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Validation Case: Drone Propeller Study

This validation case belongs to fluid dynamics. The aim of this test case is to validate the following parameters for a drone propeller:

  • Thrust and power coefficients.

The simulation results of SimScale were compared to UIUC wind tunnel measurements presented in [1].

Geometry

The geometry used for this case can be seen below:

drone propeller geometry
Figure 1: The model used for the validation case is a drone propeller.

It is a two-blade propeller represented using its dimensions in inches. This APC 11×5.5E model is an 11-inch diameter propeller with a pitch of 5.5 inches/revolution used on small UAVs and model aircrafts.

Analysis Type and Mesh

Tool Type: OpenFOAM®

Analysis Type: Steady-state, incompressible analysis with MRF rotating zone, using the k-omega SST turbulence model.

Mesh and Element Types:

The Standard mesher algorithm with tetrahedral and hexahedral cells was used to generate the mesh. Multiple meshes with incrementally increasing refinement were generated in parallel using the automatic meshing functionality.

For each run, the thrust and power coefficients were calculated and plotted below:

mesh sensitivity study testing thrust and power coefficients of propeller
Figure 2: The thrust coefficient remains stable as the amount of cells gradually increases.

A mesh sensitivity analysis has been carried out monitoring the y+ maximum values while using full resolution wall treatment with the standard mesher:

Mesh NameElement TypeNumber of cells/nodesMax y+
Mesh 13D Tetrahedral/Hexahedral4.3M cells, 1.4M nodes3.2
Mesh 23D Tetrahedral/Hexahedral6.1M cells, 2.2M nodes1.5
Mesh 33D Tetrahedral/Hexahedral13.4M cells, 4.1M nodes0.46
Table 1: The characteristics of the mesh sensitivity analysis with respect to the maximum y+ values for 6473 RPM, which is the highest input tested in this case.
mesh details around the propeller
Figure 3: Details of the mesh used for the validation study. With the mesh clip the user can zoom in and check the generated boundary layer, as well as the different refinement levels inside the domain.

Simulation Setup

Material:

  • Air
    • (\(nu\)) Kinematic viscosity: 1.529 \(e^5\ m^2/s\)
    • (\(rho\)) Density: 1.196 \(kg/ m^3\)

Boundary Conditions:

  • Inlet & outlet:
    • Total gauge pressure of 0 \(Pa\)
    • Turbulent kinetic energy with an inlet value of 3.75 \(e^{-3}\) \(m^2 \over\ s^2\)
    • Specific dissipation rate with an inlet value of 3.375 \(1 \over\ s\)
  • Slip walls on the rest of the domain faces to represent an open-air environment

Reference Solution

The UIUC\(^1\) data uses the standard definitions for propeller aerodynamic coefficients:

Thrust coefficient:

\(C_T\) = \(T \over\ (rho \times\ n^2 \times\ D^4)\)

Power coefficient:

\(C_P\) = \(P \over\ (rho \times\ n^3 \times\ D^4)\)

Where:

  • \(rho\): the density of the fluid
  • \(n\): revolutions per sec
  • \(D\): the diameter of the propeller

Result Comparison

Comparison of the thrust and power coefficients obtained from SimScale against the experimental results obtained from [1] is given below:

coefficients of power and thrust comparison for propeller
Figure 4: The graph that involves the power and thrust coefficients reveals a good agreement between the SimScale and wind tunnel results.

The SimScale results appear to be on track with the wind tunnel measurements, and below the discrepancies are calculated for each run:

RPMThrust Coefficient Discrepancy [%]Power Coefficient Discrepancy [%]
18680.42726.5879
40437.08144.6446
647310.60437.8704
Table 2: The discrepancy of the thrust and power coefficients for different RPM inputs increases when the rpm input used is also bigger.

In the following images, the results were analyzed in the post-processor. The streamlines tend to concentrate below the propeller, after being drawn by the propeller’s rotation:

acceleration and concentration of streamlines bellow the propeller
Figure 5: The blades force the air to accelerate downwards, creating an almost homogenous stream traveling across the direction of the rotational axis.

With a rotational axis’s direction towards the negative y direction, the difference in pressure on the blade is created as seen below:

pressure distribution on top and bottom of the propeller
Figure 6: The pressure distribution across the low and high pressure sides of the propeller, as well as on the cutting plane

Finally, the following cutting planes showcase the high velocity streams that are formed longitudinal to the flow.

scooping the air and accelerating the flow with propeller
Figure 7: The propeller scoops the air from the top to the bottom, increasing the velocity of the compressed air.

Note

If you still encounter problems validating you simulation, then please post the issue on our forum or contact us.

Last updated: June 11th, 2021

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