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.

Control of Separation Bubble on a Blade Leading Edge by a Stationary Bar Wake

2000 ◽  
Author(s):  
K. Funazaki ◽  
Y. Harada ◽  
E. Takahashi
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Test Model ◽  
Flat Plates ◽  
Flow Measurements ◽  
Inlet Flow ◽  
Aerodynamic Loss ◽  
Edge Separation ◽  
Blade Leading Edge

This paper describes an attempt to suppress a blade leading edge separation bubble by utilizing a stationary bar wake. This study aims at exploration of a possibility for reducing the aerodynamic loss due to blade boundary layer that is accompanied with the separation bubble. The test model used in this study consists of semi-circular leading edge and two parallel flat plates. It can be tilted against the inlet flow so as to change the characteristics of the separation bubble. Detailed flow measurements over the test model are conducted using a single hot-wire probe. Emphasis in this study is placed on the effect of bar shifting or bar clocking across the inlet flow in order to see how the bar-wake position with respect to the test model affects the separation bubble as well as aerodynamic loss generated within the boundary layer. The present study reveals a loss reduction through the separation bubble control using a properly clocked bar wake.

Interactions of Separation Bubble With Oncoming Wakes by LES

2011 ◽  
Author(s):  
S. Sarkar ◽  
Jasim Sadique
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Flat Plates ◽  
Flow Past A Cylinder ◽  
Periodic Behaviour ◽  
Rotor Stator Interaction ◽  
Multiple Bubbles

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied with the help of Large-eddy simulations (LES). A flat plate with a semicircular leading edge is employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case an infinite row of flat plates). This setup is a simplified representation of the rotor-stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of the disturbances, becomes turbulent and finally reattaches forming a bubble. In the presence of oncoming wakes, the characteristics of the separated layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase averaged results illustrate the periodic behaviour of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of separated shear layer by convective wakes forming coherent vortices, which are being shed and convect downstream. This interaction also breaks the bubble into multiple bubbles. Further, the transition of the shear layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations.

Studies on Effects of Periodic Wake Passing Upon a Blade Leading Edge Separation Bubble: Experimental Investigation Using a Simple Leading Edge Model

2003 ◽  
Author(s):  
K. Funazaki ◽  
K. Yamada ◽  
Y. Kato
Keyword(s):  
Experimental Investigation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Test Cases ◽  
Test Model ◽  
Flat Plates ◽  
Aerodynamic Interaction ◽  
Edge Separation ◽  
Wake Passing ◽  
Probe Measurements

This paper describes experimental investigation on aerodynamic interaction between incoming periodic wakes and leading edge separation bubble on a compressor or turbine blade, using a scaled leading edge model. The studies aims at expanding the range of the test conditions from that of the previous study (Funazaki and Kato [15]) in order to deepen the knowledge on how and to what extent upstream wake passing suppresses the leading edge separation bubble. Special attention is paid to the transitional behaviors of the separated boundary layers, in particular, to the emergence of wake-induced turbulence spots. Hot-wire probe measurements are then executed under five different flow conditions. The test model has a simple structure consisting of a semi-circular leading edge and two flat-plates. Cylindrical bars of the wake generator generate the periodic wakes in front of the test model. Effects of Reynolds number, Strouhal number, direction of the bar movement and incidence of the test model against the incoming flow are examined in this paper. The measurements reveal that the wake moving over the separation bubble does not directly suppress the separation bubble. Instead, wake-induced turbulence spots and the subsequent calmed regions have dominant impacts on the separation bubble suppression for the all test cases. Distinct difference of the bubble suppressing effect by the wakes is also observed when the direction of the bar movement is altered.

scholarly journals Reynolds number influence on the formation of vortical structures on a pitching flat plate

2017 ◽  
Vol 7 (1) ◽  
pp. 20160079 ◽  
Author(s):  
Alexander Widmann ◽  
Cameron Tropea
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Flow Structures ◽  
Flat Plates ◽  
Time Resolved ◽  
Flow Phenomena ◽  
Wall Interaction ◽  
The Impact

The impact of chord-based Reynolds number on the formation of leading-edge vortices (LEVs) on unsteady pitching flat plates is investigated. The influence of secondary flow structures on the shear layer feeding the LEV and the subsequent topological change at the leading edge as the result of viscous processes are demonstrated. Time-resolved velocity fields are measured using particle image velocimetry simultaneously in two fields of view to correlate local and global flow phenomena in order to identify unsteady boundary-layer separation and the subsequent flow structures. Finally, the Reynolds number is identified as a parameter that is responsible for the transition in mechanisms leading to LEV detachment from an aerofoil, as it determines the viscous response of the boundary layer in the vortex–wall interaction.

Effect of Leading-Edge Geometry on Separation Bubble on a Compressor Blade

2003 ◽  
Author(s):  
Huoxing Liu ◽  
Baojie Liu ◽  
Ling Li ◽  
Haokang Jiang
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Separation Bubble ◽  
Research Work ◽  
Wedge Angle ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Test Model ◽  
Edge Geometry ◽  
Separation Bubbles

Accurate prediction of flow field is the most important factor during the design of high performance compressors. In some cases the agreement of pressure ratio and efficiency between predicted and measured is excellent, but it is common for the efficiency to be in error by perhaps one or two percent. This error is enough to render the calculation unable to replace expensive experiment testing. One of the important matters in need of more study is the mechanism of boundary layer transition from laminar to turbulent flow. The objective of this fundamental research work is to acquire the detailed structure of separation bubbles on the suction side of the blade by using the PIV and pressure taps. This paper presents an experimental study of the influence of 2d leading-edge geometry on behavior of separation bubbles. The measurements on a nose of enlarged blade were conducted on a special large-scale experimental facility, the pressure distribution and flowfield of flow were measured. The test model used in this study consists of circular leading edge and elliptic leading edge. Results are presented for a range of incidence. The measurement result indicated that the leading edge shape has a large influence on flow details separation and transition as well as the boundary layer properties after reattached point. The wedge angle appears to be an important role in leading edge geometry parameters.

