Active Flow Control of Dynamic Stall by Means of Continuous Jet Flow at Reynolds Number of 1 × 106

2017 ◽  
Vol 140 (1) ◽  
Author(s):  
Mehran Tadjfar ◽  
Ehsan Asgari
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Pressure Difference ◽  
Active Flow Control ◽  
Velocity Ratio ◽  
Dynamic Stall ◽  
Hysteresis Curve ◽  
Small Range ◽  
Active Flow ◽  
The Impact

We have studied the influence of a tangential blowing jet in dynamic stall of a NACA0012 airfoil at Reynolds number of 1 × 106, for active flow control (AFC) purposes. The airfoil was oscillating between angles of attack (AOA) of 5 and 25 deg about its quarter-chord with a sinusoidal motion. We have utilized computational fluid dynamics to investigate the impact of jet location and jet velocity ratio on the aerodynamic coefficients. We have placed the jet location upstream of the counter-clockwise (CCW) vortex which was formed during the upstroke motion near the leading-edge; we have also considered several other locations nearby to perform sensitivity analysis. Our results showed that placing the jet slot within a very small range upstream of the CCW vortex had tremendous effects on both lift and drag, such that maximum drag was reduced by 80%. There was another unique observation: placing the jet at separation point led to an inverse behavior of drag hysteresis curve in upstroke and downstroke motions. Drag in downstroke motion was significantly lower than upstroke motion, whereas in uncontrolled case the converse was true. Lift was significantly enhanced during both upstroke and downstroke motions. By investigating the pressure coefficients, it was found that flow control had altered the distribution of pressure over the airfoil upper surface. It caused a reduction in pressure difference between upper and lower surfaces in the rear part, while substantially increased pressure difference in the front part of the airfoil.

Active Flow Control of Dynamic Stall by Means of Jet Flow at Reynolds Number 106

2016 ◽  
Author(s):  
Ehsan Asgari ◽  
Mehran Tadjfar
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Active Flow Control ◽  
Velocity Ratio ◽  
Jet Flow ◽  
Synthetic Jet ◽  
Dynamic Stall ◽  
Small Range ◽  
Active Flow ◽  
The Impact

In this study, we have examined slot location and velocity ratio of a tangential co-flow jet in dynamic stall motion of an airfoil at Reynolds number 1×106, for active flow control (AFC) purposes. The airfoil is the symmetrical NACA 0012 with a pitching motion between AOAs 5 deg. and 25 deg. about its quarter-chord with a sinusoidal motion. We have utilized Computational Fluid Dynamics (CFD) tool to numerically investigate the impact of jet location and jet velocity ratio on the aerodynamic coefficients. We have placed the jet location upstream the counter clock-wise (CCW) vortex which is formed during the upstroke motion near the leading-edge. We have also selected several other locations nearby to perform sensitivity analysis. Our results showed that placing the jet slot within a very small range upstream the CCW vortex has tremendous effects on both lift and drag, such that maximum drag which occurs at maximum incidences reduced by 80%. There was another unique observation: putting jet at separation point leads to an inverse behavior of drag hysteresis curve in upstroke and downstroke motions. Drag in downstroke motion is significantly lower than upstroke motion, whereas in uncontrolled case the converse is true. In addition, by implementing jet flow lift is significantly enhanced during both upstroke and downstroke motions. Finally, it should be indicated that this study provides initial steps in investigations of applying synthetic jet actuator (SJA) on a pitching airfoil at high Reynolds number 106 with effects of changing momentum ratio and SJA frequency, which will be presented in the near future.

Comparison of Two Active Flow Control Mechanisms of Pure Blowing and Pure Suction on a Pitching NACA0012 Airfoil at Reynolds Number of 1 × 106

2018 ◽  
Author(s):  
Ehsan Asgari ◽  
Mehran Tadjfar
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Active Flow Control ◽  
Hysteresis Loops ◽  
Leading Edge ◽  
Control Mechanisms ◽  
Airfoil Surface ◽  
Naca0012 Airfoil ◽  
Maximum Angle ◽  
Active Flow

In this study, we have applied and compared two active flow control (AFC) mechanisms on a pitching NACA0012 airfoil at Reynolds number of 1 × 106 using 2-D computational fluid dynamics (CFD). These mechanisms are continuous blowing and suction which are applied separately on the airfoil which pitches around its quarter-chord in a sinusoidal motion. The location for suction and blowing was determined in our previous study based on the formation of a counter clock-wise vortex near the leading-edge. In our current study, we have compared the effectiveness of pure blowing and pure suction in suppressing the dynamic stall vortex (DSV) which is the main contributor to the drag increase, particularly near the maximum angle of attack (AOA) and in early downstroke motion. The blowing/suction slot is considered as a dent on the airfoil surface which enables the AFC to perform in a tangential manner. This configuration would allow blowing jet to penetrate further downstream and was shown to be more effective compared to a cross-flow orientation. We have compared the two aforementioned mechanisms in terms of hysteresis loops of lift and drag coefficients and have demonstrated the dynamics of flow in controlled and uncontrolled situations.

