Features of a Laminar Separated Boundary Layer Near the Leading-Edge of a Model Airfoil for Different Angles of Attack: An Experimental Study

The evolution of a separated boundary layer over a model airfoil with semicircular leading-edge has been illustrated for angles of attack (α) varying from −3 deg to 10 deg, where the Reynolds number (Rec) based on chord is 1.6 × 105 and the inlet freestream turbulence (fst) being 1.2%. The features of boundary layer are described through measurements of velocity and surface pressure besides the flow visualization using a planar particle image velocimetry (PIV). Freestream perturbations are amplified because of enhanced receptivity of the separated boundary layer resulting in pockets of disturbances, which then propagate downstream attributing to random fluctuations near the reattachment. The separation and reattachment locations including the onset and end of transition are identified for changing α. The reattachment point changes from 18.8% to 47.7% of chord with the onset of separation at almost 7%, whereas the onset of transition moves upstream from 13.2% to 9% with increasing α. The bubble bursting occurs at α = 10 deg. The transition in the separated boundary layer occurs through Kelvin–Helmholtz (K–H) instability for α = 0 deg and 3 deg, whereas the K–H mechanism is bypassed for higher α with significant viscous effect.

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

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/09544100211021234 ◽

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.

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Tomographic Particle Image Velocimetry Measurements of a High Reynolds Number Turbulent Boundary Layer

ERCOFTAC Series - Progress in Wall Turbulence: Understanding and Modeling ◽

10.1007/978-90-481-9603-6_12 ◽

2011 ◽

pp. 113-120 ◽

Cited By ~ 1

Author(s):

Callum Atkinson ◽

Sebastien Coudert ◽

Jean-Marc Foucaut ◽

Michel Stanislas ◽

Julio Soria

Keyword(s):

Boundary Layer ◽

Reynolds Number ◽

Particle Image Velocimetry ◽

Turbulent Boundary Layer ◽

High Reynolds Number ◽

Particle Image ◽

Tomographic Particle Image Velocimetry ◽

Image Velocimetry ◽

High Reynolds

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Computational Fluid Dynamics and Particle Image Velocimetry Supported Examination of Bidirectional Velocity Probes for Measurements in Flames

Journal of Thermal Science and Engineering Applications ◽

10.1115/1.4024795 ◽

2013 ◽

Vol 6 (1) ◽

Cited By ~ 1

Author(s):

Nadir Yilmaz ◽

Brian C. Hogan ◽

Humberto Bocanegra ◽

A. Burl Donaldson ◽

Walt Gill

Keyword(s):

Fluid Dynamics ◽

Experimental Study ◽

Reynolds Number ◽

Wind Tunnel ◽

Pressure Difference ◽

Particle Image ◽

Amplification Factor ◽

Local Velocity ◽

Bernoulli’S Equation ◽

Image Velocimetry

The bidirectional velocity probe has been used in various flames to measure local velocity. The device is based on the pressure difference between a closed forward facing cavity and a closed rearward facing cavity. The probes have been noted to indicate a pressure difference greater than that which would be predicted based on Bernoulli's equation. Each device must be experimentally calibrated in a wind tunnel at similar Reynolds number to determine its “amplification factor.” This study uses PIV, flow visualization and CFD to examine the flow field around the probe, as well as an experimental study which compares various probe configurations for measurement of velocity by pressure differential. The conclusion is that the amplification factor is indeed greater than unity but use of the wind tunnel for calibration is questionable.

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Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

Experiments in Fluids ◽

10.1007/s00348-020-03039-w ◽

2020 ◽

Vol 61 (9) ◽

Author(s):

K. Fujiwara ◽

R. Sriram ◽

K. Kontis

Keyword(s):

Reynolds Number ◽

Large Scale ◽

Particle Image ◽

Leading Edge ◽

Reynolds Numbers ◽

Reattachment Length ◽

Large Scale Structures ◽

Image Velocimetry ◽

Scale Structures ◽

Sharp Leading Edge

Abstract Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

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Discrepancy in the Aerodynamic Property and Flowfield of a Symmetric Airfoil Produced by the Stationary and Moving Ground Effect

Journal of Fluids Engineering ◽

10.1115/1.4048290 ◽

2020 ◽

Vol 143 (2) ◽

Author(s):

V. Tremblay-Dionne ◽

T. Lee

Keyword(s):

Boundary Layer ◽

Particle Image Velocimetry ◽

Particle Image ◽

Ground Surface ◽

Leading Edge ◽

Ground Effect ◽

Edge Region ◽

Flow Passage ◽

Image Velocimetry ◽

Aerodynamic Property

Abstract The discrepancy in the aerodynamic property and flowfield of a symmetric airfoil produced by the stationary and moving ground effect was quantified through surface pressure and particle-image-velocimetry measurements. The results show that the stationary ground effect produced a higher lift than the moving ground due to the flow passage restriction caused by the longitudinal boundary layer developed on its ground surface. In close ground proximity, the formation of a ground vortex beneath the airfoil's leading-edge region speeded up the flow, leading to a lower lift than its moving-ground counterpart. For the moving ground, the ground vortex was absent. In close ground proximity, the moving ground effect generated a larger wake and drag than the stationary ground effect.

