In cardiovascular flows, blood transport has been revealed using the Lagrangian coherent structures (LCS). Through a geometric approach, the extracted LCS highlight mixing, stagnation, and elevated shear stress regions. In geophysical flows, graph-theoretic approaches are used to represent fluid transport as a complex flow network. By using classical graph measures, one can extract the LCS along with direct measures of local dispersion and mixing. Also, from the adjacency matrix of the flow network, we can identify the coherent sets in the flow where the fluid within each set is minimally mixed with that in other sets. Notably, the geometric approach can only detect the borders of such regions. This study shows the first application of complex networks analysis to instantaneous planar velocity fields (acquired via PIV) downstream different arrangements of healthy and repaired mitral valves. Using the in- and out-degrees of the transport matrix, the instantaneous local mixing and dispersion are highlighted. Moreover, the LCS are revealed by computing the discrete finite time entropy of the network. Finally, the flow transport matrix is partitioned using a fuzzy c-means clustering algorithm to reveal the flow coherent sets which can better reveal fluid transport mechanisms.    

Abstract

Introduction

Study aims were to compare hemodynamics and viscous energy dissipation (VED) in 3D printed mitral valves–one replicating a normal valve and the other a valve with severe mitral annular calcification (MAC). Patients with severe MAC develop transmitral gradients, without the commissural fusion typifying rheumatic mitral stenosis (MS), and may have symptoms similar to classical MS. A proposed mechanism relates to VED due to disturbed blood flow through the diseased valve into the ventricle.

Methods

A silicone model of a normal mitral valve (MV) was created using a transesophageal echocardiography dataset. 3D printed calcium phantoms were incorporated into a second valve model to replicate severe MAC. The synthetic MVs were tested in a left heart duplicator under rest and exercise conditions. Fine particles were suspended in a water/glycerol blood analogue for particle image velocimetry calculation of VED.

Results

Catheter mean transmitral gradients were slightly higher in the MAC valve compared to the normal MV, both at rest (3.2 vs. 1.3 mm Hg) and with exercise (5.9 vs. 5.0 mm Hg); Doppler gradients were 2.7 vs. 2.1 mm Hg at rest and 9.9 vs 8.2 mm Hg with exercise. VED was similar between the two valves at rest. During exercise, VED increased to a greater extent for the MAC valve (240%) versus the normal valve (127%).

Conclusion

MAC MS is associated with slightly increased transmitral gradients but markedly increased VED during exercise. These energy losses may contribute to the exercise intolerance and exertional dyspnea present in MAC patients.

Abstract

Purpose

Aortic valve replacement remains the only viable solution for symptomatic patients with severe aortic valve stenosis. Despite their improved design and long history of successful operation, bileaflet mechanical heart valves are still associated with post-operative complications leading to valve dysfunction. Thus, the flow dynamics can be highly disturbed downstream of the dysfunctional valve.

Methods

In this in vitro study, the flow dynamics downstream of healthy and dysfunctional bileaflet mechanical heart valves have been investigated using particle image velocimetry measurements. Proper orthogonal decomposition of the velocity field has been performed in order to explore the coherent flow features in the ascending aorta in the presence of a dysfunctional bileaflet mechanical heart valve.

Results

The ability of proper orthogonal decomposition derived metrics to differentiate between heathy and dysfunctional cases is reported. Moreover, reduced-order modeling using proper orthogonal decomposition is thoroughly investigated not only for the velocity field but also for higher order flow characteristics such as time average wall shear stress, oscillatory shear index and viscous energy dissipation.

Conclusion

Considering these results, proper orthogonal decomposition can provide a rapid binary classifier to evaluate if the bileaflet mechanical valve deviates from its normal operating conditions. Moreover, the study shows that the size of the reduced-order model depends on which flow parameter is required to be reconstructed.

In cardiovascular flows, Lagrangian coherent structures have been used to explore the skeleton of blood transport. Revealing these transport barriers is instrumental to quantify the mixing and stagnation of blood as well as to highlight locations of elevated strain rate on blood elements. Nevertheless, the clinical use of Lagrangian coherent structures in cardiovascular flows is rarely reported due largely to its nonintuitive nature and computational expense. Here, we explore a recently developed approach called “Lagrangian descriptors,” which quantifies the finite time Euclidean arc length of Lagrangian trajectories released from a grid of initial positions. Moreover, the finite time arc lengths of a set of trajectories capture signatures of Lagrangian coherent structures computed from the same initial condition. Remarkably, the Lagrangian descriptors approach has the most rapid computational performance among all its Lagrangian counterparts. In this work, we explore the application of Lagrangian descriptors for the first time in cardiovascular flows. For this purpose, we consider two in vitro flow models studied previously by our group: flow in an abdominal aortic aneurysm and that in a healthy left ventricle. In particular, we will demonstrate the ability of the Lagrangian descriptors approach to reveal Lagrangian coherent structures computed via the classical geometrical approach, though at a significantly reduced computational cost.

Abstract

Purpose

Identification of coherent structures in cardiovascular flows is crucial to describe the transport and mixing of blood. Coherent structures can highlight locations where minimal blood mixing takes place, thus, potential thrombus formation can be expected thither. Graph-based approaches have recently been introduced in order to describe fluid transport and mixing between multiple Lagrangian trajectories, where each trajectory serves as a node that can be connected to another trajectory based on their relative distance during the course of time.

Methods

In this study, we compute the Lagrangian trajectories from in vitro planar instantaneous velocity fields in two models of abdominal aortic aneurysms, (AAA) namely single bulge and bi-lobed. Then, we construct unweighted and undirected graphs based on the pairwise distance of Lagrangian trajectories. We report local measures of the graph namely the degree and the clustering coefficient. We also perform spectral clustering of the graph Laplacian to extract the flow coherent sets.

