1,720,997 research outputs found
Internal waves in fluid flows. Possible coexistence with turbulence
Full text linkWaves in fluid flows represents the underlying theme of this research work. Wave interactions in fluid flows are part of multidisciplinary physics. It is known that many ideas and phenomena recur in such apparently diverse fields, as solar physics, meteorology, oceanography, aeronautical and hydraulic engineering, optics, and population dynamics. In extreme synthesis, waves in fluids include, on the one hand, surface and internal waves, their evolution, interaction and associated wave-driven mean flows; on the other hand, phenomena related to nonlinear hydrodynamic stability and, in particular, those leading to the onset of turbulence. Close similarities and key differences exist between these two classes of phenomena. In the hope to get hints on aspects of a potential overall vision, this study considers two different systems located at the opposite limits of the range of existing physical fluid flow situations: first, sheared parallel continuum flows - perfect incompressibility and charge neutrality - second, the solar wind - extreme rarefaction and electrical conductivity. Therefore, the activity carried out during the doctoral period consists of two parts. The first is focused on the propagation properties of small internal waves in parallel flows. This work was partly carried out in the framework of a MISTI-Seeds MITOR project proposed by Prof. D. Tordella (PoliTo) and Prof. G. Staffilani (MIT) on the long term interaction in fluid flows. The second part regards the analysis of solar-wind fluctuations from in situ measurements by the Voyagers spacecrafts at the edge of the heliosphere. This work was supported by a second MISTI-Seeds MITOR project, proposed by D. Tordella (PoliTo), J. D. Richardson (MIT, Kavli Institute), with the collaboration of M. Opher (BU)
Dispersive-to-nondispersive transition and phase-velocity transient for linear waves in plane wake and channel flows
Full text linkExistence of non-dispersion niches of long perturbation waves in the plane Poiseuille flow. Impact on wave packets morphology
Full text linkWe consider the dispersion of 3D wavy perturbations in the plane Poiseuille flow. We focus on the wavenumbers-Reynolds numbers map. By considering the long-term evolution of these linear traveling waves, we found a sub-region nested in the dispersive part of the map where dispersion is abruptly inhibited. This region is observed at the bottom right dial of the map (Re>29840 and k<0.35) and includes non-dispersive waves moving as the basic flow. Two other regions were observed with a dispersion substantially different with respect to the surroundings. In one case, the dispersion level measured as the difference between the group speed and the phase speed is enhanced. In the other, the dispersion level is damped. Such regions contain waves with higher phase speed than waves in the surrounding area of the parameter space. This study builds on a previous one (PRE 93, 2016) where, by moving in the map from small to high wavenumbers, we show that a dispersive-to-nondispersive transition occurs in sheared flows under fixed flow conditions. The transition takes place at a specific wavenumber threshold, which splits the map in two main regions: the lower one, the dispersive one, being that hosting the nested regions above. An inference on the morphology of wave packets is presented
Voyager observations of magnetic field turbulence in the far heliosheath and in the local interstellar medium. Power spectra from high-resolution data
Full text linkVoyager 2 (V2) is in the heliosheath (HS) since the termination shock crossing in Aug 2007, while V1 is in the local interstellar medium (LISM) since Aug 2012. The fundamental processes at the basis of the observed solar wind's disordered fluctuations are still unclear. Open points regard the nature of compressive turbulence within the sectored and unipolar HS in proximity of the heliopause and in the LISM. Possibility that MHD waves give origin to turbulence in the LISM has been recently suggested by Zank, Du & Hunana [APJ 842,2017]. However, addressing these issues is a challenging task because of the data sparsity. We provide the first collection of magnetic field power spectra computed in consecutive periods after 2009 from 48s resolution data in the HS (V1, V2) and in the LISM (V1). A description of the fluctuations evolution with the heliocentric distance is given in terms of spectral decay law and anisotropy. In the HS, our observations are consistent with an anisotropic mainly inertial cascade in the frequency range [ 10^-4 , 5 ⋅ 10^-5 ] Hz, with spectral slopes from -1.7 to -1.9. Larger scales may be featured by wavy fluctuations leading to a f ^-1 decay for f < 10^-5 Hz. LISM spectra show a f^-1 power law in the whole observed range [ 10^-7 , 10^-2 ] Hz
Perturbation enstrophy decay in Poiseuille and Couette flows according to Synge's method
Full text linkIn this work we derive the conditions for no enstrophy growth for bidimensional perturbations in the plane Couette and Poiseuille flows. We follow the method of vorticity proposed by Synge in 1938 (see the Semi-Centennial Puplication of the Amer. Math. Soc., equation 12.13, and the more detailed version in the Proc. of the Fifth Inter. Congress of Applied Mechanics, pages 326-332), which is actually based on the analysis of the spatially averaged enstrophy. We find that the limit curve in the perturbation wavenumber-Reynolds number map differs from the limit for no energy growth (see e.g. Reddy 1993). In particular, the absolute stability region for the enstrophy is wider than that of the kinetic energy, and the maximum Reynolds number giving the monotonic enstrophy decay, at all wavenumbers, is 155 and 80 for the Poiseuille and Couette flows, respectively. It should be noted that in past literature the energy-based analysis was preferred to Synge's enstrophy analysis. This, possibly, for two reasons: the low diffusivity of the 1938 Vth ICAM proceedings and the objectively very complicated analytical treatment required. Nevertheless, the potentiality of this method seems high and therefore it is interesting nowadays to exploit it by means of the symbolic calculus
PHILOFLUID sito di gruppo di ricerca, webhosting del Politecnico di Torino
No full textPHILOFLUID Research Group website at the Politecnico di Torino describes the group research activity in a wide range of topics related to: turbulent flows, stratified fluid fields, hydrodynamic stability and collective behaviour of travelling waves in sheared flows, numerical and laboratory simulation of hypersonic jets, astropysics in the laboratory: mid-term stellar jets simulation, analysis of data on solar wind and related magnetic field. A few related applications are: subgrid scale (LES, SGS) modelling, turbulent mixing, dynamics of cloud-clear air interfaces, energy transport by travelling waves in microchannels, wall and wake flows, under-expanded jets, collimation of astrophysical jets as seen in an Earth laboratory under partial physical similitude. The research activities are performed in collaboration with several national and international partners. We are located at the Department of Mechanical and Aerospace Engineering (DIMEAS) of the Politecnico of Torin
Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations
Full text linkPlasma velocity and magnetic field measurements from the Voyager 2 mission are used to study solar wind turbulence in the slow solar wind at two different heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to provide information on the radial evolution of this turbulence. The magnetic helicity and the cross-helicity, which express the correlation between the plasma velocity and the magnetic field, are used to characterize the turbulence. Wave number spectra are computed by means of the Taylor hypothesis applied to time resolved single point Voyager 2 measurements. The overall picture we get is complex and difficult to interpret. A substantial decrease of the cross-helicity at smaller scales (over 1-3 h of observation) with increasing heliocentric distance is observed. At 5 AU the only peak in the probability density of the normalized residual energy is negative, near −0.5. At 29 AU the probability density becomes doubly peaked, with a negative peak at −0.5 and a smaller peak at a positive values of about 0.7. A decrease of the cross-helicity for increasing heliocentric distance is observed, together with a reduction of the unbalance towards the magnetic energy of the fluctuations. For the smaller scales, at 29 AU the normalized polarization is small and positive on average (about 0.1), but it is zero at 5 AU. For the larger scales, the polarization is low and positive at 5 AU (average around 0.1) while it is negative (around −0.15) at 29 AU
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