Andreas Fiala

Parametric Study of Fluidic Oscillators for Use in Active Boundary Layer Control

A parametric study was conducted to identify the main factors influencing the frequency produced by fluidic oscillators with the goal of using the actuator to trigger boundary layer transition through the excitation of Tollmien Schlichting waves. Test bench conditions were chosen to match the static pressure at the actuation position on the candidate blade profile for a cascade exit Mach number of 0.6 and Reynolds numbers from 60,000 to 200,000. The inlet vs. outlet pressure ratio and the position and geometry of the outlet holes were all varied. Additionally, the effect of the oscillator’s scale and the feedback channel geometry were considered. The flow at the exit was measured using a hot wire, while Kulite pressure transducers were used to measure pressure fluctuations within the device. This paper shows that fluidic oscillators can achieve frequencies of 10 kHz and that the parameters considered play an important role in the performance of these devices.Copyright

Clocking effects in a 1,5-stage axial turbine : boundary layer behaviour at midspan

This paper presents an experimental and numerical investigation on the influence of clocking on the boundary layer behaviour of the second stator in a 1.5-stage axial low pressure turbine. Surface mounted hot-film sensors were used to measure the quasi shear stress on the second stator and static pressure tappings to obtain the pressure distribution. All experiments were carried out at midspan for different clocking positions. The supporting numerical calculations were conducted with a two-dimensional Navier-Stokes solver using a finite volume discretization scheme and the v′2 f turbulence model.Copyright

Modeling Vortex Generating Jet-Induced Transition in Low-Pressure Turbines

The current design of low-pressure turbines (LPTs) with steady-blowing vortex generating jets (VGJs) uses steady computational fluid dynamics (CFD). The present work aims to support this design approach by proposing a new semiempirical transition model for injection-induced laminar-turbulent boundary layer transition. It is based on the detection of cross-flow vortices in the boundary layer which cause inflectional cross-flow velocity profiles. The model is implemented in the CFD code TRACE within the framework of the γ-Reθ transition model and is a reformulated, recalibrated, and extended version of a previously presented model. It is extensively validated by means of VGJ as well as non-VGJ test cases capturing the local transition process in a physically reasonable way. Quantitative aerodynamic design parameters of several VGJ configurations including steady and periodic-unsteady inflow conditions are predicted in good accordance with experimental values. Furthermore, the quantitative prediction of end-wall flows of LPTs is improved by detecting typical secondary flow structures. For the first time, the newly derived model allows the quantitative design and optimization of LPTs with VGJs.

Aerodynamic Measurements on a Low Pressure Turbine Cascade With Different Levels of Distributed Roughness

The effect of surface roughness on the aerodynamics of a highly loaded low-pressure turbine airfoil was investigated in a series of cascade tests conducted in a high speed facility. Profile loss and aerodynamic loading of three different surface roughnesses with a ratio of the centerline average roughness to the profile chord of 1.1 · 10−5 , 7.1 · 10−5 and 29 · 10−5 were analysed. Tests were carried out under design outlet Mach number, outlet Reynolds number ranging from 5 · 104 to 7 · 105 and inlet turbulence level of 2.5% and 5% . The mid span flow field downstream of the cascade and the loading distribution on the profile were measured for each investigated operating point using a five hole probe and surface static pressure taps. Additional measurements with a hot wire probe in the profile boundary layer under reference conditions (Re 2 th = 2 · 105 ) were also conducted. Experimental results show a loss reduction for the highest roughness under reference conditions, due to the partial suppression of the separation bubble on the suction side of the profile. At high Reynolds numbers a massive boundary layer separation on the suction side is observed for the highest roughness, along with a large increase in total pressure loss. The middle roughness tested has no effect on the loading distribution as well as on the loss behaviour of the airfoil under all investigated flow conditions.Copyright

Roughness Modeling for Turbomachinery

Fundamental concepts for roughness modeling have been further explored and advanced. A basic understanding of the effect of distributed roughness on fully developed turbulent boundary layer, its possible influence on transition, and the mechanism of local spanwise roughness on transition has been achieved. Predictions with a refinement around a spanwise roughness element have been conducted in comparison to TATMo’s turbine cascade investigated at VKI. 3d-computations document the status in comparison to the T106C measurements with spanwise roughness for all Reynolds-numbers with two different transition models. Additional validation work shows the reproduction of accurate behavior of influence of height, location, and shape of the roughness element on pitchwise averaged loss and exit angle at midspan. Beside the correct reproduction of flow quantities for the spanwise roughness element, the right assessment of distributed roughness on surfaces of an industrial configuration is important. Because a high grid resolution very near the wall on all surfaces is not always possible, the problem can be solved with the help of wall-functions. The results of the application document the significance of rough wall-function modeling for tubomachinery.Copyright

Analysis of Laminar-Turbulent Transition of a Low-Loss Generic Low Pressure Turbine Distribution

