Wake Survey Research Articles

Formation of vortices on a tubercled wing, and their effects on drag

Wake surveys of 2 swept NACA 0021 wings were conducted at angles of attack of 0°, 3°, 6°, 9°, and 12°. One wing had a smooth leading edge and the other had a tubercled leading edge. Sweeping the tubercled wing resulted in one vortex in each vortex pair being at least 4 times stronger than the other. There was little difference between the strength of the wingtip vortices of either wing at 3°. From 6° onwards tubercles reduced the strength of the wingtip vortex. The tubercle troughs tended to produce local maxima and minima in the profile and induced drag coefficients, respectively. The converse was true over the peaks. The change in the profile, induced, and total drag coefficients primarily arose from over the wingspan; there was little contribution from the wingtip region.

Drag Prediction in the Near Wake of a Circular Cylinder based on DPIV Data

This study focuses on drag prediction in the near-wake of a circular cylinder by use of mean velocity profiles and discusses the closest location where a wake survey would yield an accurate result. Although the investigation considers both the mean and fluctuating velocities, the main focus is on the mean momentum deficit which should be handled properly beyond a critical distance. Digital Particle Image Velocimetry (DPIV) experiments are performed in a Reynolds number range of 100 to 1250. Wake characteristics such as vortex formation length (L) and wake width (t) are determined and their relations to drag prediction are presented. Drag coefficients determined by momentum deficit formula are found to be in good agreement with experimental and numerical literature data in present Reynolds number regime.

Dissonant Witnessing: The “in” and “above” of Thou Proud Dream

Dissonant Witnessing:The “in” and “above” of Thou Proud Dream Damon Krometis (bio) How does a director create witnesses in the theatre? What aesthetic and storytelling tools best cultivate an experience where spectators feel emotionally and ethically transformed into the role of a witness? These were the questions that dominated my research during my MFA studies in directing at Northwestern University, and my goal was to formulate a general approach to witnessing that other directors might be able to utilize. The challenge was that while theories about what it means to be a witness in the theatre are becoming more prevalent, they are also more diverse and oppositional. One need only read Caroline Wake’s “The Accident and the Account” in order to get a sense of the breadth of this theoretical terrain. Wake surveys witnessing theories that she divides into several overlapping categories, insinuating how difficult it is to know if and how practitioners can actively create a specific condition of witnessing in the theatre. No matter which definition of witnessing that other directors chose, could I posit some core questions that could aid them in articulating a practicable approach to witnessing? The questions I found useful in my own work are rooted in creating and maintaining dissonance for my viewers. Theories on witnessing largely hinge on how a spectator is positioned between two modes of perception, which Dwight Conquergood describes as being “above the object of inquiry” and “in the thick of things” (146). He uses these phrases to describe forms of knowledge. The former allows one to learn through “empirical observation and critical analysis from a distanced perspective”; this is the privileged form of knowledge in Western cultures, as it is “rooted in paradigm” and suggests that one can archive, categorize, and clearly utilize all information (146). The latter mode refers to “knowing that is grounded in active, intimate, hands-on participation and personal connection”; it positions the knower on the “ground level,” where nuanced and unspeakable truths can be articulated (ibid.). Conquergood advocates collapsing the binaries between these modes, and asserts that the field of performance studies “is uniquely suited for the challenge of braiding together disparate and stratified ways of knowing” (152). I believe that theatre can likewise be a space where forms of knowing can commingle, deepening our personal understandings and promoting “activism, outreach, [and] connection to community” (ibid.).1 And in particular, Conquergood’s modes readily lend themselves to witnessing theory. Being “in the thick of things” allows spectators to (vicariously or vividly) encounter traumatic events, which are at the heart of most theories of witnessing. Maintaining a “distanced perspective” provides an opportunity for spectators to think critically about how trauma relates to existing sociopolitical paradigms. Somewhere in the tension between these positions exists a place where witnessing can occur. Many scholars seem to agree with this notion. In “The Accident and the Account,” Wake classifies most theories of witnessing as being about primary witnessing or secondary witnessing. Theories of primary witnessing place spectators “in the thick of things” by positioning them as either the victim, perpetrator, or bystander present at a traumatic event (5), giving them intimate knowledge of unspeakable truths. But scholars of primary witnessing likewise see the value of “critical analysis from a distanced perspective.” Tim Etchells in particular studies productions that “demand repeatedly [End Page 321] of those watching ‘be here, be here, be here’” (18). But he prefers this self-presence to result in primary witnesses “feel[ing] the weight of things and one’s own place in them” (17), an act that demands critical distance. Conversely, theories of secondary witnessing, proposed by Diana Taylor and Emma Govan, keep spectators “above the object of inquiry” by distancing them spatially or temporally from the traumatic event and prioritizing self-reflection (Wake 7–8).2 But Taylor also defines witnessing as an “involved” and “caring” form of spectatorship (1997, 25), and Govan states that witnesses must be “actively engaged with the material” (58). This suggests that to achieve the status of witness, it is not enough to observe; emotional proximity must be a part of the alchemical equation: “an epistemological point of departure and return” (Conquergood 149). Much of my...

