Repetitive Collision Research Articles

CoTree: A Side-Channel Collision Tool to Push the Limits of Conquerable Space

By introducing collision information into divide-and-conquer distinguishers, the existing collision-optimized side-channel attacks transform the given candidate space into a significantly smaller collision space, thus achieving more efficient key recovery. However, the candidates of the first several sub-keys shared by collision chains are still repeatedly detected, which happens very frequently and brings huge computational overhead. To alleviate this, we propose a highly-efficient collision-optimized attack named Collision Tree (CoTree). This collision detection tool exploits tree structure to store the chains created from the same sub-chain on the same branch, thus significantly reducing the storage requirements. It then benefits from the properties of both tree and collisions, and exploits a top-down tree building procedure and traverses each node only once when detecting their collisions with a candidate of the sub-key currently under consideration. Finally, unlike the traditional top-down node removal, CoTree launches a bottom-up branch removal procedure to remove the chains unsatisfying the collision conditions from the tree after traversing all the considered candidates of this sub-key, thus avoiding the traversal of the branches satisfying the collision condition. These strategies make our CoTree significantly alleviate the repetitive collision detection, and our experiments verify that it significantly outperforms the existing works.

Fast GPU-based Two-way Continuous Collision Handling

Step-and-project is a popular method to simulate non-penetrating deformable bodies in physically based animation. The strategy is to first integrate the system in time without considering contacts and then resolve potential intersections, striking a good balance between plausibility and efficiency. However, existing methods can be defective and unsafe when using large time steps, taking risks of failure or demanding repetitive collision testing and resolving that severely degrade performance. In this article, we propose a novel two-way method for fast and reliable continuous collision handling. Our method launches an optimization from both ends of the intermediate time-integrated state and the previous intersection-free state. It progressively generates a piecewise linear path and eventually obtains a feasible solution for the next time step. The algorithm efficiently alternates between a forward step and a backward step until the result is conditionally converged. Thanks to a set of unified volume-based contact constraints, our method offers flexible and reliable handling of various codimensional deformable bodies, including volumetric bodies, cloth, hair, and sand. Experimental results demonstrate the safety, robustness, physical fidelity, and numerical efficiency of our method, making it particularly suitable for scenarios involving large deformations or large time steps.

Effects and limitations of superhydrophobic surfaces on the heat transfer performance of a two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT), also called a gravity-assisted heat pipe, is an efficient heat transfer device that exploits boiling and condensation phase-change phenomena to transport large amounts of heat. Recently, jumping droplet condensation on a well-designed superhydrophobic (SHPo) surface has shown superior condensation heat transfer coefficient (HTC), exceeding ~380% and ~30% over conventional filmwise and dropwise condensation, respectively. However, SHPo surfaces within TPCTs have shown much lower HTCs than expected, and the exact cause of this has not been investigated yet. Here, we experimentally explored the effects and limitations of a SHPo surface in a TPCT by visualizing the internal flow patterns and condensation behavior according to heat flux. We constructed a TPCT device capable of visualizing the inside and fabricated a SHPo surface on a condenser wall of the TPCT. We revealed two important condensation characteristics that limit the condenser HTC of the SHPo surface and the TPCT's heat transfer performance. At high heat fluxes (≥ 90 kW/m2), the repetitive liquid collision led to an early flooding transition of the surface and a stepwise decrease in the condenser HTC. At low heat fluxes (< 90 kW/m2), counter-current flow generated by the evaporation and boiling caused the drag force enough to entrain growing and jumping droplets, which interrupted the removal of condensate from the surface and limited the condenser HTC. We quantified their effects on the condenser and TPCT's heat transfer performance using experiments and analytical models.

