The VTU file format is an XML-based file format used primarily within the Visualization Toolkit (VTK) ecosystem to store unstructured grid data. Unstructured grids are a flexible way to represent complex 3D geometries, such as those arising from Finite Element Method (FEM) or Computational Fluid Dynamics (CFD) simulations. A VTU file contains metadata describing the structure of the grid, including the coordinates of the points (vertices), the connectivity information (which points form which cells, like tetrahedra, hexahedra, or polygons), and the associated data arrays. These data arrays can represent scalar fields (like temperature or pressure) or vector fields (like velocity) defined over the grid cells or points. Because it is XML-based, the structure is human-readable, although the data itself is often stored in binary chunks for efficiency. VTU files are crucial for visualizing complex scientific and engineering simulations where the mesh topology is irregular and cannot be easily represented by structured grids (like rectilinear grids).