599 research outputs found
Prediction of Turbulent Shear Stresses through Dysfunctional Bileaflet Mechanical Heart Valves using Computational Fluid Dynamics
There are more than 300,000 heart valves implanted annually worldwide with
about 50% of them being mechanical valves. The heart valve replacement is often
a common treatment for severe valvular disease. However, valves may dysfunction
leading to adverse hemodynamic conditions. The current computational study
investigated the flow around a bileaflet mechanical heart valve at different
leaflet dysfunction levels of 0%, 50%, and 100%, and documented the relevant
flow characteristics such as vortical structures and turbulent shear stresses.
Studying the flow characteristics through these valves during their normal
operation and dysfunction can lead to better understanding of their
performance, possibly improved designs, and help identify conditions that may
increase the potential risk of blood cell damage. Results suggested that
maximum flow velocities increased with dysfunction from 2.05 to 4.49 ms-1 which
were accompanied by growing eddies and velocity fluctuations. These
fluctuations led to higher turbulent shear stresses from 90 to 800 N.m-2 as
dysfunctionality increased. These stress values exceeded the thresholds
corresponding to elevated risk of hemolysis and platelet activation. The
regions of elevated stresses were concentrated around and downstream of the
functional leaflet where high jet velocity and stronger helical structures
existed
Numerical Modeling of Pulse Wave Propagation in a Stenosed Artery using Two-Way Coupled Fluid Structure Interaction (FSI)
As the heart beats, it creates fluctuation in blood pressure leading to a
pulse wave that propagates by displacing the arterial wall. These waves travel
through the arterial tree and carry information about the medium that they
propagate through as well as information of the geometry of the arterial tree.
Pulse wave velocity (PWV) can be used as a non-invasive diagnostic tool to
study the functioning of cardiovascular system. A stenosis in an artery can
dampen the pulse wave leading to changes in the propagating pulse. Hence, PWV
analysis can be performed to detect a stenosed region in arteries. This paper
presents a numerical study of pulse wave propagation in a stenosed artery by
means of two-way coupled fluid structure interaction (FSI). The computational
model was validated by the comparison of the simulated PWV results with
theoretical values for a healthy artery. Propagation of the pulse waves in the
stenosed artery was compared with healthy case using spatiotemporal maps of
wall displacements. The analysis for PWV showed significance differences
between the healthy and stenosed arteries including damping of propagating
waves and generation of high wall displacements downstream the stenosis caused
by flow instabilities. This approach can be used to develop patient-specific
models that are capable of predicting PWV signatures associated with stenosis
changes. The knowledge gained from these models may increase utility of this
approach for managing patients at risk of stenosis occurrence
CEEME: compensating events based execution monitoring enforcement for Cyber-Physical Systems
Fundamentally, inherently observable events in Cyber-Physical Systems with tight coupling between cyber and physical components can result in a confidentiality violation. By observing how the physical elements react to cyber commands, adversaries can identify critical links in the system and force the cyber control algorithm to make erroneous decisions. Thus, there is a propensity for a breach in confidentiality leading to further attacks on availability or integrity. Due to the highly integrated nature of Cyber-Physical Systems, it is also extremely difficult to map the system semantics into a security framework under existing security models. The far-reaching objective of this research is to develop a science of selfobfuscating systems based on the composition of simple building blocks. A model of Nondeducibility composes the building blocks under Information Flow Security Properties. To this end, this work presents fundamental theories on external observability for basic regular networks and the novel concept of event compensation that can enforce Information Flow Security Properties at runtime --Abstract, page iii
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Magnetic Bunch Compression for a Compact Compton Source
A compact electron accelerator suitable for Compton source applications is in design at the Center for Accelerator Science at Old Dominion University and Jefferson Lab. Here we discuss two options for transverse magnetic bunch compression and final focus, each involving a 4-dipole chicane with M56 tunable over a range of 1.5-2.0m with independent tuning of final focus to interaction point \beta*=5mm. One design has no net bending, while the other has net bending of 90 degrees and is suitable for compact corner placement
