The Hyperloop system is a new concept that allows a train to travel through a near-vacuum tunnel at transonic speeds. Aerodynamic drag is one of the most important factors in analyzing such systems. The blockage ratio (BR), pod speed/length, tube pressure, and temperature affect the aerodynamic drag, but the specific relationships between the drag and these parameters have not yet been comprehensively examined. In this study, we investigated the flow phenomena of a Hyperloop system, focusing on the effects of changes in the above parameters. Two-dimensional axisymmetric simulations were performed in a large parameter space covering various BR values (0.25, 0.36), pod lengths (10.75–86 m), pod speeds (50–350 m/s), tube pressures (~100–1000 Pa), and tube temperatures (275–325 K). As BR increased, the pressure drag was significantly affected. This is because of the smaller critical Mach number for a larger BR. As the pod length increased, the total drag and pressure drag did not change significantly, but there was a considerable influence on the friction drag. As the pod speed increased, strong shock waves occurred near the end of the pod. At this point, the flows around the pod were severely choked at both BR values, and the ratio of the pressure drag to the total drag converged to its saturation level. At tube pressures above 500 Pa, the friction drag increased significantly under the rapidly increased turbulence intensity near the pod surface. High tube temperatures increase the speed of sound, and this reduces the Mach number for the same pod speed, consequently delaying the onset of choking and reducing the aerodynamic drag. The results presented in this study are applicable to the fundamental design of the proposed Hyperloop system.
A complete understanding of cardiac mechanics requires knowledge of the mechanical properties of each of the tissues that comprise the heart, Data and constitutive relations are available for the nonlinear multiaxial behavior of epicardium and noncontracting myocardium, but there have been no comparable results for endocardium. In this paper, we present biaxial mechanical data for endocardium and epicardium excised from the same bovine hearts. The data reveal that these two membranes behave differently; endocardium exhibits a greater stiffness in the low-strain range. Moreover, quantification of endocardial behavior requires a seven-parameter, polynomial-exponential pseudostrain-energy function w, whereas epicardium can be described by a four-parameter exponential w. Comparison of our current findings with previous results on canine epicardium reveals further that canine and bovine epicardium behave similarly, although the latter is more extensible. Thus there appear to be marked species differences.
We studied 19 excised, passive rabbit left ventricular walls to delineate the forms of the strain-energy functions (W) for myocardium and epicardium, to quantify residual strains across the wall, and to investigate whether the mechanical behavior of the intact wall can be predicted by accounting for the above properties. The unloaded dimensions and the stress-strain responses to equibiaxial and uniaxial loadings were obtained first for the intact wall and then individually for the epicardium and myocardium. Results show that the previously proposed W for canine myocardium and epicardium are suitable. The unloaded intact wall has residual strains: the epicardium is stretched and the myocardium shrunk from their respective isolated, unloaded states. The predicted mechanical responses of the intact wall to biaxial loadings were inaccurate when the residual strains were not taken into account. Accounting for these, however, yielded reasonable predictions. Thus information on the unloaded reference state and properties of each portion is needed to accurately predict the behavior of the intact wall.
We investigated the effects of atherosclerosis in the carotid region on cerebral haemodynamics. A total of 15 stenosis cases following NASCET criteria were modelled using patient-specific medical image data and an open-source package, SimVascular. The formulation adopted the stabilised Petrov-Galerkin scheme with Newtonian and incompressible assumptions. The boundary conditions employed pulsatile inflow and three-element lumped Windkessel outlet conditions with a rigid wall assumption. We present transitions in the represented CoW during stenosis progression using three-dimensional aortic-cerebral vasculature for the first time. This was driven by the conserved total cerebral blood flow to 50% carotid stenosis (CS) (P-value, P > 0.05), which deteriorated during subsequent stages of CS (P < 0.01), and the effective collateral capability of the communicating arteries (CoAs) activated from a degree of 75% and above (P < 0.0001). The prevalence of 'complete' CoW peaked at 50% CS and then declined. Despite the collateral flow, the ipsilateral hemispheric perfusion was moderately reduced (P < 0.01), and the contralateral perfusion was conserved (P > 0.05), revealing the ineffectiveness of collateral capability of CoW at the extreme stages of CS. We identified bulk cerebral auto-regulation effects of the conventional Windkessel model, demonstrating accurate flow reduction in the stenosed artery.
Progression of carotid stenosis (CS) significantly reduces blood flow in the affected arteries and alters both proximal and distal hemodynamics. While conventional studies consider only the stenosis region for analysis, an extended larger arterial domain of aortic–cerebral vasculature is used to avoid artificial modeling of the inlet condition to the carotid region and facilitate automatic flow redistribution during CS progression. The fluid domain was constructed and simulated using an open-source package SimVascular, and three patient models with five stenosis cases each were created using medical images. Newtonian, incompressible, and rigid-wall conditions were assumed because of the high computational burden, and boundary conditions of the lumped Windkessel and pulsatile flow rate were implemented for the outlets and inlet, respectively. We present a novel index called circulation core fraction (CCF) to quantify and visualize the stenosis-driven hemodynamics; the CCF is developed from the benchmark backward-facing step problem and compares the representative recirculation to the total volume. Thus, CCF in the post-stenotic region increases during CS progression regardless of patient-specific features whereas that in the pre-stenotic region exhibits patient-specific nature despite the incremental tendency. Streamlines with custom sources show a helical vortex with recirculation and artery-wise flow streams that vary during CS progression. We also report transitional patterns in both the pulsatility index (PI) contours and Q-criterion, where the PI values shift from high–low–high to high–low–low across the stenosis, and the latter is nearly absent at 0% and 95% but mostly present at 50% and 75% CS.
A pandemic situation of COVID-19 has made a cost-minimization strategy one of the utmost priorities for commercial airliners. A relevant scheme may involve the minimization of both the fuel- and time-related costs, and the climb trajectories of both objectives were optimized to determine the optimum aircraft cruise altitude. The Hermite-Simpson method among the direct collocation methods was employed to discretize the problem domain. Novel approaches of terminal residual analysis (TRA), and a modified version, m-σ TRA, were proposed to determine the goals. The multi-objective cruise altitude (MOCA) was different by 2.5%, compared to the one statistically calculated from the commercial airliner data. The present methods, TRA and m-σ TRA were powerful tools in finding a solution to this complex problem. The value σ also worked as a transition criterion between a single- and multi-objective climb path to the cruise altitude. The exemplary MOCA was determined to be 10.91 and 11.97 km at σ = 1.1 and 2.0, respectively. The cost index (CI) varied during a flight, a more realistic approach than the one with constant CI. With validated results in this study, TRA and m-σ TRA may also be effective solutions to determine the multi-objective solutions in other complex fields.
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