Towards self-sustained oscillations of multiple flexible vortex generators

Passive methods are widely used for flow control in engineering processes for heat and mass transfer enhancement. Using flexible vortex generators (FVGs) in such applications in order to destabilize the flow can be thought to achieve higher performances taking advantage of the fluid-structure interaction. In this paper, we discuss the assessment of getting self-sustained large oscillation amplitudes of multiple FVGs from an upstream confined laminar flow. The FVGs are located on the opposite channel walls in alternated positions, separated by a distance equal to their span and inclined in the upstream direction with an angle of 30° with respect to the wall. Five cases are studied which differ by the number of alternating FVGs in the system and investigations are also performed adding two co-planar FVGs upstream. The Reynolds number is held constant with a value of 2000 (based on the hydraulic diameter) for all the cases. The effect of increasing the degree of freedom of the system, on creating a large displacement flapping motion is numerically investigated. The results show that a minimum of three alternating FVGs is needed to produce a self-sustained and periodic flow instability, leading to large FVG displacement when the co-planar FVGs are not present. The introduction of upstream co-planar FVGs destabilizes the flow by producing vortices which act as periodic forces on the downstream FVGs. In this case, large displacement amplitudes are thus observed with two alternating FVGs added downstream. A phenomenon of inverted drafting is observed in all the cases: upstream FVGs display smaller drag force values than the downstream ones. Since the downstream FVGs oscillate in resonance with the incoming flow, motion amplitudes become higher. Moreover, it has been observed that for all the configurations studied here, the FVGs located at the same wall location oscillate in phase with each others and out-of-phase with the ones located on the opposite channel wall

Heat transfer and mixing enhancement by free elastic flaps oscillation

An original concept is proposed to enhance heat transfer and mixing quality performances by using flexible vortex generators (FVGs) for a static mixer configuration. The role of free elastic flaps oscillations on the mixing process and heat transfer in a two-dimensional laminar flow is numerically investigated. The computational domain consists of four distant FVGs mounted on two opposite walls. Two cases are studied depending on the Reynolds numbers (based on the bulk velocity and the channel height) set to 1000 and 1850. FVGs efficiencies are compared to the corresponding cases with rigid vortex generators (RVGs). In the flexible cases, flaps oscillations increase the velocity gradients and generate an unsteady laminar flow with complex coherent vortices detaching from the tip of the flaps. The mixing efficiency is quantified by the transport of a passive scalar through the channel. It is shown that oscillations in the elastic cases enhance the mixture quality up to 98% relative to that in the rigid cases. The heat transfer enhancement is also investigated showing up to a 96% increase in the Colburn factor, 56% increase in thermal performance factor and 134% increase in the overall heat transfer. As the FVGs oscillate freely without any additional external force other than that exerted by the flow itself, the implementation of such a technique shows a great potential for the performance enhancement of multifunctional heat exchangers/reactors

Heat transfer and mixing enhancement by using multiple freely oscillating flexible vortex generators

In this paper, we discuss the effect of self-sustained passive oscillations of multiple flexible vortex generators (FVG) in a two-dimensional laminar flow, on heat transfer and mixing. The FVG are located on two opposite channel walls in an alternating positions, inclined in the upstream direction with an angle of 30° with respect to the wall. The FVG oscillate freely without any external force except that provided by the flow itself. Five cases are studied and they differ by the number of alternating flaps and by the presence or absence of two co-planar flaps upstream. The Reynolds number is held constant with a value of 2000 based on the hydraulic diameter of the channel. The simulations are performed by considering a two way strongly-coupled fluid structure interaction approach. The effect of increasing the system degree of freedom, by increasing the number of flaps, resulting in a larger displacement oscillation, on heat transfer and mixing is numerically investigated. The mixing process is quantified by solving the passive scalar transport equation and calculating a mixing index. The results show that mixing is enhanced for larger flaps displacement achieving up to 99% in mixing homogeneity. Moreover, the high amplitude oscillations when compared to the results of an empty channel, show a great ability to reduce the thickness of the thermal boundary layer and to enhance heat transfer resulting in up to 275% increase in the global Nusselt number, 317% increase in the local Nusselt number and 34% increase in the thermal performance factor

The effect of self-sustained free elastic flaps oscillation on heat transfer and mixing performances

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Numerical analysis and simulation of the interaction between a Von Kármán vortex street and elastic flaps

The performance improvement of heat exchangers is a major challenge in many industries. Some of these heat exchangers use vortex generators to produce coherent structures that enhance the heat and mass transfer. The aim of this study is to numerically investigate the vortices generated by introducing flexible flaps into an oscillating and laminar flow. More specifically, a square cylinder is inserted into a two-dimensional domain such that a Von Kármán street vortex is generated. The successive wake vortices disrupt a flexible structure that oscillates and produce coherent structures. The study is performed by numerical simulations. The fluid-solid coupling dynamics and the deformation of the flexible element are simulated using a tool developed with the open source library OpenFOAM. The finite volume method is used to discretize the geometry. The problem is then solved by using the PIMPLE algorithm and a partitioned approach for fluid-structure interaction. The dynamic solver is coupled with a mesh deformation procedure based on Laplace smoothing equation with variable mesh diffusion. The solvers and the coupling method used for modeling fluid-structure interaction are detailed first. The effects of the elastic properties of the flexible structure on the oscillations and the topology of the flow are then investigated

Pion and proton showers in the CALICE scintillator-steel analogue hadron calorimeter

Showers produced by positive hadrons in the highly granular CALICE scintillator-steel analogue hadron calorimeter were studied. The experimental data were collected at CERN and FNAL for single particles with initial momenta from 10 to 80 GeV/c. The calorimeter response and resolution and spatial characteristics of shower development for proton- and pion-induced showers for test beam data and simulations using Geant4 version 9.6 are compared.Comment: 26 pages, 16 figures, JINST style, changes in the author list, typos corrected, new section added, figures regrouped. Accepted for publication in JINS