Aerodynamic response of a red-tailed hawk to discrete transverse gusts

A limiting factor in the design of smaller size uncrewed aerial vehicles is their inability to navigate through gust-laden environments. As a result, engineers have turned towards bio-inspired engineering approaches for gust mitigation techniques. In this study, the aerodynamics of a red-tailed hawk's response to variable-magnitude discrete transverse gusts was investigated. The hawk was flown in an indoor flight arena instrumented by multiple high-speed cameras to quantify the 3D motion of the bird as it navigated through the gust. The hawk maintained its flapping motion across the gust in all runs; however, it encountered the gust at different points in the flapping pattern depending on the run and gust magnitude. The hawk responded with a downwards pitching motion of the wing, decreasing the wing pitch angle to between -20∘and -5∘, and remained in this configuration until gust exit. The wing pitch data was then applied to a lower-order aerodynamic model that estimated lift coefficients across the wing. In gusts slower than the forward flight velocity (low gust ratio), the lift coefficient increases at a low-rate, to a maximum of around 2-2.5. In gusts faster than the forward flight velocity (high gust ratio), the lift coefficient initially increased rapidly, before increasing at a low-rate to a value around 4-5. In both regimes, the hawk's observed height change due to gust interaction was similar (and small), despite larger estimated lift coefficients over the high gust regime. This suggests another mitigation factor apart from the wing response is present. One potential factor is the tail pitching response observed here, which prior work has shown serves to mitigate pitch disturbances from gusts.

Influence of Pulsatility and Inflow Waveforms on Tracheal Airflow Dynamics in Healthy Older Adults

Tracheal collapsibility is a dynamic process altering local airflow dynamics. Patient-specific simulation is a powerful technique to explore the physiological and pathological characteristics of human airways. One of the key considerations in implementing airway computations is choosing the right inlet boundary conditions that can act as a surrogate model for understanding realistic airflow simulations. To this end, we numerically examine airflow patterns under the influence of different profiles, i.e., flat, parabolic, and Womersley, and compare these with a realistic inlet obtained from experiments. Simulations are performed in ten patient-specific cases with normal and rapid breathing rates during the inhalation phase of the respiration cycle. At normal breathing, velocity and vorticity contours reveal primary flow structures on the sagittal plane that impart strength to cross-plane vortices. Rapid breathing, however, encounters small recirculation zones. Quantitative flow metrics are evaluated using time-averaged wall shear stress (TAWSS) and oscillatory shear index (OSI). Overall, the flow metrics encountered in a real velocity profile are in close agreement with parabolic and Womersley profiles for normal conditions, however, the Womersley inlet alone conforms to a realistic profile under rapid breathing conditions.

Influence of expiratory flow pulsatility on the effectiveness of a surgical mask

BACKGROUND

Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport expiratory particles. The World Health Organization recommends wearing face masks to reduce the airborne transmission of diseases such as SARS-CoV-2 (COVID-19). However, masks are not perfect as flow leakage occurs around the mask, and their effectiveness under realistic (multi-pulse) coughing conditions is unknown.

OBJECTIVE

To assess the influence of expiratory flow pulsatility on the effectiveness of a surgical face mask by quantifying and classifying the flow leakage around the mask.

METHODS

A custom-built pulsatile expiratory flow simulator is used to generate single- and multi-pulsed coughing events. Flow visualization and particle image velocimetry are used to assess the penetration distance and volume of leakage flow at the top and sides of a surgical mask.

RESULTS

Leakage flow velocity profiles at the top and sides of a surgical mask take the form of a wall jet and a free-shear jet, respectively. Multi-pulsed expiratory flow events are found to generate greater leakage flow around the mask than single-pulsed events.

SIGNIFICANCE

For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough.

IMPACT STATEMENT

The novelties of this study are: First, flow field measurements are used to quantify and classify the leakage flow fields around the top and sides of a surgical mask, providing a benchmark for quantitative modeling of leakage flow velocity profiles. Second, the influence of pulsatility on the effectiveness of surgical face masks is studied by quantifying the leakage volume. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough, as multi-pulsed expiratory flow events are found to generate greater flow leakage around the mask than single-pulsed events.

Transcatheter aortic valve thrombosis: a review of potential mechanisms

Transcatheter aortic valve (TAV) thrombosis has been recognized as a significant problem that sometimes occurs as early as within 30 days after valve implantation, leading to increased concerns of stroke and long-term valve durability. In this article, a critical summary of the relevant literature on identifying potential mechanisms of TAV thrombosis from the perspective of the well-known Virchow's triad, which comprises blood flow, foreign materials and blood biochemistry, is presented. Blood flow mechanisms have been the primary focus thus far, with a general consensus on the flow mechanisms with respect to haemodynamic conditions, the influence of TAV placement and expansion and the influence of coronary flow. Less attention has been paid to the influence of blood biochemistry and foreign materials (and related endothelial damage), with little consensus among studies with regards to platelet and/or microparticle levels post-TAV implantation. Finally, we discuss the future outlook for research with unanswered scientific questions.

