Why isn't MatPIV fit for non-Newtonian fluids?

[dotancohen]

We are using the MatPIV Matlab toolkit:
http://www.math.uio.no/~jks/matpiv/

However, we are having trouble using the toolkit with viscoplastic media. It was expected to find a yielded region and a unyielded region, however, from the results of MatPIV it appeared that the whole area was flowing. This contradicts visual inspection of the system with tracers, in which flow was apparent only near moving inclusions. Is anyone familiar with MatPIV that might help us explain and possibly work around the problem? Thanks!

 

[dotancohen]

Mods: On second thought, this _might_ belong in the mechanical engineering section. Is it possible to soft-link there as well? Thanks!

 

[oswaler]

I am not personally familiar with MatPIV or the specific issue you are encountering with viscoplastic media. However, based on my understanding of non-Newtonian fluids and the capabilities of MatPIV, I can provide a potential explanation for why it may not be suitable for analyzing these types of fluids.

Firstly, MatPIV is a Matlab toolkit designed for analyzing fluid flow using particle image velocimetry (PIV) techniques. PIV is a well-established method for studying fluid flow, but it is primarily used for Newtonian fluids, which have a constant viscosity regardless of the applied shear rate. Non-Newtonian fluids, on the other hand, have a viscosity that varies with the applied shear rate and can exhibit complex behaviors such as shear-thinning or shear-thickening.

One of the main assumptions in PIV is that the fluid is incompressible and has a constant viscosity. This assumption may not hold true for non-Newtonian fluids, especially in the case of viscoplastic materials. Viscoplastic fluids have a yield stress, which means that they behave like solids until a certain stress threshold is reached, after which they start to flow like a fluid. This yield stress can significantly affect the flow behavior and cannot be captured by PIV analysis, which assumes a continuous, homogenous flow.

Furthermore, PIV analysis relies on the assumption of a laminar flow, where fluid particles move in smooth, parallel layers. Non-Newtonian fluids, particularly those with high viscosities, can exhibit turbulent flow, which can be difficult to accurately analyze using PIV techniques.

In summary, the underlying assumptions and limitations of PIV techniques make it unsuitable for accurately analyzing non-Newtonian fluids, particularly viscoplastic materials. It is possible that the results you are obtaining with MatPIV are not accurate due to these limitations. I would recommend exploring other analysis techniques that are specifically designed for non-Newtonian fluids, or consulting with experts in the field for further guidance on analyzing your specific system.