DBIAB - AERO - Journal Articleshttp://hdl.handle.net/10016/195752020-10-20T16:11:11Z2020-10-20T16:11:11ZAssessing aerodynamic force estimation with experiments and simulations of flapping-airfoil flows on the verge of three-dimensionalityMoriche Guerrero, ManuelRaiola, MarcoDiscetti, StefanoIaniro, AndreaFlores Arias, ÓscarGarcía-Villalba Navaridas, Manuelhttp://hdl.handle.net/10016/301262020-07-16T07:34:35Z2020-02-01T00:00:00ZAssessing aerodynamic force estimation with experiments and simulations of flapping-airfoil flows on the verge of three-dimensionality
Moriche Guerrero, Manuel; Raiola, Marco; Discetti, Stefano; Ianiro, Andrea; Flores Arias, Óscar; García-Villalba Navaridas, Manuel
This paper reports a combined experimental and numerical study of the flow over a rigid airfoil in flapping motion. The setup consists of a heaving and pitching airfoil at a moderate Reynolds number (Re=500-3600), at a Strouhal number St=0.1. The aim is to assess the accuracy of two-dimensional direct numerical simulations in predicting aerodynamic forces in a flow configuration, which is nominally two-dimensional but is at the verge of three-dimensionality. The assessment is carried out with experiments, including flow field and aerodynamic force measurements with particle image velocimetry and a load cell. The comparative study shows a good qualitative agreement between the experiments and the simulations at comparable Reynolds numbers both in terms of forces and flow fields, but with some quantitative differences. The quantitative discrepancies between experiments and simulation are analyzed and reduced to inherent differences between experimental and computational setups. It is observed that the significant differences are apparent almost exclusively in the wake evolution. Nonetheless, this is shown to have a minor effect on the aerodynamic force estimation. Overall, the trends observed when varying the mean pitch angle and the pitching amplitude are the same in both experiments and simulations. This suggests that two-dimensional/three-dimensional effects do not alter significantly the relationship between the unsteady flow mechanisms (i.e. leading edge vortex) and the aerodynamic forces in the parametric range considered here.
2020-02-01T00:00:00ZFrom flapping to heaving: a numerical study of wings in forward flightGonzalo Grande, AlejandroArranz Fernández, GonzaloMoriche Guerrero, ManuelGarcía-Villalba Navaridas, ManuelFlores Arias, Óscarhttp://hdl.handle.net/10016/301372020-07-16T07:34:35Z2018-11-01T00:00:00ZFrom flapping to heaving: a numerical study of wings in forward flight
Gonzalo Grande, Alejandro; Arranz Fernández, Gonzalo; Moriche Guerrero, Manuel; García-Villalba Navaridas, Manuel; Flores Arias, Óscar
Direct Numerical Simulations of the flow around a pair of flapping wings are presented. The wings are flying in forward flight at a Reynolds number Re=500, flapping at a reduced frequency K=1. Several values of the radius of flapping motion are considered, resulting in a database that shows a smooth transition from the wing rotating with respect to its inboard wingtip (flapping), to a vertical oscillation of the wing (heaving). In this transition from flapping to heaving, the spanwise-averaged effective angle of attack of the wing increases while the effect of the Coriolis and centripetal accelerations becomes weaker. The present database is analyzed in terms of the value and surface distribution of the aerodynamic forces, and in terms of 2D and 3D flow visualizations. While the former allows a decomposition of the force in pressure (i.e., the component of the force normal to the surface of the wing) and skin friction (i.e., tangential to the surface of the wing), the latter allows the identification of specific flow structures with the corresponding forces on the wing. It is found that the aerodynamic forces in the vertical direction (lift) tend to increase for wings moving with larger radius of flapping motion, becoming maximum for the heaving configuration. This is mostly due to the increase of the spanwise-averaged effective angle of attack of the wing with the radius of the flapping motion. Also, the local changes in the effective angle of attack have a strong effect on the structure of the leading edge vortex, resulting in changes in the distribution of suction along the span near the leading edge of the wing. The effect of the apparent accelerations is mostly felt on the spanwise position where the separation of the LEV occurs. On the other hand, the differences in the force in the streamwise direction (thrust/drag) between the configurations with different radius of flapping motion seems to be linked to the position of the stagnation point dividing the suction and pressure side boundary layers, which seems to be controlled by the local effective angle of attack. Finally, the results of the DNS are used to evaluate the performance of an unsteady panel method, and to explain its deficiencies.
