Wake-induced Boundary Layer Transition Research Articles

자유유동 난류강도에 따른 익형 위 후류유도 경계층 천이의 거동

Wake-induced boundary-layer transition on a NACA0012 airfoil with zero angle of attack is experimentally investigated in periodically passing wakes under the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensities at the leading edge of the airfoil are 0.5 and 3.5%, respectively. The Reynolds number (Rec) based on chord length (C) of the airfoil is , and Strouhal number (Stc) of the passing wake is about 1.4. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The patch under the high free-stream turbulence $(Tu_{\infty}

BLADEROW INTERACTIONS, TRANSITION, AND HIGH-LIFT AEROFOILS IN LOW-PRESSURE TURBINES

▪ Abstract The flow in turbomachines is unsteady due to the relative motion of the rows of blades. In the low-pressure turbine, the wakes from the upstream bladerows provide the dominant source of unsteadiness. Because much of the blade-surface boundary-layer flow is laminar, one of the most important consequences of this unsteadiness is the interaction of the wakes with the suction-side boundary layer of a downstream blade. This is important because the blade suction–side boundary layers are responsible for most of the loss of efficiency and because the combined effects of random (wake turbulence) and periodic disturbances (wake velocity defect and pressure fields) cause the otherwise laminar boundary layer to undergo transition and eventually become turbulent. This article summarizes the process of wake-induced boundary-layer transition in low-pressure turbines and the loss generation processes that result. Particular emphasis is placed on how the effects of wakes may be exploited to control loss generation and how this has enabled successful development of ultra-high-lift low-pressure turbines.

Wake-Induced Boundary Layer Transition in a Low-Speed Axial Compressor

The boundary layer on the blades of turbomachines substantially influences the loss production of the machine as well as the heat transfer between the blade surface and the fluid. For this reason the knowledge of the boundary layer structure and development plays an important role for the understanding and further improvement of turbomachines. The incoming wakes of the upstream blade rows strongly periodically influences the boundary layer on turbomachine blades. For this reason an unsteady, highly complex boundary layer behaviour can be observed. This paper presents experimental results of the unsteady boundary layer behaviour on the first stage stator blades of the four-stage Dresden Low-Speed Research Compressor. Different operating points of the compressor are investigated. Special consideration is taken on the analysis of the time-resolved structure of the incoming flow field and the unsteady boundary layer response. The wake passing triggers a periodic transition of the boundary layer starting already at the leading edge of the blade. The wake-induced transitional regions limit the extension of the laminar boundary layer in the front part of the blade. An improved description of the complex boundary layer structure is given.

A study of unsteady wake-induced boundary-layer transition with thermochromic liquid crystals

A method of using thermochromic liquid crystals has been developed to visualize the thermal footprints of artificially created turbulent spots as they propagate downstream in an otherwise laminar boundary layer. In the present study, this technique was employed to study the turbulent events, which were generated spontaneously by turbulent wakes of circular cylinders convecting in a laminar boundary layer over a heated flat plate. The experiment was conducted in a water tunnel at a free-stream velocity of 0.2 m/s. The purpose of the experiment was to simulate the unsteady wake-induced boundary-layer transition typically occurring in multistage turbomachines. The location of the onset of transition and the turbulent spot formation rate were obtained by processing the recorded colour change of thermochromic liquid crystals for the first time. The results were compared with those predicted from standard correlations.

Visualization of turbulent spots and unsteady wake-induced boundary-layer transition with thermochromic liquid crystals

A method of using thermochromic liquid crystals has been developed to visualize the thermal footprints of turbulent spots convecting downstream in an otherwise laminar boundary layer over a heated surface. This technique has been employed to visualize the development of turbulent spots under the influence of adverse pressure gradients. It has also been used to visualize the transitional events that occur during unsteady wake-induced boundary layer transition typically of those occurring in multi-stage turbomachines. The results show that liquid crystal is not only capable of providing quantitative information about the growth and development of individual spots but also allows a detailed study of formation of turbulent spots occurring naturally during a complicated transition process.

Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

Boundary Layer Transition Induced by Periodic Wake Passage. Effect of Velocity Fluctuation Caused by Wake Passage.

Detailed measurements are performed on boundary layers on a flat plate subjected to incident periodic wakes. The objective of this study is to examine whether the velocity fluctuation associated with the wake passage, which is due to a so-called negative jet, might have any effects on the wake-induced boundary layer transition. A spoked-wheel-type wake generator is used to simulate the unsteady flowfield over the suction surface of a turbine blade or a compressor blade by changing the direction of rotation of the wake generator. Wake-affected heat transfer distributions on the flat plate indicate that the wake passage promotes the boundary layer transition ; however, appreciable difference in the transition onset point appears between the two unsteady flow conditions with the normal rotation and the reverse rotation of the wake generator, emulating a turbine blade case and a compressor blade case, respectively. This difference is also confirmed by the boundary layer measurement using a hot-wire probe, which shows that a turbulent region induced within the boundary layer by the wake passage for the reverse rotation case evolves downstream at a consider-ably slower speed compared to that of the normal rotation case.