How to determine reference area for submarine drag coefficient?

I’m trying to find the drag coefficient of a submarine model I made from a frontal orientation, so I set up the simulation accordingly and then got to work with the moments and forces coefficient module. However, after carefully reading just about everything I could find on SimScale and other sources about how to calculate the reference area for your drag coefficient equation, it’s still not entirely clear to me which faces should go into calculating the reference area for my submarine. Almost everything says to just project the frontal area of the object onto a 2d plane and calculate the area of the resulting 2d sketch for a non-airfoil body, but that doesn’t make sense for my case; if I were to do that, the forward profiles of the propeller, nozzle, and stabilizing planes wouldn’t be taken to account because from the front, they’re completely obscured, and effectively wouldn’t exist to create drag at all. If I’m correct, that means if I were to replace the propeller, nozzle, and planes with the most drag inefficient part possible with a surface area equal to the sum of the three aforementioned parts, the drag coefficient would come out the same as long as said part was fully obscured from the front, which doesn’t make any sense. Does anyone have any suggestions?

Here’s the link to my project: SimScale Login

Hello @MassimoDCCC , and thanks for posting your question on the Forum!

If historical conventions exist within the field of submarine hydrodynamics, it would be better to follow these defined practices to select the correct reference area. Similarly, if your work aligns with any experimental studies or academic research papers, it’s essential to stick to the conventions used in those experiments and studies.

Therefore, I recommend checking relevant literature or any experiments that may inform the selection of surfaces for the reference area calculation. This approach will help ensure that your assessment of the drag coefficient aligns with previous studies.

If you’re only seeking a method to compare non-dimensional coefficients within your own studies, you can choose one surface area and stick to the same convention throughout your research. However, it’s important to maintain consistency with parameters. For instance, if you were investigating the aerodynamics of an aircraft, it would be logical to non-dimensionalize forces using parameters such as the wing’s span, mean aerodynamic chord length, wing surface area, etc.

I hope this information will be helpful!

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Thank you for the reply. It does seem that the projected frontal area is conventionally used as the reference area, but that does leave me with the question of how the drag of parts obscured by the frontal area are taken into account; water obviously doesn’t just disappear after flowing past the frontal area, and those obscured parts are going to create drag that’ll affect the submarine’s drag coefficient unless I’m misunderstanding something. Perhaps I should take the projected frontal area of those obscured parts and add their reference areas to the reference area of the initial projected frontal area? That would mean that all faces opposing the water flow are taken into account, but I’m not sure if that would work

Not including the surface area of parts hidden from the frontal view when dimensionalizing doesn’t imply neglecting their impact on drag computation. This process merely involves converting dimensional forces into non-dimensional forms for more convenient comparison.

CFD equations will still calculate the total drag force, while the chosen surface area will solely be utilized to non-dimensionalize these forces. Your drag force value remains unchanged.

I see, thank you for clearing this up! I’m still very new to hydrodynamics and CFD, and I appreciate your explanation.

Glad to hear that answers your question Massimo! :slight_smile:

Please feel free to post another question in case anything comes up; we’d be happy to help!