Airflow Oscillation Research Articles

Oscillatory airflow through the hypopharyngeal and supraglottic airway

Exercise-induced laryngeal obstruction (EILO) is a known cause of exertional dyspnoea, characterised by paradoxical inward collapse of laryngeal tissues. The pathophysiological mechanisms of EILO remain to be fully established, but insufficient mechanical resistance of laryngeal tissues to air-induced loads is hypothesised. It is understood that airflow and anatomic configurations of the airway play a key role in the wall pressure distribution of the larynx. While breathing is a cyclic process with directional changes of airflow, the literature is confined to steady, unidirectional airflow. It is necessary to assess the role of oscillatory airflow on the loads on the laryngeal airway. This study investigates the effect of oscillatory airflow on the laryngeal flow fields and air-induced loads. A computational fluid dynamics model of the upper respiratory tract (URT) is developed using the Reynolds-averaged Navier-Stokes equations. Five oscillatory airflow cases through a single geometry are considered, utilising sinusoidal breathing cycles with different breathing frequencies (24, 32 and 40 breaths per minute) and peak inspiratory flow rates (96, 168 and 240 L/min). Results include the airflow velocity distribution in the URT, and the air-induced pressure and forces. It is demonstrated that inspiratory velocity distribution varies with breathing frequency and intensity. The force acting on the URT walls are in-phase with the airflow rate and therefore exhibit quasi-steady behaviour. These findings are also reflected in the force vectors acting on the aryepiglottic folds and indicate that air-induced closure of the supraglottis in EILO is influenced by the breathing intensity rather than the breathing frequency.

Numerical simulation study on the effect of internal heat source on pumping ventilation

Natural ventilation is a passive building technology that can reduce energy consumption for mechanical cooling of the indoor environment. Natural ventilation technology has received renewed attention from researchers. Theoretical studies on natural ventilation have evolved from a steady state to a non-steady state. Pumping ventilation is distinct from other ways of natural ventilation in that its flow direction alternatingly changes from the indoor to the outdoor. However, all published studies did not consider the influence of internal heat sources in pumping ventilation, and there are few discussions about the fluid periodic variation in wind-thermal coupling studies. Based on a previous study, the numerical simulation of single-sided pumping ventilation with two openings in the same wall driven by wind and thermal pressure is conducted in the paper. The influence of heat source intensity and the height of the location of the heat source are analyzed. The applicability of different ventilation rate calculation formulas is discussed. The results show that the indoor airflow oscillation frequency decreases in the effect of wind and thermal pressure, compared to driven by air pressure alone. However, changing the height location and intensity of the heat source does not affect the frequency characteristics in these cases.

Hysteresis of oscillatory airflow in a supersonic intake model

Hysteresis of oscillatory airflow in a supersonic intake model

Microscale damper prototype: A preliminary study on suppressing air flow oscillations within microchannels

This research introduces a novel micro-damper designed to mitigate pressure and velocity oscillations from a piezoelectric micropump in microfluidic environments. Unlike existing research focusing on damping in incompressible liquid flows with methods like elastic films and PDMS membranes, this study proposes a novel micro-damper prototype. Integrated into a microdevice for particle granulometric separation and detection, the damper connects to a piezoelectric micropump outlet and to a focusing microchannel inlet, followed by a capacitive sensor for size-based particle counting. Preliminary analysis determined an optimal airflow velocity at w = 0.5 m/s for accurate focusing and counting under laminar conditions. The micro-damper, constrained by the piezoelectric pump’s geometry, features a 27 µm high and 1000 µm wide cross section. Its outlet supports two potential focusing microchannel inlet configurations of 30 µm or 40 µm. Distinctively, it incorporates two symmetrical backward micro-channels connecting to the atmosphere, allowing direct piezometric contact between the main flow and an infinite compliant volume. OpenFOAM simulations confirm the damper’s effectiveness in maintaining laminar outlet flow and suppressing micropump disturbances. Thus, the proposed micro-damper ensures optimal inlet conditions for subsequent microchannel processes, enabling stable particle separation and detection in controlled airflow samples.

