Low Aspect Ratio Turbines

1964 ◽  
Vol 86 (1) ◽  
pp. 13-16 ◽  
Author(s):  
Gunnar O. Ohlsson
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Aspect Ratio ◽  
Axial Distance ◽  
Mass Rate ◽  
Low Aspect Ratio ◽  
Turbine Efficiency ◽  
Aspect Ratios

Four different axial, impulse turbines with extremely low aspect ratios (between 0.07 and 0.70) were tested over wide ranges of pressure and speed ratios. The influence on mass rate of flow and efficiency of Reynolds number and axial distance between stator and rotor is given. Stator and rotor efficiency, Mach number, and flow angles, as well as other quantities, are obtained by means of a wheel with axial outlet. Semiempirical formulas are given for turbine efficiency, stator efficiency, and rotor efficiency as functions of aspect ratio.

scholarly journals Leading-Edge Vortex Characteristics of Low-Aspect-Ratio Sweptback Plates at Low Reynolds Number

2021 ◽  
Vol 11 (6) ◽  
pp. 2450
Author(s):  
Jong-Seob Han ◽  
Christian Breitsamter
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Leading Edge ◽  
Flapping Wing ◽  
Flat Plates ◽  
Leading Edge Vortex ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Oil Flow ◽  
Edge Vortex

A sweptback angle can directly regulate a leading-edge vortex on various aerodynamic devices as well as on the wings of biological flyers, but the effect of a sweptback angle has not yet been sufficiently investigated. Here, we thoroughly investigated the effect of the sweptback angle on aerodynamic characteristics of low-aspect-ratio flat plates at a Reynolds number of 2.85 × 104. Direct force/moment measurements and surface oil-flow visualizations were conducted in the wind-tunnel B at the Technical University of Munich. It was found that while the maximum lift at an aspect ratio of 2.03 remains unchanged, two other aspect ratios of 3.13 and 4.50 show a gradual increment in the maximum lift with an increasing sweptback angle. The largest leading-edge vortex contribution was found at the aspect ratio of 3.13, resulting in a superior lift production at a sufficient sweptback angle. This is similar to that of a revolving/flapping wing, where an aspect ratio around three shows a superior lift production. In the oil-flow patterns, it was observed that while the leading-edge vortices at aspect ratios of 2.03 and 3.13 fully covered the surfaces, the vortex at an aspect ratio of 4.50 only covered up the surface approximately three times the chord, similar to that of a revolving/flapping wing. Based on the pattern at the aspect ratio of 4.50, a critical length of the leading-edge vortex of a sweptback plate was measured as ~3.1 times the chord.

Reynolds-number scaling of vortex pinch-off on low-aspect-ratio propulsors

2016 ◽  
Vol 799 ◽  
Author(s):  
John N. Fernando ◽  
David E. Rival
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Time Scales ◽  
Early Stage ◽  
Reynolds Numbers ◽  
Growth Time ◽  
Flat Plates ◽  
Low Aspect Ratio ◽  
Drag Forces ◽  
Aspect Ratios

Impulsively started, low-aspect-ratio elliptical flat plates have been investigated experimentally to understand the vortex pinch-off dynamics at transitional and fully turbulent Reynolds numbers. The range of Reynolds numbers investigated is representative of those observed in animals that employ rowing and paddling modes of drag-based propulsion and manoeuvring. Elliptical flat plates with five aspect ratios ranging from one to two have been considered, as abstractions of propulsor planforms found in nature. It has been shown that Reynolds-number scaling is primarily determined by plate aspect ratio in terms of both drag forces and vortex pinch-off. Due to vortex-ring growth time scales that are longer than those associated with the development of flow instabilities, the scaling of drag is Reynolds-number-dependent for the aspect-ratio-one flat plate. With increasing aspect ratio, the Reynolds-number dependency decreases as a result of the shorter growth time scales associated with high-aspect-ratio elliptical vortex rings. Large drag peaks are observed during early-stage vortex growth for the higher-aspect-ratio flat plates. The collapse of these peaks with Reynolds number provides insight into the evolutionary convergence process of propulsor planforms used in drag-based swimming modes over diverse scales towards aspect ratios greater than one.

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

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.

Experimental Study on Flow Around Circular Cylinders With Low Aspect Ratio

2013 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Guilherme F. Rosetti ◽  
Guilherme R. Franzini ◽  
André L. C. Fujarra
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Strouhal Number ◽  
Water Channel ◽  
Circular Cylinders ◽  
Piv Measurements ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Fluctuation Frequency ◽  
Six Degree Of Freedom

Experiments were carried out in a recirculating water channel regarding the flow around stationary circular cylinders with low aspect ratio piercing the water free surface. Eight different aspect ratios were tested, namely L/D = 0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5 and 2.0; this range corresponds to aspect ratio related to circular offshore systems, such as spar and monocolumn platforms. Force was measured using a six degree-of-freedom load cell and Strouhal number is inferred through the transverse force fluctuation frequency. The range of Reynolds number covers 10,000 < Re < 50,000. PIV measurements were performed in some aspect ratio cases, namely 0.3, 0.5, 1.0 and 2.0 for Reynolds number equal to 43,000. The results showed a decrease in drag force coefficients with decreasing aspect ratio, as well as a decrease in Strouhal number with decreasing aspect ratio. The PIV showed the existence of an arch-type vortex originated in the cylinder free end.

