BACKGROUND AND AIM OF THE STUDY Cavitation occurs during mechanical heart valve closure when the local pressure drops below vapor pressure. The formation of stable gas bubbles may result in gaseous emboli, and secondarily cause transient ischemic attacks or strokes. It is noted that instantaneous valve closure, occluder rebound and high-speed leakage flow generate vortices that promote low-pressure regions in favor of stable bubble formation; however, to date no studies have been conducted for the quantitative measurement and analysis of these vortices. METHODS A Bjork-Shiley Monostrut (BSM) monoleaflet valve was placed in the mitral position of a pulsatile mock circulatory loop. Particle image velocimetry (PIV) and pico coulomb (PCB) pressure measurements were applied. Flow field measurements were carried out at t = -5, -3, -1, -0.5, 0 (valve closure), 0.3, 0.5, 0.75, 1.19, 1.44, 1.69, 1.94, 2, 2.19, 2.54, 2.79, 3.04, 3.29, 3.54, 5 and 10 ms. The vortices were quantitatively analyzed using the Rankine vortex model. RESULTS A single counter-clockwise vortex was The instantaneous formation of cavitation bubbles at mechanical heart valve (MHV) closure, which subsequently damage blood cells and valve integrity, is a well-known and widely studied phenomenon (1-4). Contributing factors seem to include the water-hammer, squeeze flow and Venturi effects, all of which are short-lived. Both, Dauzat et al. (5) and Sliwka et al. (6) have detected high-intensity transient signals (HITS) with transcranial Doppler ultrasound in the carotid and cerebral arteries of MHV recipients, while Deklunder (7) observed clinical occurrences of cerebral gas emboli that were not seen with bioprosthetic valves. These detected over the major orifice, while a pair of counter-rotating vortices was found over the minor orifice. Velocity profiles were consistent with Rankine vortices. The vortex strength and magnitude of the pressure drop peaked shortly after initial occluder-housing impact and rapidly decreased after 0.5 ms, indicating viscous dissipation, with a less significant contribution from the occluder rebound effect. The maximum pressure drop was on the order of magnitude of 40 mmHg. CONCLUSION Detailed PIV measurements and quantitative analysis of the BSM mechanical heart valve revealed large-scale vortex formation immediately after valve closure. Of note, the vortices were typical of a Rankine vortex and the maximum pressure change at the vortex center was only 40 mmHg. These data support the conclusion that vortex formation alone cannot generate the magnitude of pressure drop required for cavitation bubble formation.
Abstract In order to gain further understanding of aerodynamic forces and their effects on groups of high‐rise buildings, this study used wind‐tunnel experiments. Two square prisms were arranged both in tandem and side‐by‐side arrangement with different spacings in between. Similar experiments were carried out to study the interactions of aerodynamics between the two prisms when both were stationary, when only one prism oscillated, and finally, when both prisms oscillated. The results showed that the aerodynamic responses were either enhanced or suppressed by the spacing ratios, the oscillating frequencies, and the mutual influences of the two square prisms in various arrangements. The aerodynamics also changed due to the occurrences of different flow patterns, such as channel flow, deflected flow, pulsating flow, and so on. Obviously, the aerodynamics of the flow patterns of the two square prisms in tandem and side‐by‐side arrangements proved to be more complex than those of a single square prism.
High-speed squeeze flow during mechanical valve closure is often thought to cause cavitation, either between the leaflet tip and flat contact area in the valve housing, seating lip, or strut flat seat stop, depending on design. These sites have been difficult to measure within the housing, limiting earlier research to study of squeeze flow outside the housing or with computational fluid dynamics. We directly measured squeeze flow velocity with laser Doppler velocimetry at its site of occurrence within the St. Jude Medical (SJM), Omnicarbon (OC), and Medtronic Hall Standard (MHS) 29 mm valves in a mock circulation loop. Quartz glass provided an observation window to facilitate laser penetration. Our results showed increasing squeeze flow velocity at higher heart rates: 2.39–3.44 m/s for SJM, 3.07–4.33 m/s for OC, and 3.87–5.33 m/s for MHS. Strobe lighting technique captured the images of cavitation formation. Because these results were obtained in a mock circulation loop, one can assume this may occur in vivo resulting in valve damage, hemolysis, and thromboembolism. However, velocities of this magnitude alone cannot produce the pressure drop required for cavitation, and the applicability of the Bernoulli equation under these circumstances requires further investigation.
