Showing papers on "Drag divergence Mach number published in 2009"

Mach Wave Radiation from High-Speed Jets

Abstract: Extensive experimental evidence is now available to support the observation that there are two components of jet mixing noise. They are the fine-scale turbulence noise and the noise from the large turbulence structures of the jet flow. The large turbulence structure’s noise radiates primarily in directions with a large inlet angle around the downstream axis of the jet. The fine-scale turbulence noise dominates in the sideline and upstream directions. This study investigates the mechanism by which large turbulence structures radiate noise. It is believed that the mechanism is Mach wave radiation. Theoretical model results and physical reasoning are presented to support the Mach wave mechanism. They are further supported by experimental measurements both in the far field and in the near acoustic field. These measurements include peak noise direction, noise-source distribution along the jet column, and near-field pressure-contour pattern. A signature pattern of the near-field pressure contours associated with Mach wave radiation is identified.

Full Configuration Drag Estimation

Abstract: Accurate drag estimation is critical in making computational design studies. Drag may be estimated thousands of times during a multidisciplinary design optimization, and computational fluid dynamics is not yet possible in these studies. The current model has been developed as part of an air-vehicle conceptual-design multidisciplinary design optimization framework. Its use for subsonic and transonic aircraft configurations is presented and validated. We present our parametric geometry definition, followed by the drag model description. The drag model includes induced, friction, wave, and interference drag. The model is compared with subsonic and transonic isolated wings, and a wing/body configuration used previously in drag prediction workshops. The agreement between the predictions of the drag model and test data is good, but lessens at high lift coefficients and high transonic Mach numbers. In some cases the accuracy of this drag estimation method exceeds much more elaborate analyses.

On New Simple Low-Dissipation Scheme of AUSM-Family for All Speeds

Abstract: Compressible CFD methods for all speeds that can compute very low Mach number flows with low dissipation, as well as strong shock and expansion waves in high Mach number flows, are very attractive for aerospace applications such as sound generation by supersonic plume, flows in a rocket engine combustor, and cavitating gas-liquid flows. We present here such a new simple numerical flux of AUSM-family for all speeds with no tunable parameters, named SLAU (Simple Low-dissipation AUSM), along with its formulation. This scheme features low dissipation in low Mach number regime, while keeping robustness and non-oscillating nature at high Mach numbers, i.e., stabilities against shock-induced anomalies such as the carbuncle phenomenon. Furthermore, although this method does not use nor rely on scaling of numerical viscosity by the preconditioning matrix, its combination with the time derivative preconditioning is proved to be quite effective for fast convergence in low Mach number flows. These advantages of the present scheme will be demonstrated in applications to flow computations of wide-ranging Mach numbers.

Surface pressure and aerodynamic loads determination of a transonic airfoil based on particle image velocimetry

Abstract: The present investigation assesses a procedure to extract the aerodynamic loads and pressure distribution on an airfoil in the transonic flow regime from particle image velocimetry (PIV) measurements. The wind tunnel model is a two-dimensional NACA-0012 airfoil, and the PIV velocity data are used to evaluate pressure fields, whereas lift and drag coefficients are inferred from the evaluation of momentum contour and wake integrals. The PIV-based results are compared to those derived from conventional loads determination procedures involving surface pressure transducers and a wake rake. The method applied in this investigation is an extension to the compressible flow regime of that considered by van Oudheusden et al (2006 Non-intrusive load characterization of an airfoil using PIV Exp. Fluids 40 988–92) at low speed conditions. The application of a high-speed imaging system allows the acquisition in relatively short time of a sufficient ensemble size to compute converged velocity statistics, further translated in turbulent fluctuations included in the pressure and loads calculation, notwithstanding their verified negligible influence in the computation. Measurements are performed at varying spatial resolution to optimize the loads determination in the wake region and around the airfoil, further allowing us to assess the influence of spatial resolution in the proposed procedure. Specific interest is given to the comparisons between the PIV-based method and the conventional procedures for determining the pressure coefficient on the surface, the drag and lift coefficients at different angles of attack. Results are presented for the experiments at a free-stream Mach number M = 0.6, with the angle of attack ranging from 0? to 8?.

