Avi Seifert

Delay of Airfoil Stall by Periodic Excitation

It was recently demonstrated that oscillatory blowing can delay separation from a symmetrical airfoil much more effectively than the steady blowing used traditionally for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as, the location of the blowing slot, the steady and oscillatory momentum coefficients of the jet, the frequency of imposed oscillations, and the shape and incidence of the particular airfoil. In airfoils equipped with slotted flaps, the flow is also dependent on the geometry of the slot and on the Reynolds number in addition to the flap deflection that is considered as a part of the airfoil shape. The incremental improvements in single element airfoil characteristics are generally insensitive to a change in Reynolds number, provided the latter is sufficiently large. The imposed oscillations do not generate large oscillatory lift nor do they cause a periodic meander of the c.p. C* C D = dp Ct =

Oscillatory Control of Separation at High Reynolds Numbers

An experiment conducted in a pressurized, cryogenic wind tunnel demonstrates that unsteady flow control using oscillatory blowing (with essentially zero mass flux) can effectively delay flow separation and reattach separated flow on an airfoil at chord Reynolds numbers as high as 38 × 10 6 . Oscillatory blowing at frequencies that generate one to three vortices over the controlled region at all times are effective over the entire Reynolds number range, in accordance with previous low-Reynolds-number tests. Stall is delayed and poststall characteristics are improved when oscillatory blowing is applied from the leading-edge region of the airfoil, whereas flap effectiveness is increased when control is applied at the flap shoulder. Similar gains in airfoil performance require steady blowing with a momentum coefficient that is two orders of magnitude greater. A detailed experimental and theoretical investigation was undertaken to characterize the oscillatory blowing disturbance, in the absence of external flow, and to estimate the oscillatory blowing momentum coefficient used in the cryogenic wind tunnel experiment. Possible approaches toward closed-loop active separation control are also presented

Use of Piezoelectric Actuators for Airfoil Separation Control

Surface-mounted piezoelectric actuators are used to excite the turbulent boundary layer upstream of separation, where the actuators interact directly with the boundary layer. The actuators are rigid and do not attenuate with increased aerodynamic loading up to the maximum tested speed of 30 m/s

Active Flow Separation Control on Wall-Mounted Hump at High Reynolds Numbers

An active separation control experiment was conducted in a cryogenic pressurized wind tunnel on a wall-mounted bump at chord Reynolds numbers from 2.4 x 10 6 to 26 x 106 and a Mach number of 0.25. The model simulates the upper surface of a 20% thick Glauert-Goldschmied-type airfoil at zero incidence. The turbulent boundary layer of the tunnel sidewall flows over the model and eliminates laminar-turbulent transition from the problem. Indeed, the Reynolds number either based on the chord or boundary-layer thickness had a negligible effect on the flow and its control. Without control, a large turbulent separation bubble is formed at the lee side of the model. Periodic excitation and steady suction or blowing were applied to eliminate gradually the separation bubble. Detailed effects due to variations in the excitation frequency, amplitude, and the steady mass flux are described and compared to those of steady suction or blowing

Delta wing stall and roll control using segmented piezoelectric fluidic actuators

The separated flow around a balance-mounted, 60-deg sweptback, semispan delta wing with a sharp leading edge was controlled using zero-mass-flux periodic excitation from a segmented leading-edge slot. Excitation was generated by cavity-installed piezoelectric actuators operating at resonance with amplitude modulation (AM) and burst mode (BM) signals being used to achieve reduced frequencies (scaled with the freestream velocity and the root chord) in the range from O(1) to O(10). Results of a parametric investigation, studying the effects of AM frequency, BM duty cycle and frequency, excitation amplitude, location of the actuation along the leading edge, and optimal phase difference between the actuators, as well as the Reynolds number, are reported and discussed

Active Control of Separation from the Flap of a Supercritical Airfoil

Zero-net-mass-flux periodic excitation was applied at several regions on a simplified high-lift system to delay the occurrence of flow separation. The NASA Energy Efficient Transport supercritical airfoil was equipped with a 15% chord simply hinged leading-edge flap and a 25% chord simply hinged trailing-edge flap. Detailed flow features were measured in an attempt to identify optimal actuator placement. The current paper describes the application of active separation control at several locations on the deflected trailing-edge flap. High- and low-frequency amplitude modulation of the high-frequency excitation were used for control. It was noted that the same performance gains were obtained with amplitude modulation and required only 30% of the momentum input required by pure sine excitation

