Simulations of turbulence's smallest structures

Date:
July 8, 2021
Source:
Gauss Centre for Supercomputing
Summary:
Scientists have long used supercomputers to better understand how
turbulent flows behave under a variety of conditions. Researchers
have now include the complex but essential concept of
'intermittency' in turbulent flows.



FULL STORY ==========================================================================
When you pour cream into a cup of coffee, the viscous liquid seems to
lazily disperse throughout the cup. Take a mixing spoon or straw to the
cup, though, and the cream and coffee seem to quickly and seamlessly
combine into a lighter color and, at least for some, a more enjoyable
beverage.


==========================================================================
The science behind this relatively simple anecdote actually speaks
to a larger truth about complex fluid dynamics and underpins many of
the advancements made in transportation, power generation, and other technologies since the industrial era -- the seemingly random chaotic
motions known as turbulence play a vital role in chemical and industrial processes that rely on effective mixing of different fluids.

While scientists have long studied turbulent fluid flows, their inherent chaotic natures have prevented researchers from developing an exhaustive
list of reliable "rules," or universal models for accurately describing
and predicting turbulence. This tall challenge has left turbulence as
one of the last major unsolved "grand challenges" in physics.

In recent years, high-performance computing (HPC) resources have
played an increasingly important role in gaining insight into how
turbulence influences fluids under a variety of circumstances. Recently, researchers from the RWTH Aachen University and the CORIA (CNRS UMR 6614) research facility in France have been using HPC resources at the Ju"lich Supercomputing Centre (JSC), one of the three HPC centres comprising
the Gauss Centre for Supercomputing (GCS), to run high-resolution
direct numerical simulations (DNS) of turbulent setups including jet
flames. While extremely computationally expensive, DNS of turbulence
allows researchers to develop better models to run on more modest
computing resources that can help academic or industrial researchers
using turbulence's effects on a given fluid flow.

"The goal of our research is to ultimately improve these models,
specifically in the context of combustion and mixing applications," said
Dr. Michael Gauding, CORIA scientist and researcher on the project. The
team's recent work was just named the distinguished paper from the
"Turbulent Flames" colloquium, which happened as part of the 38th
International Symposium on Combustion.

Starts and stops Despite its seemingly random, chaotic characteristics, researchers have identified some important properties that are
universal, or at least very common, for turbulence under specific
conditions. Researchers studying how fuel and air mix in a combustion
reaction, for instance, rely on turbulence to ensure a high mixing
efficiency. Much of that important turbulent motion may stem from what
happens in a thin area near the edge of the flame, where its chaotic
motions collide with the smoother-flowing fluids around it. This area,
the turbulent-non-turbulent interface (TNTI), has big implications for understanding turbulent mixing.



========================================================================== While running their DNS calculations, Gauding and his collaborator,
Mathis Bode from RWTH Aachen, set out to specifically focus on this some
of the subtler, more complex phenomena that take place at the TNTI.

Specifically, the researchers wanted to better understand the rare but
powerful fluctuations called "intermittency" -- an irregular process
happening locally but with very high amplitude. In turbulent flames, intermittency enhances the mixing and combustion efficiency but too much
can also extinguish the flame.

Scientists distinguish between internal intermittency, which occurs
at the smallest scales and is a characteristic feature of any fully
developed turbulent flow, and external intermittency, which manifests
itself at the edge of the flame and depends on the structure of the TNTI.

Even using world-class HPC resources, running large DNS simulations
of turbulence is computationally expensive, as researchers cannot use assumptions about the fluid motion, but rather solve the governing
equations for all relevant scales in a given system -- and the scale
range increases with the "strength" of turbulence as power law. Even
among researchers with access to HPC resources, simulations oftentimes
lack the necessary resolution to fully resolve intermittency, which
occurs in thin confined layers.

For Bode and Gauding, understanding the small-scale turbulence happening
at the thin boundary of the flame is the point. "Our simulations are
highly resolved and are interested in these thin layers," Bode said. "For production runs, the simulation resolution is significantly higher
compared to similar DNS simulations to accurately resolve the strong
bursts that are connected to intermittency." The researchers were
able to use the supercomputers JUQUEEN, JURECA, and JUWELS at JSC to
build a comprehensive database of turbulence simulations. For example,
one simulation was run for multiple days on the full JUQUEEN module,
employing all 458,752 compute cores during the centre's "Big Week"
in 2019, simulating a jet flow with about 230 billion grid points.



========================================================================== Mixing and matching With a better understanding of the role that
intermittency plays, the team takes data from their DNS runs and using
it to improve less computationally demanding large eddy simulations
(LES). While still perfectly accurate for a variety of research aims,
LES are somewhere between an ab initio simulation that begins with no assumptions and a model that has already baked in certain rules about
how fluids will behave.

Studying turbulent jet flames has implications for a variety of
engineering goals, from aerospace technologies to power plants. While
many researchers studying fluid dynamics have access to HPC resources
such as those at JSC, others do not. LES models can often run on more
modest computing resources, and the team can use their DNS data to help
better inform these LES models, making less computationally demanding simulations more accurate. "In general, present LES models are not able
to accurately account for these phenomena in the vicinity of the TNTI,"
Gauding said.

The team was able to scale its application to take full advantage
of JSC computing resources partially by regularly participating in
training events and workshops held at JSC. Despite already being able
to leverage large amounts of HPC power, though, the team recognizes that
this scientific challenge is complex enough that even next-generation HPC systems capable of reaching exascale performance -- slightly more than
twice as fast as today's fastest supercomputer, the Fugaku supercomputer
at RIKEN in Japan -- may not be able to fully simulate these turbulent dynamics. However, each computational advancement allows the team to
increase the degrees of freedom and include additional physics in their simulations. The researchers are also looking at using more data-driven approaches for including intermittency in simulations, as well as
improving, developing, and validating models based on the team's DNS data.

========================================================================== Story Source: Materials provided by
Gauss_Centre_for_Supercomputing. Original written by Eric Gedenk. Note:
Content may be edited for style and length.


========================================================================== Journal References:
1. M. Gauding, M. Bode, Y. Brahami, E'. Varea,
L. Danaila. Self-similarity
of turbulent jet flows with internal and external
intermittency. Journal of Fluid Mechanics, 2021; 919 DOI:
10.1017/jfm.2021.399
2. M. Gauding, M. Bode, D. Denker, Y. Brahami, L. Danaila, E. Varea. On
the
combined effect of internal and external intermittency in turbulent
non- premixed jet flames. Proceedings of the Combustion Institute,
2021; 38 (2): 2767 DOI: 10.1016/j.proci.2020.08.022 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210708103609.htm

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