The share of renewable sources in the energy sector is increasing steadily leading to higher requirements towards plant flexibility for conventional plants to compensate for the highly fluctuating power supply. To enhance the flexibility of gas turbine power plants overspray fogging systems can be used. Opposed to this positive effect it was shown that a two-phase flow increases losses and minor turning of the blade row as shown by Ober . The reason for this change in aerodynamic performance, however, is not yet fully understood. As shown by Wurz  the velocity profile of the airflow changes in the presence of a water film which is related to an increased roughness due to the waviness of the water film. However, the experimental data base for the effect of a water film on compressor airfoils and its effect on aerodynamics is small and the possibility to capture all relevant effects in numerical codes requires an enormous effort.
Therefore the objective of this study is to show the feasibility of modeling the influence of a water film as a region of increased surface roughness to estimate the aerodynamic effects on the compressor flow more easily. In the first part of the study the focus is directed to the water wall film pattern inspected experimentally at the transonic open loop wind tunnel at Helmut-Schmidt-University in Hamburg for two different airfoils. This study reveals the areas which are covered with a continuous water film. In the second part results of CFD calculations are presented. A reverse approach is used. Instead of simulating the water film in all details the focus is put on modeling the influence of a water film present. The regions previously being detected to be covered by water are defined as rough walls. The roughness height is varied to match experimentally measured losses published by Ober . The results show the magnitude of influence of a wetted surface on the blade profile loss.