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Experimental study of eddy structures in a turbulent boundary layer using particle image velocimetry

Journal of Fluid Mechanics ◽

10.1017/s0022112005004751 ◽

2005 ◽

Vol 535 ◽

pp. 143-188 ◽

Cited By ~ 165

Author(s):

JOHAN CARLIER ◽

MICHEL STANISLAS

Keyword(s):

Boundary Layer ◽

Experimental Study ◽

Particle Image Velocimetry ◽

Turbulent Boundary Layer ◽

Particle Image ◽

Image Velocimetry ◽

Eddy Structures

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Wind tunnel study of separated and reattaching flows by particle image velocimetry and pressure measurements

Advances in Structural Engineering ◽

10.1177/1369433218824918 ◽

2019 ◽

Vol 22 (7) ◽

pp. 1769-1782 ◽

Cited By ~ 1

Author(s):

ZR Shu ◽

QS Li

Keyword(s):

Reynolds Number ◽

Particle Image Velocimetry ◽

Turbulence Intensity ◽

Particle Image ◽

Leading Edge ◽

Length Scale ◽

Pressure Measurements ◽

Pressure Coefficients ◽

Image Velocimetry ◽

The Mean

This article presents a comprehensive investigation on the separated and reattaching flows over a blunt flat plate with different leading-edge shapes by means of particle image velocimetry and surface pressure measurements. Wind tunnel tests are performed in both smooth and various turbulent flow conditions, and the separated and reattaching flows are examined as a function of Reynolds number ( Re), leading-edge shape, turbulence intensity, and turbulence integral length scale. It is shown through the particle image velocimetry and pressure measurements that the Reynolds number effect is significant regarding the mean vorticity field, but with little effect on the mean velocity field. For the effects of leading-edge shape, the distributions of pressure coefficients respond strongly to the change in leading-edge angle, and both the velocity (streamwise and vertical) and vorticity fields have a clear dependence on the leading-edge shape. For the effects of freestream turbulence, the mean pressure coefficient responds strongly to turbulence intensity, whereas the fluctuating and peak suction pressure coefficients are dependent on both turbulence intensity and integral length scale. The size of the separation bubble contracts aggressively with increasing turbulence intensity, but it remains approximately invariant in response to the change in turbulence scale in the tested range.

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Particle Image Velocimetry Investigation of the Coherent Structures in a Leading-Edge Slat Flow

Journal of Fluids Engineering ◽

10.1115/1.4038091 ◽

2017 ◽

Vol 140 (4) ◽

Cited By ~ 3

Author(s):

Patrick R. Richard ◽

Stephen John Wilkins ◽

Joseph W. Hall

Keyword(s):

Reynolds Number ◽

Particle Image Velocimetry ◽

Shear Layer ◽

Coherent Structures ◽

Particle Image ◽

Leading Edge ◽

Reynolds Numbers ◽

Image Velocimetry ◽

Lower Reynolds Number ◽

Number Case

Air traffic volume is expected to triple in the U.S. and Europe by 2025, and as a result, the aerospace industry is facing stricter noise regulations. Apart from the engines, one of the significant contributors of aircraft noise is the deployment of high-lift devices, like leading-edge slats. The unsteady turbulent flow over a leading-edge slat is studied herein. In particular, particle image velocimetry (PIV) measurements were performed on a scale-model wing equipped with a leading-edge slat in the H.J. Irving–J.C.C. Picot Wind Tunnel. Two Reynolds numbers based on wing chord were studied: Re = 6 × 105 and 1.3 × 106. A snapshot proper orthogonal decomposition (POD) analysis indicated that differences in the time-averaged statistics between the two Reynolds numbers were tied to differences in the coherent structures formed in the slat cove shear layer. In particular, the lower Reynolds number flow seemed to be dominated by a large-scale vortex formed in the slat cove that was related to the unsteady flapping and subsequent impingement of the shear layer onto the underside of the slat. A train of smaller, more regular vortices was detected for the larger Reynolds number case, which seemed to cause the shear layer to be less curved and impinge closer to the tail of the slat than for the lower Reynolds number case. The smaller structures are consistent with Rossiter modes being excited within the slat cove. The impingement of the shear layers on and the proximity of the vortices to the slat and the main wing are expected to be strong acoustic dipoles in both cases.

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Study of Laminar Horseshoe Vortex Using Particle Image Velocimetry

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.110-116.3249 ◽

2011 ◽

Vol 110-116 ◽

pp. 3249-3254

Author(s):

Zaw Zaw Oo ◽

Muhammad Younis Yamin ◽

Hua Zhang ◽

Muhammad Zaka ◽

Bo Hu

Keyword(s):

Reynolds Number ◽

Particle Image Velocimetry ◽

Saddle Point ◽

Particle Image ◽

Circular Cross Section ◽

Leading Edge ◽

Cylindrical Body ◽

Horseshoe Vortex ◽

Particle Image Velocimetry Technique ◽

Image Velocimetry

—This study investigates the upstream of the juncture flows generated by the circular cross section cylindrical body mounted on a flat plate using PIV (Particle Image Velocimetry) technique. The flow structure of laminar horseshoe vortex and a topological insight into the flow pattern of the vortex system were observed. Vortex structures for ReD(Diameter Reynolds number) 1600, 2000, 2400 and 3500 are predicted and discussed in detail. Experiments were conducted to investigate the structure of steady and periodic horseshoe vortex, the effect of Diameter Reynolds number, location of horseshoe vortex core and its variation with the change in Diameter Reynolds number and the location and nature of the saddle point located most upstream of the leading edge of the cylinder. The results revealed that (a) two different flow regimes were observed corresponding to four Reynolds number ranges; (b) the upstream vortex systems approach closer to the cylinder whereas the distance of saddle point located upstream of the leading edge of the cylinder moves away from the wall when the Reynolds number increases.

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