docs: add Magnaudet & Eames (2000) bubble-drag review; fix \tfrac lint

- add \cite Magnaudet2000 (Annu. Rev. Fluid Mech.) as a modern review covering
  the free-slip/no-slip/Levich drag regimes
- replace \tfrac (AMSmath, rejected by lint_docs.py) with \frac in the
  pressure-force volume expression -- this was the doc-reference lint failure

Author: sbryngelson

docs/documentation/case.md

@@ -986,8 +986,8 @@ When ``polytropic = 'F'``, the gas compression is modeled as non-polytropic due
 
 - `vel_model` activates translational motion of the bubbles (\cite Wilfong26): [1] tracer bubbles, which are advected with the local carrier velocity \f$\underline{u}\f$ so that \f$\dot{\underline{x}}_b = \underline{u}(\underline{x}_b)\f$; [2] Newton's second law, which integrates \f$m_b \ddot{\underline{x}}_b = \underline{F}_D + \underline{F}_p + \underline{F}_g\f$, where \f$m_b\f$ is the bubble mass and the right-hand side collects the drag, pressure, and gravity forces below. The carrier velocity at the bubble is interpolated with a Lagrange polynomial of order set by `fd_order`, which must be specified when `vel_model > 0`.
 
-    - `drag_model` selects the drag force \f$\underline{F}_D\f$ acting on the slip velocity \f$\underline{u}_{\rm rel} = \underline{u}_b - \underline{u}\f$, with bubble radius \f$a\f$ and Reynolds number \f$Re\f$: [0] no drag (default); [1] free-slip drag (clean-interface creeping-flow limit, \cite Hadamard1911, \cite Rybczynski1911), \f$\underline{F}_D = -4\pi a\,\underline{u}_{\rm rel}/Re\f$; [2] no-slip Stokes drag (rigid sphere, \cite Stokes1851), \f$\underline{F}_D = -6\pi a\,\underline{u}_{\rm rel}/Re\f$; [3] Levich drag (clean bubble at high \f$Re\f$, \cite Levich1962), \f$\underline{F}_D = -12\pi a\,\underline{u}_{\rm rel}/Re\f$.
-    - `pressure_force` (default true) enables the pressure-gradient force \f$\underline{F}_p = -V_b\,\nabla p\f$, where \f$V_b = \tfrac{4}{3}\pi a^3\f$ is the bubble volume.
+    - `drag_model` selects the drag force \f$\underline{F}_D\f$ acting on the slip velocity \f$\underline{u}_{\rm rel} = \underline{u}_b - \underline{u}\f$, with bubble radius \f$a\f$ and Reynolds number \f$Re\f$: [0] no drag (default); [1] free-slip drag (clean-interface creeping-flow limit, \cite Hadamard1911, \cite Rybczynski1911), \f$\underline{F}_D = -4\pi a\,\underline{u}_{\rm rel}/Re\f$; [2] no-slip Stokes drag (rigid sphere, \cite Stokes1851), \f$\underline{F}_D = -6\pi a\,\underline{u}_{\rm rel}/Re\f$; [3] Levich drag (clean bubble at high \f$Re\f$, \cite Levich1962), \f$\underline{F}_D = -12\pi a\,\underline{u}_{\rm rel}/Re\f$. See \cite Magnaudet2000 for a review of these bubble-drag regimes.
+    - `pressure_force` (default true) enables the pressure-gradient force \f$\underline{F}_p = -V_b\,\nabla p\f$, where \f$V_b = \frac{4}{3}\pi a^3\f$ is the bubble volume.
     - `gravity_force` (default false) enables the body force \f$\underline{F}_g = m_b\,\underline{g}\f$, with \f$\underline{g}\f$ the acceleration set by the body-force parameters.
 
 - `kahan_summation` uses Kahan compensated summation when smearing the bubble contributions onto the Eulerian void fraction, reducing the round-off sensitivity of the accumulation to the summation order. It is not compatible with `--mixed` precision builds.

docs/references.bib

@@ -704,3 +704,13 @@ @book{Levich1962
   address   = {Englewood Cliffs, NJ},
   year      = {1962}
 }
+
+@article{Magnaudet2000,
+  author  = {J. Magnaudet and I. Eames},
+  title   = {The motion of high-{R}eynolds-number bubbles in inhomogeneous flows},
+  journal = {Annual Review of Fluid Mechanics},
+  volume  = {32},
+  pages   = {659--708},
+  year    = {2000},
+  doi     = {10.1146/annurev.fluid.32.1.659}
+}