Suction and Oscillatory Blowing Actuator Modeling and Validation

Enhancing the ability to control flows in different configurations and flow conditions can lead to improved flow-related, energy-efficient systems. Certain active flow control actuators are effective at low Mach numbers, but the momentum and vorticity they provide limits their utilization to low speeds. At higher Mach numbers, robust, unsteady, effective, and practical fluidic actuators are a critical, enabling technology in any flow control system, though they are largely missing. A new actuator concept, based on the combination of steady suction and oscillatory blowing, is presented. The actuator achieves near-sonic speeds at a frequency range from 10 Hz to at least 1 kHz. It has no moving parts and therefore is expected to have superior effectiveness and reliability. The operating principles of the new actuator are presented along with two computational models and their experimental validation.

Large trucks drag reduction using active flow control

Aerodynamic drag is the cause for more than two-thirds of the fuel consumption of large trucks at highway speeds. Due to functionality considerations, the aerodynamic efficiency of the aft-regions of large trucks was traditionally sacrificed. This leads to massively separated flow at the lee-side of truck-trailers, with an associated drag penalty of at least a third of the total aerodynamic drag. Active Flow Control (AFC), the capability to alter the flow behavior using unsteady, localized energy injection, can very effectively delay boundary layer separation. By attaching a compact and relatively inexpensive “add-on” AFC device to the back side of truck-trailers (or by modifying it when possible) the flow separating from it could be redirected to turn into the lee-side of the truck, increasing the back pressure, thus significantly reducing drag. A comprehensive and aggressive research plan that combines actuator development, computational fluid dynamics and bench-top as well as wind tunnel experiments was performed. The research uses an array of 15 newly developed Suction and Oscillatory Blowing actuators housed inside a circular cylinder attached to the aft edges of a generic 2D truck model. Preliminary results indicate a net drag reduction of 10% or more.

On the evolution of the turbulent spot in a laminar boundary layer with a favourable pressure gradient

The evolution of a turbulent spot in an accelerating laminar boundary-layer flow was investigated. The type of boundary layer chosen for this experiment resembles in every respect the flow in the vicinity of a stagnation point theoretically described by Falkner and Skan. The rate of growth of the spot was significantly inhibited by the favourable pressure gradient in all three directions. It became much shorter and narrower in comparison with a similar spot generated in a Blasius boundary layer at comparable distances from its origin and comparable Reynolds numbers. The celerities of its boundaries did not scale with the local free-stream velocity as they do in the absence of a pressure gradient. Dimensional analysis was used to identify and correlate the independent variables determining the size, the convection speed, and the relative rate of growth of this spot The familiar arrowhead shape of the spot gave way to a rounded triangular shape with the trailing interface being straight and perpendicular to the direction of streaming. The familiar Tollmien-Schlichting wave packet was not observed in this pressure gradient because the surrounding boundary layer was very stable at the Re considered. Since the arrowhead shape of the spot is associated with the breakdown of the waves within the packet it cannot occur below the critical Re. The relative size of the ‘calmed region’ following the spot also diminished; however, one could only speculate as to the origin of this region.

On turbulent spots in a laminar boundary layer subjected to a self-similar adverse pressure gradient

The characteristics of a turbulent spot propagating in a laminar boundary layer subjected to a self-similar adverse pressure gradient (defined by a Falkner-Skan parameter β= -0.1) were investigated experimentally. It was observed that some small differences in the normalized shape of the undisturbed velocity profile caused by the pressure gradient had a major influence on the spreading rate of the spot at comparable Re δ* . The rate of spread of the spot in the spanwise direction was affected most dramatically by the pressure gradient where the average angle at which the tips of the spots moved outward relative to the plane of symmetry was 21°. It was noted that the strength and duration of the disturbance initiating the spots had an effect on their spanwise rate of spread. For example, a strong impulsive disturbance and a disturbance caused by a stationary three-dimensional roughness generated spots which spread at a much smaller rate. The interaction of the spot with the wave packet existing beyond its tip was enhanced by the adverse pressure gradient because the Reynolds number of the surrounding boundary layer was everywhere supercritical.