scholarly journals A Lagrangian study of interfaces at the edges of cumulus clouds

2021 ◽  
Author(s):  
Vishnu Nair ◽  
Thijs Heus ◽  
Maarten van Reeuwijk
Keyword(s):  
Specific Humidity ◽  
Cumulus Clouds ◽  
Mean Values ◽  
Eddy Simulation ◽  
Warm Cloud ◽  
Large Eddy ◽  
The Mean ◽  
Lagrangian Study ◽  
Liquid Condensate ◽  
Cloud Core

AbstractInterfaces at the edge of an idealised, non-precipitating, warm cloud are studied using Direct Numerical Simulation (DNS) complemented with a Lagrangian particle tracking routine. Once a shell has formed, four zones can be distinguished: the cloud core, visible shell, invisible shell and the environment. The union of the visible and invisible regions is the shell commonly referred to in literature. The boundary between the invisible shell and the environment is the Turbulent-NonTurbulent Interface (TNTI) which is typically not considered in cloud studies. Three million particles were seeded homogeneously across the domain and properties were recorded along individual trajectories. The results demonstrate that the traditional cloud boundary (separating cloudy and non-cloudy regions using thresholds applied on liquid condensate or updraft velocity) are some distance away from the TNTI. Furthermore, there is no dynamic difference between the traditional liquid-condensate boundary and the region extending to the TNTI. However, particles crossing the TNTI exhibit a sharp jump in enstrophy and a smooth increase in buoyancy. The traditional cloud boundary coincides with the location of minimum buoyancy in the shell. The shell pre-mixes the entraining and detraining air and analysis reveals a highly skewed picture of entrainment and detrainment at the traditional cloud boundary. A preferential entrainment of particles with velocity and specific humidity higher than the mean values in the shell is observed. Large-eddy simulation of a more realistic setup detects an interface with similar properties using the same thresholds as in the DNS, indicating that the DNS results extrapolate beyond their idealised conditions.

scholarly journals Nature versus Nurture in Shallow Convection

2010 ◽  
Vol 67 (5) ◽  
pp. 1655-1666 ◽  
Author(s):  
David M. Romps ◽  
Zhiming Kuang
Keyword(s):  
Standard Deviation ◽  
The Other ◽  
Cloud Base ◽  
Shallow Convection ◽  
Eddy Simulation ◽  
Cloud Properties ◽  
Nature Versus Nurture ◽  
Large Eddy ◽  
The Mean ◽  
Single Set

Abstract Tracers are used in a large-eddy simulation of shallow convection to show that stochastic entrainment (and not cloud-base properties) determines the fate of convecting parcels. The tracers are used to diagnose the correlations between a parcel’s state above the cloud base and both the parcel’s state at the cloud base and its entrainment history. The correlation with the cloud-base state goes to zero a few hundred meters above the cloud base. On the other hand, correlations between a parcel’s state and its net entrainment are large. Evidence is found that the entrainment events may be described as a stochastic Poisson process. A parcel model is constructed with stochastic entrainment that is able to replicate the mean and standard deviation of cloud properties. Turning off cloud-base variability has little effect on the results, which suggests that stochastic mass-flux models may be initialized with a single set of properties. The success of the stochastic parcel model suggests that it holds promise as the framework for a convective parameterization.

scholarly journals Supersaturation calculation in large eddy simulation models for prediction of the droplet number concentration

2012 ◽  
Vol 5 (3) ◽  
pp. 761-772 ◽  
Author(s):  
O. Thouron ◽  
J.-L. Brenguier ◽  
F. Burnet
Keyword(s):  
Cloud Condensation Nuclei ◽  
Simulation Models ◽  
Number Concentration ◽  
Eddy Simulation ◽  
Droplet Concentration ◽  
Large Eddy ◽  
Condensed Water ◽  
Les Model ◽  
Adjustment Scheme ◽  
Cloud Core