Numerical Simulations of Vertical Discrete Gusts Driven by Trailing Edge Blowing

A new type of gust generator generates the airflow oscillation in the wind tunnel through the Coanda effect of the unsteady trailing edge blowing, which has been shown to have strong potential for accurately simulating discrete gusts. It is necessary to study the relationship between the generated gust characteristics and the control parameters of such devices in order to optimize the design performance and improve gust simulation capabilities. By solving the compressible unsteady Reynolds-averaged Navier–Stokes (URANS) equations, the computational fluid dynamics model of the subsonic airflow past the gust generator in the wind tunnel was presented. The effects of jet momentum, frequency, and spanwise blowing ratio on gust intensity, shape, and spatial uniformity were investigated. Results indicate that the intensity of gusts is positively correlated with jet momentum and frequency. The gust shape matches well with the normalized jet momentum coefficient curve. However, when the frequency increases to above 10 Hz, the gust shape differs significantly from expectation due to the appearance of reverse wave peaks. In addition, the mechanism of the impact of the sidewall and partial spanwise blowing on gusts was revealed. In the three-dimensional situation, streamwise vortices are formed on the sidewall and at the spanwise position where the blowing stops, respectively. This results in an increase and noticeable nonuniformity in gust amplitude. When the blowing with a 15% spanwise length near the sidewall is turned off, the gust amplitude at the symmetry plane increases by nearly 40% due to the main vortex being closer to the main flow. The result provides a physical explanation for the availability of this operation to reduce gust attenuation.

The Numerical Simulation Study of Pumping Airflow Driven by Wind Pressure for Single- and Multi-Room Buildings

Pumping airflow occurs in single-sided natural ventilation buildings when the openings are located on the leeward side; the direction of airflow through the building then changes periodicity. In order to propose a calculation method of the ventilation rates for single-room buildings and analyze the pumping ventilation for multi-room buildings, the CFD method with an SST k-ω turbulence model was used to conduct numerical simulation in this study, which is verified by other experimental results. Firstly, the qualitative and quantitative characteristics of pumping ventilation were investigated for a single-room building with two openings. The results show that the steady-state method underestimates the ventilation flow rates, and the unsteady-state method captures the microstructures of the flow better. Secondly, the vortex-shedding and indoor airflow oscillation frequencies were analyzed based on transient simulation for a single-room building. It was found that both of them increase with air speed. Then, the factors affecting ventilation flow rates were analyzed. A calculation method for dimensionless ventilation rates is proposed. Finally, the pumping ventilation for a multi-room building was numerically simulated. The ventilation rate for a middle room is greater than that for a corner room, and the ventilation rate for any room in a multi-room building is greater than the single-room building given the same room size and the same incoming wind speed. The findings of this paper are helpful for the design and evaluation of natural ventilation.

Mechanisms for improving fin heat dissipation through the oscillatory airflow induced by vibrating blades

The vibrating fan is one of the promising candidates to alleviate the thermal failure of electronic devices, while its coupling mechanism in actual applications is not clear yet. In this work, the heat dissipation of vibrating fans was experimentally examined, and its inherent mechanism was demonstrated through the comparison between steady and pulsating flows based on numerical simulation. Firstly, the flow fields induced by the vibrating blades exhibit pulsating features, but the localized velocity magnitude is much higher than the pulsating flow. Besides, the large-scale vortical structures are first formed at the blade tip and then squeezed by the fins into various small-scale vortices flowing into the channels. Thus, the heat dissipation is significantly improved. Secondly, the impacts of different blade structures are identified. The trapezoidal structure can drive more airflow due to its large swept area, and the grooved structure can benefit the formation of small-scale vortices. Then, the trapezoidal-folding blade is developed as a combination of trapezoidal and grooved structures. It is found that the trapezoidal-folding blade can decrease the maximum temperature of the heat source by 7.5 °C compared to the traditional steady flow condition, which can effectively alleviate the high-temperature failure crisis.

A theoretical calculation method for critical air velocity to prevent methane draft pressure-caused airflow reversion based on oscillation theory

Methane draft pressure is a secondary disaster for mine ventilation following coal and gas outbursts, which poses a long-term threat to coal mining and workers’ safety. To study the law of methane draft pressure-caused airflow variation, oscillation theory is introduced in this study. The vibration equation is used to deduct the basic equation of airflow oscillation in parallel upward ventilated airways, which regards the entire methane and air as one vibrating object in the loop of the parallel airways. The theoretical critical air velocity to prevent airflow reversion is calculated by the airflow oscillation equation using numerical methods. Furthermore, through dimensional analysis, a formula is given to calculate the critical velocity directly. The theoretical results were compared with experiments and good agreements were obtained, which verifies the rationality of the assumptions in the theoretical deduction process. The conclusion of this study indicates that, in a short time, oscillation theory can be used to predict the critical air velocity to prevent airflow reversion, and be feasible in revealing the essence of methane draft pressure-caused airflow reversion.

