Vortex Dynamics Within the Laminar Separation Bubble Over a NACA 0018 Airfoil at Low Reynolds Numbers

Vortex Dynamics Within the Laminar Separation Bubble Over a NACA 0018 Airfoil at Low Reynolds Numbers
Author: Andrew R. Lambert
Publisher:
Total Pages: 120
Release: 2015
Genre:
ISBN:
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The structures formed in the separated shear layer within a Laminar Separation Bubble (LSB) over a NACA 0018 airfoil at a chord Reynolds number of 100,000 and Angles of Attack (AOA) of (5°, 8°, and 10°) were investigated. Techniques used during investigation include high-speed flow visualization synchronized with embedded microphones for pressure measurements. High-speed flow visualizations reveal the formation of coherent structures within the laminar separation bubble. These structures develop from disturbances that roll up into vortices, may merge, and then break down as the shear layer reattaches. Microphone measurements indicate that the growth of the structures are accompanied by growth in a band of frequencies in the fluctuating surface pressures. When simultaneous visualizations and microphone measurements were compared, it was found that a local pressure minimum indicates a vortex passing over a microphone. The merging of vortices was found to result in the merging of the associated pressure minima. To track vortices along the separation bubble, the microphone signals were cross-correlated around the minima. This tracking matches well with the reference tracking of vortices from images. The vortex dynamics at AOA = 5°, 8°, and 10° were also compared. Visualizations show that structures decrease in scale at higher angles of attack, and show greater temporal variations. The spectra of pressure fluctuations show higher-frequency activity related to the smaller scales at greater angles of attack. The vortices at all of these angles of attack develop to a peak downstream of mean transition where roll-up is complete. At reattachment although the vortices come closer to the surface, the magnitudes of the pressure fluctuations decrease as the vortices break down. Merging of vortices was also investigated using the vortex tracking technique based on surface pressure fluctuations. Merging was present at AOA = 5°, 8°, and 10°, but more prevalent at larger angles of attack. Merging occurs at a range of intervals of vortices, and does not follow a dominant frequency from the disturbance environment.

Effects of Free-stream Turbulence Intensity on Laminar Separation Bubbles

Effects of Free-stream Turbulence Intensity on Laminar Separation Bubbles
Author: Mark Istvan
Publisher:
Total Pages: 168
Release: 2017
Genre: Fluid mechanics
ISBN:
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The effects of free-stream turbulence intensity on the flow over a NACA 0018 airfoil are studied experimentally in a wind tunnel facility. A parametric study is performed over a range of chord Reynolds numbers from 100 000 to 200 000, angles of attack from 0 to 20, and free-stream turbulence intensities from 0.09% to 2.03% in order to unravel the effects of each parameter on suction side laminar separation bubble topology and the resulting changes in airfoil lift. In order to investigate the effects of free-stream turbulence intensity on the streamwise and spanwise flow development within a separation bubble, flow field measurements are made using planar Particle Image Velocimetry for an angle of attack of 4, chord Reynolds numbers of 80 000 and 125 000, and free-stream turbulence intensities between 0.10% and 1.94%. The results show that increasing the level of free-stream turbulence intensity leads to a reduction in the length of the separation bubble formed over the suction side of the airfoil. The reduction in bubble length is a result of a downstream shift in mean separation as well as an upstream shift in mean transition and, consequently, mean reattachment. At low angles of attack, the reduction in separation bubble length leads to a slight reduction in airfoil lift, while at pre-stall angles of attack the reduction in separation bubble length alleviates the loss of suction at the location of the suction peak, thereby increasing lift, and can delay stall. While the effects of turbulence intensity and chord Reynolds number on the mean flow are shown to be similar, their effects on transition are shown to be notably different. The upstream shift in mean transition with increasing turbulence intensity is shown to be the result of disturbances reaching higher amplitudes earlier upstream as the level of turbulence intensity is increased, despite increased bubble stability. This result suggests that the increased initial perturbation amplitude at elevated turbulence intensity levels is solely responsible for the upstream shift in mean transition. In contrast, the upstream shift in mean transition with increasing Reynolds number is a result of decreased bubble stability. Wavenumber-frequency spectra of velocity fluctuations in the separated shear layer show that disturbances become more broadband in both time and space with increasing turbulence intensity. In addition, the results show that as the level of free-stream turbulence intensity is increased, the spanwise coherence of shear layer rollers decreases at the location of roll-up, leading to earlier vortex breakdown. At elevated levels of turbulence intensity, streamwise streaks of low speed fluid are observed, and originate in the boundary layer upstream of the separation bubble. These streaks form as a result of the onset of bypass transition, leading to significant changes in bubble dynamics, particularly at the highest level of turbulence intensity investigated. The results suggest that the transition mechanism in the separation bubble at the highest level of turbulence intensity investigated is altered.

