Some Questions About Drag (for ships in water and airplanes in air)

[Hornbein]

I'm interested in the very basics of drag, both for ships in water and airplanes in air. Here's what I have so far :

Flow of the medium can be laminar or turbulent. As the relative speed of the vessel increases, the more likely the flow is to be turbulent.

Laminar drag increases linearly with speed.
Turbulent drag increases exponentially with speed.
Drag depends linearly on the surface area, all other things being equal.
For a large airplane about half of the energy goes into drag, the rest being the energy needed to provide lift.

Questions : Would it be reasonable to assume that turbulent drag increases with the square of the speed? If not, what are the values for water and air?

How does drag depend on the density of the medium?

How does drag depend on the mass of the boundary layer?

For a Boeing 787, at what airspeed does turbulent flow begin?

For a modern cargo ship, at what speed does turbulent flow begin?

I will be making very rough approximations so subtleties don't matter all that much. I want to keep it very simple.

 

[Filip Larsen]

If you read up on fluid dynamics, drag force, Reynolds number and similar and apply this to the characteristics of the objects and fluids you have in mind, you should be able to begin answering some of your own questions. And, as I am sure you are aware of, if you get stuck with specific questions then showing us how far you got will result in much better help or guideance on the specifics.

 

[Frabjous]

There’s a lot of simple intuition in Shape and Flow by Shapiro.

 

[A.T.]

For a large airplane about half of the energy goes into drag, the rest being the energy needed to provide lift .

What is "energy needed to provide lift"? Is that the energy needed to counter the drag loses of the wings?

In level flight at constant speed and with horizontal thrust all of the engine's thrust goes into countering the total drag.

 

[jrmichler]

The OP is attempting to learn both aerodynamics and hydrodynamics without first learning fluid dynamics. That is difficult, but here are some search terms to get started:

Laminar vs turbulent flow and drag: Search terms Reynolds number flat plate.
Aircraft drag: Search aircraft induced drag. Google will suggest further searches with parasitic in them. Follow those also.
Drag of ships: Search hull speed ratio.
Ships and airplanes: Search skin friction drag.

That should be enough to keep anybody busy for a week or three. Enjoy!

 

[Arjan82]

I would combine turbulence drag and laminar drag into friction drag. The kind of turbulence you are now talking about is in the boundary layer. The boundary layer at some point is either laminar or turbulent (let's leave the difficult topic of transition for what it is now). But a boundary layer over a whole structure can be partly laminar and partly turbulent.

By far most practical designs of aero- or hydrodynamic vehicles are considered to have a fully turbulent boundary layer (if there is some laminar flow, it is irrelevant), certainly passenger airplanes and ships. I don't know about these small drones though, although I would still expect turbulent boundary layer flow for the most part.

So, purely based on fluid dynamics you can only have three kinds of forces:
- shear force
- normal force
- body force
If this force counters the direction of movement it is called a drag. These are the only three ways you can get a fluid parcel to move. Since pressure is a normal force for a fluid parcel, and friction a shear force, two types of drags can only exist in most practical applications:
- frictional drag
- pressure drag
Gravity is a kind of body force (as would be a magnetic force if you are considering a plasma). But for most cases that can be ignored if you are considering drag (wave drag is in essence a pressure drag, but the (gravity) waves themselves can only exist because of gravity, so it is more indirect).

However, since we want to have a better understanding of what causes this drag and how to mitigate them several subcategories are defined. They can have significant overlap and are often domain specific (viscosity induced pressure drag and wave drag is common in ship hydrodynamics, parasitic drag in aerodynamics, etc.):
- viscosity induced pressure drag (i.e. drag due to flow-separation)
- wave drag (ships!)
- lift induced drag
- parasitic drag
- bluff body drag
- surface roughness drag
- interference drag
- frictional drag
- …

But all of the above acts on the body in question by either shear or pressure (or maybe a combination). If you want more details maybe you can find 'Fluid-dynamic drag' of Hoerner from 1965 (old, but the physics is still correct, and very much to the point of the OPs question).

This is to give some guidance, but I agree with @jrmichler: you should know about fluid-dynamics first!