R. B. Green

Hydrodynamic propulsion of human sperm

The detailed fluid mechanics of sperm propulsion are fundamental to our understanding of reproduction. In this paper, we aim to model a human sperm swimming in a microscope slide chamber. We model the sperm itself by a distribution of regularized stokeslets over an ellipsoidal sperm head and along an infinitesimally thin flagellum. The slide chamber walls are modelled as parallel plates, also discretized by a distribution of regularized stokeslets. The sperm flagellar motion, used in our model, is obtained by digital microscopy of human sperm swimming in slide chambers. We compare the results of our simulation with previous numerical studies of flagellar propulsion, and compare our computations of sperm kinematics with those of the actual sperm measured by digital microscopy. We find that there is an excellent quantitative match of transverse and angular velocities between our simulations and experimental measurements of sperm. We also find a good qualitative match of longitudinal velocities and computed tracks with those measured in our experiment. Our computations of average sperm power consumption fall within the range obtained by other authors. We use the hydrodynamic model, and a prototype flagellar motion derived from experiment, as a predictive tool, and investigate how sperm kinematics are affected by changes to head morphology, as human sperm have large variability in head size and shape. Results are shown which indicate the increase in predicted straight-line velocity of the sperm as the head width is reduced and the increase in lateral movement as the head length is reduced. Predicted power consumption, however, shows a minimum close to the normal head aspect ratio.

Dynamic recovery to fully attached aerofoil flow from deep stall

We present results from a comprehensive analysis of pressure data from several two-dimensional aerofoil models performing constant pitch rate ramp-down motions from the fully separated state to the fully attached state. The salient observation is that, a short while after reattachment is first observed at the leading edge, a weak disturbance travels from the leading edge to the trailing edge, and this disturbance causes a characteristic wave on the pressure data. The speed of the wave is uniform along the chord and is predominantly independent of aerofoil profile and reduced pitch rate. This contrasts with the rate of attachment, which is strongly pitch rate dependent. It is suggested that the wave indicates convection of wake fluid over the aerofoil surface.

Nonlinear Dynamics of Resonant Tunneling Optoelectronic Circuits for Wireless/Optical Interfaces

We report on experimental and modeling results on the nonlinear dynamics of a resonant-tunneling-diode-based (RTD) optoelectronic circuits that can be used as the basis of a wireless/optical interface for wireless access networks. The RTD-based circuits are optoelectronic integrated circuits that have negative differential resistance and act as optoelectronic voltage-controlled oscillators. These circuits display many of the features of classic nonlinear dynamics, including chaos and synchronization. These highly nonlinear oscillators behaves as injection-locked oscillators that can be synchronized by a small injection signal of either wireless or optical origin, and thus, can transfer phase encoded information from wireless to the optical domain or the optical to the wireless domain.

Phenomena observed during aerofoil ramp-down motions from the fully separated state

Unsteady pressure data from seven aerofoil models performing constant pitch rate “ramp-down” motions from the fully separated state to low incidence have been analysed. It is observed that the time delay between attachment at the leading edge and the occurrence of minimum normal force is, to a first order, independent of reduced pitch rate. As a result, at sufficiently high pitch rate, negative normal force may be generated at large positive incidence. In contrast to this, the re-establishmeht of attached flow to the upper surface is strongly dependent upon pitch rate. The paper also contains a description of smoke flow visualisation tests. These were at a much lower Reynolds number than the pressure data tests, and consequently are of qualitative interest. Results of flow visualisation show that during a ramp-down test the near wake is much larger than in the corresponding static case, and that attachment is significantly delayed.

The flow field around a rotor in axial descent

Measurements of the flow field around a model rotor descending axially into its own vortex wake have been performed using particle image velocimetry (PIV). At low descent rates, the expected cylindrical down-flow structure below the rotor is observed. At slightly higher descent rate, the flow enters the so-called vortex ring state (VRS) where the vorticity from the rotor accumulates into a toroidal structure near the rotor tips, and a large recirculation zone forms above the rotor disk. In the VRS, the flow below the rotor shows a significant upwards component, with a small up-flow zone penetrating right up to the rotor disk. Measurements show there to be a range of descent rates just before the onset of the VRS over which the flow may be interpreted to be in an incipient VRS condition. In this range, analyses of individual PIV measurements indicate that the flow near the rotor intermittently switches between the down-flow topology found at lower descent rates and the flow topology found in the fully developed VRS. The frequency of excursions of the flow into the VRS topology increases as the descent rate of the rotor is increased until, at high enough descent rate, the flow remains locked within its toroidal state.

