Ali Pakzad

Joseph A. Fiordilino, Ali Pakzad

The temperature in natural convection problems is, under mild data assumptions, uniformly bounded in time. This property has not yet been proven for the standard finite element method (FEM) approximation of natural convection problems with nonhomogeneous partitioned Dirichlet boundary conditions, e.g., the differentially heated vertical wall and Rayleigh-Bénard problems. For these problems, only stability in time, allowing for possible exponential growth of $\| T^{n}_{h} \| $, has been proven using Gronwall's inequality. Herein, we prove that the temperature approximation can grow at most linearly in time provided that the first mesh line in the finite element mesh is within $\mathcal{O} (Ra^{-1})$ of the nonhomogeneous Dirichlet boundary.

Ali Pakzad

Turbulence models, such as the Smagorinsky model herein, are used to represent the energy lost from resolved to under-resolved scales due to the energy cascade (i.e. non-linearity). Analytic estimates of the energy dissipation rates of a few turbulence models have recently appeared, but none (yet) study energy dissipation restricted to resolved scales, i.e. after spacial discretization with $h >$ micro scale. We do so herein for the Smagorinsky model. Upper bounds are derived on the \textit{computed} time-averaged energy dissipation rate, $\langle \varepsilon (u^h)\rangle$, for an under-resolved mesh $h$ for turbulent shear flow. For coarse mesh size $ \mathcal{O}(\mathcal{Re}^{-1}) < h < L $, it is proven, $$ \langle \varepsilon (u^h)\rangle\leq \big[ (\frac{C_s\, δ}{h})^2+ \frac{L^5}{(C_s δ)^4\,h}+\frac{L^{\frac{5}{2}}}{(C_s\, δ)^{4}}\, {h^{\frac{3}{2}}}\big]\, \frac{U^3}{L}, $$ where $U$ and $L$ are global velocity and length scale and $C_s$ and $δ$ are model parameters. This upper bound is independent of the viscosity at high Reynolds number, is in accord with the scaling theory of turbulent. This estimate suggests over-dissipation for any of $C_s>0$ and $δ>0$, consistent with numerical evidence on the effects of model viscosity (without wall damping function). Moreover, the analysis indicates that the turbulent boundary layer is a more important length scale for shear flow than the Kolmogorov microscale.