Experimental Study on the Internal Separated and Inverse Flow Between the Blades

2013 ◽  
Author(s):  
Hiroki Furuta ◽  
Osamu Terashima ◽  
Yasuhiko Sakai ◽  
Kouji Nagata ◽  
Shunsuke Ishiguro ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Energy Loss ◽  
Test Section ◽  
Velocity Fluctuation ◽  
Separation Bubble ◽  
Flow Direction ◽  
Separated Flow ◽  
Leading Edge ◽  
Hot Wire ◽  
Blade Thickness

The objective of this study is to investigate an internal separated flow between the blades of turbo machinery. Flow separation often causes undesirable phenomena such as an increase of the total pressure loss and a vibration of the machine. Therefore, understanding the characteristics of the separated flow in detail is very important for optimizing the machine to decrease the energy loss. In general, the separated flow involves a reverse flow near the solid wall in the separation bubble and its reattachment of the further downstream location. Hence, a typical hot-wire sensor is not useful for measuring the internal separated flow between the blades because it can detect only the magnitude of the flow velocity, not the flow direction. Based on this background, a self-developed tandem-type hot-wire sensor, by which both the magnitude and the flow direction can be detected, is used to measure the velocity field between the blades in this study. The tandem-type hot-wire sensor consists of two I-type hot-wire sensors and a small insulated elliptical cylinder placed between them. A calibration test is first conducted to validate its performance. Subsequently, the separated flow between the blades is measured with the tandem-type hot-wire sensor. The experimental apparatus consists of a closed-type test section which is connected to the nozzle exit of a blowout-type wind tunnel. In this test section, a sample of blade is set up. In this study, experiments are conducted with three kinds of blades with the different shapes (i.e., experiments are performed under three different conditions): a constant blade thickness from the leading edge to the trailing edge (Blade 1), a constant blade thickness from the leading edge to trailing edge but a rounded leading edge (Blade 2) and a thin blade thickness at the leading and trailing edges (Blade 3). In addition, the unsteady internal separated flow between the blades is also investigated by large-eddy simulation (LES) whose validity was fully confirmed in the previous study. The flow field and dissipation rate of the turbulent kinetic energy obtained by the simulation are discussed. Experimental and numerical results show that Blade 3 has a smallest separation bubble around the leading edge than that of Blade 1 and 2, and shows a smallest root mean square (RMS) value for the velocity fluctuation near the reattachment point than them. These differences of the size of separation bubble and velocity fluctuation were considered to be resulted in a decrease of the kinetic energy loss in the test section with Blade 3. Therefore, it can be concluded that the non-uniform thickness of the blade causes the decrease of the energy loss around the blade.

Flow Studies of the Leading Edge Stall on a Swept-Back Wing at High Incidence

1956 ◽  
Vol 60 (541) ◽  
pp. 51-60 ◽  
Author(s):  
Joseph Black
Keyword(s):  
Boundary Layer ◽  
Liquid Film ◽  
Boundary Layer Flow ◽  
Separation Bubble ◽  
Flow Direction ◽  
Separated Flow ◽  
Leading Edge ◽  
Outer Edge ◽  
Layer Flow ◽  
Wing Tip

SummaryThe flow separation on a swept-back wing with 44 degrees leading edge sweep at 18 degrees incidence has been investigated by means of detailed pressure distribution measurements over the leading edge, boundary layer flow determination with liquid film technique, and yawmeter traverses. A wool-tuft grid was also used, and a spin detector was developed to search for regions of vorticity. These tests established that the flow separates on the leading edge; over the inboard sections it re-attaches behind a “ short” separation bubble, while outboard it only re-attaches near the trailing edge, thus forming a “ long ” separation bubble, or else fails to attach. The separated flow in what has been commonly called the tip stall does in fact take the form of a “ ram's horn “ vortex with the origin, or “ tip,” located at the junction of the two bubbles on the leading edge. The vortex lies outwards across the wing surface at approximately 20 to 25 degrees to the line-of-flight before curving aft to be shed into the wake, where it extends almost from mid semi-span to the wing tip. This vortex induces considerable changes in flow direction, both on and over the wing, and also in the wake. Thus in the wake a maximum downwash of 23 degrees is induced aft of the mid semi-span, and there is an upwash of 17 degrees at the outer edge of the vortex, almost aft of the tip. A good correlation between yawmeter results and the boundary layer flow direction indications from the liquid film technique was obtained.

