Role of Mechanical Dispersion

Fluctuations in local velocity of flow disperse particles mechanically. If at the macroscopic scale, the fluctuations do not correlate, each mechanically perturbed diffusing particle follows a tortuous streamline of the flow we assume would otherwise be straight. During convection, in any interval of time, particles distribute more widely and deeply in both the horizontal and vertical directions. Because only convection creates mechanical dispersion, convection can occur when diffusion is absent.

Boundary layer dispersion interferes with diffusion when particles move slowly near solid boundaries. In experiments we can observe tracer molecules to enter and leave boundary layers both by diffusion and convection. When fluid moves slowly, diffusion is the only mechanism moving particles into or out of boundary regions. Hydrodynamic dispersion may occur as flows stagnate. Taylor dispersion results as convection combines with molecular diffusion (Ref: Mechanical Dispersion).

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