The Fluid Drag/Power Equation

[ElajRuengies]

Okay so- the Watts and Force required to overcome the drag of a fluid while moving through it is given by-

• PowerDrag = ForceDrag*Velocity = 1/2*Density of Fluid*Velocity^3*Cross Section Area*CoefficientDrag (Source)

And I've seen this equation used in a couple calcs before (here and here)

Now, like in Highschool Physics Class, air resistance is often ignored in fiction, so this equation would not be applicable for whenever a character moves fast through air, otherwise you could get some insane lifting strength values based on speed feats.

However, for instances where characters are chucked through bodies of water, is this an OK equation to use? (This is the blog I'm using it for)

 

[Dark-Carioca]

Huh... Can't say I've seen that formula before. I'm not sure on your question since throwing something through the water isn't quite the same as the other calcs there.

 

[ElajRuengies]

Huh... Can't say I've seen that formula before. I'm not sure on your question since throwing something through the water isn't quite the same as the other calcs there.

The general idea is something moving through water- since in my calc the person was about halfway in water, the cross-sectional area for them was cut in half from 0.68 m2 to 0.34 m2

 

[ElajRuengies]

I will bump this as many times as I have to before someone gives me an answer

 

[Antvasima]

The general idea is something moving through water- since in my calc the person was about halfway in water, the cross-sectional area for them was cut in half from 0.68 m2 to 0.34 m2

@Dark-Carioca @Migue79 @AbaddonTheDisappointment
What do you think about this?

 

[KLOL506]

@DontTalkDT @Executor_N0 @Mr._Bambu @DMUA @Therefir @Wokistan @Armorchompy @DemonGodMitchAubin @CloverDragon03 Thoughts?

 

[DontTalkDT]

I'm not sure if the drag coefficients in air and water are the same.
I'm also absolutely not certain about the influence of being half submerged like in your calc. Fluid dynamics is complicated.

Aside from that I'm fine with using it for reasonably low numbers.
I know of cases where one can use this formula for things to get really crazy really fast. (cough*speed of a pebble thrown 20m through water*cough)
One should definitely use some common sense and remember that turbulent behaviour and water vaporization and stuff make things uncertain if stuff gets high in value.

 

[ElajRuengies]

I'm not sure if the drag coefficients in air and water are the same.

Coefficient of drag remains the same for a given shape regardless of fluid.

I'm also absolutely not certain about the influence of being half submerged like in your calc. Fluid dynamics is complicated.

Half of the cross-section is being moved through water while the other have is being moved through air, ignoring the cross-section in the air because air resistance is ignored, plug in half the cross-section of a human for the Surface Area bit in the equation because that's how much is going through the water.

Aside from that I'm fine with using it for reasonably low numbers.
I know of cases where one can use this formula for things to get really crazy really fast. (cough*speed of a pebble thrown 20m through water*cough)

Yeah, since Power increases with the cube of the Velocity, this equation can get crazy fast, so that's fair

One should definitely use some common sense and remember that turbulent behaviour and water vaporization and stuff make things uncertain if stuff gets high in value.

100% Fair

 

[DontTalkDT]

Coefficient of drag remains the same for a given shape regardless of fluid.

I see, that's fine then.

Half of the cross-section is being moved through water while the other have is being moved through air, ignoring the cross-section in the air because air resistance is ignored, plug in half the cross-section of a human for the Surface Area bit in the equation because that's how much is going through the water.

I get the idea, I'm just not sure if it actually works that way.

Like, the drag coefficient for instance depends on the reynolds number. And that is connected to things like turbulence and other flow behaviour. But flow behaviour on the surface where a medium changes is different than in the same medium. Air and water mixes and waves are created in a different way.
Could be that the influence of such factors is negligible. I'm just saying that I don't know.

 

[ElajRuengies]

I get the idea, I'm just not sure if it actually works that way.

Like, the drag coefficient for instance depends on the reynolds number. And that is connected to things like turbulence and other flow behaviour. But flow behaviour on the surface where a medium changes is different than in the same medium. Air and water mixes and waves are created in a different way.
Could be that the influence of such factors is negligible. I'm just saying that I don't know.

Ah, I getcha-

From what I've been able to gather, the drag increases near the surface of the water due to the fact that movement there creates waves on the surface of the water, which isn't a factor when you're deeper underwater, but it appears that the time increase for swimmers is on the order of hundredths to at most a tenth of a second going by the graph, so I'd say it's mostly negligible, with the calc being a slight lowball

 

[DontTalkDT]

Ah, I getcha-

From what I've been able to gather, the drag increases near the surface of the water due to the fact that movement there creates waves on the surface of the water, which isn't a factor when you're deeper underwater, but it appears that the time increase for swimmers is on the order of hundredths to at most a tenth of a second going by the graph, so I'd say it's mostly negligible, with the calc being a slight lowball

Ah, alright. Then that should be fine.

 

[ElajRuengies]

Ah, alright. Then that should be fine.

 

[ElajRuengies]

So can I get an Ok in the comments of the blog?

 

[ElajRuengies]

Thank you for the evaluation! This thread can be closed now