Assumptions in Fluid Calculations
In order to use Poiseuille's law in fluid calculations, it is necessary to assume that the flow is laminar and that there is no turbulence. This is often an invalid assumption since many common flow phenomena exceed the critical speeds at which turbulence begins. Even below the critical speed, the existence of irregularities, angular junctions, or obstructions will produce turbulence. When the Bernoulli equation is used to calculate pressures and velocities in constrictions, similar assumptions are made. Even though the Bernoulli equation (being a statement of conservation of energy) remains valid in the case of turbulence, not all the fluid kinetic energy is contributing to the volume flowrate - the effective velocity gained from the fluid kinetic energy cannot be use in the expression "area x velocity = volume flowrate" since part of the kinetic energy is that of random, chaotic motion. Finally, the rms velocity calculated from the velocity profile of the laminar flow is used in the calculation of fluid kinetic energy in the Bernoulli equation applications.
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Index
Poiseuille's law concepts |
