Hi, In Paraview, if I want to calculate double derivative/second derivative of a variable, we have got the following options: 1) Take gradient filter twice-but this is associated with loss of accuracy and can produce some CFD mesh artefacts. Can someone please let me know if whether adding ghost cells can remove these artefacts? And how to use ghost cells? 2) Can i write double derivative code/function from theory in the programmable filter? If so, can some one let me know how to do it? Or do we have to export to MATLAB or Python if we want to calculate the double derivative efficiently? Any suggestions please.thanks
Calculating higher-order precision gradients using vmtk’s vtkvmtkUnstructuredGridGradientFilter in Python is relatively straightforward.
For an Anaconda environment, first create a virtual environment and install vmtk using the following commands:
conda create -n vmtk python=3.11
conda activate vmtk
conda install conda-forge::vmtk
Next, you can create a Hessian matrix by applying vtkvmtkUnstructuredGridGradientFilter twice to a variable with a Python program like the one below. However, this program requires modification as it takes a vtu file format as input. You will likely also need to customize the arguments and other settings.
#!/usr/bin/env python3
"""
VMTK Higher-Order Precision Gradient Calculator
This script calculates first and second-order gradients of a specified field
in a VTK unstructured grid file using VMTK's differential geometry filters.
"""
import argparse
import os
import sys
import vtk
from vmtk.vtkvmtkDifferentialGeometryPython import vtkvmtkUnstructuredGridGradientFilter
def parse_arguments():
"""Parse command line arguments."""
parser = argparse.ArgumentParser(
description="This program computes the Hessian matrix using vmtk's higher-order precision gradient calculation filter",
formatter_class=argparse.ArgumentDefaultsHelpFormatter
)
parser.add_argument(
'--input-file',
default='test.vtu',
help='Input VTK unstructured grid file'
)
parser.add_argument(
'--output-file',
default='out.vtu',
help='Output file for gradient results'
)
parser.add_argument(
'--input-array-name',
default='DistanceToCenter',
help='Scalar field to calculate gradients from'
)
parser.add_argument(
'--quadrature-order',
type=int,
default=3,
help='Quadrature order for gradient calculations'
)
return parser.parse_args()
def calculate_gradients(input_file, array_name, quadrature_order):
"""
Calculate first and second order gradients.
Args:
input_file: Path to input VTU file
array_name: Name of the scalar field for gradient calculation
quadrature_order: Order of quadrature for numerical integration
Returns:
VTK data object with calculated gradients
"""
# Check if input file exists
if not os.path.exists(input_file):
raise FileNotFoundError(f"Input file not found: {input_file}")
# Read the input file
reader = vtk.vtkXMLUnstructuredGridReader()
reader.SetFileName(input_file)
reader.Update()
# Calculate first-order gradient
gradient_filter1 = vtkvmtkUnstructuredGridGradientFilter()
gradient_filter1.SetInputConnection(reader.GetOutputPort())
gradient_filter1.SetInputArrayName(array_name)
gradient_filter1.SetGradientArrayName(f'{array_name}_grad1')
gradient_filter1.SetQuadratureOrder(quadrature_order)
# Calculate second-order gradient
gradient_filter2 = vtkvmtkUnstructuredGridGradientFilter()
gradient_filter2.SetInputConnection(gradient_filter1.GetOutputPort())
gradient_filter2.SetInputArrayName(f'{array_name}_grad1')
gradient_filter2.SetGradientArrayName(f'{array_name}_grad2')
gradient_filter2.SetQuadratureOrder(quadrature_order)
gradient_filter2.Update()
return gradient_filter2.GetOutput()
def save_results(data, output_file):
"""
Save the VTK data to an output file.
Args:
data: VTK data object to write
output_file: Path to output file
"""
# Ensure the output directory exists
output_dir = os.path.dirname(output_file)
if output_dir and not os.path.exists(output_dir):
os.makedirs(output_dir)
# Write the output file
writer = vtk.vtkXMLUnstructuredGridWriter()
writer.SetFileName(output_file)
writer.SetInputData(data)
writer.Write()
print(f"Results successfully written to {output_file}")
def main():
"""Main execution function."""
try:
# Parse command line arguments
args = parse_arguments()
# Calculate gradients
result_data = calculate_gradients(
args.input_file,
args.input_array_name,
args.quadrature_order
)
# Save results
save_results(result_data, args.output_file)
except Exception as e:
print(f"Error: {e}", file=sys.stderr)
return 1
return 0
if __name__ == "__main__":
sys.exit(main())
vmtk_test.zip (88.7 KB)