High-Speed Imaging of 2-D Ionic Diffusion Using a 16

We present an electrochemical high-speed measurement for direct observation of the mixing of ions in solution over a sensor surface. The method is applied to the determination of time-varying mixing and diffusion rates in microfluidic systems. The system generates high-speed videos of the analytes under investigation as they mix, without the need for any preliminary staining or labeling and has a pixel-to-pixel pitch of 14 μm. We demonstrate ion displacement phenomena on the surface of a Si3N4-passivated complementary metal oxide semiconductor ion-sensitive field effect transistor array device and also monitor the pH change induced by the addition of sulfuric acid to normal saline. These chemical videos are recorded with a frame rate of up to 333 frames/s. By evaluating the time-varying change in ionic concentrations across the surface of the chip, we calculate time-varying estimates of diffusivity coefficient values for the mixing analytes.

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This paper describes and presents results from static and pitch up/down wind tunnel tests conducted on a 60° delta wing at a root chord Reynolds number of 2.7 x 10 . In these tests, the wing was instrumented with 192 miniature pressure transducers which, in conjunction with a powerful multi-channel data-logging system, allowed the distribution of time-varying surface pressures to be measured at high temporal resolution. It is shown that analysis of rootmean-square (RMS) pressure on the leeward surface of the wing can provide considerable insight into the formation, position and movement of both the primary and secondary vortex structures with increasing pitch rate. The relationship between these observations and previously observed behavioural characteristics of vortex breakdown is then discussed.

16 Pixel CMOS ISFET Array on the Microfluidic Scale

Results from wind-tunnel tests conducted on a pitching 60-deg delta wing in ramp-up motion are presented and described. The wing was instrumented with 192 miniature pressure transducers, which, in conjunction with a powerful multichannel data-logging system, allowed the distribution of time-varying surface pressures to be measured at high temporal resolution for a range of pitching cases. In addition to allowing the forward progression of vortex breakdown with incidence to be tracked, it has been possible to isolate dominant postbreakdown buffet frequencies in both the static and pitching cases. It is demonstrated that, during pitch up, these frequencies differ significantly from those of the static case, indicating that alterations in flow structure may be taking place

AN ANALYSIS OF A PITCHING DELTA WING USING HIGH RESOLUTION PRESSURE MEASUREMENTS.

Quantitative and qualitative results of a series of experiments conducted oil a rotor in ground effect at low forward speeds are presented. The velocity over a wide area of the ground effect wake was measured using particle image velocimetry, and the evolution of the flow is described as the forward speed increases. The formation of a dust cloud leading to so-called helicopter brown-out was simulated through a series of flow visualisation experiments. The technique involved sprinkling a fine dust on the ground below and ahead of the rotor. Larger dust clouds were observed at lower forward speed, and the dust cloud penetrated into the areas of the flow including those where vorticity levels were of low magnitude and occasional velocity fluctuations from the mean were large.

Analysis of Unsteady Pressure Signals on a Pitching Delta Wing

This paper describes and presents results from static wind tunnel tests conducted on a 60° delta wing at a root chord Reynolds number of 2·7 x 10 6 . In these tests, the wing was instrumented with 192 miniature pressure transducers which, in conjunction with a powerful multi-channel data-logging system, allowed the distribution of time-varying surface pressures to be measured at high temporal resolution. Analysis indicates that the distribution of root mean square pressure on the leeward surface of the wing can provide considerable insight into the behaviour of both the primary and secondary vortex structures. In addition, it has been established that the frequency content of pressure signals measured in the vicinity of these vortex structures is sensitive to the vortex state. It is suggested that these data features can be directly attributed to previously observed behavioral characteristics of the vortex breakdown process.