Abstract. A new parameterization scheme is described for calculation of supersaturation in LES models that specifically aims at the simulation of cloud condensation nuclei (CCN) activation and prediction of the droplet number concentration. The scheme is tested against current parameterizations in the framework of the Meso-NH LES model. It is shown that the saturation adjustment scheme, based on parameterizations of CCN activation in a convective updraft, overestimates the droplet concentration in the cloud core, while it cannot simulate cloud top supersaturation production due to mixing between cloudy and clear air. A supersaturation diagnostic scheme mitigates these artefacts by accounting for the presence of already condensed water in the cloud core, but it is too sensitive to supersaturation fluctuations at cloud top and produces spurious CCN activation during cloud top mixing. The proposed pseudo-prognostic scheme shows performance similar to the diagnostic one in the cloud core but significantly mitigates CCN activation at cloud top.

scholarly journals Prediction of flow and dispersion in cross–ventilated buildings

2019 ◽  
Vol 128 ◽  
pp. 05002
Author(s):  
Ali Cemal Benim ◽  
Michael Diederich ◽  
Ali Nahavandi
Keyword(s):  
Computational Analysis ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Turbulence Structure ◽  
Detached Eddy Simulation ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Grid Independence

The present paper presents a detailed computational analysis of flow and dispersion in a generic isolated single–zone buildings. First, a grid generation strategy is discussed, that is inspired by a previous computational analysis and a grid independence study. Different turbulence models are appliedincluding two-equation turbulence models, the differential Reynolds Stress Model, Detached Eddy Simulation and Zonal Large Eddy Simulation. The mean velocity and concentration fields are calculated and compared with the measurements. A satisfactory agreement with the experiments is not observed by any of the modelling approaches, indicating the highly demanding flow and turbulence structure of the problem.

Study of wave effect on vorticity in Langmuir turbulence using wave-phase-resolved large-eddy simulation

2019 ◽  
Vol 875 ◽  
pp. 173-224 ◽  
Author(s):  
Anqing Xuan ◽  
Bing-Qing Deng ◽  
Lian Shen
Keyword(s):  
Large Eddy Simulation ◽  
Correlation Effect ◽  
Wave Turbulence ◽  
Langmuir Turbulence ◽  
Streamwise Vorticity ◽  
Wave Phase ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Vorticity Dynamics ◽  
The Mean

The effects of a water surface wave on the vorticity in the turbulence underneath are studied for Langmuir turbulence using wave-phase-resolved large-eddy simulation. The simulations are performed on a dynamically evolving wave-surface-fitted grid such that the phase-resolved wave motions and their effects on the turbulence are explicitly captured. This study focuses on the vorticity structures and dynamics in Langmuir turbulence driven by a steady and co-aligned progressive wave and surface shear stress. For the first time, the detailed vorticity dynamics of the wave–turbulence interaction in Langmuir turbulence in a wave-phase-resolved frame is revealed. The wave-phase-resolved simulation provides detailed descriptions of many characteristic features of Langmuir turbulence, such as elongated quasi-streamwise vortices. The simulation also reveals the variation of the strength and the inclination angles of the vortices with the wave phase. The variation is found to be caused by the periodic stretching and tilting of the wave orbital straining motions. The cumulative effect of the wave on the wave-phase-averaged vorticity is analysed using the Lagrangian average. It is discovered that, in addition to the tilting effect induced by the Lagrangian mean shear gradient of the wave, the phase correlation between the vorticity fluctuations and the wave orbital straining is also important to the cumulative vorticity evolution. Both the fluctuation correlation effect and the mean tilting effect are found to amplify the streamwise vorticity. On the other hand, for the vertical vorticity, the fluctuation correlation effect cancels the mean tilting effect, and the net change of the vertical vorticity by the wave straining is negligible. As a result, the wave straining enhances only the streamwise vorticity and cumulatively tilts vertical vortices towards the streamwise direction. The above processes are further quantified analytically. The role of the fluctuation correlation effect in the wave-phase-averaged vorticity dynamics provides a deeper understanding of the physical processes underlying the wave–turbulence interaction in Langmuir turbulence.

