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BackgroundEpidemiological contact tracing is a powerful tool to rapidly detect SARS-CoV-2 infection in persons with a close contact history with COVID-19-affected patients. However, it remains unclear whom and when should be PCR tested among the close contact subjects.MethodsWe retrospectively analyzed 817 close contact subjects, including 144 potentially SARS-CoV-2-infected persons. The patient characteristics and contact type, duration between the date of the close contact and specimen sampling, and PCR test results in PCR positive and negative persons were compared.ResultsWe found that male gender {adjusted odds ratio 1.747 [95% confidence interval (CI) 1.180–2.608]}, age ≥ 60 [1.749 (95% CI 1.07–2.812)], and household contact [2.14 (95% CI 1.388–3.371)] are independent risk factors for close contact SARS-CoV-2 infection. Symptomatic subjects were predicted 6.179 (95% CI 3.985–9.61) times more likely to be infected compared to asymptomatic ones. We could observe PCR test positivity between days 1 and 17 after close contact. However, no subject could be found with a Ct-value

Nanofluid, a liquid containing choroidal dispersion of nanometer-sized solid particles, enables high-temperature bodies to be cooled more rapidly during quenching than in pure liquid. Drastic rise of the minimum heat flux temperature (TMHF) caused by the layer of nanoparticles formed on the heat transfer surface is the key phenomenon of heat transfer enhancement. In the present work, using alumina, silica, and titanium dioxide as the nanoparticle materials, quenching experiments were carried out to explore the mechanisms of the rise of TMHF in nanofluids; stainless steel 304 and Inconel 718 were used as the materials of the specimen and distilled water was used as the base liquid. In the experiments, TMHF increased in all the nanofluids but the increasing rate was dependent significantly on the nanoparticle material and the nanoparticle layer thickness. To elucidate the mechanisms of the heat transfer enhancement, the relations of TMHF with the three basic surface parameters of roughness, wettability, and wickability were examined but no clear relationship was found. When the metal specimen of higher thermal conductivity is covered with the nanoparticle layer of lower thermal conductivity, the contact temperature during quenching should decrease and the contact duration would be dependent on the thermal properties and thickness of the nanoparticle layer. Assuming that TMHF rises with an increase in the contact duration, a new model describing the rise of TMHF in the nanofluid was proposed.

Due to the simplicity of implementation, user-initiated Wi-Fi offloading becomes more and more popular, and naturally the benefits of users become the main optimization goal. We notice the intercontact and intra-contact durations could be uncertain in reality by reason of the user mobility and network dynamics. The two uncertain durations can cause great impact on the benefit of users; however, they were either ignored or simply assumed to be deterministic in most previous works. In this paper, for the first time, we study Wi-Fi offloading problem with uncertain contact durations. The aim is to guarantee the benefit of users (delay and payment) without damaging operator's benefit (amount of the offloaded traffic) at the same time. We propose a multi-armed bandit (MAB)-based online offloading scheme (MABOO) to solve the problem and prove the near-optimality of MABOO in terms of the utility theoretically. Extensive simulations show that MABOO always approaches the optimal scheme and achieves higher utility as well as offloads more traffic compared with the minimal payment and on-the-spot-offloading schemes.

Background: A turbopump is an important and critical component in the liquid rocket engine assembly where a small deviation in the system parameters such as unbalance level, dimensional tolerances, rotor–stator clearance, etc. can have significant influence on its dynamic behavior and may lead to various faults such as rub. Hence, a detailed rotordynamic study is essential for the entire operating speed range. Purpose: Most of the earlier studies on rotor-stator rub were carried out for simplified rotor models and its implementation on a rotor assembly such as turbopump will be of immense practical relevance for researchers and engineers. Contact mechanics-based Lagrange multiplier method is used to enforce the contact constraints that incorporate compliant stator model which respond to the axial and lateral forces during contact. Methods: A full-scale cryogenic engine turbopump rotor system consisting of turbines, impellers, inducer, bearings and seals is modelled using FE framework to investigate the rotor–stator contact interaction during coast-up. The stator model the a circumferentially positioned beams around rotor disc and investigated using more realistic contact mechanics-based Lagrange multiplier method. Results: The eigenvalue analysis on the FE model gives the rotor critical speeds and the corresponding vibration modes. The constraints imposed by the stator beams and tangential friction due to rub cause a continuous phase shift in the rotor vibration during contact. The physical insights into the distinct rotor vibration features in the two lateral directions are discussed in detail. The influence of clearance level on the observed backward whirl nature of the turbine is established through a full-spectrum analysis. Conclusion: The tighter clearance between rotor and stator gives rise to dominating presence of backward whirl components that may be detrimental to the fatigue life of the shaft due to stress reversal. Duration of the contact interaction is found to decrease significantly with increase in rotor–stator clearance and decrease in friction coefficient. With tighter clearances, the influence of stator stiffness becomes more significant and the rotor response during contact exhibits nonlinear and complex vibration. [ABSTRACT FROM AUTHOR]

