Does a Rotating Wind Tunnel Impact Airflow on a Turning Vehicle?

  • #1
user079622
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Why this wind tunnel rotate?

 
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  • #2
As the video states, the airflow on the two sides of an automobile have slightly different velocities when the car goes around a turn. I am surprised that the effects of a turn would be so significant in an automobile. For an airplane, there might be more significant differences between the air velocities at the left and right wing tips. I do not know how significant the differences are.
 
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  • #3
FactChecker said:
For an airplane, there might be more significant differences between the air velocities at the left and right wing tips.
Aircraft bank when turning, but at low airspeed, the wing on the inside of the turn will stall first.
FactChecker said:
I do not know how significant the differences are.
Cross-winds are very significant for long-distance truck trailers. The skirt, along the sides below the trailer deck, significantly improves fuel economy by reducing drag.

Front wheel steering, differentially exposes the sides of the vehicle, to the airflow. That exposure is more at the front than the rear. Steering disturbs the normal airflow, just when it is needed to provide maximum down force.

A slightly diagonal model, in a linear flow tunnel, is subjected to the same airspeed effects at the front and the rear. The circular wind tunnel seems to be a compromise approximation, to a dynamic steering event.
 
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  • #4
FactChecker said:
As the video states, the airflow on the two sides of an automobile have slightly different velocities when the car goes around a turn. I am surprised that the effects of a turn would be so significant in an automobile. For an airplane, there might be more significant differences between the air velocities at the left and right wing tips. I do not know how significant the differences are.
Baluncore said:
Aircraft bank when turning, but at low airspeed, the wing on the inside of the turn will stall first.

Cross-winds are very significant for long-distance truck trailers. The skirt, along the sides below the trailer deck, significantly improves fuel economy by reducing drag.

Front wheel steering, differentially exposes the sides of the vehicle, to the airflow. That exposure is more at the front than the rear. Steering disturbs the normal airflow, just when it is needed to provide maximum down force.

A slightly diagonal model, in a linear flow tunnel, is subjected to the same airspeed effects at the front and the rear. The circular wind tunnel seems to be a compromise approximation, to a dynamic steering event.

That must be something about radial pressure gradient, because in real turn pressure gradient is zero
 
  • #5
FactChecker said:
For an airplane, there might be more significant differences between the air velocities at the left and right wing tips. I do not know how significant the differences are.
As Baluncore already said, it can be significant enough to cause "wing drop" when yawing (turning) at low speed since the inside wing effectively gets below its stall speed for the given angle of attack. And dropping a wing at low altitude can lead to crash, like that B-52 crash at Fairchild, so it is serious business.

However, I would venture the guess that "wing drop" can fully be understood to within acceptable accuracy for most planes by applying simple kinematics of the plane combined with the normal lift/drag curves (which is likely to originate back to measurements done in a regular wind tunnel), that is, I have trouble to see a wing tunnel with rotating airflow adding something new to airplane aerodynamics that cannot already be understood by traditional means.

Such a wind tunnel do seem to make more sense for cars because cars can have complex body shapes making accurate CFD airflow analysis more costly, but considering how cheap CFD analysis relative to wind tunnel measurements was already 20-30 years ago it is difficult to see what a rotating wind tunnel can offer, other than perhaps as independent experimental verification of CFD analysis methods for those airflows (I have no idea if "rotational airflow" really requires special yet-to-be-verfied CFD methods to solve, but in case it does it sure would be nice with some experimental verification).
 
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  • #7
Something that does not make sense to me in that January 2016 publication, is how the radius line of the corner, passes through the centre of the vehicle, with the axis of the vehicle perpendicular to that radius line. That assumes symmetry, with both the front and rear wheels steering, equally.
See figure 1 where Yaw; Ψ1 = Ψ2, also figure 16.

I would have expected that, the radius line of the turn, should pass along the rear axle, (of a vehicle with front wheel steering). That then brings the airflow onto the front of the vehicle, from one side, while the air departs from the rear, just beyond parallel to the vehicle axis.
 
