How do I calculate element volume weighted average of Nodal results in Mechanical?

Ayush Kumar

The average value given by Mechanical is just an average of all the Nodal results. Therefore, it is extremely dependent on the Mesh. If I have an equidistant mesh then it is correct but for the same if I have fine mesh some regions and coarse in others, the result will be wrong, because more number of sampling points are taken from the fine mesh region. Example - same model, same boundary conditions, different mesh - different results:

Ayush Kumar

The solution to this is element volume weighted average. This code expects a named selection with all the elements of interest elements_ns and please adjust line 2 and 7 as per the analysis system index in Mechanical.

Note: This works only for 3D bodies, for surfaces area weighted average can be done using element faces named selection.

# ACT reader
reader = ExtAPI.DataModel.AnalysisList[0].GetResultsData()
volume = reader.GetResult("VOLUME")
temp = reader.GetResult("TEMP")

# Mesh Data
mesh = ExtAPI.DataModel.AnalysisList[0].MeshData

total_volume = 0.0
w_element_temp = []

elements = ExtAPI.DataModel.GetObjectsByName("elements_ns")[0]

for element_id in elements.Ids:
    # nodes of that element
    nodes_element = mesh.NodeIdsFromElementIds([element_id])
    # volume of the element
    element_volume = volume.GetElementValues(element_id)[0]
    
    total_volume += element_volume
    
    # Average element temperature
    average_element_temp = sum([temp.GetNodeValues(node_id)[0] for node_id in nodes_element]) / len(nodes_element)
    
    # Weighted Temperature
    w_element_temp.append(average_element_temp * element_volume)
    
# Average temperature
avg_temp = sum(w_element_temp) / total_volume

# Free rst file
reader.Dispose()

print avg_temp

TerryR

Ayush thanks much for posting that. For others, here is a more user friendly version for calculating the real average temperature. It works off of bodies, and having a named selection is optional.

# -*- coding: utf-8 -*-

# Analysis system index (eg zero if there is just 1 analysis system in the tree)
analysis_system_index = 0

# Optional: Create a Named Selection with only the selected BODIES of interest (eg multiple bodies of same material).
named_sel_name = "Selection"

# ACT reader
reader = ExtAPI.DataModel.AnalysisList[analysis_system_index].GetResultsData()
reader.CurrentResultSet = reader.ResultSetCount  #Get final result. Can show weird values otherwise.
volume = reader.GetResult("VOLUME")
temp = reader.GetResult("TEMP")

# Bodies
all_bodies = ExtAPI.DataModel.Project.Model.Geometry.GetChildren(DataModelObjectCategory.Body, True)

# Mesh Data
mesh_data = ExtAPI.DataModel.AnalysisList[analysis_system_index].MeshData

# If the Named Selection is valid, get just those body ids.
selected_body_ids = []
named_sel = ExtAPI.DataModel.GetObjectsByName(named_sel_name)
if len(named_sel) >= 1:
    selected_body_ids = named_sel[0].Ids

print "Real average temperature for [", ExtAPI.DataModel.AnalysisList[analysis_system_index].Name, "]:"

total_volume = 0.0
total_w_element_temp = 0.0

for body in all_bodies:
    geobody = body.GetGeoBody()

    # Ignore suppressed bodies
    if geobody.Suppressed:
        continue

    # Loop thru each selected body, or all bodies if none selected
    if (geobody.Id in selected_body_ids) or len(selected_body_ids)==0:

        body_volume = 0.0
        body_w_element_temp = 0.0

        #Get body elements
        Elements= mesh_data.MeshRegionById(geobody.Id).ElementIds
        for element_id in Elements:

            # volume of the element
            element_volume = volume.GetElementValues(element_id)[0]

            body_volume += element_volume
            total_volume += element_volume

            # nodes of that element
            nodes_element = mesh_data.NodeIdsFromElementIds([element_id])

            # Average element temperature
            average_element_temp = sum([temp.GetNodeValues(node_id)[0] for node_id in nodes_element]) / len(nodes_element)

            # Weighted Temperature
            body_w_element_temp += average_element_temp * element_volume
            total_w_element_temp += average_element_temp * element_volume
            #print average_element_temp

        # Don't divide by zero on invalid bodies
        if body_volume == 0.0:
            continue

        body_avg_temp = body_w_element_temp / body_volume
        print "Body [", geobody.Name, "]:",body_avg_temp, "°C   ", body_avg_temp *9/5+32, "°F"

# Total Average temperature
if total_volume == 0.0:
    total_volume = -1 # Error
avg_temp = total_w_element_temp / total_volume

# Print if more than one body selected
if avg_temp <> body_avg_temp:
    print "Weighted average by volume for above bodies -", avg_temp, "°C", avg_temp *9/5+32, "°F"

# Free rst file
reader.Dispose()