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\DOI{10.5802/crphys.253}
\datereceived{2025-05-20}
\dateaccepted{2025-05-20}
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\dateposted{2025-06-06}
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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Geophysical and astrophysical fluid dynamics in the laboratory}
%\def\cdr@specialtitle@french{Dynamique des fluides g\'eophysiques et astrophysiques au laboratoire}
%\makeatother

\title{Foreword to the special issue:  Geophysical and astrophysical
fluid dynamics in the laboratory}

\alttitle{Avant-propos du num\'ero sp\'ecial : Dynamique des fluides
g\'eophysiques et astrophysiques au laboratoire}

\author{\firstname{Michael} \lastname{Le Bars}\CDRorcid{0000-0002-4884-6190}\IsCorresp}
\address{CNRS, Aix Marseille Univ, Centrale Marseille, IRPHE, Marseille, France}
\email[M. Le Bars]{michael.le-bars@cnrs.fr}

\author{\firstname{Stephan} \lastname{Fauve}\CDRorcid{0000-0003-2379-6737}}
\address{Laboratoire de Physique de l'\'Ecole Normale Sup\'erieure, ENS, Universit\'e PSL, CNRS, Sorbonne Universit\'e, Universit\'e de Paris, 75005 Paris, France}
\email[S. Fauve]{stephan.fauve@phys.ens.fr}

\keywords{\kwd{Geophysical fluid dynamics}
\kwd{Astrophysical fluid dynamics}
\kwd{Laboratory experiments}}

\altkeywords{\kwd{Dynamique des fluides g\'eophysiques}
\kwd{Dynamique des fluides astrophysiques}
\kwd{Exp\'eriences de laboratoire}}

\begin{abstract}
This special issue brings together 19 review articles addressing a
broad range of problems in geophysical and astrophysical fluid
dynamics, reflecting both the current state of the art and ongoing
research, with a special focus on the contribution of laboratory
experiments. This foreword outlines the general context and key
challenges of the field, and places the individual contributions in
perspective.
\end{abstract}

\begin{altabstract}
Ce num\'ero sp\'ecial r\'eunit 19 articles de revue couvrant un large
spectre de probl\'ematiques en dynamique des fluides g\'eophysiques et
astrophysiques, refl\'etant \`a la fois l'\'etat de l'art et les
recherches en cours, avec un accent particulier sur l'apport des
exp\'eriences en laboratoire. Cette pr\'eface pr\'esente le contexte
g\'en\'eral et les principaux d\'efis du domaine, et met en perspective
les diff\'erentes contributions.
\end{altabstract}

\maketitle

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{\vspace*{-.2pc}}

\section{Introduction}
Geophysical and Astrophysical Fluid Dynamics (GAFD) encompasses all
types of flows occurring in any natural system, from Earth's atmosphere
and oceans to the interiors of planets and stars, and even to\pagebreak
astrophysical structures such as accretion disks. These systems involve
a wide range of fluids---air, water, liquid metals, plasmas---and span
extensive spatial and temporal scales. Despite this diversity, GAFD
research is unified by a common set of governing equations and a shared
methodological framework: the construction of simplified models that
isolate fundamental physical mechanisms. This special issue explores
how laboratory experiments contribute to advancing our understanding of
these generic mechanisms.

GAFD often involves extreme flow regimes and complex coupled physics
that challenge numerical and theoretical approaches. While
computational tools have advanced rapidly, particularly with the
emergence of data-driven techniques, laboratory experiments remain
indispensable. They give direct access to real systems, inherently
including all physical effects, boundary conditions, and spatial and
temporal scales, without relying on parameterizations or closure
models. Laboratory experiments allow systematic exploration of
parameter space and sustained operation in extreme regimes, thereby
yielding statistically converged data. Laboratory flows emerge directly
from the fundamental equations with no a priori, and can thus reveal
rare, serendipitous and unexpected behaviors.  These strengths coexist
with inherent drawbacks: quantitative measurements are always
challenging and limited, and experiments are constrained by practical
restrictions on e.g.\ geometry, scale separation, and working fluid
properties.

\section{Topics overview}
This special issue provides an overview of the state of the art and
ongoing research in experimental GAFD. The 19 review articles highlight
the breadth of physical processes accessible in the lab and underline
the universality of the mechanisms across planetary and astrophysical
contexts. The contributions demonstrate that progress stems from both
large-scale experimental platforms and small-scale analog models.

