Flow Control of a Retreating Airfoil via NS-DBD Actuators Using Large Eddy Simulations

2014 ◽  
Author(s):  
Kelsey Shaler ◽  
Datta V. Gaitonde
Keyword(s):  
Flow Control ◽  
Shear Layer ◽  
Static Pressure ◽  
Leading Edge ◽  
Large Eddy Simulations ◽  
Surface Heating ◽  
Trailing Edge ◽  
Volumetric Heating ◽  
Large Eddy ◽  
Heating Model

The capability of Large Eddy Simulations (LES) to accurately model Nano-Second Pulsed Dielectric-Barrier Discharge (NS-DBD) plasma actuators for use as a flow control devise is demonstrated by comparing the newly-developed volumetric heating model to experimental results as well as a previously established surface heating model. The purpose of these models and corresponding experiments is to show that use of NS-DBD actuators can mitigate the presence of stall on a NACA0015 airfoil at a Reynolds number of 100,000 and 15° angle of attack in reversed-flow conditions. Actuators are placed at both the aerodynamic leading and trailing edge — the effects of which are analyzed separately — and forced at several Strouhal numbers StF=fc′U∞. The model validation is carried out by comparing the actuator pulse structure, mean value contours of various parameters, static pressure distribution (Cp) along the airfoil surface, and FFT plots of sound pressure level (SPL). The model results are then compared to the no-control simulations to provide evidence that actuation delays the onset of stall. This process is explored for both unsteady and steady quantities, including FFT plots, intantaneous flow field response, static pressure recovery, and mean quanitites, including a boundary layer analysis. It is concluded that at low Reynolds numbers reattachment occurs through enhanced turbulence of a separated, laminar shear layer; the reattachment processes is shown to take place over approximately 8 characteristic times for both actuator locations, although leading edge actuation only results in reattachment in the mean sense. Under similar situations, the volumetric and surfaec heating models showed similar recovery characteristics; however, since the volumetric model is less empirical than surface heating, it is recommended that volumetric heating be used in the future. Both heating models indicate that actuation at the aerodynamic leading edge has the greatest affect on the flow due to the laminar nature of the corresponding shear layer, as opposed to the turbulent shear layer on the trailing edge. It addition, a change in duty cycle was shown to have little effect on the results whereas an incerase in StF had a large negative effect on reattachment.

Post-stall flow control on an airfoil by local unsteady forcing

1998 ◽  
Vol 371 ◽  
pp. 21-58 ◽  
Author(s):  
JIE-ZHI WU ◽  
XI-YUN LU ◽  
ANDREW G. DENNY ◽  
MENG FAN ◽  
JAIN-MING WU
Keyword(s):  
Flow Control ◽  
Shear Layer ◽  
Separated Flow ◽  
Leading Edge ◽  
Numerical Data ◽  
Power Input ◽  
Nonlinear Process ◽  
Mode Competition ◽  
Trailing Edge ◽  
Wide Range

By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of post-stall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Kármán street.

Large Eddy Simulation of Wake-Shear Layer Interactions Over a Multi-Element Aerofoil

2020 ◽  
Author(s):  
Souvik Naskar ◽  
S. Sarkar
Keyword(s):  
Large Eddy Simulation ◽  
Shear Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Leading Edge ◽  
Aerodynamic Characteristics ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Eddy Simulation ◽  
Large Eddy

Abstract Modern commercial airliners use multi-element aerofoils to enhance take-off and landing performance. Further, multielement aerofoil configurations have been shown to improve the aerodynamic characteristics of wind turbines. In the present study, high resolution Large Eddy Simulation (LES) is used to explore the low Reynolds Number (Re = 0.832 × 104) aerodynamics of a 30P30N multi-element aerofoil at an angle of attack, α = 4°. In the present simulation, wake shed from a leading edge element or slat is found to interact with the separated shear layer developing over the suction surface of the main wing. High receptivity of shear layer via amplification of free-stream turbulence leads to rollup and breakdown, forming a large separation bubble. A transient growth of fluctuations is observed in the first half of the separation bubble, where levels of turbulence becomes maximum near the reattachment and then decay depicting saturation of turbulence. Results of the present LES are found to be in close agreement with the experiment depicting high vortical activity in the outer layer. Some features of the flow field here are similar to those occur due to interactions of passing wake and the separated boundary layer on the suction surface of high lift low pressure turbine blades.

