Francesco Cambuli

CFD–RANS analysis of the rotational effects on the boundary layer of wind turbine blades

The flow field past the rotating blade of a horizontal axis wind turbine has been modeled with a full 3–D steady–RANS approach. Flow computations have been performed using the commercial finite–volume solver Fluent. A number of blade sections from the 3–D rotating geometry were chosen and the corresponding 2–D flow computations have been carried out for comparison, for different angles of attack and in stalled conditions. In order to investigate the effects of rotation a postprocessing tool has been developed, allowing the evaluation of the terms in the boundary layer equations. Examples of the output are proposed for the analyzed flow situations. The main features of the boundary layer flow are described, for both the rotating blade and the corresponding 2–D profiles. Computed pressure distributions and aerodynamic coefficients evidence less lift losses after separation in the 3–D rotating case, mostly for the inward sections of the blade and the highest angles of attack, in agreement with the literature.

A detailed analysis of the unsteady flow within a wells turbine

Sea wave energy is one of the main renewable energy resources. Its exploitation is relatively simple and determines a minimum impact on the environment. The system that is most often used for wave energy harvesting is composed of an oscillating water column device together with a Wells turbine. When designing the Wells turbine, its interaction with the oscillating water column system must be taken into account, if the energy collected is to be maximized. The most important interaction phenomenon is the so called hysteresis effect, i.e. the time delay between the piston-like motion of the air water interface and the torque developed by the turbine. This work presents a detailed analysis of the flow within an oscillating water column system, focusing on the differences in performance and in secondary flow structures between acceleration and deceleration, and between the inflow and outflow phases. This analysis demonstrates how the hysteresis between acceleration and deceleration is caused uniquely by compressibility effects within the oscillating water column system, while differences in the flow parameters and secondary structures near the rotor are negligible, if equivalent flow conditions are compared. The effects of the oscillating water column system configuration on the performance are also highlighted.

Shroud Leakage Modeling of the Flow in a Two Stage Axial Test Turbine

Three dimensional steady multistage calculations, using mixing plane approach, are presented for two different blade geometries in a two stage axial test turbine with shrouded blades. A 3D multiblock Navier-Stokes finite volume solver (TBLOCK) has been used in all the simulations. In order to study shroud leakage flow effects the whole shroud cavity geometry has been modeled, overcoming most of the limitations of simple shroud leakage model in calculating fluid flow over complex geometries. Numerical investigations are mainly focused on assessing the ability of the solver to be used as multistage design tool for modeling leakage-mainstream flow interaction. Several calculations are compared. The first computes the main blade flow path with no modeling of the shroud cavities. The second includes the modeling of the shroud cavities for a zero leakage mass flow rate. Finally a multiblock calculation which models all the leakage flow paths and shroud cavities has been carried out for two different levels of shroud seal clearance. It is found that neglecting shroud leakage significantly alters the computed velocity profiles and loss distributions, for both the computed blade geometries. A numerically predicted shroud leakage offset loss is presented for the two considered blade geometries, focusing on the relative importance of the leakage flow, re-entry mixing losses, and inlet and exit shroud cavity effect. Results demonstrates that full calculation of leakage flow paths and cavities is required to obtain reliable results, indicating the different effects of the leakage-to-mainstream flow interaction on the blade geometries computed. Despite a slight increase in the computational time, multiblock approach in handling leakage flow problem can now-days be used as a practical tool in the blade design process and routine shroud leakage calculations.Copyright