Vortex shedding from confined micropin arrays

Abstract

The hydrodynamics in microcavities populated with cylindrical micropins was investigated using dynamic pressure measurements and fluid pathline visualization. Pressure signals were Fourier-analyzed to extract the flow fluctuation frequencies, which were in the kHz range for the tested flow Reynolds numbers (Re) of up to 435. Three different sets of flow dependent characteristic frequencies were identified, the first due to vortex shedding, the second due to lateral flow oscillation and the third due to a transition between these two flow regimes. These frequencies were measured at different locations along the chip (e.g. inlet, middle and outlet). It is established that vortex shedding initiates at the outlet and then travels upstream with increase in Re. The pathline visualization technique provided direct optical access to the flow field without any intermediate post-processing step and could be used to interpret the frequencies determined through pressure measurements. Microcavities with different micropin height-to-diameter aspect ratios and pitch-to-diameter ratios were tested. The tests confirmed an increase in the Strouhal number (associated with the vortex shedding) with increased confinement (decrease in the aspect ratio or the pitch), in agreement with macroscale measurements. The compact nature of the microscale geometry tested, and the measurement technique demonstrated, readily enabled us to investigate the flow past 4,420 pins with various degrees of confinements; this makes the measurements performed and the techniques developed here an important tool for investigating large arrays of similar objects in a flow field.

A novel method of energy efficient hotspot-targeted embedded liquid cooling for electronics: An experimental study

“…Micro pin-fins have been demonstrated to be viable for single-phase interlayer cooling of 3D-IC stacks [26,[42][43][44]. A hotspot-targeted interlayer cooling design would involve non-uniformly distributed pin fins to induce localized vortex shedding over hotspots in the individual layers of the chip stack [26,43].…”

On‐Chip Generation of Vortical Flows for Microfluidic Centrifugation

“…Copyright 2013, Macmillan Publishers Ltd.). c) Vortex shedding behind pillar arrays; the left shows the case for moderate frequencies and hence moderate Strouhal numbers Sr , whereas the right shows the case for high frequencies and hence large Sr (Reproduced with permission . Copyright 2013, Springer‐Verlag).…”

“…Another example of the use of topological features to generate recirculatory flows is through the introduction of microfabricated post arrays in a microfluidic device. The obstruction that these posts impose to a flow with sufficient inertia either causes it to deform, or, acts as a bluff body behind which fluctuations in the hydrodynamic pressure gives rise to periodic vortex shedding beyond a critical Reynolds number . In the former, the inertia in the system breaks the fore‐aft symmetry of the flow around a post that is typically expected in viscous systems (i.e., Stokes flows) such that there is sufficient fluid deformation in the flow, which, when under confinement by the channel boundaries, gives rise to net recirculation; Amini et al showed that it is possible to tune both the position as well as the structure of the recirculation by judicious choice in the lateral placement of successive pillars in the channel (Figure b) .…”

“…Further increases in the Reynolds number beyond a critical value, however, leads to boundary layer separation from the post surface, and hence vortex shedding (Figure c). Such vortex shedding, whose frequency f —as characterized by a Strouhal number , in which is the characteristic length scale of the posts and the mean velocity of the flow between them—increases with a decrease in the feature aspect ratio and increasing longitudinal confinement of the flow between the posts, i.e., smaller pitch‐to‐post diameter ratios, and was shown to be effective not only to sufficiently permeabilize cell membranes in order to deliver an exogenous payload comprising various therapeutic molecules into the cell to aid transfection, but to also lyse the cells . Additionally, topology has also been combined with geometrical features to enhance vorticity generation in microchannels …”

Low-frequency acoustic atomization with oscillatory flow around micropillars in a microfluidic device

“…Extensional flow occurs between micropillars in both horizontal and diagonal directions. Flows through a single 27 and micropillars array involve complex behaviors like vortex shedding, lateral flow oscillation, 28 and versatile flow trajectories (zigzag, laterally displaced, and dispersive) 29 are currently under active investigation. The potential application of our method for small particles encapsulation is demonstrated in Fig.…”