New combinational active control strategy for improving aerodynamic characteristics of airfoil and rotor

2019 ◽  
Vol 234 (4) ◽  
pp. 977-996
Author(s):  
Yi-yang Ma ◽  
Qi-jun Zhao ◽  
Guo-qing Zhao
Keyword(s):  
Flow Control ◽  
Control Strategy ◽  
Active Flow Control ◽  
Leading Edge ◽  
Aerodynamic Characteristics ◽  
Synthetic Jet ◽  
Dynamic Stall ◽  
Trailing Edge ◽  
Trailing Edge Flap ◽  
Cfd Method

In order to improve the aerodynamic characteristics of rotor, a new active flow control strategy by combining a synthetic jet actuator and a variable droop leading-edge or a trailing-edge flap has been proposed. Their control effects are numerically investigated by computational fluid dynamics (CFD) method. The validated results indicate that variable droop leading-edge and synthetic jet can suppress the formation of dynamic stall vortex and delay flow separation over rotor airfoil. Compared with the baseline state, Cdmax and Cmmax are significantly reduced. Furthermore, parametric analyses on dynamic stall control of airfoil by the combinational method are conducted, and it indicates that the aerodynamic characteristics of the oscillating rotor airfoil can be significantly improved when the non-dimensional frequency ( k*) of variable droop leading-edge is about 1.0. At last, simulations are conducted for the flow control of rotor by the combinational method. The numerical results indicate that large droop angle of variable droop leading-edge can better reduce the torque coefficient of rotor and the trailing-edge flap has the capability of increasing the thrust of rotor. Also, the synthetic jet could further improve the aerodynamic characteristics of rotor.

Effect of Blowing Ratio in the Near-Stagnation Region of a Three-Row Leading Edge Film Cooling Geometry Using Large Eddy Simulations

2009 ◽  
Author(s):  
Sai Shrinivas Sreedharan ◽  
Danesh K. Tafti
Keyword(s):  
Film Cooling ◽  
Density Ratio ◽  
Leading Edge ◽  
Large Eddy Simulations ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Blowing Ratio ◽  
Large Eddy ◽  
Adiabatic Effectiveness ◽  
Compound Angle

Computational studies are carried out using Large Eddy Simulations (LES) to investigate the effect of coolant to mainstream blowing ratio in a leading edge region of a film cooled vane. The three row leading edge vane geometry is modeled as a symmetric semi-cylinder with a flat afterbody. One row of coolant holes is located along the stagnation line and the other two rows of coolant holes are located at ±21.3° from the stagnation line. The coolant is injected at 45° to the vane surface with 90° compound angle injection. The coolant to mainstream density ratio is set to unity and the freestream Reynolds number based on leading edge diameter is 32000. Blowing ratios (B.R.) of 0.5, 1.0, 1.5, and 2.0 are investigated. It is found that the stagnation cooling jets penetrate much further into the mainstream, both in the normal and lateral directions, than the off-stagnation jets for all blowing ratios. Jet dilution is characterized by turbulent diffusion and entrainment. The strength of both mechanisms increases with blowing ratio. The adiabatic effectiveness in the stagnation region initially increases with blowing ratio but then generally decreases as the blowing ratio increases further. Immediately downstream of off-stagnation injection, the adiabatic effectiveness is highest at B.R. = 0.5. However, further downstream the larger mass of coolant injected at higher blowing ratios, in spite of the larger jet penetration and dilution, increases the effectiveness with blowing ratio.

scholarly journals Flow control over an airfoil using virtual Gurney flaps

2015 ◽  
Vol 767 ◽  
pp. 595-626 ◽  
Author(s):  
Li-Hao Feng ◽  
Kwing-So Choi ◽  
Jin-Jun Wang
Keyword(s):  
Flow Control ◽  
Shear Layer ◽  
Plasma Actuator ◽  
Recirculation Region ◽  
Dbd Plasma ◽  
Near Wake ◽  
Trailing Edge ◽  
Pressure Surface ◽  
Gurney Flaps ◽  
Gurney Flap