Developments in air-coupled testing hardware in impact-echo (IE) tests have enabled new levels of scanning tests for concrete bridge decks. A tooth gear IE system has been developed using tooth gears as impactors and microelectromechanical systems (MEMS). Since the tooth gear moves and generates impacts itself, this system collects a large amount of test data across the field continuously. The contact duration of two different tooth gears is evaluated and the contact mechanism is compared to a conventional steel ball impactor by a high-speed camera. The data measurements were carried out on concrete slabs, where artificial delaminations were embedded at different depths. Based on our IE experiments, reducing the pitch or increasing the number of teeth was required to decrease the contact duration and generate the thickness mode frequency from deep delaminations. Rapidly obtained time domain data were transferred to the frequency-time domain using spectrograms to identify the dominant frequency band of the signal set. The results show that the developed system enabled us to acquire high-quality data during air-coupled IE tests and spectrogram analysis provided meaningful frequency information and verified its repeatability. [ABSTRACT FROM AUTHOR]

The coefficient of restitution and contact duration of the impact of bronze, brass and steel spheres on large (extended in length and width) glass plates have been determined experimentally. The results clearly exhibit different amounts of energy dissipation mainly due to plastic deformation of the spheres and bending of the plate. Influence of the material properties and size of the spheres, impact velocity, thickness of the plate, propagation of elastic waves in the plate, and bending of the plate on the energy dissipation and contact duration have been studied and are discussed in detail. The results have been evaluated with adequate theoretical models from the literature, describing the elastic and elastic-perfectly plastic impacts, namely the models of Hertz, Zener, Hunter, Thornton and Walton-Braun. Unlabelled Image • Experimental investigation of the impact behavior of spheres on plates. • Measurement of the coefficient of restitution and contact duration. • Measurement of the yield velocity during impact. • Model-based evaluation of the experiments. [ABSTRACT FROM AUTHOR]

Most of the routing algorithm developed for DTN are encounter based algorithms in which some nodes meet with high frequency but for short period of time. Therefore, encounters cannot guarantee transmission of large number of messages during a contact. These methods also suffer from high overheads caused by the unnecessary transmissions and long delays due to suboptimal choices for relay nodes. Therefore, in this paper, we considered a duration based utility instead of frequency to achieve high transmission throughput between nodes. Further, to reduce the overheads, we propose to send multiple copies of a message towards the destination using direction based routing. In addition, we also used threshold based buffer management scheme to reduce overheads and improve packet delivery probability. The simulation results show that our proposed strategy reduces overheads as compared to the previous existing strategies while maintaining comparable delivery probability. [ABSTRACT FROM AUTHOR]

• Inter-particle contact evolution of a sheared sand was studied using X-ray μ CT. • The duration and stress-transmission characteristics of inter-particle contacts were studied. • The buckling of stress-transmission contacts was quantified. This paper presents the use of X-ray micro-tomography (X-ray μ CT) and image processing and analysis techniques to investigate the stress transmission and buckling of inter-particle contacts within a granular material. A triaxial testing of a miniature Leighton Buzzard sand (LBS) sample was carried out with full-field in-situ X-ray μ CT scanning. High-spatial-resolution CT images of the sample were acquired at several loading stages of the test. Image processing and analysis techniques were used to quantify the inter-particle contact evolution (contact gain, contact loss and contact movement), fabric, contact duration and buckling of stress-transmission contacts based on the CT images. The results indicated that contact gain and loss, and contact movement played two competing roles in determining the overall fabric evolution of the sample. Contacts with a longer duration were more likely to orient in the major principal stress direction and form a stress-transmission contact network. A gradual decrease in the buckling rate of the stress-transmission contacts was observed outside of the shear band, and a relatively stable buckling rate was observed within the shear band during the shear. The results suggested that jamming occurred outside of the shear band and that unjamming occurred within the shear band. [ABSTRACT FROM AUTHOR]

