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Is friction drag more or less in laminar boundary layer vs turbulent boundary layer?

It is written in this page of wikipedia:

The laminar boundary is a very smooth flow, while the turbulent boundary layer contains swirls or "eddies." The laminar flow creates less skin friction drag than the turbulent flow, but is less stable.

But in another page, it is said as:

The flow over a body may begin as laminar. As a fluid flows over a surface shear stresses within the fluid slow additional fluid particles causing the boundary layer to grow in thickness. At some point along the flow direction, the flow becomes unstable and becomes turbulent. Turbulent flow has a fluctuating and irregular pattern of flow which are made obvious by the formation of vortices. While the turbulent layer grows, the laminar layer thickness decreases. This results in a thinner laminar boundary layer which, relative to laminar flow, depreciates the magnitude of friction force as fluid flows over the object.

So what's the correct statement?

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  • $\begingroup$ There are duplicates here, probably on the right hand side of this page, but what you want to do, say when designing an aircraft wing, is to take advantage and control the inevitable transition, if you've actually achieved laminar flow, into turbulent flow. $\endgroup$ – user167453 Sep 12 '17 at 12:22
  • $\begingroup$ @Countto10 your comment is not clear, cau you answer exactly the question: "Is friction drag more or less in laminar boundary layer vs turbulent boundary layer?" and explain the inconsistency in two quoted paragraphs $\endgroup$ – S.Serpooshan Sep 12 '17 at 13:02
  • $\begingroup$ That's wikipedia for ya. Poorly written, vague, unreferenced, contradictory, and incorrect. $\endgroup$ – David Hammen Sep 12 '17 at 13:18
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You are focusing on skin friction. This is but one component of parasitic drag, which in turn is but one component of the total drag on an object. You need to look at all aspects of drag, and these vary dramatically based on object shape, surface texture, and speed.

A golf ball has dimples because skin friction is a small component of drag for blunt objects. Form drag (pressure drag) dominates over skin friction in the case of a golf ball. Consider an undimpled golf ball. The flow remains laminar over a good portion of a smooth ball, up to the point where the flow separates from the ball. This separation occurs early in a smooth ball, resulting in a rather large form drag. Now consider a dimpled golf ball. The dimples make the flow turbulent over almost all of the surface. This in turn moves the separation point moves well toward the rear of the ball. Skin friction increases somewhat, but pressure drag is significantly smaller for a dimpled vs undimpled golf ball. The total drag on a dimpled ball is much less than on an undimpled ball.

Now consider a wing. A wing is anything but a blunt object. Form drag is, by design, a minor contributor to the total drag force. Skin friction is much more important for a wing, so a design that maintains a laminar flow wins over a design that results in a turbulent flow.

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    $\begingroup$ In other words, to make it explicit -- the first quote is talking only about skin friction drag whereas the second quote conflates all drag (skin, form) as friction forces. So, no inconsistencies in the quotes, just poor, non-technical terminology. $\endgroup$ – tpg2114 Sep 12 '17 at 13:42
  • $\begingroup$ @tpg2114 and David: i know these types of drag... but the second quote seems some how wrong as it use "friction force"? I never saw that Form drag (= pressure drag) be called "friction", right? $\endgroup$ – S.Serpooshan Sep 12 '17 at 14:45
  • $\begingroup$ @S.Serp It's definitely playing very loose with the terminology. I think to a layman, if you called everything "drag" as "friction," they may not distinguish the difference. I usually go the other way and call all of it, including skin friction, simply "drag" -- unless I need to make distinctions clear, like when talking about why turbulent boundary layers end up causing less drag on a body even though they cause more skin friction. $\endgroup$ – tpg2114 Sep 12 '17 at 15:02
  • $\begingroup$ Nice explanation but there seems to be a bit of a gap in the explanation: Why does more turbulent flow move the separation point toward the rear of the ball? $\endgroup$ – Samuel Weir Sep 12 '17 at 16:56
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    $\begingroup$ @SamuelWeir because in turbulent boundary layer there is an exchange of momentum and energy on a bigger scale compared to a laminar. This can help to prevent reverse flow near the surface and so defer the separation. $\endgroup$ – S.Serpooshan Sep 13 '17 at 3:43
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Turbulent flow gives more (Added as edit LOCAL) friction drag other things being equal. However, the list of "other things"is quite long. When turbulence does increase friction, the basic mechanism is that turbulent mixing inside the boundary layer brings fast-moving air down toward the surface, creating a larger velocity gradient (and therefore more friction) at the surface.

There have been many attempts to design aircraft whose wings will support laminar flow and it is possible to do this under ideal conditions where the wing surface can be kept clean and smooth. In some proposals the boundary layer is removed by sucking it through the surface, which requires the suction ports to stay open. In the real world there are insects, dirt and raindrops that tend to promote turbulence. Generally, in addition to the flow being efficient you want it to be predictable and repeatable, so you may actually prefer turbulent flow.

ADDED (PARTIAL LIST OF "OTHER THINGS" TO BE TAKEN INTO ACCOUNT)

Pressure gradients outside boundary layer. Surface roughness, Surface curvature, flow divergence, heat transfer, mass transfer, Structural vibration, Noise. Temperature.

REASONS TO PREFER TURBULENT BOUNDARY LAYER

To delay separation, to promote mixing, to avoid uncertainties.

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  • $\begingroup$ i think the sentence: "other things being equal" is not true, as for example turbulent flow will defer the separation (less pressure drag) or has better heat transfer exchange properties... But the next part of first paragraph which explain why skin friction is greater seems reasonable. may you edit your post? $\endgroup$ – S.Serpooshan Sep 13 '17 at 3:49

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