Lance W. Traub

A New Class of Synthetic Jet Actuators—Part II: Application to Flow Separation Control

We present the application of the new synthetic jet actuator (SJA) to flow separation control over a NACA 0015 wing. The actuator is compact enough to fit in the interior of the wing that has a chord of 0.375 m. The wing was tested in the Texas A&M University Aerospace Engineering 3 ftX4 ft wind tunnel. An experimental investigation into the effects of the synthetic jet actuator on the performance of the wing is described. Emphasis is placed on the capabilities of the actuator to control the separation of the flow over the wing at high angles of attack. The results include force balance measurements, on surface and off surface flow visualization, surface pressure measurements, and wake surveys. All of the reported tests were performed at a free-stream velocity of 35 m/s, corresponding to a Reynolds number of 8.96×10 5 . The angle of attack was varied from -2.0 deg to 29.0 deg.

Range and Endurance Estimates for Battery-Powered Aircraft

T HE utility and cost effectiveness of small unmanned aerial vehicles (UAVs) has seen a large increase in their use, in both civilian and military applications. Depending on the particular requirements, the aircraft may be powered using a piston-gasoline engine or an electric motor. Electric propulsion appears to be favored as the UAV size diminishes or if stealth, in terms of acoustic signature, is a design requirement. Expressions to estimate the range and endurance of piston propeller and jet aircraft are well established [1,2] (the Breguet equations). Estimates for the range and endurance of electric aircraft are less well established [3,4] and may not be presented in a fashion consistent with that typically employed in the aeronautics community. Consideration of the propulsion system would suggest that equating the power delivered by the battery, accounting for losses due to the propeller, motor, and motor controller, to the power required to overcome drag would yield performance estimates. While this approach is sound, the behavior of a battery and its effective capacity, depending on the current draw (the so-called Peukert effect [5]), should be accounted for; otherwise, significant estimation errors may occur. Consequently, in this Note, expressions are derived to estimate the range and endurance of a battery-powered electric aircraft, accounting for battery discharge behavior. The impact of ignoring the Peukert effect is investigated. Parameters affecting performance are examined.

A New Class of Synthetic Jet Actuators—Part I: Design, Fabrication and Bench Top Characterization

Although the potential of synthetic jets as flow separation control actuators has been demonstrated in the existing literature, there is a large gap between the synthetic jet actuators (SJA) used in laboratory demonstrations and the SJAs needed in realistic, full-scale applications, in terms of compactness, weight, efficiency, control authority and power density. In most cases, the SJAs used in demonstrations are either too large or too weak for realistic applications. In this work, we present the development of a new class of high-power synthetic jet actuators for realistic flow control applications. The operating principle of the actuator is the same as that of crankshaft driven piston engines, which makes a significant part of the technology necessary for the actuator development available off-the-shelf. The design of the actuator is modular and scalable. Several building block units can be stacked in series to create the actuator of the desired size. Moreover, active exit slot reconfiguration, in the form of variable exit slot width, decouples the actuator frequency from the actuator jet momentum coefficient and allows the user to set the two independently (within limits). We present the design, fabrication and bench top characterization of the actuator. Several versions of the actuator were designed, built and tested, leading up to the development of a six-piston compact actuator that has a maximum power consumption of 1200 W (1.6 hp) and can produce (for the tested conditions) peak exit velocities as high as 124 m/s.

Prediction of vortex breakdown and longitudinal characteristics of swept slender planforms

A method is presented to predict high-angle-of-attack, longitudinal aerodynamic characteristics of slender wing planforms in incompressible e ow. A semiempirical approach is used to predict the location of vortex breakdown and its variation with incidence. Breakdown predictions are then coupled to vortex lift expressions based on the leading-edge suction analogy. A correction is used to account for the attenuation of vortex suction after vortex breakdown, allowing prediction of lift, drag, and pitching moment at high angles of attack. Comparisons are made with a variety of planforms, with encouraging agreement between theory and experiment being demonstrated.

Effects of Synthetic Jet Actuation on a Ramping NACA 0015 Airfoil

An experimental investigation was undertaken to evaluate the effectiveness of a synthetic jet actuator (SJA) for flow control on a pitching airfoil. A NACA 0015 profiled airfoil was ramped up at various rates with concomitant surface-pressure acquisition. A self-contained synthetic jet actuator was positioned in the interior of the airfoil. Synthetic-jet-actuation parameters included the jet-momentum coefficient and the slot exit width. The behavior of the surface pressures is investigated as well as their integrated properties. The data suggest that the effect of the SJA is to delay the onset of the dynamic-stall vortex formation to higher incidence and in some cases, for the parameters and incidence range investigated, suppress it totally. However, in the instances when formed, the synthetic jet appears to increase the loading induced by the stall vortex. At low ramp rates, the synthetic jet forcing causes the formation of a stall vortex where the unforced data showed a psuedostatic stall-type behavior

