arXiv subjects

Kerstin Avila

Publications and source records attributed to Kerstin Avila.

7 recordsLinked to original sources

arXiv2025

Experimental study of turbulent mixing in a T-shaped mixer

One of the most widespread canonical devices for fluid mixing is the T-shaped mixer, in which two opposing miscible liquid streams meet at a junction and then mix along a main channel. Laminar steady and time-periodic flows in T-shaped mixers have been thoroughly studied, but turbulent flows have received much less scrutiny despite their prevalence in applications. We here introduce a novel experimental setup with a hydraulic diameter of four centimetres that enables the optical study of turbulent mixing at small scales. Using this setup, we perform two-dimensional particle image velocimetry and planar laser-induced fluorescence measurements. First, we successfully replicate characteristic flow regimes observed in micro-scale T-shaped mixers at low Reynolds numbers. We then focus on the turbulent regime and characterize the turbulent kinetic energy and dissipation along the mixing channel. Further, we measure the scalar concentration variance and its corresponding probability density function and spectra. The latter exhibits an incipient Batchelor scaling. We estimate the mechanical-to-scalar timescale ratio and examine the link between the turbulent velocity and scalar fields. The measurement data are compared with model predictions and correlations used in engineering practice, and with our own direct numerical simulations performed with a spectral-element code.

physics.flu-dyn↗

arXiv2023

Routes to turbulence in Taylor-Couette flow

Fluid flows between rotating concentric cylinders exhibit two distinct routes to turbulence. In flows dominated by inner-cylinder rotation, a sequence of linear instabilities leads to temporally chaotic dynamics as the rotation speed is increased. The resulting flow patterns occupy the whole system and sequentially lose spatial symmetry and coherence in the transition process. In flows dominated by outer-cylinder rotation, the transition is abrupt and leads directly to turbulent flow regions that compete with laminar ones. We here review the main features of these two routes to turbulence. Bifurcation theory rationalises the origin of temporal chaos in both cases. However, the catastrophic transition of flows dominated by outer-cylinder rotation can only be understood by accounting for the spatial proliferation of turbulent regions with a statistical approach. We stress the role of the rotation number (the ratio of Coriolis to inertial forces) and show that it determines the lower border for the existence of intermittent laminar-turbulent patterns.

physics.flu-dyn↗

arXiv2022

Inertial focusing of a dilute suspension in pipe flow

The dynamics of rigid particle suspensions in a wall-bounded laminar flow present several non-trivial and intriguing features, including particle ordering, lateral transport, and the appearance of stable, preferential locations like the Segré-Silberberg annulus. The formation of more than one annulus is a particularly puzzling phenomenon that is still not fully explained. Here, we present numerical simulation results of a dilute suspension of particles in (periodic) pipe flow based on the lattice Boltzmann and the discrete element methods (DEM). Our simulations provide access to the full radial position history of the particles while traveling downstream. This allows to accurately quantify the transient and steady states. We observe the formation of the secondary, inner annulus and show that its position invariably shifts toward the Segré-Silberberg one if the channel is sufficiently long, proving that it is, in fact, a transient feature for Reynolds numbers (Re) up to 600. We quantify the variation of the channel focusing length ($L_s/2R$) with Re. Interestingly and unlike the theoretical prediction for a point-like particle, we observe that $L_s/2R$ increases with Re for both the single particle and the suspension.

physics.flu-dyn↗

arXiv2022

Data-Driven Modeling and Prediction of Non-Linearizable Dynamics via Spectral Submanifolds

We develop a methodology to construct low-dimensional predictive models from data sets representing essentially nonlinear (or non-linearizable) dynamical systems with a hyperbolic linear part that are subject to external forcing with finitely many frequencies. Our data-driven, sparse, nonlinear models are obtained as extended normal forms of the reduced dynamics on low-dimensional, attracting spectral submanifolds (SSMs) of the dynamical system. We illustrate the power of data-driven SSM reduction on high-dimensional numerical data sets and experimental measurements involving beam oscillations, vortex shedding and sloshing in a water tank. We find that SSM reduction trained on unforced data also predicts nonlinear response accurately under additional external forcing.

math.DS↗

arXiv2021

The phase transition to turbulence in spatially extended shear flows

Directed percolation(DP) has recently emerged as a possible solution to the century old puzzle surrounding the transition to turbulence. Multiple model studies reported DP exponents, however experimental evidence is limited since the largest possible observation times are orders of magnitude shorter than the flows' characteristic time scales. An exception is cylindrical Couette flow where the limit is not temporal but rather the realizable system size. We present experiments in a Couette setup of unprecedented azimuthal and axial aspect ratios. Approaching the critical point to within less than 0.1% we determine five critical exponents, all of which are in excellent agreement with the 2+1D DP-universality class. The complex dynamics encountered at the onset of turbulence can hence be fully rationalized within the framework of statistical mechanics.

physics.flu-dyn↗

arXiv2021

Phase-lag predicts nonlinear response maxima in liquid-sloshing experiments

Mass-spring models are essential for the description of sloshing resonances in engineering. By experimentally measuring the liquid's centre of mass in a horizontally oscillated rectangular tank, we show that low-amplitude sloshing obeys the Duffing equation. A bending of the response curve in analogy to a softening spring is observed, with growing hysteresis as the driving amplitude increases. At large amplitudes, complex wave patterns emerge (including wave-breaking and run up at the tank walls), competition between flow states is observed and the dynamics departs progressively from Duffing. We also provide a quantitative comparison of wave shapes and response curves to the predictions of a multimodal model based on potential flow theory (Faltinsen & Timokha 2009) and show that it systematically overestimates the sloshing amplitudes and the hysteresis. We find that the phase-lag between the liquid's centre of mass and the forcing is the key predictor of the nonlinear response maxima. The phase-lag reflects precisely the onset of deviations from Duffing dynamics and - most importantly - at resonance the sloshing motion always lags the driving by 90° (independently of the wave pattern). This confirms the theoretical 90°-phase-lag criterion (Cenedese & Haller 2020).

physics.flu-dyn↗

arXiv2015

The universality class of the transition to turbulence

Turbulence is one of the most frequently encountered non-equilibrium phenomena in nature yet characterising the transition that gives rise to it has remained an elusive task. Although in recent studies critical points marking the onset of sustained turbulence have been determined, the physical nature of the transition could not be explained. In extensive experimental and computational studies we show for the example of Couette flow that the onset of turbulence is a second order phase transition and falls into the directed percolation universality class. Consequently the complex laminar-turbulent patterns distinctive for transition in shear flows result from nearest neighbour interactions of turbulent domains and are characterised by universal critical exponents. More generally our study demonstrates that even high dimensional systems far from equilibrium like turbulence exhibit universality at onset and that here the collective dynamics obeys simple rules.

physics.flu-dyn↗