additional uncorrelated radial and axial motions. The conditioned mean flame stretch rate is positive in both cases, due to the bulk flow deceleration from the high velocity nozzle flow into the larger diameter combustor. Mean strain or velocity values conditioned on the leading flame edge are substantially different from mean values calculated at the same location. For example, the case 1 mean conditioned axial velocity at the flame leading edge is 9 m/s, while the mean axial velocity at the same location is 4 times higher, 36 m/s. In addition, the case 1 mean hydrodynamic strain rate is 3.6 times higher than the corresponding laminar flame extinction stretch rate, κext, calculated from detailed kinetics for the high velocity case, while less than half of κext in case 2. The mean strain rate predicted from bulk velocity scaling changes by a factor of 70/45 between the two cases, a value confirmed by measurements. However, the conditioned mean stretch values differ by a factor of 14. This result illustrates that additional work is needed to understand stretch scalings in such flames. Furthermore, they show that current correlations for flame stability do not capture much of the intermittency that is present in swirl flows with precessing flow features, and illustrates the need for continued work in very basic physics of how and when flames are stabilized aerodynamically.

A time domain prediction method for the vortex-induced vibrations of a flexible riser

In this paper, a time domain prediction method from experimental data is proposed for vortex-induced vibration (VIV) of flexible risers. The nonlinear factors, couplings among axial tension, VIV response in cross flow (CF) direction and the hydrodynamic force, have been taken into account in this method, with a simplified tension variation model and empirical hydrodynamic force model. The hydrodynamic force, including the excitation force in the excitation region and the damping force in the damping region are the function of excitation coefficients, non-dimensional VIV amplitude and frequency based on vibration experiment data. Iterations are performed to achieve balances between the hydrodynamic forces and the VIV responses of a riser. Moreover, a new added mass coefficient of 2.0 from model tests of flexible pipes is applied, where the predicted VIV response frequencies reveal higher accuracy. Comparison between the predicted results and the experimental results under uniform flow of 2.8 m/s and shear flow of 2.0 m/s are conducted, which verifies the feasibility and reliability of the proposed method. In addition, by comparing the prediction results with and without coupling between axial tension and VIV responses, it is found that this coupling effect is of importance to VIV prediction and can improve VIV prediction accuracy, especially under the case of high flow velocity and high vibration mode.

Computational simulation of biomolecules transport with multi-physics near microchannel surface for development of biomolecules-detection devices.

The quantitative evaluation of the biomolecules transport with multi-physics in nano/micro scale is demanded in order to optimize the design of microfluidics device for the biomolecules detection with high detection sensitivity and rapid diagnosis. This paper aimed to investigate the effectivity of the computational simulation using the numerical model of the biomolecules transport with multi-physics near a microchannel surface on the development of biomolecules-detection devices. The biomolecules transport with fluid drag force, electric double layer (EDL) force, and van der Waals force was modeled by Newtonian Equation of motion. The model validity was verified in the influence of ion strength and flow velocity on biomolecules distribution near the surface compared with experimental results of previous studies. The influence of acting forces on its distribution near the surface was investigated by the simulation. The trend of its distribution to ion strength and flow velocity was agreement with the experimental result by the combination of all acting forces. Furthermore, EDL force dominantly influenced its distribution near its surface compared with fluid drag force except for the case of high velocity and low ion strength. The knowledges from the simulation might be useful for the design of biomolecules-detection devices and the simulation can be expected to be applied on its development as the design tool for high detection sensitivity and rapid diagnosis in the future.

An Investigation on the Tribological Performances of the SiO2/MoS2 Hybrid Nanofluids for Magnesium Alloy-Steel Contacts.

Hybrid nano-materials offer potential scope for an increasing numerous novel applications when engineered to deliver availably functional properties. In the present study, the SiO2/MoS2 hybrid nanoparticles with different mass ratios were employed as lubricant additives in the base oil, and their tribological properties were evaluated using a reciprocating ball-on-plate tribometer for magnesium alloy-steel contacts. The results demonstrate that the SiO2/MoS2 hybrid nanoparticles exhibit superior lubrication performances than individual nano-SiO2 or nano-MoS2 even in high load and diverse velocity cases. The optimal SiO2/MoS2 mixing ratio and the concentration of SiO2/MoS2 hybrid nanoparticles in the base oil are 0.25:0.75 and 1.00–1.25 wt%, respectively. The excellent lubrication properties of the SiO2/MoS2 hybrid nanoparticles are attributed to the physical synergistic lubricating actions of nano-SiO2 and nano-MoS2 during the rubbing process.

