Surface flow and vortex shedding of an impulsively started wing

Huang, Rong Fung; Wu, Jen Ying; Jeng, Jie; Chen, R. C.

doi:10.1017/s002211200100489x

Cited by 79 publications

(59 citation statements)

References 24 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This suggests that the scaling exponent, n, is only weakly dependent on the angle of attack and specific airfoil profile for stalled conditions. It is interesting that, despite the variation in the frequency scaling exponent between 0.9 and 1.9 for conditions at which a separation bubble forms, the significant variation in the Reynolds number below Huang and Lin (1995); NACA 0018, present investigation; NACA 0025, Yarusevych et al (2009); SD 7003, Burgmann and Schröder (2008) which stall occurs at a particular angle of attack allows for an approximate scaling with Re 1.75 for conditions with and without reattachment.…”

Section: Resultsmentioning

confidence: 79%

“…Furthermore, values of the Strouhal number vary more significantly between angles of attack for conditions at which a separation bubble forms and are more similar in magnitude for stalled conditions. An alternative non-dimensional form of the instability frequency is the Roshko number based on the model chord length, that is, f 0 c 2 /m, which is the product of the Strouhal number and the Reynolds number (Huang and Lin 1995). In this form, the shear layer frequency is scaled by the fluid properties, allowing comparison of measurements over airfoils of different sizes in both water and air.…”

Section: Resultsmentioning

confidence: 99%

“…n . 1:9 based on data for LA2573A(LeBlanc et al 1989), NACA 0012(Huang and Lin 1995), NACA 0025(Yarusevych et al 2009), and SD 7003 (Burgmann and Schroder 2008) airfoils. The power-law fits plotted inFig.…”

mentioning

confidence: 99%

See 2 more Smart Citations

Parametric study of separation and transition characteristics over an airfoil at low Reynolds numbers

Boutilier

Yarusevych

2012

Exp Fluids

View full text Add to dashboard Cite

Time-resolved surface pressure measurements are used to experimentally investigate characteristics of separation and transition over a NACA 0018 airfoil for the relatively wide range of chord Reynolds numbers from 50,000 to 250,000 and angles of attack from 0°to 21°. The results provide a comprehensive data set of characteristic parameters for separated shear layer development and reveal important dependencies of these quantities on flow conditions. Mean surface pressure measurements are used to explore the variation in separation bubble position, edge velocity in the separated shear layer, and lift coefficients with angle of attack and Reynolds number. Consistent with previous studies, the separation bubble is found to move upstream and decrease in length as the Reynolds number and angle of attack increase. Above a certain angle of attack, the proximity of the separation bubble to the location of the suction peak results in a reduced lift slope compared to that observed at lower angles. Simultaneous measurements of the time-varying component of surface pressure at various spatial locations on the model are used to estimate the frequency of shear layer instability, maximum root-mean-square (RMS) surface pressure, spatial amplification rates of RMS surface pressure, and convection speeds of the pressure fluctuations in the separation bubble. A power-law correlation between the shear layer instability frequency and Reynolds number is shown to provide an order of magnitude estimate of the central frequency of disturbance amplification for various airfoil geometries at low Reynolds numbers. Maximum RMS surface pressures are found to agree with values measured in separation bubbles over geometries other than airfoils, when normalized by the dynamic pressure based on edge velocity. Spatial amplification rates in the separation bubble increase with both Reynolds number and angle of attack, causing the accompanying decrease in separation bubble length. Values of the convection speed of pressure fluctuations in the separated shear layer are measured to be between 35 and 50% of the edge velocity, consistent with predictions of linear stability theory for separated shear layers.

show abstract

Section: Resultsmentioning

confidence: 79%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Parametric study of separation and transition characteristics over an airfoil at low Reynolds numbers

Boutilier

Yarusevych

2012

Exp Fluids

View full text Add to dashboard Cite

show abstract

“…Nominally two-dimensional aerofoils, and, in particular, the symmetric National Advisory Committee for Aeronautics (NACA) series, have been studied the most. Huang et al [16] measured the frequency of vortex shedding in the wake of a NACA0012 over a wide range of post-stall values of a up to Re = O(10 4 ). At sufficiently large Re, the thin shear layer bounding the separation displays a Kelvin-Helmholtz instability (with St about an order of magnitude higher than vortex shedding), and ultimately becomes turbulent [17].…”

Section: Steady-state Natural and Actuated Flows (A) Separation And Vmentioning

confidence: 99%

Control of vortex shedding on two- and three-dimensional aerofoils

Colonius

Williams

2011

Phil. Trans. R. Soc. A.

