An extended version of an algebraic intermittency model for prediction of separation-induced transition at elevated free-stream turbulence level

An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin–Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables

Time resolved PIV measurements of the unsteady wake migration in a LPT blade passage: effect of the wake passing frequency

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. A very large data set has been collected: for each condition, about 9000 instantaneous flow fields have been acquired at up to 2kHz in order to resolve with great detail the vortex shedding phenomenon characterizing the separation at steady condition as well as the propagation of the coherent structures induced by the incoming wake. Instantaneous vector maps, phase averaged velocity fields and Proper Orthogonal Decomposition (POD) have been used for the in depth characterization of the different phenomena. The paper takes advantage of the properties of POD that reduces 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. POD 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), that are smeared out in the phase averaged results. Once recognized the POD modes most involved in the unsteady flow field, a procedure for the quantification of the different contributions to the overall amount of losses is proposed

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

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

Recognition of coherent structures in the boundary layer of a low-pressure-turbine blade for different free-stream turbulence intensity levels

Particle Image Velocimetry (PIV) has been adopted to analyze the instantaneous flow field developing on a high-lift turbine blade profile operating under low and elevated free-stream turbulence conditions (FSTI). Results reported in the paper allow us to analyze the dynamics leading to transition and separation of the suction side boundary layer, looking to generation, propagation and breakdown of coherent structures observed in the two different FSTI cases. To this end, measurements have been performed in two orthogonal planes. Results obtained in the blade-to-blade plane allow the detailed characterization of the propagation of Kelvin\u2013Helmholtz (KH) rolls generating, at low FSTI condition, as a consequence of a non-reattaching separation. Otherwise, data in the wall-parallel plane allow recognizing the presence of three-dimensional disuniformities induced at high FSTI by low and high speed streaks (Klebanoff mode). The sinuous breakdown of boundary layer streaks generates other complex three-dimensional coherent structures such as hairpin or cane-like vortices that induce transition. Proper Orthogonal Decomposition (POD) has been adopted to in depth characterize these structures, thus further explaining the mechanisms through which the free-stream turbulence intensity modify the transition/separation processes of the suction side boundary layer of an highly loaded low pressure turbine blade

POD Analysis of the Unsteady Behavior of a Laminar Separation Bubble

Particle Image Velocimetry (PIV) measurements have been performed in order to analyze the unsteady flow field developing along the separated flow region of a laminar separation bubble. Data have been post-processed by means of Proper Orthogonal Decomposition (POD) to improve the understanding of the physics of this complex phenomenon. The paper shows that the first two POD modes of the normal to the wall velocity component are coupled. Thus, they are representative of a vortex shedding phenomenon which is identified to be induced by Kelvin\u2013Helmholtz instability. The POD allows the phase identification of each PIV image within the vortex shedding cycle. The computed eigenvectors are used to sort the experimental snapshots and then reconstruct a phase-averaged velocity field which highlighted the motion of vortices shed close to the bubble maximum displacement. Moreover, other sources of deterministic fluctuations characterized by frequencies which are different from the one induced by the Kelvin\u2013 Helmholtz instability are also revealed. Indeed, the most energetic POD mode of the streamwise velocity component is not related to the shedding frequency, while it describes large velocity fluctuations in the shear layer region upstream of the bubble maximum displacement, where the turbulent activity is not yet present. The POD decomposition presented here identifies the large scale structures within the flow, thus separately accounts for both coherent and stochastic contributions to the overall energy of the velocity fluctuations

Heat transfer and film cooling of blade tips and endwalls

This paper investigates the flow, heat transfer and film cooling effectiveness of advanced high-pressure turbine blade tips and endwall. Two blade tip configurations have been studied, including a full rim squealer and a partial squealer with a leading edge and trailing edge cut-out. Both blade tip configurations have pressure side film cooling, and cooling air extraction through dust holes which are positioned along the airfoil camber line on the tip cavity floor. The investigated clearance gap and the blade tip geometry are typical of that commonly found in the high pressure turbine blades of heavyduty gas turbines. Numerical studies and experimental investigations in a linear cascade have been conducted at a blade exit isentropic Mach number of 0.8 and a Reynolds number of 9 7 105. The influence of the coolant flow ejected from the tip dust holes and the tip pressure side film holes has also been investigated. Both the numerical and experimental results showed that there is a complex aero-thermal interaction within the tip cavity and along the endwall. This was evident for both tip configurations. Although, the global heat transfer and film cooling characteristics of both blade tip configurations were similar, there were distinct local differences. The partial squealer exhibited higher local film cooling effectiveness at the trailing edge but also low values at the leading edge. For both tip configurations, the highest heat transfer coefficients were located on the suction side rim within the mid-chord region. However on the endwall, the highest heat transfer rates were located close to the pressure side rim and along most of the blade chord. Additionally, the numerical results also showed that the coolant ejected from the blade tip dust holes partially impinges onto the endwall. Copyright \ua9 2010 by Alstom Technology, Ltd