Flow control of tip/edge vortices

Location, strength, and structure of tip and edge vortices shed from wings and bodies can be manipulated by using flow control techniques. Flow physics of these approaches involve flow separation from the edge, roll-up into the vortex, wing flow regime, vortex instabilities, vortex–vortex interactions, and vortex–turbulence interactions. Actuators include continuous and unsteady blowing as well as suction, bleed, and control surfaces, which add momentum, vorticity, and turbulence into the vortices. It is noted that actuation may have effects on more than one aspect of the flow phenomena. A comparative review of the control of delta-wing vortices, tip vortices, and afterbody vortices is presented. The delay of vortex breakdown and the promotion of flow reattachment require different considerations for slender and nonslender delta wings and may not be possible at all. Tip vortices can be controlled to increase the effective span, to generate rolling moment, to attenuate wing rock, and to attenuate vortex–wing interactions. Although there are different approaches for each application, opportunities for future research on turbulence ingestion, bleed, and excitation of vortex instabilities exist. Recent research also indicates that active and passive flow control can be used to manipulate the afterbody vortices to reduce the drag.</p

Modified near-wakes of axisymmetric cylinders with slanted base

Near-wakes of axisymmetric cylinders with slanted base were investigated in wind tunnel experiments for an upsweep angle of 28°. Effects of splitter-plate, cavity, and flaps on the afterbody vortices and separated flow were studied by means of surface pressure and Particle Image Velocimetry measurements. The splitter plate causes more diffused afterbody vortices due to the turbulence ingestion from the separation region. When the slanted base is replaced with a deep cavity, there is weaker roll-up of vorticity due to the lack of streamwise flow and a solid surface. Varying the splitter plate length in the range tested did not have significant influence on the flowfield, apart from affecting the strength of the splitter-plate vortex. With the splitter plate and cavity, unsteadiness is dominated by the flow separation region, as opposed to the afterbody vortices in the baseline case. A pair of vertical flaps attached to the side-edges of the splitter plate can reduce the unsteadiness at the measurement plane immediately downstream of the splitter plate

Unsteady Lift and Moment of a Periodically Plunging Airfoil

To simulate the effects of gusts and maneuvers, lift, moment, and flow measurements are presented for a periodically plunging airfoil at a Reynolds number of 20,000 over a wide range of reduced frequency (k≤1.1), amplitude (A/c≤0.5), and mean angle of attack (0–20 deg). For this parameter range, the maximum lift in the cycle is determined by the circulatory lift, regardless of whether a leading-edge vortex (LEV) is formed or not, whereas the maximum nose-down moment is determined by the competition between the added mass and the arrival of the LEV near the trailing edge. The LEV generally increases the mean lift and decreases the mean moment. This increase, which is not predicted well by the reduced-order models, correlates with the maximum effective angle of attack of the motion, rather than the Strouhal number. In contrast, the amplitude and phase of lift are predicted well by the Theodorsen theory (Theodorsen, T., “General Theory of Aerodynamic Instability and the Mechanics of Flutter,” NACA TR 496, 1949), whether LEVs are present or not. Hence, LEVs have a minimal effect on the fluctuating lift. However, the amplitude of the pitching moment is not predicted well by the Theodorsen theory, if LEVs are present. The competition between the added mass and the LEV causes the nonmonotonic variation of the moment amplitude as a function of reduced frequency.<br/

Dynamic Deployment of a Minitab for Aerodynamic Load Control

Load control is the reduction of extreme aerodynamic forces produced by gusts, maneuvers, and turbulence to enable lighter, more efficient aircraft. To design an effective control system, the actuator's response in terms of amplitude and phase lagmust be known. Current load control technologies are limited to low-frequency disturbances due to their large inertia. This paper evaluates a potential high-frequency alternative: The minitab using periodic and transient deployments on a NACA0012 airfoil in wind-tunnel experiments. Periodic deployment for reduced frequencies, k ≤ 0.79 exhibits a normalized lift response amplitude, which decays with increasing k comparable to Theodorsen's circulation function but with substantially higher lag. Transient deployment, at rates as low as τdeploy = U∞;tdeploy/c = 1, illustrates a delay in aerodynamic response. The delay is larger for outward minitab motion than inward; τ; ≈ 6 and4, respectively, forα = 0 degandincreases withα. Theflowfields showthat the delay in response and the reduction in effectiveness for dynamic minitab deployment are due to delayed growth of the separated region behind the minitab. The aerodynamic response due to minitab deployment is approximated as the response of a first-order system, which is pertinent to control system design. This simple characterization for amplitude reduction and delay in response makes it well suited to load control.<br/

Lift Alleviation in Travelling Vortical Gusts

Lift alleviation by a mini-spoiler on airfoils, unswept and swept wings encountering an isolated counter-clockwise vortical gust was investigated by means of force and velocity measurements. The flow separation region behind the spoiler remains little affected during the gust encounter. The maximum lift reduction is found for the static stall angle of attack. The change in the maximum lift during the gust encounter is approximately equal to that in steady freestream. The comparison with plunging airfoils reveals that, for the same maximum gust and plunge velocity, the effectiveness of the mini-spoiler is much better in travelling gusts. This reveals the importance of the streamwise length scale of the incident gust. For the unswept wing, there is some three-dimensionality of the flow separation induced by the mini-spoiler near the wing-tip. The magnitude of the lift reduction can be estimated using the airfoil data and by making an aspect ratio correction for the reduced effective angle of attack. For the swept wing, the mini-spoiler can disrupt the formation of a leading-edge vortex induced by the incident vortex on the clean wing and can still reduce the maximum lift