An Analysis of the Wake of Pedalling Cyclists in a Tandem Formation

Aerodynamics is a critical factor in the performance of cyclists, with drag being the dominant resistive force at racing speeds. Many cycling events involve athletes riding in close proximity. Previous studies have shown that flow interactions result in significant drag reductions for tandem cyclists, inline parallel to the flow [1–5]. However, the flow mechanisms associated with these changes have not been well characterised to date.This paper reports the results of wake surveys of an athlete in single and tandem formations in the Monash Wind Tunnel. Flow measurements were phase averaged over segments of the crank cycle to investigate the dynamic evolution of the wake of a pedalling cyclist. Results show that the dominant hip vortices previously identified in the wake of a single cyclist remain dominant in the trailing cyclist wake, with minimal reduction in vorticity. Wake profiles indicate the large drag saving experienced by a trailing cyclist are not due to disruption of the wake vortices or significant streamwise momentum recovery.

An experimental study on the effects of rough hydrophobic surfaces on the flow around a circular cylinder

The present study investigates the effect that rough hydrophobic (or superhydrophobic) surfaces have on the flow separation and subsequent vortex structures in a turbulent wake behind a circular cylinder. The velocity fields were measured using two-dimensional particle image velocimetry in a water tunnel with Reynolds numbers of 0.7–2.3 × 104. The spray-coating of hydrophobic nanoparticles and roughened Teflon was used to produce the rough hydrophobic surfaces, and sandpapers with two different grit sizes were used to sand the Teflon into streamwise and spanwise directions, respectively, in order to examine the effect of the slip direction. The rough hydrophobic surface was found to enhance the turbulence in the flows above the circular cylinder and along the separating shear layers, resulting in a delay of the flow separation and early vortex roll-up in the wake. As a result, the size of the recirculation bubble in the wake was reduced by up to 40%, while the drag reduction of less than 10% is estimated from a wake survey. However, these effects are reversed as the Reynolds number increases. The surface texture normal to the flow direction (spanwise slip) was found to be more effective than that aligned to the flow (streamwise slip), supporting the suggested mechanism. In addition, the superhydrophobic surface is locally applied by varying the installation angle and that applied around the separation point is most effective, indicating that the rough hydrophobic surface directly affects the boundary layer at flow separation. In order to control the flow around a circular cylinder using rough hydrophobic surfaces, it is suggested to have a smaller roughness width, which can stably retain air pockets. In addition, a higher gas fraction and a more uniform distribution of the roughness size are helpful to enhance the performance such as the separation delay and drag reduction.

On the symmetry of proper orthogonal decomposition modes of a low-aspect-ratio plate

In this paper, the symmetry property and corresponding virtual force contribution of the proper orthogonal decomposition (POD) modes are numerically investigated for the low-Reynolds number flows passing over a low-aspect-ratio pitching-plunging plate. It is found that the flow and its POD modes have the same reflectional symmetry about the spanwise central plane. However, about the crossflow central plane, the spatio-temporal flow symmetry results in a change of symmetry pattern every two POD modes, which corresponds to odd or even multiples of the vortex shedding frequency. Based on a wake survey method for virtual forces, the POD modes are further classified into two groups, thrust- and lift-producing modes, respectively. Results have also shown that the distinct symmetry properties of these modes can be used to identify the correlation between the wake structure and the hydrodynamic force production.

