Is Air a Non-Newtonian Fluid at High Speeds?

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SUMMARY

The discussion centers on the classification of air as a non-Newtonian fluid at high speeds, particularly in relation to air resistance and its effects on vehicles. Participants clarify that air, like water, behaves as a Newtonian fluid under normal conditions, exhibiting linear stress response to deformation. The drag force experienced by objects moving through air increases with the square of velocity, particularly as speeds approach Mach 1 (768 mph). The conversation emphasizes that the apparent resistance at high speeds is primarily due to inertia and turbulence rather than non-Newtonian characteristics.

PREREQUISITES
  • Understanding of fluid dynamics principles
  • Familiarity with Newtonian vs. non-Newtonian fluids
  • Knowledge of drag force calculations, specifically FD=0.5(ρACDv²)
  • Concept of Reynolds numbers and their significance in fluid flow
NEXT STEPS
  • Research the properties of Newtonian fluids and their behavior under varying conditions
  • Explore the concept of drag coefficients and their impact on vehicle performance
  • Study turbulence in fluid dynamics and its effects on high-speed motion
  • Investigate the transition from subsonic to supersonic flow and its implications for aerodynamics
USEFUL FOR

Engineers, physicists, automotive designers, and anyone interested in the dynamics of fluid behavior at high speeds.

Anon1000
Hi guys,

I came upon a definition of Non Newtonian fluid that is any fluid which doesn't deform linearly with increasing stress. But then if you think about fluids, pretty much every fluid acts like this, depending on whether you're actually capable of generating enough force to see it happen. Water allows you to stir it pretty easily, but try to shoot a bullet into it, which has enormous speed and mass, and it stops in 2 feet straight. Air, especially when it comes to air resistance and cars, offers not just additional resistance, but exponentially additional resistance at speeds above 200mph, which is why cars need to jump an additional 200hp in power just to eek out an additional 5mph at a 250mph speed.

Anybody else confirm this?
 
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Even in the extreme conditions you describe, Newtonian fluids like water and air locally exhibit a stress tensor which is linearly dependent of the rate of deformation tensor.
 
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Google says this:
A non-Newtonian fluid is a fluid whose viscosity is variable based on applied stress or force. The most common everyday example of a non-Newtonian fluid is cornstarch dissolved in water. Behavior of Newtonian fluids like water can be described exclusively by temperature and pressure.
Cornstarch dissolved in water is a fun example. You can easily submerge your hand in bowl of it with little resistance but if you try to "punch" your fist into it you don't get very far.
Google also says:
Fluids such as water, air, ethanol, and benzene are Newtonian. This means that a plot of shear stress versus shear rate at a given temperature is a straight line with a constant slope that is independent of the shear rate. We call this slope the viscosity of the fluid. All gases are Newtonian.
I am sure if you plotted a graph of horsepower to top speed for a given automobile you would get a reasonably straight line.
 
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jerromyjon said:
I am sure if you plotted a graph of horsepower to top speed for a given automobile you would get a reasonably straight line.
Doesn't air resistance increase as the square of speed?
 
sandy stone said:
Doesn't air resistance increase as the square of speed?
At very low velocities where viscous stresses dominate, the drag force is proportional to speed.
 
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Chestermiller said:
At very low velocities where viscous stresses dominate, the drag force is proportional to speed.
I've been searching for clarification but have not found any, regarding air resistance as you approach mach speed. Is it true that as you approach mach speed there is increased resistance until you pass mach 1 (768mph)?
 
jerromyjon said:
I've been searching for clarification but have not found any, regarding air resistance as you approach mach speed. Is it true that as you approach mach speed there is increased resistance until you pass mach 1 (768mph)?
Are you talking about the drag coefficient?
 
Chestermiller said:
Are you talking about the drag coefficient?
Yes, I think so. I was simply under the impression that air resistance increases drastically up to mach 1 and then it levels back down to a linear approximation...
 
Chestermiller said:
Even in the extreme conditions you describe, Newtonian fluids like water and air locally exhibit a stress tensor which is linearly dependent of the rate of deformation tensor.

Great, if I wanted to hear the official explanation all over again I'd go back to Google. Gee thanks.

jerromyjon said:
Google says this:

Cornstarch dissolved in water is a fun example. You can easily submerge your hand in bowl of it with little resistance but if you try to "punch" your fist into it you don't get very far.
Google also says:

I am sure if you plotted a graph of horsepower to top speed for a given automobile you would get a reasonably straight line.

PRECISELY, so doesn't water experience much of the same effects whether in a belly flop or when you try to shoot a bullet into it? If you put a voluminous enough surface into it at a high enough speed, you do see an exponential increase in force pushing back, i.e. a brick wall.
 
  • #10
Chestermiller said:
At very low velocities where viscous stresses dominate, the drag force is proportional to speed.

Is that because nobody bothered to check with extremely precise instruments for a slight exponential decline in speed per force increase? You should be talking about all velocities, not just the low ones that would fit both a Newton and a non profile.
 
  • #11
Anon1000 said:
Great, if I wanted to hear the official explanation all over again I'd go back to Google. Gee thanks.

PRECISELY, so doesn't water experience much of the same effects whether in a belly flop or when you try to shoot a bullet into it? If you put a voluminous enough surface into it at a high enough speed, you do see an exponential increase in force pushing back, i.e. a brick wall.

Drag force explains your examples.

FD=0.5(ρACDv2), where:

FD = Drag force
ρ(rho) = fluid mass density
A = cross-sectional area
CD = drag coefficient
v = velocity of object relative to fluid

Drag goes up dramatically
  • when transitioning from air to water (water is much higher in density than air)
  • when area is increased (the difference between a properly executed dive and belly flop).
  • when the object is moving fast (drag increases as the square of velocity)
It isn't because air (or water, for that matter) is behaving as a non-Newtonian fluid.
 
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  • #12
Most of the apparently high 'stiffness' of water when first impacted by a solid object traveling at high speed is due to simple inertia effects . Basically you have to move water out of the way for the object to enter the water . This means that you have to accelerate the water around the object . The higher the arrival velocity of the object the higher the acceleration of the water needed and the higher the reaction force on the object is .

Once the object has some or all of it's surface in the water then viscous drag effects also come into play .

(Very much simplified)
 
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  • #13
Anon1000 said:
reat, if I wanted to hear the official explanation all over again I'd go back to Google. Gee thanks.

And if I wanted to make sure I remained ignorant, I'd lead off by insulting the people who were trying to teach me something. Just sayin'.

You seem not to be asking questions any more, but instead are pushing the idea that air is non-Newtonian. There are two prongs to your argument - in #1 you are arguing that air does not meet your expectations for how a Newtonian fluid behaves, and Chestermiller has addressed that. The second, somewhat contradictory prong is #10, where you ask about extremely subtle deviations from Newtonian behavior. This is true, there are small deviations (air is made of molecules, not a continuous fluid) but this is the same argument used against the idealizations of frictionless surfaces, stretchless ropes, ideal gasses perfect resistors, etc. Technically true, but utterly sterile when it comes to understanding nature.
 
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  • #14
Chestermiller said:
Are you talking about the drag coefficient?
The more I think about it the more I think it is turbulence I was asking about...
 
  • #15
jerromyjon said:
The more I think about it the more I think it is turbulence I was asking about...
Well, that’s very different from talking about non-Newtonian behavior. Newtonian fluids certainly do exhibit turbulent flow at high Reynolds numbers.
 
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