diff --git a/18_Rayleigh_Taylor/CASE/DATA/run.cfg b/18_Rayleigh_Taylor/CASE/DATA/run.cfg
new file mode 100644
index 0000000..89ecabe
--- /dev/null
+++ b/18_Rayleigh_Taylor/CASE/DATA/run.cfg
@@ -0,0 +1,5 @@
+[job_defaults]
+
+n_procs: 1
+n_threads: 1
+
diff --git a/18_Rayleigh_Taylor/CASE/DATA/setup.xml b/18_Rayleigh_Taylor/CASE/DATA/setup.xml
new file mode 100644
index 0000000..8f9e462
--- /dev/null
+++ b/18_Rayleigh_Taylor/CASE/DATA/setup.xml
@@ -0,0 +1,185 @@
+
+
+
+
+
+
+
+ 1
+ 10
+ 4.4
+
+
+
+
+
+
+ 1
+ 10
+ 0.1
+ 0.0005
+ 0.1
+
+
+
+ Y0 or Y1 or Z0 or Z1
+
+
+
+
+
+
+
+
+
+
+
+
+ 1.17862
+ 1
+ 5
+
+
+ 0.01
+
+
+
+
+ 1.83e-05
+ 0.001
+ 0.001
+
+
+ 0
+
+
+
+ 101325
+ 293.15
+
+
+ 0
+ -1
+ 0
+
+
+
+ 0
+ 0
+ 0
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ 0.1
+ 25
+ X0 or X1
+
+ 1
+ 0
+ 0
+
+ 1
+ 1
+
+
+
+
+ all[]
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ 1
+ 0
+
+
+
+
+
+
+
+
+
+
+
+
+
+ 2
+
+
+ 1
+
+
+
+
\ No newline at end of file
diff --git a/18_Rayleigh_Taylor/CASE/SRC/cs_user_initialization.cpp b/18_Rayleigh_Taylor/CASE/SRC/cs_user_initialization.cpp
new file mode 100644
index 0000000..3e95077
--- /dev/null
+++ b/18_Rayleigh_Taylor/CASE/SRC/cs_user_initialization.cpp
@@ -0,0 +1,152 @@
+/*============================================================================
+ * User initialization for the Rayleigh-Taylor instability (VOF).
+ *
+ * A heavy fluid (rho=5) rests on top of a light fluid (rho=1) under
+ * gravity. A sinusoidal perturbation of the interface triggers the
+ * instability, producing characteristic mushroom-shaped structures.
+ *============================================================================*/
+
+/* code_saturne version 9.1 */
+
+/*
+ This file is part of code_saturne, a general-purpose CFD tool.
+
+ Copyright (C) 1998-2025 EDF S.A.
+
+ This program is free software; you can redistribute it and/or modify it under
+ the terms of the GNU General Public License as published by the Free Software
+ Foundation; either version 2 of the License, or (at your option) any later
+ version.
+
+ This program is distributed in the hope that it will be useful, but WITHOUT
+ ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ details.
+
+ You should have received a copy of the GNU General Public License along with
+ this program; if not, write to the Free Software Foundation, Inc., 51 Franklin
+ Street, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+/*----------------------------------------------------------------------------*/
+
+#include "cs_defs.h"
+
+/*----------------------------------------------------------------------------
+ * Standard C library headers
+ *----------------------------------------------------------------------------*/
+
+#include
+#include
+
+#if defined(HAVE_MPI)
+#include
+#endif
+
+/*----------------------------------------------------------------------------
+ * PLE library headers
+ *----------------------------------------------------------------------------*/
+
+#include
+
+/*----------------------------------------------------------------------------
+ * Local headers
+ *----------------------------------------------------------------------------*/
+
+#include "cs_headers.h"
+
+/*----------------------------------------------------------------------------*/
+
+BEGIN_C_DECLS
+
+/*----------------------------------------------------------------------------*/
+/*!
+ * \file cs_user_initialization.cpp
+ *
+ * \brief Rayleigh-Taylor instability initialization (VOF).
+ */
+/*----------------------------------------------------------------------------*/
+
+/*============================================================================
+ * User function definitions
+ *============================================================================*/
+
+/*----------------------------------------------------------------------------*/
+/*!
+ * \brief Define initial conditions for the Rayleigh-Taylor problem.
+ *
+ * Sets zero velocity and a smoothed sinusoidal VOF interface between
+ * a heavy fluid on top and a light fluid below, under gravity.