In Opportunistic networks (ONs), buffer management is critical to improve the message exchanging efficiency due to the limited storage space and transmission bandwidth at the wireless edge. Current solutions make message scheduling and drop policy based on assumptions that messages can always been forwarded in a single contact, and all node pairs have the same contact rates. However, such ideal assumptions are invalid for realistic mobility traces of hand-held. Recent studies show that the single contact duration is limited and the mobility of nodes is heterogeneous in reality. In this paper, a buffer management strategy based on contact duration and heterogeneous mobility is proposed to improve the efficiency of buffer policy in the practical applications. We mainly focus on the minimization of the total expected delivery delay for all messages in ONs with resource constraints. Using the global network information including existing copies of message in the network, the distribution of pair-wise inter-contact time and contact duration between nodes, we develop a function to compute per-message utility which reflects the contribution of single message to the total expected delivery delay. Messages are scheduled or dropped according to their utilities. Simulation results show that our proposed strategy not only achieves lower delivery delay than mainstream strategies, but also keeps a high delivery ratio and a low network overhead. [ABSTRACT FROM AUTHOR]

Developments in air-coupled testing hardware in impact-echo (IE) tests have enabled new levels of scanning tests for concrete bridge decks. A tooth gear IE system has been developed using tooth gears as impactors and microelectromechanical systems (MEMS). Since the tooth gear moves and generates impacts itself, this system collects a large amount of test data across the field continuously. The contact duration of two different tooth gears is evaluated and the contact mechanism is compared to a conventional steel ball impactor by a high-speed camera. The data measurements were carried out on concrete slabs, where artificial delaminations were embedded at different depths. Based on our IE experiments, reducing the pitch or increasing the number of teeth was required to decrease the contact duration and generate the thickness mode frequency from deep delaminations. Rapidly obtained time domain data were transferred to the frequency-time domain using spectrograms to identify the dominant frequency band of the signal set. The results show that the developed system enabled us to acquire high-quality data during air-coupled IE tests and spectrogram analysis provided meaningful frequency information and verified its repeatability.

Delay-tolerant networks (DTNs) are sparse mobile ad hoc networks, in which there is typically no complete path between the source and destination. Anycast is an important group communication paradigm for numerous DTN applications such as resource discovery and information exchange in emergency or crisis situations. Unlike unicast and multicast, which have been studied extensively in DTNs, few prior works have addressed the DTN anycast routing problem. Furthermore, they often ignore the time constraint and assume long contact durations in formulating the relay selection strategy. In this paper, we study a single-copy time-constrained anycast (TCA) routing under short contact duration. We address two key issues: (1) to which next hop relay node should messages be forwarded and (2) in which order should messages be forwarded. To reduce the transmission cost, we select relay nodes from both current and past contacts based on the one-hop and two-hop delivery probabilities, respectively. We derive the delivery probability from the distribution of inter-contact time and contact duration time. We address the case of exponential and Pareto distribution, which are the most popular assumptions in literature. For the message scheduling, messages with the highest delivery probability are prioritized to be transmitted first. Extensive simulation results based on Cabspotting and MIT Reality traces show that our scheme can achieve up to 29% higher delivery rate, 24% lower delay, and 36% lower transmission cost compared to other anycast routing strategies. [ABSTRACT FROM AUTHOR]

Carbon fiber-reinforced thermoplastic (CFRTP) composites are gaining popularity in the manufacturing industry of lightweight automobiles. Common composite defects (e.g. voids and delamination) often occur inside CFRTP composites due to their inappropriate manufacturing process and long-term service. In this study, an instrumented tapping system was designed to evaluate the health condition of CFRTP composites by controlling the input force and velocity. The effective mathematical expressions of the contact duration and amplitude of the interactive force were derived to quantify the local stiffness of the tapping area. The amplitude of the interactive force was introduced innovatively as a significant evaluation parameter for a non-closed force-time curve with the time axis due to the influence of constraint condition and coupling effect of the modal shapes excited during tapping. The contact duration of the interactive force over the defective region was longer than that over the sound region. The amplitude of the interactive force applied on the defective area was observed to be lower than that on the sound area. In addition, the applicability and sensitivity of the amplitude of the interactive force was also investigated by changing the dimensions and locations of defects. [ABSTRACT FROM AUTHOR]