The Pervasive Effect of the Calmed Region

2005 ◽  
Author(s):  
R. L. Thomas ◽  
J. P. Gostelow
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Laminar Separation Bubble ◽  
Free Shear Layer ◽  
Hot Wire ◽  
Strong Suppression ◽  
Velocity Fluctuations ◽  
Laminar Separation

Experiments have been conducted relating to the interaction of imposed freestream wakes upon a flat plate laminar separation bubble under an adverse pressure gradient. Controlled wakes, representative of those seen in turbomachinery environments, were used to investigate unsteadiness effects upon a separating boundary layer that undergoes natural transition in the free shear layer under steady conditions. Hot-wire anemometry using a single hot-wire has shown leading edge boundary layer disturbances induced under each passing wake, which grow steadily via by-pass and natural transition methods into turbulent strips that convect with the flow. These disturbances are of such strength that the separated region is resisted and effectively swept away by the passing turbulence, momentarily giving rise to a wholly attached laminar boundary layer. Controlling the chord-wise proximity of neighboring wakes allowed for the investigation of the effect and extent of the calmed region behind each induced turbulent strip. Measurements have shown that a strong suppression of velocity fluctuations is seen related to the proximity of the turbulent strips. Turbulence level reductions of up to 40% have been demonstrated as wake spacing is reduced. Even for those cases where systematic wakes are sufficiently close together to prevent the development of a visible calmed region, very strong calming influences are seen in the wake induced turbulent domain that would have normally been occupied by the calmed flow.

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.

Optimal Application of Riblets on Compressor Blades and Their Contamination Behavior

2011 ◽  
Author(s):  
Christoph Lietmeyer ◽  
Karsten Oehlert ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
Particle Deposition ◽  
Smooth Surface ◽  
Separation Bubble ◽  
Suction Side ◽  
Compressor Blade ◽  
Viscous Drag ◽  
Loss Reduction ◽  
Flat Plates ◽  
Profile Loss

During the last decades, riblets have shown a potential for viscous drag reduction in turbulent boundary layers. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery type blades with ideal riblet geometry have been reported in the literature. The question where riblets must be applied on the surface of a compressor blade is still not sufficiently answered. In a first step, the profile loss reduction by ideal triangular riblets with a trapezoidal groove and a constant geometry along the surface on the suction and pressure side of a compressor blade is investigated. The results show a higher potential on the profile loss reduction by riblets on the suction side. In a second step, the effect of laser-structured ribs on the laminar separation bubble and the influence of these structures on the laminar boundary layer near the leading edge are investigated. After clarifying the best choices where riblets should be applied on the blade surface, a strategy for locally adapted riblets is presented. The suction side of a compressor blade is laser-structured with a segmented riblet-like structure with a constant geometry in each segment. The measured profile loss reduction shows the increasing effect on the profile loss reduction of this locally adapted structure compared to a constant riblet-geometry along the surface. Furthermore, the particle deposition on a riblet-structured compressor blade is investigated and compared to the particle deposition on a smooth surface. Results show a primary particle deposition on the riblet tips followed by an agglomeration. The particle deposition on the smooth surface is stochastic.

The Evolution of Streamwise Counter-Rotating Vortices in Flat Plate Boundary Layer With Leading Edge Pattern

2015 ◽  
Author(s):  
Seyed Mohammad Hasheminejad ◽  
Hatsari Mitsudharmadi ◽  
S. H. Winoto ◽  
Kim Boon Lua ◽  
Hong Tong Low
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Plate Boundary ◽  
Leading Edge ◽  
Streamwise Vortex ◽  
Streamwise Velocity ◽  
Stream Velocity ◽  
Hot Wire ◽  
Layer Flow ◽  
Different Characteristics

The evolution of streamwise counter-rotating vortices induced by different leading edge patterns is investigated quantitatively using hot-wire anemometer. A notched and triangular leading edge with the same wavelength and amplitude were designed to induce streamwise vortices over a flat plate at Reynolds number (based on the wavelength of the leading edge patterns) of 3080 corresponding to free-stream velocity of 3 m/s. The streamwise velocity at different streamwise locations collected and analyzed using a single wire probe hot-wire anemometer showed reveal different characteristics of boundary layer flow due to the presence of these two leading edge patterns. The major difference is the appearance of an additional streamwise vortex between the troughs of the notched pattern. Such vortices increase the mixing effect in the boundary layer as well as the velocity profile.

Interactions of Separation Bubble With Oncoming Wakes by Large-Eddy Simulation

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
S. Sarkar ◽  
Harish Babu ◽  
Jasim Sadique
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Constant Thickness ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Flow Past A Cylinder ◽  
Rotor Stator Interaction

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied using large-eddy simulation (LES). A series of airfoils of constant thickness with rounded leading edge are employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case, an infinite row of the model airfoils). This setup is a simplified representation of the rotor–stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of disturbances, becomes turbulent, and finally reattaches forming a separation bubble. In the presence of oncoming wakes, the characteristics of the separated boundary layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase-averaged results illustrate the periodic behavior of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of the boundary layer by convective wakes forming coherent vortices, which are being shed and convect downstream. Further, the transition of the separated boundary layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations, where viscous effect is substantial.

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