43 research outputs found
Repeller or Attractor? Selecting the Dynamical Model for the Onset of Turbulence in Pipe Flow
No full textThe collapse of turbulence, observable in shear flows at low Reynolds numbers, raises the question if turbulence is generically of a transient nature or becomes sustained at some critical point. Recent data have led to conflicting views with the majority of studies supporting the model of turbulence turning into an attracting state. Here we present lifetime measurements of turbulence in pipe flow spanning 8 orders of magnitude in time, drastically extending all previous investigations. We show that no critical point exists in this regime and that in contrast to the prevailing view the turbulent state remains transient. To our knowledge this is the first observation of superexponential transients in turbulence, confirming a conjecture derived from low-dimensional systems.Process and EnergyMechanical, Maritime and Materials Engineerin
Análisis comparativo del franqueo de obstáculos entre atletas masculinos de origen africano, americano y europeo de alto rendimiento
No full textDebido probablemente a la idiosincrasia de su desarrollo atlético, muy parejo al de las sociedades en las que viven, los atletas africanos de 3000m obstáculos tienden a tener comportamientos muy primitivos desde un punto de vista atlético a la hora de correr. Esto conlleva a que descuiden en muchos casos los aspectos técnicos de la prueba. Sin embargo, en el resto del mundo, donde quizás correr no es un hecho tan natural, se tienden a explotar mucho mejor los citados aspectos técnicos con el fin de optimizar el rendimiento.
En el presente trabajo se investiga la eficiencia del franqueo de obstáculos en la prueba de 3000 metros obstáculos en categoría masculina, haciendo una comparativa entre atletas africanos, americanos y europeos. Todo ello con el fin de comprobar las aparentes deficiencias de muchos de los atletas africanos a la hora de franquear el obstáculo. La investigación se realizó mediante el análisis del tiempo de vuelo en 13 competiciones, donde se analizaron un total de 80 atletas y 775 franqueos de obstáculos
Reduction of the Entrainment Velocity by Cloud Droplet Sedimentation in Stratocumulus
No full textThe effect of sedimentation on stratocumulus entrainment is investigated using direct numerical simulations of a cloud-top mixing layer driven by radiative and evaporative cooling. The simulations focus on the meter and submeter scales that are expected to be relevant for entrainment, and the finest grid spacing is Δx = 26 cm. The entrainment velocity is investigated from the analysis of the integrated-buoyancy evolution equation, which is exactly derived from the flow evolution equations. The analysis shows that sedimentation interacts with entrainment through two different mechanisms. As previously reported, sedimentation prevents droplets from evaporating in the entrainment zone, which in turn reduces the entrainment velocity. Here it is shown that sedimentation also promotes a positive buoyancy flux that directly opposes entrainment. The strengths of both mechanisms are characterized by two different settling numbers, which allow for predicting which meteorological conditions favor the reduction of entrainment by sedimentation. These new insights allow for including sedimentation in a parameterization of the entrainment velocity. The reduction of the entrainment velocity by sedimentation predicted by the parameterization and observed in the simulations is 3 times larger than previously reported in large-eddy simulations, which implies that meter- and submeter-scale turbulence plays an important role in the interaction of entrainment with sedimentation. On the whole, analysis and simulations indicate that stratocumulus entrainment is more sensitive to the cloud droplet number density due to sedimentation than previously thought
Direct Numerical Simulations of a Smoke Cloud–Top Mixing Layer as a Model for Stratocumuli
No full textA radiatively driven cloud-top mixing layer is investigated using direct numerical simulations. This configuration mimics the mixing process across the inversion that bounds the stratocumulus-topped boundary layer. The main focus of this paper is on small-scale turbulence. The finest resolution (7.4 cm) is about two orders of magnitude finer than that in cloud large-eddy simulations (LES). A one-dimensional horizontally averaged model is employed for the radiation. The results show that the definition of the inversion point with the mean buoyancy of hbi(zi) 5 0 leads to convective turbulent scalings in the cloud bulk consistent with the Deardorff theory. Three mechanisms contribute to the entrainment by cooling the inversion layer: a molecular flux, a turbulent flux, and the direct radiative cooling by the smoke inside the inversion layer. In the simulations the molecular flux is negligible, but the direct cooling reaches values comparable to the turbulent flux as the inversion layer thickens. The results suggest that the direct cooling might be overestimated in lessresolved models like LES, resulting in an excessive entrainment. The scaled turbulent flux is independent of the stratification for the range of Richardson numbers studied here. As suggested by earlier studies, the turbulent entrainment only occurs at the small scales and eddies larger than approximately four optical lengths (60m in a typical stratocumulus cloud) perform little or no entrainment. Based on those results, a parameterization is proposed that accounts for a large part (50%-100%) of the entrainment velocities measured in the Second Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS II) campaign. © 2013 American Meteorological Society
Cloud droplets in a bulk formulation and its application to buoyancy reversal instability
No full textMixing processes at the boundary of clouds often include typical length-scales of several metres. Such length-scales are too large for current Lagrangian models but they are also poorly resolved by typical Eulerian-based large-eddy simulations. Here, a bulk formulation is introduced for direct numerical simulations. Two main assumptions sustain this approach: the continuum approximation and the liquid-phase diffusion approximation. The formulation includes the small-scale features that originate from microscopic droplet dynamics: sedimentation, finite-time condensation/evaporation, inertial effects and the low diffusion of liquid droplets with respect to vapour. The methodology is applied to the study of the buoyancy reversal instability that occurs at the top of stratocumulus clouds as a consequence of evaporative cooling. The inclusion of sedimentation, low liquid-phase diffusion and finite-time evaporation have a negative impact on instability when compared with the equilibrium formulation. The combined effect of all these small-scale features reduces mixing at the cloud top by at least 90%. This strong reduction is explained by a condensate-free middle layer, which emerges when droplets leave the cloud interface due to sedimentation
