Index

Relaxation time function

Ver: 1
 

DESCRIPTION

For the White-Metzner model, entering this menu allows the user to select the shear-rate dependence of the relaxation time.
The general form for the relaxation time is written as
  trelax = F(g) , where g is the shear-rate.

Three laws are available for F(g) :
 

* the constant viscosity F(g) = facr * the Bird-Carreau law F(g) = facr * [ 1 + (tnatr * g)2] (expor-1)/2 * the Power law F(g) = facr * (tnatr * g) expor-1


For the Bird-Carreau and Power laws, tnatr is a natural time, which is not to be confused with the relaxation time trelax.

Before specifying material data, the user must select a relaxation time law.

By default, facr = 1.

  OPTIONS
 
-1 
 Upper level menu
1 
 Constant viscosity
2 
 Bird-Carreau law
3 
 Power law
4 
 Modify facr  = 1.0000000E+00
Always available
5 
 Modify tnatr = 1.0000000E+00
Available for non constant relaxation time laws
6 
 Modify expor = 1.0000000E+00
Available for non constant relaxation time laws
NOTES A low value for the power law index expor leads to strong non-inearities.
A Picard iteration is recommended instead of a full Newton iteration for cases where expor < 0.7.
An evolution scheme can also be used, starting from expor = 1 and decreasing expor.

The units for g and the parameters facr, tnatr and expor are :
 
Parameter
Mass
Length
Time
Temperature
        g
-
-
-1
-
        facr
1
-1
-1
-
        tnatr
-
-
   1
-
        expor
-
-
-
-


SEE ALSO