The principle of vortex flowmeter Carmen Vortex Street Production and Phenomenon To illustrate the production of Karman vortex street, we consider the flow of viscous fluid around a cylinder. When the velocity of the fluid is very low, the velocity of the fluid at the front stagnation point is zero, and the flow flows along the left and right sides of the cylinder. In the first half of the cylinder, the velocity gradually increases, the pressure decreases, the velocity in the second half decreases, and the pressure rises. The stagnation speed is zero again. The flow at this time is the same as that of the ideal fluid flow cylinder, and no vortex is generated, as shown in Figure 3-7a. As the inflow velocity increases, the pressure gradient in the second half of the cylinder increases, causing the separation of the fluid interface layer, as shown in Figure 3-7b. When the Reynolds number Re of the incoming stream increases again, reaching about 40, due to the greater retardation of the fluid micelles in the boundary layer of the rear half of the cylinder, a pair is generated at the separation point S of the boundary layer. Symmetric whirlpool with opposite rotation. As shown in Figure 3-7c. Within a certain Re number range of Re, the steady Karman vortex street and vortex shedding frequency are proportional to the fluid velocity.