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Flows and pressure differences

When a fluid flows steadily through a pipe, we observe that its pressure drops in the direction of flow (Movie 1). The stronger the current, the bigger the drop. The relation between the resistive pressure drop (called the resistive pressure difference) and the associated volume current is called the flow characteristic or resistive characteristic (Figure 1). It allows us to calculate flows if we know the associated pressure difference, or vice-versa.

The resistive flow characteristic is linear for highly viscous fluids such as oils (see Figure 1, left). Water flowing through a pipe typically leads to a nonlinear characteristic (Figure 1, right). It is found that linear characteristics are associated with laminar flows whereas the nonlinear type is the result of turbulent flow.

Interpretation

Fluids are viscous. Therefore, they have to be pushed through a horizontal pipe to keep flowing at a certain rate. This means that the pressure must be higher upstream, and lower downstream. In other words, there is a resistive pressure difference along the flowing fluid.