The efficiency of modern Turbofan engines can be significantly increased by using a gearbox between compressor and turbine of the low pressure section. Rotational speed of the low pressure turbine (LPT) in a Geared Turbofan is much higher than in normal LPT’s which lead to necessary adjustments in blade design.This work has investigated the transition behavior of a modified profile geometry for low-loss at engine cruise conditions. Typical LPT conditions have thus been chosen as baseline for the experimental work. A pressure distribution has been created on a flat plate by means of contoured walls in a low speed wind tunnel. The paper will analyze the experimental results and show additionally the numerical predictions of the test case.The experimental part of this paper describe how the blade was Mach number scaled to obtain the geometry of the wind tunnel wall contour. The pressure distribution for the incompressible test case show a very good agreement to the compressible case. Boundary layer (BL) measurements with hot-wire-anemometry have been performed at high spatial resolution under a freestream turbulence of almost 8%. Different Reynolds numbers have been investigated and will be compared with special attention being paid to the transition on the suction side by contour plots (turbulence levels, turbulent intermittency) and integral BL parameters. It was found that the transition on the suction side is not completed for small Reynolds numbers but takes place at higher velocities.In the numerical part studies by means of steady RANS simulations with k-ω – SST turbulence model and γ-Reθ transition model have been conducted. The aim is to validate the RANS solver for the low-loss LPT application. Hence, comparison is made to the measured data and the transitional behavior of the BL. Furthermore, additional parameter variations have been conducted (turbulence intensity and Reynolds number).The numerical investigations show partially a good comparison for the BL development indicating the different transition modi with increasing Reynolds number and turbulence intensity.Copyright

Modelling Vortex Generating Jet-Induced Transition in Low-Pressure Turbines

The current design of low-pressure turbines (LPTs) with steady-blowing vortex generating jets (VGJ) uses steady computational fluid dynamics (CFD). The present work aims to support this design approach by proposing a new semi-empirical transition model for injection-induced laminar-turbulent boundary layer transition. It is based on the detection of cross-flow vortices in the boundary layer which cause inflectional cross-flow velocity profiles. The model is implemented in the CFD code TRACE within the framework of the γ-Reθ transition model and is a reformulated, re-calibrated, and extended version of a previously presented model. It is extensively validated by means of VGJ as well as non-VGJ test cases capturing the local transition process in a physically reasonable way. Quantitative aerodynamic design parameters of several VGJ configurations including steady and periodic-unsteady inflow conditions are predicted in good accordance with experimental values. Furthermore, the quantitative prediction of end-wall flows of LPTs is improved by detecting typical secondary flow structures. For the first time, the newly derived model allows the quantitative design and optimization of LPTs with VGJs.Copyright

Surface Thin Film Gauge Measurements in a Two-Stage Low Pressure Turbine at Low Reynolds Number

A two-stage low pressure axial turbine has been tested in cooperation between the Institute of Aircraft Propulsion Systems (ILA) and MTU Aero Engines GmbH (MTU). The experimental results taken in the Altitude Test Facility are used to assess blade row performance of vane 2 at mid height over a range of Reynolds numbers from as low as 35,000 up to 88,000. Both Mach and Reynolds similarity are preserved. Surface thin film gauges at midspan on vane 2 suction side are used to analyse the unsteady behaviour of the boundary layer. Unsteady data from area traverses downstream of vane 2 using X-hotfilm probes complement the analysis describing the unsteady wake evolution at mid height. The nature of the unsteady transitional low Reynolds number boundary layer is discussed.Copyright

Effect of Surface Roughness on Loss Behaviour: Aerodynamic Loading and Boundary Layer Development of a Low-Pressure Gas Turbine Airfoil

Aerodynamic measurements on the linear low-pressure turbine cascade T106C were conducted in a high speed test facility, in order to investigate the effect of surface roughness on loss behaviour, aerodynamic loading, and boundary layer development. Three different roughnesses were investigated, with a ratio of the center line average roughness to the profile chord of 0.8·10−5 , 5·10−5 and 25·10−5 . Tests were carried out under design outlet Mach number (Ma 2 th = 0.6 ), outlet Reynolds number ranging from Re 2 th = 5·104 to Re 2 th = 7·105 and inlet turbulence level Tu1 = 3% and Tu1 = 6% . The flow field downstream of the cascade and the loading distribution on the profiles were measured for each investigated operating point using five hole probes and surface static pressure taps. Additional measurements with a hot-wire probe in the suction surface (SS) boundary layer were also conducted, in order to investigate the differences in boundary layer development due to surface roughness. From loss and blade loading measurements it was found that roughness has no influence on the pressure distribution on the profile, although the highest investigated roughness produces a significant loss reduction at low Reynolds numbers. Hot-wire probe surveys show that at Re 2 th = 9·104 the boundary layer for the highest roughness immediately upstream of the flow separation point on the SS is substantially thinner than for the middle roughness and the smooth profile.Copyright