Abstract
The performance of turbomachinery blade profiles, at low Reynolds numbers, is influenced by laminar separation bubbles (LSBs). Such a bubble is caused by a strong adverse pressure gradient (APG), and it makes the laminar boundary layer to separate from the curved profile surface, before it becomes turbulent. The paper consists on a joint experimental and numerical investigation on a flat plate with adverse pressure gradient. The experiment provides detailed results including distribution of wall pressure coefficient and boundary layer velocity and turbulence profiles for several values of typical influencing parameters on the behavior of the flow phenomena: Reynolds number, free stream turbulence intensity, and end-wall opening angle, which determines the adverse pressure gradient intensity. The numerical work consists on carrying out a systematic analysis, with Reynolds Average Navier-Stokes (RANS) simulations. The results of the numerical simulations are critically investigated and compared with the experimental ones in order to understand the effect of the main physical parameters on the LSB behavior. For RANS simulations, different turbulence and transition models are compared at first to identify the adaptability to the flow phenomena; then, the influence of the three aforementioned parameters on the LSB behavior is investigated under a typical aggressive adverse pressure gradient. Boundary layer integral parameters are discussed for the different cases in order to understand the flow phenomena in terms of flow time-mean properties.
Highlights
The performance of blade profiles for turbomachinery components is strongly influenced by many aerodynamic phenomena
Laminar separation bubble is of primary importance for low-pressure turbine and compressor (LPT and LPC) applications, especially when high-lift profiles operating at low Reynolds number conditions are considered
laminar separation bubbles (LSBs) is caused by a strong adverse pressure gradient, and it makes the laminar boundary layer to separate from the curved profile surface; it can have large, negative aerodynamic effects
Summary
The performance of blade profiles for turbomachinery components is strongly influenced by many aerodynamic phenomena. Li and Yang [16] investigated numerically the transition process in a separation bubble induced by an adverse pressure gradient on a flat plate with an elliptical leading edge under 2.9% freestream turbulence intensity by LES. LES study of a transitional separated bubble over a flat plate with a blunt leading edge at a very low freestream turbulence level is carried out by Yang et al [18], with result indicates that a three-dimensional secondary instability is the main mechanism at work whereas the subharmonic mode in the form of vortex pairing is hardly active. The study of Yaras [22] shows that for the plate flow with strong adverse pressure gradient, the relationship between the characteristic points of separation can be constructed by formulas, which means that it is possible to predict the overall structure of separation bubble by using Reynolds number, turbulence intensity, and integral scale as variables. These quantities are studied and validated by numerical simulations from the perspectives of timeaveraged state
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