Marzio Piller

DNS study of turbulent transport at low Prandtl numbers in a channel flow

Direct numerical simulations of the velocity and temperature fields for turbulent flow in a channel are used to examine the influence of Prandtl number Pr on turbulent transport. The Reynolds number, based on the half-height of the channel and the friction velocity, is Re τ = 150. Prandtl numbers of 1.0, 0.3, 0.1, 0.05, 0.025 were studied. The bottom and the top walls were kept at constant temperatures of + T w and − T w . The influence of Pr on Reynolds transport, on the turbulent diffusivity, α τ , and on the spectral density function of the temperature fluctuations was studied. The observation that spatial variations of the ratio of the turbulent diffusivity to the value observed at Pr = 1.0 are not large is used to propose a method for calculating average temperature fields. The decrease in α τ with decreasing Pr is related to observations of the increased damping of high-wavenumber temperature fluctuations. Molecular conductivity, at smaller Pr , is pictured to act as a filter that renders high-frequency velocity fluctuations ineffective in transporting heat.

Direct Numerical Simulation of Heat Transfer in Converging–Diverging Wavy Channels

Corrugated walls are widely used as passive devices for heat and mass transfer enhancement; they are most effective when operated at transitional and turbulent Reynolds numbers. In the present study, direct numerical simulation is used to investigate the unsteady forced convection in sinusoidal, symmetric wavy channels. A novel numerical method is employed for the simulations; it is meant for fully developed flows in periodic ducts of prescribed wall temperature. The algorithm is free of iterative procedures; it accounts for the effects of streamwise diffusion and can be used for unsteady problems. Results of two simulations in the transitional regime for Reynolds numbers based on average duct height and average velocity of Re=481 and Re=872 are reported. Time averaged and instantaneous velocity and temperature fields together with second-order statistics are interpreted in order to describe the mechanism associated with heat transfer augmentation. Heat flux distributions locate the most active areas in heat transfer and reveal the effects of convective mixing. Slanted traveling waves of high temperature are identified; peak values of Nusselt number are attained when the high-temperature fluid of the waves reaches the converging walls

Compact finite volume schemes on boundary-fitted grids

The paper focuses on the development of a framework for high-order compact finite volume discretization of the three-dimensional scalar advection-diffusion equation. In order to deal with irregular domains, a coordinate transformation is applied between a curvilinear, non-orthogonal grid in the physical space and the computational space. Advective fluxes are computed by the fifth-order upwind scheme introduced by Pirozzoli S. Pirozzoli, Conservative hybrid compact-WENO schemes for shock-turbulence interaction, J. Comp. Phys. 178 (2002) 81] while the Coupled Derivative scheme M.H. Kobayashi, On a class of Pade finite volume methods, J. Comp. Phys. 156 (1999) 137] is used for the discretization of the diffusive fluxes.Numerical tests include unsteady diffusion over a distorted grid, linear free-surface gravity waves in a irregular domain and the advection of a scalar field. The proposed methodology attains high-order formal accuracy and shows very favorable resolution characteristics for the simulation of problems with a wide range of length scales.

Analysis of 3D computed tomographic imaging of ground-wall insulation for AC rotating machines

The X-ray Computed Tomography (X-CT) has been used to reconstruct the internal features of two types of ground-wall insulation for ac rotating machines. Four samples whose insulation was designed for a rated voltage of 11 kV, were extracted from different coils and scanned using X-CT. The 3D virtual volumes have been visually inspected stacking the tomographic slices to identify the internal morphological features that is, the different materials in the inner structure, the presence of minute voids, gas bubbles, anomalous densities, wrinkled or damaged tape layers. The tomographic volumes have been segmented into different gray-levels resorting to luminosity histograms with the purpose to differentiate the structural materials and extract only specific anomalies. In particular, the binary segmentation has been adopted to investigate the distributions of the void enclosures. The use of X-CT to investigate the internal structure of complex insulation systems, opens significant perspectives in improving the insulation manufacturing quality.

Characterisation of water-oil drainage process through a reservoir rock sample by digital core analysis

Oil-water relative permeability and capillary pressure of a water-wet digital core are computed using the finite-volume/volume of fluid method and mimicking traditional laboratory steady-state experiments carried out on a water saturated sample. Commercial software is used for immiscible multiphase fluid flow simulations, carried out on a low-cost multicore workstation, and the porescale data are upscaled to derive relative permeability and capillary pressure curves as function of water saturation. The digital sample is obtained by high resolution X-ray computed tomography scanning.