Flow Separation

  • …do steady ow ever occur in nature, or have we been pursuing fantasy all along? If steady flows do occur, which ones occur? Are they stable, or will a small perturbation of the flow cause it to drift to another steady solution, or even an unsteady one? The answer to none of these questions is known. (Marvin Shinbrot Lectures on Fluid Mechanics, 1970)

3D Slip Separation

The new theory of flight builds on a new theory of flow separation which we refer to as

  • 3d slip separation

with the following two main cases:

  • 3d rotational separation with point stagnation
  • 3d parallel separation

as illustrated in the following oil film visualization of the surface flow of the flow of air around a landing gear:

landinggear landinggear2

3d rotational separation in the flow around a circular cylinder can be inspected here

We see from a view behind the cylinder a pattern of counter-rotating rolls attaching to the rear of the cylinder with a stagnation point of zero velocity at the center of each roll.  We understand that the pressure inside a roll must be low to accelerate the flow into a rotating motion.

The separation pattern arises from an instability of potential flow separation with opposing flows meeting in the back of the cylinder which develops into a more stable rotational slip separation pattern steered by alternating high and low pressure zones as illustrated in the following figure:

We compare rotational slip separation with point stagnation with the separation pattern of potential flow:

  • irrotational slip separation with line stagnation,

which is connected with a zone of high pressure required to bring the flow to stagnation along a line, as illustrated in the following picture:

We observe in the computational simulation (i) rotational slip separation and (ii) not irrotational potential flow separation, because (i) is a quasi-stable while (ii) is unstable.

The reason (ii) is unstable is the high pressure retarding the flow to line separation and a retarding flow driven by an adverse pressure gradient (pressure increases in the flow direction) is exponentially. On the other hand (i) with low pressure rolls is stable because the adverse pressure gradients are missing.

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1 thought on “Flow Separation

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