Marie-Isabelle Farinas

Asymmetry and transition to turbulence in a smooth axisymmetric constriction

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.

Asymptotically consistent numerical approximation of hemolysis

In a previous communication, we have proposed a numerical framework for the prediction of in vitro hemolysis indices in the preselection and optimization of medical devices. This numerical methodology is based on a novel interpretation of Giersiepen-Wurzinger blood damage correlation as a volume integration of a damage function over the computational domain. We now propose an improvement of this approach based on a hyperbolic equation of blood damage that is asymptotically consistent. Consequently, while the proposed correction has yet to be proven experimentally, it has the potential to numerically predict more realistic red blood cell destruction in the case of in vitro experiments. We also investigate the appropriate computation of the shear stress scalar of the damage fraction model. Finally, we assess the validity of this consistent approach with an analytical example and with some 3D examples.

Weak imposition of the slip boundary condition on curved boundaries for Stokes flow

We study the finite element approximation of two methods to weakly impose a slip boundary condition for incompressible fluid flows: the Lagrange multiplier method and Nitsche@?s method. For each method, we can distinguish several formulations depending on the values of some real parameters. In the case of a spatial domain with a polygonal or polyhedral boundary, we prove convergence results of their finite element approximations, extending previous results of Verfurth [33] and we show numerical results confirming them. In the case of a spatial domain with a smooth curved boundary, numerical results show that approximations computed on polygonal domains approximating the original domain may not converge to the exact solution, depending on the values of the aforementioned parameters and on the finite element discretization. These negative results seem to highlight Babuska@?s like paradox, due to the approximation of the boundary by polygonal ones. In particular, they seem to contradict some of Verfurth@?s theoretical convergence results.

Study of heat transfer in a horizontal cylinder with fins

Abstract The purpose of this paper is to investigate, numerically, the effect of internal fins on the flow pattern, temperature distribution and heat transfer between concentric horizontal cylinders. A Galerkin finite element method is adopted for the discretization of the governing equations. The numerical procedure consists in solving series of transient problems of increasing Ra . Results are presented using air ( Pr = 0.7) with Rayleigh numbers ranging from 10 3 to 10 6 for different fin configurations (1 and 2), geometries (sharp, round and divergent tip) and lengths ( l = 0.25, 0.5 and 0.75). They are illustrated in terms of isotherms, velocity fields, Nusselt numbers and fin efficiencies. Configuration 2 presents a heat transfer rate 10% above that of configuration 1, at Ra = 10 6 . The heat transfer is about the same for the three geometries, but the best fin efficiency is associated with the fin with a round tip.

Étude du transfert de chaleur dans un espace annulaire rhombique horizontal avec et sans ailettes

Resume Une etude numerique de la convection naturelle laminaire dans une cavite rhombique horizontale remplie d’air avec et sans ailette a ete realisee. Une methode d’elements finis adaptative est retenue pour predire les ecoulements convectifs complexes. Des simulations ont ete effectuees pour des largeurs de cavite Eg variant de 0.25 a 0.875, deux configurations d’ailettes dont la longueur va de 0.3 a 0.7 et des nombres de Rayleigh s’echelonnant entre 10 3 et 10 7 . Les resultats demontrent que le transfert de chaleur est maximal dans une cavite etroite de largeur Eg =0.25 munie d’ailettes de longueur l =0.7 (configuration 1). Dans ce cas, la conduction domine la convection. Quand, d’autre part, la convection domine, le transfert de chaleur est maximise avec Eg =0.875 et l =0.7. La conductivite equivalente k equ a ete correlee ainsi en fonction du nombre de Rayleigh pour toutes les geometries etudiees: k equ = CRa n .

Imposition of slip boundary conditions without the explicit computation of consistent normals

Computational fluid dynamics simulations often require the imposition of slip boundary conditions: zero-normal velocity. When the geometry is complex, the imposition of this type of boundary condition is not trivial since the current implementations require the computation of consistent normals. The slip boundary condition can be viewed as a constraint to the solution. Hence in the framework of the Lagrange multiplier approach, we present an accurate general implementation of consistent normals. With this new approach, these consistent normals are not computed explicitly at the expense of adding new unknowns - the Lagrange multipliers of the constraint - which can be interpreted as normal forces on the flow

Characterization of a Diffuser Flow by Time-Resolved PIV

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.

Hemodynamic characteristics of a mixed flow pump prototype: progress report of in vitro and acute animal experiments.

A new dual-inlet mixed-flow blood pump was designed and tested in our laboratory. The objective of the present study was to analyze hemodynamic characteristics of the pump prototype in vitro and during acute in vivo experiments. The mixed-flow pump was first tested in vitro and then implanted in 11 pigs and 3 calves. The left ventricular apex was cannulated with the pump and an outflow graft was anastomosed to the descending thoracic aorta. Flow and pressure probes were also implanted. Animals were killed 3 to 12 hours after surgery. In 11 pigs, pump outflow averaged 3.8 ± 0.4, 4.5 ± 0.4, 5.2 ± 0.8, 5.9 ± 0.3, and 6.5 l/min at 8,000, 9,000, 10,000, 11,000, and 12,000 pump speed in rpm. Differential pressure at the pump averaged 45 ± 6, 54 ± 8, 68 ± 16, 70 ± 12, and 85 ± 7 mm Hg at 8,000, 9,000, 10,000, 11,000 and 12,000 rpm. Mean aortic pressure averaged 64 ± 15 mm Hg throughout the procedures. In 3 calves, mean aortic pressure and left ventricular pressure remained stable during 4, 6, and 9 hours of support at 9,500, 10,000, 10,500, 11,000, and 11,500 rpm. The hemodynamic performance of our mixed-flow pump appears satisfactory during short-term support in animals. It supports similarly to axial-flow blood pumps in clinical trials. Based on these findings, an ameliorated design of this mixed-flow pump running at smaller rotational speed against a similar pressure head is under way.

Application of DOE for Optimal Turbomachinery Design

*† Design of experiments or DOE is applied in a new design methodology for optimal turbomachinery design. Through the use of Cordier’s scaling algorithm, we were able to investigate a multidimensional space of optimal prototypes, generated from a DOE and verified numerically by 3D CFD simulations. Furthermore DOE procedure helped identify an optimal pump design for a given operation point of 5 l/min – 100 mmHg (which indeed corresponds to its BEP) from design objectives such as maximizing the hydraulic efficiency (η), minimizing the rotation speed (N) and maximizing the rotor diameter (D).