Since the review of periodic flow phenomena by Berger & Wille (1972) in this journal, over twenty years ago, there has been a surge of activity regarding bluff body wakes. Many of the questions regarding wake vortex dynamics from the earlier review have now been answered in the literature, and perhaps an essential key to our new understandings (and indeed to new questions) has been the recent focus, over the past eight years, on the three-dimensional aspects of nominally two-dimensional wake flows. New techniques in experiment, using laser-induced fluorescence and PIV (Particle-Image-Velocimetry), are vigorously being applied to wakes, but interestingly, several of the new discoveries have come from careful use of classical methods. There is no question that strides forward in understanding of the wake problem are being made possible by ongoing three- dimensional direct numerical simulations, as well as by the surprisingly successful use of analytical modeling in these flows, and by secondary stability analyses. These new developments, and the discoveries of several new phenomena in wakes, are presented in this review.

Vortex Dynamics in the Cylinder Wake

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Protoplanet Migration by Nebula Tides

Gravitational interaction between a planet and a three-dimensional isothermal gaseous disk is studied. In the present paper we mainly examine the torque on a planet and the resultant radial migration of the planet. A planet excites density waves at Lindblad and corotation resonances and experiences a negative torque by the density waves, which causes a rapid inward migration of the planet during its formation in a protoplanetary disk. We formulate the linear wave excitation in three-dimensional isothermal disks and calculate the torques of Lindblad resonances and corotation resonances. For corotation resonances, a general torque formula is newly derived, which is also applicable to two-dimensional disks. The new formula succeeds in reproducing numerical results on the corotation torques, which do not agree with the previously well-known formula. The net torque of the inner and the outer Lindblad resonances (i.e., the differential Lindblad torque) is caused by asymmetry such as the radial pressure gradient and the scale height variation. In three-dimensional disks, the differential Lindblad torques are generally smaller than those in two-dimensional disks. Especially, the effect of a pressure gradient becomes weak. The scale height variation, which is a purely three-dimensional effect, makes the differential Lindblad torque decrease. As a result, the migration time of a planet is obtained as of the order of 106 yr for an Earth-size planet at 5 AU for a typical disk model, which is longer than the result of two-dimensional calculation by the factor of 2 or 3. The reflected waves from disk edges, which are neglected in the torque calculation, can further weaken the disk-planet interaction.

Three-Dimensional Interaction between a Planet and an Isothermal Gaseous Disk. I. Corotation and Lindblad Torques and Planet Migration

Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.

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Modeling Physiological Events in 2D vs. 3D Cell Culture.

We present dye visualizations of the flow generated by a sphere rolling along a solid surface in a quiescent fluid at low Reynolds numbers. Recent experimental and numerical studies of this configuration (Stewart et al. 2008, 2010) have shown a transition from steady to periodic flow at a Reynolds number Re (based on the sphere diameter and rolling speed) near 100. The unsteady flow involves the shedding of hairpin vortices, and the flow remained symmetric in the Reynolds number range considered, up to Re = 200. The present visualizations show that for higher Re the spatial symmetry of the unsteady wake is lost.

/pdf/wake-transition-of-a-rolling-sphere-24w9dq57ka.pdf

Wake transition of a rolling sphere

https://eng.monash.edu/lbe/Publications/Before%202002/ThHoSh96_etfm.pdf

Three-dimensional instabilities in the wake of a circular cylinder

Redox-active stimuli have gained a great deal of interest as an indicating factor for designing bioresponsive matrices in gene delivery. Hence, a wide range of gene carriers has been designed incorporating the redox-stimuli characteristics. The most important type of gene carriers is the class of redox responsive polymers. Among them, disulfide incorporated redox-responsive poly- ethyleneimine (PEI) and its derivatives, as a result of their outstanding DNA entrapping characteristics and their intrinsic endosomo- lytic activity, have attracted considerable attention in recent studies. The review presents the main developments of the characteristics of PEI derivatives and their applications in gene delivery. It is found that despite the uniquely stated characteristics, the noncleavable structure of conventional PEI (high molecular weight PEI: 25k), which makes it a nondegradable material, as well as the frequent inclusion of positively charged amino groups, which reduces its blood circulation period, render conventional PEI a very toxic mate- rial for gene-delivery applications. The extremely high cellular toxicity of conventional PEI has restricted its administration for real in-vivo physiological media. Recent studies have shown that employing low molecular weight PEI cross-linked by disulfide linkages (SS-PEI) and assembling low molecular weight disulfide linkages PEI (LMW SS-PEI) with bio-detachable anionic groups were two successful approaches for increasing bioavailability of the PEI-based gene carriers, while keeping outstanding cellular transfection. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42096.

/pdf/a-review-of-the-developments-of-characteristics-of-pei-3jfaym29vx.pdf

A review of the developments of characteristics of PEI derivatives for gene delivery applications

The low-Reynolds-number wake dynamics and stability of the flow past toroids placed normal to the flow direction are studied numerically. This bluff body has the attractive feature of behaving like the sphere at small aspect ratios, and locally like the straight circular cylinder at large aspect ratios. Importantly, the geometry of the ring is described by a single parameter, the aspect ratio (Ar), defined as a ratio of the torus diameter to the cross-sectional diameter of the ring. A rich diversity of wake topologies and flow transitions can therefore be investigated by varying the aspect ratio. Studying this geometry allows our understanding to be developed as to why the wake transitions leading to turbulence for the sphere and circular cylinder differ so greatly. Strouhal–Reynolds-number profiles are determined for a range of ring aspect ratios, as are critical Reynolds numbers for the onset of flow separation, unsteady flow and asymmetry. Results are compared with experimental findings from the literature. Calculated Strouhal–Reynolds-number profiles show that ring wakes shed at frequencies progressively closer to that of the straight circular cylinder wake as aspect ratio is increased from Ar =3 . For Ar > 8, the initial asymmetric transition is structurally analogous to the mode A transition for the circular cylinder, with a discontinuity present in the Strouhal–Reynolds-number profile. The present numerical study reveals a shedding-frequency decrease with decreasing aspect ratio for ring wakes, and an increase in the critical Reynolds numbers for flow separation and the unsteady flow transition. A Floquet stability analysis has revealed the existence of three modes of asymmetric vortex shedding in the wake of larger rings. Two of these modes are analogous to mode A and mode B of the circular cylinder wake, and the third mode, mode C, is analogous to the intermediate wavelength mode found in the wake of square section cylinders and circular cylinder wakes perturbed by a tripwire. Furthermore, three distinct asymmetric transition modes have been identified in the wake of small aspect ratio bluff rings. Fully developed asymmetric simulations have verified the unsteady transition for rings that exhibit a steady asymmetric wake.

/pdf/from-spheres-to-circular-cylinders-the-stability-and-flow-4cl2x2ih52.pdf

Kerry Hourigan

Papers

Wake transition of a rolling sphere

Three-dimensional instabilities in the wake of a circular cylinder

A review of the developments of characteristics of PEI derivatives for gene delivery applications

From spheres to circular cylinders: the stability and flow structures of bluff ring wakes

The beginning of branching behaviour of vortex-induced vibration during two-dimensional flow