Investigation of the symmetry-breaking instability in a T-mixer with circular cross section

This paper investigates the laminar flow inside a T-mixer composed of three pipes with a circular cross section. The flow enters the mixer symmetrically from the two aligned pipes and leaves the device from the third pipe. In similar devices, but involving rectangular channels instead of pipes, an important regime for mixing has been identified, denoted as engulfment. Despite the symmetries of the flow and of the geometry, engulfment is an asymmetric steady regime, which is observed above a critical value (Rec) of the flow Reynolds number. Conversely, for Reynolds numbers lower than Rec, the flow regime is steady and symmetric, and it is usually denoted as the vortex regime. In this paper, both the vortex and the engulfment regimes are identified for the considered geometry, and they are characterized in detail by dedicated direct numerical simulations (DNSs). Despite an apparent similitude with the behavior of T-mixers employing rectangular channels, which are the most investigated T-mixers in the literature, substantial differences are observed and highlighted here concerning both regimes, i.e., the vortex and the engulfment ones, and concerning transition between the two. Global stability analysis is finally used in synergy with DNS to investigate the onset of the engulfment regime, which is shown to be related to a symmetry-breaking bifurcation of the vortex regime

T-mixer operating with water at different temperatures: Simulation and stability analysis

In this paper we investigate the transition from the vortex to the engulfment regime in a T-mixer when the two entering flows have different viscosity. In particular we consider as working fluid water entering the two inlet channels of the mixer at two different temperatures. Contrary to the isothermal case, at low Reynolds numbers the vortex regime shows only a single reflectional symmetry, due to the nonhomogeneous distribution of the viscosity. Increasing the Reynolds number, a symmetry-breaking bifurcation drives the system to a new steady flow configuration, usually called the engulfment regime, similar to what it is possible to observe in an isothermal case. This flow regime is associated with an increase of the mixing between the two inlet streams. It is shown by direct numerical simulation (DNS) and by stability analysis that the engulfment regime is promoted by the temperature difference. Starting from the DNSs, the resulting flow fields are analyzed in detail considering different temperature jumps between the two inlet boundaries. Furthermore, dedicated linear stability analyses are carried out to investigate the instability mechanism associated with the occurrence of the engulfment regime. In particular, similarly to the case without temperature differences, the onset of engulfment is driven by the momentum equation, and the temperature field does not lead to any additional instability mechanism. However, the existence of a temperature field leads to quantitative changes of the stability characteristics and of the resulting flow fields via a variation of the viscosity coefficient

A non-linear observer for unsteady three-dimensional flows

A method is proposed to estimate the velocity field of an unsteady flow using a limited number of flow measurements. The method is based on a non-linear low-dimensional model of the flow and on expanding the velocity field in terms of empirical basis functions. The main idea is to impose that the coefficients of the modal expansion of the velocity field give the best approximation to the available measurements and that at the same time they satisfy as close as possible the non-linear low-order model. The practical use may range from feedback flow control to monitoring of the flow in non-accessible regions. The proposed technique is applied to the flow around a confined square cylinder, both in two- and three-dimensional laminar flow regimes. Comparisons are provided. with existing linear and non-linear estimation techniques

Linear stability analysis of wind turbine wakes performed on wind tunnel measurements

Wind tunnel measurements were performed for the wake produced by a three-bladed wind turbine immersed in uniform flow. These tests show the presence of a vorticity structure in the near-wake region mainly oriented along the streamwise direction, which is denoted as the hub vortex. The hub vortex is characterized by oscillations with frequencies lower than that connected to the rotational velocity of the rotor, which previous works have ascribed to wake meandering. This phenomenon consists of transversal oscillations of the wind turbine wake, which might be excited by the vortex shedding from the rotor disc acting as a bluff body. In this work, temporal and spatial linear stability analyses of a wind turbine wake are performed on a base flow obtained with time-averaged wind tunnel velocity measurements. This study shows that the low-frequency spectral component detected experimentally matches the most amplified frequency of the counter-winding single-helix mode downstream of the wind turbine. Then, simultaneous hot-wire measurements confirm the presence of a helicoidal unstable mode of the hub vortex with a streamwise wavenumber roughly equal to that predicted from the linear stability analysi

