global name ‘vtk’ is not defined

[haibianshifeng]

I installed the pypnm on my system ubuntu 14.04.

When I run the example of quasistatic flow. The error appears.

lei@lei-ThinkPad-Edge:~/pypnm/examples/quasistatic_flow_solver$ python quasistatic_drainage_imbibition.py 
porenetwork.py:69 - 2017-11-12 08:36:48,226 - pypnm.porenetwork - WARNING - Had to decrease radius of 872 tubes so that they are smaller than neighboring pores
Number of pores with volume set to zero: 26352
Number of tubes with volume set to zero: 77436
Traceback (most recent call last):
  File "quasistatic_drainage_imbibition.py", line 54, in <module>
    run_quasi_static(network)
  File "quasistatic_drainage_imbibition.py", line 12, in run_quasi_static
    simulation.create_vtk_output_folder("paraview_quasistatic_run", delete_existing_files=True)
  File "/home/lei/anaconda2/lib/python2.7/site-packages/pypnm-0.1-py2.7.egg/pypnm/flow_simulation/simulation.py", line 39, in create_vtk_output_folder
    self.vtk_writer = VtkWriter(self.network, folder_name, delete_existing_files)
  File "/home/lei/anaconda2/lib/python2.7/site-packages/pypnm-0.1-py2.7.egg/pypnm/postprocessing/vtk_output.py", line 30, in __init__
    self.polydata = self.polydata_from_network(self.network)
  File "/home/lei/anaconda2/lib/python2.7/site-packages/pypnm-0.1-py2.7.egg/pypnm/postprocessing/vtk_output.py", line 94, in polydata_from_network
    polydata = vtk.vtkPolyData()
NameError: global name 'vtk' is not defined

How to solve it?
Many Thanks!