Drag crisis

In a series of tests he found that the force loading experienced an abrupt decline at a critical Reynolds number.

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

The super-critical behavior can be described semi-empirically using statistical means or by sophisticated computational fluid dynamics software (CFD) that takes into account the fluid-structure interaction for the given fluid conditions using Large Eddy Simulation (LES) that includes the dynamic displacements of the structure (DLES) [11].

These calculations also demonstrate the importance of the blockage ratio present for intrusive fittings in pipe flow and wind-tunnel tests.

The semi-empirical descriptions of the drag crisis are often described in terms of a Strouhal bandwidth and the vortex shedding is described by broad-band spectral content.

Plot of drag coefficient against Reynolds number for rough or smooth spheres. A sharp drop is observed around Reynolds of 100000 to 1000000 for either.
The drag coefficient of a sphere drops at high Reynolds number (number 5 on the graph). The effect occurs at lower Reynolds numbers when the ball is rough (such as a golf ball with dimples) than when it is smooth (such as a table tennis ball ).