Permeability sets the linear path instability of buoyancy-driven disks

The prediction of trajectories of buoyancy-driven objects immersed in a viscous fluid is a key problem in fluid dynamics. Simple-shaped objects, such as disks, present a great variety of trajectories, ranging from zig-zag to tumbling and chaotic motions. Yet, similar studies are lacking when the object is permeable. We perform a linear stability analysis of the steady vertical path of a thin permeable disk, whose flow through the microstructure is modelled via a stress-jump model based on homogenization theory. The relative velocity of the flow associated with the vertical steady path presents a recirculation region detached from the body, which shrinks and eventually disappears as the disk becomes more permeable. In analogy with the solid disk, one non-oscillatory and several oscillatory modes are identified and found to destabilize the fluid-solid coupled system away from its straight trajectory. Permeability progressively filters out the wake dynamics in the instability of the steady vertical path. Modes dominated by wake oscillations are first stabilized, followed by those characterized by weaker, or absent, wake oscillations, in which the wake is typically a tilting induced by the disk inclined trajectory. For sufficiently large permeabilities, the disk first undergoes a non-oscillatory divergence instability, which is expected to lead to a steady oblique path with a constant disk inclination, in the nonlinear regime. A further permeability increase reduces the unstable range of all modes until quenching of all linear instabilities

An Overview of Flow Features and Mixing in Micro T and Arrow Mixers

An overview of the mixing performances of micro T mixers operating with a single fluid is presented. The focus is on the relationship between the flow features and mixing. Indeed, T mixers are characterized by a variety of regimes for increasing Reynolds numbers; they are briefly described, in particular in terms of the three-dimensional vorticity field, which can explain the different mixing performances. The effects of changes in the aspect ratio of the channels are also reviewed. The role of instability and sensitivity analyses in highlighting the mechanisms of the onsets of the different regimes is then described. These analyses also suggest possible geometrical modifications to promote mixing. We focus on that consisting of the downward tilting of the inlet channels (arrow mixers). Arrow mixers are interesting because the onset of the engulfment regime is anticipated at lower Reynolds numbers. Hence, the mixing performances of arrow mixers with varying Reynolds number are described

Ageing test of the ATLAS RPCs at X5-GIF

An ageing test of three ATLAS production RPC stations is in course at X5-GIF, the CERN irradiation facility. The chamber efficiencies are monitored using cosmic rays triggered by a scintillator hodoscope. Higher statistics measurements are made when the X5 muon beam is available. We report here the measurements of the efficiency versus operating voltage at different source intensities, up to a maximum counting rate of about 700Hz/cm^2. We describe the performance of the chambers during the test up to an overall ageing of 4 ATLAS equivalent years corresponding to an integrated charge of 0.12C/cm^2, including a safety factor of 5.Comment: 4 pages. Presented at the VII Workshop on Resistive Plate Chambers and Related Detectors; Clermont-Ferrand October 20th-22nd, 200

Optimization of RPCs read-out panel with electromagnetic simulation

With the upgrade of the RPCs [1]-[2] and the increase of its performances, the study and the optimization of the read-out panel is necessary in order to maintain the signal integrity and to reduce the intrinsic crosstalk. Through Electromagnetic Simulation, performed with CST Studio Suite, new panels design are tested and their crosstalk property are studied. The behavior of different type of panel is shown, in particular a panel with the decoupling strip connected through their characteristic impedance to the ground plane is simulated

The first level muon trigger in the central toroid of the ATLAS experiment

We present the design of the first level muon trigger in the central toroid of the ATLAS experiment at the Large Hadron Collider (LHC). A trigger is foreseen based on fast, finely segmented gaseous detectors, Resistive Plate Chambers (RPC), to unambiguously identify the interaction bunch crossing. We describe the detectors and the logic scheme of the trigger. © 1995