The flow around aligned cylinders is an archetype for several industrial devices (rod structure of the nuclear reactors, compact heat exchangers for electronic components, pin-fins heat exchangers for micro-devices ) and environmental phenomena (diffusion process close to the vegetation). Cylinders produce instabilities in the flow structures that are very sensitive to the control parameters such as the inflow velocity, the spacing between the cylinders and the fluid viscosity. The instabilities leads the transport phenomena close to the cylinders and they affect the force, the thermal balance at their surface and the diffusion process. The strong velocity gradients in confined spaces make that the experimental analysis is difficult, while the numerical simulation appears to be a promising tool for this purpose.

In this paper an accurate numerical method has been used to verify the influence of the spool velocity on the performance of a directional hydraulic valve (4/3, closed center): the flow during the opening phase of the valve has been solved by Direct Numerical Simulation (DNS), using an Immersed-Boundary (IB) technique. The present results have been compared with the ones of a previous study, based on the same numerical method, but with a stationary spool. The numerical comparisons prove that the "quasi-stationary" hypothesis is approximately correct for present commercial devices, but it is not suitable for future high-speed valves. However it is shown that, even inside the range of the spool velocities currently adopted, for small pressure drops Δp and small openings s more significant differences arise on the axial forces.

The improvement of the hydraulic valves depends on the careful analysis of the coherent structures driving the motion of the working fluid. In the past those devices have been studied by experimental tests; during the last 15 years also several numerical works have been presented, solving the flow on body-fitted computational grids by RANS methods. In this study a different approach is proposed for the axisymmetric analysis of a directional valve (4/3, closed centre): whereas the RANS techniques are based on the time-averaged equations of the flow, in the present work the unsteady Navier-Stokes equations have been solved using the Direct Numerical Simulation (DNS); the time evolution of the physics is simulated, providing important details on the instantaneous structures of the flow, affecting the valve performance. Furthermore, while in the previous numerical studies the computational domain has been discretized by conformal grids, in this case the fluid-body interaction has been represented by an immersed-boundary (IB) method on a Cartesian grid, more suitable for unsteady eddy-resolving simulations, as DNS. The analysis of the discharge coefficient and the flow forces for different openings s and pressure drops ∆p is presented in this paper. The behaviour of those global parameters is justified also considering the time-averaged and the instantaneous fields. For small openings and pressure drops the flow is steady and attached to the wall of the discharge chamber on the side of the restricted section. When s and ∆p are increased the jet separates at the restricted section and it re-attaches downstream (Coanda effect), keeping the steady state. Finally, for large openings and pressure drops the flow becomes strongly unsteady: it is organized like a free jet and is dominated by large vortices.

"In this paper a directional valve (4/3, closed center) is analyzed using a code based on the immersed boundary method and solving the Navier-. Stokes equations by a Direct Numerical Simulation (DNS).. The results are presented in terms of instantaneous and time-averaged fields, showing the Coanda effect for small valve openings, and global parameters,. such as the discharge coefficient and the flow force coefficient K."

The numerical simulations of the heat transfer around an array of isothermal circular cylinders immersed in a stream has been carried out solving the two-dimensional Navier-Stokes equations. The cylinders have been placed in a single row configuration aligned with the free stream velocity at Reynolds number 100 and Prandtl number 0.7. In Fig.1 it is shown the instantaneous temperature distribution for the case of six in-line circular cylinders at spacing ratio (s/d) equal to 4 and 3.6, where s is the center-to-center cylinder spacing and d is the cylinder diameter. In the latter case, a transition in the flow patterns occurs with the flow organized in a vortex shedding responsible for the entrainment of cold fluid in the gaps. This phenomenon makes stronger the thermal gradient close to the cylinders leading the heat transfer enhancement with the Nusselt number 25 % higher respect to the case at s/d=4. Furthermore a frequency analysis of the time dependent Nusselt number, Nui, at the i-th cylinder, shows that the main frequency is the same for all the cylinders. We found evidences that the signature of the heat transfer enhancement could be related to the phase shift between two successive cylinders (i+1- i), where the phase shift (i) is defined as the difference between the phase of each main harmonic component of the Nui respect to the phase of the signal at the first cylinder.

The effect of particles falling under gravity in a weakly turbulent Rayleigh-B´enard gas flow is studied numerically. The particle Stokes number is varied between 0.01 and 1 and their temperature is held fixed at the temperature of the cold plate, of the hot plate, or the mean between these values. Mechanical, thermal, and combined mechanical and thermal couplings between the particles and the fluid are studied separately. It is shown that the mechanical coupling plays a greater and greater role in the increase of the Nusselt number with increasing particle size. A rather unexpected result is an unusual kind of reverse one-way coupling, in the sense that the fluid is found to be strongly influenced by the particles, while the particles themselves appear to be little affected by the fluid, despite the relative smallness of the Stokes numbers. It is shown that this result derives from the very strong constraint on the fluid behavior imposed by the vanishing of the mean fluid vertical velocity over the cross sections of the cell demanded by continuity.

Numerical results for kinetic and thermal energy dissipation rates in bubbly Rayleigh-Benard convection are reported. Bubbles have a twofold effect on the flow: on the one hand, they absorb or release heat to the surrounding liquid phase, thus tending to decrease the temperature differences responsible for the convective motion; but on the other hand, the absorbed heat causes the bubbles to grow, thus increasing their buoyancy and enhancing turbulence (or, more properly, pseudoturbulence) by generating velocity fluctuations. This enhancement depends on the ratio of the sensible heat to the latent heat of the phase change, given by the Jakob number, which determines the dynamics of the bubble growth.

Heavy or light particles introduced into a liquid trigger motion due to their buoyancy, with the potential to drive flow to a turbulent state. In the case of vapor bubbles present in a liquid near its boiling point, thermal coupling between the liquid and vapor can moderate this additional motion by reducing temperature gradients in the liquid. Whether the destabilizing mechanical feedback or stabilizing thermal feedback will dominate the system response depends on the number of bubbles present and the properties of the phase change. Here we study thermal convection with phase change in a cylindrical Rayleigh-Benard cell to examine this competition. Using the Reynolds number of the flow as a signature of turbulence and the intensity of the flow, we show that in general the rising vapor bubbles destabilize the system and lead to higher velocities. The exception is a limited regime corresponding to phase change with a high latent heat of vaporization (corresponding to low Jakob number), where the vapor bubbles can eliminate the convective flow by smoothing temperature differences of the fluid.

Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a muchenhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh–Bénard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 106 and 5 × 109. We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh–Bénard convection.