A pinboard by
Raymond Ho

PhD Student, Queensland University of Technology


Fate of emboli during aortic cannulation: How does heart surgery cause brain injuries?

Heart disease is the leading natural killer in many countries and everyday there are 1000's of conducted surgical procedures to repair the arteries and organs of diseased cardiovascular patients. Cardiopulmonary bypass is an essential component of cardiovascular surgery, and has been used successfully over the last 60 years. Yet, one unexpected outcome is clearly evident, and that is brain injury during or after heart surgery. These brain injuries can be quite obvious in the form of a stroke but even more so, patients may experience subtle cognitive decline such as memory loss, unable to concentrate, or becoming delirious. These injuries have a huge impact on the patient's ability to return back to a normal life, resuming work which also becomes a significant cost to the community and families.

A critical component of cardio pulmonary bypass is aortic cannulation where filtered and oxygenated blood is returned back to the patient's main central heart artery via a simple plastic tube named a cannula. However, this returning pumped blood may cause excessive flow forces that damage the artery wall causing plaque particles to dislodge or air bubbles that accidentally enter into the system, so both air and solid particles may make their way to the arteries leading to the brain . These particles can cause blockages, which dangerously reduce blood flow thus oxygen to the brain. It is not clear how the cannula blood flow transports these particles. Therefore, this preliminary research uses a computer simulations to better understand how the emboli particles travel throughout aorta during cardiopulmonary bypass.

This numerical experiment consists of two different cannula positions, one pointing slightly forwards and the other slightly backwards within an adult's central artery; solid and oxygen particles are released and tracked as they reach the neck artery leading to the brain. The results show that slight movements in cannula tube angles could mean a significant difference of particles entering the neck arteries. And the early results so far show that oxygen is more of a problem than than solid particles entering the brain. Consequently, the value of this research to-date for medical teams, show that changes to the cannula position, whether incidental or intended, during surgery has a significant impact on emboli particulate load and ultimately may affect how well and how long the patient takes to recover.


Effect of coarctation of the aorta and bicuspid aortic valve on flow dynamics and turbulence in the aorta using particle image velocimetry

Abstract: Blood flow in the aorta has been of particular interest from both fluid dynamics and physiology perspectives. Coarctation of the aorta (COA) is a congenital heart disease corresponding to a severe narrowing in the aortic arch. Up to 85 % of patients with COA have a pathological aortic valve, leading to a narrowing at the valve level. The aim of the present work was to advance the state of understanding of flow through a COA to investigate how narrowing in the aorta (COA) affects the characteristics of the velocity field and, in particular, turbulence development. For this purpose, particle image velocimetry measurements were conducted at physiological flow and pressure conditions, with three different aorta configurations: (1) normal case: normal aorta + normal aortic valve; (2) isolated COA: COA (with 75 % reduction in aortic cross-sectional area) + normal aortic valve and (3) complex COA: COA (with 75 % reduction in aortic cross-sectional area) + pathological aortic valve. Viscous shear stress (VSS), representing the physical shear stress, Reynolds shear stress (RSS), representing the turbulent shear stress, and turbulent kinetic energy (TKE), representing the intensity of fluctuations in the fluid flow environment, were calculated for all cases. Results show that, compared with a healthy aorta, the instantaneous velocity streamlines and vortices were deeply changed in the presence of the COA. The normal aorta did not display any regions of elevated VSS, RSS and TKE at any moment of the cardiac cycle. The magnitudes of these parameters were elevated for both isolated COA and complex COA, with their maximum values mainly being located inside the eccentric jet downstream of the COA. However, the presence of a pathologic aortic valve, in complex COA, amplifies VSS (e.g., average absolute peak value in the entire aorta for a total flow of 5 L/min: complex COA: = 36 N/m2; isolated COA = 19 N/m2), RSS (e.g., average peak value in the entire aorta for a total flow of 5 L/min: complex COA: = 84.6 N/m2; isolated COA = 44 N/m2) and TKE (e.g., average peak value in the entire aorta for a total flow of 5 L/min: complex COA: = 215 N/m2; isolated COA = 100 N/m2). This demonstrates that the pathological aortic valve strongly interacts with the COA. Findings of this study indicate that the presence of both a COA and a pathological aortic valve significantly alters hemodynamics in the aorta and thus might contribute to the progression of the disease in this region. This study can partially explain the complications associated in patients with COA, in the presence of a pathological aortic valve and the consequent adverse outcome post-surgery.

