W.B. Tay

Towards patient-specific cardiovascular modeling system using the immersed boundary technique

BackgroundPrevious research shows that the flow dynamics in the left ventricle (LV) reveal important information about cardiac health. This information can be used in early diagnosis of patients with potential heart problems. The current study introduces a patient-specific cardiovascular-modelling system (CMS) which simulates the flow dynamics in the LV to facilitate physicians in early diagnosis of patients before heart failure.MethodsThe proposed system will identify possible disease conditions and facilitates early diagnosis through hybrid computational fluid dynamics (CFD) simulation and time-resolved magnetic resonance imaging (4-D MRI). The simulation is based on the 3-D heart model, which can simultaneously compute fluid and elastic boundary motions using the immersed boundary method. At this preliminary stage, the 4-D MRI is used to provide an appropriate comparison. This allows flexible investigation of the flow features in the ventricles and their responses.ResultsThe results simulate various flow rates and kinetic energy in the diastole and systole phases, demonstrating the feasibility of capturing some of the important characteristics of the heart during different phases. However, some discrepancies exist in the pulmonary vein and aorta flow rate between the numerical and experimental data. Further studies are essential to investigate and solve the remaining problems before using the data in clinical diagnostics.ConclusionsThe results show that by using a simple reservoir pressure boundary condition (RPBC), we are able to capture some essential variations found in the clinical data. Our approach establishes a first-step framework of a practical patient-specific CMS, which comprises a 3-D CFD model (without involving actual hemodynamic data yet) to simulate the heart and the 4-D PC-MRI system. At this stage, the 4-D PC-MRI system is used for verification purpose rather than input. This brings us closer to our goal of developing a practical patient-specific CMS, which will be pursued next. We anticipate that in the future, this hybrid system can potentially identify possible disease conditions in LV through comprehensive analysis and facilitates physicians in early diagnosis of probable cardiac problems.

Analysis of biplane flapping flight with tail

Numerical simulations have been performed to examine the interference effects between an upstream flapping biplane airfoil arrangement and a downstream stationary tail at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the relative distance and angle of attack between the airfoils and its tail on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that overall efficiency and average thrust per airfoil can be increased up to 17% and 126% respectively when the top and bottom airfoils come into contact during flapping. When placing the tail at a strategic position, the overall configuration generates much higher lift, although at the expense of decreased efficiency and thrust. Increasing the angle of attack of the tail also helps to increase the lift. Analysis of the vorticity plots reveals the interaction between the vortices and the airfoils and the reason behind the high thrust and lift. The results obtained from this study can be used to optimize the performance of small flapping MAVs.

Effect of Different Types of Wing-Wing Interactions in Flapping MAVs

Wing-Wing Interaction (WWI), such as the Clap and Fling Motion (CFM), occurs when two wings are flapping close together, improving performance. We intend to design a hovering Flapping Micro Aerial Vehicle (FMAV) which makes use of WWI. We investigate the effects of flexibility, kinematic motions, and two- to six-wing flapping configurations on the FMAV through numerical simulations. Results show that a rigid spanwise and flexible chordwise wing produces the highest lift with minimum power. The smoothly varying sinusoidal motion, which is visually similar to the CFM, produces similar lift in comparison to the CFM, while having lower peak power requirement. Lastly, lift produced by each wing of the two-, four-, six-wing configurations is approximately equal. Hence more wings generate higher total lift force, but at the expense of higher drag and power requirement. These results will be beneficial in the understanding of the underlying aerodynamics of WWI, and in improving the performance of our FMAV.

Biplane and Tail Effects in Flapping Flight

A numerical investigation is performed on the interference effects in single or biplane flapping airfoil propulsion in the presence of a stationary downstream tail. At a Reynolds number of 1000, this corresponds to the regime of small micro aerial vehicles. The objective of this study is to provide insight into the complex wing–tail interaction effects occurring in flapping-wing propulsion configurations. The effect of the relative distance between the airfoils, as well as the positioning and incidence angle of the tail, is investigated. Adding a tail behind a single flapping airfoil increases the efficiency and average thrust by up to 10 and 25%, respectively. For the biplane flapping airfoils without tail, overall efficiency and average thrust per airfoil increase up to 17 and 126%, respectively, with respect to the single airfoil due to the formation of a strong momentum jet. The effect of adding a tail behind the biplane flapping airfoils depends on the tail’s orientation and location. Increasing the ...

Effect of chordwise deformation on unsteady aerodynamic mechanisms in hovering flapping flight

A three-dimensional simulation of hovering flapping wings was performed using an immersed boundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex.

A comparison of robotic fish speed control based on analytical and empirical models

In this paper, a comparative study of speed control efficiencies based on two dynamical models is presented. The common structure of dynamical models is developed using first principles of physics. The complex thrust mechanism is studied using two different techniques. First, we apply a novel data-driven approach to build up the non-linear mapping between tail angular motion and thrust generated, which is essential to specify the input gradient. Further, additional experiments are designed to acquire data necessary for modelling the input delay factor causing a time-delayed input to drive the system output. Second, analytically developed Lighthills slender body theory is also employed to study thrust mechanism for comparative purposes. Though the theory has been used in robotic fish motion studies, it was originally formulated as an analytical tool for studying biological fish motion. Comparing the two theories help understand the predominant features in a robotic fish speed control. Lastly, because of the highly non-linear system dynamics, robust discrete-time Sliding Mode Controllers (SMC) are developed with the two dynamical models as basis respectively. Experimental verifications confirm that SMC based on data-driven model performs superior to the SMC based on the Lighthills theory, by efficiently controlling the robotic fishs speed to track the time-varying reference speeds.

Development of the Patient‐specific Cardiovascular Modeling System Using Immersed Boundary Technique

A computational fluid dynamics (CFD) based, patient‐specific cardiovascular modeling system is under‐developed. The system can identify possible diseased conditions and facilitate physicians’ diagnosis at early stage through the hybrid CFD simulation and time‐resolved magnetic resonance imaging (MRI). The CFD simulation is initially based on the three‐dimensional heart model developed by McQueen and Peskin, which can simultaneously compute fluid motions and elastic boundary motions using the immersed boundary method. We extend and improve the three‐dimensional heart model for the clinical application by including the patient‐specific hemodynamic information. The flow features in the ventricles and their responses are investigated under different inflow and outflow conditions during diastole and systole phases based on the quasi‐realistic heart model, which takes advantage of the observed flow scenarios. Our results indicate distinct differences between the two groups of participants, including the vortex ...