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.

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.

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.

The Fluid Dynamics of LPT Blade Separation Control Using Pulsed Jets

2001 ◽  
Author(s):  
Jeffrey P. Bons ◽  
Rolf Sondergaard ◽  
Richard B. Rivir
Keyword(s):  
Boundary Layer ◽  
Duty Cycle ◽  
Separation Bubble ◽  
Vortex Generator ◽  
Boundary Layer Separation ◽  
Skew Angle ◽  
Suction Surface ◽  
Duty Cycles ◽  
Wide Range ◽  
Vortex Generator Jets

The effects of pulsed vortex generator jets on a naturally separating low pressure turbine boundary layer have been investigated experimentally. Blade Reynolds numbers in the linear turbine cascade match those for high altitude aircraft engines and industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets (30 degree pitch and 90 degree skew angle) are pulsed over a wide range of frequency at constant amplitude and selected duty cycles. The resulting wake loss coefficient vs. pulsing frequency data add to previously presented work by the authors documenting the loss dependency on amplitude and duty cycle. As in the previous studies, vortex generator jets are shown to be highly effective in controlling laminar boundary layer separation. This is found to be true at dimensionless forcing frequencies (F+) well below unity and with low (10%) duty cycles. This unexpected low frequency effectiveness is due to the relatively long relaxation time of the boundary layer as it resumes its separated state. Extensive phase-locked velocity measurements taken in the blade wake at an F+ of 0.01 with 50% duty cycle (a condition at which the flow is essentially quasi-steady) document the ejection of bound vorticity associated with a low momentum fluid packet at the beginning of each jet pulse. Once this initial fluid event has swept down the suction surface of the blade, a reduced wake signature indicates the presence of an attached boundary layer until just after the jet termination. The boundary layer subsequently relaxes back to its naturally separated state. This relaxation occurs on a timescale which is 5–6 times longer than the original attachment due to the starting vortex. Phase-locked boundary layer measurements taken at various stations along the blade chord illustrate this slow relaxation phenomenon. This behavior suggests that some economy of jet flow may be possible by optimizing the pulse duty cycle and frequency for a particular application. At higher pulsing frequencies, for which the flow is fully dynamic, the boundary layer is dominated by periodic shedding and separation bubble migration, never recovering its fully separated (uncontrolled) state.

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.

The Response of Injection of Vortex Generator Jets to a Backward Facing Step Flow

2003 ◽  
Author(s):  
Koichi Yamagata ◽  
Manabu Saito ◽  
Tadashi Morioka ◽  
Shinji Honami
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Flow Behavior ◽  
Velocity Ratio ◽  
Vortex Generator ◽  
Step Response ◽  
Backward Facing Step ◽  
Longitudinal Vortices ◽  
Step Flow ◽  
Vortex Generator Jets

In this paper, the flow behavior of a reattachment process over a backward facing step flow is reported. The reattachment process is controlled by injection of vortex generator jets. The injection of jets upstream of the step produces the co-rotating longitudinal vortices in a separating shear layer. The experiment of the step response of the injection jet is also conducted in order to investigate the evolution process of the longitudinal vortices. A large scale of primary and counter vortices are observed, when the velocity ratio of the free stream to injected jet is 6. The detailed structure of the longitudinal vortices is clarified. The remarkable effect of the vortices on the separating shear layer downstream of the step is observed.

scholarly journals Oscillatory migratory large-scale fields in mean-field and direct simulations

2009 ◽  
Vol 5 (S264) ◽  
pp. 197-201
Author(s):  
Dhrubaditya Mitra ◽  
Reza Tavakol ◽  
Axel Brandenburg ◽  
Petri J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Large Scale ◽  
Mean Field ◽  
Direct Numerical Simulations ◽  
Mhd Equations ◽  
Perfect Conductor ◽  
Large Scale Magnetic Field ◽  
Wedge Shape ◽  
Polarity Reversals

AbstractWe summarise recent results form direct numerical simulations of both non-rotating helically forced and rotating convection driven MHD equations in spherical wedge-shape domains. In the former, using perfect-conductor boundary conditions along the latitudinal boundaries we observe oscillations, polarity reversals and equatorward migration of the large-scale magnetic fields. In the latter we obtain angular velocity with cylindrical contours and large-scale magnetic field which shows oscillations, polarity reversals but poleward migration. The occurrence of these behviours in direct numerical simulations is clearly of interest. However the present models as they stand are not directly applicable to the solar dynamo problem. Nevertheless, they provide general insights into the operation of turbulent dynamos.

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