In this project computations on an axial-flow hydrokinetic turbine will be performed using Large-Eddy Simulation (LES), coupled with an Immersed-Boundary (IB) method, resolving the flow on cylindrical grids consisting of about two billion nodes, thus an order of magnitude larger than in the most advanced studies in the literature to date. This will allow reproducing the wake flow up to several diameters downstream, providing accurate turbulence statistics and visualization of the dynamics of hub and tip vortices. Furthermore, the same project is targeted at studying the dependence of the wake features on tip speed ratio (TSR), simulating three different values of such parameter, which is defined as the ratio between the tangential velocity at the tip of the blades and the velocity of the incoming free-stream. It is worth noting that LES is especially well suited to compute flows featuring a high level of coherence, populated by large vortices. In LES the large, energy-carrying structures are resolved explicitly, as in a Direct Numerical Simulation (DNS), with no turbulence modelling and associated approximations, typical of Reynolds-averaged Navier-Stokes (RANS) methodologies, where the time-averaged equations of the flow are instead resolved. Obviously, the accuracy of the approach improves together with the resolution of the computational mesh, since more scales are resolved, having dimensions larger than the spacing of the computational grid, while only the smallest scales are modeled using a sub-grid scales (SGS) model. These features make the LES approach both very accurate and computationally demanding, especially in the framework of engineering flows, requiring very fine resolutions in both space and time and large computational resources on massively parallel supercomputers. The IFREMER geometry, whose wake is going to be investigated, is a three-bladed axial-flow hydrokinetic turbine studied via wake measurements in the framework of Round Robin tests funded by the EU within the MaRINET Initiative under the FP7. Therefore, several experimental results are already available for validation purposes on both global performance parameters, as thrust, torque and power, and wake development. Reference results on the same turbine are also provided by the numerical solutions generated via a hybrid solver, developed at the Institute of Marine Engineering in Rome, coupling the RANS approach with a Boundary Element Method (BEM). The three computations on the IFREMER turbine will be carried out for three different values of TSR = 2.0, 3.67 and 5.0. The case at TSR = 3.67 will be considered for validation. Most data on wake development are indeed available for that value of tip speed ratio. The additional lower and higher values of the same parameter will be considered for assessing the influence by TSR on coherence (vortices shed downstream by the turbine) and turbulent statistics of the wake flow.
Characterization of the wake of an axial-flow hydrokinetic turbine via LES
2020
Abstract
In this project computations on an axial-flow hydrokinetic turbine will be performed using Large-Eddy Simulation (LES), coupled with an Immersed-Boundary (IB) method, resolving the flow on cylindrical grids consisting of about two billion nodes, thus an order of magnitude larger than in the most advanced studies in the literature to date. This will allow reproducing the wake flow up to several diameters downstream, providing accurate turbulence statistics and visualization of the dynamics of hub and tip vortices. Furthermore, the same project is targeted at studying the dependence of the wake features on tip speed ratio (TSR), simulating three different values of such parameter, which is defined as the ratio between the tangential velocity at the tip of the blades and the velocity of the incoming free-stream. It is worth noting that LES is especially well suited to compute flows featuring a high level of coherence, populated by large vortices. In LES the large, energy-carrying structures are resolved explicitly, as in a Direct Numerical Simulation (DNS), with no turbulence modelling and associated approximations, typical of Reynolds-averaged Navier-Stokes (RANS) methodologies, where the time-averaged equations of the flow are instead resolved. Obviously, the accuracy of the approach improves together with the resolution of the computational mesh, since more scales are resolved, having dimensions larger than the spacing of the computational grid, while only the smallest scales are modeled using a sub-grid scales (SGS) model. These features make the LES approach both very accurate and computationally demanding, especially in the framework of engineering flows, requiring very fine resolutions in both space and time and large computational resources on massively parallel supercomputers. The IFREMER geometry, whose wake is going to be investigated, is a three-bladed axial-flow hydrokinetic turbine studied via wake measurements in the framework of Round Robin tests funded by the EU within the MaRINET Initiative under the FP7. Therefore, several experimental results are already available for validation purposes on both global performance parameters, as thrust, torque and power, and wake development. Reference results on the same turbine are also provided by the numerical solutions generated via a hybrid solver, developed at the Institute of Marine Engineering in Rome, coupling the RANS approach with a Boundary Element Method (BEM). The three computations on the IFREMER turbine will be carried out for three different values of TSR = 2.0, 3.67 and 5.0. The case at TSR = 3.67 will be considered for validation. Most data on wake development are indeed available for that value of tip speed ratio. The additional lower and higher values of the same parameter will be considered for assessing the influence by TSR on coherence (vortices shed downstream by the turbine) and turbulent statistics of the wake flow.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
