Virtual Time Research Articles

Preload Monitoring of Bolted L-Shaped Lap Joints Using Virtual Time Reversal Method.

L-shaped bolt lap joints are commonly used in aerospace and civil structures. However, bolt joints are frequently subjected to loosening, and this has a significant effect on the safety and reliability of these structures. Therefore, bolt preload monitoring is very important, especially at the early stage of loosening. In this paper, a virtual time reversal guided wave method is presented to monitor preload of bolted L-shaped lap joints accurately and simply. In this method, a referenced reemitting signal (RRS) is extracted from the bolted structure in fully tightened condition. Then the RRS is utilized as the excitation signal for the bolted structure in loosening states, and the normalized peak amplitude of refocused wave packet is used as the tightness index (TIA). The proposed method is experimentally validated by L-shaped bolt joints with single and multiple bolts. Moreover, the selections of guided wave frequency and tightness index are also discussed. The results demonstrate that the relationship between TIA and bolt preload is linear. The detection sensitivity is improved significantly compared with time reversal (TR) method, particularly when bolt loosening is at its embryo stage. The results also show that TR method is an effective method for detection of the number of loosening bolts.

Probabilistic motor sequence learning in a virtual reality serial reaction time task

The serial reaction time task is widely used to study learning and memory. The task is traditionally administered by showing target positions on a computer screen and collecting responses using a button box or keyboard. By comparing response times to random or sequenced items or by using different transition probabilities, various forms of learning can be studied. However, this traditional laboratory setting limits the number of possible experimental manipulations. Here, we present a virtual reality version of the serial reaction time task and show that learning effects emerge as expected despite the novel way in which responses are collected. We also show that response times are distributed as expected. The current experiment was conducted in a blank virtual reality room to verify these basic principles. For future applications, the technology can be used to modify the virtual reality environment in any conceivable way, permitting a wide range of previously impossible experimental manipulations.

A new method for production data analysis in shale gas reservoirs

For most shale gas wells, frequent shut-ins and nozzle size changing often occur, leading to significant discontinuities in production data. This study proposes a new production data analysis (PDA) method to address the abrupt change issue based on the virtual equivalent time. With the virtual equivalent time, the qualities of type curve matching and production data analysis can be improved when there are abrupt changes in production data. The mathematical model with a variable flow rate provides new definitions of normalized pseudopressure and pseudotime with consideration of the pressure dependent permeability for multi-fractured horizontal wells in shale gas reservoirs. Subsequently, the virtual equivalent time is calculated based on the average formation pressure to consider the abrupt changes of the production data, which also can reduce the time of superposition principle calculations. Duhamel's principle, Laplace transform and inversion, and Newman's method are employed to solve the PDA model, which is validated by analytical and numerical solutions. Then sensitivity analysis are performed on the dimensionless constrained axial modulus, which shows that the larger the dimensionless constrained axial modulus, the lower and later the curves. Finally, a field case study is carried out using the proposed method. The results show that by using the virtual equivalent time, the matching qualities are good, and the issue caused by abrupt changes is satisfactorily addressed. Therefore, it has great potential for estimating the formation parameters and predicting the well production performance more effectively and practically.

Analytical modeling of multi-source content delivery in information-centric networks

Routers in information-centric networking (ICN) are allowed to maintain the information of several repositories for every content item in their FIB. Therefore, content could be provided from different locations simultaneously, when a consumer asks for it. This multi-source property of content could benefit a network in terms of load balancing, congestion control, resource management and so on. Since exploiting multi-source content delivery has paramount importance, this work is an effort to investigate the performance of multi-source content delivery in the ICN. Existing works for modeling content transportation in ICN do not consider multi-source content delivery nor are straightforward enough to be used. Specially, the existing works do not consider the role of forwarding mechanisms in their models, an issue addressed in this paper. We develop a novel analytical model to evaluate the performance of multi-source content delivery in the ICN. We provide a recursive-style function to calculate virtual round-trip time (VRTT) between a consumer and the collection of providers which host the requested content by the consumer. Other metrics such as content transfer time can be obtained by the estimated VRTT. The proposed model was evaluated via both numerical and simulation methods. Packet-level experiments were performed by an ICN simulator (ndnSIM), and the result demonstrated the accuracy of the proposed model. Moreover, we discussed the distinct features of our approach for modeling the functionality of forwarding mechanisms in ICN, extensively.