Gravity currents impinging on bottom-mounted square cylinders: flow fields and associated forces

Abstract: The unsteady drag and lift generated by the interaction of a gravity current with a bottom-mounted square cylinder are investigated by means of high-resolution Navier–Stokes simulations. Two-dimensional simulations for Reynolds numbers (Re) O(1000) and three-dimensional simulations for Re = O(10000) demonstrate that the drag coefficient increases exponentially towards a maximum as the current meets the cylinder, then undergoes strong fluctuations and eventually approaches a quasi-steady value. The simulation results show that the maximum drag coefficient can reach a value of 3, with the quasi-steady value being O(1), which should aid in selecting a design drag coefficient for submarine structures under the potential impact of gravity currents. The transient drag and lift fluctuations after impact are associated with the Kelvin–Helmholtz vortices in the mixing layer between the gravity current and the ambient fluid. As these vortices pass over the cylinder, they cause the convection of separated flow regions along the bottom wall towards the cylinder. In two-dimensional simulations at Re = O(10000), these flow structures are seen to be unrealistically coherent and to persist throughout the interaction, thus resulting in a noticeable overprediction of the drag and lift fluctuations. On the other hand, the impact of the current on the cylinder is seen to be very well captured by two-dimensional simulations at all Re values. Three-dimensional simulations lead to excellent agreement with available experimental data throughout the flow/structure interaction. They show that the spanwise variation of the drag is determined by the gravity current's lobe-and-cleft structure at impact and by an unsteady cellular flow structure similar to that found in constant-density flows at later times. A comparison between gravity-current flows and corresponding constant-density flows shows the hydrostatic drag component to be important for gravity currents.

Baroclinic vortex influence on wave drag reduction induced by pulse energy deposition

Abstract: We present the results of numerical analysis of wave drag reduction by a single-pulse energy deposition in a supersonic flow field around a sphere. The wave drag for the sphere was reduced as a result of the interaction between a low-density core following the blast wave produced by the energy deposition and the bow shock developed in front of the sphere. We investigated the drag reduction mechanism in terms of the unsteady flow field induced by the interaction. The effects of deposited energy and deposition location on energy reduction were examined by parametric study. From the obtained results, we refined the parameters, utilizing the baroclinic source term that produced vorticity in the vortex equation when the gradients of density and pressure were not parallel. The baroclinic vortex driven by Richtmyer–Meshkov-like instability was strong enough to contribute to the temporary low-entropy shock formation that caused low wave drag for the supersonic object. We determined that the reduced energy had a l...

Reducing the wave drag of bodies in supersonic flows using porous materials

Abstract: We have experimentally studied the effect of a gas-permeable material situated in front of a blunt body on the aerodynamic drag of this system in a supersonic airflow with a Mach number of M = 4.85. It is established that the porous material significantly reduces the wave drag in the system. Explanation of the observed phenomenon is proposed.

Dissecting the Pressure Field in Tidal Flow past a Headland: When Is Form Drag “Real”?

Abstract: In the few previous measurements of topographic form drag in the ocean, drag that is much larger than a typical bluff body drag estimate has been consistently found. In this work, theory combined with a numerical model of tidal flow around a headland in a channel gives insight into the mechanisms that create form drag in oscillating flow situations. The total form drag is divided into two parts: the inertial drag, which is derived from a local potential flow solution, and the separation drag, which accounts for flow features such as eddies. The inertial drag can have a large magnitude, yet it cannot do work on the flow because its phase is in quadrature with the velocity. The separation drag has a magnitude that is nearly equal to the bluff body drag and accounts for all of the energy removed from the flow by the topography. In addition, the dependence of the form drag on the tidal excursion distance and the aspect ratio of the headlands were determined with a series of numerical experiments. This theory explains why form drag can be so large in the ocean, and it provides a method for separating the pressure field into the parts that can and cannot extract energy from the flow.

On Drag Reduction in Turbulent Channel Flow over Superhydrophobic Surfaces

Abstract: Recent simulations and experiments suggest that flow over superhydrophobic surfaces may exhibit significantly reduced drag in both laminar [8, 9] and turbulent [7, 10] regimes due to the existence of a thin layer of gas on the surface which allows a slip velocity The current paper explores this using estimates of drag based on high-resolution PIV in a low-Reynolds number turbulent channel flow

