# Difference between viscuous force and drag force?

What is the difference between viscuous force and drag force? Are they the same?

Andy Resnick
As far as I know, the terms are used interchangeably.

Gold Member
Drag comes from a number of sources. The viscous force on a surface is one of those components of drag.

Chestermiller
Mentor
Drag comes from a number of sources. The viscous force on a surface is one of those components of drag.
Yes. The OP should Google "form drag" and see what comes up.

Chet

Drag comes from a number of sources. The viscous force on a surface is one of those components of drag.
Oh I see...but what really, is the difference between them? They seem to be same.. I usually see the word "drag" used when in air, whereas the word "viscous" used in liquid

Chestermiller
Mentor
Oh I see...but what really, is the difference between them? They seem to be same.. I usually see the word "drag" used when in air, whereas the word "viscous" used in liquid
Viscous is used for both liquids and gases. Form drag is the part of the drag force that comes in when the body converts "dynamic pressure/kinetic energy" of the fluid flowing by to static pressure at its leading bluff edge.

Chet

Gold Member
Oh I see...but what really, is the difference between them? They seem to be same.. I usually see the word "drag" used when in air, whereas the word "viscous" used in liquid

The term "drag" means the force exerted on a body in motion through a fluid that opposes the motion. There are a number of sources of drag, one of which is the action of viscosity near the surface. There are other sources, though, such as the pressure difference from the front of the object to the back or shock waves or wing vortices on a plane. These all make up drag. Viscous drag is simply a part of that.

sophiecentaur
Gold Member
2020 Award
I can't find a specific mention of the term Turbulence here and it becomes a factor in drag once the laminar flow breaks up ( at speed) it's a more everyday term than Vortices. Viscous drag is there all the time- even at low speed. Delaying turbulence allows higher speeds with same engine power.

Gold Member
I can't find a specific mention of the term Turbulence here and it becomes a factor in drag once the laminar flow breaks up ( at speed) it's a more everyday term than Vortices. Viscous drag is there all the time- even at low speed. Delaying turbulence allows higher speeds with same engine power.

Turbulence itself doesn't cause drag. Turbulence causes additional momentum to be drawn lower to the wall in the boundary layer, resulting in larger viscous shear stress and therefore larger viscous drag. However, it is still viscous drag.

Chestermiller
sophiecentaur
Gold Member
2020 Award
I don't know that we are talking about the same thing but turbulence 'extracts' energy from a moving object. That corresponds to work done and, hence a drag force. Energy is needed to accelerate the fluid and that energy is not returned.
Is there a difference in terminology between us?

Gold Member
I don't know that we are talking about the same thing but turbulence 'extracts' energy from a moving object. That corresponds to work done and, hence a drag force. Energy is needed to accelerate the fluid and that energy is not returned.
Is there a difference in terminology between us?

There must be because that is not the meaning of turbulence. Even a laminar flow can extract energy from a moving object. That is exactly the nature of viscous drag. Turbulence (not so) simply increases the magnitude of this action.

sophiecentaur
Gold Member
2020 Award
Hmm. So you are saying that turbulence ( which is the random formation of vortices?) does not absorb energy?
Should I abandon the use of the description of liquid flow in terms of laminar flow ( low velocity) and turbulent flow (velocity above a threshold value)?
Perhaps I am misunderstand what you seem to be saying.

Gold Member
Hmm. So you are saying that turbulence ( which is the random formation of vortices?) does not absorb energy?
Should I abandon the use of the description of liquid flow in terms of laminar flow ( low velocity) and turbulent flow (velocity above a threshold value)?
Perhaps I am misunderstand what you seem to be saying.

Well the best I can answer there is "sort of." Yes, turbulence involves the formation of vortices, but in a very specific fashion. Yes it involves the dissipation of energy, but a laminar flow is also dissipative, just less so.

On the other hand, the demarcation between laminar and turbulent flow has only a little to do with velocity. Yes, transition tends to correlate well with increasing Reynolds number (##Re = \rho v \ell/\mu##) but there is no general Reynolds number that signifies the boundary; it is very situation-specific (the exception being most pipe flows). More so, supersonic flows tend to be more stable, particularly as the Mach number gets late. You can certainly have an object traveling at, say, Mach 8, where the flow around it remains laminar, but transitions once it slows down to Mach 2.

So yes, I think you definitely should at least abandon your description in terms of high and low velocity. The rest of what you said is true, though. You just have to be careful because all viscous flows are dissipative (so turbulence simply increases viscous dissipation and drag) and the formation of vortices alone does not imply a turbulent flow.

The term "drag" means the force exerted on a body in motion through a fluid that opposes the motion. There are a number of sources of drag, one of which is the action of viscosity near the surface. There are other sources, though, such as the pressure difference from the front of the object to the back or shock waves or wing vortices on a plane. These all make up drag. Viscous drag is simply a part of that.
Oh I see.. Thanks!

sophiecentaur
Gold Member
2020 Award
My elementary Fluid Dynamics is obviously not good enough! Surprise surprise.
The non- monotonic relationship for supersonic flight was a revelation but I guess an object that's short and at high speed can produce odd effects on wave formation by front and back sections.
Intuition really sucks in this field.

Gold Member
My elementary Fluid Dynamics is obviously not good enough! Surprise surprise.
The non- monotonic relationship for supersonic flight was a revelation but I guess an object that's short and at high speed can produce odd effects on wave formation by front and back sections.
Intuition really sucks in this field.

Intuition is great in fluid mechanics assuming you ignore viscosity. As soon as you add viscosity into the equation (literally), everything gets hairy and sometimes feels bass ackward. As an example, during re-entry, a typical spacecraft enters the atmosphere somewhere just shy of Mach 25 and the boundary layer over the surface is typically laminar. Transition generally occurs somewhere lower in the atmosphere where the Mach number is somewhere closer to the ##5 \leq M \leq 10## range, if I recall correctly.

Even more fun is that different parts of the body will transition at different times and due to different mechanisms. The transition from laminar to turbulent flow on the wings of the space shuttle, for example, has a totally different character than that under the body. What is even more interesting (to me at least) is that while the mechanism leading to transition on the underside of the fuselage of the space shuttle at Mach 10 is completely different than the mechanism leading to transition on the fuselage of, say, a Boeing 737 at Mach 0.8. However, the mechanism dominating the process on the shuttle wings at Mach 10 is nearly identical to that on the 737's wings at Mach 0.8. It's a really fun phenomenon.

Also, I'd hardly call this topic elementary, so don't be hard on yourself. They generally don't really touch on it until graduate school for those studying fluid mechanics. This is also intimately related to the reason why CFD can do a really good job at predicting lift, but not drag.