Background: For product reliability, FDA Replacement Heart Valve Guidance requires that newly developed heart valve prosthesis must be tested for 600 million cycles in an accelerated tester. We noted that the structure wear are related to the occluder closing motion. Methods: The 29mm Medtronic Hall standard valve is installed in an accelerated tester operating under the transvalular pressure of 0–120 mmHg. The leaflet motion is monitored by the laser sweeping technique and CCD laser displacement sensor. A Millar micro-tip transducer catheter is located 30 mm proximal to the valve to record the ventricular pressure waveform. A piezoelectric pressure transducer is flush mounted immediately upstream of the valve seat to record the high frequency oscillations (HFO) at the instant of valve closure. These signals were synchronize by using a digital oscilloscope of 5 MHz per channel. Results: hen the accelerated test rate was increased from 120 to 600 beats/min, the peak leaflet velocity at closing also increased from 1.9 to 3.9 m/s. From the trace of the sweeping reflection laser beam, we found that the occluder moves with a rotational motion combined with slight transitional motion for the approximately 3o before closure. After the leaflet first touched the valve ring due to the rotational contact, it adjusted its position by plane slipping to its final closure location due to the transitional contact. The HFO’s waveforms unveiled two peak bursts at the leaflet closure, individually representing the two different movement behaviors of the leaflet.
Abstract A series of experiments have been conducted to investigate the mean drag coefficient (CD ), fluctuating drag coefficient (CD‘), fluctuating lift coefficient (CL‘), spanwise correlation, vorticity of the wake, wake velocity spectrum and lift spectrum of a square prism due to forced vibration normal to approaching turbulence flows. The frequencies and amplitudes of vibration, the turbulence intensities, and the length scales of flow fields have been controlled as the parameters of the experiments. The results show that the wind coefficients are much greater when the oscillating cylinder is in the lock‐in range rather than stationary and reach peak value at the resonant frequency, while the wake vorticity attains maximum values. High turbulence intensity will weaken the vortex shedding due to the effect of shear layer mixing, so the wake vorticity and wind coefficients, CD and CL’, respectively, and the peak values of the spectrum of velocity and lift force will be decreased, while, on the contrary, CD’ will increase. As the turbulence length scale increases, the vorticity of wake and wind coefficients will increase proportional to the oscillating amplitude.
ABSTRACT The characteristics of vortex wakes of rectangular prisms, with and wihtout forced osicllation, were experimentally investigated, as placed in different uniform turbulent flows. The experimental parameters were the turbulence intensities and length scales of the uniform turbulent flows and the depth-to-width ratios and vibrating frequencies, with fixed amplitudes, of the rectangular prisms. Measurements were taken on the lateral pressure and base pressure of the prisms, the point velocity in the wake, and the phase angle between lateral pressure and body displacement. The pressure coefficients (-Cp, -Cpb, Cp′, C′pb ) are much greater when the oscillating cylinder is in lock-in range and reach peak values at resonant frequencies in a uniform flow. High turbulence intensity weakens the vortex shedding, thereby decreasing the pressure coefficients and peak values of the spectra. Increasing the turbulent length scale causes the pressure coefficients to increase. The critical depth- to-width ratio is found at H/D=0.6, while decreasing in this ratio results owing to increases in turbulence intensities.