Reconstructing Vehicle Trajectories to Support Travel Time Estimation

The primary objective of this study was to increase the sample size of public probe vehicle-based arterial travel time estimation. The complete methodology of increasing sample size using incomplete trajectory was built based on a k-Nearest Neighbors ( k-NN) regression algorithm. The virtual travel time of an incomplete trajectory was represented by similar complete trajectories. As incomplete trajectories were not used to calculate travel time in previous studies, the sample size of travel time estimation can be increased without collecting extra data. A case study was conducted on a major arterial in the city of Tucson, Arizona, including 13 links. In the case study, probe vehicle data were collected from a smartphone application used for navigation and guidance. The case study showed that the method could significantly increase link travel time samples, but there were still limitations. In addition, sensitivity analysis was conducted using leave-one-out cross-validation to verify the performance of the k-NN model under different parameters and input data. The data analysis showed that the algorithm performed differently under different parameters and input data. Our study suggested optimal parameters should be selected using a historical dataset before real-world application.

Quantifying human-environment interactions using videography in the context of infectious disease transmission.

Quantitative data on human-environment interactions are needed to fully understand infectious disease transmission processes and conduct accurate risk assessments. Interaction events occur during an individual's movement through, and contact with, the environment, and can be quantified using diverse methodologies. Methods that utilize videography, coupled with specialized software, can provide a permanent record of events, collect detailed interactions in high resolution, be reviewed for accuracy, capture events difficult to observe in real-time, and gather multiple concurrent phenomena. In the accompanying video, the use of specialized software to capture humanenvironment interactions for human exposure and disease transmission is highlighted. Use of videography, combined with specialized software, allows for the collection of accurate quantitative representations of human-environment interactions in high resolution. Two specialized programs include the Virtual Timing Device for the Personal Computer, which collects sequential microlevel activity time series of contact events and interactions, and LiveTrak, which is optimized to facilitate annotation of events in real-time. Opportunities to annotate behaviors at high resolution using these tools are promising, permitting detailed records that can be summarized to gain information on infectious disease transmission and incorporated into more complex models of human exposure and risk.

Modelling the Retreat of a Coastal Dune under Changing Winds

Gabarrou, S.; Le Cozannet, G.; Parteli, E.J.R.; Pedreros, R.; Guerber, E.; Millescamps, B.; Mallet, C., and Oliveros, C., 2018. Modelling the Retreat of a Coastal Dune under Changing Winds. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 166–170. Coconut Creek (Florida), ISSN 0749-0208.Coastal dunes can move in response to winds and cause serious hazard to human assets. Changes in wind patterns, potentially occurring as a consequence of climate change or variability, could affect rates of aeolian transport and migration velocity of coastal dunes. However, most of previous modeling studies were conducted assuming that aeolian bedforms are subject to constant wind velocities. This article presents a modeling of the mobility of the Dune du Pilat in Aquitaine, a coastal transverse dune exposed to winds of varying intensity. It applies well-established models and empirical formula for aeolian dune migration, including a model previously validated against measurements of real profiles of desert and coastal dunes. The average migration velocity of Pilat dune predicted by the models is about 3m/year, which is in good agreement with in-situ measurements. To test the response of transverse dune mobility to changing winds, virtual wind time series are generated using a stochastic model. Modelling experiments suggest that more frequent storms have less impacts than more intense winds. Due to the size of the Pilat Dune (altitude of about 100m), these result in moderate changes in the average dune velocity. This study shows that the approach of combining a stochastic model for winds with a morphodynamic dune model can provide valuable insight into how aeolian bedforms respond to changes in flow conditions potentially caused by climate change.

T115. Magnetoencephalographic imaging of ictal high-frequency oscillations (80–200 Hz) in medically refractory epilepsy

