Dynamic Characteristics of Flow Separation From a Low Reynolds Number Airfoil

2007 ◽  
Author(s):  
Daniel R. Morse ◽  
James A. Liburdy
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Large Scale ◽  
Region Of Interest ◽  
Leading Edge ◽  
Chord Length ◽  
Vortical Flow ◽  
Local Circulation ◽  
Time Resolved ◽  
Freestream Velocity

This study examines the generation of large scale vortices caused by flow separation from a flat wing at various angles of attack. Time-resolved particle image velocimetry is used to determine the evolution and convective characteristics of the large scale structures. A rectangular airfoil with aspect ratio of 0.5 is used and data are collected at a Reynolds number of 23,500, for angles of attack from 0° to 20°. Data consists of two dimensional velocity fields obtained at 500 Hz located at the airfoil centerline. The region of interest is near the separation point but fields of view extend over approximately one half of the chord length from the leading edge to document the downstream progression of the large scale vortical flow elements. The velocity data were processed to identify the vorticity field dynamics in terms of the Kelvin-Helmholtz instability occurring near the leading edge. The vortical structures are identified using vortex detection based on local circulation. The convective nature of the vortex elements are shown to consist of merging, stalling and convecting, with convective velocities on the order of 20% of the freestream velocity with an associated Stouhal number based on chord length and freestream velocity of approximately 1.0.

Vortex Detection and Characterization in Low Reynolds Number Separation

2007 ◽  
Author(s):  
Daniel R. Morse ◽  
James A. Liburdy
Keyword(s):  
Reynolds Number ◽  
Low Reynolds Number ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Chord Length ◽  
Separation Flow ◽  
Time Resolved ◽  
Freestream Velocity ◽  
Vortex Detection ◽  
Low Reynolds

This study focuses on the detection and characterization of vortices in low Reynolds number separation flow over the elliptical leading edge of a flat plate airfoil. Velocity fields were obtained using Time Resolved Particle Image Velocimetry (TRPIV). The Reynolds number based on chord length ranged from 14,700 to 66,700. Experiments were performed for velocities of 1.1, 2.0 and 5.0 m/s and angles of attack of 14°, 16°, 18° and 20°. These velocities correspond to chord length Reynolds numbers of 1.47×104, 2.68×104, and 6.70×104, respectively. A local swirl calculation was used to determine regions of high circulation, and the convection of the centers of these regions was used to determine convective velocities of these vortical structures. The streamwise convective velocity normalized by the freestream velocity is observed to range from approximately 0.4 to 0.65 over the range of angles of attack, with slightly increasing values as the angle of attack increases.

scholarly journals Numerical simulation of NACA4412 airfoil in pre-stall conditions

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Vincent Gleize ◽  
Michel Costes ◽  
Ivan Mary
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Mach Number ◽  
Skin Friction ◽  
Flow Separation ◽  
Turbulent Mixing ◽  
Design Methodology ◽  
Freestream Velocity ◽  
Content Type ◽  
Laminar Separation

Purpose The purpose of this paper is to study turbulent flow separation at the airfoil trailing edge. This work aims to improve the knowledge of stall phenomenon by creating a QDNS database for the NACA412 airfoil. Design/methodology/approach Quasi-DNS simulations of the NACA 4412 airfoil in pre-stall conditions have been completed. The Reynolds number based on airfoil chord and freestream velocity is equal to 0.35 million, and the freestream Mach number to 0.117. Transition is triggered on both surfaces for avoiding the occurrence of laminar separation bubbles and to ensure turbulent mixing in the wake. Four incidences have been considered, 5, 8 10 and 11 degrees. Findings The results obtained show a reasonably good correlation of the present simulations with classical MSES airfoil simulations and with RANS computations, both in terms of pressure and skin-friction distribution, with an earlier and more extended flow separation in the QDNS. The database thus generated will be deeply analysed and enriched for larger incidences in the future. Originality/value No experimental or HPC numerical database at reasonable Reynolds number exists in the literature. The current work is the first step in that direction.

