Python Script Tutorial: Nodes and Nodal Data

In this tutorial the access to the nodes stored in a ModelPart and their nodal data will be described. More information about nodes and nodal data can be found here.

Starting

First of all we need to create a python file with following code to import the Kratos, create a ModelPart and read it from input as described in the here :

from KratosMultiphysics import *
import KratosMultiphysics.FluidDynamicsApplication

this_model = Model()
fluid_model_part = this_model.CreateModelPart("FluidPart")

fluid_model_part.AddNodalSolutionStepVariable(VELOCITY)
fluid_model_part.AddNodalSolutionStepVariable(PRESSURE)
fluid_model_part.AddNodalSolutionStepVariable(TEMPERATURE)

fluid_model_part_io = ModelPartIO("path/to/file/example")
fluid_model_part_io.ReadModelPart(fluid_model_part)

fluid_model_part.SetBufferSize(3)

Accessing Nodes

The nodes stored in the ModelPart can be accessed using the Nodes parameter:

model_part_nodes = fluid_model_part.Nodes

Having access to the nodes make iteration over all nodes very easy. For example to print all nodes in the model part:

for node in fluid_model_part.Nodes:
    print(node)

Here is a loop over all of the nodes in a model part, which prints the ID for all of the nodes:

for node in fluid_model_part.Nodes:
    print(node.Id)

Node Coordinates

The coordinates can be accessed by X,Y,Z parameters of the node:

node_x = node.X
node_y = node.Y
node_z = node.Z

Or we can extend the previous example writing also the coordinates of all the nodes in the ModelPart:

for node in fluid_model_part.Nodes:
    print(node.Id, node.X, node.Y, node.Z)

This access is very useful in order to classify the nodes due to their position. For example we can extend the previous loop to write node information exclusively on the nodes with positive X

for node in fluid_model_part.Nodes:
    if(node.X > 0.0): # Printing the ID of all of the nodes with positive X
        print(node.Id, node.X, node.Y)

Nodal Data

The Python interface provides full access to the nodal database. The access to the historical variables is given by GetSolutionStepValue and SetSolutionStepValue passing the variable you want:

node_velocity = node.GetSolutionStepValue(VELOCITY) # node's velocity at the current time step

We can write the velocities of all the nodes:

for node in fluid_model_part.Nodes:
    node_velocity = node.GetSolutionStepValue(VELOCITY) # node's velocity at the current time step
    print(node_velocity)

you can also get a value for n time step ago, where n is the buffer size:

node_previous_velocity = node.GetSolutionStepValue(VELOCITY, 1) # node's velocity at 1 time step ago 
node_earlier_velocity = node.GetSolutionStepValue(VELOCITY, 2) # node's velocity at 2 time step ago

For getting the previous time step velocities of all the nodes:

for node in fluid_model_part.Nodes:
   print(node.GetSolutionStepValue(VELOCITY, 1)) # node's velocity at 1 time step ago

To set the historical value for a variable in a node we can use the SetSolutionStepValue. To make an example let's assume that we want to set the variable TEMPERATURE to the value of 100.0 on the nodes in our ModelPart. This is obtained immediately by typing

for node in fluid_model_part.Nodes:
    node.SetSolutionStepValue(TEMPERATURE,0,100.0)

The command above should be interpreted as: for the node pointed by iterator "it" assign to the variable TEMPERATURE at the current step (the current step is identified by 0) the value of 100.0.

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