Zhijin Wang

Functional morphology of the nasal region of a hammerhead shark

We describe several novel morphological features in the nasal region of the hammerhead shark Sphyrna tudes. Unlike the open, rounded incurrent nostril of non-hammerhead shark species, the incurrent nostril of S. tudes is a thin keyhole-like aperture. We discovered a groove running anterior and parallel to the incurrent nostril. This groove, dubbed the minor nasal groove to distinguish it from the larger, previously described, (major) nasal groove, is common to all eight hammerhead species. Using life-sized plastic models generated at 200 microm resolution from an X-ray scan, we also investigated flow in the nasal region. Even modest oncoming flow speeds stimulate extensive, but not complete, circulation within the model olfactory chamber, with flow passing through the two main olfactory channels. Flow crossed from one channel to another via a gap in the olfactory array, sometimes guided by the interlamellar channels. Major and minor nasal grooves, as well as directing flow into the olfactory chamber, can, in conjunction with the nasal bridge separating incurrent and excurrent nostrils, limit flow passing into the olfactory chamber, possibly to protect the delicate nasal structures. This is the first simulation of internal flow within the olfactory chamber of a shark.

Volumetric measurements and simulations of the vortex structures generated by low aspect ratio plunging wings

Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity.

Lift-Enhancing Vortex Flows Generated by Plunging Rectangular Wings with Small Amplitude

Experiments in a water tunnel have been carried out on low-aspect-ratio rectangular wings undergoing a small-amplitude harmonic plunge motion at Reynolds numbers 10,000 and 20,000. A series of measurement techniques have been used, including force measurements, hot film, particle image velocimetry, and volumetric velocimetry measurements, to study the lift enhancement as a function of forcing frequency. Multiple peaks in the time-averaged lift have been observed, occurring at frequencies in the order of natural vortex-shedding frequencies of the stationary wings. It is postulated that interaction between the leading-edge and trailing-edge vortices contributes to the selection of the optimal frequencies for the time-averaged lift. At a specific Strouhal number, an adverse interaction between the vortices results in a vortex dipole that directs flow upstream. A comparison between a NACA 0012 and flat-plate profile provides further insight into the advantages and disadvantages of using a thinner profile in l...

Oscillating Flexible Wings at Low Reynolds Numbers

In nature a range of techniques are used to overcome the challenges of low Reynolds number flight. In this paper two of these are considered, wing oscillations and wing flexibility. The force, deformation and flow fields of rigid and flexible wings oscillating at a fixed post-stall angle of attack of 15° and amplitude of 15% of chord are measured. The force measurements show that flexibility can increase the time-averaged lift coefficient significantly, oscillation at a Strouhal number of Src = 1.5 results in Cl = 2.43 for the flexible wing as opposed to Cl = 1.13 for the rigid wing. Simultaneously, flexibility reduces the power input coefficient, oscillation at / around the natural frequency of Src = 1.5 results in a power coefficient of Cp = 2.4 for the flexible wing as opposed to Cp = 23.2 for the rigid wing. Deformation measurements show this frequency is associated with the tip motion lagging the root motion by 90° but with an amplitude 1.84 times greater. The velocity measurements show this deformation inhibits Leading-Edge Vortex (LEV) dipole formation resulting in a stronger LEV that convects closer to the upper surface. This combined with the stronger tip vortex explains the higher lift observed for the flexible wing.

On the absence of asymmetric wakes for periodically plunging finite wings

It has previously been shown that, at high Strouhal numbers, oscillating airfoils can produce deflected jets that can create very high lift-coefficients for otherwise symmetric scenarios. These deflected jets form through pairing of the trailing-edge vortices to create asymmetric vortex couples that self-propel at an angle to the freestream, resulting in an asymmetric flow field and non-zero lift. In this paper results are presented that indicate these high-lift deflected jets cannot form for finite wings. Instead of the straight vortex tubes that pair and convect at an angle to the freestream observed for effectively infinite wings, finite wings exhibit vortex tubes that break into two branches near the tip forming double helix structures. One branch connects with the last vortex; one branch connects with the next vortex. This creates a long “daisy chain” of interconnected trailing edge vortices forming a long series of vortex loops. These symmetric flow fields are shown to persist for finite wings even to Strouhal numbers more than twice those required to produce asymmetric wakes on plunging airfoils. Two contributing reasons are discussed for why deflected jets are not observed. First the tip vortex creates three-dimensionality that discourages vortex coupling. Second, the symmetry of the circulation of the interconnected vortex loops, which has been confirmed by the experiments, is a natural consequence of the vortex topology. Therefore, the asymmetry in trailing edge vortex strength previously observed as characteristic of deflected jets cannot be supported for finite wings.