Experimental Investigation of Free Stream Turbulence Effects on Flow Separation

2003 ◽  
Author(s):  
Mahmoud Ardebili ◽  
Yiannis Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Flow Separation ◽  
Large Scale ◽  
Free Stream ◽  
Pressure Coefficient ◽  
Free Stream Turbulence ◽  
Vorticity Flux

An experimental investigation of a separated boundary layer flow has been attempted which has been created by perturbing a flat plate flow with a favorable pressure gradient immediately followed by an adverse pressure gradient. The aim of the research program is possible control of flow separation by means of free stream turbulence. The flow is configured in a large-scale low speed wind tunnel where measurements of turbulence can be obtained with high spatial and temporal resolution. A model has been designed by using CFD analysis. Mean wall pressure and vorticity flux measurements are reported in this paper. Twelve experiments with three different mesh size grids at three different Reynolds numbers have been carried out. Three bulk flow parameters seem to characterize the flow: The Reynolds number of the boundary layer, Re+, the Reynolds number of the flow through the grid, ReM, and the solidity of the grid. It was found that the pressure coefficient depends weakly on the solidity of the grids. Vorticity flux also depends on the grid used to generate free stream turbulence. The location of maximum or minimum vorticity flux moves upstream at higher ReM.

Experimental Study of Flow Separation over NACA633018 Wing with Synthetic Jet Control at Low Reynolds Numbers

2012 ◽  
Vol 29 (1) ◽  
pp. 45-52 ◽  
Author(s):  
C.-Y. Lin ◽  
F.-B. Hsiao
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Jet Control

AbstractThis paper experimentally studies flow separation and aerodynamic performance of a NACA633018 wing using a series of piezoelectric-driven disks, which are located at 12% chord length from the leading edge to generate a spanwise-distributed synthetic jets to excite the passing flow. The experiment is conducted in an open-type wind tunnel with Reynolds numbers (Re) of 8 × 104 and 1.2 × 105, respectively, based on the wing chord. The oscillations of the synthetic jet actuators (SJAs) disturb the neighboring passage flow on the upper surface of the wing before the laminar separation takes place. The disturbances of energy influence the downstream development of boundary layers to eliminate or reduce the separation bubble on the upper surface of the wing. Significant lift increase and drag decrease are found at the tested Reynolds number of 8 × 104 due to the actuators excitation. Furthermore, the effect of drag also reduces dominant with increasing Reynolds number, but the increase on lift is reduced with the Reynolds number increased.

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.

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

Flow Structure Above Stationary and Oscillating Low-Aspect-Ratio Wing

2012 ◽  
Author(s):  
Miguel R. Visbal ◽  
Daniel J. Garmann
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Structure ◽  
Large Scale ◽  
Lift Coefficient ◽  
Pressure Fluctuations ◽  
Leading Edge ◽  
Tip Vortex ◽  
Eddy Simulation ◽  
Moderate Reynolds Number

Computations have been carried out in order to describe the complex unsteady flow structure over a stationary and plunging aspect-ratio-two wing under low Reynolds number conditions (Rec = 104). The flow fields are computed employing a high-fidelity implicit large-eddy simulation (ILES) approach found to be effective for moderate Reynolds number flows exhibiting mixed laminar, transitional and turbulent regions. The evolution of the flow structure and aerodynamic loading as a function of increasing angle of attack is presented. Lift and pressure fluctuations are found to be primarily dominated by the large scale circulatory pattern established above the wing due to separation from the leading edge, and by the inherent three dimensionality of the flow induced by the finite aspect ratio. The spanwise distribution of the sectional lift coefficient revealed only a minor direct contribution to the loading exherted by the tip vortex. High-frequency, small-amplitude oscillations are shown to have a significant effect on the separation process and accompanying loads suggesting potential flow control through either suitable actuation or aero-elastic tailoring.

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