Wind tunnel measurements of the preferential concentration of inertial droplets in homogeneous isotropic turbulence

Wind tunnel measurements of the preferential concentration of inertial droplets in homogeneous... We describe an experimental setup aimed at studying turbulent-induced droplet collisions in a laboratory setting. Our goal is to reproduce conditions relevant to warm-rain formation in clouds. In these conditions, the trajectories of small inertial droplets are strongly influenced by the background air turbulence, and collisions can potentially explain the droplet growth rates and spectrum broadening observed in this type of clouds. Warm-rain formation is currently under strong scrutiny because it is an important source of uncertainty in atmospheric models. A grid at the entrance of a horizontal wind tunnel produces homogeneous isotropic turbulence at a Re λ in the range of 400–500. Water droplets are injected from the nodes of the turbulence-inducing grid at a volume fraction (ϕ) of 2.7 × 10−5 and with sizes of 10–200 μm. A complex manifold-injection system was developed to obtain uniform water droplet seeding, in terms of both water content and size distribution. We characterize the resulting droplet-laden turbulent flow, and the statistics of droplet pairs are measured and analyzed. We found that the radial distribution function (RDF), a measure of preferential concentration of droplets that plays a key role in collision kernel models, has a large peak at distances below the Kolmogorov microscale of the turbulence. At very long separations, comparable with the integral length scale of the turbulence, these RDFs show a slow decay to the average probability given by the mean droplet number density. Consistent with this result, conditional analysis shows an increased local concentration of droplets within the inertial length scale (≈ 10–100 Kolmogorov lengths). These results are in good agreement with previous experiments that found clustering of inertial droplets with St ≈ 1 at scales on the order of 10η. Ultimately, our results support the hypothesis that turbulence-induced preferential concentration and enhanced settling can lead to significant increases in the collision probability for inertial droplets in the range 10–50 μm. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Experiments in Fluids Springer Journals

Wind tunnel measurements of the preferential concentration of inertial droplets in homogeneous isotropic turbulence

Loading next page...
 
/lp/springer_journal/wind-tunnel-measurements-of-the-preferential-concentration-of-inertial-WsoyeJJbj0
Publisher
Springer-Verlag
Copyright
Copyright © 2012 by Springer-Verlag
Subject
Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics, Heat and Mass Transfer; Fluid- and Aerodynamics
ISSN
0723-4864
eISSN
1432-1114
D.O.I.
10.1007/s00348-011-1252-6
Publisher site
See Article on Publisher Site

Abstract

We describe an experimental setup aimed at studying turbulent-induced droplet collisions in a laboratory setting. Our goal is to reproduce conditions relevant to warm-rain formation in clouds. In these conditions, the trajectories of small inertial droplets are strongly influenced by the background air turbulence, and collisions can potentially explain the droplet growth rates and spectrum broadening observed in this type of clouds. Warm-rain formation is currently under strong scrutiny because it is an important source of uncertainty in atmospheric models. A grid at the entrance of a horizontal wind tunnel produces homogeneous isotropic turbulence at a Re λ in the range of 400–500. Water droplets are injected from the nodes of the turbulence-inducing grid at a volume fraction (ϕ) of 2.7 × 10−5 and with sizes of 10–200 μm. A complex manifold-injection system was developed to obtain uniform water droplet seeding, in terms of both water content and size distribution. We characterize the resulting droplet-laden turbulent flow, and the statistics of droplet pairs are measured and analyzed. We found that the radial distribution function (RDF), a measure of preferential concentration of droplets that plays a key role in collision kernel models, has a large peak at distances below the Kolmogorov microscale of the turbulence. At very long separations, comparable with the integral length scale of the turbulence, these RDFs show a slow decay to the average probability given by the mean droplet number density. Consistent with this result, conditional analysis shows an increased local concentration of droplets within the inertial length scale (≈ 10–100 Kolmogorov lengths). These results are in good agreement with previous experiments that found clustering of inertial droplets with St ≈ 1 at scales on the order of 10η. Ultimately, our results support the hypothesis that turbulence-induced preferential concentration and enhanced settling can lead to significant increases in the collision probability for inertial droplets in the range 10–50 μm.

Journal

Experiments in FluidsSpringer Journals

Published: Jan 13, 2012

References

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off