Ismet Gursul

Flexible flapping airfoil propulsion at low Reynolds numbers

Water tunnel experiments on a flexible airfoil plunging with constant amplitude have been carried out for Reynolds numbers of 0 to 27000. Peaks in thrust coefficient at intermediate values of airfoil stiffness were observed at both zero and non-zero Reynolds numbers, indicating that a degree of flexibility is beneficial at low Reynolds numbers. Time-averaged velocity fields and momentum flux data revealed a broader, higher-velocity jet in cases of optimum airfoil stiffness. Stronger vortices, separated by a larger lateral distance, characterised the corresponding instantaneous velocity fields. The flexibility causes the airfoil to pitch passively; the phase angle of the pitch was found to lead the plunge. Pitch amplitude and trailing-edge amplitude were found to be single-valued functions of pitch phase angle. The shape characteristics of the airfoil could therefore be described by the pitch phase angle only. Thrust coefficient was found to be a function of only two parameters: Strouhal number and pitch phase angle. For each Strouhal number, a peak in thrust coefficient was observed at a particular value of the pitch phase angle. The optimum pitch phase angle was found to tend to a limit of 105±5 degrees at very large Strouhal numbers. A significant thrust benefit was observed over very stiff airfoils when the optimum flexibility is utilized.

Flexible Flapping Airfoil Propulsion at Low Reynolds Numbers

Water tunnel experiments on a flexible airfoil plunging with constant amplitude have been carried out for Reynolds numbers of 0 to 27000. Peaks in thrust coefficient at intermediate values of airfoil stiffness were observed at both zero and non-zero Reynolds numbers, indicating that a degree of flexibility is beneficial at low Reynolds numbers. Time-averaged velocity fields and momentum flux data revealed a broader, higher-velocity jet in cases of optimum airfoil stiffness. Stronger vortices, separated by a larger lateral distance, characterised the corresponding instantaneous velocity fields. The flexibility causes the airfoil to pitch passively; the phase angle of the pitch was found to lead the plunge. Pitch amplitude and trailing-edge amplitude were found to be single-valued functions of pitch phase angle. The shape characteristics of the airfoil could therefore be described by the pitch phase angle only. Thrust coefficient was found to be a function of only two parameters: Strouhal number and pitch phase angle. For each Strouhal number, a peak in thrust coefficient was observed at a particular value of the pitch phase angle. The optimum pitch phase angle was found to tend to a limit of 105±5 degrees at very large Strouhal numbers. A significant thrust benefit was observed over very stiff airfoils when the optimum flexibility is utilized.

Unsteady flow phenomena over delta wings at high angle of attack

Experiments show that coherent pressure fluctuations observed on delta wings are due to the helical mode instability of the vortex breakdown flowfield. No dominant frequency in the spectra of pressure fluctuations on the wing surface was observed after the breakdown reached the apex of the wing, although the vortex shedding could be detected in the wake. Measurements of pressure fluctuations at different streamwise locations on the wing suggest that the dimensionless frequency fx/U^ is nearly constant for a given geometry, which implies increasing wavelength in the streamwise direction. For different wings, this nondimensional frequency is shown to be a function of nondimensional circulation T/U^x only. Both the wavelength of the disturbances and the core radius increase with the nondimensional circulation at a fixed streamwise location. The wavelength normalized by the core radius is around 3-4, which is much smaller than the predictions for the Q vortex.

Review of Unsteady Vortex Flows over Slender Delta Wings

Nomenclature AR = aspect ratio; amplitude ratio B = probability density function of velocity c = root chord length f = frequency k = reduced frequency; axial wave number n =w ave number in angular direction P = probability p = pressure fluctuation Re =R eynolds number based on chord length S = spectral density s = local semispan T = period t = time U∞ = freestream velocity u = axial velocity v = swirl velocity x = streamwise distance xbd = breakdown location y = spanwise distance z =v ertical distance above wing surface α = angle of attack � = circulation δ = flap angle � = sweep angle ν = kinematic viscosity τ = time constant � =fi nangle ω =v orticity; radial frequency

An Investigation of Vortex Flows over Low Sweep Delta Wings

This paper presents the results of a recent investigation into the vortex structure over a nonslender delta wing with leading edge sweep, Λ = 50°. A flow visualisation study in water tunnel experiments has shown profound sensitivity of the vortex structure to Reynolds number. As Reynolds number was reduced, the trajectory of the vortex core moved inboard toward the wing centre-line, and the onset of breakdown was noticeably delayed. The results provide the first experimental evidence of the dual vortex structure that has been observed in previous computational studies. The formation of these dual vortices was also sensitive to Reynolds number, and its formation was not observed at the very low end of values considered. Digital Particle Image Velocimetry (DPIV) measurements of the cross-flow have yielded axial vorticity data at a number of streamwise stations and incidences. At low incidences only a very weak vortex structure was observed with a strong shear layer forming very close to the wing surface. As incidence was increased the shear layer lifted from the surface but reattachment was still observed. Increasing incidence also resulted in a movement of the reattachment line towards the model centre-line, until ultimately reattachment failed.