Role of pulsatility on particle dispersion in expiratory flows

Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport respiratory particles. In this work, direct numerical simulation (DNS) of turbulent pulsatile jets, coupled with Lagrangian particle tracking of micron-sized droplets, is performed to investigate the role of secondary and tertiary expulsions on particle dispersion and penetration. Fully developed turbulence obtained from DNS of a turbulent pipe flow is provided at the jet orifice. The volumetric flow rate at the orifice is modulated in time according to a damped sine wave, thereby allowing for control of the number of pulses, duration, and peak amplitude. Thermodynamic effects, such as evaporation and buoyancy, are neglected in order to isolate the role of pulsatility on particle dispersion. The resulting vortex structures are analyzed for single-, two-, and three-pulse jets. The evolution of the particle cloud is then compared to existing single-pulse models. Particle dispersion and penetration of the entire cloud are found to be hindered by increased pulsatility. However, the penetration of particles emanating from a secondary or tertiary expulsion is enhanced due to acceleration downstream by vortex structures.

Three-dimensional extent of flow stagnation in transcatheter heart valves

The recent unexpected discovery of thrombosis in transcatheter heart valves (THVs) has led to increased concerns of long-term valve durability. Based on the clinical evidence combined with Virchow's triad, the primary hypothesis is that low-velocity blood flow around the valve could be a primary cause for thrombosis. However, due to limited optical access in such unsteady three-dimensional biomedical flows, measurements are challenging. In this study, for the first time, we employ a novel single camera volumetric velocimetry technique to investigate unsteady three-dimensional cardiovascular flows. Validation of the novel volumetric velocimetry technique with standard planar particle image velocimetry (PIV) technique demonstrated the feasibility of adopting this new technique to investigate biomedical flows. This technique was used to quantify the three-dimensional velocity field in the vicinity of a validated, custom developed, transparent THV in a bench-top pulsatile flow loop. Large volumetric regions of flow stagnation were observed in the neo-sinus throughout the cardiac cycle, with stagnation defined as a velocity magnitude lower than 0.05 m s^-1. The volumetric scalar viscous shear stress quantified via the three-dimensional shear stress tensor was within the range of low shear-inducing thrombosis observed in the literature. Such high-fidelity volumetric quantitative data and novel imaging techniques used to obtain it will enable fundamental investigation of heart valve thrombosis in addition to providing a reliable and robust database for validation of computational tools.

The Peak Index: Spirometry Metric for Airflow Obstruction Severity and Heterogeneity

Rationale: Chronic obstructive pulmonary disease (COPD) is characterized by airflow limitation. Spirometry loops are not smooth curves and have undulations and peaks that likely reflect heterogeneity of airflow.Objectives: To assess whether the Peak Index, the number of peaks adjusted for lung size, is associated with clinical outcomes.Methods: We analyzed spirometry data of 9,584 participants enrolled in the COPDGene study and counted the number of peaks in the descending part of the expiratory flow-volume curve from the peak expiratory flow to end-expiration. We adjusted the peaks count for the volume of the lungs from peak expiratory flow to end-expiration to derive the Peak Index. Multivariable regression analyses were performed to test associations between the Peak Index and lung function, respiratory morbidity, structural lung disease on computed tomography (CT), forced expiratory volume in 1 second (FEV1) decline, and mortality.Results: The Peak Index progressively increased from Global Initiative for Chronic Obstructive Lung Disease stage 0 through 4 (P < 0.001). On multivariable analysis, the Peak Index was significantly associated with CT emphysema (adjusted β = 0.906; 95% confidence interval [CI], 0.789 to 1.023; P < 0.001) and small airways disease (adjusted β = 1.367; 95% CI, 1.188 to 1.545; P < 0.001), St. George's Respiratory Questionnaire score (adjusted β = 1.075; 95% CI, 0.807 to 1.342; P < 0.001), 6-minute-walk distance (adjusted β = -1.993; 95% CI, -3.481 to -0.506; P < 0.001), and FEV1 change over time (adjusted β = -1.604; 95% CI, -2.691 to -0.516; P = 0.004), after adjustment for age, sex, race, body mass index, current smoking status, pack-years of smoking, and FEV1. The Peak Index was also associated with the BODE (body mass index, airflow obstruction, dyspnea, and exercise capacity) index and mortality (P < 0.001).Conclusions: The Peak Index is a spirometry metric that is associated with CT measures of lung disease, respiratory morbidity, lung function decline, and mortality.Clinical trial registered with www.clinicaltrials.gov (NCT00608764).

