1,721,083 research outputs found
Hydrodynamic linear stability of the two-dimensional bluff-body wake through modal analysis and initial-value problem formulation
Full text linkThe stability of the two-dimensional wake behind a circular cylinder - a free flow of general interest in differing applications (from aerodynamics to environmental physics and biology) - is studied by means of two different but complementary theoretical methods. The first part of the work is focused on the asymptotic evolution of disturbances described through modal analysis, a method which allows the determination of the asymptotic stability of a flow. The stability of the intermediate and far near-parallel wake is studied by means of a multiscale approach. The disturbance is defined as the local wavenumber at order zero in the longitudinal direction and is associated to a classical spatio-temporal WKBJ analysis. The inverse of the Reynolds number is taken as the small parameter for the multiscaling. It takes into account non-parallelism effects related to the transversal dynamics of the base flow. The first order corrections find absolute instability pockets in the first part of the intermediate wake (and not in the near wake, where the recirculating eddies are, as usually seen in literature in contrast with the near-parallelism hypothesis). These regions are present for Reynolds numbers larger than . That is in agreement with the general notion of critical Reynolds number for the onset of the first instability of about . In particular, for Re=50 and Re=100, the angular frequency obtained is in agreement with global data in literature concerning numerical and experimental results. The instability is convective throughout the domain. All the stability characteristics are vanishing in the far field, a fact that is independently confirmed by the asymptotic analysis of the Orr-Sommerfeld operator. Using asymptotic Navier-Stokes expansions for the wake inner field the entrainment evolution in the intermediate and far domain is evaluated in terms of asymptotic expansion. The maximum of entrainment is reached in the region where the absolute instability pockets are found. Downstream of this region the entrainment is decreasing and eventually vanishing in the far wake. This point confirms the validity of the multiscale approach. In the second part of the thesis the stability analysis is studied as an initial-value problem to observe the transient behaviour and the asymptotic state of perturbations initially imposed. The initial-value problem allows the formulation to be extended to the near-parallel flow configuration. The initial-value method is, however, less general than the modal analysis, since many parameters, such as the polar wavenumber, the spatial damping rate, the angle of obliquity and the symmetry of the perturbation, are involved. An exploratory analysis of these parameters permits the study of different transient configurations. Before the asymptotic (stable or unstable) state is reached, maxima and minima of the perturbation energy are observed for transients lasting hundreds of time scales. In the temporal asymptotics, the initial-value problem well reproduces modal results in terms of angular frequency and temporal growth rate. Moreover, for Reynolds numbers larger than the critical one (Re_{cr} = 47), the present method gives a good prediction, in terms of wavelength and pulsation, of the vortex shedding observed in experiments. In the framework of the initial-value problem formulation, a multiscale analysis for the stability of long waves is then proposed. Even to the lowest order, the multiscaling - whose small parameter is defined as the polar wavenumber - approximates sufficiently well the full problem solution with a relevant reduction of the computational cost. The two (modal and non-modal) analyses combined together lead to a quite complete description of the bluff-body wake stabilit
Pre-unstable set of multiple transient three-dimensional perturbation waves and the associated turbulent state in a shear flow
Full text linkshear instability, small pertubations collective behaviou
Different Impact of Heart Rate Variability in the Deep Cerebral and Central Hemodynamics at Rest: An in silico Investigation
Full text linkBackground: Heart rate variability (HRV), defined as the variability between consecutive heartbeats, is a surrogate measure of cardiac vagal tone. It is widely accepted that a decreased HRV is associated to several risk factors and cardiovascular diseases. However, a possible association between HRV and altered cerebral hemodynamics is still debated, suffering from HRV short-term measures and the paucity of high-resolution deep cerebral data. We propose a computational approach to evaluate the deep cerebral and central hemodynamics subject to physiological alterations of HRV in an ideal young healthy patient at rest.Methods: The cardiovascular-cerebral model is composed by electrical components able to reproduce the response of the different cardiovascular regions and their features. The model was validated over more than thirty studies and recently exploited to understand the hemodynamic mechanisms between cardiac arrythmia and cognitive deficit. Three configurations (baseline, increased HRV, and decreased HRV) are built based on the standard deviation (SDNN) of RR beats. For each configuration, 5,000 RR beats are simulated to investigate the occurrence of extreme values, alteration of the regular hemodynamics pattern, and variation of mean perfusion/pressure levels.Results: In the cerebral circulation, our