Direct Numerical Simulations of a Laminar Separation Bubbles on a Curved Plate: Part 2 — Flow Control Using Pulsed Vortex Generator Jets

2013 ◽  
Author(s):  
Wolfgang Balzer ◽  
Hermann F. Fasel
Keyword(s):  
Flow Control ◽  
Numerical Simulations ◽  
Large Scale ◽  
Separation Bubble ◽  
Hydrodynamic Instability ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Vortex Generator Jets

Highly-accurate direct numerical simulations (DNS) are employed to investigate active flow control of laminar boundary layer separation by means of pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. The uncontrolled flow configuration is represented by a laminar separation bubble developing on a curved-plate geometry modeling the convex suction-side curvature of the Pratt&Whitney “PackB” research blade. The simulation setup and uncontrolled flow results were presented in part I of the present paper. In this second part, particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low Reynolds number separation, as for example encountered in low-pressure turbine applications. The numerical results confirm findings of earlier flat-plate simulations, which showed that the control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the (uncontrolled) separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise-coherent structures. These structures provide the necessary entrainment of high-momentum fluid causing a much sooner reattachment of the separated flow compared to the uncontrolled flow and consequently leading to a significant reduction in performance losses.

Control of laminar separation using pulsed vortex generator jets: direct numerical simulations

2011 ◽  
Vol 676 ◽  
pp. 81-109 ◽  
Author(s):  
D. POSTL ◽  
W. BALZER ◽  
H. F. FASEL
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Hydrodynamic Instability ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Momentum Coefficient ◽  
Instability Modes ◽  
Vortex Generator Jets

Direct numerical simulations (DNS) are employed to investigate laminar boundary layer separation and its control by pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. Particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low-Reynolds-number separation, as encountered in low-pressure turbine applications. Pulsed VGJs are shown to be much more effective than steady VGJs when the same momentum coefficient is used for the actuation. From our investigations we have found that the increased control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise coherent structures. These structures provide the necessary entrainment of high-momentum fluid to successfully reattach the flow.

scholarly journals Numerical Investigation of Steady and Harmonic Vortex Generator Jets Flow Separation Control

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 94 ◽  
Author(s):  
Aria Alimi ◽  
Olaf Wünsch
Keyword(s):  
Flow Control ◽  
Large Scale ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Transition To Turbulence ◽  
Momentum Exchange ◽  
Hairpin Vortices ◽  
Flow Separation Control ◽  
Vortex Generator Jets

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient at reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures forming immediately downstream of the injection location led to the formation of hairpin-type vortices, causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs, as well. However, the increased control effectiveness for harmonic VGJs’ flow control strategy is attributed to the fact that the shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified, leading to the development of large-scale coherent structures, which are very effective at increasing the momentum exchange, thus limiting the separated region.

Role of Klebanoff modes in active flow control of separation: direct numerical simulations

2018 ◽  
Vol 850 ◽  
pp. 954-983 ◽  
Author(s):  
Shirzad Hosseinverdi ◽  
Hermann F. Fasel
Keyword(s):  
Flow Control ◽  
Numerical Simulations ◽  
Active Flow Control ◽  
Low Frequency ◽  
Secondary Instability ◽  
Direct Numerical Simulations ◽  
Striking Difference ◽  
Computational Domain ◽  
Low Levels ◽  
Active Flow

Our previous research has shown that an active flow control strategy using two-dimensional (2-D) harmonic blowing and suction with properly chosen frequency and amplitude can significantly reduce the separation region, delay transition to turbulence and can even relaminarize the flow. How such effective flow control for transition delay and relaminarization is affected by free-stream turbulence (FST) remains an open question. In order to answer this question, highly resolved direct numerical simulations (DNS) are carried out where very low-amplitude isotropic FST fluctuations are introduced at the inflow boundary of the computational domain. With FST the effectiveness of the flow control is not diminished, and the extent of the separated flow region is reduced by the same amount as for the zero FST case. However, a striking difference observed in the DNS is the fact that in the presence of even very low levels of FST, the flow transitions shortly downstream of the reattachment location of the bubble, contrary to the case without FST. It appears that this different behaviour for even very small levels of FST is caused by an interaction between the high-amplitude 2-D disturbances introduced by the flow control forcing and 3-D Klebanoff modes (K-modes) that are generated by the FST. The streamwise elongated streaks due to the K-modes cause a spanwise-periodic modulation of the basic flow and subsequently of the primary 2-D wave. The disturbances associated with this modulation exhibit strong growth and initiate the breakdown process to turbulence. Linear secondary instability investigations with respect to low-frequency 3-D disturbances are carried out based on the linearized Navier–Stokes equations. The response of the forced flow to the low-frequency 3-D disturbances reveals that the time-periodic base flow is unstable with respect to a wide range of 3-D modes. In particular, the wavelength associated with the spanwise spacing of the K-mode falls into the range of, and is in fact very close to, the most unstable 3-D disturbances. Results from the secondary instability analysis and the comparison with DNS results, support the conjecture that the forcing amplitude has a major impact on the onset and amplification rate of the K-modes: lowering the forcing amplitude postpones the onset of the growth of the K-modes and reduces the growth rate of the K-modes for a given FST intensity. The net effect of these two events is a delay of the transition onset. Nevertheless, the instability mechanism that governs the transition process is the same as previously identified, i.e. interaction of the K-mode and 2-D primary wave. Furthermore, for low levels of FST, the amplitude of the low-frequency K-modes scales linearly with the FST intensity in the approach boundary layer up to the secondary instability regime.

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 Control of Laminar Boundary-Layer Separation Using Steady and Harmonic Vortex Generator Jets

2018 ◽  
Author(s):  
Aria Alimi ◽  
Olaf Wünsch
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Laminar Boundary Layer ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Transition To Turbulence ◽  
Momentum Exchange ◽  
Layer Separation ◽  
Vortex Generator Jets

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient in reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures formed immediately downstream of injection location led to formation of hairpin-type vortices causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs as well. However, the increased control effectiveness for harmonic VGJs flow control strategy is attributed to the fact that shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified leading to development of large-scale coherent structures, which are very effective in increasing the momentum exchange, thus, limiting the separated region.

scholarly journals Control of Laminar Boundary-Layer Separation Using Steady and Harmonic Vortex Generator Jets

2018 ◽  
Author(s):  
Aria Alimi ◽  
Olaf Wünsch
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Laminar Boundary Layer ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Transition To Turbulence ◽  
Momentum Exchange ◽  
Layer Separation ◽  
Vortex Generator Jets

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient in reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures formed immediately downstream of injection location led to formation of hairpin-type vortices causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs as well. However, the increased control effectiveness for harmonic VGJs flow control strategy is attributed to the fact that shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified leading to development of large-scale coherent structures, which are very effective in increasing the momentum exchange, thus, limiting the separated region.

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