Relevance of local parallel theory to the linear stability of laminar separation bubbles

2012 ◽  
Vol 698 ◽  
pp. 468-478 ◽  
Author(s):  
Sourabh S. Diwan ◽  
O. N. Ramesh
Keyword(s):  
Linear Stability ◽  
Strong Interaction ◽  
Numerical Study ◽  
Separation Bubble ◽  
Global Analysis ◽  
Base Flow ◽  
Initial Portion ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

AbstractLaminar separation bubbles are thought to be highly non-parallel, and hence global stability studies start from this premise. However, experimentalists have always realized that the flow is more parallel than is commonly believed, for pressure-gradient-induced bubbles, and this is why linear parallel stability theory has been successful in describing their early stages of transition. The present experimental/numerical study re-examines this important issue and finds that the base flow in such a separation bubble becomes nearly parallel due to a strong-interaction process between the separated boundary layer and the outer potential flow. The so-called dead-air region or the region of constant pressure is a simple consequence of this strong interaction. We use triple-deck theory to qualitatively explain these features. Next, the implications of global analysis for the linear stability of separation bubbles are considered. In particular we show that in the initial portion of the bubble, where the flow is nearly parallel, local stability analysis is sufficient to capture the essential physics. It appears that the real utility of the global analysis is perhaps in the rear portion of the bubble, where the flow is highly non-parallel, and where the secondary/nonlinear instability stages are likely to dominate the dynamics.

On the Numerical Difficulties in Calculating Laminar Separation Bubbles

2002 ◽  
Author(s):  
Wolfgang Sanz ◽  
Max F. Platzer
Keyword(s):  
Numerical Study ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Turbulence Models ◽  
Separated Flow ◽  
Transition Model ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Transition Models ◽  
Laminar Separation Bubbles

Behaviour of laminar separation bubbles on the surfaces of compressor and turbine blades has increasingly attracted the attention of researchers and designers of turbomachinery in the last years. For the numerical investigation of laminar separation bubbles transition models are implemented into Navier-Stokes flow solvers to predict their location, extent and behaviour accurately. Several researchers conducted comparative studies to investigate the applicability of different transition models for separated-flow transition. In this work a comprehensive numerical study is carried out to investigate not only the influence of the transition model, but of the solution method in general on laminar separation bubble prediction. The flow around a NACA 0012 airfoil at different angles of attack where laminar separation bubbles were observed in experiments is chosen as test case. Different flow solvers (Osher and Roe scheme), different turbulence models as well as different solution procedures were applied together with transition models. The results show that besides the transition model other parameters like the discretisation scheme of the turbulence model or the flow solver have a comparably large influence on the computational result.

scholarly journals Instabilities in laminar separation bubbles

2013 ◽  
Vol 732 ◽  
pp. 1-4 ◽  
Author(s):  
J.-C. Robinet
Keyword(s):  
Separation Bubble ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Transition To Turbulence ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Wall Bounded Flows ◽  
Pass Through ◽  
Secondary Instabilities ◽  
Laminar Separation Bubbles

AbstractWall-bounded flows, in their transition from a laminar state to turbulence, pass through a set of particular stages characterized by different physical processes. Among wall-bounded flows, separated flows have a special place because their dynamics can either be noise amplifiers or oscillators. For several years Marxen and co-workers have been studying the evolution of two- and three-dimensional perturbations in the laminar part of a laminar separation bubble. In Marxen et al. (J. Fluid Mech., vol. 728, 2013, p. 58) they study vortex formation and its evolution in laminar–turbulent transition in a forced separation bubble. By the combined use of numerical and experimental methods, different mechanisms of secondary instabilities have been highlighted: elliptic instability of vortex cores and hyperbolic instability responsible for three-dimensionality in the braid region. This work shows, for the first time in laminar separation bubbles, the first nonlinear stages of transition to turbulence of such a flow. However, since this type of flow is very sensitive to various environmental stresses, several scenarios for transition to turbulence remain to be explored.

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