Comparison of Flying-Hot-Wire and Stationary-Hot-Wire Measurements of Flow Over a Backward-Facing Step

1999 ◽  
Vol 121 (2) ◽  
pp. 441-445 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Hot Wire Anemometry ◽  
Blunt Leading Edge

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

2021 ◽  
pp. 095441002110212
Author(s):  
K Anand ◽  
KT Ganesh
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
Pressure Gradient ◽  
Particle Image ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Field Measurements ◽  
Bench Mark ◽  
Image Velocimetry

The effect of pressure gradient on a separated boundary layer past the leading edge of an airfoil model is studied experimentally using electronically scanned pressure (ESP) and particle image velocimetry (PIV) for a Reynolds number ( Re) of 25,000, based on leading-edge diameter ( D). The features of the boundary layer in the region of separation and its development past the reattachment location are examined for three cases of β (−30°, 0°, and +30°). The bubble parameters such as the onset of separation and transition and the reattachment location are identified from the averaged data obtained from pressure and velocity measurements. Surface pressure measurements obtained from ESP show a surge in wall static pressure for β = −30° (flap deflected up), while it goes down for β = +30° (flap deflected down) compared to the fundamental case, β = 0°. Particle image velocimetry results show that the roll up of the shear layer past the onset of separation is early for β = +30°, owing to higher amplification of background disturbances compared to β = 0° and −30°. Downstream to transition location, the instantaneous field measurements reveal a stretched, disoriented, and at instances bigger vortices for β = +30°, whereas a regular, periodically shed vortices, keeping their identity past the reattachment location, is observed for β = 0° and −30°. Above all, this study presents a new insight on the features of a separation bubble receiving a disturbance from the downstream end of the model, and these results may serve as a bench mark for future studies over an airfoil under similar environment.

Investigation of the separation bubble formed behind the sharp leading edge of a flat plate at incidence

2000 ◽  
Vol 214 (3) ◽  
pp. 157-176 ◽  
Author(s):  
M J Crompton ◽  
R V Barrett
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
Laser Doppler Anemometry ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Flow Direction ◽  
Leading Edge ◽  
Laser Doppler ◽  
Static Pressure Distribution ◽  
Sharp Leading Edge

Detailed measurements of the separation bubble formed behind the sharp leading edge of a flat plate at low speeds and incidence are reported. The Reynolds number based on chord length ranged from 0.1 × 105 to 5.5 × 105. Extensive use of laser Doppler anemometry allowed detailed velocity measurements throughout the bubble. The particular advantages of laser Doppler anemometry in this application were its ability to define flow direction without ambiguity and its non-intrusiveness. It allowed the mean reattachment point to be accurately determined. The static pressure distribution along the plate was also measured. The length of the separation bubble was primarily determined by the plate incidence, although small variations occurred with Reynolds number because of its influence on the rate of entrainment and growth of the shear layer. Above about 105, the Reynolds number effect was no longer evident. The reverse flow boundary layer in the bubble exhibited signs of periodic stabilization before separating close to the leading edge, forming a small secondary bubble rotating in the opposite sense to the main bubble.

Blade Thickness Effect on Impeller Slip Factor

2010 ◽  
Author(s):  
Donghui Zhang ◽  
Jean-Luc Di Liberti ◽  
Michael Cave
Keyword(s):  
Factor Model ◽  
Numerical Study ◽  
Flow Direction ◽  
Leading Edge ◽  
Thickness Effect ◽  
Centrifugal Impeller ◽  
Trailing Edge ◽  
Slip Factor ◽  
Blade Thickness ◽  
Blockage Factor

A numerical study of the effect of the blade thickness on centrifugal impeller slip factor is presented in this paper. The CFD results show that generally the slip factor decreases as the blade thickness increases. Changing the thickness at different locations has different effects on the slip factor. The shroud side blade thickness has more effect on the impeller slip factor than the hub side blade thickness. In the flow direction, the blade thickness at 50% meridional distance is the major factor affecting the slip factor. The leading edge thickness has little effect on slip factor. There is an optimum thickness at the trailing edge for the maximum slip factor. For this impeller, the hub side thickness ratio of 0.5 between the trailing edge and the middle of the impeller gives the highest value of the slip factor, while the ratio of 0.25 at shroud side gives the highest value of the slip factor. A blockage factor is added into the slip factor model to include the aerodynamic blockage effect on the slip factor. The model explains the phenomena observed in the CFD results and the test data very well.

scholarly journals Studies on a Blade Leading Edge Separation Bubble Affected by Periodic Wakes: Its Transitional Behavior and Boundary Layer Loss Reduction

2002 ◽  
Author(s):  
K. Funazaki ◽  
Y. Kato
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Circular Cylinders ◽  
Test Model ◽  
Hot Wire ◽  
Flat Plates ◽  
Wake Effect ◽  
Time Resolved ◽  
Aerodynamic Loss

This study deals with extensive hot-wire probe measurements of wake-affected separation bubble on the leading edge of a test model. The purpose of the study is to investigate time-resolved response of the separation bubble to incoming wake passing. Another focus is placed on the wake effect on aerodynamic loss generated in the separated boundary layer, seeking any relationship between the suppression of the separation bubble on a cascade airfoil and aerodynamic gain due to the clocking in turbomachines. The test model has a semicircular leading edge and two flat-plates. Incoming wakes are generated by circular cylinders which are horizontally fixed in the wake generator. Several types of wake generating cylinders are used in order to change wake properties. The hot-wire measurements have revealed the time-resolved responses of the separated boundary layer to the wake passage. Effects of calmed regions just behind the moving wakes are also identified.

