Ahmed Body External Aerodynamics Simulation

Objective

Ahmed body is a simplified car model used for studying the flow around it and to predict the drag and lift forces. The model consists of a slanted back and a blunt front.

In this example, PyFluent API is used to perform Ahmed Body external aerodynamics simulation. which includes typical workflow of CFD Simulation as follows:

• Importing the geometry/CAD model.

• Meshing the geometry.

• Setting up the solver.

• Running the solver.

• Post-processing the results.

Import required libraries/modules

from pathlib import Path

import ansys.fluent.core as pyfluent
from ansys.fluent.core import examples
from ansys.fluent.visualization import set_config
import ansys.fluent.visualization.pyvista as pv

Specifying save path

save_path can be specified as Path(“E:/”, “pyfluent-examples-tests”) or Path(“E:/pyfluent-examples-tests”) in a Windows machine for example, or Path(“~/pyfluent-examples-tests”) in Linux.

save_path = Path(pyfluent.EXAMPLES_PATH)

Configure specific settings for this example

set_config(blocking=True, set_view_on_display="isometric")

Launch Fluent session with meshing mode

session = pyfluent.launch_fluent(mode="meshing", cleanup_on_exit=True)
session.health_check.status()

Meshing Workflow

Initialize the Meshing Workflow

workflow = session.workflow
geometry_filename = examples.download_file(
    "ahmed_body_20_0degree_boi_half.scdoc",
    "pyfluent/examples/Ahmed-Body-Simulation",
    save_path=save_path,
)
workflow.InitializeWorkflow(WorkflowType="Watertight Geometry")
workflow.TaskObject["Import Geometry"].Arguments = dict(FileName=geometry_filename)
workflow.TaskObject["Import Geometry"].Execute()

Add Local Face Sizing

add_local_sizing = workflow.TaskObject["Add Local Sizing"]
add_local_sizing.Arguments = dict(
    {
        "AddChild": "yes",
        "BOIControlName": "facesize_front",
        "BOIFaceLabelList": ["wall_ahmed_body_front"],
        "BOIGrowthRate": 1.15,
        "BOISize": 8,
    }
)
add_local_sizing.Execute()

add_local_sizing.InsertCompoundChildTask()
workflow.TaskObject["Add Local Sizing"].Execute()
add_local_sizing = workflow.TaskObject["Add Local Sizing"]
add_local_sizing.Arguments = dict(
    {
        "AddChild": "yes",
        "BOIControlName": "facesize_rear",
        "BOIFaceLabelList": ["wall_ahmed_body_rear"],
        "BOIGrowthRate": 1.15,
        "BOISize": 5,
    }
)
add_local_sizing.Execute()

add_local_sizing.InsertCompoundChildTask()
workflow.TaskObject["Add Local Sizing"].Execute()
add_local_sizing = workflow.TaskObject["Add Local Sizing"]
add_local_sizing.Arguments = dict(
    {
        "AddChild": "yes",
        "BOIControlName": "facesize_main",
        "BOIFaceLabelList": ["wall_ahmed_body_main"],
        "BOIGrowthRate": 1.15,
        "BOISize": 12,
    }
)
add_local_sizing.Execute()

Add BOI (Body of Influence) Sizing

add_boi_sizing = workflow.TaskObject["Add Local Sizing"]
add_boi_sizing.InsertCompoundChildTask()
add_boi_sizing.Arguments = dict(
    {
        "AddChild": "yes",
        "BOIControlName": "boi_1",
        "BOIExecution": "Body Of Influence",
        "BOIFaceLabelList": ["ahmed_body_20_0degree_boi_half-boi"],
        "BOISize": 20,
    }
)
add_boi_sizing.Execute()
add_boi_sizing.InsertCompoundChildTask()

Add Surface Mesh Sizing

generate_surface_mesh = workflow.TaskObject["Generate the Surface Mesh"]
generate_surface_mesh.Arguments = dict(
    {
        "CFDSurfaceMeshControls": {
            "CurvatureNormalAngle": 12,
            "GrowthRate": 1.15,
            "MaxSize": 50,
            "MinSize": 1,
            "SizeFunctions": "Curvature",
        }
    }
)

generate_surface_mesh.Execute()
generate_surface_mesh.InsertNextTask(CommandName="ImproveSurfaceMesh")
improve_surface_mesh = workflow.TaskObject["Improve Surface Mesh"]
improve_surface_mesh.Arguments.update_dict({"FaceQualityLimit": 0.4})
improve_surface_mesh.Execute()

Describe Geometry, Update Boundaries, Update Regions

workflow.TaskObject["Describe Geometry"].Arguments = dict(
    CappingRequired="Yes",
    SetupType="The geometry consists of only fluid regions with no voids",
)
workflow.TaskObject["Describe Geometry"].Execute()
workflow.TaskObject["Update Boundaries"].Execute()
workflow.TaskObject["Update Regions"].Execute()

