55. Foil feature

The identification tag for this tutorial is PDS-ABO. Pregenerated input files for this tutorial are found in the folder named PDS-ABO in the provided tutorial input files.

55.1. Tutorial overview

This tutorial covers:

  • Creating a foil feature
  • Input of custom lift and drag coefficients
  • Varying angle of attack
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Fig. 55.1 Simple foil feature model

Note

  • Foil features are used in ProteusDS to represent bodies that will provide lift and drag forces as a function of the angle of attack. Examples of foil feature applications are lifting surfaces or wings on a UAV or glider, or blades on a subsea turbine.
  • The following tutorial will setup a foil feature on a RigidBody DObject that will be kinematically fixed in space using the $Kinematic flag. The RigidBody will be rotated at a constant rate to demonstrate the change in lift and drag forces as the angle of attack is varied.

55.2. Creating the foil feature

  • Create a new project.
  • Create a new RigidBody DObject and call it foilRB.
  • Change the $Kinematic property to 1.

Note

  • Now that the kinematic mode is enabled, the mass and inertia properties of the RigidBody will not be used and can be left as default values.
  • Create a RigidBody foil feature library entry.
  • Call the new foil feature foil_10mSpan.

Note

  • Within the foil feature, reference must be made to the lift, drag and pitch moment tables.
  • To create a foil with a span of 10 m and a chord length of 1 m, set the property $Span to 10 and the property $ChordLength to 1.

Note

  • The finite span effects property allows the user to enable the finite span effects calculation. For more information on this model, please refer to the ProteusDS User Manual. For this example, finite span effects will be left off.
  • The $SpanSegments property sets the number of discretized elements along the length of the span for independent lift and drag calculations. The more segments used, the higher the resolution of total lift and drag will be, at the expense of calculation speed.
  • Set the $SpanSegments property to 20 so that each segment will be 0.5 m.

Note

  • For more advanced foils, the $PitchSlope and $ChordSlope properties can be used to make foils that twist (varying pitch over span) or lengthen/shorten (varying chord length over span).

55.3. Input of custom lift, drag and pitch moment coefficients

Note

  • The lift, drag, and pitch moment coefficients need to be provided in an external (.dat) file. The file must be formatted correctly or the foil feature may not perform as expected.
  • The first row of the external coefficient tables must always start with a 0 value followed by the Reynolds Number values that will correspond to each column of the coefficients tables. Each of the following rows in the file are led with the angle of attack (alpha) value followed by the lift, drag, or pitch moment coefficient for the corresponding alpha and Re values. Values must be provided for the range of alpha values from 0 to 360 degrees. All alpha values should be positive numbers, negative values are not interpolated within ProteusDS.
  • The table format is outlined in Fig. 55.2.
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Fig. 55.2 Example of a foil lift coefficients table

  • Copy the files, NACA0015_CL.dat, NACA0015_CD.dat, and NACA0015_CM.dat from the tutorial required files folder into the current project folder.
  • Enter the properties $FoilCLDataFile NACA0015_CL.dat, $FoilCDDataFile NACA0015_CD.dat, $FoilCMMDataFile NACA0015_CM.dat into the foil feature.

Note

  • Open the coefficient files in a text editor to view the required format for foil coefficient tables.

55.4. Orientation of the foil feature frame

Note

  • The orientation of the foil feature frame is important as it will determine the angle of attack and the direction of the lift and drag forces. The foil feature frame is always set such that the x direction is forward (into flow), the y direction is in the direction of positive lift and the z direction is along the span. The feature orientation can be seen in Fig. 55.3.
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Fig. 55.3 Foil feature frame

  • Add the foil feature to the foilRB RigidBody using the property $Foil foil 10mSpan 0 0 0 -90 0 0.

Note

  • The -90 degree rotation about the x axis is required to orient the foil span parallel to the seabed. Now the foil feature y axis is pointing up as this is the desired direction for positive lift.

55.5. Set the RigidBody state and uniform current

Note

  • Before running the simulation, the RigidBody state must be set and a current must be provided.
  • Set the RigidBody state position to be at (0,0,10) m and the orientation to be at (0,-10,0) deg. Set the rate of rotation to (0,1,0) deg/sec so that the RigidBody will slowly pitch upwards at 1 deg/sec.

Note

  • Slowly pitching the RigidBody will demonstrate the results of changing the angle of attack of the foil.
  • In the environment input file, change the $CurrentProfile to 1, set the $CurrentSpeed to 1 and set the $CurrentHeading to 180. This will create a uniform 1 m/s current in the -x direction, directly into the foil.
  • In the simulation input file, change the simulation length to 20 seconds and then run the simulation.

55.6. Foil feature visualization in PST

Note

  • Unlike other features such as the cylinder feature, the foil feature does not currently visualize in PST. However, running the simulation will create a visualization mesh to use in future simulations.
  • To get the visualization mesh from PST, copy the files foil_10mSpanFoil.mtl and foil_10mSpanFoil.obj from Results/SolverData from the results folder into the input folder.
  • Add a custom mesh feature to the foilRB RigidBody by setting the property $CustomMesh foilCustomMesh 0 0 0 -90 0 0.

Note

  • It is important that the foil custom mesh feature has the exact same location and orientation as the foil feature.
  • Click on the custom mesh feature and press F12 to create the new feature.
  • Turn off $HydroLoading, $WindLoading, $BuoyancyFroudeKrylov, and $SoilLoading.

Note

  • Now that all of the loading properties are turned off, this custom mesh is strictly a visualization mesh that will help to ensure that the foil orientation is correct by visualizing in PST and PostPDS.
  • Set the property $CustomMeshFile foil_10mSpanFoil.obj.
  • Open the visualizer in PST. The foil feature should now be visible.

Note

  • Note that the foil feature visualization will match the span and chord of the specified foil feature, however, the shape of the visualization foil will always be that of a NACA0015 foil regardless of the coefficients used.
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Fig. 55.4 Foil feature in PST visualizer

  • Rerun the simulation. The results will be identical to the previous results.

55.7. Foil feature results

  • Once the simulation is complete, navigate to the foil feature results folder located in Results/foilRB/foil_10mSpan0.
  • The AOA.dat file will present the angles of attack for each segment along the span of the foil. In this case, the angle of attack for all segments will be the same as there is no twist in the foil. This file demonstrates that the angle of attack changes from -10 degrees to +10 degrees over the course of the 20 second simulation.
  • The liftForce.dat, dragForce.dat, and pitchMoment.dat files state the lift, drag force, and the pitch moment for each foil segment. The forces.dat file states the total force in all 6 degrees of freedom for the foil feature in the foil feature frame.

Note

  • The lift force, drag force, and angle of attack can be seen on in Fig. 55.5.
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Fig. 55.5 Foil feature results