Mixing Driven by Radiative and Evaporative Cooling at the Stratocumulus Top
No full textThe stratocumulus-top mixing process is investigated using direct numerical simulations of a shear-free cloud-top mixing layer driven by evaporative and radiative cooling. An extension of previous linear formulations allows for quantifying radiative cooling, evaporative cooling, and the diffusive effects that artificially enhance mixing and evaporative cooling in high-viscosity direct numerical simulations (DNS) and many atmospheric simulations. The diffusive cooling accounts for 20% of the total evaporative cooling for the highest resolution (grid spacing ~14 cm), but this can be much larger (~100%) for lower resolutions that are commonly used in large-eddy simulations (grid spacing ~5 m). This result implies that the κ scaling for cloud cover might be strongly influenced by diffusive effects. Furthermore, the definition of the inversion point as the point of neutral buoyancy allows the derivation of two scaling laws. The in-cloud scaling law relates the velocity and buoyancy integral scales to a buoyancy flux defined by the inversion point. The entrainment-zone scaling law provides a relationship between the entrainment velocity and the liquid evaporation rate. By using this inversion point, it is shown that the radiative-cooling contribution to the entrainment velocity decouples from the evaporative-cooling contribution and behaves very similarly as in the smoke cloud. Finally, evaporative and radiative cooling have similar strengths, when this strength is measured by the integrated buoyancy source. This result partially explains why current entrainment parameterizations are not accurate enough, given that most of them implicitly assume that only one of the two mechanisms rules the entrainment
Evaporative cooling amplification of the entrainment velocity in radiatively driven stratocumulus
No full textEvaporative cooling monotonically increases as the thermodynamical properties of the inversion allow for more evaporation in shear-free radiatively driven stratocumulus. However, the entrainment velocity can deviate from the evaporative cooling trend and even become insensitive to variations in the inversion properties. Here the efficiency of evaporative cooling at amplifying the entrainment velocity is quantified by means of direct numerical simulations of a cloud top mixing layer. We demonstrate that variations in the efficiency modulate the effect of evaporative cooling on entrainment, explaining the different trends. These variations are associated with the evaporation of droplets in cloud holes below the inversion point. The parametrization of the efficiency provides the evaporative amplification of the entrainment velocity as a function of a single parameter that characterizes the inversion. The resulting entrainment velocities match our experiments and previous measurements to within ±25%. The parametrization also predicts the transition to a broken-cloud field consistently with observations. ©2015. American Geophysical Union
Long-resident droplets at the stratocumulus top
No full textTurbulence models predict low droplet-collision rates in stratocumulus
clouds, which should imply a narrow droplet size distribution and little
rain. Contrary to this expectation, rain is often observed in stratocumuli.
In this paper, we explore the hypothesis that some droplets can grow well
above the average because small-scale turbulence allows them to reside at
cloud top for a time longer than the convective-eddy time t*.
Long-resident droplets can grow larger because condensation due to longwave
radiative cooling, and collisions have more time to enhance droplet growth.
We investigate the trajectories of 1 billion Lagrangian droplets in direct
numerical simulations of a cloudy mixed-layer configuration that is based on
observations from the flight 11 from the VERDI campaign. High resolution is
employed to represent a well-developed turbulent state at cloud top. Only
one-way coupling is considered. We observe that 70 % of the droplets spend
less than 0.6t* at cloud top before leaving the cloud, while 15 % of
the droplets remain at least 0.9t* at cloud top. In addition, 0.2 % of
the droplets spend more than 2.5t* at cloud top and decouple from the
large-scale convective eddies that brought them to the top, with the result
that they become memoryless. Modeling collisions like a Poisson process leads
to the conclusion that most rain droplets originate from those memoryless
droplets. Furthermore, most long-resident droplets accumulate at the
downdraft regions of the flow, which could be related to the closed-cell
stratocumulus pattern. Finally, we see that condensation due to longwave
radiative cooling considerably broadens the cloud-top droplet size
distribution: 6.5 % of the droplets double their mass due to radiation in
their time at cloud top. This simulated droplet size distribution matches the
flight measurements, confirming that condensation due to longwave radiation
can be an important mechanism for broadening the droplet size distribution in
radiatively driven stratocumuli
Edge state in pipe flow experiments
No full textRecent numerical studies suggest that in pipe and related shear flows, the region of phase space separating laminar from turbulent motion is organized by a chaotic attractor, called an edge state, which mediates the transition process. We here confirm the existence of the edge state in laboratory experiments. We observe that it governs the dynamics during the decay of turbulence underlining its potential relevance for turbulence control. In addition we unveil two unstable traveling wave solutions underlying the experimental flow fields. This observation corroborates earlier suggestions that unstable solutions organize turbulence and its stability border.Postprint (published version
