Antonio Posa

Experimental and Computational Wake Characterization of a Vertical Axis Wind Turbine

This work visualizes the wake of a vertical axis wind turbine (VAWT) both experimentally and computationally. In the experiments, a scale VAWT model is placed inside a wind tunnel under a controlled laboratory setting. A motor rotates the scale model at a constant angular speed. Stereo particle image velocimetry (PIV) is used to visualize the wake of the turbine. When designing the scaled system, the turbine size, angular velocity, and wind speed were selected to most closely match the dimensionless parameters of the original full-scale system. The experiments are coordinated with a series of computations. A large-eddy simulation approach is utilized to resolve all the energy carrying eddies in the wake. Wake profiles from both experiments and computations show that the wakes contain a large oppositely signed pair of vortices from each foil as well as several smaller vortical structures. This structure is similar to that of a spinning cylinder, but the smaller structures present create a distinct difference. This study is among the first attempts to gain an understanding of the fundamental wake structure of a single VAWT.

An adaptive reconstruction for Lagrangian, direct-forcing, immersed-boundary methods

Abstract Lagrangian, direct-forcing, immersed boundary (IB) methods have been receiving increased attention due to their robustness in complex fluid-structure interaction problems. They are very sensitive, however, on the selection of the Lagrangian grid, which is typically used to define a solid or flexible body immersed in a fluid flow. In the present work we propose a cost-efficient solution to this problem without compromising accuracy. Central to our approach is the use of isoparametric mapping to bridge the relative resolution requirements of Lagrangian IB, and Eulerian grids. With this approach, the density of surface Lagrangian markers, which is essential to properly enforce boundary conditions, is adapted dynamically based on the characteristics of the underlying Eulerian grid. The markers are not stored and the Lagrangian data-structure is not modified. The proposed scheme is implemented in the framework of a moving least squares reconstruction formulation, but it can be adapted to any Lagrangian, direct-forcing formulation. The accuracy and robustness of the approach is demonstrated in a variety of test cases of increasing complexity.

Influence of the Spool Velocity on The Performance of a Directional Hydraulic Valve

Abstract 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.