Add Boundary Layers

add_boundary_layers = workflow.TaskObject["Add Boundary Layers"]
add_boundary_layers.AddChildToTask()
add_boundary_layers.InsertCompoundChildTask()
workflow.TaskObject["smooth-transition_1"].Arguments.update_dict(
    {
        "BLControlName": "smooth-transition_1",
        "NumberOfLayers": 14,
        "Rate": 1.15,
        "TransitionRatio": 0.5,
    }
)
add_boundary_layers.Execute()

Generate the Volume Mesh

generate_volume_mesh = workflow.TaskObject["Generate the Volume Mesh"]
generate_volume_mesh.Arguments.update_dict({"VolumeFill": "poly-hexcore"})
generate_volume_mesh.Execute()

Switch to the Solver Mode

session = session.switch_to_solver()

Mesh Visualization

Solver Setup and Solve Workflow

Define Constants

density = 1.225
inlet_velocity = 30
inlet_area = 0.11203202

Define Materials

session.tui.define.materials.change_create("air", "air", "yes", "constant", density)
session.settings.setup.models.viscous.model = "k-epsilon"
session.settings.setup.models.viscous.k_epsilon_model = "realizable"
session.settings.setup.models.viscous.options.curvature_correction = True

Define Boundary Conditions

inlet = session.settings.setup.boundary_conditions.velocity_inlet["inlet"]
inlet.turbulence.turb_intensity = 0.05
inlet.momentum.velocity.value = inlet_velocity
inlet.turbulence.turb_viscosity_ratio = 5

outlet = session.settings.setup.boundary_conditions.pressure_outlet["outlet"]
outlet.turbulence.turb_intensity = 0.05

Define Reference Values

session.settings.setup.reference_values.area = inlet_area
session.settings.setup.reference_values.density = density
session.settings.setup.reference_values.velocity = inlet_velocity

Define Solver Settings

session.tui.solve.set.p_v_coupling(24)

session.tui.solve.set.discretization_scheme("pressure", 12)
session.tui.solve.set.discretization_scheme("k", 1)
session.tui.solve.set.discretization_scheme("epsilon", 1)
session.tui.solve.initialize.set_defaults("k", 0.000001)

session.settings.solution.monitor.residual.equations["continuity"].absolute_criteria = (
    0.0001
)
session.settings.solution.monitor.residual.equations["x-velocity"].absolute_criteria = (
    0.0001
)
session.settings.solution.monitor.residual.equations["y-velocity"].absolute_criteria = (
    0.0001
)
session.settings.solution.monitor.residual.equations["z-velocity"].absolute_criteria = (
    0.0001
)
session.settings.solution.monitor.residual.equations["k"].absolute_criteria = 0.0001
session.settings.solution.monitor.residual.equations["epsilon"].absolute_criteria = (
    0.0001
)

Define Report Definitions

session.settings.solution.report_definitions.drag["cd-mon1"] = {}
session.settings.solution.report_definitions.drag["cd-mon1"] = {
    "zones": ["wall_ahmed_body_main", "wall_ahmed_body_front", "wall_ahmed_body_rear"],
    "force_vector": [0, 0, 1],
}
session.parameters.output_parameters.report_definitions.create(name="parameter-1")
session.parameters.output_parameters.report_definitions["parameter-1"] = {
    "report_definition": "cd-mon1"
}

session.settings.solution.monitor.report_plots.create(name="cd-mon1")
session.settings.solution.monitor.report_plots["cd-mon1"] = {"report_defs": ["cd-mon1"]}

Initialize and Run Solver

session.settings.solution.run_calculation.iter_count = 5
session.settings.solution.initialization.initialization_type = "standard"
session.settings.solution.initialization.standard_initialize()
session.settings.solution.run_calculation.iterate(iter_count=5)

Post-Processing Workflow

session.results.surfaces.iso_surface.create(name="xmid")
session.results.surfaces.iso_surface["xmid"].field = "x-coordinate"
session.results.surfaces.iso_surface["xmid"] = {"iso_values": [0]}

graphics_session1 = pv.Graphics(session)
contour1 = graphics_session1.Contours["contour-1"]
contour1.field = "velocity-magnitude"
contour1.surfaces_list = ["xmid"]
contour1.display("window-1")

contour2 = graphics_session1.Contours["contour-2"]
contour2.field.allowed_values
contour2.field = "pressure-coefficient"
contour2.surfaces_list = ["xmid"]
contour2.display("window-2")

Simulation Results Visualization

Velocity Magnitude Contour

Pressure Coefficient Contour

Save the case file

save_case_data_as = Path(save_path) / "ahmed_body_final.cas.h5"
session.settings.file.write(file_type="case-data", file_name=str(save_case_data_as))

Close the session

session.exit()

References

[1] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake,SAE-Paper 840300,1984