High-fidelity simulations of bubble, droplet and spray formation in breaking waves

2016 ◽  
Vol 792 ◽  
pp. 307-327 ◽  
Author(s):  
Zhaoyuan Wang ◽  
Jianming Yang ◽  
Frederick Stern
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Power Law ◽  
Wave Breaking ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Small Scale ◽  
High Fidelity ◽  
Spray Formation ◽  
High Fidelity Simulations

High-fidelity simulations of wave breaking processes are performed with a focus on the small-scale structures of breaking waves, such as bubble/droplet size distributions. Very large grids (up to 12 billion grid points) are used in order to resolve the bubbles/droplets in breaking waves at the scale of hundreds of micrometres. Wave breaking processes and spanwise three-dimensional interface structures are identified. It is speculated that the Görtler type centrifugal instability is likely more relevant to the plunging wave breaking instabilities. Detailed air entrainment and spray formation processes are shown. The bubble size distribution shows power-law scaling with two different slopes which are separated by the Hinze scale. The droplet size distribution also shows power-law scaling. The computational results compare well with the available experimental and computational data in the literature. Computational difficulties and challenges for large grid simulations are addressed.

Cloud Droplet Size Distributions from Observations of Glory and Cloudbow during the EUREC4A Campaign

2020 ◽  
Author(s):  
Veronika Pörtge ◽  
Tobias Kölling ◽  
Tobias Zinner ◽  
Linda Forster ◽  
Bernhard Mayer
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Cloud Droplet ◽  
Droplet Size Distribution ◽  
Cloud Microphysics ◽  
Small Scale ◽  
Remote Sensing Technique ◽  
Cloud Droplet Size Distribution ◽  
Cloud Radiative Effect ◽  
Spherical Droplets

<p>The cloud droplet size distribution determines the evolution of clouds and their impact on weather and climate. First, droplet size determines<br>the cloud radiative effect. Second, evolution of clouds and formation of precipitation are determined by droplet size and the shape of the size distribution. Therefore, measurements of the size distribution are important to further our understanding of clouds and their role in the earth system. We present a remote sensing technique for droplet size and width of the size distribution based on polarized observations of the glory and the cloudbow.<br>Glory and cloudbow are caused by backscattering of sunlight by spherical droplets in liquid clouds. This backscattering results in colorful concentric rings surrounding the observer’s shadow; the formation is described quantitatively by Mie theory. The rings of the glory appear in an angular range of 170° – 180° scattering angle and the larger cloudbow rings in a range of about 130° – 160° . The angular radius of the rings is the most accurate and direct measure of the droplet size at cloud edge. In addition, the sharpness of the rings conveys information about the width of the droplet size distribution. The visibility of glory and cloudbow is significantly enhanced by the use of polarized observations which reduce the contribution of multiple scattering.<br>The specMACS sensor of LMU Munich has been upgraded recently by a polarization-sensitive wide-angle imager which was operated for the first time on the HALO aircraft during the EUREC4A campaign. The newly installed sensor offers a high spatial and temporal resolution, allowing to study small-scale variability of cloud microphysics at cloud top with a resolution of about 20 m. specMACS measurements and first retrieval results using the glory-cloudbow technique are presented.</p>

Lagrangian Droplet Dynamics in the Subsiding Shell of a Cloud Using Direct Numerical Simulations

2015 ◽  
Vol 72 (10) ◽  
pp. 4015-4028 ◽  
Author(s):  
Vincent E. Perrin ◽  
Harmen J. J. Jonker
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Evaporative Cooling ◽  
Mixing Layer ◽  
Droplet Size Distribution ◽  
Small Scale ◽  
Cumulus Clouds ◽  
Droplet Dynamics ◽  
Turbulent Dissipation ◽  
Environmental Air

