scholarly journals Turbulent Systolic Flow Downstream of a Bioprosthetic Aortic Valve: Velocity Spectra, Wall Shear Stresses, and Turbulent Dissipation Rates

2020 ◽  
Vol 11 ◽  
Author(s):  
Barna Becsek ◽  
Leonardo Pietrasanta ◽  
Dominik Obrist
Keyword(s):  
Aortic Valve ◽  
Shear Stresses ◽  
Turbulent Dissipation ◽  
Dissipation Rates ◽  
Wall Shear ◽  
Velocity Spectra

scholarly journals Computational Analysis of Wall Shear Stress Patterns on Calcified and Bicuspid Aortic Valves: Focus on Radial and Coaptation Patterns

Fluids ◽  
2021 ◽  
Vol 6 (8) ◽  
pp. 287
Author(s):  
Huseyin Enes Salman ◽  
Levent Saltik ◽  
Huseyin C. Yalcin
Keyword(s):  
Aortic Valve ◽  
Three Dimensional ◽  
Shear Stresses ◽  
Aortic Valves ◽  
Bicuspid Valve ◽  
Valve Function ◽  
Valvular Diseases ◽  
Wall Shear ◽  
Stress Patterns ◽  
Valve Formation

Calcification and bicuspid valve formation are important aortic valve disorders that disturb the hemodynamics and the valve function. The detailed analysis of aortic valve hemodynamics would lead to a better understanding of the disease’s etiology. We computationally modeled the aortic valve using simplified three-dimensional geometry and inlet velocity conditions obtained via echocardiography. We examined various calcification severities and bicuspid valve formation. Fluid-structure interaction (FSI) analyses were adapted using ANSYS Workbench to incorporate both flow dynamics and leaflet deformation accurately. Simulation results were validated by comparing leaflet movements in B-mode echo recordings. Results indicate that the biomechanical environment is significantly changed for calcified and bicuspid valves. High flow jet velocities are observed in the calcified valves which results in high transvalvular pressure difference (TPG). Wall shear stresses (WSS) increased with the calcification on both fibrosa (aorta side) and ventricularis (left ventricle side) surfaces of the leaflet. The WSS distribution is regular on the ventricularis, as the WSS values proportionally increase from the base to the tip of the leaflet. However, WSS patterns are spatially complex on the fibrosa side. Low WSS levels and spatially complex WSS patterns on the fibrosa side are considered as promoting factors for further calcification and valvular diseases.

scholarly journals Computational Investigation of Wall Shear Stress Patterns on Calcified Aortic Valve Leaflets

2020 ◽  
Author(s):  
Huseyin Enes Salman ◽  
Huseyin Cagatay Yalcin
Keyword(s):  
Aortic Valve ◽  
Patient Specific ◽  
Shear Stresses ◽  
Aortic Leaflet ◽  
Attachment Region ◽  
Potential Risk Factors ◽  
Wall Shear ◽  
Dynamic Structures ◽  
Interaction Approach ◽  
Valve Diseases

Background: Aortic valve diseases affect about 25% of the population over 65 years of age. Aortic valve separates the left ventricle from the aorta, and consists of three half-moon shaped leaflets. The leaflets are highly dynamic structures which open during the ventricle contraction and close during the ventricle relaxation. Calcification on leaflet surfaces results in poor valve functioning which deteriorates the valve hemodynamics. Wall shear stresses (WSS) on the leaflet surfaces are considered to be strongly related with the initiation and progression of calcification. Aim: To investigate the effect of altered hemodynamics on the valve leaflet calcification, and to understand the role of WSS patterns in the progression of the aortic valve diseases. Methods: We investigate the hemodynamics of aortic valves using computational modeling. Fluid-structure interaction approach is employed to accurately determine the complex dynamic motion of valve leaflets. A 3D patient-specific aortic valve model is generated. Using finite element modeling, blood flow velocities, pressures, and WSS values are determined within the entire model, employing numerical techniques to obtain the characteristics of altered hemodynamics and spatial WSS patterns. Results: In case of calcification, WSS values are increased at both surfaces of the leaflets. On the ventricularis surface, there is a spatially-regular WSS distribution, which gradually increase from the leaflet attachment region to the leaflet tip. However, a spatially-complex WSS distribution is observed on the aortic leaflet surface. Conclusion: Relatively low WSS levels and spatiallycomplex WSS patterns on the aortic leaflet surface are observed as potential risk factors for the initiation and progression of aortic valve calcification.

scholarly journals Estimation of Kinetic Energy Dissipation from Breaking Waves in the Wave Crest Region

2017 ◽  
Vol 47 (5) ◽  
pp. 1145-1150 ◽  
Author(s):  
J. H. Lee ◽  
J. P. Monty ◽  
J. Elsnab ◽  
A. Toffoli ◽  
A. V. Babanin ◽  
...  
Keyword(s):  
Energy Dissipation ◽  
Kinetic Energy ◽  
Isotropic Turbulence ◽  
Breaking Waves ◽  
Wave Crest ◽  
Shear Stresses ◽  
Surface Shear ◽  
Phase Dependence ◽  
Dissipation Rates ◽  
Velocity Spectra

AbstractWave-induced turbulence due to breaking in the absence of surface shear stresses is investigated experimentally. A high-fidelity particle image velocimetry (PIV) technique is used to measure the turbulence near the water surface, inside the wave crests. The spatial velocity vector fields of the breaking waves acquired from PIV provide accurate vertical velocity profiles near the air–water interface, as well as wavenumber velocity spectra beneath the breaking waves at different depths. These velocity spectra exhibit a Kolmogorov interval at high wavenumbers, indicating the presence of isotropic turbulence and permitting an estimation of energy dissipation rates. The depth dependence of dissipation rates of the breaking waves generated in the laboratory shows a scaling similar to that found in wind-forced breaking waves in the field. A phase dependence in the dissipation rates of turbulence kinetic energy is also observed, which should be considered to improve the accuracy of the estimated and modeled wave energy dissipation.

