Direct Numerical Simulations of Laminar-to-Turbulent Transition in Laminar Separation Bubbles in Three-Dimensional Boundary-Layer

2016 ◽  
Author(s):  
Shirzad Hosseinverdi ◽  
Hermann F. Fasel
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Separation Bubbles ◽  
Dimensional Boundary ◽  
Laminar Separation Bubbles

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.

Direct numerical simulations of laminar separation bubbles: investigation of absolute instability and active flow control of transition to turbulence

2014 ◽  
Vol 747 ◽  
pp. 141-185 ◽  
Author(s):  
Martin Embacher ◽  
H. F. Fasel
Keyword(s):  
Numerical Simulations ◽  
Three Dimensional ◽  
Secondary Instability ◽  
Direct Numerical Simulations ◽  
Absolute Instability ◽  
Two Dimensional ◽  
Periodic Forcing ◽  
Laminar Separation ◽  
Flow Response ◽  
Separation Bubbles

AbstractLaminar separation bubbles generated on a flat plate by an adverse pressure gradient are investigated using direct numerical simulations (DNSs). Two-dimensional periodic forcing is applied at a blowing/suction slot upstream of separation. Control of separation through forcing with various frequencies and amplitudes is examined. For the investigation of absolute instability mechanisms, baseflows provided by two-dimensional Navier–Stokes calculations are analysed by introducing pulse disturbances and computing the three-dimensional flow response using DNS. The primary instability of the time-averaged flow is investigated with a local linear stability analysis. Employing a steady flow solution as baseflow, the nonlinear and non-parallel effects on the self-sustained disturbance development are illustrated, and a feedback mechanism facilitated by the upstream flow deformation is identified. Secondary instability is investigated locally using spatially periodic baseflows. The flow response to pulsed forcing indicates the existence of an absolute secondary instability mechanism, and the results indicate that this mechanism is dependent on the periodic forcing. Results from three-dimensional DNS provide insight into the global instability mechanisms of separation bubbles and complement the local analysis. A forcing strategy was devised that suppresses the temporal growth of three-dimensional disturbances, and as a consequence, breakdown to turbulence does not occur. Even for a separation bubble that has transitioned to turbulence, the flow relaminarizes when applying two-dimensional periodic forcing with proper frequencies and amplitudes.

scholarly journals On the Calculation of Laminar Separation Bubbles Using Different Transition Models

1997 ◽  
Author(s):  
Wolfgang Sanz ◽  
Max F. Platzer
Keyword(s):  
Transition Process ◽  
Navier Stokes ◽  
Laminar Separation ◽  
Controlled Diffusion ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Separation Bubbles ◽  
Turbomachinery Blades ◽  
Transition Models ◽  
Laminar Separation Bubbles

Laminar separation bubbles are commonly observed on turbomachinery blades and therefore require effective methods for their prediction. The location and size of the bubbles is critically dependent on the laminar-to-turbulent transition process. Therefore, in this paper the transition models of Solomon et al., Abu-Ghannam & Shaw, Mayle, Calvert, and Choi & Kang are incorporated into an upwind-biased Navier-Stokes solver and the computed results are compared with the measurements of Elazar & Shreeve in a cascade with controlled-diffusion blades. It is found that none of the models predicts the measured bubbles very well, although most of them give reasonable results as long as transition is predicted to occur within the bubble.

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).

Review: Laminar-to-Turbulent Transition of Three-Dimensional Boundary Layers on Rotating Bodies

1994 ◽  
Vol 116 (2) ◽  
pp. 200-211 ◽  
Author(s):  
Ryoji Kobayashi
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Three Dimensional ◽  
Speed Ratio ◽  
Stream Velocity ◽  
Turbulent Transition ◽  
Centrifugal Instability ◽  
Crossflow Instability ◽  
Dimensional Boundary ◽  
Axisymmetric Bodies

The laminar-turbulent transition of three-dimensional boundary layers is critically reviewed for some typical axisymmetric bodies rotating in still fluid or in axial flow. The flow structures of the transition regions are visualized. The transition phenomena are driven by the compound of the Tollmien-Schlichting instability, the crossflow instability, and the centrifugal instability. Experimental evidence is provided relating the critical and transition Reynolds numbers, defined in terms of the local velocity and the boundary layer momentum thickness, to the local rotational speed ratio, defined as the ratio of the circumferential speed to the free-stream velocity at the outer edge of the boundary layer, for the rotating disk, the rotating cone, the rotating sphere and other rotating axisymmetric bodies. It is shown that the cross-sectional structure of spiral vortices appearing in the transition regions and the flow pattern of the following secondary instability in the case of the crossflow instability are clearly different than those in the case of the centrifugal instability.

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