scholarly journals Numerical investigation of the low-frequency flow oscillation over a NACA-0012 aerofoil at the inception of stall

2019 ◽  
Vol 11 ◽  
pp. 175682931983368 ◽  
Author(s):  
Yasir A ElAwad ◽  
Eltayeb M ElJack
Keyword(s):  
Experimental Data ◽  
Flow Field ◽  
Separation Bubble ◽  
Lift Coefficient ◽  
Angle Of Attack ◽  
Low Frequency ◽  
Laminar Separation Bubble ◽  
Flow Oscillation ◽  
Laminar Separation ◽  
Naca 0012

High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of [Formula: see text], Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of [Formula: see text]. The mean lift coefficient peaks at the angle of attack of [Formula: see text], in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack ([Formula: see text]). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of [Formula: see text]. The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively.

Oscillating behaviour of laminar separation bubble formed on an aerofoil near stall

2004 ◽  
Vol 108 (1081) ◽  
pp. 153-163 ◽  
Author(s):  
K. Rinoie ◽  
N. Takemura
Keyword(s):  
Separation Bubble ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Chord Length ◽  
Flow Visualisation ◽  
Laminar Separation Bubble ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Naca 0012 ◽  
Small Separation

Abstract Laminar separation bubbles formed on NACA 0012 aerofoil near the onset of a stall were investigated to clarify the behaviour of the laminar separation bubble. Measurements were done at a chord Reynolds number of 1·3 × 105. Mean velocity measurements indicate that the long bubble of about 35% chord length is formed at α = 11·5° after the short bubble burst occurred. However, the instantaneous flow visualisation picture indicates that the flow is strongly oscillating at this angle of attack. The phase averaging technique has been applied to analyse this oscillating behaviour. The results indicate that the flow is oscillating between a small separation-reattachment bubble formed near the leading-edge at about a 10% chord length and a large separated region extending over the aerofoil surface. It is suggested that this small separation-reattachment bubble has a similar flow structure to that of the short bubble formed at a lower angle of attack.

scholarly journals Laminar Separation Bubble and Flow Topology of NACA 0015 at Low Reynolds Number

CFD Letters ◽  
2021 ◽  
Vol 13 (10) ◽  
pp. 36-51
Author(s):  
Mohamed Ibren ◽  
Amelda Dianne Andan ◽  
Waqar Asrar ◽  
Erwin Sulaeman
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Flow Field ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Angle Of Attack ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Naca 0015

The development of sophisticated unmanned aerial vehicles and wind turbines for daily activities has triggered the interest of researchers. However, understanding the flow phenomena is a strenuous task due to the complexity of the flow field. The engaging topic calls for more research at low Reynolds numbers. The computational investigations on a two-dimensional (2D) airfoil are presented in this paper. Numerical simulation of unsteady, laminar-turbulent flow around NACA 0015 airfoil was performed by using shear-stress transport (SST) model at relatively low Reynolds number (8.4 × 104 to 1.7 × 105) and moderate angles of attack (0 ≤ α ≤ 6). In general, on the suction side, with increasing Reynolds number and angles of attack, separation, and reattachment point shifts upstream and concurrently shrinking the size of the laminar bubble. However, On the pressure side, the laminar bubble is seen to move toward the trailing edge at the relatively same size as the angle of attack increases. Moreover, the variations in the angle of attack have more influence on the laminar separation bubble characteristics as compared to the Reynolds number. The reattachment points were barely observed for the range of the angles of attack studied. At very high angles of attack, it is recommended to simulate the flow field using large eddy simulation or direct numerical simulation since the flow is considered three-dimensional and detached from the surface thus forming a complex phenomenon.