AbstractFlow control over a NACA 0012 airfoil is carried out using a dielectric barrier discharge (DBD) plasma actuator at the Reynolds number of 20 000. Here, the plasma actuator is placed over the pressure (lower) side of the airfoil near the trailing edge, which produces a wall jet against the free stream. This reverse flow creates a quasi-steady recirculation region, reducing the velocity over the pressure side of the airfoil. On the other hand, the air over the suction (upper) side of the airfoil is drawn by the recirculation, increasing its velocity. Measured phase-averaged vorticity and velocity fields also indicate that the recirculation region created by the plasma actuator over the pressure surface modifies the near-wake dynamics. These flow modifications around the airfoil lead to an increase in the lift coefficient, which is similar to the effect of a mechanical Gurney flap. This configuration of DBD plasma actuators, which is investigated for the first time in this study, is therefore called a virtual Gurney flap. The purpose of this investigation is to understand the mechanism of lift enhancement by virtual Gurney flaps by carefully studying the global flow behaviour over the airfoil. First, the recirculation region draws the air from the suction surface around the trailing edge. The upper shear layer then interacts with the opposite-signed shear layer from the pressure surface, creating a stronger vortex shedding from the airfoil. Secondly, the recirculation region created by a DBD plasma actuator over the pressure surface displaces the positive shear layer away from the airfoil, thereby shifting the near-wake region downwards. The virtual Gurney flap also changes the dynamics of laminar separation bubbles and associated vortical structures by accelerating laminar-to-turbulent transition through the Kelvin–Helmholtz instability mechanism. In particular, the separation point and the start of transition are advanced. The reattachment point also moves upstream with plasma control, although it is slightly delayed at a large angle of attack.

scholarly journals Evaluation of Injection Strategies in Supersonic Nozzle Flow

Aerospace ◽  
2021 ◽  
Vol 8 (12) ◽  
pp. 369
Author(s):  
Bernhard Semlitsch ◽  
Mihai Mihăescu
Keyword(s):  
Penetration Depth ◽  
Static Pressure ◽  
Supersonic Nozzle ◽  
Large Eddy Simulations ◽  
Nozzle Flow ◽  
Nozzle Throat ◽  
Vortex Pairs ◽  
Large Eddy ◽  
Injection Strategies ◽  
Injection Pressures

The ability to manipulate shock patterns in a supersonic nozzle flow with fluidic injection is investigated numerically using Large Eddy Simulations. Various injector configurations in the proximity of the nozzle throat are screened for numerous injection pressures. We demonstrate that fluidic injection can split the original, single shock pattern into two weaker shock patterns. For intermediate injection pressures, a permanent shock structure in the exhaust can be avoided. The nozzle flow can be manipulated beneficially to increase thrust or match the static pressure at the discharge. The shock pattern evolution of injected stream is described over various pressure ratios. We find that the penetration depth into the supersonic crossflow is deeper with subsonic injection. The tight arrangement of the injectors can provoke additional counter-rotating vortex pairs in between the injection.

Impact of Elevated Kevin-Helmholtz Billows on the Atmospheric Boundary Layer

2021 ◽  
Author(s):  
Qingfang Jiang
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Shear Layer ◽  
Exponential Growth ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Two Dimensional ◽  
Large Eddy ◽  
Saturation Stage ◽  
The Impact

AbstractThe impact of Kelvin-Helmholtz billows (KHBs) in an elevated shear layer (ESL) on the underlying atmospheric boundary layer (BL) is examined utilizing a group of large-eddy simulations. In these simulations, KHBs develop in the ESL and experience exponential growth, saturation, and exponential decay stages. In response, strong wavy motion occurs in the BL, inducing rotor circulations near the surface when the BL is stable. During the saturation stage, secondary instability develops in the ESL and the wavy BL almost simultaneously, followed by the breakdown of the quasi-two-dimensional KH billows and BL waves into three-dimensional turbulence. Consequently, during and after a KH event, the underlying BL becomes more turbulent with its depth increased and stratification weakened substantially, suggestive of significant lasting impact of elevated KH billows on the atmospheric BL. The eventual impact of KHBs on the BL is found to be sensitive to both the ESL and BL characteristics.

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