Advances in computational science offer a principled pipeline for predictive modeling of cardiovascular flows and aspire to provide a valuable tool for monitoring, diagnostics and surgical planning. ...Such models can be nowadays deployed on large patient-specific topologies of systemic arterial networks and return detailed predictions on flow patterns, wall shear stresses, and pulse wave propagation. However, their success heavily relies on tedious pre-processing and calibration procedures that typically induce a significant computational cost, thus hampering their clinical applicability. In this work we put forth a machine learning framework that enables the seamless synthesis of non-invasive in-vivo measurement techniques and computational flow dynamics models derived from first physical principles. We illustrate this new paradigm by showing how one-dimensional models of pulsatile flow can be used to constrain the output of deep neural networks such that their predictions satisfy the conservation of mass and momentum principles. Once trained on noisy and scattered clinical data of flow and wall displacement, these networks can return physically consistent predictions for velocity, pressure and wall displacement pulse wave propagation, all without the need to employ conventional simulators. A simple post-processing of these outputs can also provide a relatively cheap and effective way for estimating Windkessel model parameters that are required for the calibration of traditional computational models. The effectiveness of the proposed techniques is demonstrated through a series of prototype benchmarks, as well as a realistic clinical case involving in-vivo measurements near the aorta/carotid bifurcation of a healthy human subject.
•We introduce physics-informed neural networks to solve conservation laws in graph topologies.•We show how non-dimensionalization and normalization can mitigate vanishing gradient pathologies.•For the first time, a physics-informed neural network is applied to real noisy clinical data.•Arterial pressure is predicted from MRI data of blood velocity and wall displacement.•Our model also allows for calibrating boundary conditions of conventional flow simulators.
Dramatic irregularities in the plasma density of the ionosphere, first discovered by their effects on radio wave propagation in 1938, despite decades of investigation, still remain puzzling. Their ...deleterious effects on radio wave communication, satellite command and control, and GPS navigation strongly motivate better understanding of their nature. Many aspects of such irregularities are now understood, but the mechanism(s) of their formation and their detailed nature remain a topic of great interest. In this work, detailed time resolved measurements of lightning generated waves sometimes show dispersionless, strongly attenuated propagation with substantial propagation delays. These characteristics of the electromagnetic wave propagation in the hypothetical two‐phase bubble/non‐bubble lower ionosphere parallel the characteristics of acoustic wave propagation through two‐phase liquid/vapor foams. This motivates the suggestion that the bottomside layer of the ionosphere may sometimes be foamy.
Plain Language Summary
Just as ocean waves breaking at the interface between sea and land produce copious bubbles and foam, recent satellite data suggests a similar phenomenon at the interface between neutral atmosphere and the charged plasma of the ionosphere. Lightning generated electromagnetic waves passing through the lower ionosphere observed by low altitude satellites are found to have the same characteristics as acoustic waves passing through foamy water. This hypothetical foam in the lower ionosphere apparently strongly absorbs some radio wave frequencies and seems to prevent most such waves from escaping the foam to pass through to the upper ionosphere.
Key Points
Dispersionless, highly attenuated, lightning generated electromagnetic waves are observed in the lower ionosphere
The propagation of these electromagnetic waves has characteristics of acoustic wave propagation through two‐phase foams
Such foamy plasma bubbles may cover approximately 85% of the bottomside of the equatorial nightside ionosphere
•Determination of crack tip position and FPZ in concrete based on CBM model and DIC technique.•Observation for the whole fatigue crack propagation process in concrete based on DIC method.•Discussion ...for the evolution laws of FPZ and non-cohesive crack.•Description for da/dN can be divided into descending and ascending segments.
A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bažant Crack Band Model (CBM) and digital image correlation (DIC) technique, which was described as: firstly, the critical crack opening displacement of concrete (wcr) was determined based on CBM and the basic mechanical properties; secondly, DIC method was used to obtain the displacement field on the surface of specimens; thirdly, the position where the crack opening displacement equaled to wcr was defined as the crack tip position. In the fatigue crack propagation tests, the whole fatigue crack was FPZ at the beginning of fatigue loading until the non-cohesive crack grew and the length of FPZ decreased. The length of FPZ tended to be stable when the number of loading cycles was close to half of the fatigue life. Fatigue crack propagation curves (a-N curves) were obtained and fitted by logistic function to reduce the influence of the data scattering. Fatigue crack propagation rate da/dN was separated into descending and ascending crack propagation functions based on the increment of crack Δa and stress intensity factor ΔK, respectively. The da/dN-Δa curves for the descending segment showed obvious scatter. Paris’ law was applied to describe fatigue crack propagation rate well for the ascending segment.
•The influence mechanism of the circular hole on crack propagation behaviour was studied.•The cause for the turning point during crack propagation was analyzed using AUTODYN software.•The effect of ...the circular hole on the dynamic propagation toughness (DPT) was measured.•The dynamic fracture toughness versus crack speeds were analyzed.
