Talk:Flow separation

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Boundary layer separation[edit]

"An example of this is the Tacoma Narrows Bridge. The shedding vortices reached the resonance frequency of the bridge and ultimately caused it to collapse"

TACOMA bridge failure IS NOT due to a resonance but to a flutter phenomenon: see "Tacoma Narrows Bridge" on Wikipedia:

"The bridge's spectacular self-destruction is often used as an object lesson in the necessity to consider both aerodynamics and resonance effects in civil and structural engineering. However the effect that caused the destruction of the bridge should not be confused with forced resonance (as from the periodic motion induced by a group of soldiers marching in step across a bridge).[8] In the case of the Tacoma Narrows Bridge, there was no periodic disturbance. The wind was steady at 42 mph (67 km/h). The frequency of the destructive mode, 0.2 Hz, was neither a natural mode of the isolated structure nor the frequency of blunt-body vortex shedding of the bridge at that wind speed. The event can only be understood while considering the coupled aerodynamic and structural system which requires rigorous mathematical analysis to reveal all the degrees of freedom of the particular structure and the set of design loads imposed"

Technically the "effects of separation" section is dealing more with wakes from bluff bodies than boundary layer separation. Also, a Karman vortex street is a low-Reynolds number phenomenon, and vortex shedding != Karman vortex street (however, Karman vortex streets are a subset of vortex shedding). —Preceding unsigned comment added by 158.39.25.27 (talk) 10:32, 3 July 2008 (UTC)[reply]

"The viscosity of the fluid causes the boundary layer separation.[1] It is well known that as the Reynolds number increases, the likelihood of the boundary separating increases." -- While true for laminar flow, this could easily be misunderstood since high reynolds numbers normally imply turbulent flow rather than laminar flow. Generally, turbulent flow is _less_ likely to separate than laminar flow is, around similar objects. —Preceding unsigned comment added by 163.1.207.136 (talk) 22:04, 6 November 2009 (UTC)[reply]

What is flow separation? The article does not say. — Preceding unsigned comment added by 88.91.45.128 (talk) 09:05, 23 October 2013‎

I would argue that the flow in the second picture is separated in all three instances because an inflection point in the profile can be seen. select a better picture from the hordes when googling 'flow separation'. — Preceding unsigned comment added by 86.137.83.225 (talk) 10:39, 28 March 2014‎

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Displacement thickness[edit]

"When the boundary layer separates, its displacement thickness increases sharply," The way this is worded suggests to me that "displacement thickness" also applies after separation. Can someone clarify? Cheers.Pieter1963 (talk) 01:55, 22 February 2020 (UTC)[reply]

Whereabouts in the article does this quote come from? I haven’t found it. Dolphin (t) 04:42, 23 February 2020 (UTC)[reply]

In section "Effects of bl separation"Pieter1963 (talk) 16:52, 23 February 2020 (UTC)[reply]

The article Boundary layer thickness describes several analytical methods for determining the thickness of the boundary layer; “displacement thickness” is one such method, “momentum thickness” is another. All these methods will indicate a sharp increase in BL thickness beyond the separation point. I think this article (Flow separation) could be improved by eliminating the word “displacement” because the point being made is equally true of momentum thickness, energy thickness etc. Perhaps the first sentence should say “When the BL separates, its effective thickness increases sharply.”

These analytical methods can be used to determine BL thickness at each point downstream from the leading edge of the solid body. These methods can continue to be applied beyond the separation point. Where the BL is attached to the solid surface the flow can be considered “efficient”, causing only a small drag force and having a relatively minor effect on the flow of the fluid past the solid body. Where the BL is separated from the solid surface the flow is much less “efficient” - separation causes a significant increase in (pressure) drag and significantly alters the effective shape of the solid body, partly because the effective thickness includes the thickness of the layer of reversed flow that is trapped between the BL and the solid surface. Dolphin (t) 23:12, 24 February 2020 (UTC)[reply]

The article says there are two types of bl, laminar and turbulent. As the bl thickness "exists" beyond separation should it say 3 types: laminar,turbulent and separated?Pieter1963 (talk) 20:52, 25 February 2020 (UTC)[reply]

The distinction between laminar and turbulent applies to fluid flows in general, not just the boundary layer. Boundary layers can be described as either attached or detached (separated). The point (in two-dimensional flow) at which the boundary layer becomes detached is the separation point.

I will look closely at the article. You may be right in pointing out that “two types of boundary layer - laminar and turbulent” may be clumsy and in need of refinement. Dolphin (t) 14:17, 26 February 2020 (UTC)[reply]

Thank you for your replies. Does the term bl still apply at the surface under the separated region or not because there is no general drift in the streamwise direction?Pieter1963 (talk) 19:18, 26 February 2020 (UTC)[reply]

I have never seen the term BL applied to the region of reversed flow, stagnant fluid etc that lies between the solid surface and the fluid moving in the streamwise direction. For the purpose of contemplating the BL, determining its thickness, velocity distribution profile etc, the streamwise flow of the fluid including an attached BL must be considered steady flow. The motion of the fluid between the solid surface and the streamwise flow is unsteady. I think a detached BL could also only be considered unsteady. Dolphin (t) 23:03, 26 February 2020 (UTC)[reply]

I have just found this: https://www.aps.org/units/dfd/resources/upload/prandtl_vol58no12p42_48.pdf In fig.3 there are 2 light blue streams (Anderson calls them dark regions). He calls them shear layers (boundary region between 2 flows of widely different velocities, page 997 in his book: https://ia600906.us.archive.org/12/items/FundamentalsOfAerodynamics6thEdition/Fundamentals%20of%20Aerodynamics%2C%206th%20Edition.pdf So perhaps it's best not to refer in the article to the different bl thicknesses once the bl has separated and become a shear layer.Pieter1963 (talk) 01:21, 28 February 2020 (UTC)[reply]

I agree. Once the BL has separated, any analytical determination of thickness is an effective thickness, not the thickness of a layer as shown in diagrams of the classical layer. A separated BL is an unsteady flow region that must be random in shape and ultimately mixing with the surrounding regions. Feel free to eliminate mention of the thickness of the detached (separated) BL. Dolphin (t) 03:43, 29 February 2020 (UTC)[reply]