Page 166 - Mechanical Engineer's Data Handbook

FLUID MECHANICS 155

Inlet
I Throat

Values of C,

Cd
Venturi 0.974.99
Orifice plate 0.60
Nozzle 0.92 to 0.98

4.4 Viscosity and laminar flow

V+dV At
4.4. I viscosity
In fluids there is cohesion and interaction between
molecules which results in a shear force between
adjacent layers moving at different velocities and
between a moving fluid and a fixed wall. This results in
friction and loss of energy.
The following theory applies to so-called ‘laminar’
or ‘viscous’ flow associated with low velocity and high
viscosity, i.e. where the Reynold’s number is low.
Flat plate moving overjixed plate of area A
Dejnition of viscosity
V
In laminar flow the shear stress between adjacent Force to move plate F=7A=pA-
layers parallel to the direction of flow is proportional Y
to the velocity gradient. A

Let :
V=velocity
y = distance normal to flow
p =dynamic viscosity
dV dV
Shear stress 7=constant-=p- --Iy Flua.mkKitypmfile
dY dY