A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

• Published in: International journal of turbomachinery, propulsion and power Vol. 2; no. 4; p. 17
• Main Authors: Lengani, Davide, Simoni, Daniele, Ubaldi, Marina, Zunino, Pietro, Bertini, Francesco
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.12.2017
• Subjects: Boundary layer; Boundary layers; Energy dissipation; Fluid dynamics; Fluid flow; Fluids; Kinetic energy; Migration; Particle image velocimetry; Proper Orthogonal Decomposition; Turbines; turbulent kinetic energy production; unsteady losses; Velocity measurement; Vortex shedding; Vortices; wake-boundary layer interaction; Wakes; Italy
• ISSN: 2504-186X; 2504-186X
• DOI: 10.3390/ijtpp2040017

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation by means of a new procedure based on proper orthogonal decomposition (POD). The paper takes advantage of the properties of POD that reduce the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. Proper orthogonal decomposition is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e., the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), which are smeared out in the phase averaged results. A procedure exploiting the biorthogonality condition of the POD modes, and the related temporal coefficients, has been developed for the quantification of the contribution due to the different POD modes to the overall turbulence kinetic energy production, or, equivalently, the mean flow energy dissipation rate. Results into the paper clearly show that losses due to wake migration, boundary layer and vortex shedding related phenomena can be distinguished and separately quantified for the different tested conditions.