Specificity of ictal high-frequency oscillations (HFOs) in identifying epileptogenic abnormality is significant, compared to the spikes and interictal HFOs. The objectives of the study were to detect and to localize ictal HFOs by magnetoencephalography (MEG) for identifying the seizure onset zone (SOZ), evaluate the cortical excitability from preictal to ictal transition, and establish HFO concordance rates with other modalities and postsurgical resection. Sixty-seven patients with drug-resistant epilepsy had at least 1 spontaneous seizure each during MEG acquisition, and analysis was carried out on 20 seizures from 20 patients. Ictal MEG data were bandpass filtered (80–200 Hz) to visualize, review, and analyze the HFOs co-occurring with ictal spikes. Source montages were generated on both hemispheres, mean fast Fourier transform was computed on virtual time series for determining the preictal to ictal spectral power transition, and source reconstruction was performed with sLORETA and beamformers. The concordance rates of ictal MEG HFOs (SOZ) was estimated with 4 reference epileptogenic regions. In each subject, transient bursts of high-frequency oscillatory cycles, distinct from the background activity, were observed in the periictal continuum. Time–frequency analysis showed significant spectral power surge (85–160 Hz) during ictal state (P < .05) compared to preictal state, but there was no variation in the peak HFO frequencies (P > .05) for each subgroup and at each source montage. HFO source localization was consistent between algorithms (k = 0.8570 ± 0.138), with presumed epileptogenic zone (EZ) comparable to other modalities. In patients who underwent surgery (n = 6), MEG HFO SOZ was concordant with the presumed EZ HFOs could reliably be detected in the MEG periictal state, and its sources were accurately localized in almost 95% of the patients with medically refractory epilepsy. During preictal to ictal transition, HFOs exhibited dynamic augmentation in intrinsic epileptogenicity. Spatial overlap of ictal HFO sources was consistent with EZ determinants including the presumed EZ and the surgical resection area.

Analytical Stability Criterion in Haptic Rendering: The Role of Damping

A new closed-form equation for stability analysis of a haptic device is presented using frequency response function analysis via continuous time model of the system. Desired impedance of the virtual environment (VE), time delay, and zero-order hold are considered, beside the continuous model of the haptic device. The developed equations provide critical virtual damping and stiffness of the VE versus the mass and viscous friction of the haptic device, sampling time, and time delay. Unlike prior work in this field, the developed equations are valid without any limitation on the values of the time delay and virtual damping. It is shown that they cover available well-known equations in literature for special cases of small values of virtual damping and time delay. The resulting equations are of practical usefulness in many fields, such as surgery simulation for avoiding instability during virtual tool interaction with high-stiffness VEs. The proposed analytical derivation can also be used for studying the effect of operator, sensors, actuator dynamics, and velocity filtering. Simulation and experimental results on the KUKA light weight robot show that the proposed criterion can accurately predict the stability boundaries.

Multi-spacecraft observations and transport simulations of solar energetic particles for the May 17th 2012 event

Context. The injection, propagation and arrival of solar energetic particles (SEPs) during eruptive solar events is an important and current research topic of heliospheric physics. During the largest solar events, particles may have energies up to a few GeVs and sometimes even trigger ground-level enhancements (GLEs) at Earth. These large SEP events are best investigated through multi-spacecraft observations. Aims. We aim to study the first GLE-event of solar cycle 24, from 17th May 2012, using data from multiple spacecraft (SOHO, GOES, MSL, STEREO-A, STEREO-B and MESSENGER). These spacecraft are located throughout the inner heliosphere, at heliocentric distances between 0.34 and 1.5 astronomical units (au), covering nearly the whole range of heliospheric longitudes. Methods. We present and investigate sub-GeV proton time profiles for the event at several energy channels, obtained via different instruments aboard the above spacecraft. We investigated issues caused by magnetic connectivity, and present results of three-dimensional SEP propagation simulations. We gathered virtual time profiles and perform qualitative and quantitative comparisons with observations, assessed longitudinal injection and transport effects as well as peak intensities. Results. We distinguish different time profile shapes for well-connected and weakly connected observers, and find our onset time analysis to agree with this distinction. At select observers, we identify an additional low-energy component of Energetic Storm Particles (ESPs). Using well-connected observers for normalisation, our simulations are able to accurately recreate both time profile shapes and peak intensities at multiple observer locations. Conclusions. This synergetic approach combining numerical modelling with multi-spacecraft observations is crucial for understanding the propagation of SEPs within the interplanetary magnetic field. Our novel analysis provides valuable proof of the ability to simulate SEP propagation throughout the inner heliosphere, at a wide range of longitudes. Accurate simulations of SEP transport allow for better constraints of injection regions at the Sun, and thus, better understanding of acceleration processes.