Heat Transfer for High Aspect Ratio Rectangular Channels in a Stationary Serpentine Passage With Turbulated and Smooth Surfaces

2013 ◽  
Author(s):  
Matthew A. Smith ◽  
Randall M. Mathison ◽  
Michael G. Dunn
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Behavior ◽  
Reynolds Numbers ◽  
Hydraulic Diameter ◽  
Internal Cooling ◽  
Number Range ◽  
Aspect Ratios ◽  
Parallel Configuration

Heat transfer distributions are presented for a stationary three passage serpentine internal cooling channel for a range of engine representative Reynolds numbers. The spacing between the sidewalls of the serpentine passage is fixed and the aspect ratio (AR) is adjusted to 1:1, 1:2, and 1:6 by changing the distance between the top and bottom walls. Data are presented for aspect ratios of 1:1 and 1:6 for smooth passage walls and for aspect ratios of 1:1, 1:2, and 1:6 for passages with two surfaces turbulated. For the turbulated cases, turbulators skewed 45° to the flow are installed on the top and bottom walls. The square turbulators are arranged in an offset parallel configuration with a fixed rib pitch-to-height ratio (P/e) of 10 and a rib height-to-hydraulic diameter ratio (e/Dh) range of 0.100 to 0.058 for AR 1:1 to 1:6, respectively. The experiments span a Reynolds number range of 4,000 to 130,000 based on the passage hydraulic diameter. While this experiment utilizes a basic layout similar to previous research, it is the first to run an aspect ratio as large as 1:6, and it also pushes the Reynolds number to higher values than were previously available for the 1:2 aspect ratio. The results demonstrate that while the normalized Nusselt number for the AR 1:2 configuration changes linearly with Reynolds number up to 130,000, there is a significant change in flow behavior between Re = 25,000 and Re = 50,000 for the aspect ratio 1:6 case. This suggests that while it may be possible to interpolate between points for different flow conditions, each geometric configuration must be investigated independently. The results show the highest heat transfer and the greatest heat transfer enhancement are obtained with the AR 1:6 configuration due to greater secondary flow development for both the smooth and turbulated cases. This enhancement was particularly notable for the AR 1:6 case for Reynolds numbers at or above 50,000.

Numerical Prediction of Contaminant Removal from Cavity in Horizontal Channel by Constrained Interpolated Profile Method

2014 ◽  
Vol 695 ◽  
pp. 384-388
Author(s):  
Nor Azwadi Che Sidik ◽  
A.S. Ahmad Sofianuddin ◽  
K.Y. Ahmat Rajab
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Removal Rate ◽  
Contaminant Removal ◽  
Square Cavity ◽  
Profile Method ◽  
Aspect Ratios ◽  
Contaminants Removal ◽  
Parabolic Flow ◽  
High Removal Rate

In this paper, Constrained Interpolated Profile Method (CIP) was used to simulate contaminants removal from square cavity in channel flow. Predictions were conducted for the range of aspect ratios from 0.25 to 4.0. The inlet parabolic flow with various Reynolds number from 50 to 1000 was used for the whole presentation with the same properties of contaminants and fluid. The obtained results indicated that the percentage of removal increased at high aspect ratio of cavity and higher Reynolds number of flow but it shows more significant changes as increasing aspect ratio rather than increasing Reynolds number. High removal rate was found at the beginning of the removal process.

Power Augmentation of a HAWT by Mie-type Tip Vanes, considering Wind Tunnel Flow Visualisation, Blade-Aspect Ratios and Reynolds Number

2003 ◽  
Vol 27 (3) ◽  
pp. 183-194 ◽  
Author(s):  
Yukimaru Shimizu ◽  
Edmond Ismaili ◽  
Yasunari Kamada ◽  
Takao Maeda
Keyword(s):  
Reynolds Number ◽  
Wind Tunnel ◽  
Aspect Ratio ◽  
Reynolds Numbers ◽  
Flow Visualisation ◽  
Velocity Measurements ◽  
Horizontal Axis Wind Turbine ◽  
Aspect Ratios ◽  
Detailed Surface ◽  
Blade Tips

Wind tunnel results are reported concerning the effects of blade aspect ratio and Reynolds number on the performance of a horizontal axis wind turbine (HAWT) with Mie-type1 tip attachments. The flow behaviour around the blade tips and the Mie-type tip vanes is presented. Detailed surface oil film visualization and velocity measurements around the blade tips, with and without Mie vanes, were obtained with the two-dimensional, Laser-Doppler Velocimetry method. Experiments were performed with rotors having blades with different aspect ratio and operating at different Reynolds numbers. The properties of the vortices generated by the Mie vanes and the blade tips were carefully studied. It was found that increased power augmentation by Mie vanes is achieved with blades having smaller aspect ratio and smaller Reynolds number.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

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