Using a Novel In Vitro Fontan Model and Condition-Specific Real-Time MRI Data to Examine Hemodynamic Effects of Respiration and Exercise

Several studies exist modeling the Fontan connection to understand its hemodynamic ties to patient outcomes (Chopski in: Experimental and Computational Assessment of Mechanical Circulatory Assistance of a Patient-Specific Fontan Vessel Configuration. Dissertation, 2013; Khiabani et al. in J Biomech 45:2376-2381, 2012; Taylor and Figueroa in Annu Rev Biomed 11:109-134, 2009; Vukicevic et al. in ASAIO J 59:253-260, 2013). The most patient-accurate of these studies include flexible, patient-specific total cavopulmonary connections. This study improves Fontan hemodynamic modeling by validating Fontan model flexibility against a patient-specific bulk compliance value, and employing real-time phase contrast magnetic resonance flow data. The improved model was employed to acquire velocity field information under breath-held, free-breathing, and exercise conditions to investigate the effect of these conditions on clinically important Fontan hemodynamic metrics including power loss and viscous dissipation rate. The velocity data, obtained by stereoscopic particle image velocimetry, was visualized for qualitative three-dimensional flow field comparisons between the conditions. Key hemodynamic metrics were calculated from the velocity data and used to quantitatively compare the flow conditions. The data shows a multi-factorial and extremely patient-specific nature to Fontan hemodynamics.

The Fluid Mechanics of Transcatheter Heart Valve Leaflet Thrombosis in the Neosinus

BACKGROUND

Transcatheter heart valve (THV) thrombosis has been increasingly reported. In these studies, thrombus quantification has been based on a 2-dimensional assessment of a 3-dimensional phenomenon.

METHODS

Postprocedural, 4-dimensional, volume-rendered CT data of patients with CoreValve, Evolut R, and SAPIEN 3 transcatheter aortic valve replacement enrolled in the RESOLVE study (Assessment of Transcatheter and Surgical Aortic Bioprosthetic Valve Dysfunction With Multimodality Imaging and Its Treatment with Anticoagulation) were included in this analysis. Patients on anticoagulation were excluded. SAPIEN 3 and CoreValve/Evolut R patients with and without hypoattenuated leaflet thickening were included to study differences between groups. Patients were classified as having THV thrombosis if there was any evidence of hypoattenuated leaflet thickening. Anatomic and THV deployment geometries were analyzed, and thrombus volumes were computed through manual 3-dimensional reconstruction. We aimed to identify and evaluate risk factors that contribute to THV thrombosis through the combination of retrospective clinical data analysis and in vitro imaging in the space between the native and THV leaflets (neosinus).

RESULTS

SAPIEN 3 valves with leaflet thrombosis were on average 10% further expanded (by diameter) than those without (95.5±5.2% versus 85.4±3.9%; P<0.001). However, this relationship was not evident with the CoreValve/Evolut R. In CoreValve/Evolut Rs with thrombosis, the thrombus volume increased linearly with implant depth (R²=0.7, P<0.001). This finding was not seen in the SAPIEN 3. The in vitro analysis showed that a supraannular THV deployment resulted in a nearly 7-fold decrease in stagnation zone size (velocities <0.1 m/s) when compared with an intraannular deployment. In addition, the in vitro model indicated that the size of the stagnation zone increased as cardiac output decreased.

CONCLUSIONS

Although transcatheter aortic valve replacement thrombosis is a multifactorial process involving foreign materials, patient-specific blood chemistry, and complex flow patterns, our study indicates that deployed THV geometry may have implications on the occurrence of thrombosis. In addition, a supraannular neosinus may reduce thrombosis risk because of reduced flow stasis. Although additional prospective studies are needed to further develop strategies for minimizing thrombus burden, these results may help identify patients at higher thrombosis risk and aid in the development of next-generation devices with reduced thrombosis risk.

Valve mediated hemodynamics and their association with distal ascending aortic diameter in bicuspid aortic valve subjects

PURPOSE

Valve mediated hemodynamics have been postulated to contribute to pathology of the ascending aorta (AAo). The objective of this study is to assess the association of aortic valve morphology and hemodynamics with downstream AAo size in subjects with bicuspid aortic valve (BAV) disease.