Experimental and Numerical Methods for Transition and Drag Predictions of Laminar Airfoils

The friction component is responsible for more than 40% of typical civil aircraft drag. As a consequence, the issue of laminar flow has been of prime importance in aeronautics for many years now. This article is focused on Tollmien–Schlichting-induced transition and drag predictions of two-dimensional laminar airfoils obtained with experimental and numerical methods. In 2012, a test campaign in the ONERA-S2MA wind tunnel, including infrared acquisitions, pressure sensors, and wake analyses, allowed substantial data to be obtained on such airfoils in transonic conditions. To complete this study, two-dimensional fluid dynamics computations have been performed, either with a Reynolds-averaged Navier–Stokes solver using transition criteria or with a boundary-layer code combined with direct stability analysis. Furthermore, experimental (wake survey) and numerical (far-field theory) techniques allowing airfoil drag breakdown have been employed. Wind-tunnel and computational fluid dynamics transition predictions have been compared. Good agreement has been observed but the transition criteria may show some limitations in particular situations, such as long separation bubble development. The gains in lift and drag due to laminar flow have been quantified (natural vs triggered transition). Concerning drag reduction, the importance of the viscous pressure component has been highlighted. Finally, the effects of parameters such as angle of attack, Mach number, and Reynolds number on transition location and drag have been investigated.

Calibration and Use of -Hole Velocity Probes

A generalized calibration process is presented for multihole, pressure-based velocity probes that is independent of the number of holes and probe geometry, allowing the use of probes with large numbers of holes. The calibration algorithm is demonstrated at low speeds with a conventional seven-hole pressure probe and a novel 19-hole pressure probe. Because the calibration algorithm is independent of probe configuration, it is very tolerant of data corruption and imperfections in the probe-tip geometry. The advantages of using probes with large numbers of holes is demonstrated in a conventional wing wake survey. The 19-hole probe offers a higher angular sensitivity than a conventional seven-hole probe and can accurately measure velocity components even when an analytical calibration scheme is used. The probe can also provide local estimates of the diagonal components of the crossflow velocity gradient tensor in highly vortical flows.

Flow topology in the wake of a cyclist and its effect on aerodynamic drag

AbstractThree-dimensional flows around a full-scale cyclist mannequin were investigated experimentally to explain the large variations in aerodynamic drag that are measured as the legs are positioned around the $360^\circ $ crank cycle. It is found that the dominant mechanism affecting drag is not the small variation in frontal surface area over the pedal stroke but rather due to large changes in the flow structure over the crank cycle. This is clearly shown by a series of detailed velocity field wake surveys and skin friction flow visualizations. Two characteristic flow regimes are identified, corresponding to symmetrical low-drag and asymmetrical high-drag regimes, in which the primary feature of the wake is shown to be a large trailing streamwise vortex pair, orientated asymmetrically in the centre plane of the mannequin. These primary flow structures in the wake are the dominant mechanism driving the variation in drag throughout the pedal stroke. Topological critical points have been identified on the suction surfaces of the mannequin’s back and are discussed with velocity field measurements to elucidate the time-average flow topologies, showing the primary flow structures of the low- and high-drag flow regimes. The proposed flow topologies are then related to the measured surface pressures acting on the suction surface of the mannequin’s back. These measurements show that most of the variation in drag is due to changes in the pressure distribution acting on the lower back, where the large-scale flow structures having the greatest impact on drag develop.

Forces and flow structures evolution on a car body in a sudden crosswind

A vehicle driver is commonly exposed to strong side air flows, for example when passing through a wind gust. The aerodynamic efforts generated in these situations may induce undesired lateral deviations, which can lead to dramatic effects, if the driver is surprised. In order to simulate a sudden yaw angle change on a moving vehicle, a double wind tunnel facility, adapted from the one of Ryan, Dominy, 2000. Wake Surveys Behind a Passenger Car Subjected to a Transient Cross-wind Gust. SAE Technical Paper No. 2000-01-0874 is developed. Two Windsor car body models, differing from their rear geometry, are analysed. The transient evolution of the side force and yaw moment aerodynamic coefficients are interpreted in connection with the unsteady development of the flow, based on TR-PIV and stereoscopic PIV measurements. Our analysis shows that the region which is most sensitive to crosswind is located at the rear part of the leeward flank. However, changes in the rear geometry (from squareback to fastback body) only affect the established lateral coefficients values while transient duration and the force overshoots appear not to be significantly modified. Furthermore, the circulation of the most energetic leeward vortex appears to be correlated with the lateral coefficients transient evolutions.

