Dimensionless characterisation of viscoelastic flows

The Deborah number Db has been defined by Metzner, White and Denn as  

Characteristic process time Characteristic fluid time 

In the above example of the packed bed, Db would be 10 for the jet and the nozzle Db would be 10 . The smaller the value of Db, the more likely is elasticity to be of practical significance. 

Unfortunately, this group Db depends on the assignment of a single characteristic time to the fluid (perhaps a. relaxation time). While this has led to some success, it pe s to be inadequate for many viscoelastic materials which show different relaxation behaviour under differing conditions. 

For further information on viscoelastic behaviour, reference should again be made to specialist sources  

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