Inspired by this video, what happens when 2 air streams beside each other going the same direction, but one going faster, interact? Do they make turbulence and or vortices as said in the video?

I’m sure turbulence is created because of what the video said, but why is it created, and is it possible for vortices to also form?

(When I say beside each other, I mean the air streams right beside each other, not separated by any gap)

  • $\begingroup$ "going the same direction, but one going faster": For this matter, the direction doesn't count, only the absolute value of the velocity difference. $\endgroup$
    – mins
    Feb 29 at 18:31

2 Answers 2


Look at this image of a turbine core exhaust as it mixes with the slower airflow that bypasses the core. You can see the shear layer from the high speed difference grow into small scale instabilities, then become larger as mixing progresses. There are vortices at numerous locations in the flow.

Mixing occurs as the fluids interact with each other. Right at the nozzle the flows are already mixing because molecules even in "stationary" fluids are moving in all directions and easily cross the boundary between the flows, although in a very thin layer at this point. As molecules from the slow flow collide with those from the fast flow and vice versa, the thickness of the layer of colliding molecules continues to grow until the energy differential has been eliminated. In a fully mixed flow all the molecules are back to being in a "stationary" state; there are no longer two discernable flows.

enter image description here

  • $\begingroup$ Outstanding picture. On can see that once the faster flow loses velocity (and the surrounding air gains velocity), some serious mixing (turbulence) occurs. Near the outlet, the high velocity air will draw surrounding air into the stream, but eddys will not form until further downstream. "There are no longer 2 discernable flows", but rotational energy in the form of vorticies can remain. $\endgroup$ Feb 26 at 20:31

Two streams next to one another form a boundary layer -- just like a boundary layer between a flow and a solid wall.

In the 'middle' of a flow, nearby velocities must be parallel. Hopefully this is intuitive, if they aren't parallel, then the conservation of mass is violated.

When the velocities are parallel, but different magnitude, a shear force exists. Imagine a block (cube) of flow. One face has velocity $V$, the opposite face has $V+\Delta V$. If you think of the cube as a block of jello, it will shear due to the $\Delta V$.

Those shear forces related to $\Delta V$ are viscous in origin. It is the friction between the 'layers' of flow that cause the shear force. The shear force tries to slow down the fast stream -- and speed up the slow one.

In the middle of most flows, these $\Delta V$ are all small and we can generally think of the bulk of the flow as inviscid. These shear forces are generally small everywhere.

At a wall, the $\Delta V$ is large -- the velocity at the wall is zero, so a strong boundary layer forms where the effects of friction are strong and turbulence may appear.

When you have two streams of flow that meet (and are parallel), a shear layer will form.

Previously, I have ignored viscosity and I have described this to you as a slip line. I.e. if we let the flow 'slip' over one another, we allow the $\Delta V$ to exist without viscoscity.

When we consider that friction exists, we observe the formation of a shear layer. Like a boundary layer on a body, it will grow in thickness as it moves downstream. It is trying to speed up the slow flow -- and slow down the fast flow.

  • $\begingroup$ Ah I see, thanks for your answer. So a shear layer is created like you said, but does that necessarily mean that turbulence is created? $\endgroup$
    – Wyatt
    Feb 26 at 18:16
  • $\begingroup$ No, you can have a laminar shear layer under certain circumstances. $\endgroup$ Feb 26 at 19:48
  • $\begingroup$ Oh okay yeah that makes sense. Also, I’m assuming that the Re number has something to do with it? $\endgroup$
    – Wyatt
    Feb 27 at 4:02

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