scholarly journals On the Effects of Lateral Openings on Courtyard Ventilation and Pollution—A Large-Eddy Simulation Study

Atmosphere ◽  
2019 ◽  
Vol 10 (2) ◽  
pp. 63 ◽  
Author(s):  
Tobias Gronemeier ◽  
Matthias Sühring
Keyword(s):  
Air Quality ◽  
Negative Impact ◽  
Flow Direction ◽  
Urban Environments ◽  
High Concentration ◽  
Eddy Simulation ◽  
Recreation Areas ◽  
Large Eddy ◽  
The Mean ◽  
Upstream Flow

Courtyards are an omnipresent feature within the urban environment. Residents often use courtyards as recreation areas, which makes them crucial for the physical and psychological comfort of the urban population. However, considering that courtyards represent enclosed cavities, they are often poorly ventilated spaces and pollutants from neighboring traffic, once entrained, can pose a serious threat to human health. Here, we studied the effects of lateral openings on courtyard pollution and ventilation. Therefore, we performed a set of large-eddy simulations for idealized urban environments with different courtyard configurations. While pollutant concentration and ventilation are barely modified by lateral openings for wide courtyards, lateral openings have a significant effect on the mean concentration, the number of high-concentration events and the ventilation within narrower and deeper courtyards. The impacts of lateral openings on air quality within courtyards strongly depend on their orientation with respect to the flow direction, as well as on the upstream flow conditions and upstream building configuration. We show that lateral openings, in most cases, have a negative impact on air quality; nevertheless, we also present configurations where lateral openings positively impact the air quality within courtyards. These outcomes may certainly contribute to improve future urban planning in terms of health protection.

Large Eddy Simulation of Round Impinging Jets With Large Stand-Off Distance

2014 ◽  
Author(s):  
Mehrdad Shademan ◽  
Vesselina Roussinova ◽  
Ron Barron ◽  
Ram Balachandar
Keyword(s):  
Large Eddy Simulation ◽  
Flow Characteristics ◽  
Impinging Jet ◽  
Impinging Jets ◽  
Vortical Structures ◽  
Nozzle Height ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Good Agreement

Large Eddy Simulation (LES) has been carried out to study the flow of a turbulent impinging jet with large nozzle height-to-diameter ratio. The dynamic Smagorinsky model was used to simulate the subgrid-scale stresses. The jet exit Reynolds number is 28,000. The study presents a detailed evaluation of the flow characteristics of an impinging jet with nozzle height of 20 diameters above the plate. Results of the mean normalized centerline velocity and wall shear stress show good agreement with previous experiments. Analysis of the flow field shows that vortical structures generated due to the Kelvin-Helmholtz instabilities in the shear flow close to the nozzle undergo break down or merging when moving towards the plate. Unlike impinging jets with small stand-off distance where the ring-like vortices keep their interconnected shape upon reaching the plate, no sign of interconnection was observed on the plate for this large stand-off distance. A large deflection of the jet axis was observed for this type of impinging jet when compared to the cases with small nozzle height-to-diameter ratios.

LARGE EDDY SIMULATION OF THE PHENOMENOLOGY OF THE FLOW THROUGH A LINEAR BLADE CASCADE – THE MEAN FIELD

2017 ◽  
Vol 20 (1) ◽  
pp. 1-20
Author(s):  
Karima Heguehoug ép. Benkara-Mostefa ◽  
Zoubir Nemouchi ◽  
Lahouari Adjlout
Keyword(s):  
Large Eddy Simulation ◽  
Mean Field ◽  
Eddy Simulation ◽  
Blade Cascade ◽  
Large Eddy ◽  
Flow Through ◽  
The Mean

Wind-Driven Natural Ventilation in an Areaway-Attached Basement with a Single-Sided Opening for Residential Purposes

