1,721,007 research outputs found
T-shaped micromixers aligned in a row: characterization of the engulfment regime
No full textAccurate control of mixing between two fluids is a fundamental aspect in many applications and generally implies the use of small devices operating at low velocities. This is often achieved using micromixers which, due to the combination of small dimensions and low velocities, work in the laminar regime and generally process very limited flow rates. The flow rates can be increased using more mixers at the same time. In this respect it is appealing to use several micromixers connected in a unique device. In this paper we propose and characterize a simple strategy to connect several T-mixers together by aligning them in a row and feeding them by shared inlet channels. Since engulfment is the preferred flow regime for mixing, we investigate the proposed devices specifically focusing on the onset and on the properties of the engulfment regime. This investigation is carried out by a combined use of numerical simulation and linear stability analysis. The results reported here show that the proposed strategy can lead to compact devices in which the single mixers manifest engulfment, although some important differences in comparison with isolated T-mixers may exist depending on the spacing between the inlet/outlet channels in the device
Stability properties of the mean flow after a steady symmetry-breaking bifurcation and prediction of the nonlinear saturation
No full textIn this paper, it is shown that when a flow undergoes a steady bifurcation breaking one reflection symmetry, the mean flow obtained by averaging the two possible asymmetric flow fields resulting from the instability remains marginally stable in the postcritical regime. This property is demonstrated rigorously through an asymptotic analysis which closely follows that proposed in Sipp and Lebedev (J Fluid Mech 792:620–657, 2007) for a Hopf bifurcation with focus on wakes. In the case of wakes, the marginal stability of the mean flow is well known and had several consequences documented in the literature. To the authors’ knowledge, the marginal stability of mean flows after a symmetry-breaking pitchfork bifurcation is demonstrated here for the first time. As an example of possible consequences of marginal stability, the self-consistent model proposed for wakes in Mantič-Lugo et al. (Phys Rev Lett 113:084501, 2014) and relying on marginal stability is also applied here to the symmetry-breaking instability of the flow in a channel with a sudden expansion. For this specific case, the marginal stability of the mean flow is first demonstrated by dedicated direct numerical simulations; successively, it is shown that the resulting self-consistent model predicts the nonlinear saturation of the instability with remarkable accuracy
Linear Stability and Receptivity analysis of the vortex shedding in flows past confined square cylinders
No full textAnalysis of the inversion of the von Karman street in the wake of a confined square cylinder
No full textThis study considers the incompressible 2D laminar flow around a square cylinder symmetrically positioned in a channel. In this type of flow, even if vortices of opposite sign are alternately shed from the body into the wake as in the unconfined case, an inversion of their position with respect to the flow symmetry line takes place further downstream. Thanks to a dedicated numerical investigation, an interpretation of the inversion is given in terms of interference between the wake and the vorticity of the incoming flow, which is shown to play a dominant role in the phenomenon
Flow induced by a rotating cone: Base flow and convective stability analysis
Full text linkThe steady flow over a rotating cone is investigated theoretically and numerically in order to improve the traditional von Kármán solution by proposing a self-similar correction which is an explicit function of the cone angle. The effect of the correction on the linear stability analysis of the rotating-cone flow is successively investigated through a weakly divergent approach. Both the base flow correction and the results of the stability analysis are validated against dedicated numerical simulations. As for the base flow, the comparison shows a clear improvement obtained by using the proposed correction in comparison with the classical von Kármán solution. As for the stability properties of the flow, the com-parison against the reference simulations shows a good agreement among all the approaches for large azimuthal wave numbers, but a better performance is obtained with the weakly divergent approach for lower wave numbers. The latter approach provides a lower critical Reynolds number than that predicted by parallel theory and, most importantly, changes the interplay between modes I and II with respect to what predicted by the parallel stability calculations. Finally, it is observed that the proposed correction of base flow has a slight effect on the stability analysis of the considered cases, but it may have important effects for low cone angles. Thus, while the classical Kármán solution is appropriate for large cone angles, the proposed correction is recommended for future stability analyses of slender cones
Sensitivity analysis and passive control of the secondary instability in the wake of a cylinder
No full textThe stability properties of selected flow configurations, usually denoted as base flows, can be significantly altered by small modifications of the flow, which can be caused, for instance, by a non-intrusive passive control. This aspect is amply demonstrated in the literature by ad hoc sensitivity studies which, however, focus on configurations characterised by a steady base flow. Nevertheless, several flow configurations of interest are characterised by a time-periodic base flow. To this purpose, we propose here an original theoretical framework suitable to quantify the effects of base-flow variations in the stability properties of saturated time-periodic limit cycles. In particular, starting from a Floquet analysis of the linearised Navier–Stokes equations and using adjoint methods, it is possible to estimate the variation of a selected Floquet exponent caused by a generic structural perturbation of the base-flow equations. This link is expressed concisely using the adjoint operators coming from the analysis, and the final result, when applied to spatially localised disturbances, is used to build spatial sensitivity and control maps. These maps identify the regions of the flow where the placement of a infinitesimal small object produces the largest effect on the Floquet exponent and may also provide a quantification of this effect. Such analysis brings useful insights both for passive control strategies and for further characterising the investigated instability. As an example of application, the proposed analysis is applied here to the three-dimensional flow instabilities in the wake past a circular cylinder. This is a classical problem which has been widely studied in the literature. Nevertheless, by applying the proposed analysis we derive original results comprising a further characterisation of the instability and related control maps. We finally show that the control maps obtained here are in very good agreement with control experiments documented in the literature
Investigation of the symmetry-breaking instability in a T-mixer with circular cross section
Full text linkThis 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
Numerical investigation of the AFRODITE transition control strategy
No full textThe experiments carried out within the AFRODITE[2] project are aimed at exploring the effectiveness of properly shaped velocity miniature vortex generators in delaying transition to turbulence in a boundary layer. The present work details the direct numerical simulation setup designed to support and reproduce the AFRODITE experiments and provide results showing that the proposed DNS is in good agreement with the experiments. The results of the DNS also show that even a minimal delay of the transition point results in an overall gain in terms of drag when MVGs are installed on the plate
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