KATEDRA MATERILOVHO INENRSTV A CHEMIE

MATERIALS ENGINEERING viscosity

Viskosity (Internal friction of fluids)

Property of fluid characterized by internal friction.

Viscosity is dependent on internal forces between atoms

or molecules. Fluids with higher internal forces has higher

viscosities.

Ideal fluid has no viscosity.

There is no relation between

viscosity and density fluids

with similar densities can

have different viscosities.

Viscosity

- We have two plates with large area S. The first is fixed and

the second is moved by force F.

- There is a fluid between these two plates.

- Fluid si moving with velocity u and gradient du/dy

Viscosity

Problem can be described by equation

velocity gradient

where F is applied force, S is area of plates, is dynamic

vicosity, u is velocity and y is distance from the plate

Problem can be rewritten to the form

Where is shear stress

Coefficient (eta) [N.s/m2] or [Pa.s] - dynamic viscosity

S

F=

y

uSF

=

y

u

=

binoybnairNoteBefore this SI unit was standardised, dynamic viscosity was measured in 'poise' where 1 Pa.s = 10 poise

binoybnairNote

Viscosity

Newtonian fluid constant viscosity water, most of gases

Bingham plastic material that behaves as a solid at low stresses but flows as a viscous fluid at high stresses.

Shear thickening viscosity increases with the rate of shear.

Shear thinning viscosity decreases with the rate of shear.

Dynamic viscosity x kinematic viskosity

Viskosity increases with increasing pressure.

Viscosity decreases with increasing temperature.

Dynamic viskosity (eta) [N.s/m2] or [Pa.s]

Kinematic viskosity (nu) [m2 s]

Dynamic viscosity divided by density of fluid

y

u

=

=

binoybnairNote1 m2.s = 10000 stokes

Measurement of viscosity

- Method of falling ball

Hopplers viskosimeter

time measurement of balls fall in fluid. Ball has defined

density and its movement has to be slow enough to

assure there is no existence of vortex flow

(minimal falling time)

Measurement of viscosity

Applied forces

Gravitation - G

Lifting force - FA

Stokes force - FS

Calculation of dynamic viscosity

t [s] falling time

1 [g cm-3] density of ball

2 [g cm-3] density of fluid at temperature T

K [mPa cm3 g-1] - constant

Measurement of viscosity

KOL

Measure density of water and glycerol

Measure dynamic viscosity of water and glycerol

Calculate kinematic viscosity of water and glycerol

Compare results with tabular values

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