  • #8
Baluncore said:
I would have expected that, the radius line of the turn, should pass along the rear axle, (of a vehicle with front wheel steering).
So in terms of fig. 1 you would expect ##\psi_2 \approx 0##? If so I would assume (without having fully read the paper) that the scale model simply have to be placed in a slightly rotated position such that the desired airflow entry or exit angle is approximated. Knowing the radius of curvature and the scale model size and rear axle position calculating such offset angle should be simple geometry. If rotated, it may be that larger models also needs to have their center shifted slightly radially in- or outwards to keep it in the middle of the desired flow.
 
  • #9
Filip Larsen said:
If so I would assume (without having fully read the paper) that the scale model simply have to be placed in a slightly rotated position such that the desired airflow entry or exit angle is approximated.
Rotating the model so that the rear axle line passes through the centre of rotation of the flow, is exactly the same as moving the model forward, by about one third of a vehicle length, to achieve the same thing.

Rotating the model is not shown in figure 1, neither is moving it forward.
 
  • #10
Baluncore said:
Rotating the model so that the rear axle line passes through the centre of rotation of the flow, is exactly the same as moving the model forward, by about one third of a vehicle length, to achieve the same thing.
I agree, but I am not sure where you are going with this?

Any model obviously needs to be fixated in the tunnel in some controlled (or at least measured) position and orientation, which is true for measurements in any wind tunnel, so if the paper does not address this issue I would guess that the authors imply this aspect is just "business as usual" left for the design of the specific experiment for a specific shape in a well-known (i.e. here curved) airflow.
 
  • #11
Baluncore said:
I would have expected that, the radius line of the turn, should pass along the rear axle, (of a vehicle with front wheel steering). That then brings the airflow onto the front of the vehicle, from one side, while the air departs from the rear, just beyond parallel to the vehicle axis.
Can you draw this?
 
  • #12
user079622 said:
Can you draw this?
Yes, quickly. The two situations, advanced and rotated, on the same diagram.
(Whoops, rotated vehicle should have front wheels turned).

Cornering-wind.jpg
 
  • #13
Baluncore said:
Yes, quickly. The two situations, advanced and rotated, on the same diagram.
(Whoops, rotated vehicle should have front wheels turned).

View attachment 342525
What is advanced and what is rotated?
 
  • #14
user079622 said:
What is advanced and what is rotated?
Advance the vehicle in a straight line, until the axis of the back axle, points to the centre of rotation of the airflow.
Rotate the vehicle about its centre of mass, until the axis of the back axle, points to the centre of rotation of the airflow.
 
  • #15
Baluncore said:
Advance the vehicle in a straight line, until the axis of the back axle, points to the centre of rotation of the airflow.
Rotate the vehicle about its centre of mass, until the axis of the back axle, points to the centre of rotation of the airflow.
Straight line and car moves in turn. Why rotate around com. Dont understand.
 
  • #16
user079622 said:
Straight line and car moves in turn. Why rotate around com. Dont understand.
You must be much more specific in your question.

Do you understand that a turning vehicle, with front wheel steering, has air impacting the side of the vehicle at the front, but not impacting the side of the vehicle where the back axle is mounted?
 
  • #17
Baluncore said:
You must be much more specific in your question.

Do you understand that a turning vehicle, with front wheel steering, has air impacting the side of the vehicle at the front, but not impacting the side of the vehicle where the back axle is mounted?
Airflow is perpendicular at all positions at rear axle, is that you want to say?
 
  • #18
user079622 said:
Airflow is perpendicular at all positions at rear axle, is that you want to say?
Perpendicular to what?
 
  • #19
Baluncore said:
Perpendicular to what?
to rear axle
 
  • #20
user079622 said:
Straight line and car moves in turn. Why rotate around com. Dont understand.
Baluncore said:
You must be much more specific in your question.
user079622 said:
Airflow is perpendicular at all positions at rear axle, is that you want to say?
No, I do not want to say anything so restrictive.
I asked you this question, that you have not answered.
Baluncore said:
Do you understand that a turning vehicle, with front wheel steering, has air impacting the side of the vehicle at the front, but not impacting the side of the vehicle where the back axle is mounted?
I was trying to establish what you: "Dont understand".
 

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