Abstract This study investigates the droplet dynamics at the lateral cloud–environment interface in shallow cumulus clouds. A mixing layer is used to study a small part of the cloud edge using direct numerical simulation combined with a Lagrangian particle tracking and collision algorithm. The effect of evaporation, gravity, coalescence, and the initial droplet size distribution on the intensity of the mixing layer and the evolution of the droplet size distribution is studied. Mixing of the droplets with environmental air induces evaporative cooling, which results in a very characteristic subsiding shell. As a consequence, stronger horizontal velocity gradients are found in the mixing layer, which induces more mixing and evaporation. A broadening of the droplet size distribution is observed as a result of evaporation and coalescence. Gravity acting on the droplets allows droplets in cloudy filaments detrained from the cloud to sediment and remain longer in the unsaturated environment. While this effect of gravity did not have a significant impact in this case on the mean evolution of the mixing layer, it does contribute to the broadening of the droplet size distribution and thereby significantly increases the collision rate. Although more but smaller droplets result in more evaporative cooling, more droplets also increase small-scale fluctuations and the production of turbulent dissipation. For the smallest droplets considered with a radius of 10 μm, the authors found that, although a more pronounced buoyancy dip was present, the increase in dissipation rate actually led to a decrease in the turbulent intensity of the mixing layer. Extrapolation of the results to realistic clouds is discussed.

scholarly journals Air entrainment and bubble statistics in breaking waves

2016 ◽  
Vol 801 ◽  
pp. 91-129 ◽  
Author(s):  
Luc Deike ◽  
W. Kendall Melville ◽  
Stéphane Popinet
Keyword(s):  
Size Distribution ◽  
Bubble Size ◽  
Dissipation Rate ◽  
Unit Length ◽  
Bubble Radius ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Bubble Size Distribution ◽  
Breaking Wave ◽  
Bubble Plume

We investigate air entrainment and bubble statistics in three-dimensional breaking waves through novel direct numerical simulations of the two-phase air–water flow, resolving the length scales relevant for the bubble formation problem, the capillary length and the Hinze scale. The dissipation due to breaking is found to be in good agreement with previous experimental observations and inertial scaling arguments. The air entrainment properties and bubble size statistics are investigated for various initial characteristic wave slopes. For radii larger than the Hinze scale, the bubble size distribution, can be described by $N(r,t)=B(V_{0}/2{\rm\pi})({\it\varepsilon}(t-{\rm\Delta}{\it\tau})/Wg)r^{-10/3}r_{m}^{-2/3}$ during the active breaking stages, where ${\it\varepsilon}(t-{\rm\Delta}{\it\tau})$ is the time-dependent turbulent dissipation rate, with ${\rm\Delta}{\it\tau}$ the collapse time of the initial air pocket entrained by the breaking wave, $W$ a weighted vertical velocity of the bubble plume, $r_{m}$ the maximum bubble radius, $g$ gravity, $V_{0}$ the initial volume of air entrained, $r$ the bubble radius and $B$ a dimensionless constant. The active breaking time-averaged bubble size distribution is described by $\bar{N}(r)=B(1/2{\rm\pi})({\it\epsilon}_{l}L_{c}/Wg{\it\rho})r^{-10/3}r_{m}^{-2/3}$, where ${\it\epsilon}_{l}$ is the wave dissipation rate per unit length of breaking crest, ${\it\rho}$ the water density and $L_{c}$ the length of breaking crest. Finally, the averaged total volume of entrained air, $\bar{V}$, per breaking event can be simply related to ${\it\epsilon}_{l}$ by $\bar{V}=B({\it\epsilon}_{l}L_{c}/Wg{\it\rho})$, which leads to a relationship for a characteristic slope, $S$, of $\bar{V}\propto S^{5/2}$. We propose a phenomenological turbulent bubble break-up model based on earlier models and the balance between mechanical dissipation and work done against buoyancy forces. The model is consistent with the numerical results and existing experimental results.

Turbulence Effects of Collision Efficiency and Broadening of Droplet Size Distribution in Cumulus Clouds

2018 ◽  
Vol 75 (1) ◽  
pp. 203-217 ◽  
Author(s):  
Sisi Chen ◽  
M. K. Yau ◽  
Peter Bartello
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Droplet Size Distribution ◽  
Droplet Growth ◽  
Small Scale ◽  
Cumulus Clouds ◽  
Collision Efficiency ◽  
Cloud Droplets ◽  
R Ratio ◽  
Condensational Growth