+ *
+ * \param[in, out] domain pointer to a cs_domain_t structure
+ */
+/*----------------------------------------------------------------------------*/
+
+void
+cs_user_initialization(cs_domain_t *domain)
+{
+ const cs_lnum_t n_cells = domain->mesh->n_cells;
+
+ /* Do not reinitialize on restart */
+ if (domain->time_step->nt_prev > 0)
+ return;
+
+ /* ---- Parameters ---- */
+
+ const cs_real_t Lx = 1.0; /* domain length in x */
+ const cs_real_t y0 = 1.0; /* interface height (middle of Ly=2) */
+ const cs_real_t eps = 0.03; /* perturbation amplitude */
+ const cs_real_t thickness = 0.005; /* interface smoothing thickness */
+
+ /* ---- Mesh coordinates (v9.x format) ---- */
+
+ const cs_real_t (*cell_cen)[3] =
+ domain->mesh_quantities->cell_cen;
+
+ /* ---- Get VOF field ---- */
+
+ cs_field_t *vf = cs_field_by_name_try("void_fraction");
+ if (vf == nullptr)
+ vf = cs_field_by_name_try("volume_fraction");
+ if (vf == nullptr)
+ vf = cs_field_by_name_try("alpha");
+
+ if (vf == nullptr)
+ bft_error(__FILE__, __LINE__, 0,
+ "VOF fraction field not found.");
+
+ /* ---- Velocity field ---- */
+
+ cs_real_t *vel = CS_F_(vel)->val;
+
+ /* ---- Initialization loop ---- */
+
+ for (cs_lnum_t c_id = 0; c_id < n_cells; c_id++) {
+
+ const cs_real_t x = cell_cen[c_id][0];
+ const cs_real_t y = cell_cen[c_id][1];
+
+ /* 1) Perturbed interface */
+ const cs_real_t y_int =
+ y0 + eps * sin(2.0 * M_PI * x / Lx);
+
+ /* 2) Smoothed VOF (heavy fluid above) */
+ const cs_real_t s = (y - y_int) / thickness;
+
+ cs_real_t alpha =
+ 0.5 * (1.0 + tanh(s));
+
+ /* Clamp */
+ if (alpha < 0.0) alpha = 0.0;
+ if (alpha > 1.0) alpha = 1.0;
+
+ vf->val[c_id] = alpha;
+
+ /* 3) Initial velocity = 0 everywhere */
+ vel[3*c_id + 0] = 0.0;
+ vel[3*c_id + 1] = 0.0;
+ vel[3*c_id + 2] = 0.0;
+ }
+}
+
+END_C_DECLS
\ No newline at end of file
diff --git a/18_Rayleigh_Taylor/README.md b/18_Rayleigh_Taylor/README.md
new file mode 100644
index 0000000..89c79a7
--- /dev/null
+++ b/18_Rayleigh_Taylor/README.md
@@ -0,0 +1,56 @@
+Rayleigh-Taylor Instability
+===========================
+
+2D Rayleigh-Taylor instability simulated with the VOF (Volume of Fluid)
+method in code_saturne.
+
+A heavy fluid rests on top of a light fluid under gravity. Any
+perturbation of the interface grows exponentially, producing
+characteristic mushroom-shaped structures as the heavy fluid sinks
+and the light fluid rises.
+
+Setup
+-----
+
+- Multiphase model: VOF (Volume of Fluid)
+- Turbulence: off (laminar)
+- Mesh: 64 x 192 x 1 Cartesian (generated at runtime)
+- Domain: [0, 1] x [-1, 2] x [0, 1]
+- Periodicity: x-direction
+- Boundary conditions:
+ - x0, x1: periodic
+ - y0, y1, z0, z1: symmetry
+- Gravity: g = -1 m/s^2 (y-axis)
+- Time stepping: adaptive (CFL = 1), dt_ref = 0.0005 s, t_final = 4.4 s
+
+Fluid properties:
+
+| Property | Light fluid | Heavy fluid |
+|------------|-------------|-------------|
+| Density | 1.0 | 5.0 |
+| Viscosity | 0.001 | 0.001 |
+
+Atwood number: At = (rho_h - rho_l) / (rho_h + rho_l) = 0.667
+
+Initial conditions (set in `cs_user_initialization.cpp`):
+
+| Parameter | Value |
+|-------------------------------|-----------------|
+| Interface height y0 | 1.0 m |
+| Perturbation amplitude eps | 0.03 m |
+| Perturbation wavelength | Lx (2 pi mode) |
+| Interface smoothing thickness | 0.005 m |
+
+The VOF fraction is initialized with a smoothed sinusoidal interface.
+Velocity is zero everywhere; the instability is driven purely by gravity.
+
+Reference
+---------
+
+Lord Rayleigh, "Investigation of the character of the equilibrium of an
+incompressible heavy fluid of variable density", Proc. London Math. Soc.,
+14:170-177, 1883.
+
+G. I. Taylor, "The instability of liquid surfaces when accelerated in a
+direction perpendicular to their planes", Proc. R. Soc. Lond. A,
+201(1065):192-196, 1950.
diff --git a/18_Rayleigh_Taylor/smgr.xml b/18_Rayleigh_Taylor/smgr.xml
new file mode 100644
index 0000000..c5665b7
--- /dev/null
+++ b/18_Rayleigh_Taylor/smgr.xml
@@ -0,0 +1,7 @@
+
+
+
+
+
+
+