In-situ exfoliated graphene for high-performance water-based lubricants

In this study, the tribological behavior of in-situ exfoliated graphene for water-based lubricants is investigated. The aqueous graphene dispersions show high stability due to the assistance of a non-ionic surfactant, Triton X-100. The steel–steel ball-plate tribotest results reveal that the in-situ exfoliated graphene possesses brilliant frictional and anti-wear properties. Compared with pure deionized water, the graphene-enhanced lubricants can offer 81.3% and 61.8% reduction in friction coefficient and wear scar diameter (i.e. a reduction in wear volume by two orders of magnitude), respectively, owing to the formation of a protective graphene layer at the sliding contact zone. The performance of in-situ exfoliated graphene is also better than that of graphene oxide at the same additive concentration. Furthermore, the as-prepared lubricants exhibit enhanced lubrication effect even in high load and diverse velocity cases. It is proposed that the surfactant, Triton X-100, eases the deposition procedure of the graphene layer by adjusting the dispersion wettability of the sliding surfaces, thus leading to the superior tribological properties.

A HYDRODYNAMICAL SOLUTION FOR THE “TWIN-TAILED” COLLIDING GALAXY CLUSTER “EL GORDO”

The distinctive cometary X-ray morphology of the recently discovered massive galaxy cluster "El Gordo" (ACT-CT J0102-4915; z=0.87) indicates that an unusually high-speed collision is ongoing between two massive galaxy clusters. A bright X-ray "bullet" leads a "twin-tailed" wake, with the SZ centroid at the end of the Northern tail. We show how the physical properties of this system can be determined using our FLASH-based, N-body/hydrodynamic model, constrained by detailed X-ray, Sunyaev-Zel'dovich (SZ), and Hubble lensing and dynamical data. The X-ray morphology and the location of the two Dark Matter components and the SZ peak are accurately described by a simple binary collision viewed about 480 million years after the first core passage. We derive an impact parameter of ~300 kpc, and a relative initial infall velocity of ~2250 km/sec when separated by the sum of the two virial radii assuming an initial total mass of 2.15x10^(15) Msun and a mass ratio of 1.9. Our model demonstrates that tidally stretched gas accounts for the Northern X-ray tail along the collision axis between the mass peaks, and that the Southern tail lies off axis, comprising compressed and shock heated gas generated as the massive component plunges through the main cluster. The challenge for LCDM will be to find out if this physically extreme event can be plausibly accommodated when combined with the similarly massive, high infall velocity case of the "Bullet cluster" and other such cases being uncovered in the new SZ based surveys.

Mechanical Behavior of Polycrystalline Aluminum under Penetration with Extremely Large Loading Rates via Molecular Dynamics Simulation

In this study, polycrystalline aluminum nanoscale thin sheets are constructed by sputter deposition simulations with the molecular dynamics (MD) simulation. Subsequently, the penetration problem of a conical rigid projectile moving through the aluminum thin sheet is simulated by the MD technique. The MD simulations adopted the interatomic potential of a tight-binding type. During the deposition simulation, in order to include the ion-ion interactions, the pair-wise Moliere potential was adopted to model the interaction between working gas argon and deposited atoms. The as-deposited films did not show clear grain boundaries, but after thermal annealing, grains grow and form nanocrystalline structure with a grain size of 8 nm. The thin sheets consisted of the face-centered cubic phases of crystal unit cells, separated by grain boundaries. For the penetration simulations, four velocities were chosen 102, 103, 104 and 105 m/s. The first two velocities are called high velocity case and the rest two velocities are the hypervelocity case. Our results show that, as the penetration rate increases, more stresses are required to move the projectile through the Al film due to temperature effects from the high velocity to hypervelocity case. In addition, defects, such as dislocations, increase during the projectile penetration. In the high velocity case, the penetrated hole in the film may be recovered, but not in the hypervelocity case. The temperature difference increased in the hypervelocity case is significantly than that in the high velocity case.

Linear stability analysis of supersonic axisymmetric jets

Stabilities of supersonic jets are examined with different velocities, momentum thicknesses, and core temperatures. Amplification rates of instability waves at inlet are evaluated by linear stability theory (LST). It is found that increased velocity and core temperature would increase amplification rates substantially and such influence varies for different azimuthal wavenumbers. The most unstable modes in thin momentum thickness cases usually have higher frequencies and azimuthal wavenumbers. Mode switching is observed for low azimuthal wavenumbers, but it appears merely in high velocity cases. In addition, the results provided by linear parabolized stability equations show that the mean-flow divergence affects the spatial evolution of instability waves greatly. The most amplified instability waves globally are sometimes found to be different from that given by LST.