Streamlining the time trial apparel of cyclists: The Nike swift spin project

Abstract: This paper documents the development of aerodynamic apparel for the Tour de France individual time trial (TT), the Olympic TT, and track cycling races. A wind tunnel and metric balance were used to measure the drag force (Fd) and wind tunnel air velocity on cylinders, limb models, and live cyclists clad in samples or suits sewn with one or more of 200 stretch fabrics. A concurrent measurement of model dimensions and frontal areas provided the non‐dimensional drag coefficient (Cd) and Reynolds Numbers (Re) that characterized the ability of the various fabrics and suits to reduce frictional drag and induce a drag crisis (DC) or premature flow transition. DC defines a critical air velocity over the body segments at which the airflow transitions from laminar to turbulent, yielding a smaller wake behind the body segment and a corresponding decrease in Fd. A number of fabrics triggered DC on cylinders and limb segments, reducing cylinder and limb Cd by over 40 per cent. Several methods of lowering the Fd of cyc...

Supersonic Drag Performance of Truncated Cones with Repetitive Energy Depositions

Abstract: The drag performance of truncated cones in a supersonic flow of Mach number 2 with repetitive energy depositions is evaluated by using computational fluid dynamics. The calculated result shows a quasisteady flowfield: a virtual spike, which is supported by an axi-symmetric recirculation, is formed in front of the truncated cone. The recirculation is generated due to baroclinic interaction between a bow shock wave and a heated bubble produced by energy depositions. The reduction of the drag over the truncated cone is attributed to the virtual spike so formed. The time averaged drag of the truncated cone depends on the amount of deposited energy, repetition frequency and the area of a truncation surface. The averaged drag can be smaller than that of a sharp cone with the same apex angle, maintaining the energy savings due to drag reduction.

Drag force on quasi-axisymmetric scramjets at various flight Mach numbers: theory and experiment

Abstract: The net axial force on a non-fuelled quasi-axisymmetric scramjet model designed for operation at Mach 6 was measured in the T4 Stalker tube at The University of Queensland using a single-component stress wave force balance. The design used was a variant of a model that was tested previously at Mach 6. The new model was equipped with a modified thrust nozzle that was designed to improve the performance of the nozzle. Tests were performed to measure the drag force on the model for Mach 6, Mach 8 and Mach 10 shock tunnel nozzles for a range of flow conditions. The nozzle-supply enthalpy was varied from 3 to 10 MJ/kg and the nozzle-supply pressure from 35 to 45 MPa. For the test model, the drag coefficient increased with increasing nozzle-supply enthalpy. The test results are compared with a force prediction method based on simple hypersonic theories and three-dimensional CFD. The test results are in good agreement with the predictions over the wide range of conditions tested. The re-designed model has a more efficient nozzle but this comes at the expense of increased drag associated with the modifications required for the cowl. The results indicate that this type of vehicle design is not likely to be suitable for flight above Mach 8.

Shape optimization of the airfoil comprising a cross flow fan

Abstract: Purpose – Fanwing airfoil is a new lift‐generating section invented in 1997 by Patrick Peebles. The early shape of the airfoil has not changed until now. So far, no research has been done to change or modify the airfoil shape in order to improve its aerodynamic performance. In this paper, possibility of changing the airfoil shape to improve its aerodynamic performance is studied. For this purpose, six different geometric shapes of the airfoil are investigated numerically to determine the best airfoil on the basis of lift and drag coefficients. Flow over the airfoil is solved by developing a computational fluid dynamics (CFD) code. The purpose of this paper is to find a more efficient configuration for the Fanwing airfoil with lower power consumption and better performance.Design/methodology/approach – Flow over the airfoil is investigated by CFD. At the airfoil solid walls, the no slip condition is applied. Re‐Normalization Group k‐e model is used for turbulence modeling. The pressure‐velocity coupling is...

Analytic Drag Prediction for Cambered Wings with Partial Leading Edge Suction

Abstract: Rapid estimation of a wing's drag polar accounting for the combined effects of planform and profile drag represents a significant challenge, but is of necessity for preliminary analysis and design. In this paper, methodology is presented culminating in simple expressions allowing the combination of these drag components. Information required is knowledge of the wing's sectional characteristics as well as the planform's lift-curve slope and inviscid span efficiency factor. Airfoil profile drag is interpreted as a loss of leading-edge suction. Effects of camber are explicitly accounted for. The method also lends itself to be used as a design tool in which camber and suction may be optimized to minimize drag, depending on the operating conditions. Comparisons of the method with the experimental data show encouraging agreement.