View full text Add to dashboard Cite

We review and expand on the control of separated flows over flat plates and aerofoils at low Reynolds numbers associated with micro air vehicles. Experimental observations of the steady-state and transient lift response to actuation, and its dependence on the actuator, aerofoil geometry and flow conditions, are discussed and an attempt is made to unify them in terms of their excitation of periodic and transient vortex shedding. We also examine strategies for closed-loop flow and flight control using actuation of leading-edge vortices.

show abstract

“…Recent theoretical analyses (Anderson et al, 1998;Triantafyllou et al, 1993Triantafyllou et al, , 1991Wang, 2000), computational analyses (Jones and Platzer, 1996;Tuncer and Platzer, 1996), and experimental analyses (Anderson et al, 1998;Huang et al, 2001) indicate that isolated flapping foils can produce high thrust coefficients together with very high efficiency if the kinematics are appropriately configured. Specifically, wingbeat frequency (f), stroke double amplitude (a) and flight speed (U) should combine to give a dimensionless Strouhal number (St=fa/U) at which wake formation is energetically efficient and a leading-edge vortex (LEV) is formed on each downstroke (Taylor et al, 2003).…”

mentioning

confidence: 99%

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarilyviaangle of attack

Thomas

Taylor

Srygley

et al. 2004

Journal of Experimental Biology

285

208

View full text Add to dashboard Cite

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,that dragonflies fly by using unsteady aerodynamic mechanisms to generate high-lift, leading-edge vortices. In normal free flight, dragonflies use counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing downstroke, attached flow on the forewing upstroke, and attached flow on the hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats with highly separated downstroke flows, with a single LEV attached across both the fore- and hindwings. We use smoke visualizations to distinguish between the three simplest local analytical solutions of the Navier–Stokes equations yielding flow separation resulting in a LEV. The LEV is an open U-shaped separation, continuous across the thorax, running parallel to the wing leading edge and inflecting at the tips to form wingtip vortices. Air spirals in to a free-slip critical point over the centreline as the LEV grows. Spanwise flow is not a dominant feature of the flow field – spanwise flows sometimes run from wingtip to centreline, or vice versa –depending on the degree of sideslip. LEV formation always coincides with rapid increases in angle of attack, and the smoke visualizations clearly show the formation of LEVs whenever a rapid increase in angle of attack occurs. There is no discrete starting vortex. Instead, a shear layer forms behind the trailing edge whenever the wing is at a non-zero angle of attack, and rolls up, under Kelvin–Helmholtz instability, into a series of transverse vortices with circulation of opposite sign to the circulation around the wing and LEV. The flow fields produced by dragonflies differ qualitatively from those published for mechanical models of dragonflies, fruitflies and hawkmoths, which preclude natural wing interactions. However, controlled parametric experiments show that, provided the Strouhal number is appropriate and the natural interaction between left and right wings can occur, even a simple plunging plate can reproduce the detailed features of the flow seen in dragonflies. In our models, and in dragonflies, it appears that stability of the LEV is achieved by a general mechanism whereby flapping kinematics are configured so that a LEV would be expected to form naturally over the wing and remain attached for the duration of the stroke. However, the actual formation and shedding of the LEV is controlled by wing angle of attack, which dragonflies can vary through both extremes, from zero up to a range that leads to immediate flow separation at any time during a wing stroke.

show abstract

Surface flow and vortex shedding of an impulsively started wing

Cited by 79 publications

References 24 publications

Parametric study of separation and transition characteristics over an airfoil at low Reynolds numbers

Parametric study of separation and transition characteristics over an airfoil at low Reynolds numbers

Control of vortex shedding on two- and three-dimensional aerofoils

Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarilyviaangle of attack

Contact Info

Product

Resources

About