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

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

Turbulent Flows Over Rough Forward Facing Steps

2014 ◽  
Author(s):  
Weijie Shao ◽  
Martin Agelin-Chaab
Keyword(s):  
Turbulent Flows ◽  
Large Scale ◽  
Reattachment Length ◽  
Particle Image Velocimetry Technique ◽  
Large Scale Structures ◽  
Image Velocimetry ◽  
The Mean ◽  
Scale Structures ◽  
Scale Turbulence ◽  
Proper Orthogonal

This paper reports an investigation of the effects of rough forward facing steps on turbulent flows. The surfaces of the rough steps were covered with sandpapers. A particle image velocimetry technique was used to conduct measurements at the mid-plane of the test section and at several locations downstream to 68 step heights. A Reynolds number of Reh = 4800 and δ/h = 4.7 were employed, where h is the mean step height and δ is the incoming boundary layer thickness. The results indicate that mean reattachment length decreases with increasing roughness. In addition, the effect of the step roughness decreases with downstream distance. The proper orthogonal decomposition results showed that the step roughness affects even the large scale structures. Furthermore, the reconstructed turbulence quantities suggest that the step roughness suppresses the large scale turbulence.

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

2017 ◽  
Vol 140 (4) ◽  
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.

Measurement of Velocity Profiles of Laminar to Turbulent Water Flows in a Tube Using Particle Image Velocimetry

2004 ◽  
Author(s):  
Meredith R. Martin
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Particle Image ◽  
Glass Tube ◽  
Reynolds Numbers ◽  
Velocity Profiles ◽  
Reynolds Number Flow ◽  
Image Velocimetry ◽  
Dye Visualization ◽  
Number Flow

The transition from laminar to turbulent in-tube flow is studied in this paper. Water flow in a glass tube with an inside diameter of 21.7 mm was investigated by two methods. First, a dye visualization test using a setup similar to the 1883 experiment of Osborne Reynolds was conducted. For the dye visualization, Reynolds numbers ranging from approximately 1000 to 3500 were tested and the transition from laminar to turbulent flow was observed between Reynolds numbers of 2500 and 3500. For the second method, a particle image velocimetry (PIV) system was used to measure the velocity profiles of flow in the same glass tube at Reynolds numbers ranging from approximately 500 to 9000. The resulting velocity profiles were compared to theoretical laminar profiles and empirical turbulent power-law profiles. Good agreement was found between the lower Reynolds number flow and the laminar profile, and between the higher Reynolds number flow and turbulent power-law profile. In between the flow appeared to be in a transition region and deviated some between the two profiles.

Flow Instabilities in Gas Turbine Chute Seals

2020 ◽  
Vol 142 (2) ◽  
Author(s):  
Joshua T. M. Horwood ◽  
Fabian P. Hualca ◽  
Mike Wilson ◽  
James A. Scobie ◽  
Carl M. Sangan ◽  
...  
Keyword(s):  
Large Scale ◽  
Computational Study ◽  
Leading Edge ◽  
Large Scale Structures ◽  
Strong Shear ◽  
Time Marching ◽  
Flow Through ◽  
Scale Structures ◽  
Large Scale Flow ◽  
Secondary Flow Field

Abstract The ingress of hot annulus gas into stator–rotor cavities is an important topic to engine designers. Rim-seals reduce the pressurized purge required to protect highly stressed components. This paper describes an experimental and computational study of flow through a turbine chute seal. The computations—which include a 360 deg domain—were undertaken using dlrtrace's time-marching solver. The experiments used a low Reynolds number turbine rig operating with an engine-representative flow structure. The simulations provide an excellent prediction of cavity pressure and swirl, and good overall agreement of sealing effectiveness when compared to experiment. Computation of flow within the chute seal showed strong shear gradients which influence the pressure distribution and secondary-flow field near the blade leading edge. High levels of shear across the rim-seal promote the formation of large-scale structures at the wheel-space periphery; the number and speed of which were measured experimentally and captured, qualitatively and quantitatively, by computations. A comparison of computational domains ranging from 30 deg to 360 deg indicates that steady features of the flow are largely unaffected by sector size. However, differences in large-scale flow structures were pronounced with a 60 deg sector and suggest that modeling an even number of blades in small sector simulations should be avoided.

Large-scale structures in a developed flow over a wavy wall

2003 ◽  
Vol 478 ◽  
pp. 257-285 ◽  
Author(s):  
AXEL GÜNTHER ◽  
PHILIPP RUDOLF VON ROHR
Keyword(s):  
Velocity Field ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Water Channel ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Wavy Surface ◽  
Large Scale Structures ◽  
Stress Maximum ◽  
Scale Structures

We address – motivated in part by the findings of Gong et al. (1996) and Miller (1995) – the role of streamwise-oriented large-scale structures in a developed flow between a sinusoidal bottom wall and a flat top wall. Particle image velocimetry (PIV) is used to examine the spatial variation of the velocity in different planes of the flow through a water channel with an aspect ratio of 12:1. The wave amplitude is equal to one tenth of the wall wavelength, Λ, and Reynolds numbers between 500 and 7300, defined with the bulk velocity and the half-height of the channel, are considered. To examine streamwise-oriented structures, the spanwise variation of the velocity field is studied in a plane parallel to the top wall, and in one that intersects the wavy surface at an uphill location. From a proper orthogonal decomposition (POD) of the streamwise velocity fluctuations, we obtain the dominant eigenfunctions with a characteristic spanwise scale of O(1.5Λ), which agrees with the scale of perturbations for the streamwise velocity at laminar conditions. A decomposition of the turbulent velocity field close to the uphill section of the wavy surface reveals smaller structures at a location that coincides with the Reynolds shear stress maximum.

Flow Instabilities in Gas Turbine Chute Seals

2019 ◽  
Author(s):  
Joshua T. M. Horwood ◽  
Fabian P. Hualca ◽  
Mike Wilson ◽  
James A. Scobie ◽  
Carl M. Sangan ◽  
...  
Keyword(s):  
Large Scale ◽  
Computational Study ◽  
Leading Edge ◽  
Large Scale Structures ◽  
Strong Shear ◽  
Time Marching ◽  
Flow Through ◽  
Scale Structures ◽  
Large Scale Flow ◽  
Secondary Flow Field

Abstract The ingress of hot annulus gas into stator-rotor cavities is an important topic to engine designers. Rim-seals reduce the pressurised purge required to protect highly-stressed components. This paper describes an experimental and computational study of flow through a turbine chute seal. The computations — which include a 360° domain — were undertaken using DLR TRACE’s time-marching solver. The experiments used a low Reynolds number turbine rig operating with an engine-representative flow structure. The simulations provide an excellent prediction of cavity pressure and swirl, and good overall agreement of sealing effectiveness when compared to experiment. Computation of flow within the chute seal showed strong shear gradients which influence the pressure distribution and secondary-flow field near the blade leading edge. High levels of shear across the rim-seal promote the formation of large-scale structures at the wheel-space periphery; the number and speed of which were measured experimentally and captured, qualitatively and quantitatively, by computations. A comparison of computational domains ranging from 30° to 360° indicate that steady features of the flow are largely unaffected by sector size. However, differences in large-scale flow structures were pronounced with a 60° sector and suggest that modelling an even number of blades in small sector simulations should be avoided.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Hassan Iftekhar ◽  
Martin Agelin-Chaab
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Step Height ◽  
Reattachment Length ◽  
Mean Velocity ◽  
The Mean

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

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