Aerodynamic Investigations of an Advanced Over-the-Wing Nacelle Transport Aircraft Configuration

Abstract: The transonic aerodynamic characteristics of an advanced, Over-the-Wing Nacelle (OWN), subsonic transport configuration are assessed using both inviscid Euler and viscous Navier-Stokes CFD. Results of the assessments are compared to a similar configuration with a more traditional Under-the-Wing Nacelle (UWN) installation and a similar reference Wing-Body (WB). The engine installation and inboard wing section of the OWN are designed to create a channel between the nacelle and fuselage that accelerates the upper surface wing flow, increases leading edge suction, and subsequently reduces the overall drag. CFD results were used to examine the span loadings of the three configurations and allow normalization of their wing twist distributions. Qualitative observations and quantitative drag computations are performed for the three configurations at a cruise Mach number of 0.78, an approximate altitude of 35,000 ft, and a CL of 0.44. It was found that the OWN’s inboard wing channel section is effective in producing favorable leading edge suction but that the overall drag, compared to the UWN, is higher by 32 counts. Although the source of this drag excess could not be precisely identified, it was isolated in the nacelle region. CFD solutions were obtained for several transonic Mach numbers to assess the drag rise characteristics of the three configurations. Inviscid Euler results showed similar trends to previous studies where the OWN is higher in drag at lower Mach numbers but enjoys a much milder drag rise than the UWN or WB and has lower drag at higher Mach numbers. Viscous Navier-Stokes results however showed that the effect of shock-induced flow separation significantly increases the drag rise but that the OWN still likely experiences it less severely than the UWN. Overall, the results of this study indicate that the Over-theWing Nacelle concept under consideration appears to be an aerodynamically competitive engine installation to the more traditional Under-the-Wing pylon installation.

Star-Body Waveriders with Multiple Design Mach Numbers

Abstract: range and maneuver capability. A baseline Mach 6, conventional star body, an equivalent volume cone, and three multiple-design-Mach-number star bodies are analyzed using three-dimensional, full Navier–Stokes computational fluid dynamics for Mach numbers of 4, 5, 6, 7, and 8 and an altitude of 100,000 ft. As expected, the conventional, single-design-Mach-number star body has higher skin-friction drag, lowerwave drag, andlower total drag than the equivalent volumecone. Because the volumes of the multiple-design-Mach-number star bodiesare greaterthan that of the cone, they have higher total drag than the cone, but the wave drag and drag coefficient are lower than the equivalent cone for some of the new configurations at higher Mach numbers. Before performing the analysis, it was assumed that the star body sectors at an on-design flow condition would have the greatest pressure levels. However, the computational fluid dynamics analysis showed that the star-body sectors with the maximum flow deflection (center wedge) angle produced the highest pressures, even at offdesign flow conditions, where three-dimensional effects and pressure loss due to detached shocks and flow spillage are present. The asymmetric, two-, and tripledesign-Mach-number star bodies produced lift that can be used for downrange and cross-range capability.

On the dynamics of drop acceleration at the early stage of velocity relaxation in a shock wave

Abstract: The early stage of velocity relaxation of a water drop in a flow behind a shock wave under conditions of drop deformation and breakup is studied experimentally. The aerodynamic drag of the drop is measured on the basis of the dynamics of motion of its leading edge, and a strong dependence of the drag on the observation time is demonstrated. These data are compared with the drag obtained on the basis of the dynamics of the center of mass of the drop. The drag of the center of mass is shown to be comparable with the drag of a solid sphere and to be substantially smaller than the value determined from the leading edge motion. A possible physical mechanism of this effect is proposed, which is based on the delay of the beginning of motion of the center of mass of the drop, owing to certain specific features of stabilization of external and internal flows at the early stage of velocity relaxation.

Numerical Study on Drag Reduction for Grid-Fin Configurations

Abstract: A grid fin, or lattice fin, consists of an outer frame supporting an inner grid of intersecting planar surfaces of small chord. At transonic Mach numbers normal shocks form at the back of the lattice cells thus choking the flow through the cells and causing a significant increase in drag force. In order to reduce the transonic flow choking, an improved, sweptback grid fin configuration is proposed in the present study. Viscous computational fluid dynamic (CFD) simulations were performed to investigate the flow characteristics of a vehicle with baseline and sweptback grid fins at transonic and supersonic Mach numbers in the range 0.817- 2.0, at zero angle of attack. Good agreement (within the error of 4%) is observed for the computed drag coefficients with data available in literature. The present numerical results indicate the sweptback grid fins reduce the flow chocking. This translates in a grid-fin drag reduction of about 12% for all the Mach numbers investigated in the present study.