Research of the Resistance Model in Solid Armature Railgun

During the past few decades, various calculation models have been introduced into studying solid armature railgun. In this paper, four models, including constant resistance, speed resistance, current resistance, and multifactor resistance, were studied in detail. To verify their validity, experimental and simulation methods were adopted in both low- and high-velocity cases. By conducting a 3-D eddy current electromagnetic field simulation with the finite-element numerical method, the self-inductance gradient ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> ' <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> ) was got. The control equations of these different resistance models can be solved by the Runge-Kutta method. Comparisons of experimental and simulation results of these different resistance models indicated that an appropriate resistance model according to the experimental condition can improve the accuracy of simulation analysis. The friction coefficient resistance model can describe armature resistance during the movement process in both low- and high-velocity cases.

FDTD Analysis with Overset Grid Generation Method for Rotating Body and Evaluation of Its Accuracy

This paper presents an alternative approach for the analysis of EM field by a rotating body with FDTD method and Overset Grid Generation method, considering Lorentz transformation for the higher velocity cases. This approach has been previously proposed for the case of linear and uniformly moving body against/to the incident wave. Here, the approach is expanded to a rotating body which includes the interpolation technique in the space and time increment along the cylindrical rotation at the fixed axis. First, the grid size ratios between the main mesh and the sub-mesh are studied. The appropriate choice of the grid size ratio is obtained. Then, the modulations of the EM field when the incident wave hits the rotating body in high velocity cases are analyzed. The relationship of the phase shift and the velocity is further observed. The observed EM fields are compared with the theoretical results and achieved good agreements in high relative velocities. The assessment of the numerical errors in a rotating environment is also highlighted. This numerical approach may have numerous situations to which it can be applied. This may be involved with the design of rotating devices such as microactuator, commutator and others.

Inertial waves in a rotating annulus with inclined inner cylinder: comparing the spectrum of wave attractor frequency bands and the eigenspectrum in the limit of zero inclination

We investigate theoretically inertial waves inside a liquid confined between two co-rotating coaxial cylinders of finite length. We consider the case of small viscosity and high angular velocity (i.e., small Ekman numbers), a parameter range of interest for many geophysical applications. In this case, inertial waves propagating in the container show multiple reflections at the walls before the waves can be damped by weak diffusion. We allow for the inner cylinder wall to be parallel or inclined with respect to the annulus’ vector of rotation (truncated cone). For the limit of zero viscosity, the wave propagation is governed by a boundary value problem that is composed of a linear second-order hyperbolic partial differential equation and the impermeability boundary conditions. For the special case of vertical cylinder walls (no inclination of the inner cylinder), this boundary value problem is separable, the corresponding eigenmodes can analytically be found and they are regular. However, when the inner cylinder wall is inclined, the hyperbolicity of the governing equation leads to internal shear layers (corresponding to singularities for the inviscid case). The geometrical structure of the shear layers can be explained by inertial waves, trapped on limit cycles denoted as wave attractors. The shape of the limit cycles depends on the wave frequency. In fact, the spectrum of regular modes, existing for the case of vertical cylinder walls, vanishes almost completely when the inner wall is inclined. Instead of a spectrum of discrete frequencies and regular eigenmodes, a spectrum of wave attractor frequency bands and singular eigenmodes exist. The question addressed here is whether the spectrum of wave attractor intervals collapses to the discrete frequency spectrum when the inclination angle of the inner cylinder goes to zero. To answer this question, the attractor frequency intervals are evaluated numerically for a series of decreasing cylinder inclination angles and are compared with the analytically found eigenspectrum for the case of zero inclination. Goal is to better understand the asymptotic behavior of the problem for decreasing inclination angles. This understanding helps to interpret results from laboratory experiments with geometries that differ from the perfect annulus with parallel cylinder walls.

Lifting line theory for wing-in-ground effect in proximity to a free surface

Although it has some limitations in applications, the classical Prandtl lifting line theory remains a standard methodology for evaluating lifting problems in free space. It is of theoretical interest in revealing lifting mechanisms. It is therefore, interesting to generalize the classical lifting line theory to cases more general than just the free space problem. In this article, we present the Prandtl lifting line theory for wing-in-ground effect (WIG) near a free surface. While, the fundamental methodology being similar to the classical lifting line theory, it turns out that the difficulty lies in finding the three-dimensional Green’s function for the system of horseshoe vortices operating over the deformable free surface. Linear free surface boundary conditions are applied to deal with the two-dimensional lifting problem solved by the singularity distribution method and the three-dimensional correction found by placing a system of horseshoe vortices on the wing. This approach was validated against published data. Excellent agreement is found among results obtained from this study, experiments and numerical simulations. Extensive numerical examples are carried out to show the features of lift coefficients in the vicinity of a free surface. As expected, the free surface can be represented by a rigid wall for the case of high velocity. Finally, the free surface effect on WIG is discussed.