Analysis and characterisation of momentum and thermal wakes of elliptic cylinders

Abstract

Non-canonical wakes of two-dimensional elliptic cylinders are analysed numerically for their near- and far-wake characteristics. The governing equations are solved using an immersed boundary method based projection scheme. The wakes are then classified into three distinct types according to diverse flow and thermal properties. An unexpected mean temperature evolution along the centreline of the wake is observed for certain wake states. In order to explain this unusual variation, novel heat transport models are constructed based on the vortex dynamics. These models are derived by considering vorticity is acted by flow, which has shear and swirl. Mechanisms of the primary vortex street breakdown and formation of the secondary vortex street are also proposed based on these models. A new phenomenon namely ‘dual near-wall instantaneous recirculation’ is observed, and its appearance is found to be a function of length of the primary von Kármán vortex street. The same phenomenon is also found to be responsible for the secondary peak in the Nusselt number variation along the circumference of the cylinder. Despite varied differences between the wake types, it is observed that the transitions occur through a supercritical Hopf bifurcation in all of them, at least in the von Kármán region of the wake. Low-frequency unsteadiness observed in the far wakes is examined through a signal decomposition method. Our results show that the secondary low frequency is resulting from the transition region which has a negative instability slope. Finally, onset of the primary vortex street breakdown and its scale in terms of Reynolds number is computed.

Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

“…This is because the cold oncoming flow makes contact with the cylinder surface first on the front stagnation point. As the flow warms up, the rest of the cylinder surface is exposed to flow with higher temperature than the one that has impinged on the front stagnation point (Paul et al 2016). This observation is valid only in the cylinder case, when there is no oncoming turbulence.…”

A two-dimensional numerical study of film boiling over an elliptical cylinder in the mixed regime under aiding and orthogonal saturated liquid flow configurations

“…Low values of the liquid cross-flow Reynolds number (Re l = ρ l UD/μ l ) in the range 10-170 are considered such that film boiling occurs in the mixed regime where the flow inertia is accompanied by a significant effect of buoyancy. The selected values of Re l also conform to the critical limits reported for flow transition to a three-dimensional nature in single phase flow over an ellipse with different aspect ratios (Thompson et al 2014;Paul et al 2016), which is necessary with regard to the present two-dimensional numerical model and different angles of incidence investigated including θ = 90 • . At this stage it is important to define the characteristic geometric dimension D used to calculate the various non-dimensional parameters in this study.…”

“…The investigation has also been extended to different wall superheats and Re l values (10-170) as presented subsequently for both liquid cross-flow orientations. However, for each geometric configuration, the magnitude of Re l is carefully selected to conform to the critical limits reported for flow transition to a three-dimensional nature in single phase flow (Thompson et al 2014;Paul et al 2016). Corresponding to the various conditions, the magnitude of the Froude number varies as √ Fr = 0.086-1.86, which again ascertains a significant influence of buoyancy in addition to flow inertia.…”

“…Further, an ellipse represents the simplest deviation from a circular body as a non-canonical form with features such as aspect ratio (Γ ) and angle of incidence (θ ) offering possibilities to study the interfacial dynamics and heat transfer for external flow film boiling in a comprehensive manner. It can also be noted that many important aspects including wake characteristics even for single-phase flow over an ellipse have been brought to the fore only in recent studies (Paul et al 2016;Rao et al 2017) as compared with an extensive effort dedicated towards understanding flow over a circular cylinder. Additionally, the interfacial dynamics in film boiling is prominently governed by Rayleigh-Taylor and Kelvin-Helmholtz instabilities, which can result in the heat transfer behaviour being counter-intuitive to that expected for single phase flows under certain conditions, as shown in some previous studies (Singh & Premachandran 2019, 2020) by the present authors.…”

Turbulent wake behind a concave curved cylinder

“…the introductions in Leontini, Jacono & Thompson (2015) and Paul et al. (2016). Depending on whether the major or the minor axis of the ellipse is aligned with the incoming flow, an elliptic cylinder can appear either more slender or more bluff compared to a circular cylinder, leading to different flow features.…”