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In this work, direct numerical simulation (DNS) is used to investigate how airfoil shape affects wake structure and performance during a pitching-heaving motion. First, a class-shape transformation (CST) method is used to generate airfoil shapes. CST coefficients are then varied in a parametric study to create geometries that are simulated in a pitching and heaving motion via an immersed boundary method-based numerical solver. The results show that most coefficients have little effect on the propulsive efficiency, but the second coefficient does have a very large effect. Looking at the CST basis functions shows that the effect of this coefficient is concentrated near the 25% mark of the foils chord length. By observing the thrust force and hydrodynamic power through a period of motion it is shown that the effect of the foil shape change is realized near the middle of each flapping motion. Through further inspection of the wake structures, we conclude that this is due to the leading-edge vortex attaching better to the foil shapes with a larger thickness around 25% of the chord length. This is verified by the pressure contours, which show a lower pressure along the leading edge of the better performing foils. The more favorable pressure gradient generated allows for higher efficiency motion.

Studying the airflows and the resultant aerodynamic pressure/force in the pharyngeal airway is critical for understanding the pathophysiology of snoring and sleep apnea. In this work, an experiment-driven computational study was conducted to examine the aerodynamics in human pharyngeal airway. An anatomically accurate pharynx model associated with different uvula kinematics was reconstructed from human magnetic resonance image (MRI) and high-speed photography. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver was adopted to simulate the corresponding unsteady flows in all their complexity. Analyses were performed on vortex dynamics and pressure fluctuations in the pharyngeal airway and force oscillations on the pharyngeal wall under the influence of varying airway obstructions, uvula flapping mode, and uvula flapping frequencies. It was found the vortex formation, aerodynamic pressure, and pharyngeal wall force were significantly affected by the width of the pharyngeal airway. By contrast, the influences from the uvula flapping mode were insignificant when other parameters were similar. Fast Fourier transformation (FFT) and continuous wavelet transform (CWT) analysis of the pressure time history revealed the existence of higher order harmonics of base frequency with significant pressure amplitudes and energy intensities. It was also found the airway pressure and pharyngeal wall force oscillate more dramatically at higher uvula flapping frequencies, which tends to promote the collapse of pharyngeal wall and initiates sleep apnea.

Recently, it was demonstrated in an inviscid flow that burst-and-coast or intermittent swimming can save energy when compared with continuous swimming and that the energy savings are maximized for ...

Numerical studies are presented on the propulsive performance and vortex dynamics of multiple hydrofoils pitching in an in-line configuration. The study is motivated by the quest to understand the hydrodynamics of multiple fin–fin interactions in fish swimming. Using the flow conditions (Strouhal and Reynolds numbers) obtained from a solitary pitching foil of zero net thrust, the effect of phase differences between neighboring foils on the hydrodynamic performance is examined both in position-fixed two- and three-foil systems at Reynolds number R e = 500 . It is found that the three-foil system achieves a thrust enhancement up to 118% and an efficiency enhancement up to 115% compared to the two-foil system. Correspondingly, the leading-edge vortex (LEV) and the trailing-edge vortex (TEV) of the hindmost foil combine to form a ‘2P’ wake structure behind the three-foil system with the optimal phase differences instead of a ‘2S’ wake, a coherent wake pattern observed behind the optimal two-foil system. The finding suggests that a position-fixed three-foil system can generate a ‘2P’ wake to achieve the maximum thrust production and propulsive efficiency simultaneously by deliberately choosing the undulatory phase for each foil. When increasing Reynolds number to 1000, though the maximum thrust and propulsive efficiency are not achieved simultaneously, the most efficient case still produces more thrust than most of the other cases. Besides, the study on the effects of three-dimensionality shows that when the foils have a larger aspect ratio, the three-foil system has a better hydrodynamic performance, and it follows a similar trend as the two-dimensional (2D) foil system. This work aids in the future design of high-performance underwater vehicles with multiple controlled propulsion elements.

Unsteady uvula motions and the resultant pressure oscillations within the pharyngeal airway are critical for the pathology of snoring and sleeping apnea. In this paper, an immersed-boundary-method based direct numerical simulation flow solver was adopted to simulate the unsteady flows in an anatomically accurate pharynx model reconstructed from human magnetic resonance images (MRI) with prescribed uvula oscillation and airway obstruction. In order to study the influence of uvula length on the aerodynamics of pharyngeal airflow, simulations were conducted using various uvula models with scaled uvula lengths at 25%, 50%, 75%, and 100% of the original length, respectively. Analyses of vortex dynamics, pressure oscillations, and the aerodynamic force of uvula were conducted. It was found the length of uvula had significant impacts on vortex development as well as aerodynamic pressure/force. Shorter uvula induced weaker pressure oscillations and fewer vortices in the airway. Further fast Fourier transform analysis of pressures from different pressure probes showed higher-order harmonic waves other than the base frequency of uvula motion. This study is expected to bring understanding of snoring and sleep apnea and provide guidance for surgery.

The fluid dynamics of air flow in the pharynx is critical to the vibration of the uvula and to the generation of the snoring sound. In this work, a combined experimental and computational approach was conducted to study the aerodynamics of the flow field in the human airway. An anatomically accurate pharynx model associated with different uvula kinematics was reconstructed from human magnetic resonance images (MRI) and high-speed photography. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver was used to simulate the corresponding unsteady flows in all their complexity. Analysis has been performed on vortex dynamics and pressure oscillation at various points of interest. Computations with varying airway obstructions, uvula kinematics, and vibrating frequencies were conducted to study the effect of the three factors on the vortex formation and pressure oscillation, respectively. It is found that the vortex formation is significantly affected by the airway width. The fast Fourier transformation (FFT) analysis of the pressure time history revealed the existence of higher order harmonics of the base frequency at significant amplitudes. It was also found that the pressure tended to oscillate more violently at higher uvula vibrating frequencies. Results from this work are expected to bring novel understanding on the sound producing in patients with sleep apnea and provide guidance for surgical interventions.

Three-dimensional numerical simulations are used to investigate the hydrodynamic performance and the wake patterns of a sunfish in steady swimming. Immersed boundary method for deformable attaching bodies (IBM-DAB) are used to handle complex moving boundaries of one solid body (fish body) attached with several membranes (fins). The effects of the vortices shed from both the dorsal and anal fins on the hydrodynamic performance of the caudal fin are analyzed by prescribing an undulatory swimming kinematics to a full body sunfish model. Results show that both the dorsal fin vortices and the anal fin vortices can increase the thrust and efficiency of the caudal fin comparing to caudal fin only case. This is because the dorsal/anal fin not only can feed vorticity into the caudal fin wake via vortex shedding, but also can modulate the flow in the downstream in a way of forming a jet with stronger backward component.

Influence of the human body on a 5.8GHz wideband antenna is reported in this paper. Limited to 30×30 mm2 area on a PCBoard with er = 4.4, proposed antenna has two U-shaped, two F-shaped and two L-shaped slots as additional resonators to achieve wide-band operation. Measured-10 dB bandwidth is 2.68GHz (5.63 GHz~8.31 GHz). When the antenna applied to wireless body area network (WBAN) application, the characteristic of proposed antenna had been degraded to near to the-10 dB return loss. The performance of the designed antenna on body was characterized through simulations using the Finite Element Method and Method of Moment.