scholarly journals Thermal Chains and Entrainment in Cumulus Updrafts. Part I: Theoretical Description

2020 ◽  
Vol 77 (11) ◽  
pp. 3637-3660 ◽  
Author(s):  
Hugh Morrison ◽  
John M. Peters ◽  
Adam C. Varble ◽  
Walter M. Hannah ◽  
Scott E. Giangrande
Keyword(s):  
Vertical Velocity ◽  
Convective Available Potential Energy ◽  
Air Entrainment ◽  
Chain Structure ◽  
Theoretical Description ◽  
Entrainment Rate ◽  
Eddy Simulation ◽  
Environmental Relative Humidity ◽  
Lateral Mixing ◽  
Environmental Air

AbstractRecent studies have shown that cumulus updrafts often consist of a succession of discrete rising thermals with spherical vortex-like circulations. In this paper, a theory is developed for why this “thermal chain” structure occurs. Theoretical expressions are obtained for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. Analysis of these expressions suggests that the thermal chain structure arises from enhanced lateral mixing associated with intrusions of dry environmental air below an updraft’s vertical velocity maximum. This dry-air entrainment reduces buoyancy locally. Consequently, the updraft flow above levels of locally reduced buoyancy separates from below, leading to a breakdown of the updraft into successive discrete thermals. The range of conditions in which thermal chains exist is also analyzed from the theoretical expressions. A transition in updraft structure from isolated rising thermal, to thermal chain, to starting plume occurs with increases in updraft width, environmental relative humidity, and/or convective available potential energy. Corresponding expressions for the bulk fractional entrainment rate ε are also obtained. These expressions indicate rather complicated entrainment behavior of ascending updrafts, with local enhancement of ε up to a factor of ~2 associated with the aforementioned environmental-air intrusions, consistent with recent large-eddy simulation (LES) studies. These locally large entrainment rates contribute significantly to overall updraft dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Part II of this study compares results from the theoretical expressions to idealized numerical simulations and LES.

scholarly journals Measurement of Convective Entrainment Using Lagrangian Particles

2013 ◽  
Vol 70 (1) ◽  
pp. 266-277 ◽  
Author(s):  
Kyongmin Yeo ◽  
David M. Romps
Keyword(s):  
Direct Measurement ◽  
Cloud Base ◽  
Entrainment Rate ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Environmental Air ◽  
Cumulus Congestus ◽  
Entrained Air ◽  
Eulerian Methods ◽  
Good Agreement

Abstract Lagrangian particle tracking is used in a large-eddy simulation to study an individual cumulus congestus. This allows for the direct measurement of the convective entrainment rate and of the residence times of entrained parcels within the cloud. The entrainment rate obtained by Lagrangian direct measurement is found to be higher than that obtained using the recently introduced method of Eulerian direct measurement. This discrepancy is explained by the fast recirculation of air in and out of cloudy updrafts, which Eulerian direct measurement is unable to resolve. By filtering these fast recirculations, the Lagrangian calculation produces a result in very good agreement with the Eulerian calculation. The Lagrangian method can also quantify some aspects of entrainment that cannot be probed with Eulerian methods. For instance, it is found that more than half of the air that is entrained by the cloud during its lifetime is air that was previously detrained by the cloud. Nevertheless, the cloud is highly diluted by entrained air: for cloudy air above 2 km, its mean height of origin is well above the cloud base. This paints a picture of a cloud that rapidly entrains both environmental air and its own detritus.

scholarly journals Thermal Chains and Entrainment in Cumulus Updrafts. Part II: Analysis of Idealized Simulations

2020 ◽  
Vol 77 (11) ◽  
pp. 3661-3681 ◽  
Author(s):  
John M. Peters ◽  
Hugh Morrison ◽  
Adam C. Varble ◽  
Walter M. Hannah ◽  
Scott E. Giangrande
Keyword(s):  
Vertical Velocity ◽  
Conceptual Models ◽  
Deep Convection ◽  
Three Dimensional ◽  
Entrainment Rate ◽  
Vertical Profiles ◽  
Passive Tracer ◽  
Tracer Concentration ◽  
Environmental Relative Humidity