High-frequency operation of pulsatile ventricular assist devices: A computational study on circular and elliptically shaped pumps

2019 ◽  
Vol 42 (12) ◽  
pp. 725-734 ◽  
Author(s):  
Christian Loosli ◽  
Stephan Rupp ◽  
Bente Thamsen ◽  
Mathias Rebholz ◽  
Gerald Kress ◽  
...  
Keyword(s):  
High Frequency ◽  
Flow Patterns ◽  
Ventricular Assist Devices ◽  
Shear Stresses ◽  
Residence Times ◽  
Pump Frequency ◽  
Ventricular Assist ◽  
Assist Devices ◽  
High Frequency Operation ◽  
Wall Shear

Pulsatile positive displacement pumps as ventricular assist devices were gradually replaced by rotary devices due to their large volume and high adverse event rates. Nevertheless, pulsatile ventricular assist devices might be beneficial with regard to gastrointestinal bleeding and cardiac recovery. Therefore, aim of this study was to investigate the flow field in new pulsatile ventricular assist devices concepts with an increased pump frequency, which would allow lower stroke volumes to reduce the pump size. We developed a novel elliptically shaped pulsatile ventricular assist devices, which we compared to a design based on a circular shape. The pump size was adjusted to deliver similar flow rates at pump frequencies of 80, 160, and 240 bpm. Through a computational fluid dynamics study, we investigated flow patterns, residence times, and wall shear stresses for different frequencies and pump sizes. A pump size reduction by almost 50% is possible when using a threefold pump frequency. We show that flow patterns inside the circular pump are frequency dependent, while they remain similar for the elliptic pump. With slightly increased wall shear stresses for higher frequencies, maximum wall shear stresses on the pump housing are higher for the circular design (42.2 Pa vs 18.4 Pa). The calculated blood residence times within the pump decrease significantly with increasing pump rates. A smaller pump size leads to a slight increase of wall shear stresses and a significant improvement of residence times. Hence, high-frequency operation of pulsatile ventricular assist devices, especially in combination with an elliptical shape, might be a feasible mean to reduce the size, without any expectable disadvantages in terms of hemocompatibility.

Response of cultured nasal goblet cells to wall shear stresses

2006 ◽  
Vol 39 ◽  
pp. S602
Author(s):  
N. Even-Tzur ◽  
U. Zaretsky ◽  
M. Wolf ◽  
D. Elad
Keyword(s):  
Goblet Cells ◽  
Shear Stresses ◽  
Wall Shear

Preston-Static Tubes for the Measurement of Wall Shear Stress

1994 ◽  
Vol 116 (3) ◽  
pp. 645-649 ◽  
Author(s):  
Josef Daniel Ackerman ◽  
Louis Wong ◽  
C. Ross Ethier ◽  
D. Grant Allen ◽  
Jan K. Spelt
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Convenient Method ◽  
Pipe Flow ◽  
Total Pressure ◽  
Static Pressure ◽  
Shear Stresses ◽  
Streamline Curvature ◽  
Syringe Needle ◽  
Wall Shear

We present a Preston tube device that combines both total and static pressure readings for the measurement of wall shear stress. As such, the device facilitates the measurement of wall shear stress under conditions where there is streamline curvature and/or over surfaces on which it is difficult to either manufacture an array of static-pressure taps or to position a single tap. Our “Preston-static” device is easily and conveniently constructed from commercially available regular and side-bored syringe needles. The pressure difference between the total pressure measured in the regular syringe needle and the static pressure measured in the side-bored one is used to determine the wall shear stress. Wall shear stresses measured in pipe flow were consistent with independently determined values and values obtained using a conventional Preston tube. These results indicate that Preston-static tubes provide a reliable and convenient method of measuring wall shear stress.

scholarly journals Experimental investigation of an annular diffuser for axial fans at different inflow profiles

2017 ◽  
Vol 21 (suppl. 3) ◽  
pp. 553-564
Author(s):  
Johannes Walter ◽  
Dieter Wurz ◽  
Stefan Hartig ◽  
Martin Gabi
Keyword(s):  
Power Plants ◽  
High Efficiency ◽  
Outer Wall ◽  
Pressure Recovery ◽  
Shear Stresses ◽  
Axial Fan ◽  
Mean Velocity ◽  
Annular Diffuser ◽  
Axial Fans ◽  
Wall Shear

Axial fans are used in power plants for fresh air supply and flue gas transport. A typical configuration consists of an axial fan and annular diffuser which connects the fan to the following piping. In order to achieve a high efficiency of the con-figuration, not only the components have to be optimized but also their interaction. The present study focuses on the diffuser of the configuration. Experiments are performed on a diffuser-piping configuration to investigate the influence of the velocity profile at the fan outlet on the pressure recovery of the configuration. Two different diffuser inlet profiles are generated, an undisturbed profile and a profile with the typical outlet characteristics of a fan. The latter is generated by the superposition of screens in the inlet zone. The tests are conducted at a high Reynolds number (Re ? 4?105). Mean velocity profiles and wall shear stresses are measured with hydraulic methods (Prandtl and Preston tubes). The results show that there is a lack of momentum at the outer wall of the diffuser and high shear stresses at the inner wall in case of the undisturbed inflow profile. For the typical fan outlet profile it is vice versa. There are high wall shear stresses at the outer wall while the boundary layer of the inner wall lacks momentum. The pressure recovery of the undisturbed inflow configuration is in good agreement with other studies.

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