Artificial prostheses create non-physiologic flow conditions with stress forces that may induce blood cell damage, particularly hemolysis. Earlier computational fluid dynamics (CFD) prediction models based on a quantified power model showed significant discrepancies with actual hemolysis experiments. These models used the premise that shear stresses act as the primary force behind hemolysis. However, additional studies have suggested that extensional stresses play a more substantial role than previously thought and should be taken into account in hemolysis models. We compared extensional and shear stress flow fields within the contraction of a short capillary with sharp versus tapered entrances. The flow field was calculated with CFD to determine stress values, and hemolysis experiments with porcine red blood cells were performed to correlate the effects of extensional and shear stress on hemolysis. Our results support extensional stress as the primary mechanical force involved in hemolysis, with a threshold value of 1000 Pa under exposure time less than 0.060 ms.
Abstract OBJECTIVES Handmade trileaflet expanded polytetrafluoroethylene valved conduit developed using the flip-over method has been tailored for pulmonary valve reconstruction with satisfactory outcomes. We investigated the in vitro performance of the valve design in a mock circulatory system with various conduit sizes. In our study, the design was transformed into a transcatheter stent graft system which could fit in original valved conduits in a valve-in-valve fashion. METHODS Five different sizes of valved polytetrafluoroethylene vascular grafts (16, 18, 20, 22 and 24 mm) were mounted onto a mock circulatory system with a prism window for direct leaflets motion observation. Transvalvular pressure gradients were recorded using pressure transducers. Mean and instant flows were determined via a rotameter and a flowmeter. Similar flip-over trileaflet valve design was then carried out in 3 available stent graft sizes (23, 26 and 28.5 mm, Gore aortic extender), which were deployed inside the valved conduits. RESULTS Peak pressure gradient across 5 different sized graft valves, in their appropriate flow setting (2.0, 2.5 and 5.0 l/min), ranged from 4.7 to 13.2 mmHg. No significant valve regurgitation was noted (regurgitant fraction: 1.6–4.9%) in all valve sizes and combinations. Three sizes of the trileaflet-valved stent grafts were implanted in the 4 sizes of valved conduits except for the 16-mm conduit. Peak pressure gradient increase after valved-stent graft-in-valved-conduit setting was <10 mmHg in all 4 conduits. CONCLUSIONS The study showed excellent in vitro performance of trileaflet polytetrafluoroethylene valved conduits. Its valved stent graft transformation provided data which may serve as a reference for transcatheter valve-in-valve research in the future.
Cavitation with mechanical valve closure has been observed both in vitro and in vivo when local pressure drops below vapor pressure. Under physiological conditions, adequate data on random cavitation formation is difficult to obtain. We used accelerated testing to 600 bpm, a transvalvular pressure of 120 mmHg, synchronized high-speed videography, and high-frequency pressure measurements to study cavitation intensity and behavior. The Medtronic Hall Standard 29mm (MHS), Medtronic Hall D-16 29mm (MHD), and Omni Carbon 29mm (OC) were studied with a Millar micro-tip transducer catheter, a PCB high-fidelity piezoelectric pressure transducer, and a CCD high-speed video camera to indicate downstream pressure changes with valve closure and cavitation intensity and distribution. MHS creates a cloud of bubbles near the seat stop due to squeeze flow, followed by peripheral cavitation between the housing ring and leaflet edge due to occluder rebound and Venturi effect; cavitation lasts for 400 μs. MDH, without a seat stop and squeeze flow, has a water hammer effect causing more distributed bubbles on the occluder surface followed by similar rebound and Venturi effects, lasting 350 μs. The OC valve also has the water hammer, but a cloud of localized bubbles does not appear until 50 μs after rebound. The tension wave and Venturi effect both contribute to cavitation lasting 350 μs. The underlying major contributor to cavitation varies based on valve design. However, all three valves tested here show significant rebound effect intensifying cavitation, implying future designs must account for this phenomenon.
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