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While reading the Wikipedia article on Drag Crisis, I found:

The drag crisis is associated with a transition from laminar to turbulent boundary layer flow adjacent to the object.

While, the Wikipedia article on Turbulence states:

In general terms, in turbulent flow, unsteady vortices of many sizes appear which interact with each other; consequently, drag due to friction effects increases.

If drag (due to friction effects) increases due to turbulence, then why does drag crisis occur when flow shifts from laminar to turbulent? Shouldn't drag be less for laminar as compared to turbulent flow? Why else would surfaces smoothen due to drag over time?

The Wikipedia article on Drag Crisis also goes on to say:

For cylindrical structures, this transition is associated with a transition from well-organized vortex shedding to randomized shedding behavior for super-critical Reynolds numbers, eventually returning to well-organized shedding at a higher Reynolds number with a return to elevated drag force coefficients.

I am having trouble understanding this.


A related question, Drag Crisis and Terminal Velocity?, examines the relationship between drag coefficient and terminal velocity.

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  • $\begingroup$ Why would anybody consider this a crisis? $\endgroup$
    – D. Halsey
    Commented May 16, 2023 at 14:00

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Indeed, the frictional drag associated with a turbulent boundary layer is greater than that of a laminar boundary layer.

But, for a non-profiled object, most of the drag is form drag: the boundary layer separate and the pressure in the wake is much lower than the pressure in front of the object.

In the case of a turbulent boundary layer, the separation of the boundary layer is delayed by more efficient exchanges with the main flow. As a result, the wake behind the object is narrower and the area where the flow looks like a perfect flow is larger.

By exaggerating a little, we can say that we are approaching the situation described by d'Alembert's paradox : no drag for a perfect flow.

Hope it can help and sorry for my poor english.

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Your given link explains this.

The drag crisis is associated with a transition from laminar to turbulent boundary layer flow adjacent to the object. For cylindrical structures, this transition is associated with a transition from well-organized vortex shedding to randomized shedding behavior for super-critical Reynolds numbers, eventually returning to well-organized shedding at a higher Reynolds number with a return to elevated drag force coefficients.

I understand it like this. Vortex shedding frequency depends on Reynolds number, like this :

$$ f_{shedding}(\text{Re}) = \frac {\text{St} \cdot \text{Re} \cdot \nu}{L^2} \tag 1$$

Where $\text{St}$ is Strouhal number, $\nu$ - kinematic viscosity, and $L$ characteristic linear dimension.

For very high vortex shedding frequencies (high $Re$ numbers), flow approaches laminar behavior.

It's like imagine you are going with jeep in a "bumpy" off-road at low speeds, so you feel every pit and clod in the road, but when your driving speed increases sharply, then you feel less jittering in amplitude.

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