Results

Local graph measures reveal fluid regions of high mixing such as vortex boundaries. Through spectral clustering, the fluid is partitioned into a reduced number of coherent sets where within each set, inner mixing of fluid is maximized while the fluid mixing between different coherent sets is minimized. The approach reveals multiple coherent sets adjacent to the AAA bulge that have sustained this adjacency to the wall through their coherent motion during one cardiac cycle.

Conclusion

Identifying coherent sets enables tracking their transport during the cardiac cycle and identify their role in the flow dynamics. Moreover, the size and the transport of the long-residing coherent sets inside the AAA bulges can be deduced which may aid in predicting thrombus formation at such a location.

Voltage source Multilevel Inverters (MLIs) are vital components for medium voltage and high-power applications due to their advantages like modularity and better power quality. However, the number of components used is significant. In this paper, an improved asymmetrical multilevel inverter topology is proposed producing 17-levels output voltage utilizing two dc sources. The circuit is developed to reduce the number of isolated dc-sources used without reducing output levels. The circuit utilizes six two-quadrant switches, three four-quadrant switches and four capacitors. The capacitors are self-balancing and do not require extra attention, i.e. the control system is simple for the proposed MLI. Detailed analysis of the topology under linear and non-linear loading conditions is carried out. Comparison with other similar topologies shows that the proposed topology is superior in device count, power quality, Total Standing Voltage (TSV), and cost factor. The performance of the topology is validated for different load conditions through MATLAB/Simulink environment and the prototype developed in the laboratory. Furthermore, thermal analysis of the circuit is done, and the losses are calculated via PLECS software. The topology offers a total harmonic distortion (THD) of 4.79% in the output voltage, with all the lower order harmonics being less than 5% complying with the IEEE standards.

In this work, we propose a high-quality control solution for islanded microgrids with multiparallel power converters; it uses a full state-variable direct model predictive control (FSV-DMPC) and has a simple structure. Unlike the conventional cascaded control loops, the proposed FSV-DMPC solution tracks the optimal reference generated by a robust droop loop using a unified cost function. This proposal enables the FSV-DMPC to be inserted into the entire control framework with plug-and-play capability; it is robust to parameter variations while also guaranteeing dynamics and stability. We conduct a deep analysis of the proposed approach, taking into account both the characteristics of the solution and the bounded stability of the system. Through comprehensive comparative studies with a classical double-loop linear controller, we validate that our solution achieves superior output voltage regulation during the load transients in terms of voltage error and settling time. Meanwhile, similar steady-state performances are accomplished for both methods. Finally, we verify our approach experimentally in different scenarios through a lab-constructed microgrid test bench. Experimental data confirm that the proposed approach achieves excellent steady-state and transient performances and obtains accurate load sharing.

A two-level voltage source inverter (2L-VSI) when working under overmodulation regulation potentially has low performance, especially during large reference or load changes. To address this problem, a new calculation method is proposed in this article by determining a new reference vector from the boundary of linear-modulation zone as a best vector. The reference vector is synthesized by one or two vectors on the boundary line. Therefore, this technique simplifies the calculations to obtain the duty cycles and uses these duty cycles to identify the overmodulation zones. The proposed method is applied to the modulated model predictive control (MMPC) and the space vector modulation, respectively, in a 2L-VSI. A comprehensive assessment mathematically proves that both methods are in consistence with each other. Simulations and experiments are carried out to show that the proposed methods can reduce the computational burden and implementation complexity and improve the performance of the 2L-VSI in terms of fast dynamic response and smooth transition between the linear-modulation zone and overmodulation zone.

Finite-control-set model predictive control (FCSMPC) techniques have been widely applied for power electronics, and motor drive. Furthermore, the principles of FCS-MPC have been extended to phase-locked loop (PLL), which called finiteposition-set PLL (FPS-PLL), for sensorless control of permanentmagnet synchronous generators (PMSGs) in wind turbine applications (WTAs). However, 64 iterations are essential to find the optimal rotor position, i.e., high computational burden. In this article, two computationally efficient (CE) FPS-PLLs are proposed for encoderless control of PMSGs in WTAs. The first CE-FPS-PLL1 reduces the number of iterations to 36 with slightly better accuracy than the FPS-PLL, while the second (novel) CE-FPS-PLL2 calls for only 24 iterations to find the best rotor position with significantly better accuracy than the FPS-PLL. The performance of the proposed CE-FPS-PLLs has been experimentally investigated, and compared with that of the FPS-PLL, and classical PLL using a 14.5-kW PMSG. Furthermore, the robustness of the proposed CE-FPS-PLLs is investigated against variations of the PMSG parameters.

 Electrospraying is a process that uses electrostatic force to break up a liquid into droplets by using a strong electric field. There are different modes of electrospraying depending on the electric field strength and the liquid flow rate for a specified liquid. The simple-jet mode is among these modes, which can produce monodisperse droplets. This article is aimed at analyzing electrospraying in simple-jet mode of a pesticide—solution issued from the nozzle of a capillary of the spray—system. The analysis includes the formation of a jet charged by conduction at low applied voltages (below corona onset) and by corona discharge at voltages higher than the corona onset value. The electric field distribution in the space between the charged jet and the target is calculated at voltages below and above the corona onset value. The disintegration of the jet into droplets is assessed to determine the jet length and radius as well as the charge and radius of droplets. The current–voltage characteristic of the spray system is calculated and checked experimentally. The agreement between the calculated and measured currents in the spray system at the same applied voltage is satisfactory.