Pulsatile Flow Past a Cylinder: An Experimental Model of Flow in an Artificial Lung

Abstract

The focus of this study is an experimental apparatus that serves as a model for studying blood flow in a total artificial lung (TAL), a prototype device intended to serve as a bridge to lung transplantation or that supports pulmonary function during the treatment of severe respiratory failure. The TAL consists of hollow cylindrical fibers that oxygen-rich air flows through and oxygen-poor blood flows around. Because gas diffusivity in the TAL is very small, a convection mechanism dominates the gas transport, which is why we focus on the velocity around the fibers (modeled as a 0.05-cm-in-diameter and 5-cm-long cylinder). We designed a low-speed water tunnel to study the flow mechanism around the cylinder, across which the flow is generated by a linear actuator that allows different flow patterns to mimic the flow in a TAL. We tested the flow in the test section by numerical simulation and by the particle image velocimetry method to study the flow profile. The results show a uniform flow near the centerline of the water tunnel where the cylinder is placed. This decreases the effects of free-stream turbulence in the shear layers and reduces the uncertainty in determining the flow patterns around the cylinder. Knowledge gained from the flow around one cylinder (fiber) is beneficial for understanding vortex formation around multiple cylinders. We present a summary of vortex formation behind a cylinder for Reynolds numbers (Re) of 1, 3, and 5 and Stokes numbers (Ns) of 0.18 to 0.37; results show that higher Re and Ns favor vortex formation. These findings regarding the parameter range for vortex formation may provide principles for designing artificial lungs to enhance convective mixing. We anticipate that the pulsatile flow circuit presented here can be used to mimic the flow not only in TALs but in other physiological systems.

An investigation of pulsatile flow past two cylinders as a model of blood flow in an artificial lung

“…Flow reversal was observed downstream of the tandem cylinders for Re = 1 and Ns = 0.18. This flow reversal was not observed around a single cylinder that immersed in the same pulsatile flow condition [24]. Vortex size varied during the cycle of the pulsatile flow (Fig.…”

“…This approach used an experimental setup similar to previous investigation of a single circular cylinder in pulsatile cross-flow [24]. This model was scaled up for easier flow visualization, and the relevant dimensionless numbers were matched to the anticipated range in MC3’s BioLung® (Michigan Critical Care Consultants, Ann Arbor, MI).…”

“…However, they used a small device that makes the velocity field very hard to examine and investigated only one configuration of the fibers (staggered). Lin et al [24] presented the first experimental study of vortex formation around a cylinder in pulsatile flow at Re = 1 to 5. It was demonstrated that vortices occur behind the cylinder at low Reynolds number ( Re =1) in pulsatile flow, while no vortex was observed in this low Reynolds numbers in steady flow.…”

Micro-fibers shape effects on gas exchange in Total Artificial Lung

“…Several numerical and bench-top experiments have been thought through to optimize the device ranging from varying pulsatile parameters [6], [7], [8], [9], [10], mechanical motions of fibers [11] and addition of compliance chamber [12]. In present work we numerically investigate the fiber cross-section shape effects on gas exchange and device resistance.…”

Pulsatility role in cylinder flow dynamics at low Reynolds number

“…A fundamental striking difference between the pulsatile inflow and the uniform flow past the cylinder at low Re appears in the relative size of the separated zone. Lin et al 17,18 instance when vortex reaches their maximum size. For a given Re, the attached separated zone grows exponentially with the amplitude of pulsation.…”