Hole defects and crack defects usually coexist in a real engineering rock masses, they together affect the structural security of rock masses. With the purpose of investigating the effect of circular hole on crack initiation and propagation process under dynamic loads, a circular opening specimen with straight crack cavity (COSSCC) model was proposed in this study and polymethyl methacrylate was chosen to make the specimens. A series of dynamic experiments were carried out utilizing a drop hammer impact test system (DHITS), crack speeds and crack initiation time were determined utilizing crack propagation gauge (CPG). The maximum principal stress field evolution in the crack evolution process was simulated using a modified finite element difference method software, and dynamic fracture toughness was calculated by a traditional finite element method software. Some significantly conclusions can be obtained: As the inclination angle θ of the circular hole is less than 10o, crack propagation trajectories will propagate to the surface of the circular hole; As the inclination angle θ of circular hole is 20o and 30o, crack propagation trajectories will turn in the direction of circular hole but not reach the circular hole; As circular hole inclination angle θ is 40o and 50o, the crack shows a pure mode I crack propagation. As crack propagates near the circular hole, crack propagation speeds increase significantly. The dynamic propagation toughness during crack propagation is not constant and is less than dynamic initiation toughness (DIT) and the dynamic propagate toughness (DPT) is inversely proportional to the crack speeds.
In this paper, we present and evaluate a new propagation model for heterogeneous networks. The designed model is multiband, multienvironment, and is usable for short and long distance. For this ...research, a measurement campaign was conducted in Tunis (Tunisia) using continuous wave analog technology. It concerns the most used bands (450, 850, 1800, 2100, and 2600 MHz) in rural, suburban, and urban environments. Measurements are split into two independent and random sets. The first one is used for model training, whereas the second is used for model validation. The new model is based on neural networks, uses back propagation algorithm, and obtains its inputs from Standard Propagation Model, to which we have added more parameters such as frequency, environment type, land use distribution, and diffraction loss. Model variables are computed from accurate Digital Terrain Model and Land Used maps with 2-m resolution. The statistical analysis has shown that the developed model is accurate as we obtained the following metrics: 0.235-dB absolute mean error, 6.850-dB standard deviation, and 85% correlation factor. The obtained simulation results are then compared to SPM and ITU-R P.1812-4 prediction, which are taken as reference to highlight the benefit of the new model.
This paper presents and analyzes the results of millimeter-wave 60-GHz frequency range propagation channel measurements that are performed in various indoor environments for continuous-route and ...direction-of-arrival (DOA) measurement campaigns. The statistical parameters of the propagation channel, such as the number of paths, the RMS delay spread, the path loss, and the shadowing, are inspected. Moreover, the interdependencies of different characteristics of the multipath channel are also investigated. A linear relationship between the number of paths and the delay spread is found, negative cross correlation between the shadow fading and the delay spread can be established, and an upper bound exponential model of the delay spread and the path loss is developed to estimate the worst case of the RMS delay spread at given path loss. Based on the DOA measurements that are carried out in a room line of sight (LOS) and in a corridor with both LOS and nonline-of-sight (NLOS) scenarios, radio-wave propagation mechanisms are studied. It is found that considering the direct wave and the first-order reflected waves from smooth surfaces is sufficient in the LOS cases. Transmission loss is very high; however, diffraction is found to be a significant propagation mechanism in NLOS propagation environments. The results can be used for the design of 60-GHz radio systems in short-range wireless applications.
The effects of rock inherent heterogeneity and grain size on hydraulic fracture initiation and propagation for different propagation regimes are investigated through two dimensional discrete element ...modeling. Random particle assembly is used to mimic rock inherent heterogeneity in the numerical model while regular particle assembly is used as the reference. The rock inherent heterogeneity mainly affects the hydraulic fracture net pressure in the viscosity dominated regime and the effect is more profound in the toughness dominated regime. In the toughness dominated regime, in addition to the increase of net pressure relative to the regular particle sample, the hydraulic fracture profiles in the random particle sample also show larger tortuosity and asymmetry caused by the local heterogeneity, and the fracture growth of one of the wings can be temporarily arrested. Numerical simulations show that the effective toughness of the random particle sample is larger than that of the regular particle sample. This is caused by tortuosity, in which case the net pressure in the random particle sample is also affected by the local geometrical arrangements of the particles. Also, the apparent toughness is influenced by the magnitude of initial stress, which comes in addition to the tensile strength of the contact bond and the particle radius. The effect of stress anisotropy has limited effect on the hydraulic fracture propagation for both the viscosity and toughness dominated regimes.
Metasurfaces (MSs) have been used pervasively to manipulate the wavefront of structured beams propagating in free space. However, they also enable us to create nontrivial waveguide boundaries which ...offer novel opportunities for the manipulation of the mode profile and propagation constant of electromagnetic waves confined in the space enclosed by them. Here, we study a waveguide that is formed by a general class of MSs, which are both electrically and magnetically polarizable, support multiple resonances and exhibit temporal and spatial dispersion. We investigate the opportunities offered by tailoring the temporal dispersion of the surface conductivities and study three different cases of spatial dispersion, namely, no dispersion, an ideal case of very specific dispersion of the form <inline-formula> <tex-math notation="LaTeX">\sigma _{se}\propto k_{t} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">\sigma _{sm}\propto 1/k_{t} </tex-math></inline-formula>, and the realistic dispersion of an actual physical implementation based on a metal-backed cut-wire unit cell. Both analytical derivations and full-wave numerical simulations are performed for addressing different scenarios and verifying the results. We show that the proposed metasurface waveguides can enable: 1) the ability to shape the mode profile from concave, to flat, to convex and 2) the possibility of both superluminal and subluminal propagation, as well as both normal and anomalous group velocity dispersion. An example application of controlling the mode profile along propagation is showcased, highlighting the potential of MSs as waveguide boundaries.
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