IRIS Pol. Torinohttps://iris.polito.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Mon, 01 Jun 2020 16:45:15 GMT2020-06-01T16:45:15Z1071Turbulent anisotropic transport in a model cloud interfacehttp://hdl.handle.net/11583/2681657Titolo: Turbulent anisotropic transport in a model cloud interface
Abstract: Small-scale turbulence in cumuli can make a significant contribution to the droplet growth by coalescence (Grabowski & Wang, ARFM 2013, Wang & Grabowski Atm.Sci.Lett. 2009), with a consequent significant impact of the onset of warm rain. We study the transport of energy and water vapour at the interface between two turbulent regions with a different kinetic energy and vapour concentration in a stratified environment through direct numerical simulations by applying the Boussinesq approximation. This configuration (Iovieno et al. JoT 2014, Gallana et al. JoP:CS 2015) mimics the inhomogeneity observed at the edge of a cloud between a more energetic cloud and the calmer drier ambient. Both stable and unstable stratifications are considered. In presence of a stable stratification we show the onset of two intermittent layers which confine a low kinetic energy sublayer, which acts as a barrier and blocks entrainment. On the opposite situation, a fast growth of an
intermittent mixing layer enhances the entrainment till the bouyancy forces overcome inertial forces. We consider also the motion of droplets subject to the Stokes drag, gravitational acceleration and condensational growth. Their motion is coupled with the advection-diffusion of vapour in the air by using Mason’s model (Villancourt et al., J.Atm.Sci. 2000, Devenish et al. QJRMS 2012, Kumar et al. TCFD 2013). Unlikely most simulations, which use the "ghost collision approximation" (e.g. Ayala et al. 2008), we assume coalescence of the droplets. By means of this formulation, we will verigy our ansatz that the concentration of fluid elements across the cloud interface (shearless mixing layer) can ehnance collision rate and coalescence. We will show how the flow inhomogeneity
influences the droplet growth and their spatial distribution. Collision kernels and the spectral density function of particle size will be also given.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11583/26816572016-01-01T00:00:00ZTemperature fluctuations in numerical simulations of particle-laden isotropic turbulence with two-way couplinghttp://hdl.handle.net/11583/2696509Titolo: Temperature fluctuations in numerical simulations of particle-laden isotropic turbulence with two-way coupling
Abstract: The two-way coupling between fluid and particle temperature fluctuations in forced steady isotropic turbulence is investigated by means of direct numerical simulations with sub-Kolmogorov inertial particle Lagrangian tracking. The aim is to determine the sensitivity of the temperature field statistics to the presence of particles, parametrized by the Stokes number (St) and the thermal Stokes number (Stθ). As shown by Béc et al. PRL, 2014), the inertia of particles enhances mixing and heat transfer because particles cluster in regions of sharp temperature gradients out of the Lagrangian coherent structures. This trend strongly affects the small-scale dynamics of the temperature field and the dissipation rate of the temperature variance. Moreover, in presence of a two-way coupling, the finite mass and heat capacity of particles allows them to carry temperature increments across the scales of the flow influencing the temperature statistics at all scales. The resulting non trivial behavior is highlighted by means of two-particle statistics and two-point statistics of the temperature field.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11583/26965092017-01-01T00:00:00ZApplication of the nonuniform fast Fourier transform to the direct numerical simulation of two-way coupled particle laden flowshttp://hdl.handle.net/11583/2722444Titolo: Application of the nonuniform fast Fourier transform to the direct numerical simulation of two-way coupled particle laden flows
Abstract: We present the application of the Nonuniform Fast Fourier Transform (NUFFT) to the pseudo-spectral Eulerian–Lagrangian direct numerical simulation of particle-laden flows. In the two-way coupling regime, when the particle feedback on the flow is taken into account, a spectral method requires not only the interpolation of the flow fields at particle positions, but also the Fourier representation of the particle back-reaction on the flow fields on a regular grid. Even though the direct B-spline interpolation is a well-established tool, to the best of our knowledge the reverse projection scheme has never been used, replaced by less accurate linear reverse interpolation or Gaussian regularization. We propose to compute the particle momentum and temperature feedback on the flow by means of the forward NUFFT, while the backward NUFFT is used to perform the B-spline interpolation. Since the backward and forward transformations are symmetric and the (non local) convolution computed in physical space is removed in Fourier space, this procedure satisfies all constraints for a consistent interpolation scheme, and allows an efficient implementation of high-order interpolations. The resulting method is applied to the direct numerical simulation of a forced and isotropic turbulent flow with different particle Stokes numbers in the two-way coupling regime. A marked multifractal scaling is observed in the particle statistics, which implies that the feedback from the particles on the fields is far from being analytic and therefore only high-order methods, like the one here proposed, can provide an accurate representation.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11583/27224442018-01-01T00:00:00ZOn the condensational growth of droplets in isotropic turbulencehttp://hdl.handle.net/11583/2761034Titolo: On the condensational growth of droplets in isotropic turbulence
