The purpose of this paper is to present the results of an uncertainty analysis study for commonly used turbulence models in Reynolds-Averaged Navier-Stokes codes due to the epistemic uncertainty in closure coefficients for attached and separated wall-bounded flows. Sensitivity analysis is performed to rank the uncertainty contribution of each coefficient to various output quantities of interest. In particular, an uncertainty and sensitivity analysis focusing on determining the similarities and differences between the results based on the structure of the flow (i.e., attached vs. separated) is performed. Comparison between two wall-bounded flow cases is performed including low-speed attached flow over a 2D zero pressure gradient flat plate and a 2D NASA wall-mounted hump case with separated flow. In total, two turbulence models were considered in the analysis: the Spalart-Allmaras Model and Wilcox (2006) κ-ω Model. The flow solver used in the study is Fun3D, a code developed by NASA Langley Research Center. The uncertainty quantiflcation approach involves stochastic expansions based on non-intrusive polynomial chaos to efficiently propagate the uncertainty. Sobol indices are used to rank the relative contribution of each closure coefficient to the total uncertainty for several output flow quantities. The results of the current study identify a set of closure coefficients for each turbulence model which contribute most to the uncertainty for both attached and separated low speed flows. These major conclusions are also consistent with the flndings of a previous work focusing on transonic wall-bounded flows.