Roughened aeroengine blade surfaces lead to increased friction losses and reduced efficiency of the individual blades. The surface roughness also affects the wake flow of the blade and thus the inflow conditions for the subsequent compressor or turbine stage. To investigate the impact of surface roughness on a turbulent blade wake, we conducted velocity field measurements by means of stereo particle image velocimetry in the wake of a roughened turbine blade in a linear cascade wind tunnel. The turbine blade was roughened at different chordwise locations. The influence of the chordwise location of the added surface roughness was examined by comparing their impact on the width and depth of the wake and, the positions and distribution of vortical structures in the wake. Additionally, the friction loss coefficients for different surface roughness positions were estimated directly from the velocity field. 1. Introduction The aerodynamics of multistage turbomachinery is extremely complex due to the inherent unsteadiness and three-dimensionality of the internal flow. Designing aerodynamically efficient aeroengines is a challenging task that requires detailed knowledge about unsteady boundary development and transition [1, 2], the structure of blade wakes [3, 4], and the interaction of these wakes with downstream blades [5, 6]. The dominating unsteadiness of the internal engine flow results from the relative motion of rotor and stator blades and is periodic in nature. Additionally, turbulent inflow conditions and variations of the blades’ geometry and surface roughness within a turbine or compressor stage lead to aperiodic fluctuations of the flow. Although the stochastic fluctuations are generally smaller in amplitude than the periodic variations, they can have significant influence on the aerodynamic [7, 8], aeroacoustic [9], and aeroelastic properties [10, 11] of turbines and compressors. Surface roughness on aeroengine blades can be induced by manufacturing or repair processes and by operational and environmental conditions. It reduces the blades’ efficiency by increasing the boundary layer momentum loss and blade skin friction and by precipitating boundary layer transition. At high Reynolds numbers, surface roughness also raises the odds for flow separation to occur, leading to further increased losses. The influence of surface roughness on a particular blade does not only affect that blade’s efficiency, but also reaches even further by affecting the wake flow and thus the inflow conditions for subsequent compressor or turbines stages. To allow for
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