A Global Approach to Turbomachinery Flow Control: Passage Vortex Control

A flow control scheme was implemented in a low-pressure turbine cascade that simultaneously mitigated profile and endwall losses using midspan vortex generator jets (VGJs) and endwall suction. The combined system had an approximate zero-net mass flux. During the design, a theoretical model was used that effectively predicted the trajectory of the passage vortex using inviscid results obtained from two-dimensional computational fluid dynamics (CFD). The model was used in the design of two flow control approaches: the removal and redirection approaches. The emphasis of the removal approach was the direct application of flow control along the passage vortex (PV) trajectory. The redirection approach attempted to alter the trajectory of the PV with the judicious placement of suction holes. A potential flow model was created to aid in the design of the redirection approach. The model results were validated using flow visualization and particle image velocimetry (PIV) in a linear turbine cascade. Detailed total pressure loss wake surveys were measured while matching the suction and VGJ mass flow rates for the removal and redirection approaches at ReCx = 25,000 and blowing ratio, B, of 2. When compared with the no control results, the addition of VGJs and endwall suction reduced the wake losses by 69% (removal) and 68% (redirection). The majority of the total pressure loss reduction resulted from the spanwise VGJs, while the suction schemes provided modest additional reductions (<2%). At ReCx = 50,000, the endwall control effectiveness was assessed for a range of suction rates without midspan VGJs. Area-averaged total pressure loss reductions of up to 28% were measured in the wake at ReCx = 50,000, B = 0, with applied endwall suction (compared to no suction at ReCx = 50,000), at which point the loss core of the PV was almost completely eliminated.

Drag reduction of large wind turbine blades through riblets: Evaluation of riblet geometry and application strategies

Wind tunnel experiments were performed to quantify the drag reduction on a wind turbine airfoil partially or fully covered with riblets. A full-scale 2.5 MW wind turbine airfoil section, typical for the near tip, was placed in the free stream flow of the wind tunnel at the Saint Anthony Falls Laboratory, University of Minnesota. Various sizes and geometries of experimental riblets were provided by 3M Company and tested at angles of attack ranging from 0° ≤ α ≤ 10° (0.25 ≤ CL ≤ 1.14) and at a Reynolds number of Re = 2.2 × 106. Mean drag was measured via wake survey (momentum deficit) and with a sensitive force balance. Lift was measured directly from the force balance. Tests included the cases of complete and partial riblet coverage on the wing. Results indicated that riblets could provide an overall reduction of skin friction drag, and that the amount of the decrease varied with riblet height and geometry. Partial riblet coverage appears in some cases more efficient than its complete coverage counterpart. The percentage of drag the riblets reduced varied greatly and in some cases the riblets were even detrimental to the airfoil. The most efficient riblet for a completely covered airfoil was found to be the V-groove shape of 100 μm height. It produced a reduction of roughly 6% in the operational range expected in a turbine airfoil. On the other hand, the most efficient riblet size for partial coverage was also a V-groove shape and seemed to shift slightly to a smaller peak height of 80 μm. This configuration produced a reduction of roughly 4% in the range of angle of attack that is typical for operation in the field. The average non-dimensional square root of the groove cross-section, l+, defined in terms of the drag coefficient at design angle of attack for the optimum riblet configuration in the fully coverage case was found to be l+≈10, which is very close to the optimum value found for planar surfaces. Based on our results we propose a formulation for the optimum riblet size in airfoil considering the mean drag coefficient and chord length Reynolds number. Even though the optimum full coverage case showed better performance that the partial case, the additional drag reduction benefit may be offset by the additional application cost.

Virtual Shaping on NACA 0015 by Means of a High Momentum Coefficient Synthetic Jet

Results concerning flow control on a NACA 0015 airfoil using high power synthetic jets are presented for low incidences and for Reynolds numbers ranging from 132000 to 425000. The forcing was operated through a spanwise slit positioned near the leading edge at x/c = 1.25% or at x/c = 10% on the upper surface. Static pressure distribution measurements around the airfoil, wake surveys and smoke flow visualizations were performed. Pressure distributions were significantly modified around the injection location, showing an area of intense suction which increased the lift and strongly affected the drag. Flow visualizations highlighted that the intense suction was due to a virtual shaping effect caused by the formation of a recirculation bubble capable of displacing the streamlines. Low momentum deficits in the wake velocity distributions and, in certain conditions, jet-like flow were observed for the forced cases. Finally, a scaling law relating the bubble size to the forcing intensity is proposed.

