Renato Ricci

Analysis of boundary layer separation phenomena by infrared thermography Use of acoustic and/or mechanical devices to avoid or reduce the laminar separation bubble effects

Aerodynamic bodies operating at low Reynolds numbers are normally subjected to local boundary layer separation phenomena; these strongly modify airfoils behavior. The body geometry and the incoming flow angle of attack define the pressure pattern over the surface in a way that there will be sections with a favorable pressure gradient and others with a negative one, the latter causing the flow to slow down and sometimes to separate. A laminar separation, followed by a turbulent transition in the separated shear layer and a subsequent turbulent reattachment, causes the Laminar Separation Bubble (LSB) phenomenon. The LSB presence induces an aerodynamic drag increase and an efficiency decrease; this problem is of interest in many application fields including wind turbine energy production, where a LSB may induce less energy output and occasionally mechanical problems due to pulsating pressure variations (bubble bursting phenomena). This paper illustrates an infrared measurement approach, useful for the instantaneous observation of the boundary layer pattern on an airfoil surface, as well as an analysis of the effects of both an acoustic and a mechanical system on the behavior of the LSB. These latter induce disturbances in the developing boundary layer in order to promote the turbulent transition and contrasting the LSB presence. The first one involves the use of a subwoofer and the analysis is performed on an Eppler 205 airfoil; the second one makes use a Micro Electro Mechanical System (MEMS) and the IR observation is carried out on a WT01 airfoil developed for small wind turbines (about 1 kW). The results show a pronounced effectiveness for both the methodologies particularly when using MEMS.

On the development of OpenFOAM solvers based on explicit and implicit high-order Runge–Kutta schemes for incompressible flows with heat transfer

Abstract Open-source CFD codes provide suitable environments for implementing and testing low-dissipative algorithms typically used to simulate turbulence. In this research work we developed CFD solvers for incompressible flows based on high-order explicit and diagonally implicit Runge–Kutta (RK) schemes for time integration. In particular, an iterated PISO-like procedure based on Rhie–Chow correction was used to handle pressure–velocity coupling within each implicit RK stage. For the explicit approach, a projected scheme was used to avoid the “checker-board” effect. The above-mentioned approaches were also extended to flow problems involving heat transfer. It is worth noting that the numerical technology available in the OpenFOAM library was used for space discretization. In this work, we additionally explore the reliability and effectiveness of the proposed implementations by computing several unsteady flow benchmarks; we also show that the numerical diffusion due to the time integration approach is completely canceled using the solution techniques proposed here.

An OpenFOAM Solver for Forced Convection Heat Transfer Adopting Diagonally Implicit Runge–Kutta Schemes

Nowadays open-source CFD codes provide suitable environments for the implementation and testing low-dissipative algorithms typically used for turbulence simulation. Therefore in this research work, we have developed a CFD solver for incompressible fluid flow and forced convection heat transfer based on high-order diagonally implicit Runge–Kutta (RK) schemes for time integration. In particular, an iterated PISO-like procedure based on Rhie–Chow correction was used for handling pressure–velocity coupling within each RK stage. It is worth emphasizing that for space discretization, the numerical technology available within the well-known OpenFOAM library was used. The first aim of this work was to explore the reliability and effectiveness of OpenFOAM library for convective heat transfer problems using high-fidelity numerics. This is a point of interest since we cannot find similar papers in the available literature. The accuracy of the considered algorithm was evaluated studying several flow benchmarks. Hence, we also provide a further contribution to the literature involving forced convection heat transfer around bluff bodies at low Reynolds numbers. Lastly, this paper is only a first step toward turbulent heat transfer simulation in complex configurations by means of DNS/LES techniques.

On the improvement of computational performance of a vapor-liquid equilibria solver for mixtures

This work deals the computational performance improvement of vapor–liquid equilibria solver for fluids mixtures. The code here developed is based on the chemical potential equality (expressed in terms of fugacity) and implements Soave–Redlich–Kwong and Peng–Robinson equations of state with classical van der Waals mixing rules. To reduce the bulk of the computational effort required by the solver we propose the following approaches: (i) exploit high-order methods for the solution of Rachford–Rice equation; (ii) develop an efficient programming methodology for the sub-routines devoted to the fugacity coefficients computation in order to reduce their overall impact on the CPU-time exploiting the parallelism at CPU level, i.e. CPU pipelining, and cache blocking. In this paper we have carefully evaluated the effectiveness of the aforementioned approaches performing a suite of computations of the equilibrium properties of several literature mixtures. The pros and cons of the strategies here suggested are outlined and discussed.

Study of the flow field past dimpled aerodynamic surfaces: numerical simulation and experimental verification

This work presents the study of the flow field past of dimpled laminar airfoil. Fluid dynamic behaviour of these elements has been not still deeply studied in the scientific community. Therefore Computational Fluid-Dynamics (CFD) is here used to analyze the flow field induced by dimples on the NACA 64-014A laminar airfoil at Re = 1.75 105 at α = 0°. Reynolds Averaged Navier–Stokes (RANS) equations and Large-Eddy Simulations (LES) were compared with wind tunnel measurements in order to evaluate their effectiveness in the modeling this kind of flow field. LES equations were solved using a specifically developed OpenFOAM solver adopting an L–stable Singly Diagonally Implicit Runge–Kutta (SDIRK) technique with an iterated PISO-like procedure for handling pressure-velocity coupling within each RK stage. Dynamic Smagorinsky subgrid model was employed. LES results provided good agreement with experimental data, while RANS equations closed with approach overstimates laminar separation bubble (LSB) extension of dimpled and un–dimpled configurations. Moreover, through skin friction coefficient analysis, we found a different representation of the turbulent zone between the numerical models; indeed, with RANS model LSB seems to be divided in two different parts, meanwhile LES model shows a LSB global reduction.