Abstract: The role of thermal inertia of droplets in the broadening of the droplet size distribution in homogeneous and isotropic turbulence is investigated. A new model for the condensational growth of water droplets, which takes into account the finite thermal relaxation time of droplets, is formulated. Results from direct numerical simulations with vanishing mean supersaturation in the two-way coupling regime show an increase of droplet size variance due to the increased fluctuations in the supersaturation field seen by each particle, which produce a differentiation of the growth conditions.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11583/27610342019-01-01T00:00:00ZMultiscale fluid–particle thermal interaction in isotropic turbulencehttp://hdl.handle.net/11583/2764613Titolo: Multiscale fluid–particle thermal interaction in isotropic turbulence
Abstract: We use direct numerical simulations to investigate the interaction between the temperature field of a fluid and the temperature of small particles suspended inthe flow, employing both one- and two-way thermal coupling, in a statisticallystationary, isotropic turbulent flow. Using statistical analysis, we investigate thisvariegated interaction at the different scales of the flow. We find that the varianceof the carrier flow temperature gradients decreases as the thermal response timeof the suspended particles is increased. The probability density function (PDF) ofthe carrier flow temperature gradients scales with its variance, while the PDF ofthe rate of change of the particle temperature, whose variance is associated withthe thermal dissipation due to the particles, does not scale in such a self-similarway. The modification of the fluid temperature field due to the particles is examinedby computing the particle concentration and particle heat fluxes conditioned on themagnitude of the local fluid temperature gradient. These statistics highlight thatthe particles cluster on the fluid temperature fronts, and the important role playedby the alignments of the particle velocity and the local fluid temperature gradient.The temperature structure functions, which characterize the temperature fluctuationsacross the scales of the flow, clearly show that the fluctuations of the carrier flowtemperature increments are monotonically suppressed in the two-way coupled regimeas the particle thermal response time is increased. Thermal caustics dominate theparticle temperature increments at small scales, that is, particles that come intocontact are likely to have very large differences in their temperatures. This is causedby the non-local thermal dynamics of the particles: the scaling exponents of theinertial particle temperature structure functions in the dissipation range reveal verystrong multifractal behaviour. Further insight is provided by the flux of temperatureincrements across the scales. Altogether, these results reveal a number of non-trivialeffects, with a number of important practical consequences
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11583/27646132019-01-01T00:00:00ZModulation of fluid temperature fluctuations by inertial particles in turbulencehttp://hdl.handle.net/11583/2761036Titolo: Modulation of fluid temperature fluctuations by inertial particles in turbulence
Abstract: We investigate the effect of the thermal two-way coupling between the fluid temperature field and point-like particles on the temperature statistics in homogeneous and isotropic steady turbulence by means of direct numerical simulations (DNS). Results show that, on average, particles dissipate the variance of the temperature fluctuations modulating the fluid temperature gradients. The temperature gradient normalized Probability Density Functions (PDF) collapse to a single curve for all Stokes and thermal Stokes numbers. On the other hand, the normalized PDF of the fluid temperature-particle temperature increments, which cause the thermal dissipation, shows a strong dependence on the thermal Stokes number. Inertial particles preferentially cluster in the region of sharp fluid temperature variation and easily cross these thin temperature gradient sheets causing large heat fluxes. The impact of the particle thermal feedback across the scales of the flow is examined through the fluid temperature and particle temperature Eulerian structure functions.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11583/27610362019-01-01T00:00:00ZIs vortex stretching the main cause of the turbulent energy cascade?http://hdl.handle.net/11583/2777652Titolo: Is vortex stretching the main cause of the turbulent energy cascade?
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11583/27776522019-01-01T00:00:00Z