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We analyse the vortex structures of the mean turbulent flow field in a 90-degree bend pipe at bulk Reynolds number 5300 and radius of curvature 2.5 times the pipe radius. The Reynolds averaged Navier–Stokes equations are solved for the mean turbulent flow field in the finite volume framework of ANSYS Fluent. The Reynolds stress model is adopted for closure of turbulence quantities. Notwithstanding any scale resolving simulation, we show that a detailed exploration of the flow, turbulence, and vortex structures in the mean turbulent flow field inside a pipe bend is possible, through the paradigm based on complex conjugate eigenvalues of the velocity gradient tensor. The convex contour of the inner side of the bend is found to be the key vortex-generating surface. We find that the vortices start to gain swirling strength just after entering the bend. The strength further decays, rather than getting enhanced, within the remaining part of the bend. The outer side of the bend, on the other hand, appears to maintain a straining flow regime. Fluid particles move between the vortex region and the straining region through the strain-dominated vortex regions, the structure of which is also unveiled through the eigenvalue-based vortex defining paradigm. Both the straining and the vortex zone inject turbulence. However, the vortex-induced turbulence suffers significant dissipation within the bend region. The vortices transported downstream of the bend help in generating turbulence to recover the loss of turbulence in the bend region. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we analyze the consequences of substrate wettability on the convective heat transfer characteristics in microchannel flows. However, in sharp contrast to other reported studies, we do not presume a slip-length a priori, but take an explicit accounting of the substrate wettability through an order parameter consideration. The order parameter formalism also allows us to consider co-existences of bulk and interfacial phases and takes the consequent thermo-physical property variations aptly into account. Our results reveal explicit dependence of the heat transfer characteristics, such as the thermal entrance length and the Nusselt number, on the equilibrium contact angle. Further, we present an analysis that theoretically justifies the role of wettability by decomposing the thermophysical properties and flow variables into a base part and a wettability-dependant part, with rigorous justification from a perturbation-based analysis of the order parameter evolution equations. These considerations, along with our simulation predictions, reveal that the effect of substrate wettability on bulk thermal characteristics of microchannel transport is primarily advection-dominated, although a combined advection–diffusion mechanism controls the corresponding interfacial transport in the near-wall regime. Our results bring out an explicit dependence of the heat transfer characteristics on the substrate wettability, and in particular, reveal a trend of decreasing Nusselt number with increasing value of the contact angle. [Copyright &y& Elsevier]