Bunch Splitting Simulations for the JLEIC Ion Collider Ring
We describe the bunch splitting strategies for the proposed JLEIC ion collider ring at Jefferson Lab. This complex requires an unprecedented 9:6832 bunch splitting, performed in several stages. We outline the problem and current results, optimized with ESME including general parameterization of 1:2 bunch splitting for JLEIC parameters
Challenges in Applying the DCF Method for Investment Property Valuation in Sri Lanka: Insights from a Delphi Study
This study aimed to identify the challenges of implementing the Discounted Cash Flow (DCF) valuation method for investment properties in Sri Lanka. Through a mixed methodological approach involving the Delphi technique and structured interviews, insights were gathered from industry valuation experts via two Delphi rounds. The study's key findings were derived from the consensus reached among these experts, focusing on data- and valuer-bound factors. One of the primary challenges identified in the Sri Lankan context is the lack of training, which significantly hinders the knowledge and understanding required for implementing the DCF method effectively. Other significant hurdles included obtaining relevant data and accurately determining the discount rate. Imperfections in available data, the absence of a centralized digital data system, and challenges associated with increasing cash flows and market uncertainties also hindered the adoption of the DCF method in Sri Lanka. This study contributes to the existing DCF literature and provides valuable insights for practitioners and future researchers in the field of property valuation in Sri Lanka
Predicting the remaining life of timber bridges
This paper documents the current state of knowledge relating to the deterioration of timber bridges in Australia. The aim of this research, was to comprehend the present state of knowledge regarding maintenance of timber bridges and address any gap in knowledge. This involved: identifying key defects in timber, investigat-ing the inspection methods utilised to detect these faults and finding the preven-tive measures used to mitigate bridge deterioration. Enclosed are figures which demonstrate how simple industry practices and procedures implemented by each states’ governing authority can reduce these impacts and concludes with an em-pirical model for predicting the remaining lifespan of a bridge
Enforcing Information Flow Security Properties in Cyber-Physical Systems: A Generalized Framework Based on Compensation
This paper presents a general theory of event compensation as an information flow security enforcement mechanism for Cyber-Physical Systems (CPSs). The fundamental research problem being investigated is that externally observable events in modern CPSs have the propensity to divulge sensitive settings to adversaries, resulting in a confidentiality violation. This is a less studied yet emerging concern in modern system security. A viable method to mitigate such violations is to use information flow security based enforcement mechanisms since access control based security models cannot impose restrictions on information propagation. Further, the disjoint nature of security analysis is not appropriate for systems with highly integrated physical and cyber infrastructures. The proposed compensation based security framework is foundational work that unifies cyber and physical aspects of security through the shared semantics of information flow. A DC circuit example is presented to demonstrate this concept
The Influence of the Aortic Root Geometry on Flow Characteristics of a Bileaflet Mechanical Heart Valve
Bileaflet mechanical heart valves have one of the most successful valve
designs for more than 30 years. These valves are often used for aortic valve
replacement, where the geometry of the aortic root sinuses may vary due to
valvular disease and affect valve performance. Common geometrical sinus changes
may be due to valve stenosis and insufficiency. In the current study, the
effect of these geometrical changes on the mean flow and velocity fluctuations
downstream of the valve and aortic sinuses were investigated. The study focused
on the fully-open leaflet position where blood velocities are close to their
maximum. Simulation results were validated using previous experimental laser
Doppler anemometry (LDA) measurements. Results showed that as the stenosis and
insufficiency increased there were more flow separation and increased local
mean velocity downstream of the leaflets. In addition, the detected elevated
velocity fluctuations were associated with higher Reynolds shear stresses
levels, which may increase the chances of blood damage and platelet activation
and may lead to increased risk of blood clot formation
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