2018-11-01T00:00:00ZExtended proper orthogonal decomposition of non-homogeneous thermal fields in a turbulent pipe flowAntoranz Perales, AntonioIaniro, AndreaFlores Arias, ÓscarGarcía-Villalba Navaridas, Manuelhttp://hdl.handle.net/10016/301342020-07-16T07:34:35Z2018-03-01T00:00:00ZExtended proper orthogonal decomposition of non-homogeneous thermal fields in a turbulent pipe flow
Antoranz Perales, Antonio; Ianiro, Andrea; Flores Arias, Óscar; García-Villalba Navaridas, Manuel
This manuscript analyzes the role of coherent structures in turbulent thermal transport in pipe flows. A Proper Orthogonal Decomposition (POD) analysis is performed on a direct numerical simulation dataset with non-homogeneous boundary conditions, heated on the upper side, representative of solar receivers (Antoranz et al., 2015, Int. J. Heat Fluid Flow, 55). Three flow conditions are analyzed: with friction Reynolds number equal to 180 and Prandtl number equal to 0.7 and 4 and with friction Reynolds number equal to 360 and Prandtl number equal to 0.7. Both POD and extended POD modes are presented and compared. This allows to visualize the main flow modes in terms of both turbulent kinetic energy and temperature fluctuations, analyzing their contribution to the turbulent transport of heat. The POD analysis shows that the temperature fluctuations are described by a more compact modal subspace than the turbulent kinetic energy. The effect of increasing the Reynolds number is to produce a thinner boundary layer, with a slightly less compact representation of both kinetic energy and temperature fluctuations. The increase of the Prandtl number, instead, results in a thinner thermal boundary layer with a greater scale separation between thermal fluctuations and kinetic energy. Temperature POD modes together with velocity extended POD modes are used to analyze and quantify the mode contribution to turbulent thermal transport. Results show that the correlation between velocity and temperature is such that it is possible to describe roughly 100% of the turbulent heat transport and temperature fluctuations with only 40% of the kinetic energy. For the cases with Pr = 0.7, the first extended POD mode is a large vertical jet flanked by a pair of counter-rotating vortices near the heated part of the pipe. This single structure accounts for up to 10% of the turbulent heat transport.
2018-03-01T00:00:00ZA numerical study of the flow around a model winged seed in auto-rotationArranz Fernández, GonzaloGonzalo Grande, AlejandroUhlmann, MarkusFlores Arias, ÓscarGarcía-Villalba Navaridas, Manuelhttp://hdl.handle.net/10016/301252020-07-16T07:34:35Z2018-07-16T00:00:00ZA numerical study of the flow around a model winged seed in auto-rotation
Arranz Fernández, Gonzalo; Gonzalo Grande, Alejandro; Uhlmann, Markus; Flores Arias, Óscar; García-Villalba Navaridas, Manuel
In this study the flow around a winged-seed in auto-rotation is characterized using direct numerical simulations (DNS) at Reynolds number in the range 80-240, based on the descent speed and a characteristic chord length. In this range, the flow is approximately steady when observed from a reference frame fixed to the seed. For all cases, the flow structure consists of a wing tip vortex which describes a helical path, a vortex shed behind the nut of the seed and a stable leading edge vortex above the wing surface which merges with the tip vortex. With increasing Reynolds number, the leading edge vortex becomes more intense and gets closer to the wing surface. The simulation results also show the formation of a spanwise flow on the upper surface of the wing, moving fluid towards the wing tip in a region downstream and beneath the leading edge vortex. This spanwise flow is rather weak inside the core of the leading edge vortex, and the analysis of the streamlines show a very weak transport of vorticity along the vortex for the cases under consideration. The analysis of the flow suggests that the stabilization of the leading edge vortex is mainly due to non-inertial accelerations, although viscous effects may contribute, specially at lower Re. Furthermore, the leading edge vortex has been characterized by analysing the flow variables averaged along cross-sections of the vortex. While some quantities, like the spanwise velocity or the pressure inside the vortex, are rather insensitive to the threshold used to define the leading edge vortex, the same is not true for the circulation of the vortex or its averaged spanwise vorticity, due to the viscous nature of the vortex. Finally, it is observed that the spanwise vorticity scales with the angular rotation of the seed for the different Re.
2018-07-16T00:00:00Z