An experimental study is reported which investigates the wall shear stress (WSS) distribution in a transparent model of the human aorta comparing a bileaflet mechanical heart valve (BMHV) with a trileaflet mechanical heart valve (TMHV) in physiological pulsatile flow. Elastic micro-pillar WSS sensors, calibrated by micro-Particle-Image-Velocimetry measurement, are applied to the wall along the ascending aorta (AAo). Peak WSS values are observed almost twice in BMHV compared to TMHV.Flow field analyses illuminate that these peaks are linked to the jet-like flows generated in the valves interacting with the aortic wall. Not only the magnitude but also the impact regions are specific for the different valve designs. The side-orifice jets generated by BMHV travel along the aortic wall in the AAo and cause a whole range impact, while the jets generated by TMHV impact further downstream in the AAo generating less severe WSS.

It is widely acknowledged that rotary positive displacement machines exhibit highly unsteady flow fields that affect their performance. The presence of the operational clearances impacts this unsteady flow field and further affects the performance. However, the exact nature of these unsteady flow mechanisms remains largely unknown that necessitates both detailed experimental investigations and computational modelling. Thus, the present study employs both optical visualization and unsteady Reynolds-Averaged Navier Stokes (URANS) computational modelling methods while focussing on investigating the transient flow field inside a Roots blower, a general type of the rotary positive displacement machine. Straight lobes in a Roots blower provide convenient optical access to experimentally analyse internal flow and compare it with the predictions obtained by standard computational models. In the first part of this paper, this study covers the lowspeed experimental investigations using i) High-Speed Camera (HC), ii) the continuous High-Speed Particle Image Velocimetry (CPIV) and, iii) the instantaneous PIV (IPIV) obtained with a double pulse laser and a double shutter camera. Relative merits from these techniques are discussed with respect to the Roots blower unsteady flow mechanisms. In addition, computational analyses are performed using a combination of in-house and commercial modelling methods and the results are compared against the experiments. The results confirm the existence of highly three-dimensional and unsteady flow field where certain distinct flow mechanisms originating from the operational clearances impact the performance of the Roots blower. The study also highlights challenges of the experimental and computational methods used for evaluation of positive displacement machines that impact the accuracy of results.

An experimental study is reported which investigates the head-on collision of a laminar vortex ring of diameter D (Γ = 3000) on a fakir-like surface composed of circular posts of height h/D=0.068 located on a planar bed. Lattices of the posts in hexagonal and random distribution (average porosity of ϵ=0.94 in the layer) are compared to each other with respect to the plain wall. Prior to impact, the vortex ring develops the early state of natural azimuthal instabilities of different mode numbers N=5-7 competing with each other. While impacting with the rough wall, a secondary ring is observed which is pushed outwards and is not wrapped around the primary ring as in flat wall impact. Between both rings of opposite sign vorticity, a strong fluid rebound is induced. The hexagonal lattice causes the rapid growth of further secondary vortex structures in a regular mode number N=6 arrangement at the outer edge of the primary ring in form of six lobes which are aligned with the orientations of preferential "pathways" in the layer. At the outer tip of the lobes radial wall-jets are generated. Rotating the fakir geometry around the centre of impact also rotates the jets' location and direction accordingly. A surface with random lattice of the posts at the same average number density is not able to repeat this observation and no regular secondary flow pattern is visible until full breakdown of the ring. The results show that a tailored arrangement of such posts can be used for near-wall flow control when patterns of preferred pathways in the posts' layer lock-on with existing instability modes such as in impacting jet flows or in turbulent boundary layer flows.

Background—Physiological helical flow in the ascending aorta has been well documented in the last two decades, accompanied by discussions on possible physiological benefits of such axial swirl. Recent 4D-MRI studies on healthy volunteers have found indications of early generation of helical flow, early in the systole and close to the valve plane. Objectives—Firstly, the aim of the study is to investigate the hypothesis of premature swirl existence in the ventricular outflow tract leading to helical flow in the valve plane, and second to investigate the possible impact of two different mechanical valve designs on the preservation of this early helical flow and its subsequent hemodynamic consequences. Methods—We use a pulse duplicator with an aortic arch and High-Speed Particle Image Velocimetry to document the flow evolution in the systolic cycle. The pulse-duplicator is modified with a swirl-generating insert to generate early helical flow in the valve plane. Special focus is paid to the interaction of such helical flow with different designs of mechanical prosthetic heart valves, comparing a classical bileaflet mechanical heart valve, the St. Jude Medical Regent valve (SJM Regent BMHV), with the Triflo trileaflet mechanical heart valve T2B version (Triflo TMHV). Results—When the swirl-generator is inserted, a vortex is generated in the core flow, demonstrating early helical flow in the valve plane, similar to the observations reported in the recent 4D-MRI study taken for comparison. For the Triflo trileaflet valve, the early helical flow is not obstructed in the central orifice, similar as in the case of the natural valve. Conservation of angular momentum leads to radial expansion of the core flow and flattening of the axial flow profile downstream in the arch. Furthermore, the early helical flow helps to overcome separation at the outer and inner curvature. In contrast, the two parallel leaflets for the bileaflet valve impose a flow straightener effect, annihilating the angular momentum, which has a negative impact on kinetic energy of the flow. Conclusion—The results imply better hemodynamics for the Triflo trileaflet valve based on hydrodynamic arguments under the discussed hypothesis. In addition, it makes the Triflo valve a better candidate for valve replacements in patients with a pathological generation of nonaxial velocity in the ventricle outflow tract.

Background - Physiological helical flow in the ascending aorta has been well documented in the last two decades, accompanied by discussions on possible physiological benefits of such axial swirl. Recent 4D-MRI studies on healthy volunteers have shown indication of early generation of helical flow, early in the systole and already close to the valve plane. Objectives - Firstly, the aim of the study is to investigate the hypothesis of premature swirl existence in the ventricular outflow tract leading to already helical flow in the valve plane, and second to investigate the possible impact of two different mechanical valves design on the preservation of this early helical flow and its subsequent hemodynamic consequences. Methods - We use a pulse duplicator with an aortic arch and High Speed Particle Image Velocimetry to document the flow evolution in the systolic cycle. The pulse-duplicator is modified with a swirl-generating insert to generate early helical flow in the valve plane. Special focus is laid on the interaction of such helical flow with different designs of mechanical prosthetic heart valves, comparing a classical bileaflet mechanical heart valve, the St Jude Medical Regent valve (SJM Regent BMHV) with the Triflo trileaflet mechanical heart valve T2B version (Triflo TMHV). Results – When the swirl-generator is inserted, a vortex is generated in the core flow demonstrating early helical flow in the valve plane, similar as observed in the recent 4-D-MRI study taken for comparison. For the Triflo trileaflet valve, the early helical flow is not obstructed in the central orifice, similar as in the case of the natural valve. Conservation of angular momentum leads to radial expansion of the core flow and flattening of the axial flow profile downstream in the arch. Furthermore, the early helical flow helps to overcome separation at the outer and inner curvature. In contrast, the two parallel leaflets for the bileaflet valve impose a flow straightener effect, annihilating the angular momentum with negative impact on kinetic energy of the flow. Conclusion - The results imply better hemodynamics for the Triflo trileaflet valve based on hydrodynamic arguments under the discussed hypothesis. In addition, it makes the Triflo valve a better candidate for replacements in patients with pathological generation of nonaxial velocity in ventricle outflow tract.