This study experimentally investigates the stationary and dynamic characteristics of a tandem cylinder configuration in which the downstream cylinder is confined to rotate about a pivot point centered at a fixed upstream cylinder with a specified cylinder-to-cylinder spacing. Results were obtained for Reynolds numbers 1.3×104≤Re≤1.4×105, cylinder spacings ℓ=L/D=2,4, and, for the dynamic case, mass ratios 0.5 ≤ m* ≤ 2. The positional dependence of the quasi-static moment of the fluid forces on the downstream cylinder about the pivot point was determined by measuring the torque required to hold the downstream cylinder stationary. This moment data were subsequently used to evaluate the validity of using a torsional wake stiffness model to predict the frequency response of a freely oscillating tethered cylinder. This model assumes the wake of the upstream cylinder acts as a linear torsional spring, generating a restorative moment that drives the downstream cylinder to the upstream cylinder’s wake centerline. In the subcritical regime, 1×104≲Re≲9×104, the wake stiffness model predicts the system frequency with reasonable accuracy, with results being best for larger mass ratios. A fundamental change occurs for the critical regime with Re≳1.0×105. For ℓ=4, the frequency response drops off and then becomes negligible for Re≳1.2×105. For ℓ=2, the slope of the stationary moment changes sign, creating a regime of “negative stiffness” near the wake centerline, and the dynamic system maintains robust, high-amplitude limit-cycle oscillations, in contrast to the ℓ=4 case. For both spacings, the wake stiffness model is not accurate in the critical flow regime. These findings suggest that for Re≳1×105, the wake-structure interaction may be governed by unsteady, motion-dependent fluid dynamics that necessitate a fully dynamic energy approach for accurate prediction.

Localized vortices can have significant influence on transport of inhaled particles through the upper respiratory tract. These vortices have complex three-dimensional structure with details dependent on the anatomical geometry. Using a highly simplified model, we demonstrate that changes in transport characteristics with geometric distortion can be estimated by accounting merely for the net strength and location of the vorticity in a two-dimensional projection. Test cases consider 30 L/min inhaled airflow containing suspended spherical water droplets from 1 micrometer to 30 micrometers in diameter through (1) a healthy upper respiratory tract and (2) a distorted variation mimicking a glottic tumor. The reduced-order model approximates the system by a two-dimensional potential flow with embedded point vortices having features derived from Large Eddy Simulations of inhaled airflow through anatomically realistic, tomography-based, three-dimensional tracts. The effects of vorticity and particle size on changes in particle transport are shown to be consistent between the reduced-order model and the full-scale simulations.

We introduce a new definition of vortex formation length for uniform steady flow past a circular cylinder using Proper Orthogonal Decomposition (POD) of the pressure field. Previous definitions each identify a single characteristic length for a given value of Reynolds number. We use the leading modes of the pressure POD to define upper and lower bounds on the vortex formation length. Identifying a range for the vortex formation length is consistent with the hysteresis observed in the critical spacing of two tandem cylinders.

When two cylinders submerged in a uniform flow are arranged in tandem, the downstream cylinder can oscillate in response to the wake from the upstream cylinder. In this investigation, the downstream cylinder is allowed to oscillate freely around the center of a fixed upstream cylinder, mimicking a pendulum-like motion. Our findings suggest that the wake stiffness concept, initially verified for relatively large cylinder spacing (ℓ≥4) and linear transverse cylinder motion, is also relevant for characterizing the wake-induced vibration (WIV) response observed for two tandem tethered cylinders situated in close proximity (2≤ℓ≤4) for Reynolds numbers in the range 1.0×104≲Re≲1.2×105, with tests conducted up to Re≈1.4×105. For small cylinder spacing (ℓ≤2.5), the downstream cylinder attains a maximum oscillation angle amplitude and exhibits consistent vibration, providing reliable potential for energy harvesting. We also explore hysteresis in the WIV response, which is observed to depend on the history of Reynolds number variation. Our findings reveal hysteresis at both the onset and termination of oscillation.

This article contains flow visualization data from experiments conducted in an inclined gravity-driven soap film intersected by a circular cylinder undergoing controlled transverse oscillations at a Reynolds number of Re≈235. The dimensionless frequency and amplitude of cylinder oscillation were varied systematically over the ranges 0.2<f*<1.8 and 0.1<A*<1.3. A high-speed camera was used to capture the interference fringe patterns reflected from the soap film. These videos show the structure of the wake behind the cylinder, including the initial formation of vortices and the extended ‘vortex street’. Several wake patterns were identified, including the classic 2S, P+S, 2P, and 2T patterns, which are discussed in detail in the accompanying research article titled “The wake of a transversely oscillating circular cylinder in a flowing soap film at low Reynolds number” [1]. The videos presented in this article can be accessed through the Virginia Tech University Libraries’ Repository at https://doi.org/10.7294/14448027.v5 [2].