Plasma Flow Control of Cylinders in a Tandem Configuration

The tandem cylinder in cross flow represents a canonical flow field geometry that is relevant to the landing gear noise generation process. In this paper, plasma flow control in the form of dielectric barrier discharge plasma actuators is use to control the tandem cylinder flow field with the objective of reducing unsteady pressure fluctuations on the downstream cylinder. Two types of plasma actuators are applied to the upstream cylinder. These include spanwise-oriented plasma actuators and plasma streamwise vortex generators. Flow control experiments were performed over the Reynolds number range 22,000 € ≤ Re D € ≤ 172,000 and the results obtained at both the lowest and highest Reynolds numbers are reported in this paper. In each case, baseline flow field measurements are compared with those using plasma flow control. Results include, global flow visualization, unsteady pressure measurements on the downstream cylinder and nonintrusive near wake surveys using laser Doppler velocimetry. Both methods of plasma flow control are demonstrated to dramatically reduce pressure fluctuations on the downstream cylinder. Nomenclature C p = pressure coefficient D = Cylinder diameter L = Cylinder streamwise separation distance P s = Surface pressure

Bluff-body drag reduction using a deflector

A passive flow control on a generic car model was experimentally studied. This control consists of a deflector placed on the upper edge of the model rear window. The study was carried out in a wind tunnel at Reynolds numbers based on the model height of 3.1 × 105 and 7.7 × 105. The flow was investigated via standard and stereoscopic particle image velocimetry, Kiel pressure probes and surface flow visualization. The aerodynamic drag was measured using an external balance and calculated using a wake survey method. Drag reductions up to 9% were obtained depending on the deflector angle. The deflector increases the separated region on the rear window. The results show that when this separated region is wide enough, it disrupts the development of the counter-rotating longitudinal vortices appearing on the lateral edges of the rear window. The current study suggests that flow control on such geometries should consider all the flow structures that contribute to the model wake flow.

An Examination of Trapped Bubbles for Viscous Drag Reduction on Submerged Surfaces

Viscous drag reduction on a submerged surface can be obtained both in the limit of an unbroken gas film coating the solid and in the nanobubble or perhaps microbubble coating regime when an air layer is created with superhydrophobic coatings. We examine an intermediate bubble size regime with a trapped-bubble array (TBA) formed in a tap water environment using electrolysis to grow and maintain bubbles in thousands of millimeter-sized holes on a solid surface. We show that even though surface tension is sufficient to stabilize bubbles in a TBA against hydrostatic and shear forces beneath a turbulent boundary layer, no drag reduction is obtained. Drag measurements were acquired over Reynolds numbers based on plate length ranging from 7.2×104<ReL<3.1×105 using either a force balance for plates mounted in a vertical orientation, or by performing a momentum integral balance using a wake survey for a flat plate mounted in either vertical or horizontal orientation. In that the drag forces were small, emphasis was placed on minimizing experimental uncertainty. For comparison, the flow over a flat plate covered on one side by a large uninterrupted gas film was examined and found to produce large drag reductions of up to 32%.

Wake Measurement Downstream of a Hybrid Wing Body Model with Blown Flaps

Flow-field measurements were obtained in the wake of a full-span Hybrid Wing Body model with internally blown flaps. The test was performed at the NASA Langley 14 x 22 Foot Subsonic Tunnel at low speeds. Off-body measurements were obtained with a 7-hole probe rake survey system. Three model configurations were investigated. At 0deg angle of attack the surveys were completed with 0deg and 60deg flap deflections. At 10deg angle of attack the wake surveys were completed with a slat and a 60deg flap deflection. The 7-hole probe results further quantified two known swirling regions (downstream of the outboard flap edge and the inboard/outboard flap juncture) for the 60deg flap cases with blowing. Flowfield results and the general trends are very similar for the two blowing cases at nozzle pressure ratios of 1.37 and 1.56. High downwash velocities correlated with the enhanced lift for the 60deg flap cases with blowing. Jet-induced effects are the largest at the most inboard station for all (three) velocity components due in part to the larger inboard slot height. The experimental data are being used to improve computational tools for high-lift wings with integrated powered-lift technologies.

