Pulsatility role in cylinder flow dynamics at low Reynolds number

Abstract

We present dynamics of pulsatile flow past a stationary cylinder characterized by three non-dimensional parameters: the Reynolds number (Re), non-dimensional amplitude (A) of the pulsatile flow velocity, and Keulegan-Carpenter number (KC = Uo/Dωc). This work is motivated by the development of total artificial lungs (TAL) device, which is envisioned to provide ambulatory support to patients. Results are presented for 0.2 ≤ A ≤ 0.6 and 0.57 ≤ KC ≤ 2 at Re = 5 and 10, which correspond to the operating range of TAL. Two distinct fluid regimes are identified. In both regimes, the size of the separated zone is much greater than the uniform flow case, the onset of separation is function of KC, and the separation vortex collapses rapidly during the last fraction of the pulsatile cycle. The vortex size is independent of KC, but with an exponential dependency on A. In regime I, the separation point remains attached to the cylinder surface. In regime II, the separation point migrates upstream of the cylinder. Two distinct vortex collapse mechanisms are observed. For A < 0.4 and all KC and Re values, collapse occurs on the cylinder surface, whereas for A > 0.4 the separation vortex detaches from the cylinder surface and collapses at a certain distance downstream of the cylinder. The average drag coefficient is found to be independent of A and KC, and depends only on Re. However, for A > 0.4, for a fraction of the pulsatile cycle, the instantaneous drag coefficient is negative indicating a thrust production.

Novel hole-pillar spacer design for improved hydrodynamics and biofouling mitigation in membrane filtration

“…5 . Figure 5 A (location L 1 ) indicates clearly that the asymmetric nature of the boundary layer profiles is seen for the commercial spacer, while those of the pillar and hole-pillar spacers are perfectly symmetric as seen in cylinder flow dynamics 43 . However, the average profile values calculated along the channel height are roughly similar for all spacers.…”

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

“…3(d)). The collapse of vorticies are attributed to the accelerating free stream velocity (favorable pressure gradient), whereas growth is due to larger adverse pressure gradient [13] during retardation phase. Similar flow field, as in Fig.…”

“…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].…”

Membrane filtration performance enhancement and biofouling mitigation using symmetric spacers with helical filaments

“…For all helical spacers, the downstream filament region had alternating patterns of the high and low velocities. Furthermore, flow velocities for standard spacer were visually perturbed indicating highly unsteady state, while for helical spacers the velocity profiles were well-defined suggesting a quasi-periodic state [55].…”

“…It is expected that the presence of helices on the filaments would produce more flow instability comparing to standard spacer equipped with smooth cylindrical filaments. As a result of this instability, the attached vortices behind the helical filament would destabilize faster (compared to cylindrical filament) and produce vortex shedding resulting in reduction of drag force (which is directly proportional to pressure drop) as observed in flow past bluff bodies and flow past rigid cylinders[55,56]. Consequently, the spacer with three helices which will have more unsteadiness comparing to those with one or two helices has the least pressure drop.The percentage of the pressure drop decrease between the helical and standard spacers was the highest for the spacer with 3-helical (65% and 64.6%, respectively), followed by 1-helical (54.9% and 54.4%, respectively) and 2-helical spacers (46.9% and 45.8%, respectively) at both tested feed flow velocities (U0= 0.162 m/s and U0 = 0.188 m/s).…”