Convection is a classical and long-standing research topic in fluid
mechanics, with particular features in GAFD. Natural systems are often
expected to operate in so-called ultimate regimes, where diffusive
effects become negligible. While the existence of such a regime remains
a subject of debate in the context of Rayleigh--B\'enard convection, a
radiatively driven convection setup has recently demonstrated an
ultimate regime in a novel
configuration~\cite{CRPHYS_2024__25_S3_A14_0}. Rotation plays a
dominant role in GAFD, and its influence on convection has been studied
experimentally for over seventy years, remaining a very active area of
research~\cite{CRPHYS_2024__25_S3_A23_0}. Furthermore, many natural
systems---including oceans, gas giants, and stellar 
interiors---feature multiple buoyancy sources, leading to complex
double-diffusive convection regimes that are especially prone to
laboratory investigation~\cite{CRPHYS_2024__25_S3_A5_0}.

Phase-change dynamics are fundamental across planetary layers but
remain challenging to parameterize due to complex interactions between
fluid mechanics and out-of-equilibrium thermodynamics. Laboratory
investigations address phenomena ranging from planetary surface
ablation~\cite{CRPHYS_2024__25_S3_A1_0} to core crystallization
processes~\cite{CRPHYS_2024__25_S3_A11_0}. The resulting solid boundary
topographies, spanning multiple scales, increase the complexity of GAFD
flows and offer a valuable platform for controlled experimental
studies~\cite{CRPHYS_2024__25_S3_A21_0}.

Multiphase flows pose major challenges for numerical modeling and are
ubiquitous in natural systems. Experimental studies target applications
ranging from marine plastic transport~\cite{CRPHYS_2024__25_S3_A19_0}
and dune morphodynamics~\cite{CRPHYS_2024__25_S3_A9_0} to soil
erosion~\cite{CRPHYS_2024__25_S3_A20_0} and metal-silicate
differentiation during planetary
accretion~\cite{CRPHYS_2024__25_S3_A22_0}.

Stratification and rotation are fundamental components of GAFD, driving
instabilities such as baroclinic modes~\cite{CRPHYS_2024__25_S3_A3_0}.
They govern complex flow regimes including jet
dynamics~\cite{CRPHYS_2024__25_S3_A17_0} and mixing in stratified
turbulence~\cite{CRPHYS_2024__25_S3_A4_0}. They also sustain internal
waves---gravity and inertial, respectively---that share many
characteristics but exhibit key
differences~\cite{CRPHYS_2024__25_S3_A7_0}. Internal waves play a
central role in geophysical and astrophysical dynamics, particularly
through their nonlinear interactions that sustain wave
turbulence~\cite{CRPHYS_2024__25_S3_A2_0} and generate mean flow
oscillations, exemplified by the quasi-biennial
oscillation~\cite{CRPHYS_2024__25_S3_A6_0}.

Magnetohydrodynamics (MHD) is a central component of GAFD, yet remains
particularly challenging to investigate experimentally. Ambitious
laboratory setups, often pushing the limits of current technology, have
been developed to explore MHD phenomena. The dynamics of accretion
disks has been a subject of intense experimental
effort~\cite{CRPHYS_2024__25_S3_A8_0}. Dynamo action remains a
long-standing goal, with several past attempts yielding limited
success. Nonetheless, notable progress includes the laboratory-scale
observation of magnetic field
reversals~\cite{CRPHYS_2024__25_S3_A10_0}. New experimental platforms
are now entering the arena offering new opportunities for
exploration~\cite{CRPHYS_2024__25_S3_A18_0}.

\section{Conclusion}
This special issue is intended for researchers in GAFD, scientists from
adjacent disciplines, and early-career or prospective students. It also
aims to foster dialogue across traditionally compartmentalized
fields---including atmospheric sciences, oceanography, deep geophysics,
planetology, astrophysics, fluid mechanics, and nonlinear physics.
Finally, it highlights the role of experimental GAFD as a powerful tool
for education and outreach, thanks to accessible and illustrative
setups that make complex processes tangible for students and the
broader public.

In conclusion, laboratory experiments remain a cornerstone of GAFD.
They complement observations, theory, and numerical simulations,
providing unique access to physical insight. This role is increasingly
critical as we seek to understand the changing dynamics of our planet
and the surprising behaviors emerging from newly explored planetary and
stellar systems.

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