scholarly journals Structural changes of laminar separation bubbles induced by global linear instability

2010 ◽  
Vol 655 ◽  
pp. 280-305 ◽  
Author(s):  
D. RODRÍGUEZ ◽  
V. THEOFILIS
Keyword(s):  
Separation Bubble ◽  
Three Dimensional ◽  
Flow Fields ◽  
Basic Flow ◽  
Laminar Separation Bubble ◽  
Flow Topology ◽  
Linear Superposition ◽  
Global Mode ◽  
Laminar Separation ◽  
Composite Flow

The topology of the composite flow fields reconstructed by linear superposition of a two-dimensional boundary layer flow with an embedded laminar separation bubble and its leading three-dimensional global eigenmodes has been studied. According to critical point theory, the basic flow is structurally unstable; it is shown that in the presence of three-dimensional disturbances the degenerate basic flow topology is replaced by a fully three-dimensional pattern, regardless of the amplitude of the superposed linear perturbations. Attention has been focused on the leading stationary eigenmode of the laminar separation bubble discovered by Theofilis et al. (Phil. Trans. R. Soc. Lond. A, vol. 358, 2000, pp. 3229–3324); the composite flow fields have been fully characterized with respect to the generation and evolution of their critical points. The stationary global mode is shown to give rise to a three-dimensional flow field which is equivalent to the classical U-shaped separation, defined by Hornung & Perry (Z. Flugwiss. Weltraumforsch., vol. 8, 1984, pp. 77–87), and induces topologies on the surface streamlines that are resemblant to the characteristic stall cells observed experimentally.

Structure and dynamics of a laminar separation bubble near a wingtip

2021 ◽  
Vol 929 ◽  
Author(s):  
Connor E. Toppings ◽  
Serhiy Yarusevych
Keyword(s):  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Surface Flow ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Flow Topology ◽  
Laminar Separation ◽  
The Mean

The three-dimensional flow topology of a laminar separation bubble forming on the suction surface of a semispan wing with an aspect ratio of $2.5$ and NACA 0018 airfoil section is characterised experimentally using surface pressure measurements and particle image velocimetry at a chord Reynolds number of $125\ 000$ . In the inboard region of the wing, the separation bubble is essentially two-dimensional, and the transition process in the separated shear layer leads to periodic vortex shedding, which dominates the bubble dynamics, similar to two-dimensional separation bubbles. However, progressive spanwise changes in the mean structure and vortex dynamics occur near the wingtip, leading to an open separation and eventual suppression of the bubble. In the immediate proximity of the wingtip, the boundary layer remains attached, no vortex shedding occurs and the flow remains laminar, terminating separation bubble formation. Despite variations in the mean separation bubble topology and vortex dynamics along the span, the fundamental shedding characteristics remain nearly invariant across the portion of the wing where vortex shedding occurs, and the flow appears to lock onto a common instability mode across the span, leading to minimal changes in the mean bubble characteristics despite notable changes in the effective angle of attack along the span. A comparison with available surface flow visualisations from previous studies indicates that the observed changes to the mean bubble footprint along the span of the wing are similar across different geometries and flow characteristics, suggesting similarities in the three-dimensional bubble topology and dynamics on finite wings.

Stability and Transition Over a Low-Reynolds Number Airfoil

2016 ◽  
Author(s):  
Paul Ziadé ◽  
Pierre E. Sullivan
Keyword(s):  
Reynolds Number ◽  
Stability Analysis ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Peak Frequency ◽  
Laminar Separation Bubble ◽  
Airfoil Surface ◽  
Laminar Separation

Large-eddy simulation and linear stability analysis were performed on a NACA 0025 airfoil at a chord Reynolds number of 105 and four angles of attack. The computations showed that the initial vortex roll-up quickly breaks down to three-dimensional turbulence. Flow separation was observed at all angles, whereas only the lowest angle of attack formed a laminar separation bubble due to flow transition occuring close to the airfoil surface. A Chebyshev collocation method was employed to solve the viscous and inviscid stability equations. Linear stability analysis demonstrated that high-frequency disturbances occur in the laminar separation bubble case, whereas lower frequencies are present for the fully separated angles of attack. The maximum disturbance growth rates were dampened with the addition of viscosity but negligible change in peak frequency was noted.