MATERIALS AND METHODS

Four-dimensional flow MRI at 1.5 or 3 Tesla was used to evaluate the hemodynamics in the proximal AAo of 52 subjects: size-matched controls with tricuspid aortic valves (n = 24, mid ascending aorta [MAA] diameter = 38.0 ± 4.9 mm) and BAV patients with aortic dilatation (n = 14 right and left coronary leaflet fusion [RL]-BAV, MAA diameter = 38.1 ± 5.3 mm; n = 14 right and noncoronary leaflet fusion [RN]-BAV, MAA diameter = 36.5 ± 6.6 mm). A validated semi-automated technique was used to evaluate hemodynamic metrics (flow angle, flow displacement, and jet quadrant) and valve morphology (orifice circularity) for all subjects. Regression analysis of these metrics to AAo diameter was performed.

RESULTS

RN-BAV subjects displayed a stronger correlation between hemodynamic metrics in the proximal AAo with diameter in the distal AAo compared with size-matched tricuspid aortic valve (TAV) controls and RL-BAV subjects. The distal AAo diameter was found to be strongly correlated to the upstream flow displacement (R²_adjusted = 0.75) and flow angle (R²_adjusted = 0.66) measured at the sino-tubular junction (STJ). Orifice circularity was also strongly correlated (R²_adjusted = 0.53) to the distal AAo diameter in RN-BAV subjects. For TAV controls and RL-BAV subjects, correlations were weaker (R²_adjusted < 0.2).

CONCLUSION

Hemodynamics in the STJ were strongly correlated to the distal AAo diameter for the RN-BAV subjects. Hemodynamic metrics were more strongly correlated to the downstream aortic size when compared with valve morphology metrics.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:246-254.

On the Mechanics of Transcatheter Aortic Valve Replacement

Transcatheter aortic valves (TAVs) represent the latest advances in prosthetic heart valve technology. TAVs are truly transformational as they bring the benefit of heart valve replacement to patients that would otherwise not be operated on. Nevertheless, like any new device technology, the high expectations are dampened with growing concerns arising from frequent complications that develop in patients, indicating that the technology is far from being mature. Some of the most common complications that plague current TAV devices include malpositioning, crimp-induced leaflet damage, paravalvular leak, thrombosis, conduction abnormalities and prosthesis-patient mismatch. In this article, we provide an in-depth review of the current state-of-the-art pertaining the mechanics of TAVs while highlighting various studies guiding clinicians, regulatory agencies, and next-generation device designers.

The hemodynamic effects of acute aortic regurgitation into a stiffened left ventricle resulting from chronic aortic stenosis

Acute aortic regurgitation (AR) post-chronic aortic stenosis is a prevalent phenomenon occurring in patients who undergo transcatheter aortic valve replacement (TAVR) surgery. The objective of this work was to characterize the effects of left ventricular diastolic stiffness (LVDS) and AR severity on LV performance. Three LVDS models were inserted into a physiological left heart simulator. AR severity was parametrically varied through four levels (ranging from trace to moderate) and compared with a competent aortic valve. Hemodynamic metrics such as average diastolic pressures (DP) and reduction in transmitral flow were measured. AR index was calculated as a function of AR severity and LVDS, and the work required to make up for lost volume due to AR was estimated. In the presence of trace AR, higher LVDS had up to a threefold reduction in transmitral flow (13% compared with 3.5%) and a significant increase in DP (2-fold). The AR index ranged from ∼42 to 16 (no AR to moderate AR), with stiffer LVs having lower values. To compensate for lost volume due to AR, the low, medium, and high LVDS models were found to require 5.1, 5.5, and 6.6 times more work, respectively. This work shows that the LVDS has a significant effect on the LV performance in the presence of AR. Therefore, the LVDS of potential TAVR patients should be assessed to gain an initial indication of their ability to tolerate post-procedural AR.

Long-Term Durability of Carpentier-Edwards Magna Ease Valve: A One Billion Cycle In Vitro Study

BACKGROUND

Durability and hemodynamic performance are top considerations in selecting a valve for valve replacement surgery. This study was conducted in order to evaluate the long-term mechanical durability and hydrodynamic performance of the Carpentier-Edwards PERIMOUNT Magna Ease Bioprostheses, through 1 billion cycles (equivalent to 25 years).

METHODS

In vitro valve hydrodynamic performance, durability, and quantitative flow visualization were conducted in accordance with ISO 5840:2005 heart valve standard. The study valves were subjected to accelerated valve cycling to an equivalent of 25 years of wear. Hydrodynamic evaluations at intervals of 100 million cycles (2.5 years) were performed on the study valves. New uncycled Magna Ease valves were used as hydrodynamic controls in this study. A quantitative assessment of the fluid motion downstream of the control and study valves was performed using particle image velocimetry. The results between the test and control valves were compared to assess valve performance after an equivalent of 25 years of wear.