A Lightweight Detection Algorithm For Collision-Optimized Divide-and-Conquer Attacks

By introducing collision information into divide-and-conquer attacks, several existing works transform the original candidate space, which may be too large to enumerate, into a significantly smaller collision space, thus making key recovery possible. However, the inefficient collision detection algorithms and fault tolerance mechanisms make them time-consuming and their success rate low. Moreover, they may still leave very huge chain spaces that makes it difficult for key recovery. In this article, we exploit collision attack to optimize Template Attack (TA), and propose a Lightweight Collision Detection (LCD) algorithm. The proposed method exploits a jump detection mechanism to efficiently reduce the repetitive collision detections on chains with the same prefix sub-chains. We then introduce guessing theory to reorder the collision detection of the sub-keys according to their guessing lengths, and provide us with an evaluation tool. Finally, we design a highly efficient fault tolerance mechanism for our LCD to allow flexible thresholds adjustment, and further optimize sieving mechanism to efficiently extract the best chains with the largest number of collisions. Experimental results fully demonstrate LCD's superiority.

Collision modeling and two-echelon collaborative control of non-contact ultra-quiet Stewart spacecraft

This article presents the collision modeling and two-echelon collaborative control of a novel non-contact ultra-quiet Stewart spacecraft. It has internal instability because of its non-contact configuration. To overcome the instability, the equivalent modeling of repetitive collision process with small releasing velocity is established at first. Then, a two-echelon collaborative control is developed to guarantee both collision rejection and stability performance. Finally, the numerical simulation is conducted to demonstrate the validity and effectiveness of the proposed design.

Interventions for Hip Pain in the Maturing Athlete

Context:Femoroacetabular impingement (FAI) alters hip mechanics, results in hip pain, and may lead to secondary osteoarthritis (OA) in the maturing athlete. Hip impingement can be caused by osseous abnormalities in the proximal femur or acetabulum. These impingement lesions may cause altered loads within the hip joint, which result in repetitive collision damage or sheer forces to the chondral surfaces and acetabular labrum. These anatomic lesions and resultant abnormal mechanics may lead to early osteoarthritic changes.Evidence Acquisition:Relevant articles from the years 1995 to 2013 were identified using MEDLINE, EMBASE, and the bibliographies of reviewed publications.Level of Evidence:Level 4.Results:Improvements in hip arthroscopy have allowed FAI to be addressed utilizing the arthroscope. Adequately resecting the underlying osseous abnormalities is essential to improving hip symptomatology and preventing further chondral damage. Additionally, preserving the labrum by repairing the damaged tissue and restoring the suction seal may theoretically help normalize hip mechanics and prevent further arthritic changes. The outcomes of joint-preserving treatment options may be varied in the maturing athlete due to the degree of underlying OA. Irreversible damage to the hip joint may have already occurred in patients with moderate to advanced OA. In the presence of preexisting arthritis, these patients may only experience fair or even poor results after hip arthroscopy, with early conversion to hip replacement. For patients with advanced hip arthritis, total hip arthroplasty remains a treatment option to reliably improve symptoms with good to excellent outcomes and return to low-impact activities.Conclusion:Advances in the knowledge base and treatment techniques of intra-articular hip pain have allowed surgeons to address this complex clinical problem with promising outcomes. Traditionally, open surgical dislocations for hip preservation surgery have shown good long-term results. Improvements in hip arthroscopy have led to outcomes equivalent to open surgery while utilizing significantly less invasive techniques. However, outcomes may ultimately depend on the degree of underlying OA. When counseling the mature athlete with hip pain, an understanding of the underlying anatomy, degree of arthritis, and expectations will help guide the treating surgeon in offering appropriate treatment options.

Voicing and silence periods in daily and weekly vocalizations of teachers

The National Center for Voice and Speech data bank on voice dosimetry was used to study the distributions of continuous voicing periods and silence periods in 31 teachers over the duration of two weeks. Recordings were made during all awake hours of the day. Voicing periods were grouped into half-decades, ranging from 0.0316 0.10 s for the shortest periods of phonation to 31.6 100 s for the longest periods of phonation. Silence periods were grouped into similar half-decades, but ranged up to periods of several hours. On average, the teachers had 1,800 occurrences of voicing (onset followed by offset) per hour at work and 1,200 occurrences per hour while not at work. Voicing occurred 23% of the total time at work, dropping to 13% during off-work hours and 12% on weekends. The greatest accumulation of voicing occurred in the 0.316 1.0 s voicing periods, whereas the greatest accumulation of silence occurred in the 3 10 s silence periods. The study begins to lay the groundwork for understanding vocal fatigue in terms of repetitive motion and collision of tissue, as well as recovery from such mechanical stress.