Lift Enhancement by Means of Small-Amplitude Airfoil Oscillations at Low Reynolds Numbers

Force and particle image velocimetry measurements were conducted on a NACA 0012 airfoil undergoing small-amplitude sinusoidal plunge oscillations at a poststall angle of attack and Reynolds number of 10,000. With increasing frequency of oscillation, lift increases and drag decreases due to the leading-edge vortices shed and convected over the suction surface of the airfoil. Within this regime, the lift coefficient increases approximately linearly with the normalized plunge velocity. Local maxima occur in the lift coefficient due to the resonance with the most unstable wake frequency, its subharmonic and first harmonic, producing the most efficient conditions for high-lift generation. At higher frequencies, a second mode of flowfield occurs. The leading-edge vortex remains nearer the leading edge of the airfoil and loses its coherency through impingement with the upward-moving airfoil. To capture this impingement process, high-fidelity computational simulations were performed that showed the highly transitional nature of the flow and a strong interaction between the upper and lower-surface vortices. A sudden loss of lift may also occur at high frequencies for larger amplitudes in this mode.

Jet switching phenomenon for a periodically plunging airfoil

An experimental investigation has been carried out on rigid and flexible airfoils oscillating in still fluid. It was found that the vortex pairs generated by the oscillating airfoil move at an angle to the chordwise direction. The deflection angle of the induced jet was observed to change periodically in time. The switching period was found to increase with increasing airfoil stiffness and to decrease with increasing heave frequency and increasing amplitude. Over the range of frequency, amplitude, and stiffness tested, the switching period was found to be two orders of magnitude greater than the heave period. The development of the vorticity field for upward and downward deflected jets, as well as the transition between the two modes, was captured with the particle image velocimetry measurements. The deflection of the jet, and thus the jet switching effect, was found to diminish with increasing free stream velocity (decreasing Strouhal number).

Buffeting Flows over a Low-Sweep Delta Wing

An experimental study was conducted with the aim of understanding the unsteady vortex flows and buffeting response of a nonslender delta wing with 50-deg leading-edge sweep angle. Particle image velocimetry and laser Doppler velocimetry measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. It was found that there is a profound effect of Reynolds number on the structure of vortical flows. The breakdown of the leading-edge vortices is delayed significantly, and the vortices form more inboard at low Reynolds numbers. The secondary vortex effectively splits the primary vortex into two separate concentrations of vorticity, resulting in a dual vortex structure at small incidences. This dual vortex structure diminishes, and a single primary vortex is observed at higher incidences. At higher Reynolds numbers (on the order of 3 × 10 4) the flow approaches an asymptotic state, with further increases in the Reynolds number resulting in only small variations in the location of vortex core and breakdown. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with increasing incidences. However, the maximum buffeting occurs prior to the stall, after the vortex breakdown has reached the apex of the wing. The largest velocity fluctuations near the wing surface are observed along the reattachment line. Hence, the shear-layer reattachment, rather than the vortex breakdown phenomenon, is the most important source of increasing buffet in the prestall region as incidence is increased. The velocity fluctuations in the reattachment region have similar dominant frequencies as slender wings in spite of the differences in the physical nature of the flow. With further increase in incidence, the shear-layer reattachment becomes impossible, resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing-tip acceleration.

Buffeting Flows over Delta Wings

Experimental evidence suggests that vortex breakdown is not the only source of buffeting of delta wings and fins. Other unsteady flow phenomena that contribute to buffeting at high angles of attack are fluctuations of vortex breakdown location and vortex shedding. Flow visualization and velocity measurements were carried out over a delta wing, over a wide range of angles of attack, to understand the transition between the helical mode instability and the vortex shedding. It was found that this transition is abrupt, as indicated by a jump in the frequency parameter, and that it occurs at the angle of attack at which breakdown reached the apex. The unsteady nature of vortex breakdown location was investigated by flow visualization for the interaction of vortex breakdown with a rigid flat plate. Although there are indications of a feedback effect on vortex breakdown, the amplitude of the fluctuations of breakdown location is smaller for impinging flows

Unsteady Aerodynamics of Membrane Airfoils

An experimental study of unsteady aerodynamics of two-dimensional membrane airfoils has been conducted. Measurements of membrane shape with a high-speed camera were complemented with the measurements of unsteady velocity field with a Particle Image Velocimetry (PIV) system and flow visualization. Vibrations of the membrane and mode shapes were investigated as a function of angle of attack and free stream velocity. While the mean membrane shape is not very sensitive to angle of attack, the amplitude and mode of the vibrations of the membrane depend on the relative location and the magnitude of the unsteadiness of the separated shear layer. The results indicate strong coupling of unsteady flow with the membrane oscillations. There is evidence of coupling of membrane oscillations with the vortex shedding in the wake, in particular, for the post-stall incidences. Comparison of rigid and flexible membrane airfoils shows that the flexibility might delay the stall.