Francesca Maria Susin

Hysteretic behavior of the flow under a vertical sluice gate

In the paper, the phenomenon of hysteresis which can develop in a supercritical channel flow approaching an obstacle is analyzed, and a simple theory to predict the occurrence of hysteresis is described. The results of an in-depth theoretical and experimental study of the case of flow under a vertical sluice gate in a rectangular channel are then presented. Possible flow regimes in the vicinity of a gate are classified on the basis of the nondimensional gate opening and the Froude number of the undisturbed approaching flow. It is shown that within a wide range of flow parameters both undisturbed and free outflow conditions may exist for the same gate opening. Within this range, the actual regime depends on the previous history of the flow, thus implying the hysteretic character of the flow. It is worth noting that a subcritical approaching flow may also exhibit such a hysteretic behavior provided the Froude number is greater than approximately 0.8. This occurrence, which has not previously been reported in the literature, is probably a result of the contraction affecting the flow issuing from under the gate. The results of an extensive series of experiments, performed over a wide range of flow parameters, are detailed in the paper and confirm theoretical predictions.

Bed friction effects on the stability of a stationary hydraulic jump in a rectangular upward sloping channel

The issue of the bed friction effect on the stability of a stationary hydraulic jump in a rectangular upward sloping channel is investigated through a combined theoretical and experimental approach. The theoretical stability criterion proposed by Defina and Susin [Phys. Fluids 15, 3883 (2003)] is generalized to include rough wall flows. The results of an extensive series of experiments are then presented. The adopted experimental procedure is detailed, and results are compared with theoretical predictions. It is shown that the proposed stability criterion successfully predicts both the stable and unstable behavior of the jump for smooth and rough wall flow, at least in the range of small upward bottom slopes.

Stability of a stationary hydraulic jump in an upward sloping channel

A simple theoretical criterion to predict stability of a stationary hydraulic jump in a rectangular channel is presented. It is shown that stability condition depends on the Froude number of the flow, as well as on the bottom slope and friction along the channel. In particular, it is found that in an upward sloping channel the jump may either be unstable (i.e., if the jump is slightly displaced from its equilibrium location, then it moves further away from it) or stable (i.e., it returns to its original location). This result improves previous theoretical findings reported in the literature.

Physiological vortices in the sinuses of Valsalva: An in vitro approach for bio-prosthetic valves

Purpose The physiological flow dynamics within the Valsalva sinuses, in terms of global and local parameters, are still not fully understood. This study attempts to identify the physiological conditions as closely as possible, and to give an explanation of the different and sometime contradictory results in literature. Methods An in vitro approach was implemented for testing porcine bio-prosthetic valves operating within different aortic root configurations. All tests were performed on a pulse duplicator, under physiological pressure and flow conditions. The fluid dynamics established in the various cases were analysed by means of 2D Particle Image Velocimetry, and related with the achieved hydrodynamic performance. Results Each configuration is associated with substantially different flow dynamics, which significantly affects the valve performance. The configuration most closely replicating healthy native anatomy was characterised by the best hemodynamic performance, and any mismatch in size and position between the valve and the root produced substantial modification of the fluid dynamics downstream of the valve, hindering the hydrodynamic performance of the system. The worst conditions were observed for a configuration characterised by the total absence of the Valsalva sinuses. Conclusion This study provides an explanation for the different vortical structures described in the literature downstream of bioprosthetic valves, enlightening the experimental complications in valve testing. Most importantly, the results clearly identify the fluid mechanisms promoted by the Valsalva sinuses to enhance the ejection and closing phases, and this study exposes the importance of an optimal integration of the valve and root, to operate as a single system.

STABILITY OF A STRATIFIED VISCOUS SHEAR FLOW IN A TILTED TUBE

The present investigation is concerned with the effects of viscosity on the stability of a bounded stratified shear flow with Prandtl number Pr≫1. Theoretical results obtained from the solution of the Orr–Sommerfeld equation extended to stratified fluids are compared with experiments performed in a tilting tube filled with water and brine. Theoretical analysis shows that a complete stabilization of the flow field with respect to infinitesimal disturbances is attained, irrespective of the Richardson number J, as the Reynolds number Re decreases below 75. This damping action of viscosity is shown to appreciably reduce the critical Richardson number Jc with respect to the inviscid limit Jc=0.25, even at moderately high Re. On the other hand, the destabilizing action enhanced by viscosity through the diffusion of momentum leads to a viscous mode of instability that may develop if J decreases below a threshold value. An extensive series of experiments has been carried out in a long tilting tube in order to ver...

Numerical study of the Guderley and Vasilev reflections in steady two-dimensional shallow water flow

We present high-resolution numerical solutions of the depth-averaged two-dimensional inviscid shallow water equations which provide new information on shock reflection configuration within the von Neumann paradox conditions. The computed flow field and shock wave patterns close to the triple point for the Guderley and the Vasilev reflections confirm the four-wave theory. We suggest that the most likely Guderley reflection model is a four-wave pattern with a compression wave that originates along the downstream boundary of the supercritical patch. The compression wave, after being refracted by the slip stream, turns the flow behind the Guderley stem further toward the wall until critical condition is achieved.

