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Does air that is accelerating have more turbulence? It makes sense why air decelerating would make turbulent flow (e.x, the air during pressure recovery over the rear of an airfoil). Would speeding it up make it turbulent? Also, does the air being in a confined space (for example a tube with airflow) have any effect on this? Compared to unconfined airflow, of course.

(All of this assuming there is no object obstructing flow)

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No.

Turbulence means that air has significant speed components in directions orthogonal to its primary direction of movement. A negative pressure gradient accelerates air, and it does so only in the direction of that pressure drop. This means that the whole air picks up speed in one direction only and the speed components orthogonal to this direction become smaller relative to the main speed component. This reduces turbulence.

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  • $\begingroup$ As an example, one way that a wind tunnel reduces (undesirable) turbulence is by pulling air with a fan at its exhaust rather than shoving air into its intake. Thus, the air is being stretched (accelerated) rather than squashed (decelerated). $\endgroup$ Commented Jan 25 at 16:44
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    $\begingroup$ I see, that makes sense, thanks! You really have to watch out where you read info, because I got this whole idea from here. $\endgroup$
    – Wyatt
    Commented Jan 25 at 16:53
  • $\begingroup$ @CamilleGoudeseune yes, and maybe even pulling it out the sides (of the wind tunnel) through little holes too $\endgroup$ Commented Jan 25 at 18:45
  • $\begingroup$ @RobertDiGiovanni, neat! I found porosity for reducing resonances, but not powered flow through the holes "pulling the air out." osti.gov/servlets/purl/1184574 Another question? $\endgroup$ Commented Jan 25 at 19:17
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    $\begingroup$ @CamilleGoudeseune Laminar windtunnels have a very wide cross section and a throat ahead of the test section so air will accelerate and be laminar around the model. And yes, open tunnels will suck in air with a fan at the aft end of the tunnel. $\endgroup$ Commented Jan 25 at 20:10
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Well, airflow is relevant to surrounding air so ... wait, what is turbulence?

disturbed flow in a moving stream of air, manifested by variations in speed an direction

Does accelerating air have more turbulence. One would think not. It is harder to turn when accelerating.

Can accelerating air cause more turbulence in the surrounding air? In the aviation sense, air drawn into the accelerated flow will also be accelerated.

Pressure differential is required to accelerate air. Once the pressure differential "pump" (the wing) is past, there will be turbulence as the entire downwash interacts with even more air and friction inevitably dissipates the energy of the moving air.

Vorticies can last a considerably long time.

does being in a confined space have any effect on this?

yes, because the "confined" space (if it is rigid like a tube) will interact with the moving stream, causing turbulence near the interface as friction slows the fluid stream.

This is spot on with observations of what happens with accelerated air flow over a wing. The boundary layer will become more and more turbulent towards the trailing edge, but the upper portion of the accelerated airstream is only interacting with air.

As a result, the slowing and increased turbulence of the boundary layer eventually causes the low pressure area over the wing to collapse if Angle of Attack exceeds a critical value.

Interestingly, as seen with helicopter rotor wash (acceleration), the upper boundary of wing accelerated airflow also draws air (momentum) from above, contributing to down wash.

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