| | When a fluid enters a region where it flows faster than in the region it comes from, its pressure drops (Movies 1 and 2, Figure 1, see the drop in pressure from B to D, from the inside of the tank to the beginning of the pipe). This is most easily seen in Photo 1 if we connect the water levels in the riser pipes along the outlet pipe. Extrapolating the level (pressure) to the tank shows what the pressure of the water in the outlet pipe right at the outlet is lower than the pressure at the bottom of the tank. When it enters a region where it flows more slowly, the pressure rises. Changing flow speeds are the result of narrowing or widening of the channel through which the fluid flows.
Interpretation Consider water flowing from a wide tank into a thin pipe, or blood flowing from the heart into the aorta. The same flow must pass through a narrower conduit than before. Liquids are incompressible, so they must speed up. To make a fluid speed up from one point to another along its path, it must be pushed from behind (like we have to push bodies so they speed up). So the pressure is higher where the fluid comes from. In the case of a decrease of flow speed, the pressure is lower at the point from where the fluid comes. | |