Predicting Transitional Separation Bubbles

2004 ◽  
Vol 127 (3) ◽  
pp. 497-501
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper describes the modifications made to a successful attached flow transition model to produce a model capable of predicting both attached and separated flow transition. This transition model is used in combination with the Fluent CFD software, which is used to compute the flow around the blade assuming that it remains entirely laminar. The transition model then determines the start of transition location and the development of the intermittency. These intermittency values weight the laminar and turbulent boundary layer profiles to obtain the resulting transitional boundary layer parameters. The ERCOFTAC T3L test cases are used to validate the predictions. The T3L blade is a flat plate with a semi-circular leading edge, which results in the formation of a separation bubble the length of which is strongly dependent on the transition process. Predictions were performed for five T3L test cases for differing free-stream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

Transition Over C4 Leading Edge and Measurement of Intermittency Factor Using PDF of Hot-Wire Signal

1995 ◽  
Author(s):  
Birinchi K. Hazarika ◽  
Charles Hirsch
Keyword(s):  
Separated Flow ◽  
Leading Edge ◽  
Threshold Value ◽  
Angle Of Incidence ◽  
Bypass Transition ◽  
Hot Wire ◽  
Suction Surface ◽  
Intermittency Factor ◽  
Conditional Averaging

The variation of intermittency factors in the transition region of a C4 leading edge flat plate is measured at three incidence angles in a low turbulence free-stream. During the determination of intermittency factor the threshold value of the detector function and the validity of conditional averaging are verified by a method based on the direct application of PDF of the hot-wire output. As the angle of incidence is increased, the transition progressively moves through all the three modes on the suction surface : at zero incidence the bypass transition, at 2° incidence the natural transition and at 4° incidence the separated-flow transition occur respectively. All the three modes of transition exhibited the chord-wise intermittency factor variation in accordance with Narasimha’s universal intermittency distribution, thus the method based on spot production rate is applicable to all the three modes of transition. In the transition zone of the attached boundary layers, the conditionally averaged inter-turbulent profiles are fuller than the Blasius profile while the conditionally averaged turbulent profiles follow a logarithmic profile with a variable additive parameter.

Predicting Transitional Separation Bubbles

2004 ◽  
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
Attached Flow

This paper describes the modifications made to a successful attached flow transition model to produce a model capable of predicting both attached and separated flow transition. This transition model is used in combination with the Fluent CFD software, which is used to compute the flow around the blade assuming that it remains entirely laminar. The transition model then determines the start of transition location and the development of the intermittency. These intermittency values weight the laminar and turbulent boundary layer profiles to obtain the resulting transitional boundary layer parameters. The ERCOFTAC T3L test cases are used to validate the predictions. The T3L blade is a flat plate with a semi-circular leading edge, which results in the formation of a separation bubble the length of which is strongly dependent on the transition process. Predictions were performed for five T3L test cases for differing freestream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

On the steady separated flow around an inclined flat plate

1997 ◽  
Vol 333 ◽  
pp. 403-413 ◽  
Author(s):  
W. W. H. YEUNG ◽  
G. V. PARKINSON
Keyword(s):  
Flat Plate ◽  
Bluff Body ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Source Model ◽  
Pressure Distributions ◽  
Wide Range ◽  
Trailing Edges ◽  
Edge Separation

An inviscid analytic model is proposed for the steady separated flow around an inclined flat plate. With the plate normal to the stream, the model reduces to the wake-source model of Parkinson & Jandali originally developed for flow external to a symmetrical two-dimensional bluff body and its wake. At any other inclination, the Kutta condition is satisfied at both leading and trailing edges of the plate, and, in the limit that the angle of attack approaches zero, classical airfoil theory is recovered. A boundary condition is formulated based on some experimental results of Abernathy, but no additional empirical information is required. The predicted pressure distributions on the wetted surface for a wide range of angle attack are found to be in good agreement with experimental data, especially at smaller angles of attack. An extension to include a leading-edge separation bubble is explored and results are satisfactory.

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