RESULTS

All study valves met the ISO 5840 requirements for effective orifice area, 1.81 ± 0.06 cm(2) and 2.06 ± 0.17 cm(2), and regurgitant fraction, 1.11% ± 0.87% and 2.5% ± 2.34%, for the 21 mm and 23 mm study valves, respectively. The flow characterization of the control valves and the billion-cycle valves demonstrated that the valves exhibited similar flow characteristics. The velocity and shear stress fields were similar between the control and study valves.

CONCLUSIONS

The Magna Ease valves demonstrated excellent durability and hydrodynamic performance after an equivalent of 25 years of simulated in vitro wear. All study valves successfully endured 1 billion cycles of simulated wear, 5 times longer than the standard requirement for a tissue valve as stipulated in ISO 5840.

Is Iron Deficiency Anaemia, a risk Factor For Wheeze Associated Respiratory Tract Infection in Children?

Introduction: Wheezing is the common in children with cumulative prevalence of almost 50% by the age of 6 yrs. Iron deficiency anaemia is an independent risk factor for lower respiratory tract infections (LRTI), however may also play an important role in wheeze associated respiratory tract infection. The present study aimed at finding a correlation between iron deficiency anaemia and wheeze associated respiratory tract infection. Materials and Methods: This case control study was conducted at a multidisciplinary tertiary care hospital from Jul 2012 to Jun 2013 among children six months to five years and having ≥2 episodes of wheeze associated respiratory tract infections. The clinical presentation and laboratory profile were recorded on a predesigned performa. Iron deficiency anaemia was diagnosed by haemoglobin <11.0 gm%, serum ferritin<12ng/ml and RDW>15%. The statistical analysis was done using SPSS, Version 15.0. Results: A total of 100 children were enrolled in the study, of which 50 cases and 50 matched controls. Majority of subjects in both groups were males (n=31;62%),with male to female ratio of 1.6:1. Proportion of cases with iron deficiency anaemia were higher among cases(32%) as compared to controls(24%) but the difference between two groups was not significant statistically (p=0.373) despite having higher risk(OR=1.49;95% CI=0.62-3.59). Mean monocyte, oeosinophil and absolute oeosinophil count was found to be significantly higher in cases as compared to controls (p<0.05). Conclusion: The present study do not suggest a possible linkage between iron deficiency anemia and wheeze associated respiratory infection, however its role as causative factor needs to be explored through long-term prospective and intervention studies on a large population.J Nepal Paediatr Soc 2014;34(3):195-199 DOI: http://dx.doi.org/10.3126/jnps.v34i3.11378

High Transcatheter Valve Replacement May Reduce Washout in the Aortic Sinuses: an In-Vitro Study

BACKGROUND AND AIM OF THE STUDY

Transcatheter aortic valve replacements (TAVRs) are performed using fluoroscopic guidance, which makes precise positioning challenging. The aim of the present study was to investigate the effect of TAV positioning on flow characteristics in the ascending aorta and sinuses.

METHODS

A commonly used TAV design with a supra-annular support section was investigated using particle image velocimetry (PIV) under physiological flow and pressure conditions. A bioprosthetic valve served as a control and mimicked the native aortic valve. The valve assembly was mounted in a custom-designed chamber with an axisymmetric sinus, the design of which was based on anatomic dimensions. The TAV was deployed in the supra-annular (high) and sub- annular (low) implantation positions and studied at two cardiac outputs (5.0 and 2.5 l/min).

RESULTS

The TAV showed good systolic flow characteristics with wide forward flow jets in the ascending aorta (V = 1.5 m/s at 5.0 1/min; V = 1.0 m/s at 2.5 1/min). In the high implantation, the physical spacing between the leaflet free edge and sinotubular junction was reduced (< 10 mm), causing a weaker sinus vortex and a lower washout. A larger region of low velocity (< 0.1 m/s) in the sinus was observed at high implantation at all time points, particularly at a cardiac output of 2.5 l/min. In the low implantation, good sinus washout was observed.

CONCLUSION

For optimal sinus washout, sub- annular TAV deployment is recommended, particularly for patients with smaller sinuses. An impaired cardiac output may also require sub- annular deployment for adequate sinus washout. The study results confirmed the need for precise TAV deployment tailored to patient-specific annular and sinus dimensions, as sub-optimal positioning may inhibit coronary perfusion and cause potential regions of stasis near the aortic annulus.