This paper aims to investigate and quantify the turbulence effect on droplet collision efficiency and explore the broadening mechanism of the droplet size distribution (DSD) in cumulus clouds. The sophisticated model employed in this study individually traces droplet motions affected by gravity, droplet disturbance flows, and turbulence in a Lagrangian frame. Direct numerical simulation (DNS) techniques are implemented to resolve the small-scale turbulence. Collision statistics for cloud droplets of radii between 5 and 25 μm at five different turbulence dissipation rates (20–500 cm2 s−3) are computed and compared with pure-gravity cases. The results show that the turbulence enhancement of collision efficiency highly depends on the r ratio (defined as the radius ratio of collected and collector droplets r/ R) but is less sensitive to the size of the collector droplet investigated in this study. Particularly, the enhancement is strongest among comparable-sized collisions, indicating that turbulence can significantly broaden the narrow DSD resulting from condensational growth. Finally, DNS experiments of droplet growth by collision–coalescence in turbulence are performed for the first time in the literature to further illustrate this hypothesis and to monitor the appearance of drizzle in the early rain-formation stage. By comparing the resulting DSDs at different turbulence intensities, it is found that broadening is most pronounced when turbulence is strongest and similar-sized collisions account for 21%–24% of total collisions in turbulent cases compared with only 9% in the gravitational case.

scholarly journals Small Scale Physical and Bio-Chemical Processes Affecting the Transport of Oil after a Spill

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Joseph Katz ◽  
CJ Beegle-Krause ◽  
Michel Boufadel ◽  
Marcelo Chamecki ◽  
Vijay John ◽  
...  
Keyword(s):  
Crude Oil ◽  
Size Distribution ◽  
Targeted Delivery ◽  
Breaking Waves ◽  
Halloysite Nanotubes ◽  
Small Scale ◽  
Oil Droplets ◽  
Form Complex ◽  
Oil Water ◽  
The Impact

Abstract A series of GOMRI-sponsored experimental and computational studies have discovered, elucidated and quantified the impact of small-scale processes on the dispersion, transport and weathering of crude oil slicks and subsurface plumes. Physical interfacial phenomena occurring at micron-scales include the formation of particle-stabilized emulsions, penetration of particles into oil droplets, formation of compound water-containing oil droplets during plume breakup, and the mechanisms affecting the breakup of oil into micro-droplet by tip streaming resulting from the drastic reduction in interfacial tension upon introduction of dispersant. Efforts aimed at development targeted delivery of surfactants have introduced solvent-free halloysite nanotubes that can be filled with surfactants, and preferentially released at oil-water interface. Buoyant surfactant-based gels, which enhance their encounter rates with oil slicks and adhere to weathered oil have also been developed. Studies of oil-bacteria interactions during early phases of biodegradation and shown how the bacteria, some highly active, attach to the oil-water interfaces and form complex films. Clay-decorated droplets sequester these bacteria and promote the propagation of these biofilm. Long extracellular polymeric substance (EPS) streamers generated by these biofilms form connected networks involving multiple droplets and debris, as well as increase the drag on the oil droplets. At 0.01–10 m scales, the generation of subsurface and airborne crude oil droplets by breaking waves, subsurface plumes and raindrop impact have been quantified. For waves, premixing the oil with dispersant reduces the droplets sizes to micron- and submicron-scales, and changes the slope of their size distribution. Without dispersant, the droplet diameters can be predicted based on the turbulence scales. With dispersant, the droplets are much smaller than the turbulence scales owing to the abovementioned tip-streaming. Aerosolization of oil is caused both by the initial splash and by subsequent bubble bursting, as entrained bubbles rise to the surface. Introduction of dispersant increases the airborne nano-droplet concentration by orders of magnitude, raising health questions. Dispersant injection also reduces the size of droplets in subsurface plumes, affecting the subsequent dispersion of these plume by currents and turbulence. Advancements have also been made in modeling of dissolution of oil in plumes, as well as in applications of Large Eddy Simulations (LES) to model plumes containing oil droplets and gas bubbles. The new multiscale framework, which accounts for the droplet size distribution and mass diffusion, can simulate the near- and far-fields of plumes, and predict the effect of vertical mixing promoted by turbulence on the transport of dispersed oil.

scholarly journals Effect of Inlet Tangential Port Area on the Performance of Small – Scale Simplex Atomizer

2012 ◽  
Author(s):  
Selvan G. Muthu ◽  
H. S. Muralidhara ◽  
Vinod Kumar Vyas ◽  
Kanth T. P. Dinesh ◽  
S. Kumaran ◽  
...  
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Cone Angle ◽  
Droplet Size Distribution ◽  
Small Scale ◽  
Sauter Mean Diameter ◽  
Spray Cone Angle ◽  
Spray Cone ◽  
Mean Diameter ◽  
Coefficient Of Discharge