The fluid motion produced by a spatially periodic array of identical, axisymmetric, thin-cored vortex rings is investigated.It is well known that such an array moves uniformly without change of shape or form in the direction of the central axis of symmetry, and is therefore an equilibrium solution of Euler’s equations. In a frame of reference moving with the system of vortex rings, the motion of passive fluid particles is investigated as a function of the two nondimensional parameters that define this system: $$\varepsilon=a/R$$ , the ratio of minor radius to major radius of the torus-shaped vortex rings, and $$\lambda=L/R$$ , the separation of the vortex rings normalized by their radii. Two bifurcations in the streamline topology are found that depend significantly on $$\varepsilon$$ and $$\lambda$$ ; these bifurcations delineate three distinct shapes of the “atmosphere” of fluid particles that move together with the vortex ring for all time. Analogous to the case of an isolated vortex ring, the atmospheres can be “thin-bodied” or “thick-bodied”. Additionally, we find the occurrence of a “connected” system, in which the atmospheres of neighboring rings touch at an invariant circle of fluid particles that is stationary in a frame of reference moving with the vortex rings.

The classic problem of three point vortex motion on the plane is revisited by using the interior angles of the vortex triangle, $$\theta_{j}$$ , $$j=1,2,3$$ , as the key system variables instead of the lengths of the triangle sides, $$s_{j}$$ , as has been used classically.Similar to the classic approach, the relative vortex motion can be represented in a phase space, with the topology of the level curves characterizing the motion. In contrast to the classic approach, the alternate formulation gives a compact, consistent phase space representation and facilitates comparisons of vortex motion in a co-moving frame.This alternate formulation is used to explore the vortex behavior in the two canonical cases of equal vortex strength magnitudes, $$\Gamma_{1}=\Gamma_{2}=\Gamma_{3}$$ and $$\Gamma_{1}=\Gamma_{2}=-\Gamma_{3}$$ .

An inclined gravity-driven soap film channel was used to study the wake patterns formed behind a transversely oscillating cylinder at Re = 235 +/- 14. The natural frequency of vortex shedding from a stationary cylinder, f(St), was used to identify the oscillation frequencies of interest. The (dimensionless) frequency, f* = f/f(St), and amplitude, A* = A/D, of the cylinder's motion was varied over a large portion of the fundamental synchronization region (i.e., for f* approximate to 1), and a 'map' of wake patterns was constructed in (f*, A*) space. Lock-on between the frequency of the cylinder's motion and the dominant frequency of the resulting vortex wake was observed for a large range of this parameter space, predominantly manifested as synchronized '2S' and '2P' wake modes. Synchronized 'P+S', '2T', and 'transitional' wakes were also found in smaller regions of parameter space. Unsynchronized 'coalescing' and 'perturbed von Karman' wakes were observed as the oscillation frequency became sufficiently different from f(St). The wake patterns and vortex formation processes found in this study, particularly the 2P mode wakes, bear a strong resemblance to previous three-dimensional experimental results, despite the physical constraint from the soap film that limits three-dimensional effects in the wake. (C) 2021 Elsevier Ltd. All rights reserved.

We show the validity of using a flowing soap film system as a two-dimensional laboratory model of flow past a circular cylinder at low Reynolds numbers through a novel combination of qualitative wake visualizations and quantitative velocity measurements and through a new quantitative method for determining the relative film thickness. We verify the correlation between interference fringe patterns and underlying flow structures by simultaneously obtaining digital particle image velocimetry (DPIV) data and interferograms. We introduce a quantitative soap film thickness measurement method using background-oriented schlieren (BOS) to provide a spatially resolved (relative) thickness field. Vortex cores in the cylinder wake appear as low thickness zones, and the minimum thickness regions identified with BOS are shown to coincide with the vortex centers identified in phase-matched interferograms. The vortex positions and circulations in the soap film correlate well with those reported in the literature for the case of low-Reynolds-number flow past a circular cylinder.

The response of cells to physical or chemical stimuli is complex, unfolding on time-scales from seconds to days, with or without de novo protein synthesis, and involving signaling processes that are transient or sustained. By combining the technology of microfluidics that supports fast and precise execution of a variety of cell handling operations, with that of mass spectrometry detection that facilitates an accurate and complex characterization of the protein complement of cells, in this work, we developed a platform that supports (near) real-time sampling and proteome-level capturing of cellular responses to a perturbation such as treatment with mitogens. The geometric design of the chip supports three critical features: (a) capture of a sufficient number of cells to meet the detection limit requirements of mass spectrometry instrumentation, (b) fluid delivery for uniform stimulation of the resident cells, and (c) fast cell recovery, lysis and processing for accurate sampling of time-sensitive cellular responses to a stimulus. COMSOL simulations and microscopy were used to predict and evaluate the flow behavior inside the microfluidic device. Proteomic analysis of the cellular extracts generated by the chip experiments revealed that the identified proteins were representative of all cellular locations, exosomes, and major biological processes related to proliferation and signaling, demonstrating that the device holds promising potential for integration into complex lab-on-chip work-flows that address systems biology questions. The applicability of the chips to study time-sensitive cellular responses is discussed in terms of technological challenges and biological relevance.

