Non-Newtonian and Viscoelastic Fluids

1.Non Newtonian Fluids  viscosity does not depend on on the stress state and velocity of the flow. On the other hand the viscosity of non-Newtonian fluids is dependent on shear rate or shear rate history.

In Shear thickening non-Newtonian fluid, apparent viscosity increases with increased stress. Such as,

Application in Houdini,  In example below, increased pressure make fluid more viscose after  a certain pressure threshold. Red color represents viscosity, blue color velocity.

nonNewtonianStressTickening.gif

  1. import fluid source with viscosity attribute.
  2. import collisions
  3. gasMatchField for creating “pressureNormalised” attr. This attribute will take pressure field, manipulates with control attributes and transfer to viscosity attribute.
  4. gasFieldVop,
  5. import pressure field
  6. ramp it for enabling threshold(very low pressures does not affect viscosity)
  7. viscosity multiplier parameter
  8. multiplication pressure with viscosity multiplier variable
  9. viscosity min parameter for making min viscosity level even in low pressure
  10. add operant
  11. output pressureNormalised attr
  12. gasFieldToParticle for setting particle viscosity attr by pressureNormalised

nonNewtonian.png

2.Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. 

Application in Houdini. System below  is based on Yancy Lindquist‘s tutorial about elasticity. In order to have elasticity effect, measure of the distortion that the fluid in the voxel has undergone(straint) is used.

elasticity

  1. strain field is created
  2. prevStrain field is created
  3. particles strain attributes is transfered to prev strain field
  4. in gasCalculate microsolver previousStrain field is copied to strain field
  5. gas strain integrate microsolver
  6. gas strain forces microsolver
  7. gas calculate microsolver for difference between strain and previous strain
  8. strain field is send back to particles

Control of Elasticity can be achieved by Elasticitic modulus (in gasstrainforces microsolver),  alpha and gamma (in gasstrainintegrate microsolver.)

Elastic Modulus  provides the scale factor for how to translate a certain amount of distortion into a restorative force.  A larger value will cause the fluid to spring back faster. fluidelasticitymodulus

Alpha This is the rate of plastic flow. The current strain is dissipated at this rate per second.fluidelasticityalpha

Gamma  threshold for plastic flow. When the norm of the strain exceeds gamma, the strain is dissipated according to the alpha term. This causes the fluid to lose its history and permanently enter its new configuration.fluidelasticitygamma

References

https://en.wikipedia.org/wiki/Non-Newtonian_fluid
https://en.wikipedia.org/wiki/Viscoelasticity
https://cmivfx.com/products/344-houdini-frictional-viscosities

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