Leading-edge Bubble Research Articles

Numerical Investigation of Deep Dynamic Stall for a Helicopter Blade Section

For helicopters in forward flight at high advance ratios, because of the cyclic variation of the angle of attack, the flow over the retreating blade can stall. This phenomenon, which is known as dynamic stall, limits the maximum airspeed of the helicopter. Physical understanding of the highly complex unsteady flow can serve as a catalyst for the development of novel flow control strategies that counter the detrimental effects of dynamic stall. For the present implicit large-eddy simulations, the complex flow over a rotor blade section with a Sikorsky SCC-A09 airfoil is decomposed into a pitching, surging, and yawing flow (dynamic crossflow). The heaving/plunging motion resulting from the unsteady blade bending under load is disregarded. Instead, the centrifugal and Coriolis accelerations are modeled. Three different combinations of the elementary flows are considered: 1) pitching and surging (baseline); 2) pitching, surging, and rotational accelerations; and 3) pitching, surging, and yawing. Simulations for an outboard blade section at a chord-based Reynolds number and a reduced frequency exhibit all relevant stages of dynamic stall reported in the literature: The bursting of a laminar leading-edge bubble is followed by the emergence of a strong dynamic stall vortex, which is eventually shed. When the harmonic yawing motion is added, the laminar bubble bursts earlier and the dynamic stall vortex is shed sooner. The addition of the rotational accelerations reduces the coherence of the dynamic stall vortex and, as a result, diminishes the cycle-to-cycle variations of the lift coefficient.

Modulation of wake flow and aerodynamic behaviors around a square cylinder using an upstream control bar

This investigation studies the flow patterns, vortex-shedding frequency, turbulence intensity (T.I.), drag coefficient (CD) and lift coefficient (CL) around a square cylinder using an upstream control bar. The flow behaviors were investigated using various Reynolds numbers (Re), rotation angles (θ) and spacing ratios (L/w). The surface oil-flow visualization was utilized to observe the flow structures. The surface oil-flow patterns were classified into three characteristic flow modes – twin-bubble, leading-edge bubble, and uninfluenced – using various Re and L/w at θ=0°. Four characteristic flow modes – uninfluenced, twin-bubble, leading-edge bubble, and attached flow – were defined by changing L/w and θ while Re=6×104. Hot-wire anemometry was used to measure the vortex-shedding frequency behind the square cylinder. With the influence of upstream control bar, the highest Strouhal number of 0.225 and the lowest T.I. of 10.4% occurs in the leading-edge bubble mode at θ=0°. Additionally, the linear pressure scanner was utilized to detect the surface pressure on the square cylinder, and the aerodynamic parameters were determined from the surface-pressure profiles. In the leading-edge bubble mode, CD has the lowest and CL has the highest value at θ=0°.

Characteristic flow patterns and aerodynamic performance on a forward-swept wing

This investigation elucidates the effects of Reynolds number (Re) and angle of attack (α) on the boundary-layer flow patterns, aerodynamic performance, flow behaviors and vortex shedding. This investigation applies a finite NACA 0012 forward-swept wing with the forward-sweep angle (φ) of 15°. The Reynolds numbers were tested in the range of 4.6 × 104 < Re < 105. The wing chord length is 6 cm and the semi-wing span is 30 cm, such that the full-span wing aspect ratio is 10. The surface oil-flow scheme was utilized to visualize the boundary-layer flow structures. The hot-wire anemometer was applied to measure the vortex-shedding frequency behind the forwardswept wing. Furthermore, a force-moment sensor was applied to measure the aerodynamic loadings. The surface oil-flow patterns are classified into six characteristic flow modes — separation, separation bubble, secondary separation, leading-edge bubble, bubble extension and bluff-body wake modes. Additionally, the output of force-moment sensor and the visualized boundary-layer flow configurations indicate that the aerodynamic performance is closely related to the boundary-layer flow behaviors. Furthermore, the boundary-layer flow stalled in the leading-edge bubble mode. Moreover, the vortex-shedding frequency behind the forward-swept wing shows that the vortexshedding frequency at low α exceeds that at high α.

Numerical Investigation of Separation Control for Wing Sections

For Reynolds numbers that are typical for general aviation aircraft separation is typically turbulent. In this paper the question is addressed if flow control strategies that are successful at low-Reynolds number conditions remain effective at higher Reynolds numbers. Towards this end hybrid simulations based on a one-equation renormalization group model were carried out for a modified NACA643-618 airfoil at a chord Reynolds number of one million. For ten degrees angle of attack a short laminar separation bubble develops near the leading edge and the flow separates turbulent from the suction side downstream of the leading edge bubble. Active flow control by harmonic blowing through a spanwise slot was investigated. The control was found to be only mildly effective or even counterproductive. In particular, the disturbances that were introduced by the control were only weakly amplified or even dampened. The unresolved eddy viscosity in the separated boundary layer lowers the effective Reynolds number and ma...