Oscillating behaviour of laminar separation bubble formed on an aerofoil near stall

2004 ◽  
Vol 108 (1081) ◽  
pp. 153-163 ◽  
Author(s):  
K. Rinoie ◽  
N. Takemura
Keyword(s):  
Separation Bubble ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Chord Length ◽  
Flow Visualisation ◽  
Laminar Separation Bubble ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Naca 0012 ◽  
Small Separation

AbstractLaminar separation bubbles formed on NACA 0012 aerofoil near the onset of a stall were investigated to clarify the behaviour of the laminar separation bubble. Measurements were done at a chord Reynolds number of 1·3 × 105. Mean velocity measurements indicate that the long bubble of about 35% chord length is formed at α = 11·5° after the short bubble burst occurred. However, the instantaneous flow visualisation picture indicates that the flow is strongly oscillating at this angle of attack. The phase averaging technique has been applied to analyse this oscillating behaviour. The results indicate that the flow is oscillating between a small separation-reattachment bubble formed near the leading-edge at about a 10% chord length and a large separated region extending over the aerofoil surface. It is suggested that this small separation-reattachment bubble has a similar flow structure to that of the short bubble formed at a lower angle of attack.

Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

2016 ◽  
Vol 798 ◽  
pp. 5-26 ◽  
Author(s):  
Andrea Sansica ◽  
Neil D. Sandham ◽  
Zhiwei Hu
Keyword(s):  
Boundary Layer ◽  
Linear Stability ◽  
Flow Instability ◽  
Separation Bubble ◽  
Low Frequency ◽  
Boundary Layer Transition ◽  
Separation Point ◽  
Oblique Shock Wave ◽  
Laminar Separation Bubble ◽  
Laminar Separation

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

Numerical and experimental study of the laminar separation bubble over SS007 airfoil for micro aerial vehicles

2020 ◽  
Vol 92 (8) ◽  
pp. 1125-1131
Author(s):  
Somashekar V. ◽  
Immanuel Selwyn Raj A.
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Lift Coefficient ◽  
Laminar Separation Bubble ◽  
Micro Aerial Vehicle ◽  
Aerodynamic Efficiency ◽  
Content Type ◽  
Laminar Separation ◽  
Aerodynamic Performances ◽  
Aerial Vehicle

Purpose This paper aims to deal with the numerical investigation of laminar separation bubble (LSB) characteristics (length and height of the bubble) of SS007 airfoil at the chord Reynolds number of Rec = 0.68 × 105 to 10.28 × 105. Design/methodology/approach The numerical simulations of the flow around SS007 airfoil were carried out by using the commercial fluid dynamics (CFD) software, ANalysis system (ANSYS) 15. To solve the governing equations of the flow, a cell-centred control volume space discretisation approach is used. Wind tunnel experiments were conducted at the chord-based Reynolds number of Rec = 1.6 × 105 to validate the aerodynamic characteristics over SS007 airfoil. Findings The numerical results revealed that the LSB characteristics of a SS007 airfoil, and the aerodynamic performances are validated with experimental results. The lift and drag coefficients for both numerical and experimental results show very good correlation at Reynolds number 1.6 × 105. The lift coefficient linearly increases with the increasing angle of attack (AOA) is relatively small. The corresponding drag coefficient was found to be very small. After the formation of LSB which leads to burst to cause airfoil stall, the lift coefficient decreases and increases the drag coefficient. Practical implications Low Reynolds number and LSB characteristics concept in aerodynamics is predominant for both civilian and military applications. These include high altitude devices, wind turbines, human powered vehicles, remotely piloted vehicles, sailplanes, unmanned aerial vehicle and micro aerial vehicle. In this paper, the micro aerial vehicle flight conditions considered and investigated the LSB characteristics for different Reynolds number. To have better aerodynamic performances, it is strongly recommended to micro aerial vehicle (MAV) design engineers that the MAV is to fly at 12 m/s (cruise speed). Social implications MAVs and unmanned aerial vehicles seem to give some of the technical challenges of nature conservation monitoring and law enforcement a versatile, reliable and inexpensive solution. Originality/value The SS007 airfoil delays the flow separation and improves the aerodynamic efficiency by increasing the lift and decreasing the drag. The maximum increase in aerodynamic efficiency is 12.5% at stall angle of attack compared to the reference airfoil at Re = 2 × 105. The results are encouraging and this airfoil could have better aerodynamic performance for the development of MAV.

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