Fluids in Motion

FLUIDS IN MOTION

Streamlines are hypothetical lines without width drawn parallel to all points to the motion of the fluid. Figure 2.4 illustrates their use in depicting the flow of a fluid past a cylinder. If the flow at any position does not vary with time, it is steady and the streamlines retain their shape. In steady flow, a change in the spacing of the streamlines indicates a change in velocity because, by definition, no fluid can cross a streamline. In the regions on the upstream side of the cylinder, the velocity of the fluid is increasing. On the downstream side, the reverse occurs. The maximum velocity occurs in the fluid adjacent to regions B and D. At points A and C, the fluid is at rest. As velocity increases, the pressure decreases. The pressure field around an object is the reverse of the velocity field. This may appear to contradict common experience. However, it follows from the principle of conservation of energy and finds expression in Bernoulli’s theorem.

FIGURE 2.4 Flow of a fluid past a cylinder.