Investigation of flow separation in a transonic-fan linear cascade using visualization methods
An extensive experimental study into the nature of the separated flows on the blade suction surface of modern transonic fans is described in this paper. The study was a subtask of a larger experimental effort focused on blade flutter excited by flow separation in the blade tip region. The tip sections of airfoils on transonic fan blades are designed for precompression and consequently they differ from sections on the rest of the blade. The blade tip section was modeled by a low aspect ratio blade and therefore most of the blade tested was exposed to the secondary flow effects. The aim of this work was to supply reliable data on flow separation on transonic fan blades for validation of future analytical studies. The experimental study focused on two visualization techniques: surface flow visualization using dye oils and schlieren (and shadowgraph) flow visualization. The following key observations were made during the study. For subsonic inlet flow, the flow on the suction surface of the blade was separated over a large portion of the blade, and the separated area increased with increasing inlet Mach number. For the supersonic inlet flow condition, the flow was attached from the leading edge up to the point where a bow shock from the upper neighboring blade imposed on the blade surface. Downstream, there was a separated flow region in which air flowed in the direction opposite the inlet flow. Finally, past the separated flow region, the flow reattached to the blade surface. For subsonic inlet flow, the low cascade solidity resulted in an increased area of separated flow. For supersonic flow conditions, the low solidity resulted in an improvement in flow over the suction surface.
On Flowfield Periodicity in the NASA Transonic Flutter Cascade: Part I — Experimental Study
An extensive study to improve flow uniformity and periodicity in the NASA Transonic Flutter Cascade is presented here. The results are reported in two independent parts dealing with the experimental approach and the analytical approach. The first part, the Experimental Study, focuses first on the data sets acquired in this facility in the past and explains several discrepancies, particularly the questions of actual flow incidence and cascade backpressure levels. Next, available means for control and modifications of the cascade flowfield, boundary layer bleed and tailboard settings are presented in detail. This is followed by experimental data sets acquired in modified test facility configurations that were based on analytical predictions of the cascade flowfield. Finally, several important conclusions about improving the cascade flowfield uniformity and blade load periodicity are summarized. The most important conclusions are: (1) boundary layer bleed system does not improve the cascade flow periodicity; (2) carefully match the tunnel wall contours to the expected shape of cascade streamlines; (3) measure actual flow incidence for each cascade configuration rather than rely on the tunnel geometry; and (4) the current cascade configuration exhibits a very high blade load uniformity over six blades from blade #2 to blade #7, and the facility is now ready for unsteady pressure data acquisition.Copyright © 2000 by ASME
The Influence of Tailboards on Unsteady Measurements in a Linear Cascade
The present paper shows the comparison between steady-state and unsteady results from two versions of a flow solver and measurements in a linear cascade at transonic flow conditions for forced vibration of a single blade. Experiments have been conducted in a linear test facility with a cascade composed of five turbine blades. The center blade was forced to oscillate in a pure bending mode with a frequency of 160 Hz. Tailboards are fixed in this test facility at the trailing edges of the border blades to create a periodic flow at the outlet of the cascade. A 2D inviscid flow solver for unsteady flows through vibrating blade rows was adapted to simulate the flow conditions in the linear cascade. The initial adaptation of the flow solver was the limitation to only one vibrating blade. Comparisons with measurements showed discrepancies which were hypothesized to be due to the presence of the tailboards which can create reflections of pressure waves and shocks. Therefore, as a second modification of the program, the periodic boundaries downstream of the blades were replaced by solid walls. Based on these computations it can be concluded that for transonic flows the tailboards have a significant influence on both the steady-state and unsteady flow in the outlet region as well as on the blade in the shock region. In particular, the unsteady data measured in a linear cascade has to be analyzed very carefully to estimate the influence of the tailboards. NOMENCLATURE