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SUMMARYIn the present study, a computational investigation was carried out to understand the influence of flexibility on the aerodynamic performance of a hovering wing. A flexible, two-dimensional, two-link model moving within a viscous fluid was considered. The Navier-Stokes equations governing the fluid dynamics were solved together with the equations governing the structural dynamics by using a strongly coupled fluid-structure interaction scheme. Harmonic kinematics was used to prescribe the motions of one of the links, thus effectively reducing the wing to a single degree-of-freedom oscillator. The wing's flexibility was characterized by the ratio of the flapping frequency to the natural frequency of the structure. Apart from the rigid case, different values of this frequency ratio (only in the range of 1/2 to 1/6) were considered at the Reynolds numbers of 75, 250 and 1000. It was found that flexibility can enhance aerodynamic performance and that the best performance is realized when the wing is excited by a non-linear resonance at 1/3 of the natural frequency. Specifically, at Reynolds numbers of 75, 250 and 1000, the aerodynamic performance that is characterized by the ratio of lift coefficient to drag coefficient is respectively increased by 28%, 23% and 21% when compared with the corresponding ratios of a rigid wing driven with the same kinematics. For all Reynolds numbers, the lift generated per unit driving power is also enhanced in a similar manner. The wake capture mechanism is enhanced, due to a stronger flow around the wing at stroke reversal, resulting from a stronger end of stroke vortex at the trailing edge. The present study provides some clues about how flexibility affects the aerodynamic performance in low Reynolds number flapping flight. In addition, it points to the importance of considering non-linear resonances for enhancing aerodynamic performance.

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We have performed large-eddy simulations of spatially developing homogeneous isotropic turbulence advected past an interface where the grid is suddenly coarsened or refined by a factor of two in each direction. We have compared simulations in which the filter width is proportional to the grid size and is discontinuous at the coarse-fine grid interface, to others in which the filter width varies gradually between the values corresponding to the coarse and fine grids. The Smagorinsky and Lagrangian-dynamic eddy viscosity (LDEV) models were used to parameterize the unresolved subgrid scales. A sudden refinement of the grid does not result in significant flow perturbation: small scales are gradually generated and the flow is generally quite smooth across the interface. When the grid is suddenly coarsened, on the other hand, a considerable energy pileup at small scales is observed near the interface. At lower resolutions, the extent of this high-energy zone grows. Increasing the eddy viscosity upstream of the interface by smoothly increasing the filter width is beneficial. For fine-to-coarse interfaces, the LDEV model with a smoothly varying filter width yields more accurate results than other models. Explicit filtering of the advective term is also beneficial: by increasing the length scale of the turbulence upstream of the grid discontinuity, interpolation and aliasing errors are reduced and better agreement with single-grid results is obtained.

Mechanical hemolysis is a major concern in the design of cardiovascular devices, such as prosthetic heart valves and ventricular assist devices. The primary cause of mechanical hemolysis is the impact of the device on the local blood flow, which exposes blood elements to non-physiologic conditions. The majority of existing hemolysis models correlate red blood cell (RBC) damage to the imposed fluid shear stress and exposure time. Only recently more realistic, strain-based models have been proposed, where the RBC's response to the imposed hydrodynamic loading is accounted for. In the present work we extend strain-based models by introducing a high-fidelity representation of RBCs, which is based on existing coarse-grained particle dynamics approach. We report a series of numerical experiments in simple shear flows of increasing complexity, to illuminate the basic differences between existing models and establish their accuracy in comparison to the high-fidelity RBC approach. We also consider a practical configuration, where the flow through an artificial heart valve is computed. Our results shed light on the strengths and weaknesses of each approach and identify the key gaps that should be addressed in the development of new models.