Forces between molecules make fluid stick together {viscosity}|.
causes
In liquids, van der Waals forces cause viscosity. In gases, non-ideal molecular collisions cause viscosity.
pressure
Gas viscosity increases if pressure increases.
temperature
Temperature increase increases gas viscosity and decreases liquid viscosity.
factors
Fluid viscosity depends on fluid density, pressure, temperature, and velocity. In pipe, pipe-opening size affects viscosity. Intermolecular forces tend to pull fluid sideways in pipes and contribute to turbulence. Fluid sideways pressure P equals viscosity V times velocity change dv divided by length change dl: P = v * dV/dl.
Pipe flow with incompressible fluid has two regions. A thin layer {boundary layer} touches tube or obstruction and has viscous effects, because surface interacts thermally and mechanically with fluid. Center has flow with no turbulence.
Temperature, viscosity, and fluid depth relate {Rayleigh number}. Reynolds number and Rayleigh number together account for flow effects, viscosity, thermal conductivity, linear-expansion or volume-expansion coefficient, fluid depth, and temperature gradient.
Fluids have ratios {Reynolds number} of internal force to viscous force. Reynolds number measures fluid momentum change. If Reynolds number is small, smooth pipe decreases drag, because flow is laminar. If Reynolds number is high, vortices in smooth pipe increase drag.
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Date Modified: 2022.0225