Direct numerical simulations of turbulence in elasto-viscoplastic fluids

The project will take place at lab. MTFC - Multiphysic and Turbulent Flow Computation, University of Liege, Belgium (Numerical approach, Vincent TERRAPON) in close collaboration with IMT Nord Europe, French (Experiment approach, S. Amir BAHRANI). 

 

Context: While water and air are Newtonian fluids (the viscous stress is linearly proportional to the rate of deformation), many fluids are characterized by a much more complex rheology, where the viscous stress can depend not only nonlinearly on the local rate of strain, but also on the history of past deformations. Such fluids can exhibit shear thinning (decreasing apparent viscosity with increasing shear rate), viscoplasticity (deformation occurs only above a threshold called yield stress), elasticity (memory effect, part of the deformation can be recovered), etc. This non-Newtonian rheology is a direct consequence of the complex microstructure of such fluids, that typically contain “elements” or “particles” that are not small molecules and whose dynamics plays a critical role. 

Examples are ubiquitous in daily life, including food (mayonnaise, mustard, etc.), biofluids (blood, saliva, etc.), personal care products (toothpaste, nail polish, etc.), and many others (drilling mud, paint, polymer solutions, polymer melts, etc.). While for most applications the Reynolds number is usually low, in some cases it can be sufficiently high that the flow is in a turbulent state. It has been observed that the complex rheology of such fluids has a strong impact, not only of the turbulence characteristics, but also on the transition path to turbulence. For instance, elasticity has the ability to create turbulence even in the absence of inertia, or to strongly damp inertial turbulence.

In the present case, we are interested in the flow of sewage sludge during wastewater treatment, where the fluid has an elasto-viscoplastic rheology. It has been shown experimentally that elasticity and viscoplasticity have a strong impact on the critical Reynolds number of transition, and on the pathway to turbulence. To complement these experimental results, we are interested in performing numerical simulations of transitional and turbulent pipe flows of such elasto-viscoplastic fluids.

Objectives :
 The main objective of this project is to implement an elasto-viscoplastic rheological model in an existing CFD code, to perform preliminary direct numerical simulations of a turbulent pipe or channel flow at moderate Reynolds number and to analyze and characterize the resulting turbulent state.

Tasks :
- Review the literature
- Familiarize yourself with the solver (Fortran 90)
- Implement the Saramito model
- Perform direct numerical simulations of Newtonian, visoelastic and elasto-viscoplastic fluids for one specific case
- Analyze the results and identify the differences and commonalities between the different rheology models
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Candidates having a background in Physics, Fluid Mechanics, Soft Matter, Process Engineering and Energetic and with an interest in numerical methods and programing.