Estimating the drag coefficients of meteorites for all mach number regimes.

Abstract: Accurate values for the drag coefficient are essential to determine the trajectories that meteoroids follow during atmospheric entry. Most models that describe the descent of meteors or witnessed falls find that a constant drag coefficient, Γ, of 0.7 reproduces the observations rather well (e.g. [1], [2]). However, experimental data obtained from studies of regular solids indicate that the drag coefficient is a strong function of Mach number, especially in the low supersonic and subsonic Mach regimes (see Figure 1). For instance, Γ for a sphere ranges from less than 0.5 at subsonic speeds, rapidly increases to ~1 in the transonic regime, and levels off to about 0.9 in the hypersonic limit. When estimating the aerodynamic properties of meteorites, it is helpful to assume that they are aerodynamically similar to a simple shape. Although meteorites are not perfect spheres, it is likely that their drag coefficients follow a similar trend, such that Γ drops by a factor of ~2 as the bolide slows from supersonic to subsonic speeds. These generalizations provide a framework for a meteorite’s drag coefficient function to be dependent on Mach number, such that Γ decreases as the meteorite’s speed decreases. If the Γ of 0.7 represents every portion of the bolide’s flight, current models may be significantly overestimating the drag in the subsonic regime, and underestimating the drag in the supersonic and hypersonic regimes. To address this issue, we present a numerical method to calculate Γ as a function of Mach number. This method can be used to more accurately model the trajectories of meteors, particularly the dark-flight portion. First we gather drag coefficient data for spheres in every Mach number regime [3], [4]. It is especially important to have experimental data during Mach number regime changes. Spheres are aerodynamically simple objects, and drag coefficient data from past experiments is plentiful. We can derive a best fit function over the data points to estimate drag values in between measurements. This best fit function is composed of two base functions that connect in the subsonic regime. During the majority of the subsonic regime, Γ is represented by a quadratic function. At speeds slightly less than Mach one, the best fit function switches to the second base function, which is a sum of two exponentials:

Drag Reduction Due to Cut-Corners at the Front-Edge of a Rectangular Cylinder With the Length-to-Breadth Ratio Being Less Than or Equal to Unity

Abstract: The flow past a rectangular cylinder with small cut-corners at the front-edge is investigated to discuss a relation between drag reduction and the cutout dimension The rectangular shape is selected in eleven kinds of the length-to-breadth ratio from 2/6 to 6/6 (square prism) with the small rectangular-shaped cut-corners at the front-edge The wind tunnel experiment is carried out to obtain time-averaged hydrodynamic forces measured by the force transducer at Re ≈ 50,000 The contour map of the hydrodynamic coefficients with respect to the cutout dimension are shown to investigate the relation between the drag reduction and the cutout shape In the contour map for the zero angle of attack, the region of the effective drag reduction achieved, in which the value of the drag coefficient is less than that of a circular cylinder at the same Reynolds number, is observed to become wide with the increase in the length-to-breadth ratio and it is independent of the angle of attack, α, within α being small Furthermore, it is shown that there is a condition in which the drag reduction of C D≤ 15 can be achieved even when the Strouhal number is less than

Frictional drag reduction by wavy advection of deformable bubbles

Abstract: Bubbles can reduce frictional drag in wall turbulence, and its effect is expected to use for ships and pipelines to save their power consumptions. A number of basic experiments have been carried out to date for finding out the best condition for enhancing the drag reduction. One issue that remains at present is the difference of the performance between steady and unsteady status in terms of bubble concentration. All the experiments in the past deal with the steady effect, i.e., the drag reduction is evaluated as a function of mean void fraction or given gas flow rate of continuous injection. Despite to this, the actual phenomena highly depend on local interaction between two phases upon unsteady manner. We focus on this point and elucidate the influence of time-fluctuating void fraction on the total response to the drag reduction. This view is in fact important to estimate the persistency of the bubble-based drag reduction in the flow direction since bubbles formulate wavy advection during their migration. Our experiments are designed to measure the above-mentioned effect from laminar, transitional, and turbulent flows in a horizontal channel. For avoiding the contamination effect that worsens the reproducibility of the experiment, Silicone oil is used as carrier fluid. The oil also simulates the high Weber number bubble condition because of low surface tension. The unsteady interaction between the wavy advection of bubbles and the local skin friction, a synchronized system is constructed to connect the high-speed camera with the shear transducer, which can evaluate the interaction at 1000 fps. From the results, we confirm that the drag reduction is provided at Re>3000 in the turbulent flow regime, and also the total drag reduction is enhanced by the presence of the waves.