Voicing and silence periods in daily and weekly vocalizations of teachers

The National Center for Voice and Speech (NCVS) data bank on voice dosimetry was used to study the distributions of continuous voicing periods and silence periods in 31 teachers over the duration of two weeks. Recordings were made during all awake hours of the day. Voicing periods were grouped into half decades, ranging from 0.0316 to 0.10 s for the shortest periods of phonation to 31.6-100 s for the longest periods of phonation. Silence periods were grouped into similar half decades, but ranged up to periods of several hours. On average, the teachers had 1800 occurrences of voicing (onset followed by offset) per hour at work and 1200 occurrences per hour while not at work. Voicing occurred 23% of the total time at work, dropping to 13% during off-work hours and 12% on weekends. The greatest accumulation of voicing occurred in the 0.316-1.0 s voicing periods, whereas the greatest accumulation of silence occurred in the 3-10 s silence periods. The study begins to lay the groundwork for understanding vocal fatigue in terms of repetitive motion and collision of tissue, as well as recovery from such mechanical stress.

Thursday, October 31, 2001 3:47–4:17 pm Focused Review: Cervical Spine Surgery Complications: 3:47 Neck pain in demolition derby drivers

Purpose of study: The purpose of this study was to investigate the incidence of neck pain and associated symptoms in an unusual population of car drivers with high-risk behaviors for whiplash disorder.Methods used: Fifty members of the Demolition Derby Association returned a neck pain questionnaire that had been contacted through the association's internet website. Questions included the participant's exposure to demolition activity (years of participation, estimated number of collisions, approximate impact velocity, and so forth; demolition-related acute and chronic neck pain symptoms, divided into worst and average events and nondemolition neck pain history.of findings: Of the 50 respondents, (44 male, 6 female), 40 completed the entire survey. Average age was 27.8 years (range, 18 to 46). Sixty-six percent were employed full time. Drivers had participated in an average of 11 events over the previous 3 years and an average of 30 events over their total careers. They reported an average of 48 car collisions per event, 54% of these being rear-end collisions. Eighty-four the respondents were restrained drivers all of the time. Thirty-six percent respondents reported that they never wear neck protective gear. Participants estimated average collision speed was 26 miles/hour. Their estimated average maximum collision speeds were 45 miles/hour. Ten of the 50 respondents reported injuries requiring a physician's evaluation, although only 3 reported visits to a physician regarding neck pain. Two of 40 reported their worst postparticipation neck pain to have lasted more than 3 months, 1 of 40 lasting more than 1 year, but for the majority the worst neck pain lasted less than 21 days (27 or 40 less than 21 days, 9 of 40 none). Three participants reported mild chronic persistent neck pain (never went away) as a result of demolition driving experience. The remainder reported none. The average episode of neck pain was moderate or severe for 8 of 40 participants, but for all respondents average pain episodes lasted less than 21 days. Thirteen of 40 respondents reported experiencing some form of chronic neck pain they felt was unrelated to derby competition (33 none, 10 mild, 2 moderate, 1 severe).Relationship between findings and existing knowledge: Considerable controversy surrounds the understanding of whiplash injury with regard to its significance in the causation of chronic symptomology. Some studies suggest that as many as 50% of collision victims report persistent pain 6 months after the collision. By contrast, studies performed in sociological settings where no remuneration for injury exists suggested recovery by 95% of subjects within 6 weeks or less. Participants of demolition derby sport willingly subject themselves to repetitive car collision trauma and might be subject to a high incidence of whiplash. Demolition derby participants reported having undergone high numbers of car collisions in their lifetime at low to medium velocity collision speeds. In spite of this repeated traumatic exposure, only 3 of 40 respondents attributed any chronic neck pain to the sport itself, and those who did rated it mild.Overall significance of findings: The possible injury to the cervical spine resulting from motor vehicle collision appears to result in significantly less chronic pain in this setting where there is little or no medicolegal remuneration. Further study of this unique population of car drivers offers the possibility of greater understanding of whiplash disorder.Disclosures: No disclosures.Conflict of interest: No conflicts.

