Pulsatile flow past an oscillating cylinder

Abstract

A fundamental study to characterize the flow around an oscillating cylinder in a pulsatile flow environment is investigated. This work is motivated by a new proposed design of the total artificial lung (TAL), which is envisioned to provide better gas exchange. The Navier-Stokes computations in a moving frame of reference were performed to compute the dynamic flow field surrounding the cylinder. Cylinder oscillations and pulsatile free-stream velocity were represented by two sinusoidal waves with amplitudes A and B and frequencies ω(c) and ω, respectively. The Keulegan-Carpenter number (K(c)=U(o)∕Dω(c)) was used to describe the frequency of the oscillating cylinder while the pulsatile free-stream velocity was fixed by imposing ω∕K(c)=1 for all cases investigated. The parameters of interest and their values were amplitude (0.5D<A<D), the Keulegan-Carpenter number (0.33<K(c)<1), and the Reynolds number (5<Re<20) corresponding to operating conditions of the TAL. It was observed that an increase in amplitude and a decrease in K(c) are associated with an increase in vorticity (up to 246%) for every Re suggesting that mixing could be enhanced by the proposed TAL design. The drag coefficient was found to decrease for higher amplitudes and lower K(c) for all cases investigated. In some cases the drag coefficient values were found to be lower than the stationary cylinder values (A=0.5, K(c)=0.3, and Re=10 and 20). A lock-in phenomenon (cylinder oscillating frequency matched the vortex shedding frequency) was found when K(c)=1 for all cases. This lock-in condition was attributed to be the cause of the rise in drag observed in that operating regime. For optimal performance of the modified TAL design it is recommended to operate the device at higher fiber oscillation amplitudes and lower K(c) (avoiding the lock-in regime).

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

“…The gas exchange is modeled by a time dependent convection-diffusion equation coupled to the Navier-Stokes [6] equation. The governing equations are non-dimensionalized based on free-stream parameters [11], [13]. The resulting non-dimensionlised numbers Numerical computations are carried out by utilizing a composite grid approach [14].…”

“…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.…”

Oscillatory oblique stagnation-point flow towards a plane wall

“…In this paper, we present an exact solution describing unsteady flow at a stagnation point on a wall, in which the dividing streamline makes an oblique, time-dependent angle with the wall. Such a description may be useful in studying more complex unsteady flow phenomena, for example, the flow at the rear stagnation point which appears on a cylinder held fixed in a pulsatile stream [2]. A physical example of unsteady stagnation-point flow at a wall is provided by pulsatile blood flow through a dividing artery, or blood flow through an end-to-side anastomosis [e.g .…”

Pulsatility role in cylinder flow dynamics at low Reynolds number

“…Few studies regarding TAL from a physiological perspective have been done to identify mass transport, 10,11 TAL resistance 12 and impedance of the device. 13 More recently, 14 we have proposed a design modification in the basic TAL design, which involve fiber oscillations. The new design has shown potential of higher gas exchange and better control on the devise resistance.…”

“…15 The scales used to non-dimensionalize the governing equations are the same as used in our earlier work. 14 16 The amplitude is kept <1 to avoid flow reversal at the inlet. These variations in KC and A values result in total of 48 simulations.…”