Session: 05-11 Turbine Facility Sensors & Diagnostics

Paper Number: 127971

127971 - On the Application of Background Oriented Schlieren to a Transonic Low-Reynolds Turbine Cascade 

High-speed low-pressure turbines are a key-technology for the future generation of aircraft engines, characterized by ultra-high bypass ratio, and typically operating at transonic exit Mach numbers and low Reynolds numbers. These flow conditions are quite challenging to reproduce in ground testing facilities, hence in literature there is a significant lack of experimental data.

This shortage of data is even more pronounced for the utilization of optical qualitative and quantitative measurement techniques within a turbine cascade. In this regard, Schlieren based flow visualization approaches enable the characterization of density varying flow features that commonly exist in compressible flow scenarios. The application of grayscale Schlieren imaging with a collimated light beam setup requires either a double side optical access and considerably large lenses, or mirrors organized in complex single optical window setups. On the other hand, background oriented Schlieren (BOS) offers a unique opportunity to investigate density gradients from a qualitative and a quantitative standpoint, featuring a non-intrusive setup that is relatively simple and affordable.

An experimental campaign on the SPLEEN (Secondary and Leakage Flow Effects in High-Speed Low-PrEssurE TurbiNes) C1 cascade has been performed in the S1/C high-speed transonic cascade rig of the von Karman Institute over a range of engine-representative cascade exit Reynolds and Mach numbers, namely Re2 = [70k – 120k – 140k] and M2 = [0.90 – 0.95 – 1.0].

Components of the image acquisition setups are varied to allow time-averaged and time-resolved BOS flow visualizations. The application of BOS to the low-Reynolds transonic cascade presents severe challenges because of the low-density and low-transonic flow conditions, resulting in considerably small displacement of illumination features over the background patterns. The data has been processed by means of the cross-correlation algorithm, extensively optimized for PIV measurements, and by means of an optical flow algorithm, recently in-depth discovered for BOS applications. The optical flow technique provides an opportunity to enhance the resolution and the sensitivity of the pixel motion, especially needed.

The results obtained from the two different techniques have been comparatively assessed. The boundary layer thickness and shock location are found to be in good agreement with both image processing techniques. The time-resolved measurements have been further analyzed by means of a shock displacement reconstruction technique and Proper Orthogonal Decomposition (POD) to understand the spatially distributed temporal modes that characterize the shock wave and the specific values of the frequency associated to each mode. The paraxial approximation has been introduced for this specific case to associate the detected pixel displacement with the correspondent density gradient. The BOS data have been compared against available numerical simulations and experimental data of time-resolved blade surface pressure and quasi-shear stress.

Presenting Author: Alexandre Halby von Karman Institute for Fluid Dynamics

Presenting Author Biography: Alexandre Halby is a PhD candidate at the prestigious von Karman Institute for Fluid Dynamics. He earned his Master of Science in Mechanical Engineering from Politecnico di Milano, Italy, and embarked his journey at the von Karman Institute when he began his master's thesis in May 2022.

Since June 2023, Alexandre has been engaged in his doctoral research centered on the investigation of transonic low-pressure turbines. Alexandre had the opportunity to actively participate to some turbomachinery test campaigns, where he conducted turbulence measurements upstream of a low-pressure turbine stage and he has also applied optical techniques to investigate the flow behavior in a low-pressure turbine cascade.

Authors:

Alexandre Halby von Karman Institute for Fluid Dynamics
Bora Orcun Cakir von Karman Institute for Fluid Dynamics
Lorenzo Da Valle von Karman Insititute for Fluid Dynamics
Gustavo Lopes von Karman Institute for Fluid Dynamics
Mizuki Okada von Karman Institute for Fluid Dynamics
Sergio Lavagnoli von Karman Institute for Fluid Dynamics