Low Reynolds Number

Low Reynolds Number
Author: Mustafa Serdar Genç
Publisher: BoD – Books on Demand
Total Pages: 176
Release: 2012-04-04
Genre: Science
ISBN: 9535104926
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This book reports the latest development and trends in the low Re number aerodynamics, transition from laminar to turbulence, unsteady low Reynolds number flows, experimental studies, numerical transition modelling, control of low Re number flows, and MAV wing aerodynamics. The contributors to each chapter are fluid mechanics and aerodynamics scientists and engineers with strong expertise in their respective fields. As a whole, the studies presented here reveal important new directions toward the realization of applications of MAV and wind turbine blades.

Experimental Measurements of the Laminar Separation Bubble on an Eppler 387 Airfoil at Low Reynolds Numbers

Experimental Measurements of the Laminar Separation Bubble on an Eppler 387 Airfoil at Low Reynolds Numbers
Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
Total Pages: 148
Release: 2018-07-17
Genre:
ISBN: 9781723048913
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An experimental investigation was conducted to measure the flow velocity in the boundary layer of an Eppler 387 airfoil. In particular, the laminar separation bubble that this airfoil exhibits at low Reynolds numbers was the focus. Single component laser Doppler velocimetry data were obtained at a Reynolds number of 100,000 at an angle of attack of 2.0 degree. Static Pressure and flow visualization data for the Eppler 387 airfoil were also obtained. The difficulty in obtaining accurate experimental measurements at low Reynolds numbers is addressed. Laser Doppler velocimetry boundary layer data for the NACA 663-018 airfoil at a Reynolds number of 160,000 and angle of attack of 12 degree is also presented. Cole, Gregory M. and Mueller, Thomas J. Unspecified Center AIRFOILS; BOUNDARY LAYER SEPARATION; BUBBLES; FLOW VELOCITY; LAMINAR BOUNDARY LAYER; ANGLE OF ATTACK; FLOW VISUALIZATION; LASER DOPPLER VELOCIMETERS; LOW REYNOLDS NUMBER; STATIC PRESSURE...

Direct Numerical Simulations of a Cambered NACA 65(1)-412 Airfoil at Low Reynolds Numbers

Direct Numerical Simulations of a Cambered NACA 65(1)-412 Airfoil at Low Reynolds Numbers
Author: Bjoern Fabian Klose
Publisher:
Total Pages: 216
Release: 2021
Genre:
ISBN:
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This dissertation is a comprehensive account of the low-Reynolds number (Re) flow over a cambered airfoil for a wide range of angles of attack with a focus on the dynamics of boundary layer separation and transition. The unsteady and complex phenomena of the transitional flow are analyzed through a combination of direct numerical simulations (DNS), large-eddy simulations (LES), experiments, and development of Lagrangian theory and methods. A discontinuous Galerkin spectral element method (DGSEM) is used to model the compressible Navier-Stokes equations in two and three dimensions. The DGSEM generates high-order accurate results with low dispersion and diffusion errors and has been developed to include kinetic-energy conserving volume fluxes, tools to efficiently track Lagrangian fluid tracers, and computation of higher wall-normal velocity derivatives. The code is benchmarked through a series of Navier-Stokes flows using different DG variants and polynomial orders. High-fidelity DNS in three dimensions show that the transitional flow over a cambered NACA 65(1)-412 airfoil at Re = 20,000 swiftly changes from a state of laminar separation at mid-chord without reattachment to a laminar separation bubble (LSB) at the leading edge with a turbulent boundary layer. The bifurcation occurs within an angle-of-attack change of two degrees and is accompanied by a rapid increase of the lift and decrease of the drag force, which is observed in computations and experiments likewise. Each flow regime is governed by different dynamics, instabilities, and wake structures that change with the transition location of the separated shear layer. The kinematic aspects of flow separation are further investigated in the Lagrangian frame, where the initial motion of upwelling fluid material from the wall is related to the long-term attracting manifolds in the flow field. An objective finite-time diagnostic for instabilities in shear flows based on the curvature of Lagrangian material lines is introduced. By defining a flow instability in the Lagrangian frame as the increased folding of lines of fluid particles, subtle perturbations and unstable growth thereof are detected early based solely on the curvature change of material lines over finite time.

Numerical Study of Intermittent Bursting of a Laminar Separation Bubble on a NACA 643-618 Airfoil

