Proposal of a correlation for boundary layer flow over real roughened surfaces

The effects of surface roughness on boundary layer features are analyzed through scaled facsimiles (flat plates) of real turbine blades. The main aim of the present work is to exploit the effect of surface roughness with particular reference to the steam turbine applications. Four rough plates and a smooth plate were tested at different high Reynolds number conditions and for different incidence angles of the flow respect to the plates. Roughness characteristics of the plates are typical of the turbine blades, in order to fully understand the aerodynamic efficiency improvement induced by a specific surface texture and to estimate the availability and convenience of manufacturing. All experimental results regarding the smooth plate are compared with the numerical ones obtained from the 2D cascade code MISES.Velocity profiles and boundary layer features are presented for all plates. Skin friction coefficient is estimated and laws of the wall are proposed for different roughness profiles. The effect of the grooves (riblets) is also investigated. The experimental data related to surface texture are analyzed and the most representative roughness parameters are underlined; it is found one unique roughness parameter which is representative of the real surface texture. The law of the wall for rough surfaces is correlated with the roughness parameter proposed. Moreover, a correlation between technical (sandgrain) and real surface roughness is proposed. Pressure gradient effect is also analyzed.