Influence of vortical flow induced by a wall-mounted short cylinder with a streamlined front side on a horizontal air–solid two-phase flow

To induce large oscillations of air flow and suspend particles easily near the particle supply point in a low air velocity region of horizontal air-solid two-phase flow, a short cylinder model with a streamlined front side (refer to streamlined model) is proposed and is fixed on the bottom or top of the pipe in front of the particle feed. The streamlined front side of the model is designed to reduce the flow resistance, and the rectangular rear section of the model generates wake vortical flow so as to induce large airflow oscillation. The test tube consists of a horizontal pipe having an inner diameter of 80 mm and a length of 5 m. Spherical polyethylene particles with an average diameter of 2.3 mm and density of 978 kg/m3 were used as the test solid particles. Average air velocities and particle mass flow rates ranged from 9 to 16 m/s and 0.11 to 0.51 kg/s, respectively. Compared with the non-model air-solid two-phase flow, the streamlined model has reduced the pressure drop, critical and minimum conveying air velocities, power consumption, and additional pressure drop. This model achieved the highest reduction rate with a minimum air velocity of approximately 6% and an additional pressure loss of approximately 37.5%. According to particle image velocimetry measurement of air-solid two-phase flow, the time-averaged particle velocity and fluctuating particle velocity generated by the streamlined model are larger than those in the non-model flow, so even low air velocities can easily convey particles.

Multi-scale analysis on particle dynamics in horizontal pneumatic conveying with oscillating air flow

This study focuses on multi-scale particle dynamics of horizontal pneumatic conveying equipped with two types of soft fins at the air velocity of the minimum pressure drop. The particle velocity fields of non-fin and using soft fin's cases are measured by high-speed particle image velocimetry. Then the particle fluctuation velocities are decomposed into various scales based on one-dimensional orthogonal wavelet decomposition. It is found that the non-uniform case of Fin260 yields the highest axial- and vertical- particle velocities and fluctuation energies, giving rise to the lowest pressure drop and conveying air velocity. The overall relative energy distributions of wavelet components suggest that the particle fluctuation energies become more concentrated at wavelet levels 3 and 4 by using fins, especially for the case of non-uniform Fin260. The energy concentration on specific wavelet levels indicates a more organized particle motions under the effect of air turbulence induced by non-uniform soft fins. The correlation analysis suggests that the spatial correlation of wavelet component is associated with the contribution of wavelet component to the measured particle fluctuation energy. The oscillating soft fins modulate the air turbulence and promote the particle motions to be spatially correlated in a range of specific frequency.

Theoretical and Experimental Study on Rapid Decompression Oscillation in Altitude Chamber

Given the frequent airflow excitation phenomenon caused by the rapid decompression of the altitude chamber, the mathematical model of the nonlinear system of the rapid decompression of the altitude chamber is established. The polynomial parameter method is used to evaluate the characteristics of airflow oscillation, and a rapid decompression test system is built. The test results verify the pressure oscillation phenomenon of the numerical simulation. This paper proves the phenomenon of fluid-induced vibration in the process of rapid decompression and determines that the main factors affecting the induced oscillation are the diameter of the pipe (throat), pressure difference between the two chambers, and initial pressure conditions. Specifically, this study establishes safety and reliability for preventing engineering accidents caused by resonance in the altitude chamber.

Feedback effects of pesticides flow atomization on the operating characteristics of pulsed engines

Traditional pulse-jet fogger have problems such as poor atomization or unstable operation. A pulse-jet fogger test device with an adjustable liquid flow rate was designed. Four experimental settings were used, no spraying and spraying with three liquid flow rates (11 L/h, 14 L/h, and 20 L/h). Pressure waveform in the combustion chamber and smoke inhalation depth tests showed following results: under different chemical fluid flow conditions, influences of the thermal atomization of the chemical liquid were observed in the pulse engine. Feedback effects were mainly changes in parameters and during each pulsation period, the turning time of the airflow in the pulse engine had different effects. The fuel consumption rate and pressure peak-to-peak value under 20 L/h chemical fluid flow were higher than other working conditions, and the positive and negative pressure amplitude fluctuated the least and had the best pressure stability. The 14 L/h chemical liquid flow demonstrated the worst stability, with the most significant fluctuations in pressure amplitude and large cyclical changes and poor consistency of the maximum depth of smoke inhalation in each cycle in the atomization section, with a high maximum relative difference of 34.6%, causing significant turbulence in the airflow oscillation system.