Characteristic Flow Field Behind a Square-Cylinder Using Upstream Mesh Fences

The flow behaviors around a square cylinder were modulated using the passive mesh fence. The effects of Reynolds number (Re) and rotation angle (θ) on the square-cylinder flow fields using different turbulence intensity (TI) were also investigated. Additionally, various steel mesh fences with different mesh densities were installed between the nozzle outlet and the test-section inlet to adjust the free-stream TI. The Reynolds number and turbulence intensity used in this investigation are 3.0 × 104 ≤ Re ≤ 1.0 × 105 and 0.32% ≤ TI ≤ 0.82%. The flow fields are visualized using the surface oil-flow visualization scheme. Furthermore, the flow patterns are classified as—leading-edge bubble, separation bubble, separation, leading-edge separation, and boundary-layer attached modes. Specifically, the leading-edge bubble mode does not exist while θ and TI are low. Moreover, a hot-wire anemometer was placed in the wake to detect the vortex-shedding frequency. The experimental results indicate that Strouhal number (St) decreases with increasing the free-stream TI while TI &lt; 0.45%. However, St approaches a constant as TI &gt; 0.45%. Furthermore, the surface pressure was detected using a pressure scanner and the drag coefficient (CD) was obtained using the surface-pressure profile. The experimental results also reveal that CD decreases with increasing the free-stream TI. However, the change rate of CD for TI &lt; 0.45% exceeds that for TI &gt; 0.45%.

Experimental study of vortex generators effects on low Reynolds number airfoils in turbulent flow

In the present work, we study the aerodynamic effects of triangular vortex generators, as passive flow control devices, placed on the upper surface of an airfoil submitted to a low Reynolds number turbulent flow. In the experiments, different configurations of those devices have been studied. An Eppler 387 airfoil was used. The tests were performed in a turbulent boundary layer wind tunnel using a two component aerodynamic balance and flow visualisation systems. Turbulent flow characterisation was made by means of hot wire anemometry. Calculations of local turbulent intensity as well as temporal and spatial turbulent scales were made. Vortex generators were located at 10% and 20% of the airfoil chord from the leading edge, modifying its angle of incidence refereed to the free stream. The results show changes in the aerodynamic section coefficients, Cl, Cd and Cl, for the different vortex generator configurations. Neither hysteresis effects, nor leading edge bubbles were found in the experiments.

Sail aerodynamics: Understanding pressure distributions on upwind sails

The pressure distributions on upwind sails is discussed and related to the flow field around the headsail and the mast/mainsail. Pressures measured on several horizontal sections of model-scale and full-scale sails are used to provide examples. On the leeward side of the sails, leading edge separation and turbulent reattachment occurs, sometimes followed by trailing edge separation. On the windward side, leading edge separation occurs on the mast/mainsail and, at low angles of attack, it can also occur on the headsail. Differences were found between the leading edge bubbles on the two sails. Pressure trends for different angles of attack are presented, and these can be explained in terms of standard aerodynamic theory, particularly in terms of short and long leading edge separation bubble types. It was found that the pressure distributions measured on mainsails at full- and model-scale showed good agreement on both the windward and leeward sides.

Boundary-Layer Flow Effects on Aerodynamic Performance of Forward-Swept Wings

AbstractThe effects of a forward-sweep angle (φ), Reynolds number (R), and angle of attack (α) on the boundary-layer flow and aerodynamic performance of a finite forward-swept wing (NACA 0012 airfoil) were experimentally investigated. The Reynolds number and the forward-sweep angle were 2.5×104<R<105 and 0°≤φ≤45°, respectively. The chord length was 6 cm, and the semiwing span was 30 cm; therefore, the semiwing aspect ratio was 5. The boundary-layer flow structures were visualized using the surface-oil-flow technique. Seven boundary-layer flow modes were identified by changing R, α, and φ: separation, separation bubble, secondary separation, leading-edge bubble, three-dimensional (φ=0°)/bubble-extension flow, and bluff-body wake modes. Furthermore, a six-component balance measured the aerodynamic loadings. The aerodynamic performance related closely to the boundary-layer flow modes. The dimensionless bubble length (Lb/C) indicated that a high forward-sweep angle conducted a longer bubble length than that a...

