#### Date of Award

8-2013

#### Document Type

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Legacy Department

Mathematical Science

#### Advisor

Lee, Hyesuk

#### Committee Member

Cox , Chris

#### Committee Member

Ervin , Vince

#### Abstract

This dissertation studies two important problems in the mathematics of computational fluid dynamics. The first problem concerns the accurate and efficient simulation of incompressible, viscous Newtonian flows, described by the Navier-Stokes equations. A direct numerical simulation of these types of flows is, in most cases, not computationally feasible. Hence, the first half of this work studies two separate types of models designed to more accurately and efficient simulate these flows. The second half focuses on the defective boundary problem for non-Newtonian flows. Non-Newtonian flows are generally governed by more complex modeling equations, and the lack of standard Dirichlet or Neumann boundary conditions further complicates these problems. We present two different numerical methods to solve these defective boundary problems for non-Newtonian flows, with application to both generalized-Newtonian and viscoelastic flow models.

Chapter 3 studies a finite element method for the 3D Navier-Stokes equations in velocity-

vorticity-helicity formulation, which solves directly for velocity, vorticity, Bernoulli pressure and

helical density. The algorithm presented strongly enforces solenoidal constraints on both the velocity (to enforce the physical law for conservation of mass) and vorticity (to enforce the mathematical law that div(curl)= 0). We prove unconditional stability of the velocity, and with the use of a (consistent) penalty term on the difference between the computed vorticity and curl of the computed velocity, we are also able to prove unconditional stability of the vorticity in a weaker norm. Numerical experiments are given that confirm expected convergence rates, and test the method on a benchmark problem.

Chapter 4 focuses on one main issue from the method presented in Chapter 3, which is the question of appropriate (and practical) vorticity boundary conditions. A new, natural vorticity boundary condition is derived directly from the Navier-Stokes equations. We propose a numerical scheme implementing this new boundary condition to evaluate its effectiveness in a numerical experiment.

Chapter 5 derives a new, reduced order, multiscale deconvolution model. Multiscale deconvolution models are a type of large eddy simulation models, which filter out small energy scales and model their effect on the large scales (which significantly reduces the amount of degrees of freedom necessary for simulations). We present both an efficient and stable numerical method to approximate our new reduced order model, and evaluate its effectiveness on two 3d benchmark flow problems.

In Chapter 6 a numerical method for a generalized-Newtonian fluid with flow rate boundary conditions is considered. The defective boundary condition problem is formulated as a constrained optimal control problem, where a flow balance is forced on the inflow and outflow boundaries using a Neumann control. The control problem is analyzed for an existence result and the Lagrange multiplier rule. A decoupling solution algorithm is presented and numerical experiments are provided to validate robustness of the algorithm.

Finally, this work concludes with Chapter 7, which studies two numerical algorithms for viscoelastic fluid flows with defective boundary conditions, where only flow rates or mean pressures are prescribed on parts of the boundary. As in Chapter 6, the defective boundary condition problem is formulated as a minimization problem, where we seek boundary conditions of the flow equations which yield an optimal functional value. Two different approaches are considered in developing computational algorithms for the constrained optimization problem, and results of numerical experiments are presented to compare performance of the algorithms.

#### Recommended Citation

Galvin, Keith, "Advancements In Finite Element Methods For Newtonian And Non-Newtonian Flows" (2013). *All Dissertations*. 1136.

http://tigerprints.clemson.edu/all_dissertations/1136