Numerical investigation of the response of turbulent swirl non-premixed flames to air flow oscillations

The response of swirl non-premixed flames to air flow oscillations is studied using Large-Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model, focusing on the physical mechanisms leading to the heat release rate oscillations observed in a parallel experimental study. Cases relatively close to blow-off and characterized by different amplitude of the flow oscillations are considered. Numerical results are in good agreement with the experiment in terms of both mean flame shape and heat release rate response. Simulations show that the oscillation of the air flow leads to an axial movement and fragmentation of the flame that are more pronounced with increasing amplitude of the forcing. The flame response is characterized by fluctuations of the flame area, time-varying local extinction and lift-off from the fuel injection point. LES-CMC, due to the inherent capability to capture burning state transitions, predicts properly the flame transfer function as a function of the amplitude of the air flow oscillations. This suggests that the response mechanism for this flame is not only due to time-varying flame area, but also local extinction and re-ignition. This study demonstrates that LES-CMC is a useful tool for the analysis of the response of flames of technical interest to large velocity oscillations and for the prediction of the flame transfer function in conditions close to blow-off.

Effect of Sparger Characteristics on Bubble-Formation Dynamics under Oscillatory Air Pattern

The size of bubbles generated from a porous sparger is at least an order of magnitude larger than the pore diameter under the steady air pattern. Recent studies have shown that the bubble size can be significantly reduced when the steady air flow is replaced by an oscillatory pattern. However, the effectiveness of oscillatory air flow on reducing bubble size under different sparger characteristics is yet to be studied. This work fundamentally investigates the response of bubble size to sparger characteristics under an oscillatory air pattern by segregating the bubble formation subprocesses into bubble detachment and consequent coalescence. The results show that bubble size significantly decreased with hydrophilic plates regardless of contact angle, owing to depressed coalescence, while no impact was observed for a hydrophobic plate. The influence of the oscillatory air pattern on decreasing bubble size was weakened as the chamber volume was increased, and above a critical volume the bubble-formation process became similar with that under a steady air pattern. An optimum plate thickness was obtained for bubble generation by avoiding weeping, and meanwhile taking full advantage of the momentum force by the oscillatory airflow. The outcomes show that the oscillatory air pattern in determining bubble formation closely depends on the sparger characteristics, which should be appropriately determined.

Extreme Sawtooth-Sign in Motor Neuron Disease (MND) suggests Laryngeal Resistance to Forced Expiratory Airflow.

The impact of laryngeal dysfunction on airflow has not been well characterized in motor neuron disease (MND). This study aimed to detect and characterize extreme airflow oscillations informally observed during volitional cough and forced vital capacity (FVC) tasks in individuals with MND who demonstrated neurolaryngeal impairments including reduced speed and extent of vocal fold abduction compared to healthy controls during volitional cough expulsion. The extreme airflow oscillations in the MND group, when viewed as a flow-volume loop, appeared similar to the "sawtooth-sign." If the airflow oscillations are periodic in a range similar to phonation, they may reflect reduced laryngeal patency. Volitional cough and FVC airflow data (3 trials each) from 12 participants with MND with bulbar/laryngeal involvement (3 F; ages 45-76) and 12 healthy controls (6 F; ages 41-68) were analyzed for periodicity. Percent and absolute durations of periodicity of the flow oscillations were calculated by an algorithm applied to the airflow signals. In addition, the frequency, magnitude, and kurtosis of the periodic airflow oscillations were described and compared between groups. In both volitional cough and FVC trials, the percent of airflow periodicity during forced expiration was significantly higher (z = 3.54) in individuals with MND, adjusted for age and sex. Periodic airflow accounted for on average 28% of the total time in participants with MND and was within a frequency range similar to phonation. Magnitude of the airflow oscillations was also larger for participants with MND (z = 3.46), and kurtosis of airflow was smaller (z = -4.70) during forced expiration, indicating persistent airflow oscillations throughout exhalation. The significantly larger-magnitude, lower-kurtosis, and more prominent presence of sawtooth-like airflow periodicity within a frequency range similar to phonation observed in individuals with MND with neurolaryngeal impairments suggests glottic airflow resistance during forced expiration.

