1 Investigation of Film Cooled Rough Surfaces using Large Eddy Simulation Prasad Kalghatgi and Sumanta Acharya Turbine Innovation and Energy Research (TIER) Center College of Engineering Louisiana State University Baton Rouge, LA 70803 Abstract The use of “dirty” fuels with increased particulate concentrations leads to enhanced surface deposition and erosion on airfoil surfaces and can produce rough surfaces that can alter film cooling characteristics significantly. Both large scale and small scale roughness has been observed as a result of deposition and erosion. The study of film cooled rough surfaces has been predominantly experimental till now. For computational investigations, the resolution and meshing requirements of surface asperities is a major challenge. In the present paper the effect of roughness on the film cooling has been investigated numerically using Large Eddy Simulation. Effect of small scale (micro) roughness and large scale (macro) roughness have been studied. Macro roughness is modeled using an Immersed boundary method, and a roughness element model is used for micro roughness over immersed surfaces. Results of different roughness scenarios are analyzed and compared with the baseline smooth case. Introduction Significant changes in the film cooling performance due to roughness has been reported in the published literature [1][2] . Goldstein et al. [3] have reported a decrease in adiabatic film cooling effectiveness at lower blowing ratios and a significant improvement in adiabatic film cooling effectiveness at higher blowing ratios (in the range where jet lift off is observed). Bogard et al. [1] reported 50-60 % increase in heat transfer rate to the blade due to surface roughness. Due to these significant effects, predicting the cooling performance on rough surfaces is necessary to properly design cooling circuits. The topography of realistic rough surfaces is quite complex and may contain multiple scales simultaneously. Exact roughness modeling therefore is extremely difficult. In earlier studies, Reynolds-Averaged Navier Stokes (RANS) and Detached Eddy Simulation (DES) approaches, with adaptive grids, have been used to study the effect of roughness [4] . However, restrictions arise in the 3D modeling of exact surface roughnesses by using a body fitted grid due to the large number of mesh points required and mesh quality due to the rough-surface topography. Roughness elements and 2D ribs (grooves, d or k type roughness) have been the most popular approaches in roughness modeling. A Roughness Element Model was used to model roughness effects in a channel flow by Miyake at al. [5] in which the effect of roughness is modeled by the equivalent drag produced by an array of virtual cones introduced in the flow field. Bogard et al. [1] have used scaled conical roughness elements to simulate effect of roughness on heat transfer rate to the turbine blade. In this paper, we present a computationally cost-effective procedure of undertaking simulations over rough surfaces and apply this to film cooled rough surfaces to better understand the impact of roughness on film cooling.