Drag Force Balance of a Blunt and Divergent Trailing-Edge Airfoil

M odifying the trailing edge of an airfoil can lead to significant improvements in its aerodynamic performance. Divergent trailing-edge (DTE) airfoils were first introduced to improve the liftto-drag ratio of supercritical airfoils at cruise conditions for commercial aircraft [1]. They are a natural evolution of Gurneyflaps used on low-speed airfoils to increase airfoil lift and reduce profile drag [2]. DTE airfoils are characterized by upper and lower surfaces that diverge from each other over about the aft 30% of chord forming a blunt trailing. This DTE approach was extended to low-speed applications such as wind turbine blades [3]. DTE profiles on wind turbine blades can improve aerodynamic performance and structural strength compared to profiles with sharp trailing edges. Critical to enhancing the aerodynamic performance of these airfoils and optimizing their geometrical configuration is the accurate characterization of surface flow properties as well as the precise determination of their aerodynamic characteristics. The profile drag of airfoils can be determined using three different approaches: dynamic force balance, wake surveys, and direct measurements of the two drag components, that is, skin friction and pressure drag. Direct measurements of drag components along the airfoil surface also provide useful information for characterizing the surface flow (i.e., the boundary-layer state). However, the accuracy of the available direct skin-friction measurement methods was previously questioned [4]. Recently, the oil film interferometry method that relies on local thinning of an oil film under shear stress has been proven to accurately measure skin friction [5]. In the present study, direct measurements of both surface pressure and skin friction are employed to 1) characterize the physics of the surface flow around a DTE airfoil, and 2) estimate accurately the profile drag from direct measurements and compare it with that estimated from a survey in the airfoil far wake.

Flow Physics of a Race Car Wing With Vortex Generators in Ground Effect

This paper experimentally investigates the use of vortex generators for separation control on an inverted wing in ground effect using off-surface flow measurements and surface flow visualization. A typical racing car wing geometry is tested in a rolling road wind tunnel over a wide range of incidences and ride heights. Rectangular vane type of sub-boundary layer and large-scale vortex generators are attached to the suction surface, comprising counter-rotating and corotating configurations. The effects of both device height and spacing are examined. The counter-rotating sub-boundary layer vortex generators and counter-rotating large-scale vortex generators suppress the flow separation at the center of each device pair, while the counter-rotating large-scale vortex generators induce horseshoe vortices between each device where the flow is separated. The corotating sub-boundary layer vortex generators tested here show little evidence of separation control. Increasing the spacing of the counter-rotating sublayer vortex generator induces significant horseshoe vortices, comparable to those seen in the counter-rotating large-scale vortex generator case. Wake surveys show significant spanwise variance behind the wing equipped with the counter-rotating large-scale vortex generators, while the counter-rotating sub-boundary layer vortex generator configuration shows a relatively small variance in the spanwise direction. The flow characteristics revealed here suggest that counter-rotating sub-boundary layer vortex generators can provide effective separation control for race car wings in ground effect.

Effects of Surface Flats on the Performance of a Remotely Piloted Aircraft

T HE characterization of the aerodynamic performance of airfoils has been the subject of focused study since the 1910s. An implicitly assumed requirement is that the model fidelity (experimental or computational) is such that it is an exact rendering of the proposed full-scale flight article. In many instances, however, the actual wing profile may not be representative of its design. Generally, this becomes a greater issue as the size and dimensions of the vehicle reduce. With the proliferation of small-scale unmanned aerial vehicles, such concerns have increased in relevance. Wing geometry deviationsmay be due to damage, poor assembly, or simplified structure and design. This is commonly seen in radio-controlled (R/C) aircraft. Weight and cost considerations result in aircraft composed largely of balsa and plywood with aMonoKote skin covering. Generally, the wing skin is not supported between the ribs or the leadingand trailing-edge spars. This naturally results in flats when the skin is attached and stressed. As a result, the airfoil design and that actually implemented on the aircraft may be significantly different. Although wing/airfoil contour issues or alterations due to effects such as battle damage [1,2] have been investigated, little information appears available on a systematic study of contour effects thatmay result fromR/C modeling techniques. Consequently, a low-speed wind-tunnel investigation has been undertaken to quantify the effect of surface flats on the behavior of a Clark-Y airfoil. Data presented include surface pressure, wake survey, integral lift and drag coefficients, and surface flow visualization.