Lagrangian coherent structures (LCS) constitute one of the many tools commonly used to analyze and better explain time dependent flow behavior. They are defined as the ridge curves of the Finite-Time Lyapunov Exponent (FTLE) field, which are usually computed on structured cartesian interpolation grids with a large number of nodes to ensure a proper definition of the field. The main contribution of this work is the introduction of adapted unstructured anisotropic interpolation grids to enhance the definition of the FTLE field, and consequently the precision and quality of extracted LCS lines. Since these lines correspond to the most anisotropic feature of the FTLE field, anisotropic adaptation of the interpolation grid allows the proper concentration of nodes along the LCS lines and keeps the total number of nodes low.

The flow through a smooth axisymmetric constriction (a stenosis in medical applications) of 75% restriction in area is measured using stereoscopic and time-resolved particle image velocimetry (PIV) in the Reynolds number range Re ~ 100–1100. At low Reynolds numbers, steady flow results reveal an asymmetry of the flow downstream of the constriction. The jet emanating from the throat of the nozzle is deflected towards the wall causing the formation of a one-sided recirculation region. The asymmetry results from a Coanda-type wall attachment already observed in symmetric planar sudden expansion flows. When the Reynolds number is increased above the critical value of 400, the separation surface cannot remain attached and an unsteady flow regime begins. Low-frequency axial oscillations of the reattachment point are observed along with a slow swirling motion of the jet. The phenomenon is linked to a periodic discharge of the unstable recirculation region inducing alternating laminar and turbulent flow phases. The resulting flow is highly non-stationary and intermittent. Discrete wavelet transforms are used to discriminate between the large-scale motions of the mean flow and the vortical and turbulent fluctuations. Continuous wavelet transforms reveal the spectral structure of flow disturbances. Temporal measurements of the three velocity components in cross-sections are used with the Taylor hypothesis to qualitatively reconstruct the three-dimensional velocity vector fields, which are validated by comparing with two-dimensional PIV measurements in meridional planes. Visualizations of isosurfaces of the swirling strength criterion allow the identification of the topology of the vortices and highlight the formation and evolution of hairpin-like vortex structures in the flow. Finally, with further increase of the Reynolds number, the flow exhibits less intermittency and becomes stationary for Re ~ 900. Linear stochastic estimation identifies the predominance of vortex rings downstream of the stenosis before breakdown to turbulence.

Computational fluid dynamics is extensively used in the design methodology of medical devices. However, for such applications, the predictive capabilities of CFD codes are highly dependent upon geometry, which most of the time is extremely complex, and flow conditions. The study concerns a ventricular assist device (VAD) where the exit flow, generated through a diffuser, is of particular importance for blood damage predictions. The difficulty to predict the flow lies in the fact that the Reynolds number range includes the transition Reynolds number of the separated diffuser flow as well as the critical Reynolds number of pipe flows. In order to choose the appropriate CFD methodology in terms of flow hypothesis and turbulence model, an experimental setup of the diffuser was built to run PIV velocity measurements and to analyze the flow pattern with the influence of Reynolds number. The flow is described with mean and variance values of the in-plane velocity components and timeresolved results are used to visualize the development of unsteady phenomena introduced in the diffuser separated region. An optimal filter is also used to remove noise in measured velocity vector fields.