CD226 is an activating receptor expressed on natural killer (NK) cells, CD8+ T cells, and other immune cells. Upon binding to its ligands expressed on target cells, CD226 activates intracellular signaling that triggers cytokine production and degranulation in NK cells. However, the role of CD226 in contact dynamics between NK and cancer cells has remained unclear. Our time-lapse images showed that individual wild-type CD226+ NK cells contacted B16F10 melanoma cells for 23.7 min, but Cd226−/− NK cells only for 12.8 min, although both NK cell subsets showed equal contact frequency over 4 h. On the surface of B16F10 cells, CD226+ cells stayed at the same site with oscillating movement (named stable contact), while Cd226−/− NK cells moved around at a velocity of 4 μm/min (named unstable contact). Consequently, Cd226−/− NK cells did not kill B16F10 cells in vitro and did not inhibit their metastasis into the lung in vivo. Taken together, our data demonstrate that CD226 enables prolonged stable interaction between NK and cancer cells, which is needed for efficient killing of cancer cells.

To cope with increasing vehicular traffic and extensive application demands on vehicular cellular networks, opportunistic vehicular networks are used to disseminate mobile data by high-capacity device-to-device communication contacts when vehicles come into the communication range of each other, which offloads significant traffic from the cellular network. The current opportunistic vehicular data transmission models often do not take into account the fact that the contact duration is usually very short, due to the high mobility of vehicles, which will limit the amount of data that can be transmitted during one opportunistic contact. In this paper, we consider a mobile data offloading system that integrates cellular network and vehicular opportunistic communications. Based on this proposed system, we establish a mathematical framework to study the problem of contact-aware optimal resource allocation for mobile data offloading by the explicit consideration of the contact duration. Based on theoretical analysis of this problem, we derive an optimal scheme for mobile data offloading to optimally allocate the network resources in terms of opportunistic contacts and offloading helpers’ storage. By carrying out extensive simulation using two realistic urban vehicular traces, we demonstrate the effectiveness of our contact duration aware optimal offloading scheme, in comparison with a wide range of existing schemes. [ABSTRACT FROM AUTHOR]

This paper presents the results from experiments and numerical simulations on the ballistic impact of 10 mm thick aramid fabric-epoxy composite laminates by a 7.62 mm armor piercing projectile at varying impact velocities. Post perforated residual velocity (RV), contact duration of projectile with the target and ballistic limit (BL) of composite were simulated using a finite element code HyperWorks-Radioss. Interaction of projectile with composite laminates was captured by high speed video. The predicted ballistic parameter from simulation compared well with the precision experimental results. The simulated energy and stress distribution during impact of projectile on composite laminate showed marked difference with variation in impact velocities (SV). The magnitude and duration of stress as well as the contact force was found to increase when projectile impacted at lower SV thus enhancing the extent of delaminating and the core damage area and the trend was reversed for higher SV. The findings corroborate the bullet inflicted damage of composite laminates experimentally determined by ultrasonic C-scan. [ABSTRACT FROM AUTHOR]

The soft-sphere model design of the discrete element method is based on types of dynamic systems for determining the contact forces. These forces are computed considering the solution of equation of motion for the overlap between the particles during the contact. The dynamic systems can be modeled as linear or nonlinear mass-spring-damper. In the present work we derive two new approximate relations for determining the damping coefficient and duration of contact for a specific non-linear soft-sphere contact model where the contact force is continuous at the start and end of the contact. The proposed relations are applied in three different problems: single freely falling particle, 3D bubbling fluidized bed and 3D hopper. All results are obtained using the MFIX computational code and are compared with literature data showing good agreement. In these simulations, when we compare the nonlinear contact force models, the proposed approximate relations reduced the computational time. This behavior is caused because the proposed contact model simulates better the contact process between particles. [ABSTRACT FROM AUTHOR]

