How to write cell data changed over time into one file by `vtk.vtkXMLUnstructuredGridWriter`?

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I have a triangle mesh, and want to write some cell data which changed over time into a test.vtu files by vtk python interface.

import numpy as np
import vtk
import vtk.util.numpy_support as vnp

mesh =vtk.vtkUnstructuredGrid()
# creat points, cellType, cells, (here we have NC cells)
mesh.SetPoints(points)
mesh.SetCells(cellType, cells)
writer = vtk.vtkXMLUnstructuredGridWriter()
writer.SetFileName("test.vtu")
writer.SetInputData(mesh)
writer.SetNumberOfTimeSteps(100)
writer.Start()
for i in range(100):
    # how to add the celldata array of current time level into mesh?
    # cdata = mesh.GetCellData()
    # val = ... # a numpy array
    # d = vnp.numpy_to_vtk(val)
    # cdata.AddArray(val)
    writer.WriteNextTime(i)
writer.Stop()

The code in the loop which was commented can not work well. Any suggestion? Thanks very much.