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.

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.

Three-Dimensional and Rotational Aerodynamics on the NREL Phase VI Wind Turbine Blade

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Alvaro Gonzalez ◽  
Xabier Munduate
Keyword(s):  
Wind Turbine ◽  
Turbine Blade ◽  
Three Dimensional ◽  
Separated Flow ◽  
Leading Edge ◽  
Wind Turbine Blade ◽  
Trailing Edge ◽  
Rotating Blade ◽  
Nrel Phase Vi ◽  
Edge Separation

This work undertakes an aerodynamic analysis over the parked and the rotating NREL Phase VI wind turbine blade. The experimental sequences from NASA Ames wind tunnel selected for this study respond to the parked blade and the rotating configuration, both for the upwind, two-bladed wind turbine operating at nonyawed conditions. The objective is to bring some light into the nature of the flow field and especially the type of stall behavior observed when 2D aerofoil steady measurements are compared to the parked blade and the latter to the rotating one. From averaged pressure coefficients together with their standard deviation values, trailing and leading edge separated flow regions have been found, with the limitations of the repeatability of the flow encountered on the blade. Results for the parked blade show the progressive delay from tip to root of the trailing edge separation process, with respect to the 2D profile, and also reveal a local region of leading edge separated flow or bubble at the inner, 30% and 47% of the blade. For the rotating blade, results at inboard 30% and 47% stations show a dramatic suppression of the trailing edge separation, and the development of a leading edge separation structure connected with the extra lift.

Sting-free measurements on a magnetically supported right circular cylinder aligned with the free stream

2008 ◽  
Vol 596 ◽  
pp. 49-72 ◽  
Author(s):  
HIROSHI HIGUCHI ◽  
HIDEO SAWADA ◽  
HIROYUKI KATO
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Free Stream ◽  
Aerodynamic Force ◽  
Cross Flow ◽  
Leading Edge ◽  
Magnetic Suspension ◽  
Recirculation Region ◽  
Mean Velocity ◽  
Wide Range

The flow over cylinders of varying fineness ratio (length to diameter) aligned with the free stream was examined using a magnetic suspension and balance system in order to avoid model support interference. The drag coefficient variation of a right circular cylinder was obtained for a wide range of fineness ratios. Particle image velocimetry (PIV) was used to examine the flow field, particularly the behaviour of the leading-edge separation shear layer and its effect on the wake. Reynolds numbers based on the cylinder diameter ranged from 5×104 to 1.1×105, while the major portion of the experiment was conducted at ReD=1.0×105. For moderately large fineness ratio, the shear layer reattaches with subsequent growth of the boundary layer, whereas over shorter cylinders, the shear layer remains detached. Differences in the wake recirculation region and the immediate wake patterns are clarified in terms of both the mean velocity and turbulent flow fields, including longitudinal vortical structures in the cross-flow plane of the wake. The minimum drag corresponded to the fineness ratio for which the separated shear layer reattached at the trailing edge of the cylinder. The base pressure was obtained with a telemetry technique. Pressure fields and aerodynamic force fluctuations are also discussed.

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.

Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow

2010 ◽  
Vol 654 ◽  
pp. 65-97 ◽  
Author(s):  
RUPESH B. KOTAPATI ◽  
RAJAT MITTAL ◽  
OLAF MARXEN ◽  
FRANK HAM ◽  
DONGHYUN YOU ◽  
...  
Keyword(s):  
Nonlinear Dynamics ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separated Flow ◽  
Leading Edge ◽  
Synthetic Jet ◽  
Forced Flow ◽  
Separation Control ◽  
Computational Domain ◽  
Nonlinear Coupling

A novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing on the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier–Stokes simulations of this flow at a chord Reynolds number of 6 × 104 have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of ‘lock-on’ states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 × 105.

Experiments With Three-Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils

2004 ◽  
Vol 128 (2) ◽  
pp. 251-260 ◽  
Author(s):  
Douglas G. Bohl ◽  
Ralph J. Volino
Keyword(s):  
Reynolds Number ◽  
Flow Control ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Turbine Airfoils

The effectiveness of three-dimensional passive devices for flow control on low pressure turbine airfoils was investigated experimentally. A row of small cylinders was placed at the pressure minimum on the suction side of a typical airfoil. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered under low freestream turbulence conditions. Streamwise pressure profiles and velocity profiles near the trailing edge were documented. Without flow control a separation bubble was present, and at the lower Reynolds numbers the bubble did not close. Cylinders with two different heights and a wide range of spanwise spacings were considered. Reattachment moved upstream as the cylinder height was increased or the spacing was decreased. If the spanwise spacing was sufficiently small, the flow at the trailing edge was essentially uniform across the span. The cylinder size and spacing could be optimized to minimize losses at a given Reynolds number, but cylinders optimized for low Reynolds number conditions caused increased losses at high Reynolds numbers. The effectiveness of two-dimensional bars had been studied previously under the same flow conditions. The cylinders were not as effective for maintaining low losses over a range of Reynolds numbers as the bars.