Measurements of Fluctuating Pressures on a Circular Cylinder in Subsonic Crossflow

Abstract: A circular cylinder is tested in crossflow over the subsonic speed range. Time-resolved pressure distributions give information on surface pressure fluctuations; the corresponding drag and base drag coefficients are provided. Measured base pressure fluctuations at low Mach numbers are in agreement with the findings of other researchers. Flow changes at higher subsonic velocities and into the transonic range are described. Illustrations are drawn from the observations of other researchers, enabling physical explanations to be given to assist toward a more general modeling of the problem. At Mach numbers above 0.6 the changing strength of the vortices reduces the base drag coefficient up to a Mach number of 0.9, at which the onset of sonic flow increases the drag. Strouhal number variation is compared with the measurements of other authors. The paper concentrates on providing reliable measurements of vortex shedding and base pressure over the subsonic speed range rather than attempting to provide universal correlations.

Reducing drag penalty in the three-dimensional supersonic biplane:

Abstract: The two-dimensional (2D) supersonic biplane is well known for its shock wave cancellation effect and zero wave drag at supersonic speed. Nonetheless, in a three-dimensional (3D) setting, this favourable shock wave interaction becomes disturbed in the Mach cone regions at the wing tips of the supersonic biplane, which results in a severe drag penalty near the wing tips. However, this can be alleviated by appropriately designing the planform of the biplane. This study was performed to investigate the patterns of the 3D shock wave interaction using computational fluid dynamics by considering the 3D supersonic biplane with different planforms. The drag was shown to become large when the wing has a high sweepback angle, and it became small when the wing has a small taper ratio. However, an excessively low taper ratio could also lead to a drag greater than that of the rectangular supersonic biplane. It was found that the low drag 3D supersonic biplane has a small sweepback angle and an adequate taper ra...

Drag reduction by a forward facing aerospike for a large angle blunt cone in high enthalpy flows

Abstract: Drag reduction studies are conducted using a flat disc tipped aerospike for a 120-degree apex angle blunt cone model in high enthalpy flows. Accelerometer based force balance is used for the drag force measurement in the newly established free piston driven shock tunnel, HST3. Drag reduction upto about 58 percent has been achieved for Mach 8 flow of 5 MJ/kg specific enthalpy at zero degree angle of attack.

Numerical Investigation of Turbulent Flow Around a Stepped Airfoil at High Reynolds Number

Abstract: This study presents the numerical simulation of flow development around NACA-2412 airfoil which utilized the backward facing step to explore the possibility of enhancing airfoil aerodynamic performance by trapped vortex lift augmentation. This article concentrate on the effect of separated flow and following vortex formation which is created by backward facing step on pressure distribution and subsequently on lift and drag coefficient. Reynolds number that based on the free stream velocity and airfoil chord is 5.7×106 . The two-equation shear stress transport (SST) k-ω turbulence model of Menter is employed to determine accurately turbulent flow, as well as the recirculation pattern along the airfoil. The Reynolds-averaged Navier Stokes (RANS) equations are solved numerically using finite volume based solution with second-order upwind Roe’s scheme. Steps are located on both suction side and pressure side of the airfoil, at different locations, different lengths and various depths in order to determine their effects on lift, lift to drag ratio and near stall behavior. The modeling results showed that all stepped airfoil cases studied experienced higher drag compared to the base airfoil. Considerable lift enhancement was found for airfoil with backward facing step on pressure side at all values of angle of attack because of trapped vortex. The results suggest that the steps on the lower surface that extended back to trailing edge can lead to more enhancement of lift to drag ratio for some angles of attack; while the rear locations for the step on upper surface was found to have negative effect on lift to drag ratio. Based on this study, the backward facing step on suction surface offers no discernable advantages over the conventional airfoil but showed some positive effect on delaying stall.© 2009 ASME