Investigation of the quasi-periodic airflow oscillation and ventilation performance of an enclosed environment with a jet air supply

Unsteady airflows lead to better mixing in ventilated enclosures, facilitating indoor pollutant dispersion and exhausting. In this study, an air jet ventilated enclosure and its flow characteristics were investigated using experimental measurements and computational fluid dynamics simulations, focusing on the quasi-periodic fluctuation of the airflow field and the advantages of such an oscillation pattern for the ventilation efficiency. Starting from a non-converged Reynolds-averaged Navier–Stokes (RANS) simulation, we then used the unsteady RANS method (URANS). As validated by sampling the high-frequency velocity data of representative positions in the jet-ventilated space, the URANS results reflected the unsteady flow characteristics more precisely than the RANS results, further confirming that the supply jet air swung up and down quasi-periodically. The ventilation performance of such an actual quasi-periodic airflow was evaluated by purging a gaseous pollutant from the enclosure, and the results were compared with those in the average steady-state airflow field to showcase the different performance of the unsteady ventilation system.

Spray response on a model prefilmer under unsteady airflows of various frequencies

An appealing concept for jet engine combustors is the Lean Premixed Prevaporized (LPP) combustor, which operates at high pressures. The low NOx emissions achieved by lean combustion are one of the targets for modern aircraft engines. However, these types of combustors can introduce thermoacoustic instabilities that can potentially damage the engine and reduce its lifespan. Since the potential instabilities on the fuel spray characteristics, i.e. the spray mass flux, can affect the flame stability, the need arises to investigate the spray response under an unsteady airflow. For this study, a model prefilmer was experimentally investigated to produce a two-dimensional droplet flow without swirl flow. An acoustic forcing in the range of 100–500 Hz was introduced into the airflow, characterized by a hot wire Constant Temperature Anemometry (CTA) setup. Droplet characteristics, namely the droplet diameter distribution and velocity, were determined using a Phase Doppler Anemometry (PDA) setup, while the acquired data were phase-averaged in one period of the airflow oscillation. The influence of the excitation frequency and the air-to-liquid ratio (ALR) on the spray was studied: the spray responded to the acoustic excitation and therefore critical performance parameters, such as the spray mass flux, oscillated indicating potential problems regarding the flame stability.

Three-dimensional numerical simulation of air-flow in inkjet print-zones

Air-flow oscillations in inkjet print-zones give rise to undesirable print defects due to misplacement of low Stokes number satellite droplets. The problem is studied numerically using a novel dispersed-phase continuum method, that, due to the separation of length scales, permits the force exerted by the multitude of high Stokes number main droplets to be modelled as a continuous smooth field. The oscillations seem to be linked to the deformation of the primary vortex upstream of the print-zone. It is also shown that introducing extra flow in the direction of the paper motion is an effective remedy for the oscillations.

Using the lattice Boltzman method and GPU computing to model flow in organ pipes

The lattice Boltzman method (LBM) has been used to study oscillating air flow in musical instruments since the early 1990s. Most of the work has focused on flow into organ pipes and flow behavior around the instrument’s labium. The current work intends to use GPU computing to simulate oscillations in 3D modeled and printed organ pipes of different cross-sectional geometries. These pipes will be used in experimental validation measurements of flow at the pipe exit (see TCMU general session for this work). The domain space required to visualize each outlet of air (around the labium and the pipe’s open end) simultaneously with a sufficient resolution was larger than could fit in our GPU's memory. The resolution and space requirements of the simulations has required the use of high-memory GPUs (a physical machine as well as Google Cloud GPU have been tested). The physical and computational parameter optimization, GPU processes, and the simulation difficulties encountered will be discussed. Finally, preliminary comparisons between experiment and model will be shared.

Bubble Size in a Flotation Column with Oscillatory Air Supply in the Presence of Frothers

ABSTRACT Oscillatory air supply has demonstrated advantages over conventional steady air supply in froth flotation with diffused aeration, but the interaction between oscillatory air supply and frother in determining bubble size remained unclear. In the present work, we measured the size of air bubbles in a laboratory scale flotation column with common frothers such as MIBC, Dowfroth 250, and polypropylene glycol with an average molecular weight of 425 (PPG 425), with conventional steady air supply and with oscillatory air supply. The oscillatory air supply was converted from steady airflow using a fast-switching solenoid valve with the main operating parameters being switching frequency and on/off time ratio. In the absence of frother, the Sauter mean bubble diameter (d 32) remained the same with oscillatory air supply replacing steady air supply, but the d 32 decreased by 13–35% in the presence of frother. It was also found that the critical coalescence concentration (CCC) of each frother were reduced by 11–69% using oscillatory air supply to replace steady air supply. The degree of reduction in d 32 or the CCC appeared to be dependent on the oscillatory airflow condition (i.e., frequency and on/off time ratio) and frother type. The present work also confirms the recent findings by others that the CCC is not solely a material property of frothers.