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Register a new userActive Flow Control of NACA 0012 airfoil using Sawtooth Direct Current Augmented Dielectric Barrier Discharge Plasma Actuator
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Dielectric barrier discharge (DBD) plasma actuators are an attractive option for separation control, lift enhancement, and drag reduction. Some plasma actuators feature optimized electrode shapes, electrical waveforms to maximize the aerodynamic forces at higher angles of attack. Here, we analyze the performance of a direct current augmented DBD (DBD - DCA) actuator with a sawtooth shape exposed electrode. The active electrode was positioned at 18% chord and the electrode at 48% chord of NACA 0012 airfoil. Wind tunnel experiments were conducted at wind speeds of 15 - 25 m/s, corresponding to Reynolds number Re = 201k - 335k. Lift coefficient (C$_L$), drag coefficient (C$_D$), and pitching moment coefficients (C$_M$), were measured with and without plasma actuation for angles of attack $alpha$ = 0$^o$ - 8$^o$ and the DCA electrode potential ($varphi_{DC}$) was varied from 0 kV to -15 kV. With energized DCA electrode, the C$_L$ increases up to 0.03 and the C$_D$ decreases by 50% at 15 m/s flow speeds and 0$^o$ angle of attack, the results are similar throughout the range of $alpha$. The effect of the actuator at higher Re diminishes, suggesting that the maximum control authority could be achieved at lower wind speeds.
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The flow structure obtained when Localized Arc Filament Plasma Actuators (LAFPA) are employed to control the flow issuing from a perfectly expanded Mach 1.3 nozzle is elucidated by visualizing coherent structures obtained from Implicit Large-Eddy Simulations. The computations reproduce recent experimental observations at the Ohio State University to influence the acoustic and mixing properties of the jet. Eight actuators were placed on a collar around the periphery of the nozzle exit and selectively excited to generate various modes, including first and second mixed (m = +/- 1 and m = +/- 2) and axisymmetric (m = 0). In this fluid dynamics video http://ecommons.library.cornell.edu/bitstream/1813/13723/2/Alljoinedtotalwithmodetextlong2-Datta%20MPEG-1.m1v, http://ecommons.library.cornell.edu/bitstream/1813/13723/3/Alljoinedtotalwithmodetextlong2-Datta%20MPEG-2.m2v}, unsteady and phase-averaged quantities are displayed to aid understanding of the vortex dynamics associated with the m = +/- 1 and m = 0 modes excited at the preferred column-mode frequency (Strouhal number 0.3). The unsteady flow in both contains a broad spectrum of coherent features. For m = +/- 1, the phase-averaged flow reveals the generation of successive distorted elliptic vortex rings with axes in the flapping plane, but alternating on either side of the jet axis. This generates a chain of structures where each interacts with its predecessor on one side and its successor on the other. Through self and mutual interaction, the leading segment of each loop is pinched and passes through the previous ring before rapidly breaking up, and the mean jet flow takes on an elliptic shape. The m = 0 mode exhibits relatively stable roll-up events, with vortex ribs in the braid regions connecting successive large coherent structures.
A comprehensive and detailed overview of the flow topology over a cambered NACA 65(1)-412 airfoil at Re = 20,000 is presented for angles of attack ranging from 0{deg} to 10{deg} using high-order direct numerical simulations. It is shown that instabilities bifurcate the flow and cause it to change at a critical angle of attack from laminar separation without reattachment over a laminar separation bubble at the trailing edge to a bubble at the leading edge. The transition of the flow regimes is governed by the Karman vortex shedding of the pressure side boundary layer at the trailing edge, Kelvin-Helmholtz instabilities within the separated shear layer on the suction side, as well as three-dimensional instabilities of elliptic flow within the vortex cores and hyperbolic flow in the shear layer between subsequent Karman vortices. As the suction side shear layer transitions and reattaches, the interaction of the two and three-dimensional instabilities results in three-dimensional tubular structures and large-scale turbulent puffs. The formation and shifting of the laminar separation bubble defines the far-wake topology several chord-lengths behind the airfoil and is accompanied by a sudden increase of the lift force and decrease in the drag that underscores the sensitive nature of low-Reynolds number airfoil aerodynamics. Lift and drag polars are presented for direct numerical simulations, wind tunnel experiments, and simplified numerical procedures where incorrect prediction of the force coefficients is caused by the failure to correctly model the low-pressure region at the trailing edge that is caused by the time-dependent generation of the Karman vortices.
We investigate superfluid flow around an airfoil accelerated to a finite velocity from rest. Using simulations of the Gross--Pitaevskii equation we find striking similarities to viscous flows: from production of starting vortices to convergence of airfoil circulation onto a quantized version of the Kutta-Joukowski circulation. We predict the number of quantized vortices nucleated by a given foil via a phenomenological argument. We further find stall-like behavior governed by airfoil speed, not angle of attack, as in classical flows. Finally we analyze the lift and drag acting on the airfoil.
In this paper, the problem of compressible flow over a thin airfoil located near the ground is studied. A singular integral equation, also known as Possio equation, that relates the pressure jump along the airfoil to its downwash is derived. The derivation of the equation utilizes Laplace transform, Fourier transform, method of images, and theory of Mikhlin multipliers. The existence and uniqueness of solution to the Possio equation is verified for the steady state case and an approximate solution is obtained. The aerodynamic loads are then calculated based on the approximate solution. Moreover, the divergence speed of a continuum wing structure located near the ground is obtained based on the derived expressions for the aerodynamic loads.
We propose two sets of initial conditions for magnetohydrodynamics (MHD) in which both the velocity and the magnetic fields have spatial symmetries that are preserved by the dynamical equations as the system evolves. When implemented numerically they allow for substantial savings in CPU time and memory storage requirements for a given resolved scale separation. Basic properties of these Taylor-Green flows generalized to MHD are given, and the ideal non-dissipative case is studied up to the equivalent of 2048^3 grid points for one of these flows. The temporal evolution of the logarithmic decrements, delta, of the energy spectrum remains exponential at the highest spatial resolution considered, for which an acceleration is observed briefly before the grid resolution is reached. Up to the end of the exponential decay of delta, the behavior is consistent with a regular flow with no appearance of a singularity. The subsequent short acceleration in the formation of small magnetic scales can be associated with a near collision of two current sheets driven together by magnetic pressure. It leads to strong gradients with a fast rotation of the direction of the magnetic field, a feature also observed in the solar wind.
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