Session: 32-03 CFD Studies

Paper Number: 153663

Response of Transitional Boundary Layers Over Low-Pressure Turbine Blades to Varying Characteristics of Free-Stream Turbulence 

The low-pressure turbines (LPTs) in modern aircraft engines typically operate at low Reynolds numbers of O(10⁵). Consequently, the boundary layers over high-lift/ultra-high-lift LPT blades are prone to separation under strong local adverse pressure gradients. Intermittent free-stream turbulence (FST), periodic wakes shed by the moving upstream blades and surface roughness due to in-service degradation can suppress the separation bubble, however at the cost of an increased turbulent wetted area. Literature have shown that the intensity of free-stream disturbances affects the transition process [1, 2], altering the boundary layer losses significantly. However, the range over which the intensity is investigated is small (0.1-5%, typically) and the understanding on the influence of integral length scales of FST on the boundary layer development is limited. Hence in this work we will explore the role of both intensity and integral length scales of FST on the transitional and turbulent boundary layer growth over a LPT blade. The range of turbulent intensity and integral length scale will be varied from 4-15% and 8-24% (in terms of axial chord), respectively, to characterize the boundary layer behavior under conditions of industrial relevance. To this end, we perform a series of wall-resolved Large Eddy Simulations (LES) of flow over a LPT blade (designed at the VKI) under low-speed conditions. An in-house unstructured and massively parallel high-fidelity solver – AVBP, developed at CERFACS, is employed for this study. The wall-adapting local eddy-viscosity (WALE) sub-grid scale model is used for the LES computations. To impose free-stream-turbulence at the inlet, the synthetic turbulence method of Bailly et al. [3] with a Passot-Pouquet spectrum [4] is adopted. Flow statistics for various test cases will be compared based on the time-averaged flow quantities and boundary layer integral parameters, and the boundary layer losses will be analyzed in detail. Furthermore, the influence of free-stream turbulence parameters on the near-wall velocity profiles and Reynolds stresses will be explored. Wherever possible, the results will be compared and contrasted against the available Reynolds Averaged Navier-Stokes (RANS) predictions from the VKI thereby providing accurate turbulent characteristics for RANS turbulence models. It is expected that this study will aid in a better understanding of the behavior of blade surface boundary layers that are exposed to free-stream disturbances representative of engine relevant conditions.

[1] Durovic, K., Vincentiis, L.D., Simoni, D., Lengani, D., Pralits, J., Henningson, D.S., and Hanifi, A. (2021), “Free-Stream Turbulence-Induced Boundary-Layer Transition in Low-Pressure Turbines,” ASME J. Turbomach., 143(8), 081015.

[2] Lengani, D. and Simoni, D. (2015), “Recognition of coherent structures in the boundary layer of a low-pressure-turbine blade for different free-stream turbulence intensity levels,” Int. J. Heat Fluid Flow, 54, pp. 1–13.

[3] Bailly, C., Gloerfelt, X. and Bogey, C. (2002), “Report on stochastic noise source modelling”, Projet JEAN, Technical report.

[4] Passot, T. and Pouquet, A. (1987) “Numerical Simulation of Compressible Homogeneous Flows in the Turbulent Regime.” J. Fluid Mech., 181, pp. 441 – 466.

Presenting Author: Ananth S M CERFACS

Presenting Author Biography: My research areas of interest involve transition, low-pressure turbine flows, passive flow control using roughness and riblets, compressor flow instabilities (surge and rotating stall), scale-resolving simulations and high-performance computing employing GPUs. At CERFACS, as a post-doctoral researcher, I am investigating - the boundary layer transition over low-pressure turbine blades employing high-fidelity Large Eddy Simulations and turbine averaging methods. I hold a PhD degree (jointly with IIT Madras and the University of Melbourne) and my dissertation is on 'Roughened Ultra High-Lift Blades with Grooved for Drag Reduction'.

https://scholar.google.com/citations?user=WDUf3IQAAAAJ&hl=en&oi=ao

Authors:

Ananth S M CERFACS
Nicolas Odier CERFACS
Florent Duchaine CERFACS
Giacomo Pastorino von Karman Institute for Fluid Dynamics
Elissavet Boufidi von Karman Institute for Fluid Dynamics
Fabrizio Fontaneto von Karman Institute for Fluid Dynamics