An experimental investigation was conducted to study the effects of increased area of inlet tangential ports on the performance of small scale simplex atomizer. The spray characteristics of three different simplex atomizer representing increasing area of inlet tangential ports are examined using water as a working fluid. Measurements of coefficient of discharge, spray cone angle, Sauter mean diameter and droplet size distribution were carried out over wide range of injection pressure. Coriolis mass flow meter was used to measure coefficient of discharge. Spray cone angle was measured by image processing technique. Sauter mean diameter and droplet size distributions were measured by Malvern droplet sizing instrument. It was observed that with increase in area of inlet tangential ports the size of air core produced along the center line reduced, which increases the coefficient of discharge. Spray cone angle decreases with increase in area of inlet tangential ports. It was found that increase in area of inlet tangential ports reduces swirl strength inside swirl chamber, which results in increasing Sauter mean diameter. Better droplet size distribution was observed for lower area of inlet tangential port configuration. The obtained experimental results were compared with experimental correlations available in literatures. Deviations in the obtained experimental results and experimental correlations was observed. This is due to difference in the size of atomizer used and difference in experimental techniques used between the present work and other investigations.

scholarly journals Slamming Force Contribution due to Plunging Breakers on Circular, Square and Diamond Cylinders

2019 ◽  
Author(s):  
Xin Wang ◽  
Arun Kr Dev ◽  
Longbin Tao ◽  
De Wang Chia ◽  
Yali Zhang
Keyword(s):  
Cross Section ◽  
Wave Breaking ◽  
Extreme Event ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Offshore Structures ◽  
Design Guidelines ◽  
Cylindrical Shape ◽  
Offshore Structure ◽  
Slamming Load

Abstract Plunging breakers, unlike non-breaking waves, impose additional slamming load on the offshore structures. This additional slamming load is considered an extreme event and is one of the most devastating forces that an offshore structure could encounter during its operational lifecycle. Whilst there are design guidelines for offshore structures pertaining to breaking waves, however it is limited to only cylindrical shape. The amount of slamming load contribution by the plunging jet is also dependent on the cross section geometries of the offshore structures. Different geometries would give rise to different air entrainment phenomenon during wave breaking and therefore affecting the slamming load contributions. In this research, JONSWAP spectrum was used to create plunging breakers via the focusing method at Newcastle University’s Wind Wave and Current tank. The crux of this research is to investigate the wave-breaking impact load on cylindrical structures with different cross section geometries commonly used in the offshore industry.

scholarly journals Long-resident droplets at the stratocumulus top

2016 ◽  
Vol 16 (10) ◽  
pp. 6563-6576 ◽  
Author(s):  
Alberto de Lozar ◽  
Lukas Muessle
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Large Scale ◽  
Turbulence Models ◽  
Longwave Radiation ◽  
Droplet Size Distribution ◽  
Droplet Growth ◽  
Small Scale ◽  
Radiative Cooling ◽  
Stratocumulus Clouds

Abstract. Turbulence 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.

Time Evolution of the Droplet Size Distribution in Dropwise Condensation

2020 ◽  
Author(s):  
Maofei Mei ◽  
Feng Hu ◽  
Chong Han ◽  
Yan Sun ◽  
Dongdong Liu
Keyword(s):  
Size Distribution ◽  
Droplet Size ◽  
Power Law ◽  
Droplet Size Distribution ◽  
Droplet Growth ◽  
Size Distributions ◽  
Dropwise Condensation ◽  
Dominant Feature ◽  
Droplet Size Distributions ◽  
Instantaneous Distribution

Abstract Droplet growth processes during dropwise condensation are simulated with a help of computer. We focus on instantaneous and time-averaged characteristics of droplet size distributions. Based on simulation results, shift of a single peak from small to large size is a significant characteristic for the instantaneous distribution before the first departure. Once condensing surface was refreshed time and again by shedding droplets, then coexistence, shift and combination of multiple peaks is the dominant feature. This indicates that the instantaneous droplet size distribution highly depends on growth time and target area. The findings can explain why different distribution characteristics were reported in experiments. Different from the instantaneous distribution, time-averaged size distributions for coalesced droplets follow a power-law style due to a collaboration of coalescence events and re-nucleation behaviors. However, the size range for the power-law distributions were affected by nucleation density. This requires an appropriate usage of the empirical or fractal model to predict theoretically heat transfer rate of dropwise condensation. The present work provides a comprehensive understanding of the instantaneous and time-averaged characteristics of droplet size distributions.

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