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Basics of Fluid Mechanics, 2014a

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9.4. SUMMARY OF DIMENSIONLESS NUMBERS 311<br />

Table -9.8. Common Dimensionless Parameters <strong>of</strong> <strong>Fluid</strong> <strong>Mechanics</strong> (continue)<br />

Standard System<br />

Name Symbol Equation Interpretation Application<br />

Prandtl<br />

Number<br />

Pr<br />

ν<br />

α<br />

viscous diffusion rate<br />

thermal diffusion rate<br />

Prandtl is fluid<br />

property important<br />

in flow due to<br />

thermal forces<br />

Reynolds<br />

Number<br />

Re<br />

ρUl<br />

μ<br />

inertia forces<br />

viscous forces<br />

In most fluid mechanics<br />

issues<br />

Rossby<br />

Number<br />

Ro<br />

U<br />

ωl 0<br />

inertia forces<br />

Coriolis forces<br />

In rotating fluids<br />

Shear<br />

Number<br />

Sn<br />

τ c l c<br />

μ c U c<br />

actual shear<br />

“potential” shear<br />

shear flow<br />

Stokes<br />

Number<br />

Stk<br />

t p<br />

t K<br />

particle<br />

relaxation<br />

time<br />

Kolmogorov time<br />

In aerosol flow<br />

dealing with<br />

penetration <strong>of</strong><br />

particles<br />

Strouhal<br />

Number<br />

St<br />

ωl<br />

U<br />

“unsteady” effects<br />

inertia effect<br />

The effects <strong>of</strong><br />

natural or forced<br />

frequency in all<br />

the field that is<br />

how much the<br />

“unsteadiness” <strong>of</strong><br />

the flow is<br />

Taylor<br />

Number<br />

Ta<br />

ρ 2 ω i 2 l 4<br />

μ 4<br />

centrifugal forces<br />

viscous forces<br />

Stability <strong>of</strong> rotating<br />

cylinders Notice<br />

l has special<br />

definition<br />

Weber<br />

Number<br />

We<br />

ρU 2 l<br />

σ<br />

inertia force<br />

surface tension force<br />

For large Re and<br />

surface tension involve<br />

problem<br />

The dimensional parameters that were used in the construction <strong>of</strong> the dimensionless<br />

parameters in Table 10.2 are the characteristics <strong>of</strong> the system. Therefore there are<br />

several definition <strong>of</strong> Reynolds number. In fact, in the study <strong>of</strong> the physical situations<br />

<strong>of</strong>ten people refers to local Re number and the global Re number. Keeping this point<br />

in mind, there several typical dimensions which need to be mentioned. The typical body<br />

force is the gravity g which has a direction to center <strong>of</strong> Earth. The elasticity E in case<br />

<strong>of</strong> liquid phase is B T , in case <strong>of</strong> solid phase is Young modulus. The typical length is<br />

denoted as l and in many cases it is referred to as the diameter or the radius. The<br />

15 This number is named by Reiner, M. (1964), “The Deborah Number”, Physics Today 17 (1): 62,<br />

doi:10.1063/1.3051374. Reiner, a civil engineer who is considered the father <strong>of</strong> Rheology, named this<br />

parameter because theological reasons perhaps since he was living in Israel.

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