The overall efficiency of a jet engine and hence the specific fuel consumption is strongly determined by the aerodynamic efficiency of the Low Pressure Turbine (LPT). The LPT is therefore a main target for aerodynamic improvements. Due to the fact that most of the LPT profile losses (about 2/3) are caused in the suction surface boundary layer [3, 21] and these are strongly linked to the state of the boundary layer, it is of utmost importance to know if and, if the case, where transition occurs. Especially on LPT airfoils with typically low Reynolds numbers this is a crucial point of interest in order to improve the profile loss predictions. Precise knowledge of the transition process is also crucial for computational fluid dynamics (CFD) validation. Only with well calibrated transition models confident loss predictions are to be expected. The area of transition on profiles with boundary layer separation is usually obvious by measuring the pressure distribution. Thereby, the localization of transition can be imprecise due to the fact that the separation and the reattachment point do not always coincide with transition start and end for different bubble types [7, 14]. However, with increasing turbulence or less deceleration transition with separation turns to bypass transition [14]. Without separation bubble, the accurate detection of transition from the pressure distribution turns out to be difficult or even impossible. Hence analyses with more sophisticated measurement techniques have to be performed to determine onset and end of transition. Basically two measurement methods are convenient to examine the transition process: Today, commonly hot films are used to obtain a semiquantitative quasi wall shear stress. In addition RMS and skewness of the signal can be used to localise transition onset and end [2, 8, 10, 15, 25]. With a Preston tube, the total pressure close to the surface can be measured and, with the local static pressure on the surface, the dynamic pressure close to the wall can be determined [11, 12, 15]. Both methods are well established to detect boundary layer transition. However, conducting the data with a Preston probe is much less effort than with hot films. The data presented here are the results of experiments which were conducted to detect boundary layer transition on a profile without separation, designed for low Reynolds numbers. The data is also used to compare and validate the methods against each other and to investigate the physical limitations. This paper describes and compares these two experimental methods to characterise the transition process on the suction surface of a low pressure turbine cascade showing no evidences of boundary layer separation in the pressure distribution. For that purpose a typical range of operation points is examined to compare the methods against each other.    «

The overall efficiency of a jet engine and hence the specific fuel consumption is strongly determined by the aerodynamic efficiency of the Low Pressure Turbine (LPT). The LPT is therefore a main target for aerodynamic improvements. Due to the fact that most of the LPT profile losses (about 2/3) are caused in the suction surface boundary layer [3, 21] and these are strongly linked to the state of the boundary layer, it is of utmost importance to know if and, if the case, where t...    »