Pub.: 15 Mar '14, Pinned: 27 Aug '17

Characterization of Flow Vectoring Phenomenon in Adjacent Synthetic Jets using CFD and PIV

Abstract: Publication date: Available online 28 September 2016 Source:Computers & Fluids Author(s): Sajad Alimohammadi, Eoin Fanning, Tim Persoons, Darina B. Murray Flow vectoring by a pair of synthetic jets is suitable for modification of the global flow characteristics with practical applications in active flow control and adaptive heat convection. The interaction of a pair of synthetic jets, with a separation distance s = 3.3D, stroke length L0 =29D, and Reynolds number Re =300, are investigated numerically using computational fluid dynamics (CFD) and experimentally using particle image velocimetry (PIV). To achieve the most realistic calculation of the flow induced by synthetic jets, a full unsteady RANS simulation is performed of the internal flow in two cavities as well as the external jet flow using a dynamic mesh technique. The results for the intricate flow vectoring phenomenon show a reasonable quantitative agreement with PIV measurements, with a maximum deviation from PIV measurements of 14% for stream-wise centreline velocity in 10 < y/D < 20. The effect of phase difference between the pair of jets on the vectoring of the merged jet is investigated for δ ∅ = 0 ∘ , 60 ∘ and 130 ∘ . The merged jet is vectored in the direction of the cavity that is leading in phase, with a similar trend shown by the experimental and numerical results of instantaneous and time-averaged vortical structures. This leads to a better physical understanding of the fluid mechanics of adjacent synthetic jets, and will enhance the theoretical basis needed to promote their practical application.

Pub.: 04 Oct '16, Pinned: 27 Aug '17

Data-driven CFD modeling of turbulent flows through complex structures

Abstract: The growth of computational resources in the past decades has expanded the application of Computational Fluid Dynamics (CFD) from the traditional fields of aerodynamics and hydrodynamics to a number of new areas. Examples range from the heat and fluid flows in nuclear reactor vessels and in data centers to the turbulence flows through wind turbine farms and coastal vegetation plants. However, in these new applications complex structures are often exist (e.g., rod bundles in reactor vessels and turbines in wind farms), which makes fully resolved, first-principle based CFD modeling prohibitively expensive. This obstacle seriously impairs the predictive capability of CFD models in these applications. On the other hand, a limited amount of measurement data is often available in the systems in the above-mentioned applications. In this work we propose a data-driven, physics-based approach to perform full field inversion on the effects of the complex structures on the flow. This is achieved by assimilating observation data and numerical model prediction in an iterative Ensemble Kalman method. Based on the inversion results, the velocity and turbulence of the flow field can be obtained. A major novelty of the present contribution is the non-parametric, full field inversion approach adopted, which is in contrast to the inference of coefficient in the ad hoc models often practiced in previous works. The merits of the proposed approach are demonstrated on the flow past a porous disk by using both synthetic data and real experimental measurements. The spatially varying drag forces of the porous disk on the flow are inferred. The proposed approach has the potential to be used in the monitoring of complex system in the above mentioned applications.

Pub.: 28 Nov '16, Pinned: 27 Aug '17

Numerical simulations of aerosol delivery to the human lung with an idealized laryngeal model, image-based airway model, and automatic meshing algorithm

Abstract: The authors proposed a new method to automatically mesh computed tomography (CT)-based three-dimensional human airway geometry for computational fluid dynamics (CFD)-based simulations of pulmonary gas-flow and aerosol delivery. Traditional methods to construct and mesh realistic geometry were time-consuming, because they were done manually using image-processing and mesh-generating programs. Furthermore, most of CT thoracic image data sets do not include the upper airway structures. To overcome these issues, the proposed method consists of CFD grid-size distribution, an automatic meshing algorithm, and the addition of a laryngeal model along with turbulent velocity inflow boundary condition attached to the proximal end of the trachea. The method is based on our previously developed geometric model with irregular centerlines and cross-sections fitted to CT segmented airway surfaces, dubbed the “fitted-surface model.” The new method utilizes anatomical information obtained from the one-dimensional tree, e.g., skeleton connectivity and branch diameters, to efficiently generate optimal CFD mesh, automatically impose boundary conditions, and systematically reduce simulation results. The aerosol deposition predicted by the proposed method agreed well with the prediction by a traditional CT-based model, and the laryngeal model generated a realistic level of turbulence in the trachea. Furthermore, the computational time was reduced by factor of two without losing accuracy by using the proposed grid-size distribution. The new method is well suited for branch-by-branch analyses of gas-flow and aerosol distribution in multiple subjects due to embedded anatomical information.

Pub.: 10 Feb '17, Pinned: 27 Aug '17

Numerical study on secondary flows of viscoelastic fluids in straight ducts: Origin analysis and parametric effects

Abstract: Numerical simulations were conducted on viscoelastic fluid flows in straight ducts with different cross sections, for which the origin of secondary flows and influences of material parameters and flow passage geometrical configuration were numerically investigated. The Giesekus constitutive model was chosen to describe the viscoelastic fluid with the second normal stress difference N2, and solved by embedding UDF (User-defined Function) into the CFD (computational fluid dynamics) code FLUENT. The origin of such kind of secondary flow was theoretically studied from the perspective of the budget of vorticity energy for the first time. Sufficient and necessary condition for the existence of secondary flow was then developed in terms of N2, the gradient of N2 and cross-sectional geometry ϑ (i.e., generation term EΩEΩ). Moreover, helicity density was considered as an excellent indicator of secondary flow pattern. Effects of material properties (including anisotropic parameterα, solvent viscosity ratio β and relaxation time λ) and flow passage geometrical configuration (aspect ratio of cross sectionsr, the number of polygon sides n  ) on secondary flow strength and pattern were investigated with EΩEΩ. Finally, a universal variable σs was proposed to describe non-circularity of cross section, based on which the results for ducts with different cross sections can be normalized together.

Pub.: 18 Apr '17, Pinned: 27 Aug '17