Modeling the non-linear response of mixed culture biofilm structures to turbulent flow
Towler, Brett William
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Microbial biofouling of wetted interfaces can negatively impact the hydrodynamic performance of pressurized conduits. These impacts are due, in part, to the material properties of biofilm, yet few studies have examined this polymeric substance in the context of a constitutive relation. The goal of this research was two-fold; 1) to determine a suitable constitutive model for a mixed-culture biofilm and 2) use this material model in a numerical simulation to evaluate biofilm mechanical behavior in response to varying hydrodynamic conditions. Creep tests revealed that these biofilms may be classified as viscoelastic fluids. Furthermore, results indicated the presence of viscous, time-dependent and instantaneous components to the biofilm compliance functions. A regression analysis (r2 = 0.8819) supported the treatment of these samples as linear viscoelastic fluids within the stress range of 0.1 Pa to 0.5 Pa. A specific linear viscoelastic constitutive equation was then determined by fitting experimental results to analytical solutions using an optimization algorithm. It was found that the Burger material model closely approximated the behavior of all samples. A numerical fluid-structure interface model was then developed and employed in a parametric study to investigate biofilm behavior. The effect of the Burger material parameters, mean flow velocity and biofilm size were examined. Simulations showed that weaker or softer biofilms (characterized by lower elastic moduli) were highly susceptible to lift forces. Additionally, polar diagrams were generated by plotting the coefficients of drag versus lift. The plots suggested that in the first few seconds after loading, the deformation paths taken by hemispherical biofilms are largely insensitive to specific material coefficients. Moreover, the diagrams illustrated that the effects of biofilm strength, size and channel velocity on displacement were predictable. These relationships may lead to the development of a simple, yet accurate method for predicting the hydrodynamic forces acting on an attached biofilm.