AERODYNAMIC PERFORMANCE AND SHEDDING CHARACTERISTICS ON A SWEPT-BACK WING

A NACA 0012 finite swept-back wing with a sweep-back angle of 15° was utilized to investigate the effects of angle of attack (α) and the chord Reynolds number (Rec) on the vortex shedding and aerodynamic coefficients. A hot-wire anemometer was applied to measure the vortex-shedding frequency. The projected Strouhal number (Std) at various angles of attack was determined and discussed. The relationship between Std and α is regressed as: Std = -0.0008 α + 0.209, for 22° < α < 90°. Four characteristic surface-flow patterns: separation bubble, leading-edge bubble, bubble burst, and turbulent separation were classified by changing α and Re. The behavior of surface-flow structures significantly affects the lift, drag, and moment coefficients. The lift coefficient (CL) increases with α in the separation bubble and leading-edge bubble regimes. The maximum increase rate of CL with respect to α (d(CL)/dα) is 1.52 π/rad. Occurring in the leadingedge bubble regime. However, the maximum increase rate of drag coefficient (CD) with respect to α (d(CD)/dα) is 0.49 π/rad. Occurring in the bubble-burst regime. The steep-drop of moment coefficient at stall in the unswept-wings is not observed in the swept-back wings.

Reynolds number effects on flow characteristics and aerodynamic performances of a swept-back wing

A NACA 0012 wing model with sweep-back angle of 38° was studied. This finite wing has an aspect ratio of 10. Surface-flow fields were visualized using the smoke wire and surface oil-flow schemes. According to the smoke-streak flow patterns for the low Reynolds number ( Re < 1.5 × 10 4 ), five characteristic flow modes are defined — attached surface flow, separation, separation vortex, separation near the leading-edge and bluff-body wake. Based on the surface oil-flow patterns for the high Reynolds numbers ( Re > 3 × 10 4 ), six characteristic flow modes are defined — laminar separation, separation bubble, leading-edge bubble, bubble extension, bubble burst and turbulent boundary layer. The velocity field around the wing was quantified using the particle image velocimetry (PIV). Moreover, the topological scheme was utilized to verified the surface-flow patterns. A six-component balance was used to measure the aerodynamic loadings. The aerodynamic performances are closely related to the characteristic surface-flow patterns.

Performance of Two-Equation Turbulence Models for Flat Plate Flows With Leading Edge Bubbles

This paper investigates the performance of the popular k-ω and SST turbulence models for the two-dimensional flow past the flat plate at shallow angles of incidence. Particular interest is paid to the leading edge bubble that forms as the flow separates from the sharp leading edge. This type of leading edge bubble is most commonly found in flows past thin airfoils, such as turbine blades, membrane wings, and yacht sails. Validation is carried out through a comparison to wind tunnel results compiled by Crompton (2001, “The Thin Aerofoil Leading Edge Bubble,” Ph.D. thesis, University of Bristol). This flow problem presents a new and demanding test case for turbulence models. The models were found to capture the leading edge bubble well with the Shear-Stress Transport (SST) model predicting the reattachment length within 7% of the experimental values. Downstream of reattachment both models predicted a slower boundary layer recovery than the experimental results. Overall, despite their simplicity, these two-equation models do a surprisingly good job for this demanding test case.

Influence of Boundary Layer Behavior on Aerodynamic Coefficients of a Swept-Back Wing

The effects of the Reynolds number and angle of attack on the boundary layer and the aerodynamic performance of a finite swept-back wing are studied experimentally. The cross-sectional profile of the wing is NACA 0012 (aspect ratio=10), and the sweep-back angle is 15 deg. The Reynolds number is set in the range of 30,000–130,000. The boundary layer field is visualized with surface oil-flow techniques. Six characteristic flow regimes—laminar separation, separation bubble, leading-edge bubble, bubble burst, turbulent separation, and bluff-body wake—are categorized and studied by considering the Reynolds numbers and angles of attack. The characteristic behaviors of boundary layer significantly affect the lift, drag, and moment coefficients. The bubble length shortens significantly in the separation bubble and leading-edge bubble regimes as the angle of attack rises. The aerodynamic performances demonstrate that the swept-back wing model has no hysteresis.