The Delay Tolerant Network (DTN) is a novel Wireless Sensor Network architecture for an opportunistic network environment, in which environment end-to-end connection cannot be set up constantly between source and destination nodes pairs. In this paper, we have proposed a novel routing algorithm based on a hybrid of message delivery probability and message redundancy to reduce the communication overhead while keeping the high message delivery ratio. In this algorithm, the message delivery probability is calculated by the combined impact of meeting frequency and length of contact duration. Further, the maximum number of copies of the message is designated in the source node, and the forwarding task of message copies is assigned to relay nodes based on the pattern of a binary tree, so that multi-path parallel transmission can be implemented on message forwarding. Simulated results showed that the proposed routing algorithm can achieve a higher efficiency of message delivery than the related existing routing algorithms and it can also reduce the communication overhead significantly in general DTNs. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents new experimental techniques utilizing a smart debris device for direct measurement of the impact acceleration forces associated with tsunami-borne debris that impact inland structures. The resulting experimental data will lead to advanced predictive capabilities of such forces for use in design guidelines. The measured debris acceleration data were used to calculate impact forces (mass × acceleration). An image processing technique was used to detect the debris impact angle. The debris impact tests were conducted using a disc-shaped smart debris device with masses of 550, 800 and 1000 g. For calculation of the debris force it was found necessary to include the mass of entrained water. The impact acceleration was found to be a function of debris mass, velocity, and contact duration. An equation is developed to allow estimation of the debris velocity for a known distance between the debris pick-up location by a tsunami and the structure. [ABSTRACT FROM PUBLISHER]

The recent popularization of hand-held mobile devices, such as smartphones, enables the inter-connectivity among mobile users without the support of Internet infrastructure. When mobile users move and contact each other opportunistically, they form a delay tolerant network (DTN), which can be exploited to share data among them. Data replication is one of the common techniques for such data sharing. However, the unstable network topology and limited contact duration in DTNs make it difficult to directly apply traditional data replication schemes. In this paper, we recognize the deficiency of existing data replication schemes which treat the complete data item as the replication unit, and propose to replicate data at the packet level using erasure coding techniques. Our study consists of two cases based on the operating spectrum: unlicensed spectrum and licensed spectrum. For both cases, we analytically formulate the data replication problem as a mixed integer programming problem and propose a practical algorithm which operates in a fully distributed manner. Extensive simulations on both synthetic and realistic traces show that our scheme outperforms other existing replication schemes in terms of successful data retrieval probability in various scenarios. [ABSTRACT FROM PUBLISHER]

An experimental investigation has been conducted to collect fundamental data and to develop more understanding of the effect of steel reinforcement distribution on the dynamic response of reinforced concrete plates. Five high strength concrete (HSC) plates are tested using free-fall low-velocity impact technique. A total of 10 impact tests are conducted, including two tests on each specimen. The impact loads are applied at the mid-point of the HSC plates by dropping a 475 kg steel weight from a constant height of 4.15 m. Two parameters are investigated namely: the main bottom steel reinforcement ratio (1.0, 2.0, and 3.0%); and the steel reinforcement arrangement (single or doubly reinforced plates). The top reinforcement of all doubly reinforced plates is kept constant as shrinkage reinforcement of 10 mm diameter spaced at 210 mm. The experimental results are evaluated focusing on the impact force characteristics and the impact behaviors of reinforced concrete plates. The test program was successful in providing a simple method for validating impact test setup using impulse–momentum theorem. Results showed that the change of reinforcement ratio and/or reinforcement arrangement has no significant effect on impulse and absorbed energy values for same impact loading condition. Additionally crack pattern and failure mode are found to be more dependent on the reinforcement arrangement rather than reinforcement ratio. [ABSTRACT FROM AUTHOR]

The paper analyses the impact of the road micro-profile on the duration and the type of the vehicle wheel contact with the road surface driving at different speed. The selected vehicle bicycle model describes vertical displacements of front and rear wheels and their suspension as well as the impact of the vehicle body motion and longitudinal oscillation. International Roughness Index (IRI) and micro-profile irregularities of the road section analysed in the paper were identified using specialized road testing equipment. The experimental investigations measuring the vehicle suspension displacement and the body acceleration were carried out. Frequency characteristics of suspension motion and regularities of vertical movement of the wheel were identified after dividing the investigated road section according to driving modes. The analysis into the wheel contact with the road surface and identified correlations enable to determine the vehicle stability on selected quality roads.