2011 ◽  
Vol 383-390 ◽  
pp. 5344-5349
Author(s):  
Zhen Bu
Keyword(s):  
Natural Ventilation ◽  
Ventilation Rate ◽  
Navier Stokes ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Effective Ventilation ◽  
The Mean ◽  
Ventilation Rates ◽  
Les Model ◽  
Good Agreement

This paper discusses the sustainability of the areaway-attached basement concept with the attentions focused on wind-driven single-sided natural ventilation. First, numerical simulations were performed on an areaway-attached basement with a single-sided opening. Two CFD approaches: Reynolds averaged Navier-Stokes (RANS) and large-eddy simulation (LES) were used and compared with the previous experimental results of effective ventilation rate. A good agreement between the measurement and LES model was found and RANS model tends to underestimate the ventilation rates. Furthermore, Based on LES with the inflow turbulent fluctuations, the mean airflow patterns within and around the areaway-attached basement was investigated for different wind incidence angles to examine the influences of wind direction on ventilation performances.

Large Eddy Simulation of Turbulent Heat Transfer in Curved-Pipe Flow

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Changwoo Kang ◽  
Kyung-Soo Yang
Keyword(s):  
Heat Transfer ◽  
Pipe Flow ◽  
Temperature Fluctuations ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Subgrid Scale ◽  
Curved Pipe ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean

In the present investigation, turbulent heat transfer in fully developed curved-pipe flow has been studied by using large eddy simulation (LES). We consider a fully developed turbulent curved-pipe flow with axially uniform wall heat flux. The friction Reynolds number under consideration is Reτ  = 1000 based on the mean friction velocity and the pipe radius, and the Prandtl number (Pr) is 0.71. To investigate the effects of wall curvature on turbulent flow and heat transfer, we varied the nondimensionalized curvature (δ) from 0.01 to 0.1. Dynamic subgrid-scale models for turbulent subgrid-scale stresses and heat fluxes were employed to close the governing equations. To elucidate the secondary flow structures due to the pipe curvature and their effect on the heat transfer, the mean quantities and various turbulence statistics of the flow and temperature fields are presented, and compared with those of the straight-pipe flow. The friction factor and the mean Nusselt number computed in the present study are in good agreement with the experimental results currently available in the literature. We also present turbulence intensities, skewness and flatness factors of temperature fluctuations, and cross-correlations of velocity and temperature fluctuations. In addition, we report the results of an octant analysis to clarify the correlation between near-wall turbulence structures and temperature fluctuation in the vicinity of the pipe wall. Based on our results, we attempt to clarify the effects of the pipe curvature on turbulent heat transfer. Our LES provides researchers and engineers with useful data to understand the heat-transfer mechanisms in turbulent curved-pipe flow, which has numerous applications in engineering.

scholarly journals Predicting Unsteady Loads in Marine Propulsor Crashback Using Large Eddy Simulation

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
Hyunchul Jang ◽  
Aman Verma ◽  
Krishnan Mahesh
Keyword(s):  
Large Eddy Simulation ◽  
Reverse Flow ◽  
Reverse Direction ◽  
Unsteady Forces ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Submarine Hull ◽  
Marine Propulsor ◽  
Good Agreement

Propulsor crashback is an off-design operating condition where a propulsor rotates in the reverse direction to yield negative thrust. Crashback is characterized by the interaction of the free stream with the reverse flow generated by propulsor rotation. This causes a highly unsteady vortex ring which leads to flow separation and unsteady forces and moments on the blades. Large eddy simulation (LES) is performed for marine propulsors in crashback for various configurations and advance ratios and validated against experiments. The predictive capability of LES as a tool for propulsor crashback is demonstrated on an open propulsor, open propulsor with a submarine hull, and ducted propulsor with and without stator blades. LES is in good agreement with experiments for the mean and RMS levels, and spectra of the unsteady loads on the propulsors.

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