Block iterative frequency-based lattice Boltzmann algorithm for microscale oscillatory flow

A recent article (Yong Shi and John E. Sader, Phys. Rev. E 81, 036706 (2010)) developed a linearized frequency-based lattice Boltzmann (LB) model to describe oscillatory Stokes flow in microelectromechanical systems (MEMS). Nonetheless, its numerical algorithm is formulated in the conventional time-marching form with addition of virtual time. In this article, we propose a different algorithm to solve the linearized LB model in the frequency domain using the block iteration scheme integrating the tri-diagonal matrix method, Gaussian Seidel-based alternative direction iteration and over relaxing technique. This change in the LB algorithm leads to direct modeling of microscale oscillatory flow in the frequency domain without mimicking a numerical time evolution. Through simulating the one-dimensional oscillatory Couette flow and two-dimensional flow around an oscillating circular cylinder, we examined numerical accuracy of this block iterative LB algorithm proposed in this article. Importantly, we also explored its computational efficiency in comparison to that of the conventional time-marching LB algorithm and several simplified block-iterative LB algorithms. Our results demonstrate the block iterative LB algorithm in this article is a useful alternative numerical solver exhibiting nearly 2nd order accuracy for simulating frequency-dependent oscillatory flow in MEMS. The simulations also reveal rich possibilities in construction of the block iterative LB algorithm through combining the LB theory in the frequency domain with advanced CFD numerical techniques.

Performance evaluation of IEEE 802.15.4 with real time queueing analysis

To provide a tool for performance evaluation of IEEE 802.15.4 with sleep mode enabled, a novel model based on real time queueing analysis is proposed in this paper. A low-rate wireless personal area network (LR-WPAN), composed of multiple nodes which send packets to the coordinator, is considered. The queueing behaviour of IEEE 802.15.4 node with sleep mode enabled differs from others because the packet arrivals in sleep period accumulate at the beginning of the active period, which makes a heavier load in the beginning than at any other time. This model analyses this behaviour by dividing the active portion of the superframe into backoff slots and then using an embedded discrete-time Markov chain model. The concept of virtual service time is introduced into this model which makes the proposed queueing model novel and different from typical ones. The accuracy of the proposed model is validated by Monte Carlo simulations in existing typical application scenarios, which indicates that the proposed queueing model can accurately evaluate the performance of IEEE 802.15.4 in the context of the application scenarios described in the simulations.

Integration of computer-assisted fracture reduction system and a hybrid 3-DOF-RPS mechanism for assisting the orthopedic surgery

In this paper, we present study to integrate virtual fracture bone reduction simulation tool with a novel hybrid 3-DOF-RPS external fixator to relocate back bone fragments into their anatomically original position. A 3D model of fractured bone was reconstructed and manipulated using 3D design and modeling software, PhysiGuide. The virtual reduction system was applied to reduce a bilateral femoral shaft fracture type 32-A3. Measurement data from fracture reduction and fixation stages were implemented to manipulate the manipulator pose in patient’s clinical case. The experimental result presents that by merging both of those techniques will give more possibilities to reduce virtual bone reduction time, improve facial and shortest healing treatment.

Temporal Integration of Emulation and Network Simulators on Linux Multiprocessors

Integration of emulation and simulation in virtual time requires that emulated execution bursts be ascribed a duration in virtual time and that emulated execution and simulation executions be coordinated within this common virtual time basis. This article shows how the open-source tool TimeKeeper for coordinating emulations in virtual time can be integrated with three different existing software emulations/simulations (CORE, Mininet, and EMANE) and with two existing network simulators (ns-3 and S3F). The integration does not require modification to those tools. However, the information that TimeKeeper needs to administer these emulations has to be extracted from each. We discuss the issues and challenges we encounter there, and the solutions. The S3F integration is specialized and shows how we can treat bursts of emulated execution just like an event handler in a discrete-event simulation. Through these case studies, we show the impact that the time dilation factor has on available resources, execution time, and fidelity of causality and that deleterious behaviors suffered under best-effort management of emulation processes can be corrected by integration with TimeKeeper. The key contribution is that we have shown how, using TimeKeeper, it is possible to bring virtual time to many existing emulators without needing to change them.

Recording of Coincidence Signals in a Software Medium

A virtual coincidence unit, a virtual counter and a virtual timer were developed via a software in this study. Different frequency signals supplied from a function generator were sent to a real coincidence unit. The same signals were transmitted to the virtual coincidence unit via a digitizer. Output signals of the real and virtual coincidence units were counted by real and the virtual counters. Count periods were set through real and virtual timers. For different count periods, coincidence counts obtained from the virtual counter were compared with those of the real one. It was seen that obtained results were highly compatible with each other for low frequency (25 – 55 Hz) signals.