For steady flow past two stationary circular cylinders that are aligned in tandem with their axes perpendicular to the flow direction, the wake pattern, vortex shedding frequency, and forces on the cylinders typically show an abrupt change as a function of the dimensionless center-to-center cylinder spacing over a range of critical spacing values. For Re≲190, the flow is expected to be laminar and two dimensional, and hence well behaved, yet prior experimental and computational results do not agree regarding the value of the critical spacing, ℓc. Using flow visualization techniques in a flowing soap film system with Re=99(±8), we found the experimental critical spacing range to be 2.9≲ℓc≲4.1, lower than prior experimental results and consistent with published computational results.

Mosquitoes transport liquid foods into the body using two muscular pumps in the head. In normal drinking, these pumps reciprocate in a stereotyped pattern of oscillation, with a high frequency but small stroke volume. Do mosquitoes modulate their neuromotor programs for pumping to produce different drinking modes? More broadly, what are the mechanical consequences of a two-pump system in insects? To address these questions, we used synchrotron x-ray imaging and fluid mechanical modeling to investigate drinking performance in mosquitoes. X-ray imaging of the pumps during drinking revealed two modes of pumping: continuous reciprocation with multiple small strokes, and a newly discovered 'burst mode' involving a single, large-volume stroke. Results from modeling demonstrate that burst mode pumping creates a very large pressure drop and high volume flow rate, but requires a massive increase in power, suggesting that continuous pumping is more economical for drinking. Modeling also demonstrates that, from one mode of pumping to the other, the mechanical role of the individual pumps changes. These results suggest that the advantage of a two-pump system in insects lies in its flexibility, enabling the animal to pump efficiently or powerfully as demanded by environmental considerations.

The motion of three interacting point vortices in the plane can be thought of as the motion of three geometrical points endowed with a dynamics. This motion can therefore be re-formulated in terms of dynamically evolving geometric quantities, viz. the circle that circumscribes the vortex triangle and the angles of the vortex triangle. In this study, we develop the equations of motion for the center, $Z$, and radius, $R$, of this circumcircle, and for the angles of the vortex triangle, $A$, $B$, and $C$. The equations of motion for $R$, $A$, $B$ and $C$ form an autonomous dynamical system. A number of known results in the three-vortex problem follow readily from the equations, giving a new geometrical perspective on the problem.

Ovarian cancer cells are exposed to physical stress in the peritoneal cavity during both tumor growth and dissemination. Ascites build-up in metastatic ovarian cancer further increases the exposure to fluid shear stress. Here, we used a murine, in vitro ovarian cancer progression model in parallel with immortalized human cells to investigate how ovarian cancer cells of increasing aggressiveness respond to < 1 dyne/cm(2)of fluid-induced shear stress. This biophysical stimulus significantly reduced cell viability in all cells exposed, independent of disease stage. Fluid shear stress induced spheroid formation and altered cytoskeleton organization in more tumorigenic cell lines. While benign ovarian cells appeared to survive in higher numbers under the influence of fluid shear stress, they exhibited severe morphological changes and chromosomal instability. These results suggest that exposure of benign cells to low magnitude fluid shear stress can induce phenotypic changes that are associated with transformation and ovarian cancer progression. Moreover, exposure of tumorigenic cells to fluid shear stress enhanced anchorage-independent survival, suggesting a role in promoting invasion and metastasis.

We investigate the finite-time collapse of three point vortices in the plane utilizing the geometric formulation of three-vortexmotion from Krishnamurthy, Aref and Stremler (2018) Phys. Rev. Fluids 3, 024702. In this approach, the vortex system is described in terms of the interior angles of the triangle joining the vortices, the circle that circumscribes that triangle, and the orientation of the triangle. Symmetries in the governing geometric equations of motion for the general three-vortex problem allow us to consider a reduced parameter space in the relative vortex strengths. The well-known conditions for three-vortex collapse are reproduced in this formulation, and we show that these conditions are necessary and sufficient for the vortex motion to consist of collapsing or expanding self-similar motion. The geometric formulation enables a new perspective on the details of this motion. Relationships are determined between the interior angles of the triangle, the vortex strength ratios, the (finite) system energy, the time of collapse, and the distance traveled by the configuration prior to collapse. Several illustrative examples of both collapsing and expanding motion are given.