The present paper discusses the experimental results of a big experimental data set describing transitional boundary layers. Time resolved Particle image velocimetry measurements have been adopted to survey the boundary layer developing over a flat plate under prescribed adverse pressure gradients typical of turbomachinery component. The tests have been performed varying the pressure gradient, the Reynolds number and the inlet free-stream turbulence intensity (FSTI). Under such conditions the natural transition does not occur and the boundary layer undergoes transition by two different mechanisms: the separated flow transition, occurring at low Reynolds number and low FSTI, or the, so called, by-pass transition that is induced by the high free-stream turbulence intensity. The dynamics of this two types of transition are discussed in details focusing on the mechanisms that lead to the production of turbulence kinetic energy (TKE) and hence of losses.

Two exemplary cases, referring to the two different types of transition, are discussed in details by means of proper orthogonal decomposition (POD) and principal axis analysis. The POD is used at first to provide the statistical representation of the flow structures that occur for the two different types of transition. It confirms that, in the case of laminar separation bubble, the most energetic and coherent structures are related to low frequency fluctuations of the bubble shear layer and to the shedding of vortical structures due to the Kelvin-Helmholtz instability. For the by-pass type, the first POD modes highlight the presence of boundary layer streaks, that are structures elongated in the streamwise direction, and their breakdown to turbulence forming hairpin vortices.

The properties of POD are further exploited to compute the turbulence production (i.e., the mean flow energy dissipation) due the dynamical features observed for the different types of transition. To this purpose, reduced order model representations of the flow field are provided and their quota to the total turbulence kinetic energy production is isolated. This analysis is completed by the inspection of the eigenvectors of the strain rate and Reynolds stress tensors, that provide the principal axes of the deformation of the mean flow and turbulence properties. Therefore, the regions where the eigenvectors of the two tensors are aligned are related to the largest production of turbulence kinetic energy and losses as well. For the separated flow transition, it is shown that the eigenvectors of strain rate and shear tensor are almost perfectly aligned downstream of the bubble maximum displacement. This corresponds to the region where the reduced order model reconstruction of the Kelvin-Helmholtz shed vortices provides the largest quota to the overall TKE production. For the high FSTI induced transition type, the eigenvectors of the shear and stress tensors do not present the same exact direction. In this case the maximum loss production is related to the local maximum Reynolds shear stress region that is ascribed to the breakdown of the boundary layer streaks to hairpin vortices.


Experimental Investigation on the Loss Production Mechanisms in Transitional Boundary Layers

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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