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.

scholarly journals Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence

2008 ◽  
Vol 602 ◽  
pp. 175-207 ◽  
Author(s):  
L. E. JONES ◽  
R. D. SANDBERG ◽  
N. D. SANDHAM
Keyword(s):  
Numerical Simulations ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Transition Process ◽  
Direct Numerical Simulations ◽  
Transition To Turbulence ◽  
Absolute Instability ◽  
Laminar Separation ◽  
Velocity Magnitude ◽  
Separation Bubbles

Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Rec=5×104 and incidence 5° are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.

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.

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.

Enhancement of disturbance wave amplification due to the intrinsic three-dimensionalisation of laminar separation bubbles

2018 ◽  
Vol 123 (1268) ◽  
pp. 1492-1507 ◽  
Author(s):  
D. Rodríguez ◽  
E. M. Gennaro
Keyword(s):  
Convective Instability ◽  
Three Dimensional ◽  
High Amplitude ◽  
Recirculation Region ◽  
Streamwise Vorticity ◽  
Laminar Separation ◽  
Wave Amplification ◽  
Disturbance Waves ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

ABSTRACTPrevious studies demonstrated that laminar separation bubbles (LSBs) in the absence of external disturbances or forcing are intrinsically unstable with respect to a three-dimensional instability of centrifugal nature. This instability produces topological modifications of the recirculation region with the introduction of streamwise vorticity in an otherwise purely two-dimensional time-averaged flows. Concurrently, the existence of spanwise inhomogeneities in LSBs have been reported in experiments in which the amplification of convective instability waves dominates the physics. The co-existence of the two instability mechanisms is investigated herein by means of three-dimensional parabolised stability equations. The spanwise waviness of the LSB on account of the primary instability is found to modify the amplification of incoming disturbance waves in the linear regime, resulting in a remarkable enhancement of the amplitude growth and a three-dimensional arrangement of the disturbance waves in the aft portion of the bubble. Present findings suggest that the oblique transition scenario should be expected in LSBs dominated by the convective instability, unless high-amplitude disturbances are imposed.

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.

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