Inertial hydrodynamic particle–bubble interaction in flotation

The flotation rate of particles of different size and with different contact angles, provided by surface modification of the surface of broken quartz particles, with single monodisperse bubbles was investigated. A monotonic increase of collection efficiency was observed with increasing particle size at high particle hydrophobicity where attachment and stability are at a maximum. At smaller particle hydrophobicity the dependence of collection efficiency on particle diameter for E col is interpreted as a manifestation of particle rebound. At the moment of inertial impact a particle deforms the bubble surface, creates a water layer between particle and bubble and causes the particle to recoil from the bubble surface. These phenomena as well as the influence of centrifugal force were incorporated in a model of particle–bubble inertial hydrodynamic interaction complicated by bubble surface mobility. The purpose of this chapter is an experimental test of the model based on the Generalised Sutherland Equation (GSE), incorporating the angular dependence of attachment efficiency as well as the influence of hydrophobicity in the model. Attachment is impossible at the bubble front pole (angle θ=0) vicinity because of particle rebound and at θ> θ t ( θ t angle of tangency for the particle grazing trajectory) because of the influence of centrifugal forces. With increasing particle diameter and decreasing hydrophobicity, the attachment area confined between angles θ t and θ 1 ( θ 1< θ t) shrinks, causing the collection efficiency E col to decrease and determining the slope of the descending branch of the collection efficiency–particle diameter dependence. This is quantified by means of a modification of the GSE accounting for the shrinkage of the attachment area and attachment at repetitive collision.

Linking by collision initiated in the absence of preexisting reentrant tachycardia

Dynamic functional block in 1 limb of a reentrant circuit (“linking”) can be maintained by either repetitive interference or collision of successive impulses entering the circuit. Occurrence of linking by collision during attempted overdrive pacing of reentrant tachycardias accounts for the entrainment phenomenon. To investigate whether linking by collision can be initiated in the absence of preexisting tachycardia, a human reentrant circuit model was studied. The model consisted of the atrioventricular node and His-Purkinje system as anterograde limb and an electronic stimulator that served as “retrograde limb” by initiating a paced atrial impulse at a predetermined ventriculoatrial interval following each sensed ventricular depolarization. In 3 patients with intact ventriculoatrial conduction, “reentrant tachycardia” was initiated by a ventricular extrastimulus (V 2), which retrogradely blocked bilaterally below the His bundle. When this same V 2 was followed, instead, by a paced V 2V 2 train at a cycle length equal to the programmed ventriculoatrial interval of the “tachycardia,” it could be shown that each beat of the train not only “traversed” the simulated “retrograde limb” but also retrogradely collided with a prior circulating impulse in the anterograde limb of the circuit, thereby constituting linking by collision at a supra-Hisian level with inability of even a single “reentrant cycle” to be completed; “tachycardia” became manifest only after termination of the V 2V 2 train. The findings suggest the existence of a unique mechanism for initiation of certain clinical reentrant tachycardias during incremental pacing.

Trajectory studies of hyperthermal Xe scattering from GaAs(110)

We present the results of stochastic classical trajectory simulations of the scattering of a nonreactive gas, xenon, from a semiconductor surface of known structure and electronic and vibrational properties, GaAs(110). The range of incident energies considered is 1 to 8 eV, in order to make direct comparison with results of molecular beam experiments reported in the accompanying paper. We employed a 48-atom three-layer slab of GaAs, with periodic boundary conditions in two dimensions and frictional and stochastic forces in the third (surface normal) dimension. Pairwise additive Lennard-Jones potentials describe the gas–surface interaction. The calculations reproduce the large energy exchange and surprising structural sensitivity observed experimentally. Energy transfer is dominated by an initial binary interaction of the Xe with a single Ga or As atom. The repetitive collision nature of this binary encounter produces angular scattering patterns very similar to those obtained for scattering of light atoms, including ‘‘rainbow maxima.’’ The mechanism of energy deposition may have implications to ‘‘gentle’’ sputtering by neutral atom bombardment and to collision induced excitation of electron–hole pairs.