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MultiSurface Aerodynamics Hydrofoil Calculations

The lifting forces computed by MultiSurface Aerodynamics for a hydrofoil at a finite depth submergence were compared to the experimental data found in NACA Report 1232, "A theoretical and experimental investigation of the lift and drag characteristics of hydrofoils at subcritical and supercritical speeds." by K.L. Waldin et. al. The results were obtained for the subcritical hydrofoils only (MultiElement Airfoils does not consider supercritical hydrofoils).

To simulate finite depth, an image hydrofoil was used to model the free surface at the air-water interface. This was implemented in MultiSurface Aerodynamics as a bi-plane (see Fig 1. below). Using the method of images, MultiSurface can be used to analyze general hydrofoil shapes in shallow water. The experiments were carried out for two different hydrofoils; one with an aspect ratio of 10 and the other with an aspect ratio of 4 (presented in Part 2). In all cases, the NACA 64A-412 airfoil was used as the hydrofoil cross section.


Fig 1. Biplane arrangemet to simulate free surface.
The top wing models the free surface

The hydrofoil was simulated in MultiSurface Aerodynamics using a grid of 16 X 24. This grid was adequate to obtain a good definition of the airfoil camber line. The NACA 64A-412 airfoil was generated using the NACA 6-digit airfoil generator built into the software.

In Part 1, we will look at the comparisons of the lift for the hydrofoil with aspect ratio =10. The depth of submergence of .84 and 3.84 of the chord length were considered.

Results for the Aspect Ratio=10 Hydrofoil at a depth of 0.84*Chord.

Lift at Speed= 20 fps
AOA   MultiSurface Aerodynamics    Experiment     % Difference  

0.0

381 lb

390 lb

2.6 %

2.0

614 lb

620 lb

1.0 %

4.0

845 lb

890 lb

5.0 %

Lift at Speed = 30 fps
AOA   MultiSurface Aerodynamics    Experiment     % Difference  

0.0

857 lb

870 lb

1.5%

2.0

1380 lb

1420 lb

2.8 %

4.0

1902 lb

2000 lb

4.9%

Results for the Aspect Ratio=10 Hydrofoil at a depth of 3.84*Chord.

Lift at Speed= 20 fps
AOA   MultiSurface Aerodynamics    Experiment     % Difference  

0.0

435 lb

480 lb

9.4%

2.0

721 lb

750 lb

3.9 %

4.0

1006 lb

1100 lb

8.5%

Lift at Speed = 30 fps
AOA   MultiSurface Aerodynamics    Experiment     % Difference  

0.0

979 lb

1050 lb

6.76%

2.0

1622 lb

1720 lb

5.7 %

4.0

2263 lb

2450 lb

7.6%

Conclusions
The results show that MultiSurface Aerodynamics obtained good results for both depths of submergence. The bi-plane model also proved to effectively model the lift developed by a hydrofoil near the surface.

Other Results
Please click here for another comparison with experiment.
Please click here to view Screen Shots of MultiSurface Aerodynamics.

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