Drag crisis

Between about Rop = 350,000 and 1 X 10 , the drag coefficient drops dramatically in a drag crisis owing to the transition to turbulent flow in the boundary layer around the particle, which delays aft separation, resulting in a smaller wake and less drag. Beyond Re = 1 X 10 , the drag coefficient may be estimated from (Clift, Grace, and Weber) ... 

Drag coefficients may be affected by turbulence in the free-stream flow the drag crisis occurs at lower Reynolds numbers when the free... 

Stream is turbulent. Torobin and Guvin AlChE J., 7,615-619 [1961]) found that the drag crisis Reynolds nnmber decreases with increasing free-stream turbulence, reaching a valne of 400 when the relative turbulence intensity, dejlned as VuVC/jj is 0.4. Here Vu is the rms fluctuating velocity and C/r is the relative velocity between the particle and the fluid. 

As Re increases, the adjacent vortices are elongated and shedding of vortices occurs (Karman s vortices are formed). Finally, for Re > 1000, the remote wake becomes completely turbulent [117]. At the same time, the separation point moves toward the midsection and even a bit farther upstream. For such values of Re, we can speak about a pronounced boundary layer. In a large part of the boundary layer, the flow remains laminar [486], Strong turbulence within the boundary layer occurs for considerably higher Reynolds numbers (Re 2x 105), at which the cylinder drag drops rapidly [117], This phenomenon is called the drag crisis. 

At Re = 1.5 x 105, one can observe the drag crisis characterized by a sharp decrease in the drag coefficient the boundary layer becomes more and more turbulent the separation point shifts abruptly to the aft area. 

Developed turbulence in the boundary layer on a cylinder. Developed turbulence within the boundary layer takes place at higher Reynolds numbers Re = 105 and is accompanied by the drag crisis. According to [522], the cylinder drag first decreases sharply to Cf = 0.3 at Re = 3.5 x 105 and then begins to increase and again enters the self-similar regime characterized by the constant value... 

These relations all form tangents to the curve in Fig. 4.3 in their effective range. Kelbaliyev and Ceylan [6] came up with a single correlation, which would fit the drag curve very nicely for Reynolds numbers smaller than that of the drag crisis. Their relation is in the form of... 

Hypertensive crisis occurs with tyramine-containing foods and some drags (dietary tyramine is normally inactivated in the gut by MAO). Moclobemide has very limited potential for this interaction. 

For Re 300000 the resistance crisis occurs. The drag force depends on wake dimensions and these dimensions also depend upon the position of the separation line between the wake and the boundary layer. As Re increases, the turbulent separation point moves back as a consequence the wake dimensions diminish and the drag coefficient drops sharply from 0.4 to 0.1 (Fig. 20). All of these considerations are reported in detail by Torobin and Gauvin Boulos, Bhattacharyya and... 

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