Update 3_numerical.rst

Author: hweifluids

source/3_numerical.rst

@@ -18,7 +18,7 @@ To obtain this map, one must numerically integrate the underlying dynamical syst
    \mathbf{x}(t_n)=\mathbf{x}_n\,,  
 
 where :math:`\sigma = \pm1` selects forward or backward integration.
-During the time integration process, the algorithm frequently queries the flow velocity vector :math:`\mathbf{v}(\mathbf{x},t)` at specific locations and moments with very high precision requirements.
+During the time integration process, the algorithm frequently queries the flow velocity vector :math:`\mathbf{u}(\mathbf{x},t)` at specific locations and moments with very high precision requirements.
 However, since the data grid is inherently spatially discretized, high-order interpolation methods are required by the program to keep numerical stability and obtain a physically meaningful flow map.
 Furthermore, when querying the velocity, special wall treatment must be applied at the boundaries to avoid value discontinuities and to represent certain real physical conditions.