48 research outputs found
Pre-unstable set of multiple transient three-dimensional perturbation waves and the associated turbulent state in a shear flow
shear instability, small pertubations collective behaviou
Visibility analysis of boundary layer transition
We study the transition to turbulence in a flat plate boundary layer by means of visibility analysis of velocity time-series extracted across the flow domain. By taking into account the mutual visibility of sampled values, visibility graphs are constructed from each time series. The latter are, thus, transformed into a geometrical object, whose main features can be explored using measures typical of network science that provide a reduced order representation of the underlying flow properties. Using these metrics, we observe the evolution of the flow from laminarity to turbulence and the effects exerted by the free-stream turbulence. Different from other methods requiring an extensive amount of spatiotemporal data (e.g., full velocity field) or a set of parameters and thresholds arbitrarily chosen by the user, the present network-based approach is able to identify the onset markers for transition by means of the streamwise velocity time-series alone. Published under an exclusive license by AIP Publishing
Effects of Atrial Fibrillation on the Coronary Flow at Different Heart Rates: A Computational Approach
Atrial fibrillation (AF) has several effects on the cardiovascular system responses. This study focuses on the consequences of AF on the coronary blood flow, by exploiting a computational approach. 2000 heartbeat periods (RR) were simulated for 5 different mean heart rates (HR), ranging from 50 to 130 bpm. The resulting flow rate signals at the coronary level were analysed through a specific set of hemodynamic parameters. Three main results emerge during AF: (i) maximal coronary flow rates modify with HR, (ii) the coronary perfusion begins to be impaired when exceeding 90-110 bpm, and (iii) the coronary perfusion pressure is not a good estimate of the coronary blood flow at HRs higher than 90-110 bpm
An exploratory analysis of the transient and long-term behavior of small three-dimensional perturbations in the circular cylinder wake
An initial-value problem (IVP) for arbitrary small three-dimensional vorticity perturbations imposed on a free shear flow is considered. The viscous perturbation equations are then combined in terms of the vorticity and velocity, and are solved by means of a combined Laplace–Fourier transform in the plane normal to the basic flow. The perturbations can be uniform or damped along the mean flow direction. This treatment allows for a simplification of the governing equations such that it is possible to observe long transients, which can last hundreds time scales. This result would not be possible over an acceptable lapse of time by carrying out a direct numerical integration of the linearized Navier–Stokes equations. The exploration is done with respect to physical inputs as the angle of obliquity, the symmetry of the perturbation, and the streamwise damping rate. The base flow is an intermediate section of the growing two-dimensional circular cylinder wake where the entrainment process is still active. Two Reynolds numbers of the order of the critical value for the onset of the first instability are considered. The early transient evolution offers very different scenarios for which we present a summary for particular cases. For example, for amplified perturbations, we have observed two kinds of transients, namely (1) a monotone amplification and (2) a sequence of growth–decrease–final growth. In the latter case, if the initial condition is an asymmetric oblique or longitudinal perturbation, the transient clearly shows an initial oscillatory time scale. That increases moving downstream, and is different from the asymptotic value. Two periodic temporal patterns are thus present in the system. Furthermore, the more a perturbation is longitudinally confined, the more it is amplified in time. The long-term behavior of two-dimensional disturbances shows excellent agreement with a recent two-dimensional spatio-temporal multiscale model analysis and with laboratory data concerning the frequency and wave length of the parallel vortex shedding in the cylinder wake
Characterizing the cardiovascular functions during atrial fibrillation through lumped-parameter modeling
Atrial fibrillation (AF), causing irregular and rapid heartbeats, is the most common
arrhythmia. Due to the widespread impact on the population and the disabling symptoms
related to rapid heart rate, AF is a subject of growing interest under several aspects:
statistical analyses on the heartbeat distributions, risk factors, impact on quality of life,
correlation with other cardiac pathologies. However, several key points on the
consequences induced by AF on the cardiovascular system are still not completely
understood. The proposed work aims at quantifying the impact of AF on the most relevant
cardiovascular parameters by means of a lumped-parameter modeling, paying particular
attention to the stochastic nature of the irregular heartbeats and the reduced contractility of
the heart. The global response leads to a rather impressive overall agreement with the
clinical state-of-the-art measures regarding AF: reduced cardiac output with correlated
arterial hypotension, as well as higher left atrial volume and pressure values are some of
the most representative outcomes emerging during AF. Moreover, new insights on
hemodynamic parameters such as cardiac flow rates, which are difficult to measure and
almost never offered in literature, are here provided
Experimental investigation of vertical turbulent transport of a passive scalar in a boundary layer: Statistics and visibility graph analysis
The dynamics of a passive scalar plume in a turbulent boundary layer is
experimentally investigated via vertical turbulent transport time-series. Data
are acquired in a rough-wall turbulent boundary layer that develops in a
recirculating wind tunnel set-up. Two source sizes in an elevated position are
considered in order to investigate the influence of the emission conditions on
the plume dynamics. The analysis is focused on the effects of the meandering
motion and the relative dispersion. First, classical statistics are
investigated. We found that (in accordance with previous studies) the
meandering motion is the main responsible for differences in the variance and
intermittency, as well as the kurtosis and power spectral density, between the
two source sizes. On the contrary, the mean and the skewness are slightly
affected by the emission conditions. To characterize the temporal structure of
the turbulent transport series, the visibility algorithm is exploited to carry
out a complex network-based analysis. Two network metrics -- the average peak
occurrence and the assortativity coefficient -- are analysed, as they can
capture the temporal occurrence of extreme events and their relative intensity
in the series. The effects of the meandering motion and the relative dispersion
of the plume are discussed in the view of the network metrics, revealing that a
stronger meandering motion is associated with higher values of both the average
peak occurrence and the assortativity coefficient. The network-based analysis
advances the level of information of classical statistics, by characterizing
the impact of the emission conditions on the temporal structure of the signals
in terms of extreme events and their relative intensity. In this way, complex
networks provide -- through the evaluation of network metrics -- an effective
tool for time-series analysis of experimental data