Compressor blade leading edges in subsonic compressible flow

The flow around the leading edge of a compressor blade is interesting and important because there is such a strong interaction between the viscous boundary layer flow and the inviscid flow around it. As the velocity of the inviscid flow just outside the boundary layer is increased from subsonic to supersonic, the type of viscous-inviscid interaction changes; this has important effects on the boundary layer downstream and thus on the performance of the aerofoil or blade. An investigation has been undertaken of the flow in the immediate vicinity of a simulated compressor blade leading edge for a range of inlet Mach numbers from 0.6 to 0.95. The two-dimensional aerofoil used has a circular leading edge on the front of a flat aerofoil. The incidence, Reynolds number and level of free-stream turbulence have been varied. Measurements include the static pressure around the leading edge and downstream and the boundary layer profile far enough downstream for the leading edge bubble to have reattached. Schlieren pictures were also obtained. The flow around the leading edge becomes supersonic when the inlet Mach number is 0.7 for the zero-incidence case; for an inlet Mach number of 0.95 the peak Mach number was approximately 1.7. The pattern of flow around the leading edge alters as the Mach number is increased, and at the highest Mach number tested here the laminar separation bubble is removed. Positive incidence, raised free-stream turbulence or increased Reynolds number at intermediate inlet Mach numbers tended to promote flow patterns similar to those seen at the highest inlet Mach number. Both increased free-stream turbulence and increased Reynolds number, for the same Mach number and incidence, produced thinner shear layers including a thinner boundary layer well downstream. The measurements were supported by calculations using the MSES code (the single aerofoil version of the MISES code); the calculations were helpful in interpreting the measured results and were demonstrated to be accurate enough to be used for design purposes.

Effects of Freestream Turbulence on Wing-Surface Flow and Aerodynamic Performance

The effects of freestream turbulence intensity on the surface-e ow characteristics, including separation, bubble, leading-edge bubble, turbulent separation, and three-dimensional e ow, as well as the corresponding aerodynamic performance, including lift, drag, and stall, of a cantilever NACA 0012 wing model have been studied in a wind tunnel. Different wire-mesh screens were placed between the nozzleand test section to produce various freestream turbulence intensities. The characteristic regimes of surface e ow were identie ed by using the surface-oil e ow technique. The aerodynamic lift and drag were measured with a PC-based force/moment sensing system. The variationsofliftanddragwerefoundtobecloselyrelatedtothebehaviorofsurfacee ow.Thefreestreamturbulence signie cantlyaffectsthesurfacee owandaerodynamicperformance.Thesurfacee owandlift/dragatlowfreestream turbulenceareapparentlydifferentfrom thatatlargefreestreamturbulenceintensity.Theine uenceisdrasticwhen turbulence intensities are smaller than about 0.45%, whereas it becomes insignie cant when turbulence intensities are greater than 0.45%. Details of the ine uences are discussed and illustrated.

The aerodynamics of hovering insect flight. IV. Aerodynamic mechanisms

Theoretical considerations and available experimental studies are combined for a discussion on the aerodynamic mechanisms of lift generation in hovering animal flight. A comparison of steady-state thin-aerofoil theory with measured lift coefficients reveals that leading edge separation bubbles are likely to be a prominent feature in insect flight. Insect wings show a gradual stall that is characteristic for thin profiles at Reynolds numbers (Re) less than about 105. In this type of stall, flow separates at the sharp leading edge and then re-attaches downstream to the upper wing surface, producing a region of limited separation enclosing a recirculating flow. The resulting leading edge bubble enhances the camber and thickness of the thin profile, improving lift at low Re. Some of the results for bird wing profiles indicate that the complications of leading edge bubbles might even be found in the fast forward flight of birds.

Some Experiments on the Reattachment of a Laminar Boundary Layer Separating From a Rearward Facing Step on a Flat Plate Aerofoil

Summary:The results of experiments on the reattachment of a laminar boundary layer, separating from a rearward facing step in a flat plate aerofoil, are correlated with the properties of the short leading edge bubble which forms on thin aerofoils near the stall.The experiments, comprising pressure measurements, Pitot explorations, liquid film and smoke studies, indicate that for all Reynolds numbers above the value given by the Owen-KIanfer criterion the reattachment is turbulent behind a stationary air reverse flow vortex bubble. It is also found that the reattachment is laminar for Reynolds numbers below the critical, which further supports Crabtree's interpretation of the Owen-KIanfer criterion in terms of the condition for the growth of turbulent bursts.

Transition in a Separated Laminar Boundary Layer

PROGRESS in understanding the mechanism of transition from laminar to turbulent flow in a boundary layer has been largely confined to flow along a flat plate. The present investigation is concerned with transition in a separated laminar boundary layer. A hot wire investigation of flow near the short leading-edge bubble on a RAE-103 airfoil section led to a study of a boundary-layer transition in the flow past a step. The step investigation represented airfoil bubble conditions approximately and to a larger scale.