Computations of turbulent and transitional flows in rotating machinery applications are very challenging due to complexity of the geometry, which usually consists of multiple rotating and stationary parts. The application of well-established, body-fitted methods frequently utilizes overset grids and different reference frames, which have an adverse impact on the overall accuracy and cost-efficiency of the method. In the present work we explore the feasibility of performing computations of such flows using a single reference frame and an immersed-boundary approach. In particular, we report one of the first large-eddy simulation in this class of flows, where a structured cylindrical coordinate solver with optimal conservation properties is utilized in conjunction with an immersed-boundary method. To evaluate the accuracy of the computations the results are compared to the experimental measurements in [1]. Results using the standard Smagorinsky model and the Filtered Structured Function model are presented. We demonstrate that the overall approach is well suited for the flow under consideration and the results with the more advanced subgrid scale model are in good agreement with the experiment. We also briefly discuss some of the features of the instantaneous flow dynamics, to provide a glimpse of the wealth of information that can be extracted from such computations.

reduced flow-rates in turbopumps produce significant unsteady phenomena, characterized by separation and back-flow. In this study an LES approach coupled with an immersed-boundary methodology is utilized to investigate the changes in the flow physics, when compared to nominal flow-rates. The present methodology has been already validated for the design case through comparison with PIV experiments in the literature. It will be shown that for a reduced flow rate (40% of the design one) separation phenomena are generated on the suction side of the rotor blades and on the pressure side of the stator ones. Significant spanwise non-uniformity is produced in the diffuser channels, with a displacement of the flow towards the hub side and back-flow on the shroud side. The values of turbulent kinetic energy are increased by an order of magnitude at off-design conditions and the main source of turbulence is not anymore the flow from the suction side and the trailing edge of the rotor blades: most turbulence is generated now at the leading edge of the diffuser blades. The increased interaction between rotating and stationary parts implies also a stronger dependence of the flow features on the relative position between impeller and diffuser blades.

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.

The thermal performance of hollow bricks, as elements of the building envelope, is mainly linked to the presence of air cavities. The heat propagates within those components through the three basic modes of transmission: conduction in the solid matrix, convection of the air cavity, direct radiation between surfaces that face in the cavities. The rigorous treatment of such heat exchange is very complex even in thermal conditions close to the environment with moderate temperature differences. In engineering practice a simplified calculation method is used, as incorporated in national and international standards, which reduce heat transfer into the cavity to an equivalent heat conduction that results in the transport of the same thermal power. UNI EN ISO 6946 and UNI 10355 follow this method by proposing a simplified model of computation that lead to different results. Given the importance that the correct assessment of the insulation performance of the envelop in the context of energy certification of buildings it was considered useful to undertake a detailed analysis of the reliability of the estimation methods of heat transfer in air cavities of hollow bricks. The study was conducted with the use of different tools, numerical and experimental. An experimental study was conducted in the laboratory with heat transmission measurements, in specimens with parallelepiped cavities, through the guarded hot plate apparatus according to UNI EN 12664. The analysis of the results, numerical and experimental, compared with the predictions obtained from simplified models of technical standards, have called attention to the order of magnitude of the calculation accuracy obtainable with such procedures. For certain geometric configurations and boundary conditions, the application of the standard procedures can lead to large errors of evaluation of the thermal resistance of the cavity.

Given the importance of the assessment of the insulation performance of the building envelope in the context of energy certification of buildings, a detailed analysis of the reliability of the methods of evaluation of heat transfer in the air cavities of hollow blocks has been carried out. An experimental study was conducted in the laboratory. Heat transfer measurements on specimens with parallelepiped cavities were done, through a guarded hot plate device according to UNI EN 12664. A numerical analysis of the heat transfer in the specimens through the software ANSYS FLUENT was carried out. The analysis of the numerical and experimental results when compared with the predictions obtained from simplified models of technical standards, have called attention to the order of magnitude of the calculation accuracy obtainable with such procedures. For some geometric configurations and boundary conditions, the application of the standards can lead to large errors of evaluation of the thermal resistance of the cavity.

"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.