Laminar Separation Bubbles in Two-Dimensional Straight-Diverging-Straight Channel Flows

2015 ◽  
pp. 177-182
Author(s):  
Mamta Jotkar ◽  
José Miguel Pérez ◽  
Vassilis Theofilis ◽  
Rama Govindarajan
Keyword(s):  
Channel Flows ◽  
Two Dimensional ◽  
Straight Channel ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

The two classes of primary modal instability in laminar separation bubbles

2013 ◽  
Vol 734 ◽  
Author(s):  
Daniel Rodríguez ◽  
Elmer M. Gennaro ◽  
Matthew P. Juniper
Keyword(s):  
Three Dimensional ◽  
Stream Velocity ◽  
Two Dimensional ◽  
Global Instability ◽  
Laminar Separation ◽  
Centrifugal Instability ◽  
Separation Bubbles ◽  
Dimensional Instability ◽  
Shed Light ◽  
Laminar Separation Bubbles

AbstractThe self-excited global instability mechanisms existing in flat-plate laminar separation bubbles are studied here, in order to shed light on the causes of unsteadiness and three-dimensionality of unforced, nominally two-dimensional separated flows. The presence of two known linear global mechanisms, namely an oscillator behaviour driven by local regions of absolute inflectional instability and a centrifugal instability giving rise to a steady three-dimensionalization of the bubble, is studied in a series of model separation bubbles. These results indicate that absolute instability, and consequently a global oscillator behaviour, does not exist for two-dimensional bubbles with a peak reversed-flow velocity below $12\hspace{0.167em} \% $ of the free-stream velocity. However, the three-dimensional instability becomes active for recirculation levels as low as ${u}_{rev} \approx 7\hspace{0.167em} \% $. These findings suggest a route to the three-dimensionality and unsteadiness observed in experiments and simulations substantially different from that usually found in the literature of laminar separation bubbles, in which two-dimensional vortex shedding is followed by three-dimensionalization.

Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

2012 ◽  
Vol 53 (1) ◽  
pp. 173-186 ◽  
Author(s):  
Daniele Simoni ◽  
Marina Ubaldi ◽  
Pietro Zunino ◽  
Francesco Bertini
Keyword(s):  
Synthetic Jet ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

Dynamic Stall Flow Structure and Forces on Symmetrical Airfoils at High Angles of Attack and Rotation Rates

2018 ◽  
Vol 141 (5) ◽  
Author(s):  
R. R. Leknys ◽  
M. Arjomandi ◽  
R. M. Kelso ◽  
C. H. Birzer
Keyword(s):  
Flow Structure ◽  
Boundary Layer Separation ◽  
Vortex Structures ◽  
Dynamic Stall ◽  
Aerodynamic Load ◽  
Laminar Separation ◽  
Rotation Rates ◽  
Constant Pitch ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

This article describes a direct comparison between two symmetrical airfoils undergoing dynamic stall at high, unsteady reduced frequencies under otherwise identical conditions. Particle image velocimetry (PIV) was performed to distinguish the differences in flow structure between a NACA 0021 and a NACA 0012 airfoil undergoing dynamic stall. In addition, surface pressure measurements were performed to evaluate aerodynamic load and investigate the effect of laminar separation bubbles and vortex structures on the pressure fields surrounding the airfoils. Airfoil geometry is shown to have a significant effect on flow structure development and boundary layer separation, with separation occurring earlier for thinner airfoil sections undergoing constant pitch-rate motion. Inertial forces were identified to have a considerable impact on the overall force generation with increasing rotation rate. Force oscillation was observed to correlate with multiple vortex structures shedding at the trailing-edge during high rotation rates. The presence of laminar separation bubbles on the upper and lower surfaces was shown to dramatically influence the steady-state lift of both airfoils. Poststall characteristics are shown to be independent of airfoil geometry such that periodic vortex shedding was observed for all cases. However, the onset of deep stall is delayed with increased nondimensional pitch rate due to the delay in initial dynamic-stall vortex.

Sign in / Sign up

Export Citation Format

Share Document