AbstractResearch has suggested that the structure of deep convection often consists of a series of rising thermals, or “thermal chain,” which contrasts with existing conceptual models that are used to construct cumulus parameterizations. Simplified theoretical expressions for updraft properties obtained in Part I of this study are used to develop a hypothesis explaining why this structure occurs. In this hypothesis, cumulus updraft structure is strongly influenced by organized entrainment below the updraft’s vertical velocity maximum. In a dry environment, this enhanced entrainment can locally reduce condensation rates and increase evaporation, thus eroding buoyancy. For moderate-to-large initial cloud radius R, this breaks up the updraft into a succession of discrete pulses of rising motion (i.e., a thermal chain). For small R, this leads to the structure of a single, isolated rising thermal. In contrast, moist environments are hypothesized to favor plume-like updrafts for moderate-to-large R. In a series of axisymmetric numerical cloud simulations, R and environmental relative humidity (RH) are systematically varied to test this hypothesis. Vertical profiles of fractional entrainment rate, passive tracer concentration, buoyancy, and vertical velocity from these runs agree well with vertical profiles calculated from the theoretical expressions in Part I. Analysis of the simulations supports the hypothesized dependency of updraft structure on R and RH, that is, whether it consists of an isolated thermal, a thermal chain, or a plume, and the role of organized entrainment in driving this dependency. Additional three-dimensional (3D) turbulent cloud simulations are analyzed, and the behavior of these 3D runs is qualitatively consistent with the theoretical expressions and axisymmetric simulations.

scholarly journals An Analytic Description of the Structure and Evolution of Growing Deep Cumulus Updrafts

2017 ◽  
Vol 74 (3) ◽  
pp. 809-834 ◽  
Author(s):  
Hugh Morrison
Keyword(s):  
Conceptual Models ◽  
Convective Available Potential Energy ◽  
Three Dimensional ◽  
Passive Scalar ◽  
Analytic Description ◽  
Moist Convection ◽  
Analytic Expressions ◽  
Environmental Relative Humidity ◽  
Environmental Air ◽  
Negatively Buoyant

Abstract New theoretical analytic expressions are derived for the evolution of a passive scalar, buoyancy, and vertical velocity in growing, entraining moist deep convective updrafts. These expressions are a function of updraft radius, height, convective available potential energy (CAPE), and environmental relative humidity RH. They are quantitatively consistent with idealized three-dimensional moist updraft simulations with varying updraft sizes and in environments with differing RH. In particular, the analytic expressions capture the rapid decrease of buoyancy with height due to entrainment for narrow updrafts in a dry environment despite large CAPE. In contrast to the standard entraining-plume model, the theoretical expressions also describe the effects of engulfment of environmental air between the level of free convection (LFC) and height of maximum buoyancy (HMB) required by mass continuity to balance upward acceleration of updraft air (i.e., dynamic entrainment). This organized inflow sharpens horizontal gradients, thereby enhancing smaller-scale lateral turbulent mixing below the HMB. For narrow updrafts in a dry environment, this enhanced mixing leads to a negatively buoyant region between the LFC and HMB, effectively cutting off the region of positive buoyancy at the HMB from below so that the updraft structure resembles a rising thermal rather than a plume. Thus, it is proposed that a transition from plume-like to thermal-like structure is driven by dynamic entrainment and depends on updraft width (relative to height) and environmental RH. These results help to bridge the entraining-plume and rising-thermal conceptual models of moist convection.

A Simple Parameterization for Detrainment in Shallow Cumulus

2008 ◽  
Vol 136 (2) ◽  
pp. 560-576 ◽  
Author(s):  
Wim C. de Rooy ◽  
A. Pier Siebesma
Keyword(s):  
Mass Flux ◽  
Eddy Simulation ◽  
Shallow Cumulus ◽  
Wide Range ◽  
Flux Profile ◽  
Current Mass ◽  
Lateral Mixing ◽  
Environmental Air ◽  
Bulk Parameter ◽  
Les Model

Abstract For a wide range of shallow cumulus convection cases, large-eddy simulation (LES) model results have been used to investigate lateral mixing as expressed by the fractional entrainment and fractional detrainment rates. It appears that the fractional entrainment rates show much less variation from hour to hour and case to case than the fractional detrainment rates. Therefore, in the parameterization proposed here, the fractional entrainment rates are assumed to be described as a fixed function of height, roughly following the LES results. Based on the LES results a new, more flexible parameterization for the detrainment process is developed that contains two important dependencies. First, based on cloud ensemble principles it can be understood that deeper cloud layers call for smaller detrainment rates. All current mass flux schemes ignore this cloud-height dependence, which evidently leads to large discrepancies with observed mass flux profiles. The new detrainment formulation deals with this dependence by considering the mass flux profile in a nondimensionalized way. Second, both relative humidity of the environmental air and the buoyancy excess of the updraft influence the detrainment rates and, therefore, the mass flux profiles. This influence can be taken into account by borrowing a parameter from the buoyancy-sorting concept and using it in a bulk sense. LES results show that with this bulk parameter, the effect of environmental conditions on the fractional detrainment rate can be accurately described. A simple, practical but flexible parameterization for the fractional detrainment rate is derived and evaluated in a single-column model (SCM) for three different shallow cumulus cases, which shows the clear potential of this parameterization. The proposed parameterization is an attractive and more robust alternative for existing, more complex, buoyancy-sorting-based mixing schemes, and can be easily incorporated in current mass flux schemes.