Dynamic Cost-Load Aware Service Broker Load Balancing in Virtualization Environment

Cloud computing offers diverse services such as servers, storage and applications to the end users as per the requirement with pay per use concept. These services are provided by the cloud service provider through datacenters with high configured servers. However, Datacenters which are providing these services are geographically distributed gets overloaded if multiple requests arrive simultaneously from the same location. The complexity involved in guaranteeing these services to achieve reliable operation under peak loads is a challenging task. Many requests with variable workloads may affects the performance of data centers. Cloud Service provider has to make decisions for allocating resources for these requests by choosing appropriate Datacenter. In this regard, the Service broker act as mediator between the service provider and cloud user for choosing the best datacenter. Certain policies are used by a service broker to direct the user requests to appropriate data center. Therefore, a heuristic based approach called dynamic cost-load aware service brokering policy is proposed to reduce overall processing time, Cost of Virtual Machine and response time by assigning user requests in efficient method.

Improved-Precision Time Synchronization Protocol for WSNs Based on Averaging Consensus Control

Accurate and simple time synchronization is a crucial requirement for decentralized wireless sensor networks (WSNs) to function well specially in harsh environments. The averaging consensus control protocol (ACCP), that uses single hop communication, performs well in such environments. This paper provides rigorous analysis of its time synchronization error which is by basic definition the difference between the gateway node time and the average virtual times of the other nodes in the network. Also, the paper presents a modified synchronizer protocol of the ACCP that further improves the precision of the time synchronization in WSNs. The improved time precision protocol is achieved reducing the forced response part of the time synchronization error. The proposed protocol is compared by simulations and by experiments and shown to achieve a synchronization error that is orders of magnitude less than these resulted from other known protocols which are. Finally, the proposed protocol was found to utilize the least memory and consume the least energy relative to some existing protocols.

Chirp technique Based under Water Communication Systems

In this paper, by using the multi chirp rate signals the multi user communication is achieved in the underwater acoustic systems. In general, the time, frequency is assigned for the single users for the communication systems like TDMA (Time division multiple access), CDMA (Code division multiple access) and FDMA (Frequency division multiple access). In proposed work, the multi chirp rate is assigned for the multi users for the marine systems. By using FFT (Fractional Fast Fourier Transform), the multi chirp rate signals are separated. Inter symbol interference (ISI) is caused by the multi patch is improved by the Virtual Time Reverse Mirror (VTRM).

Cloud-Centric and Logically Isolated Virtual Network Environment Based on Software-Defined Wide Area Network

Recent development of distributed cloud environments requires advanced network infrastructure in order to facilitate network automation, virtualization, high performance data transfer, and secured access of end-to-end resources across regional boundaries. In order to meet these innovative cloud networking requirements, software-defined wide area network (SD-WAN) is primarily demanded to converge distributed cloud resources (e.g., virtual machines (VMs)) in a programmable and intelligent manner over distant networks. Therefore, this paper proposes a logically isolated networking scheme designed to integrate distributed cloud resources to dynamic and on-demand virtual networking over SD-WAN. The performance evaluation and experimental results of the proposed scheme indicate that virtual network convergence time is minimized in two different network models such as: (1) an operating OpenFlow-oriented SD-WAN infrastructure (KREONET-S) which is deployed on the advanced national research network in Korea, and (2) Mininet-based experimental and emulated networks.

An enhanced authentication scheme in mobile RFID system

The popularization of wireless networks and mobile applications has increased the importance of RFID technologies. However, since wireless networks does not guarantee transmission channel security, putting private user information at risk for unintentional disclosure. Previous research has introduced a security mechanism to provide privacy and authentication. This mechanism is based on quadratic residue, does not require a secure channel and fits EPC Class-1 Gen-2 specifications. However, this mechanism cannot resist replay attacks, and lacks an efficient means of its server is not able to find determining validating values, making it difficult to implement. This paper proposes an improvement scheme that uses virtual IDs and time parameters. It does not need a secure channel, fits EPC Class-1 Gen-2 specifications, is resistant to replay attacks, and can efficiently find validation information. The proposed scheme is applied to mobile devices as a proof of concept for use in wireless/mobile RFID systems.