Control of afterbody vortices by blowing

An experimental study has been performed to examine the effect of blowing via a pair of jets from the upswept face of a slanted base cylinder, upon the development of the afterbody vortex flow field. Jet vortices initially interact with the shear layer separated from the upswept section. The nature of the early interactions between the jet and shear layer is highly dependent upon the blowing direction. For most blowing configurations tested, the shear layer becomes wrapped around the jet vortex. The most significant evidence of modification of the afterbody vortex pair was found when initiating a jet vortex further outboard and closer to the shear layer. The jet/vortex interactions initially cause the afterbody vortices to form further outboard, at a greater distance from the surface, and with a smaller vortex core. There is a reduction in the circulation over the first half of the afterbody length, before it recovers to baseline levels at the trailing edge. The vortex pair appears more diffuse near the trailing edge, but this is a result of increased meandering.

Frequency lock-in phenomenon for self-sustained roll oscillations of rectangular wings undergoing a forced periodic pitching motion

The free-to-roll behaviour of rigid and membrane rectangular wings with an aspect ratio of two was studied in wind tunnel experiments conducted at a chord Reynolds number of Rec = 46 000. Self-excited roll oscillations resulting from the fluid-structure interaction were studied in forced sinusoidal pitching motion in order to simulate gust encounters of small air vehicles. For the dynamic pitching cases, the frequency and phase of the self-excited roll oscillations can become synchronized (or locked-in) with the fundamental pitching frequency and its subharmonics. This is believed to be the first documented example of synchronization for this type of fluid-structure interaction. Depending on the amplitude and frequency of excitation (pitching motion), there are regions of decreased roll oscillations, which may be important for the gust response of small vehicles.

Attenuation of Self-Excited Roll Oscillations of Low-Aspect-Ratio Wings by Using Acoustic Forcing

Attenuation of self-excited roll oscillations of low-aspect-ratio wings using acoustic excitation was studied in a wind tunnel. For a rectangular flat-plate wing with an aspect ratio of 2, roll oscillations can be completely suppressed, and the onset of the roll oscillations can be delayed with external acoustic excitation. Similar results were also obtained for wings with two different airfoil profiles. Velocity measurements indicated that acoustic excitation could restore a symmetric vortex flow over the free-to-roll wings, thus eliminating the self-excited roll oscillations. The effect of excitation is most noticeable for the side of the wing that sees a larger effective angle of attack due to the rolling motion. Acoustic excitation energizes the shear-layer instabilities and results in reattachment or smaller separated flow region closer to wing surface, thus in turn suppressing the roll oscillations.

Active control of self-induced roll oscillations of a wing using synthetic jet

Active control of self-excited roll oscillations of a rectangular flat plate wing with an aspect ratio of two was studied experimentally in a wind tunnel, using synthetic jet excitation near the leading edge. It was found that, by activating the synthetic jet excitation at an optimum frequency of St = 1, large amplitude roll oscillations could be attenuated and the onset of the oscillation can be delayed by up to Δαmax = 3.5° for extremely small values of momentum coefficient. High frame-rate Particle Image Velocimetry (PIV) measurements revealed a strong resonance between the synthetic jet excitation and shear layer instabilities. The resonance energizes the shear layer separated from the leading edge and results in a local flow field that is more typical of lower wing incidence, thus effectively suppressing roll oscillations.

Lift Enhancement of a Flat-Plate Airfoil by Steady Suction

The effect of suction on an airfoil surface at various locations downstream of the leading edge of a thin flat-plate airfoil was studied in a wind tunnel at a low Reynolds number. At poststall angles of attack, substantial lift enhancement and delay of stall can be achieved if a large separation bubble is generated by reattaching the massively separated flow near the trailing edge. The effects of location and volumetric flow rate of suction were investigated by means of force and velocity field measurements. There is an optimal location of suction around xs/c=0.4, which generates the maximum lift coefficient for suction coefficients less than 3%. When suction is applied closer to the leading edge, it may be possible to reattach the flow for smaller suction coefficients, but the resulting small separation bubble causes smaller lift increase. Large separation bubbles are needed for the maximum time-averaged lift enhancement, however, they exhibit shear layer flapping, intermittent reattachment, and larger l...