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The Stokes number (Stk), named after dimensionless number characterising the behavior of particles suspended in a fluid flow. The Stokes number is defined as the ratio of the characteristic time of a particle (or droplet) to a characteristic time of the flow or of an obstacle, or
where t_0 is the relaxation time of the particle (the time constant in the exponential decay of the particle velocity due to drag), u_0 is the fluid velocity of the flow well away from the obstacle and l_0 is the characteristic dimension of the obstacle (typically its diameter). A particle with a low Stokes number follows fluid streamlines (perfect advection), while a particle with a large Stokes number is dominated by its inertia and continues along its initial trajectory.
In the case of Stokes flow, which is when the particle (or droplet) Reynolds number is low enough that the particle drag coefficient is inversely proportional to the Reynolds number itself, the characteristic time of the particle can be defined as
where \rho_d is the particle density, d_d is the particle diameter and \mu_g is the gas dynamic viscosity.^{[1]}
In experimental fluid dynamics, the Stokes number is a measure of flow tracer fidelity in particle image velocimetry (PIV) experiments where very small particles are entrained in turbulent flows and optically observed to determine the speed and direction of fluid movement (also known as the velocity field of the fluid). For acceptable tracing accuracy, the particle response time should be faster than the smallest time scale of the flow. Smaller Stokes numbers represent better tracing accuracy; for \mathrm{Stk} \gg 1, particles will detach from a flow especially where the flow decelerates abruptly. For \mathrm{Stk} \ll1, particles follow fluid streamlines closely. If \mathrm{Stk} \ll 0.1, tracing accuracy errors are below 1%.^{[2]}
For example, the selective capture of particles by an aligned, thin-walled circular nozzle is given by Belyaev and Levin^{[3]} as:
where c is particle concentration, u is speed, and the subscript 0 indicates conditions far upstream of the nozzle. The characteristic distance is the diameter of the nozzle. Here the Stokes number is calculated,
where V_{s} is the particle's settling velocity, d is the sampling tubes inner diameter, and g is the acceleration of gravity.
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