results show that HRV has overall a stronger impact on pressure than flow rate mean values but similarly alters pressure and flow rate in terms of extreme events. By comparing reduced and increased HRV, this latter induces a higher probability of altered mean and extreme values, and is therefore more detrimental at distal cerebral level. On the contrary, at central level a decreased HRV induces a higher cardiac effort without improving the mechano-contractile performance, thus overall reducing the heart efficiency.Conclusions: Present results suggest that: (i) the increase of HRV per se does not seem to be sufficient to trigger a better cerebral hemodynamic response; (ii) by accounting for both central and cerebral circulations, the optimal HRV configuration is found at baseline. Given the relation inversely linking HRV and HR, the presence of this optimal condition can contribute to explain why the mean HR of the general population settles around the baseline value (70 bpm)
Hydrodynamics linear stability theory. A comparison between Orr-Sommerfeld modal and initial value problem analyses
No full textA review of multiscale 0D–1D computational modeling of coronary circulation with applications to cardiac arrhythmias
Full text linkComputational hemodynamics is becoming an increasingly important tool in clinical applications and surgical procedures involving the cardiovascular system. Aim of this review is to provide a compact summary of state of the art 0D–1D multiscale models of the arterial coronary system, with particular attention to applications related to cardiac arrhythmias, whose effects on the coronary circulation remain so far poorly understood. The focus on 0D–1D models only is motivated by the competitive computational cost, the reliability of the outcomes for the whole cardiovascular system, and the ability to directly account for cardiac arrhythmias. The analyzed studies show that cardiac arrhythmias by their own are able to promote significant alterations of the coronary hemodynamics, with a worse scenario as the mean heart rate (HR) increases. The present review can stimulate future investigation, both in computational and clinical research, devoted to the hemodynamic effects induced by cardiac arrhythmias on the coronary circulation
Energy spectrum in shear flows. A general pre-unstable largeset of multiple transient three dimensional waves and theturbulent state
No full textextended abstrac
New insights into spatial characterization of turbulent flows: a complex network-based analysis
Full text linkDespite much progress has been made, several mechanisms about turbulence dynamics are still unclear. We propose an innovative approach based on complex networks theory, which combines elements from graph theory and statistical physics, providing a powerful framework to investigate complex systems.The network is built on a forced isotropic turbulent field, by evaluating the temporal correlation of the kinetic energy for pairs of nodes within the Taylor microscale, λ. Among all the parameters analyzed, the degree centrality, k, is one of the most meaningful, representing how a node is linked to the others. We observe 3D patterns of high k values, which can be interpreted as regions of spatial coherence. The turbulent network exhibits typical behaviors of real and spatial networks (scale-free property). Similarly to other physical systems where complex networks successfully apply, our approach can give new insights for the spatial characterization of turbulence
An exploratory analysis of the transient and long-term behavior of small three-dimensional perturbations in the circular cylinder wake
Full text linkAn 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
Large-to-small scale frequency modulation analysis in wall-bounded turbulence via visibility networks
Full text linkScale interaction is studied in wall-bounded turbulence by focusing on the frequency modulation (FM) mechanism of the large scales on small-scale velocity fluctuations. Different from the analysis of amplitude modulation (AM), FM has been less investigated owing to the difficulty of developing robust tools for broadband signals. To tackle this issue, the natural visibility graph approach is proposed in this work to map the full velocity signals into complex networks. We show that the network degree centrality is able to capture the signal structure at local scales directly from the full signal, thereby quantifying FM. Velocity signals from numerically-simulated turbulent channel flows and an experimental turbulent boundary layer are investigated at different Reynolds numbers. A correction of Taylor's hypothesis for time series is proposed to overcome the overprediction of near-wall FM obtained when local mean velocity is used as the convective velocity. Results provide network-based evidence of the large-to-small FM features for all the three velocity components in the near-wall region, with a reversal mechanism emerging far from the wall. Additionally, scaling arguments, in view of the quasi-steady quasi-homogeneous hypothesis, are discussed, and a delay time between the large and small scales is detected that is very close to the characteristic time of the near-wall cycle. Results show that the visibility graph is a parameter-free tool that turns out to be effective and robust to detect FM in different configurations of wall-bounded turbulent flows. Based on the present findings, the visibility network-based approach can represent a reliable tool to systematically investigate scale interaction mechanisms in wall-bounded turbulence
- …