Multiple States In Open Channel Flow

Steady flow regimes in a free surface flow approaching an obstacle are described and extensively discussed. Attention is focused on the phenomenon of hydraulic hysteresis, and a simple one-dimensional theory to predict its occurrence in a supercritical channel flow is proposed. It is shown that in many cases knowledge of the Froude number of the undisturbed approaching flow and of a geometric characteristic of the obstacle allows for a reliable prediction of the flow state. In the region of multiple regimes, however, the previous history of the flow must also be known. Three different obstacles in a rectangular channel are considered, namely a sill, a vertical sluice gate, and a circular cylinder, and the theoretical boundaries of the hysteresis region are specified for each obstacle. The experimental results show that the theoretical predictions are consistent with experiments in the case of obstacles that do not affect channel width (i.e. sills and gates). On the contrary, in the case of channel contraction, a further parameter, which the presented theory does not account for, was found to affect the behavior of the flow, namely the ratio of undisturbed flow depth to contraction width. Finally, in the case of a vertical sluice gate it was found that hysteresis develops in a subcritical undisturbed approaching flow as well.

In vitro hemodynamic testing of Amplatzer plugs for paravalvular leak occlusion after transcatheter aortic valve implantation.

OBJECTIVE We aimed to in-vitro test Amplatzer devices (Amplatzer Vascular Plug II and Amplatzer Vascular Plug III, SJM, St. Paul, MN) in closing PVL generated by transcatheter balloon expandable aortic valve prosthesis in order to quantify the effective treatment of PVL. BACKGROUND Transcatheter aortic valve implantation (TAVI) procedures represent the treatment of choice for high risk patients. Despite evolving technologies paravalvular leak (PVL) is still a major unaddressed issue. This severe complication significantly impairs long-term survival. Percutaneous treatment of this complication is usually performed with the implantation of not specifically designed and not approved vascular devices. METHODS A 26 mm Sapien XT (Edwards Lifesciences, Irvine, CA) was implanted in a rubber aortic root and a semi-elliptical shape PVL was created. The vascular occluder devices were implanted in the PVL and hemodynamic performance was tested in a pulse duplicator according to international standard ISO 5840-3:2013. Different type of comparison tests together with high speed camera recording allowed us to define the global efficiency of the occluders and their interaction with the transcatheter prosthesis. RESULTS The results revealed that the use of vascular plugs was not per se sufficient to produce an effective or substantial reduction of PVL with a maximum efficiency of less than 50%. Recorded video showed clearly that the vascular plug always interfered with the leaflet of the prosthetic valve. CONCLUSIONS Currently used devices do not guarantee effective treatment of PVL and may otherwise compromise the structural integrity of the prosthetic valve implanted. Specifically designed devices are required. CONDENSED ABSTRACT Despite evolving technologies, paravalvular leak (PVL) is still a major unaddressed issue after transcatheter aortic valve implantation. Percutaneous treatment of this complication is usually performed with the implantation of Amplatzer devices not specifically designed and not approved for this specific use. We tested Amplatzer devices in a pulse duplicator to occlude PVL generated after implantation of a 26 mm SAPIENT XT prosthesis. The results revealed that the use of vascular plugs was not per se sufficient to produce an effective or substantial reduction of PVL. The video showed clearly that the vascular plug always interfered with the leaflet of the prosthetic valve.

Comparative classification of thrombotic formations on bileaflet mechanical heart valves by phonographic analysis

Haemodynamic performance of bileaflet mechanical heart valves can be severely affected by the formation of thrombotic deposits. Hence, early detection of thrombi is fundamental for a prompt diagnosis and adequate therapy. This article aims at designing a novel diagnostic and prognostic tool able to detect valvular thrombosis at early stages of formation, i.e., before the appearance of critical symptoms in patients who can be effectively treated by pharmacological therapy, preventing re-operation. This approach relies on the acquisition of the acoustic signals produced by mechanical heart valves in the closing phase; the corresponding power spectra are then analysed by means of artificial neural networks trained to identify the presence of thrombi and classify their occurrence. Five commercial bileaflet mechanical heart valves were investigated in vitro in a Sheffield Pulse Duplicator; for each valve six functional conditions were considered, each corresponding to a risk class for patients (one normofunctioning and five thrombosed): they have been simulated by placing artificial deposits of increasing weight and different shape on the valve leaflet and on the annular housing; the case of one completely blocked leaflet was also investigated. These six functional conditions represent risk classes: they were examined under various hydrodynamic regimes. The acoustic signals produced by the valves were acquired by means of a phonocardiographic apparatus, then analysed and classified. The ability to detect and classify thrombotic formations on mechanical valve leaflet would allow ranking patients by assigning them to one of the six risk classes, helping clinicians in establish adequate therapeutic approaches.

A Red Blood Cell Model to Estimate the Hemolysis Fingerprint of Cardiovascular Devices

One of the most relevant and open issues within cardiovascular prosthetic hemodynamic performance is a realistic quantification of the damage sustained by red blood cells (RBCs). Specifically, the optimal design of bileaflet mechanical heart valves (BMHVs) requires both low shear stresses along the leaflets and short particle resident times. This study approaches RBC damage estimation by developing a numerical model of RBCs and computing the damage sustained by a set of passive RBCs immersed within in vitro flows. The RBC is modeled as an ellipsoidal shell with size dependent on age. Mechanically, a viscous hyper-elastic model was adopted to compute the stress-deformation transmitted by the experimental flow field to the RBC layer. The rupture parameters were calibrated using experimental results on real RBCs submitted to Couette flow. Moreover, the integrated hemolysis index (HI) through a BMHV was computed for a set of RBCs injected in a flow field derived from an in vitro study and for multiple RBC passages. The main results are (1) a good capability of the RBC model to replicate in vitro experiments performed with real RBCs, finding realistic rupture parameters; (2) the spatial distribution for the HI, maximal along the leaflet boundary layer and for long resident times; (3) 90% of HI is produced by the less damaging trajectories, which are favored by local flow dynamics; (4) cumulated HI in 8 days is about 0.01% smaller than the clinical warning threshold, the latter being obtained only after a period of time comparable with the RBC lifetime.