Experimental Investigations on the Efficiency of Active Flow Control in a Compressor Cascade With Periodic Non-Steady Outflow Conditions

2017 ◽  
Author(s):  
Marcel Staats ◽  
Wolfgang Nitsche
Keyword(s):  
Flow Field ◽  
Flow Control ◽  
Flow Separation ◽  
Active Flow Control ◽  
Experimental Investigations ◽  
Compressor Cascade ◽  
Steady Regime ◽  
Stator Blade ◽  
Active Flow ◽  
The Impact

We present results of experiments on a periodically unsteady compressor stator flow of the type which would be expected in consequence of pulsed combustion. A Reynolds number of Re = 600000 was used for the investigations. The experiments were conducted on the two-dimensional low-speed compressor testing facility in Berlin. A choking device downstream the trailing edges induced a periodic non-steady outflow condition to each stator vane which simulated the impact of a pressure gaining combuster downstream from the last stator. The Strouhal number of the periodic disturbance was Sr = 0.03 w.r.t. the stator chord length. Due to the periodic non-steady outflow condition, the flow-field suffers from periodic flow separation phenomena, which were managed by means of active flow control. In our case, active control of the corner separation was applied using fluidic actuators based on the principle of fluidic amplification. The flow separation on the centre region of the stator blade was suppressed by means of a fluidic blade actuator leading to an overall time-averaged loss reduction of 11.5%, increasing the static pressure recovery by 6.8% while operating in the non-steady regime. Pressure measurements on the stator blade and the wake as well as PIV data proved the beneficial effect of the active flow control application to the flow field and the improvement of the compressor characteristics. The actuation efficiency was evaluated by two figures of merit introduced in this contribution.

Impact of the Precessing Vortex Core on NOx Emissions in Premixed Swirl-Stabilized Flames: An Experimental Study

2020 ◽  
Author(s):  
Finn Lückoff ◽  
Moritz Sieber ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Flow Control ◽  
Large Scale ◽  
Active Flow Control ◽  
Premixed Flame ◽  
Nox Emissions ◽  
Precessing Vortex Core ◽  
Flame Dynamics ◽  
Swirl Flows ◽  
Active Flow ◽  
The Impact

Abstract The reduction of polluting NOx emission remains a driving factor in the design process of swirl-stabilized combustion systems, to meet strict legislative restrictions. In reacting swirl flows, hydrodynamic coherent structures, such as periodic large-scale vortices in the shear layer, induce zones with increased heat release rate fluctuations in connection with temperature peaks, which lead to an increase of NOx emissions. Such large-scale vortices can be induced by the helical coherent structure known as precessing vortex core (PVC), which influences the flow and flame dynamics of reacting swirl flows under certain operating conditions. We developed an active flow control system, which allows for a targeted actuation of the PVC, to investigate its impact on important combustion properties. In this study, the direct active flow control is used to actuate a PVC of arbitrary frequency and amplitude, which facilitates a systematic study of the impact of the PVC on NOx emissions. In the course of the present work, a perfectly premixed flame, which slightly damps the PVC, is studied in detail. Since the PVC is slightly damped, it can be precisely excited by means of open-loop flow control. In connection with time-resolved OH*-chemiluminescence and stereoscopic PIV measurements, the flame and flow response to PVC actuation as well as the impact of the actuated PVC on flow and flame dynamics are characterized. It turns out that the PVC rolls up the inner shear layer, which results in an interaction of PVC-induced vortices and flame. This interaction considerably influences the measured level of NOx emissions, which grow with increasing PVC amplitude in a perfectly premixed flame. Nearly the same increase is to be seen for a partially premixed flame. This in contrast to previous studies, where the PVC is assumed to reduce the NOx emissions due to vortex-enhanced mixing.

Low Reynolds Number Laminar Airfoil with Active Flow Control

2010 ◽  
Author(s):  
Michael Thake ◽  
Nathan Packard ◽  
Carlos Bonilla ◽  
Jeffrey Bons
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Low Reynolds Number ◽  
Active Flow Control ◽  
Low Reynolds ◽  
Active Flow

Iterative Learning Active Flow Control Applied to a Compressor Stator Cascade With Periodic Disturbances

2015 ◽  
Author(s):  
Simon J. Steinberg ◽  
Rudibert King ◽  
Marcel Staats ◽  
Wolfgang Nitsche
Keyword(s):  
Flow Control ◽  
Active Flow Control ◽  
Pressure Sensors ◽  
Iterative Learning ◽  
Suction Surface ◽  
Periodic Disturbances ◽  
Control Command ◽  
Detonation Engine ◽  
Active Flow ◽  
The Impact

