Session: 41-07 Turbine Secondary Flows and Interactions II

Paper Number: 101925

101925 - Coherent Turbulent Stresses in Unsteady Forced Transonic Nozzle With Shock-Induced Separation 

Modern turbomachines are known to exhibit highly unsteady flows in which turbulence plays an important role. In transonic regime, a shock-wave/boundary layer interaction occurs and leads to flow separation if the pressure gradient imposed by the shock is strong enough. The separation is further magnified by the blade curvature and turbulence is considerably amplified downstream. In addition, rotor-stator interaction, which causes the backpressure to oscillate, impacts the interaction and the turbulence behaviors. These effects strongly limit the engine aerodynamic performance and accounting for them in the design process would give room for improvement.

Whereas current turbomachine designs rely on Reynolds Averaged Navier-Stokes simulations, the complex features of transonic flow can only be captured using high-fidelity methods such as Large-Eddy Simulations and Direct Numerical Simulations. Nevertheless, these approaches are computationally expensive, especially if one considers the whole span of the blade. A way to circumvent this problem is to investigate simpler configurations that can reproduce the same flow features.

In this work, the analysis performed is therefore based upon the results from an Implicit Large-Eddy Simulation of the transonic flow over a bump. Such a configuration is relevant to study turbomachinery flows and has already been investigated experimentally. By choosing appropriate pressure conditions, a shock-wave develops over the bump and interacts with the boundary layer. Moreover, the back pressure is set to fluctuate in order to mimic rotor-stator interaction at a realistic reduced frequency.

The solver employed is built around the flux reconstruction, a recent approach for developing high-order schemes. In flows involving discontinuities such as shocks, care must be taken to ensure robustness. For this purpose, an artificial viscosity technique is used and has been modified so that the turbulence is not affected. A previous work on a canonical oblique shock-wave/boundary layer interaction assessed the ability of the solver and the performance of the shock-capturing scheme.

The impact of the forced conditions on the flow features is assessed. In a triple decomposition framework, harmonic (or coherent) components of the flow are extracted using a conditional averaging technique. The condition here is derived from the separation point location. The coherent turbulent stresses downstream of the interaction are investigated. To get a better understanding of this coherent flow field, its temporal evolution is analyzed. Budgets are also studied to put in light dominant terms. The data will serve as a basis to develop a reduced-order model of the coherent turbulent stresses that will be able to accurately consider the impact of unsteady potential effect and should be used early in the design process.

Presenting Author: Nicolas Goffart Cadence Design Systems Belgium

Presenting Author Biography: Nicolas Goffart is an engineer graduated from the Faculty of Engineering of Mons (Belgium) in energy engineering and also holds the engineering degree of ISAE-SUPAERO (France) where he majored in aerodynamics. He is now a PhD student working in Cadence Design Systems Belgium on high-fidelity simulations and reduced-order modeling for shock-wave/boundary layer interactions.

Authors:

Nicolas Goffart Cadence Design Systems Belgium
Benoît Tartinville Cadence Design Systems Belgium
Sergio Pirozzoli La Sapienza Unviersity of Rome