diff --git a/src/bqn/ns.bqn b/src/bqn/ns.bqn
new file mode 100644
index 0000000..6514ee4
--- /dev/null
+++ b/src/bqn/ns.bqn
@@ -0,0 +1,5 @@
+cn ← {
+ _p ⇐ {(-´𝔽¨)⋈(+´𝔽¨)⟜⌽}
+ Cis ⇐ (⍉•math.Cos≍•math.Sin)¯π×↕⊸÷
+}
+FFT ← {𝕊⁼: ⋈⟜-´˘{≠÷˜·𝔽𝔾∘𝔽}𝕊𝕩; (1=≠)◶⟨(+∾-)⟜(⊢×cn._p˘cn.Cis∘≠)´(𝕊¨⊢⊔˜2|⊒˜), ⊢⟩𝕩}
diff --git a/src/index.org b/src/index.org
index f03b06c..6a62bec 100644
--- a/src/index.org
+++ b/src/index.org
@@ -36,5 +36,6 @@
5. [[./si.org][Scheming a mise-en-abîme in BQN]]
6. [[./aoc24.org][Coding in Advent]]
7. [[./nn.org][The miniaturist's neural network]]
+8. [[./ns.org][Navier-Stokes simulator]]
#+HTML:
diff --git a/src/nm.org b/src/nm.org
deleted file mode 100644
index 3c1f134..0000000
--- a/src/nm.org
+++ /dev/null
@@ -1,24 +0,0 @@
-# -*- eval: (face-remap-add-relative 'default '(:family "BQN386 Unicode" :height 180)); -*-
-#+TITLE: A numerical bonsai display
-#+HTML_HEAD:
-#+HTML_HEAD:
-
-** Foreword
-
-** Bibliography
-
-These are my favorite books on numerical analysis, in descending order of preference.
-I use the first two very often, and consult the others to deepen my understanding:
-
-1. [[https://www.equalsharepress.com/][Numerical Methods for Scientific Computing: The Definitive Manual for Math Geeks]]
-2. [[https://link.springer.com/book/10.1007/978-3-031-08121-7][Numerical Analysis: A Graduate Course]]
-3. [[https://epubs.siam.org/doi/book/10.1137/1.9781421407944][Matrix computations]]
-4. [[https://numerical.recipes/book.html][Numerical Recipes]]
-5. [[https://epubs.siam.org/doi/10.1137/1.9781611976557][Numerical Linear Algebra with Julia]]
-6. [[https://link.springer.com/book/10.1007/978-0-387-40065-5][Numerical Optimization]]
-
-#+BEGIN_EXPORT html
-
-#+END_EXPORT
diff --git a/src/ns.org b/src/ns.org
new file mode 100644
index 0000000..37db5f7
--- /dev/null
+++ b/src/ns.org
@@ -0,0 +1,44 @@
+# -*- eval: (face-remap-add-relative 'default '(:family "BQN386 Unicode" :height 180)); -*-
+#+TITLE: Navier-Stokes simulator (WIP)
+#+HTML_HEAD:
+#+HTML_HEAD:
+#+HTML_HEAD:
+
+** Foreword
+
+We want to solve the following [[https://en.wikipedia.org/wiki/Derivation_of_the_Navier%E2%80%93Stokes_equations][system]] of partial differential equations
+in two dimensions for an incompressible fluid, imposing periodic boundary conditions:
+
+\begin{equation*}
+ \begin{aligned}
+ \nabla \cdot \mathbf{q} &= 0,\\
+ \frac{\partial \mathbf{q}}{\partial t}
+ + \nabla \cdot (\mathbf{q} \otimes \mathbf{q}) &=
+ -\nabla p + \frac{1}{\text{Re}} \nabla^2 \mathbf{q},
+ \end{aligned}
+\end{equation*}
+
+** Simulator
+
+The numerical solution will be obtained using this zero-dependency BQN code:
+
+#+begin_src bqn :tangle bqn/ns.bqn :exports code
+ cn ← {
+ _p ⇐ {(-´𝔽¨)⋈(+´𝔽¨)⟜⌽}
+ Cis ⇐ (⍉•math.Cos≍•math.Sin)¯π×↕⊸÷
+ }
+ FFT ← {𝕊⁼: ⋈⟜-´˘{≠÷˜·𝔽𝔾∘𝔽}𝕊𝕩; (1=≠)◶⟨(+∾-)⟜(⊢×cn._p˘cn.Cis∘≠)´(𝕊¨⊢⊔˜2|⊒˜), ⊢⟩𝕩}
+#+end_src
+
+#+RESULTS:
+: (function block)
+
+#+BEGIN_EXPORT html
+
+#+END_EXPORT