scholarly journals Improving Parameterization of Entrainment Rate for Shallow Convection with Aircraft Measurements and Large-Eddy Simulation

2016 ◽  
Vol 73 (2) ◽  
pp. 761-773 ◽  
Author(s):  
Chunsong Lu ◽  
Yangang Liu ◽  
Guang J. Zhang ◽  
Xianghua Wu ◽  
Satoshi Endo ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Vertical Velocity ◽  
Dissipation Rate ◽  
Principal Component ◽  
Entrainment Rate ◽  
Shallow Convection ◽  
Cumulus Clouds ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Multiple Variables

AbstractThis work examines the relationships of entrainment rate to vertical velocity, buoyancy, and turbulent dissipation rate by applying stepwise principal component regression to observational data from shallow cumulus clouds collected during the Routine Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign over the ARM Southern Great Plains (SGP) site near Lamont, Oklahoma. The cumulus clouds during the RACORO campaign simulated using a large-eddy simulation (LES) model are also examined with the same approach. The analysis shows that a combination of multiple variables can better represent entrainment rate in both the observations and LES than any single-variable fitting. Three commonly used parameterizations are also tested on the individual cloud scale. A new parameterization is thus presented that relates entrainment rate to vertical velocity, buoyancy, and dissipation rate; the effects of treating clouds as ensembles and humid shells surrounding cumulus clouds on the new parameterization are discussed. Physical mechanisms underlying the relationships of entrainment rate to vertical velocity, buoyancy, and dissipation rate are also explored.

scholarly journals Large-Eddy Simulation Study of Ultra-High Fuel Injection Pressure on Gasoline Sprays

2020 ◽  
Author(s):  
S. Wadekar ◽  
A. Yamaguchi ◽  
M. Oevermann
Keyword(s):  
Large Eddy Simulation ◽  
Droplet Size ◽  
Injection Pressure ◽  
Air Entrainment ◽  
Entrainment Rate ◽  
Eddy Simulation ◽  
High Injection ◽  
Large Eddy ◽  
The Impact ◽  
Injection Pressures

Abstract The development of gasoline spray at ultra-high injection pressures was analyzed using Large-Eddy simulation (LES). Two different nozzle hole geometries, divergent and convergent shape, were considered to inject the fuel at injection pressures ranging from 200 to 1500 bar inside a constant volume spray chamber maintained at atmospheric conditions. The discrete droplet phase was treated using a Lagrangian formulation together with the standard spray sub-models. The numerical results were calibrated by reproducing experimentally observed liquid penetration length and efforts were made to understand the influence of ultra-high injection pressures on the spray development. The calibrated model was then used to investigate the impact of ultra-high injection pressures on mean droplet size and droplet size distribution. In addition, the spray-induced large-scale eddies and entrainment rate were evaluated at different ultra-high injection pressures. Overall, simulation results showed a good agreement with available measurement data. At ultra-high injection pressures mean droplet sizes were significantly reduced and comprised very high velocities. Integral length scales of spray-induced turbulence and air entrainment rate into the spray were larger at higher injection pressure compared to lower ones. Graphic abstract

The Effect of Nozzle Type on Air Entrainment by Plunging Water Jets

2002 ◽  
Vol 37 (3) ◽  
pp. 599-612 ◽  
Author(s):  
Tamer Bagatur ◽  
Ahmet Baylar ◽  
Nusret Sekerdag
Keyword(s):  
Penetration Depth ◽  
Oxygen Transfer ◽  
Free Water ◽  
Air Entrainment ◽  
Water Jet ◽  
Transfer Efficiency ◽  
Entrainment Rate ◽  
Water Jets ◽  
Air Entrainment Rate ◽  
Oxygen Transfer Efficiency