Numerical Study of Intermittent Bursting of a Laminar Separation Bubble on a NACA 643-618 Airfoil
Author: Antoine Michael Diego Jost
Publisher:
Total Pages: 250
Release: 2014
Genre:
ISBN:
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2D flows over a NACA 643-618 at Re=64,200 at a=8.64°, 11.24°, and 13.84° are simulated numerically using FLUENT. Spatial and temporal resolution studies indicate the need for a sufficiently fine spatial and temporal resolution to properly capture the dominant flow features (e.g. vortex shedding, bubble bursting). At a=8.64°, a long laminar separation bubble (LSB) containing a small counter rotating region is present. Vortex shedding results in immediate roll up and separation from the surface of the airfoil; there is no vortex pairing. Vortex shedding and bubble flapping is determined by correlating probe data and instantaneous flow fields. A discernable fundamental frequency, corresponding to vortex shedding, is present in aerodynamic coefficients data and vorticity probe data. Vertical and horizontal flapping is noticeable. The periodicity of the flow is further supported through the local intermittency measure (LIM); LIM is dominated by an intermittency value of one, indicating perfect periodicity. Time averaging is well posed and independent of time interval. Intermittent bubble bursting and vortex shedding is present at a=11.24°. The LSB changes from a short to a long bubble. A low frequency bubble bursting is detected from aerodynamic coefficient data and probe data. Bubble bursting is quasi-periodic with a clistribution of frequency around 5.542 Hz, indicative of intermittency. At short or long bubble instances, the primary reattachment and vortex sheddling outline has time dependent shape, motion, and location. Vortex shedding involves vortex pairing at time dependent locations. Location dependent vortex shedding frequencies and associated frequency distribution in the probe data and aerodynamic coefficient demonstrates intermittent vortex shedding. LIM is dominated by regions of peak intermittency at low frequencies and high frequencies, indicating a high degree of intermittency. Low frequency LIM is in agreement with the vorticity magnitude spikes encountered in probe data for probes located in the vicinity of the LSB. Time averaged pressure and skin friction coefficient distributions for different time intervals produce different results; time averaging is ill-posed as the averages are time interval dependent. Flow at a=13.84, may be considered as a transition of the behavior exhibited at a=11.24° to higher angle of attack.

An Experimental Analysis of the Laminar Separation Bubble at Low Reynolds Numbers

An Experimental Analysis of the Laminar Separation Bubble at Low Reynolds Numbers
Author: Karla Marie Swift
Publisher:
Total Pages: 96
Release: 2009
Genre:
ISBN:
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This paper is an investigation into the laminar separation bubble that frequently plagues airborne vehicles operating in the low Reynolds number regime -- experimentally found to be typically present in flows with Reynolds numbers below 106 (Lissaman 1983). The specific application driving the present investigation is the fixed wing performance of unmanned micro air vehicles (MAVs), defined by their maximum chord length of 6 inches and current cruising speeds of 10-20 meters per second (Mueller 2001). A basic generic model was chosen for this investigation: a circular arc (section of 16 inch diameter PVC pipe) with sharp leading and trailing edges having a chord length of 9.3 inches and height of 1.5 inches. This airfoil model was tested in the UTSI water tunnel at Reynolds numbers of 27,000 and 45,000. The goal of this study was to gain some insight into the boundary layer behaviour through the use of dye injection for flow visualization and hot film anemometry for quantitative velocity measurements. Small diameter cylinders were then statically placed upstream of the model to determine their interaction with the laminar separation bubble and its effects on the boundary layer downstream over the airfoil model. The length and height of the laminar separation bubble was found to be impacted with a small cylindrical wire placed upstream at all Reynolds numbers and angles of attack with the exception of an 18 degree angle of attack at the higher Reynolds number. However, these changes did not result in a substantial or distinguishable improvement in the downstream separation point. The laminar separation bubble was found to be nearly or completely eliminated when a thermocouple wire was placed upstream of the leading edge. Although the elimination of the bubble would result in only a minor decrease in drag and increase in lift, there would be a possible improvement in the stability of the leading edge stall and possible reduction or elimination in the hysteresis associated with stall.

An Experimental Investigation of Airfoils with Laminar Separation Bubbles and Effects of Distributed Suction

An Experimental Investigation of Airfoils with Laminar Separation Bubbles and Effects of Distributed Suction
Author: Redha Wahidi
Publisher:
Total Pages:
Release: 2009
Genre: Aerofoils
ISBN:
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In an effort to understand the behavior of the laminar separation bubbles on NACA 0012 and Liebeck LA2573a airfoils at different Reynolds numbers and angles of attack, the boundary layers on the solid airfoils were investigated by measuring the mean and fluctuating components of the velocity profiles over the upper surfaces of the airfoils. Surface pressure measurements were carried out to complete the mapping of the laminar separation bubble and to calculate the lift generated by the airfoils. The experiments were carried out at Reynolds numbers of 150,000 and 250,000. The locations of separation, transition and reattachment were determined as functions of angle of attack and Reynolds number for the two airfoils. The drag was estimated from wake pressure measurements based on the momentum deficit generated by the airfoil. The size and location of the laminar separation bubble on the LA2573a airfoil did not show significant changes with Reynolds number and angle of attack for values of the angle of attack between 0 and 6 degrees. The baseline results of the size and location of the laminar separation bubble on the LA2573a airfoil were used to design a suction distribution. This suction distribution was designed based on Thwaites' criterion of separation. The effects of applying suction on the size and location of the laminar separation bubble were investigated. The results showed that the suction distribution designed in this work was effective in controlling the size of the laminar separation bubble, maintaining an unseparated laminar boundary layer to the transition point, and controlling the location of transition. The effects of different suction rates and distributions on the drag were investigated. Drag reductions of 14-24% were achieved. A figure of merit was defined as drag reductions divided by the equivalent suction drag to assess the worthiness of utilizing suction on low Reynolds number flows. The values of the figure of merit were around 4.0 which proved that the penalty of using suction was significantly less than the gain obtained in reducing the drag.