Stability and receptivity characteristics of a laminar separation bubble on an aerofoil

2010 ◽  
Vol 648 ◽  
pp. 257-296 ◽  
Author(s):  
L. E. JONES ◽  
R. D. SANDBERG ◽  
N. D. SANDHAM
Keyword(s):  
Acoustic Waves ◽  
Feedback Loop ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Leading Edge ◽  
Two Dimensions ◽  
Absolute Instability ◽  
Laminar Separation Bubble ◽  
Trailing Edge ◽  
Laminar Separation

Stability characteristics of aerofoil flows are investigated by linear stability analysis of time-averaged velocity profiles and by direct numerical simulations with time-dependent forcing terms. First the wake behind an aerofoil is investigated, illustrating the feasibility of detecting absolute instability using these methods. The time-averaged flow around an NACA-0012 aerofoil at incidence is then investigated in terms of its response to very low-amplitude hydrodynamic and acoustic perturbations. Flow fields obtained from both two- and three-dimensional simulations are investigated, for which the aerofoil flow exhibits a laminar separation bubble. Convective stability characteristics are documented, and the separation bubble is found to exhibit no absolute instability in the classical sense; i.e. no growing disturbances with zero group velocity are observed. The flow is however found to be globally unstable via an acoustic-feedback loop involving the aerofoil trailing edge as a source of acoustic excitation and the aerofoil leading-edge region as a site of receptivity. Evidence suggests that the feedback loop may play an important role in frequency selection of the vortex shedding that occurs in two dimensions. Further simulations are presented to investigate the receptivity process by which acoustic waves generate hydrodynamic instabilities within the aerofoil boundary layer. The dependency of the receptivity process to both frequency and source location is quantified. It is found that the amplitude of trailing-edge noise in the fully developed simulation is sufficient to promote transition via leading-edge receptivity.

Large-Eddy Simulations of Owl-Like Wing Under Low Reynolds Number Conditions

2013 ◽  
Author(s):  
Katsutoshi Kondo ◽  
Hikaru Aono ◽  
Taku Nonomura ◽  
Akira Oyama ◽  
Kozo Fujii ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Suction Side ◽  
Flow Fields ◽  
Pressure Side ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Large Eddy ◽  
Drag Ratio ◽  
Lift To Drag Ratio

Flow fields around an owl-like wing and aerodynamic characteristics at a chord Reynolds number of 23,000 are investigated using three-dimensional implicit large-eddy simulation. The cross sectional profile of the owl wing model named “owl-like wing” is constructed based on the owl wing at 40% of the span length from the root. It consists of flat upper surface, large camber, and thin geometry. Results show that at low angles of attack (α), separation, transition, and reattachment are observed in the instantaneous flow fields on the pressure side. The laminar separation bubbles can be seen in time- and span-averaged flow fields. It is likely that lift and drag generation is correlated with the location of separation points on the suction side. However, it has little influence on behavior of CL-α curve. On the other hand, at high angles of attack, the flow on the pressure side is fully attached. The flow on the suction side is similar to that of the pressure side at low angles of attack. It is found that unlike the case of the flow at the low angles of attack, the laminar separation bubble on the suction side affects the response of CL to variation of α. Furthermore, it is possible to decrease the drag and to increase the lift when the location of the laminar separation bubble is well organized by an appropriate airfoil surface geometry. Also, the deeply concaved lower surface contributes to lift enhancement. From those factors mentioned above, the owl-like wing gains higher lift-to-drag ratio comparing with conventional thin and thick symmetrical airfoils such as NACA0002 and NACA0012. Indeed, maximum lift-to-drag ratio of the owl-like wing is approximately 23 at the angle of attack of 6.0 degrees at Reynolds number of 23,000.

Sign in / Sign up

Export Citation Format

Share Document