On the interaction between fronts and vortices

International audienceWe investigate the formation and evolution of fronts and vortices in a two-layer stably-stratified fluid experiment. In a rotating tank with a differentially rotating rigid lid, we create a vertical shear across a density interface resulting in a baroclinic front. Different instabilities, such as baroclinic, Kelvin-Helmoltz or Rossby-Kelvin instabilities (RK), appear depending on the regime in Froude, Rossby and dissipation numbers. In the mean time, vortices naturally form that interact with the front. Using space-time analyses and Fourier filter analyses on particle Image Velocimetry (PIV) measurements and dye observations (LIF) for the RK and baroclinic unstable regimes, we investigate the interaction between small-waves with the baroclinic life cycle and the formation of these vortices. We characterise their size, the lifetime and the behaviour of vortices depending on the initial flow parameters

Numerical study of time-periodic mixing ventilation: effect of amplitude

© Healthy Buildings Europe 2017. All rights reserved. Conventional ventilation methods often supply fresh air to the room with a steady supply flow rate, which might induce stagnant recirculation cells and stagnation zones that are characterized by high concentrations of contaminants. This paper presents computational fluid dynamics (CFD) simulations of isothermal mixing ventilation with a transient supply flow rate in a generic room. The time-periodic supply flow rate is described by a sine function with different amplitudes. It is shown that time-periodic forcing triggers the stagnant recirculation cells to continuously shift throughout the room, thereby reducing average (passive gaseous) concentration levels in the occupied zone volume compared to the steady supply of fresh air. The concentrations are lower when larger amplitudes are applied. The analysis indicates a reduction in contaminant concentration by up to 24% and an increase of the contaminant removal effectiveness by 24% for time-periodic supply conditions compared to steady supply, which offers new perspectives on healthy and sustainable ventilation for, among others, residential buildings, office rooms, ship cabins, train cabins and cars.status: publishe

Instantaneous characteristics of interacting opposing plane jets in a generic enclosure measured with PIV

Knowledge of the instantaneous flow behaviour of interacting opposing jets, in addition to knowledge of the mean flow, is important for science and practice. Whereas studies often focused on axisymmetric jets, analyses for plane jets are scarce in general and for plane jets in an enclosed domain (i.e. not a (semi-)open environment) in particular, as e.g. encountered in airplane cabin ventilation. In this paper, 2D particle image velocimetry measurements are performed to study isothermal interacting opposing plane wall jets and plane free jets in a generic empty reduced-scale water-filled enclosure. Inlet Reynolds numbers vary from 3450 to 4650. The analyses encompass an inspection of the global flow patterns (in the vertical midplane) and of the flow components (e.g. interaction zone, merged jet, return flows, recirculation cells), using distribution plots, correlation functions and fast Fourier transforms. Vortical structures are also visualised and tracked over time. It is shown that the transient interaction of the opposing wall jets drives a merged jet that resembles a flapping turbulent plane jet. Remarkable are the occasional deviating (more unstable) flow patterns that appear. Furthermore, many vortical structures are present that could enhance mixing within the enclosure. The opposing free jets mainly show quasi-periodic oscillations with a given frequency (Strouhal number around 3.3 × 10–3), comparable to opposing plane free jets mentioned in the literature. Also in this configuration, many different vortices are present that can grow considerably large while transported through the flow domain. Both configurations show a potential for contaminant lock-up (stagnation zones)

Golf en wervel generatie aan barocline instabiele fronten

International audienc

Golf en wervel generatie aan barocline instabiele fronten

International audienc

CFD study of the flow field driven by interacting jets in a generic enclosure

Mixing ventilation flows consisting of interacting jets (as in airplane cabins) can be highly turbulent and unsteady. The aim of this study is to assess the performance of Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) simulations in predicting the isothermal flow resulting from interacting wall jets in a generic enclosure. Three k-ε turbulence models are employed: the standard k-ε and RNG k-ε models with near-wall modelling, and a low-Reynolds number version of the k-ε model. Comparison of the simulations with (time-averaged) PIV measurements reveals a good overall prediction of the velocity and turbulent kinetic energy by all models. Local deviations from the experiments can be attributed to the inability of the steady RANS method to capture the unsteady jet interaction

Airplane cabin mixing ventilation with time-periodic supply: Contaminant mass fluxes and ventilation efficiency

Airplane cabin ventilation is essential to ensure passengers' well-being. The conventional ventilation method is mixing ventilation with a statistically steady supply, which, according to former studies, has reached its limits regarding, for example, the ventilation efficiency. However, the effect of a statistically unsteady (time-periodic) supply on the mixing ventilation efficiency has remained largely unexplored. This research uses computational fluid dynamics (CFD) with the large eddy simulation (LES) approach to study isothermal time-periodic mixing ventilation in a section of a single-aisle airplane cabin model, in which the air exhaled by the passengers functions as (passive) contaminants. Two time-periodic supply strategies are evaluated. The induced time-periodic airflow patterns promote an efficient delivery of fresh air to the passenger zone and affect the passengers' expiratory plumes. This results in increased mean contaminant mass fluxes, causing a strong reduction of the mean contaminant concentrations in the passenger zone (up to 23%) and an increased contaminant extraction from the cabin. Mean velocities increase with up to 55% but remain within the comfortable range. It is shown that the ventilation efficiency improves; that is, the contaminant removal effectiveness and air change efficiency (in the full cabin volume) increase with up to 20% and 7%, respectively

PIV measurements of opposing-jet ventilation flow in a reduced-scale simplified empty airplane cabin

Mixing ventilation in airplane cabins is driven by overhead supply jets, which may be directed towards each other, interact and merge before entering the passenger area. Although many studies already investigated airplane cabin mixing ventilation, detailed knowledge on the fundamental flow components (i.e. supply jets, interaction zone, merged jet and secondary flows) is rather scarce, but can be useful to obtain new insights into mixing ventilation design optimisation. This paper presents time-averaged 2D particle image velocimetry (PIV) measurements of the flow driven by isothermal opposing plane jets in a reduced-scale water-filled generic airplane cabin. Two configurations of opposing jets, i.e. ceiling jets and lateral jets, are examined with inlet Reynolds numbers ranging from 3,450 to 4,650. The analysis focuses on the mean velocity and Reynolds stresses of the whole flow field and the aforementioned fundamental flow components. Decay rates of the supply jets and decay and growth rates of the merged jet are analysed and the size of the interaction zone is assessed. The supply jet velocity decays very fast due to the interaction with the opposing jet. Within the interaction zone, the turbulence levels are high and turbulence is highly anisotropic. A comparison between the merged jet and a self-similar turbulent plane free jet is presented as well. The measurement data is also valuable for (sub-configuration) computational fluid dynamics (CFD) validation purposes