Erratum on Flow Characteristics in a Transonic Ultra-Low-Aspect-Ratio Axial Turbine Vane

Micro aerial vehicles are currently receiving growing interest because of their broad applications in many fields. In their flight tests, the onset of unwanted large amplitude roll oscillations was reported, which resulted in difficulties with flight control, and this has become one of the major challenges of current micro aerial vehicles design. In this review type of article, the low Reynolds number flow characteristics of a low-aspect-ratio wing are reviewed, and the self-induced roll oscillations are discussed with special attention being payed to the interaction between the three-dimensional flow structure and wing in reciprocatively rolling motion. The roll attenuation methods are introduced via flow control approaches, which can suppress the roll oscillations effectively by manipulating the leading-edge flow separation and tip vortices of the low-aspect-ratio wings.

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Effects of Mach Number and Secondary Flows on Ultra-Low Aspect Ratio Radial Outflow Turbine Cascade Aerodynamics

Volume 2E: Turbomachinery ◽

10.1115/gt2020-14146 ◽

2020 ◽

Author(s):

Aki Grönman ◽

Jonna Tiainen ◽

Antti Uusitalo

Keyword(s):

Mach Number ◽

Aspect Ratio ◽

Turbine Cascade ◽

Penetration Length ◽

Axial Turbine ◽

Low Aspect Ratio ◽

Aspect Ratios ◽

Flow Phenomena ◽

Axial Turbines ◽

Radial Outflow

Abstract Radial outflow turbines are an alternative for axial turbines for example in heat recovery applications. They are, however, also often characterized by ultra-low aspect ratios. In these designs, the secondary losses dominate the overall loss share, and under a certain aspect ratio, the secondary structures from the hub and shroud begin to interact. This interaction causes a decrease in aerodynamic performance. Previous studies have suggested that the general flow phenomena between radial outflow and axial turbines could share several similarities due to observed trends in performance prediction. The blade outlet Mach number is known to affect the spanwise positions of the secondary vortices in axial turbine blading and therefore, its effect is also tested here for an ultra-low aspect ratio radial outflow turbine cascade. In addition, there are currently no cascade level experimental data publicly available, and the suitability of axial turbine loss correlations under these conditions remains an open question. From this background, the current study presents an experimental, numerical, and loss correlation analysis of the effects of an isentropic Mach number in a radial outflow turbine cascade. An experimental campaign is used to validate the numerical model both quantitatively and qualitatively. In addition, the validity of the axial turbine loss correlation is extended to ultra-low aspect ratios by introducing a new variable called penetration length. The main findings are: 1. The flow phenomena do not differ significantly from what has been observed with axial turbines, 2. The effect of penetration length calculation method on the loss breakdown is relatively low, and 3. With ultra-low aspect ratio radial outflow turbines, the loss breakdown is markedly changed when the extended Benner’s approach is employed.

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Unsteady winglet-cavity tip on leakage flow in a high-pressure turbine stage of low-aspect ratio

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1177/0954406217741517 ◽

2017 ◽

Vol 232 (20) ◽

pp. 3708-3721 ◽

Cited By ~ 1

Author(s):

Zhihua Zhou ◽

Shaowen Chen ◽

Songtao Wang

Keyword(s):

High Pressure ◽

Aspect Ratio ◽

Flow Characteristics ◽

Leakage Flow ◽

Turbine Stage ◽

High Pressure Turbine ◽

Tip Leakage ◽

Low Aspect Ratio ◽

Passage Vortex ◽

Blade Tip

In an unshrouded high-pressure turbine, the upstream vane wake, vane–blade interaction and blade tip leakage flow indicate complex and unsteady flow characteristics. Considering a high-pressure turbine stage of low-aspect ratio, the effects of the flat tip, cavity tip and winglet-cavity tip on the unsteady flow characteristics are investigated by numerical simulation. The exit Mach number and Reynolds number based on the chord of vane are 0.9 and 5.5×105, respectively. The pressure ratio of stage is 2.4. The time-resolved results indicate that the winglet-cavity tip scheme has smaller time variation of the total performance parameters and obtains a smaller tip leakage mass flow and a higher turbine stage efficient than the other cases. The vane–rotor interaction affects the pressure distribution at the region of blade leading edge remarkably and leads to a large time variation of tip leakage mass flow at the front of blade tip. The unsteady aerodynamic performance is analyzed with entropy-increase distribution at stage outlet and the vane–rotor interaction is discussed by the entropy-increase phase–time and phase–phase diagrams at blade outlet. Compared with the flat tip and cavity tip, the upper passage vortex loss is reduced obviously by the winglet-cavity tip. Thus, it is evaluated that the improvement of turbine stage coefficient with winglet-cavity tip results from the reduction of the upper passage vortex loss.

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Effect of Endwall Contouring on Performance of Ultra-Low Aspect Ratio Transonic Turbine Inlet Guide Vanes

Volume 6: Turbo Expo 2007, Parts A and B ◽

10.1115/gt2007-28210 ◽

2007 ◽

Cited By ~ 1

Author(s):

Toyotaka Sonoda ◽

Martina Hasenja¨ger ◽

Toshiyuki Arima ◽

Bernhard Sendhoff

Keyword(s):

Aspect Ratio ◽

Secondary Flow ◽

Flow Characteristics ◽

Design Concept ◽

Guide Vanes ◽

Low Aspect Ratio ◽

Inlet Guide Vanes ◽

Turbine Inlet ◽

Transonic Turbine ◽

Endwall Contouring

In our previous work on ultra-low aspect ratio transonic turbine inlet guide vanes for a small turbofan engine [1], we used numerical stochastic design optimization to propose the new design concept of an extremely aft-loaded airfoil to improve the difficult-to-control aerodynamic loss. At the same time, it is well known that endwall contouring is an effective method for reducing the secondary flow loss. In the literature, both “axisymmetric” and “non-axisymmetric” endwall geometries have been suggested. Almost all of these geometric variations have been based on the expertise of the turbine designer. In our current work, we employed a stochastic optimization method — the evolution strategy (ES) — to optimize and analyze the effect of the axisymmetric endwall contouring on the inlet guide vanes’ performance. In the optimization, the design of the endwall contour was divided into three different approaches: 1) only hub contour, 2) only tip contour, and 3) hub and tip contour together with the possibility to observe the correlation between hub/tip changes with regard to their joint influence on the pressure loss. Furthermore, three dimensional flow mechanisms, related to a secondary flow near the endwall region in the low-aspect ratio transonic turbine IGV, was investigated, based on above optimization results. A design concept and secondary flow characteristics for the low-aspect ratio full annular transonic turbine IGV is discussed in this paper.

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