Preliminary parametric study of gurney-flap dependencies

Introduction T HE Gurney flap1,2 is typically a small plate, which is attached at or near the trailing edge of an airfoil on the pressure side. The flap has been shown to be a highly effective small-scale (typically 0.5–1.5% of the chord) modification that can achieve significant lift and pitching-moment generation.1,2 The Gurney functions by essentially increasing the downward deflection of the trailingedge flow, facilitated through the formation of a series of counterrotating vortices similar to that of a von Karman vortex street. A subsequent effect is an apparent violation of the trailing-edge Kutta condition; experimental data show that finite loading is carried to the trailing edge. The Gurney flap increases the effective chord and camber of the airfoil, so by augmenting circulation. Liebeck3 suggested a flow pattern where a “virtual” cusped trailing edge is formed downstream of the Gurney from the shear layers merging downstream of the flap. The final pressure recovery would then occur off-surface, which is analogous to violation of the Kutta condition. Experimental and computational studies exploring the effect of Gurney flaps have been undertaken covering effects of flap height,4 angle,5 effects on multi-element airfoils,2 etc. In this Note, the database is increased through evaluation of the effects of flap porosity, inclination, and spacing from the surface. It would also be useful for experimental design and conceptual understanding to have correlations that relate Gurney-flap geometric parameters to performance. Consequently, such correlations are developed.

Experimental Investigation of Annular Wing Aerodynamics

A low-speed wind-tunnel investigation was conducted to explore the behavior of annular (ring) wings. Effects of aspect ratio as well as gap were investigated. Ring wings using a low Reynolds number Eppler section and a NACA 0012 profile were manufactured and tested. Measurements were recorded using a six-component sting balance. Experimental and theoretical trend comparisons were effected using a vortex-lattice code. The experimental results indicate wing efficiency factors well above 1 are achievable. The effect of gap was to increase the wing lift-curve slope as well as efficiency. The large increases in aerodynamic efficiency were generally mitigated by the significant minimum drag coefficient. Pitching moment characteristics were unfavorable and were dominated by dissimilar stall behavior between the upper and lower wing sections.

Experimental Investigation of Pressure Measurement and Airfoil Characteristics at Low Reynolds Numbers

A low-speed wind-tunnel investigation is presented that elucidates the effects of pressure tapping layout on observed pressure distributions. The characteristics of a 16% thick S8036 profile used in the study are explored. The set Reynolds number Re range was 75,000,100,000,150,000 and 200,000. Effects of streamwise, inclined, and wing-root taps were studied. Results encompassing pressure plots, lift coefficients, and surface flow visualization are presented. All data are compared to numerical predictions using Xfoil. The data show good agreement between experiment and Xfoil for most cases; most disagreement is in the location and extent of the laminar separation bubble. The results show that the streamwise taps may cause premature transition depending on the test Reynolds number and wing incidence. For a given angle of attack, agreement between the streamwise and inclined pressure ports improved with the Reynolds number, while for Re ≤ 100,000, the accord improved with wing incidence. Tappings at the wing root generally indicated reduced pressure compared to those along the span, although agreement between all three tap locations was reasonable downstream of the bubble reattachment As cited in the literature, the effect of increasing Reynolds number was to shorten the laminar bubble, not by altering the location of laminar separation but by earlier transition and reattachment. Incidence was noted to move the bubble toward the leading edge and to reduce its length slightly. Both the laminar and turbulent sections of the bubble were seen to demonstrate Reynolds number dependency. Reynolds number effects were also present in the magnitude of the loading upstream of bubble reattachment, but were not evident from the location of reattachment aft.

Aerodynamic Characteristics of a Gurney/Jet Flap at Low Reynolds Numbers

A low-speed wind-tunnel investigation has been undertaken to establish the effect of a Gurney flap used in conjunction with a jet flap at low Reynolds numbers. The characteristics of the combination were explored as a potential control effector for small unmanned aerial vehicles. Measurements included force balance, wake survey, and flow visualization. The results indicate that the jet flap maintains a theoretically determined dependence on the jet momentum coefficient, even at a low Reynolds number (160,000). A numerical lifting-line program was modified to allow estimate of the rolling moment that may be generated by ailerons composed of a Gumey/jet flap. The numerical data suggest that the jet flap is capable of generating significant rolling moments with realistic jet momentum coefficients.

Effects of Vortex Generators on an Airfoil at Low Reynolds Numbers

A low-speed wind-tunnel investigation is presented detailing the effects of vortex generators on an airfoil at low Reynolds numbers (80,000 and 160,000). Six different static vortex generator layouts were tested. In addition, an oscillatory (or active) vortex generator was designed and tested. Force balance measurements were recorded and interpreted with the aid of surface flow visualization. The data suggest that the static vortex generators function similarly to those at higher Reynolds numbers; increasing the maximum lift coefficient and increasing the stall angle. Different static vortex generator configurations appear preferable at the two tested Reynolds number ranges. The oscillating vortex generator did not appear effective in its present configuration.