Direct Numerical Simulations of the Effect of Free-Stream Turbulence on 'Long' Laminar Separation Bubbles

2012 ◽  
Author(s):  
Shirzad Hosseinverdi ◽  
Wolfgang Balzer ◽  
Hermann Fasel
Keyword(s):  
Numerical Simulations ◽  
Free Stream ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

Direct Numerical Simulations of Laminar Separation Bubbles on a Curved Plate: Part 1 — Simulation Setup and Uncontrolled Flow

2013 ◽  
Author(s):  
Wolfgang Balzer ◽  
Hermann F. Fasel
Keyword(s):  
Numerical Simulations ◽  
Active Flow Control ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Low Pressure Turbine ◽  
Physical Mechanisms ◽  
Separation Bubbles ◽  
Significant Performance ◽  
Lifting Surfaces ◽  
Laminar Separation Bubbles

The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. For a modern low-pressure turbine (LPT) stage, in particular when designed for high blade loadings, laminar separation at cruise conditions can result in significant performance degradation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with laminar separation and the formation of laminar separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, laminar separation (part I) and its control (part II) were investigated numerically by employing highly-resolved, high-order accurate direct numerical simulations (DNS).

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.

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