Design of non-uniformly distributed annular fins for a shell-and-tube thermal energy storage unit

A review on fluid dynamics of flapping foils

While the wake of a circular cylinder and, to a lesser extent, the normal flat plate have been studied in considerable detail, the wakes of elliptic cylinders have not received similar attention. However, the wakes from the first two bodies have considerably different characteristics, in terms of three-dimensional transition modes, and near- and far-wake structure. This paper focuses on elliptic cylinders, which span these two disparate cases. The Strouhal number and drag coefficient variations with Reynolds number are documented for the two-dimensional shedding regime. There are considerable differences from the standard circular cylinder curve. The different three-dimensional transition modes are also examined using Floquet stability analysis based on computed two-dimensional periodic base flows. As the cylinder aspect ratio (major to minor axis) is decreased, mode A is no longer unstable for aspect ratios below 0.25, as the wake deviates further from the standard Benard–von Karman state. For still smaller aspect ratios, another three-dimensional quasi-periodic mode becomes unstable, leading to a different transition scenario. Interestingly, for the 0.25 aspect ratio case, mode A restabilises above a Reynolds number of approximately 125, allowing the wake to return to a two-dimensional state, at least in the near wake. For the flat plate, three-dimensional simulations show that the shift in the Strouhal number from the two-dimensional value is gradual with Reynolds number, unlike the situation for the circular cylinder wake once mode A shedding develops. Dynamic mode decomposition is used to characterise the spatially evolving character of the wake as it undergoes transition from the primary Benard–von Karman-like near wake into a two-layered wake, through to a secondary Benard–von Karman-like wake further downstream, which in turn develops an even longer wavelength unsteadiness. It is also used to examine the differences in the two- and three-dimensional near-wake state, showing the increasing distortion of the two-dimensional rollers as the Reynolds number is increased.

/pdf/low-reynolds-number-wakes-of-elliptical-cylinders-from-the-463b64cw63.pdf

Low-Reynolds-number wakes of elliptical cylinders: from the circular cylinder to the normal flat plate

This paper discusses the dynamic behavior of water drops impacting on inclined superhydrophobic surfaces. For a normal impact on a smooth hydrophobic surface, the spreading (or expansion) and retraction dynamics of an impacting drop varies from complete rebound to splashing depending on its Weber number, (We(d)), calculated using the impact speed and diameter d of the drop. For a slanted impact, on the other hand, the impact dynamics depends on two distinct Weber numbers, based on the velocity components normal, (We(nd)), and tangential, (We(td)), to the surface. Impact on superhydrophobic surfaces is even more complicated as the surfaces are covered with micro- to nano-scale texture. Therefore, we develop an expression for an additional set of two Weber numbers, (We(na), We(ta)), which are counterparts to the first set but use the gap distance a between asperities on the textured surface as the characteristic length. We correlate the derived Weber numbers with the impact dynamics on tilted surfaces covered with three different types of texture: (i) posts, (ii) ridges aligned with and (iii) ridges perpendicular to the impact direction. Results suggest that the first two Weber numbers, (We(nd), We(td)), affect the impact dynamics of a drop such as the degree of drop deformation as long as the superhydrophobicity remains intact. On the other hand, the Weber number We(na) determines the transition from the superhydrophobic Cassie-Baxter regime to the fully-wetted Wenzel regime. Accuracy of our model becomes lower at a high tilting angle (75°), due to the change in the transition mechanism.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-007553.pdf

Drop impact on inclined superhydrophobic surfaces

Study on solidification performance of PCM by longitudinal triangular fins in a triplex-tube thermal energy storage system

Study on the effect of multiple spiral fins for improved phase change process

The properties of asymmetric wake patterns behind a flat plate inclined at angles of attack 20°, 25°, and 30° are investigated. The Reynolds number based on the inflow velocity and the plate width is 1000. Both two-dimensional and three-dimensional calculations are performed by direct numerical simulations. Compared to the three-dimensional simulations, the two-dimensional calculations predict a significantly lower pressure on the rear surface of the plate, which consequently leads to very high drag and lift forces on the plate. The asymmetric mean wake flow, turbulence properties, and coherent patterns in the three-dimensional simulations are analysed by time- and phase-averaged techniques. Unlike the symmetric wake flow, the vortices shed from the leading and trailing edges of an inclined plate possess unequal strength with the trailing edge vortex having higher strength. It is observed that the present three-dimensional simulations predict results which compare well with the experimental data. In addit...

Vortex shedding in flow past an inclined flat plate at high incidencea)

Drag reduction of a rapid vehicle in supercavitating flow

The effects of superhydrophobic surfaces (SHSs), which consist of microgrates oriented transverse to the flow direction, on the onset of three-dimensional instability of flow past a bluff body were studied using Floquet analysis. The SHS was modelled on an air–water interface with a shear-free condition. The results showed that SHSs increased the vortex shedding frequency. Floquet analysis revealed that modes B and S were suppressed dramatically by the partial-slip condition compared with a regular no-slip body; however, mode A was less affected. Correspondingly, the critical spanwise wavelengths were not significantly affected by SHSs. A similar phenomenon was observed in flow past a circular cylinder coated by SHSs. The results also revealed that modes B and S were collapsed into mode A due to the increased width of the air–water region for flow past an elongated body. Furthermore, the critical Reynold numbers of different modes were diversely affected by gas fraction (GF) variations. The unstable modes with short wavelengths, such as modes B and S , stabilized with increasing GF. Conversely, the opposite was seen for the unstable mode A with a longer wavelength. The exact critical Reynolds number depended on the geometric configuration, which should be between the critical values of the two extreme cases. The application of SHSs could modify the transition route from two- to three-dimensionality by alternating different unstable modes. As the wavelength of the unstable mode decreases, the inhibition of three-dimensional instability becomes more efficient by SHSs.

Influence of slip on the three-dimensional instability of flow past an elongated superhydrophobic bluff body

The vortex-induced vibration (VIV) in a Newtonian fluid has been widely studied due to its significance in many industrial applications. However, it could be significantly affected by the addition of polymer. In this work, we investigate the viscoelastic effect on the VIV of a circular cylinder whose motion is confined in the cross-flow direction. The Arbitrary Lagrangian–Eulerian (ALE) based finite volume method is used to carry out the two-dimensional simulations at the Reynolds number (Re) range of 30–500. The nonlinear FENE-P model is used to characterize the polymeric viscoelastic fluids. The results show that the addition of polymers in a Newtonian fluid can suppress the VIV amplitude of the cylinder since the polymer can significantly modify the vortex pattern and inhibit the fluctuation in flow. Both higher Weissenberg Number (We) and larger maximum polymer extensibility can reinforce this tendency. Furthermore, the corresponding reduced velocity for the peak of maximum vibration amplitude increases as the increase of Weissenberg number. This is because that the viscoelasticity can reduce the characteristic frequency in the wake flow. Finally, the viscoelasticity can increase the critical Re of the structure vibrations implying a more stable flow.

Dan Yang

Papers

Study on the effect of multiple spiral fins for improved phase change process

Vortex shedding in flow past an inclined flat plate at high incidencea)

Drag reduction of a rapid vehicle in supercavitating flow

Influence of slip on the three-dimensional instability of flow past an elongated superhydrophobic bluff body

Numerical study on the vortex-induced vibration of a circular cylinder in viscoelastic fluids