A New Unsteady-Based Turbulence Model to Predict Shear Layer Rollup and Breakdown

2004 ◽  
Author(s):  
D. Scott Holloway ◽  
James H. Leylek
Keyword(s):  
Shear Layer ◽  
Reynolds Stress ◽  
Turbulence Model ◽  
Turbine Blades ◽  
Turbulence Models ◽  
Separated Flow ◽  
Leading Edge ◽  
Tip Clearance ◽  
Aircraft Carrier ◽  
Flow Through

This paper documents the computational investigation of the unsteady rollup and breakdown of a turbulent separated shear layer. This complex phenomenon plays a key role in many applications, such as separated flow at the leading edge of an airfoil at off-design conditions; flow through the tip clearance of a rotor in a gas turbine; flow over the front of an automobile or aircraft carrier; and flow through turbulated passages that are used to cool turbine blades. Computationally, this problem poses a significant challenge in the use of traditional RANS-based turbulence models for the prediction of unsteady flows. To demonstrate this point, a series of 2-D and 3-D unsteady simulations have been performed using a variety of well-known turbulence models, including the “realizable” k-ε model, a differential Reynolds stress model, and a new model developed by the present authors that contains physics that account for the effects of local unsteadiness on turbulence. All simulations are fully converged and grid independent in the unsteady framework. A proven computational methodology is used that takes care of several important aspects, including high-quality meshes (2.5 million finite volumes for 3-D simulations) and a discretization scheme that will minimize the effects of numerical diffusion. To isolate the shear layer breakdown phenomenon, the well-studied flow over a blunt leading edge (Reynolds number based on plate half-thickness of 26,000) is used for validation. Surprisingly, none of the traditional eddy-viscosity or Reynolds stress models are able to predict an unsteady behavior even with modifications in the near-wall treatment, repeated adaption of the mesh, or by adding small random perturbations to the flow field. The newly developed unsteady-based turbulence model is shown to predict some important features of the shear layer rollup and breakdown.

On the coherent structures and stability properties of a leading-edge separated aerofoil with turbulent recirculation

2011 ◽  
Vol 683 ◽  
pp. 395-416 ◽  
Author(s):  
V. Kitsios ◽  
L. Cordier ◽  
J.-P. Bonnet ◽  
A. Ooi ◽  
J. Soria
Keyword(s):  
Shear Layer ◽  
Bluff Body ◽  
Time History ◽  
Leading Edge ◽  
Numerical Data ◽  
Water Tunnel ◽  
Shear Layer Instability ◽  
Flow Configuration ◽  
Temporal Properties ◽  
Statistical Measures

AbstractThe present study is motivated by a need to produce stability modes to assist in the understanding and control of unsteady separated flows. The flow configuration is a NACA 0015 aerofoil with laminar leading-edge separation and turbulent recirculation. In previous water tunnel experiments, this flow configuration was measured in an unperturbed (uncontrolled) separated state, and a harmonically perturbed (controlled) reattached state. This study presents numerical data of the unperturbed case, and recovers stability modes to describe the evolution of perturbations in this environment. The unperturbed flow is numerically generated using large eddy simulation. Its temporal properties are quantified via a Fourier analysis of the velocity time history at selected points in space. The leading-edge shear layer instability is characterized by instantaneous vortex structures, and the bluff body shedding is illustrated by proper orthogonal decomposition modes. Statistical measures of the velocity field agree well with the water tunnel measurements. Finally a stability analysis is undertaken using a triple decomposition to distinguish between the time averaged field, the unsteady scales of motion, and a coherent wave (perturbation). This analysis identifies that perturbations in the region immediately downstream of the separated shear layer have the highest spatial growth rates. The associated frequency is of the order of the sub-harmonic of the shear layer instability.

Flow control effectiveness of synthetic jet on a stalled airfoil

2011 ◽  
Vol 225 (9) ◽  
pp. 2106-2114 ◽  
Author(s):  
D H Luo ◽  
X J Sun ◽  
D G Huang ◽  
G Q Wu
Keyword(s):  
Flow Control ◽  
Active Flow Control ◽  
Leading Edge ◽  
Synthetic Jet ◽  
Control Technique ◽  
Recirculation Region ◽  
Maximum Increase ◽  
Suction Control ◽  
Wide Range ◽  
Flow Phenomena

Synthetic jet has attracted much attention for its use as an effective active flow control technique. With the aim of achieving a deeper understanding of the mechanisms and the critical fluid flow phenomena associated with synthetic jet, 2D simulations are conducted, in which a synthetic jet is placed on a NACA 0012 airfoil’s upper surface near the leading edge simulating the periodic blowing and suction control at Re = 5 × 105 and an angle of attack of 18°. Important control parameters, such as jet amplitude and frequency are investigated over a wide range and the jet width is also considered. Numerical results show that the recirculation region on the airfoil is decreased with a maximum increase in the lift by about 40 per cent and a reduced drag. The synthetic jet is found to be particularly effective at high forcing levels when the forcing frequency fe is close to the natural shedding frequency of the airfoil, and however become ineffective at low forcing levels. Use of higher forcing frequencies further decreases the drag of the airfoil but also reduces the lift.

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.

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