This contribution presents the capability of iterative learning active flow control to decrease the impact of periodic disturbances in an experimental compressor stator cascade with sidewall actuation. The periodic disturbances of the individual passage flows are generated by a damper flap device that is located downstream of the trailing edges of the blades. These mimic the throttling effect of periodically closed combustion tubes in a pulsed detonation engine. For the purpose of rejecting this disturbance the passage flow is manipulated by fluidic actuators that introduce an adjustable amount of pressurized air through slots in the sidewalls of the cascade. Pressure sensors that are mounted flush to the suction surface of the middle blade provide information on the current flow situation. This data is fed back in real-time to an optimal iterative learning controller. By learning from period to period the controller modifies the actuation amplitude such that, eventually, a control command trajectory is calculated that reduces the impact of the periodic disturbance on the flow in an optimal manner.

Active Flow Control of Separated Turbulent Flow Over a Hump Using RANS, DES, and LES

2008 ◽  
Author(s):  
Subhadeep Gan ◽  
Urmila Ghia ◽  
Karman Ghia
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Flow Separation ◽  
Turbulence Modeling ◽  
Active Flow Control ◽  
Separated Flow ◽  
Experimental Results ◽  
Complex Flow ◽  
Active Flow ◽  
Turbulent Separated Flow

Most practical flows in engineering applications are turbulent, and exhibit separation. Losses due to separation are undesirable because they generally have adverse effects on performance and efficiency. Therefore, control of turbulent separated flows has been a topic of significant interest as it can reduce separation losses. It is of utmost importance to understand the complex flow dynamics that leads to flow separation and come up with methods of flow control. In the past, passive flow-control was mostly implemented that does not require any additional energy source to reduce separation losses but it leads to increasing viscous losses at higher Reynolds number. More recent work has been focused primarily on active flow-control techniques that can be turned on and off depending on the requirement of flow-control. The present work is focused on implementing flow control using steady suction in the region of flow separation. The present work is Case 3 of the 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop, http://cfdval2004.larc.nasa.gov/case3.html, conducted by NASA for the flow over a wall-mounted hump. The flow over a hump is an example of a turbulent separated flow. This flow is characterized by a simple geometry, but, nevertheless, is rich in many complex flow phenomena such as shear layer instability, separation, reattachment, and vortex interactions. The baseline case has been successfully simulated by Gan et al., 2007. The flow is simulated at a Reynolds number of 371,600, based on the hump chord length, C, and Mach number of 0.04. The flow control is being achieved via a slot at approximately 65% C by using steady suction. Solutions are presented for the three-dimensional RANS SST, steady and unsteady, turbulence model and DES and LES turbulence modeling approaches. Multiple turbulence modeling approaches help to ascertain what techniques are most appropriate for capturing the physics of this complex separated flow. Second-order accurate time derivatives are used for all implicit unsteady simulation cases. Mean-velocity contours and turbulent kinetic energy contours are examined at different streamwise locations. Detailed comparisons are made of mean and turbulence statistics such as the pressure coefficient, skinfriction coefficient, and Reynolds stress profiles, with experimental results. The location of the reattachment behind the hump is compared with experimental results. The successful control of this turbulent separated flow causes a reduction in the reattachment length, compared with the uncontrolled case. The effects of steady suction on flow separation and reattachment are discussed.

Large Eddy Simulation of Active Flow Control Over SD7003 at Reynolds Number of 60,000

2020 ◽  
Author(s):  
Djavad Kamari ◽  
Mehran Tadjfar ◽  
Ali Tarokh
Keyword(s):  
Reynolds Number ◽  
Large Eddy Simulation ◽  
Flow Control ◽  
Turbulent Viscosity ◽  
Active Flow Control ◽  
Smagorinsky Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Active Flow

Abstract Large Eddy Simulation for active flow control (AFC) by employing synthetic and continuous blowing is done to investigate their effects on resizing separation. The flow around an SD7003 airfoil at Reynolds number of 60,000 and angles of attack of 13° is considered where a widespread separation occurs at post stall. In this work, the Dynamic Smagorinsky model is used as to calculate the turbulent viscosity.

Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions

2021 ◽  
Vol 143 (2) ◽  
Author(s):  
Tobias Werder ◽  
Robert Liebich ◽  
Karl Neuhäuser ◽  
Clara Behnsen ◽  
Rudibert King
Keyword(s):  
Phase Shift ◽  
Flow Control ◽  
Gas Turbines ◽  
Active Flow Control ◽  
Operating Conditions ◽  
Extremum Seeking ◽  
Experimental Investigations ◽  
Compressor Cascade ◽  
Active Flow ◽  
The Impact

Abstract As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.

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