Abstract In this study, for the plunging water jet aeration system using various inclined nozzle types, bubble penetration depth, air entrainment rate, water jet expansion, effect of water jet circumference at impact point, oxygen transfer coefficient and oxygen transfer efficiency which changed depending on the water jet velocity, were researched in an air-water system. Numerous studies were conducted with circular nozzles. The present study describes new experiments performed with different nozzle types. Three types of nozzles were examined, i.e., those with circular, ellipse and rectangle duct with rounded ends. Experimental results showed that water jets produced with ellipse and rectangle duct with rounded ends nozzles have very different flow characteristics, entrainment patterns on free water jet surface, and submerged water jet region within the receiving tank. Higher air entrainment rate and oxygen transfer efficiency was observed in the rectangle duct with rounded ends nozzle due to water jet expansion. Bubble penetration depth, however, is lower for the rectangle duct with rounded ends nozzle than for the other nozzles. The ellipse nozzle provided the highest bubble penetration depth. These results showed that it is appropriate to use ellipse nozzle in aeration of deep pool and rectangle duct with rounded ends nozzle in the applications where high bubble concentration is desirable.

scholarly journals Wake behind a three-dimensional dry transom stern. Part 1. Flow structure and large-scale air entrainment

2019 ◽  
Vol 875 ◽  
pp. 854-883 ◽  
Author(s):  
Kelli Hendrickson ◽  
Gabriel D. Weymouth ◽  
Xiangming Yu ◽  
Dick K.-P. Yue
Keyword(s):  
Froude Number ◽  
Flow Structure ◽  
Large Scale ◽  
Three Dimensional ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Variable Density ◽  
Entrainment Rate ◽  
Two Phase ◽  
Density Spectrum

We present high-resolution implicit large eddy simulation (iLES) of the turbulent air-entraining flow in the wake of three-dimensional rectangular dry transom sterns with varying speeds and half-beam-to-draft ratios $B/D$. We employ two-phase (air/water), time-dependent simulations utilizing conservative volume-of-fluid (cVOF) and boundary data immersion (BDIM) methods to obtain the flow structure and large-scale air entrainment in the wake. We confirm that the convergent-corner-wave region that forms immediately aft of the stern wake is ballistic, thus predictable only by the speed and (rectangular) geometry of the ship. We show that the flow structure in the air–water mixed region contains a shear layer with a streamwise jet and secondary vortex structures due to the presence of the quasi-steady, three-dimensional breaking waves. We apply a Lagrangian cavity identification technique to quantify the air entrainment in the wake and show that the strongest entrainment is where wave breaking occurs. We identify an inverse dependence of the maximum average void fraction and total volume entrained with $B/D$. We determine that the average surface entrainment rate initially peaks at a location that scales with draft Froude number and that the normalized average air cavity density spectrum has a consistent value providing there is active air entrainment. A small parametric study of the rectangular geometry and stern speed establishes and confirms the scaling of the interface characteristics with draft Froude number and geometry. In Part 2 (Hendrikson & Yue, J. Fluid Mech., vol. 875, 2019, pp. 884–913) we examine the incompressible highly variable density turbulence characteristics and turbulence closure modelling.

scholarly journals Representation of Boundary Layer Moisture Transport in Cloud-Resolving Models

2012 ◽  
Vol 140 (11) ◽  
pp. 3682-3698 ◽  
Author(s):  
Chin-Hoh Moeng ◽  
Akio Arakawa
Keyword(s):  
Vertical Velocity ◽  
Moisture Transport ◽  
Deep Convection ◽  
Cloud Layer ◽  
Cloud System ◽  
Small Scale ◽  
Closure Model ◽  
Eddy Simulation ◽  
Gradient Model ◽  
Local Gradient

Abstract One of the important roles of the PBL is to transport moisture from the surface to the cloud layer. However, how this transport process can be accounted for in cloud-resolving models (CRMs) is not sufficiently clear and has rarely been examined. A typical CRM can resolve the bulk feature of large convection systems but not the small-scale convection and turbulence motions that carry a large portion of the moisture fluxes. This study uses a large-eddy simulation of a tropical deep-convection system as a benchmark to examine the subgrid-scale (SGS) moisture transport into a cloud system. It is shown that most of the PBL moisture transport to the cloud layer occurs in the areas under low-level updrafts, with rain, or under cloudy skies, although these PBL regimes may cover only a small fraction of the entire cloud-system domain. To develop SGS parameterizations to represent the spatial distribution of this moisture transport in CRMs, three models are proposed and tested. An updraft–downdraft model performs exceptionally well, while a statistical-closure model and a local-gradient model are less satisfactory but still perform adequately. Each of these models, however, has its own closure issues to be addressed. The updraft–downdraft model requires a scheme to estimate the mean SGS updraft–downdraft properties, the statistical-closure model needs a scheme to predict both SGS vertical-velocity and moisture variances, while the local-gradient model requires estimation of the SGS vertical-velocity variance.

Sign in / Sign up

Export Citation Format

Share Document