Orifice Aspect Ratio Research Articles

Numerical simulation of the effect of jet small orifice structure on cavitation characteristic and jet impact flow field

The jet impingement-negative pressure deamination reactor (JI-NPDR) is a continuous, non-clogging, and efficient deamination equipment characterized by its simple structure. Research has demonstrated that the fluid passing through the jet orifices in the jet impact section of the reactor leads to partial cavitation. This situation directly impacts the flow characteristics before jet impact and the gas-liquid separation behavior in the negative pressure separation section. Therefore, this work proposes to utilize the CFD numerical simulation method to analyze the fluid cavitation evolution rules under the aspect ratio l/d structure of different jet orifices and to investigate the influence of orifice structure on the jet impact flow field. The results indicate that an increase in the liquid flow rate and the small orifice aspect ratio l/d promotes the conversion of pressure energy to kinetic energy, leading to an increase in cavitation intensity in the jet orifice. Furthermore, the evolution behavior of the jet impact flow field can be effectively controlled based on the l/d structural parameter of the small orifice aspect ratio. The research results can provide fundamental data support for the structural optimization design and engineering application of the JI-NPDR.

Dynamics of bubbles detached from non-circular orifices: Confinement effect of orifice boundary

In practice, gas usually escapes from non-circular orifices due to machining accuracy, plugging, and other reasons, especially gas leakage in the natural environment. Compared to circular orifices, the bubble behavior and dynamics of non-circular orifices have rarely been investigated. This paper presents three-dimensional imaging system results to obtain comprehensive information on bubble dynamics generated from five elliptical orifices with equal cross-sectional areas and different aspect ratios. Computational fluid dynamics studies using the coupled level set and volume of fluid methods were adopted to assist in the flow field analysis. Experimental data were analyzed statistically to study the effects of orifice wall structure on bubble flow pattern, size, trajectory, and other dynamic parameters. The results show that the equivalent bubble diameter increases with increasing orifice aspect ratio, as does the randomness of the bubble flow field, while the bubble flow pattern complexity increases. A force analysis as the bubble is generated is also conducted. The wall structure of the orifice affects the bubble migration effect to some extent. The work presented in this article provides valuable fundamental information for a range of heater and desalination applications in chemical engineering, wastewater treatment, and immersion leakage identification.

AERATED CIRCULAR AND ELLIPTICAL LIQUID JETS IN A GASEOUS CROSSFLOW

In this study, the atomization of an effervescent atomizer with an elliptical and circular orifice was investigated experimentally in the gaseous crossflow. The shadowgraph technique was used to visualize the near field of the spray atomization. To measure the spray penetration height in crossflow, image processing was performed on the shadowgraph images based on two different approaches: (i) a threshold analysis of average spray images and (ii) standard deviation images of spray. By comparing these two approaches, it is found that the penetration heights obtained from the standard deviation images were much higher since these images were able to capture small droplets more accurately. Moreover, based on the standard deviation images of spray, an empirical correlation for the spray penetration height was developed as a function of gas-liquid mass flowrate ratio, liquid-air momentum flux ratio, orifice aspect ratio, and downstream location. The laser diffraction technique was used to analyze the particle size of the aerated elliptical and aerated circular jets in a gaseous crossflow. For each orifice shape, the effect of gas-liquid ratio and downstream location on the Sauter mean diameter (SMD) was studied. The results show that the aerated circular jet penetrates higher into the gaseous crossflow than the aerated elliptical jet. Besides, the aerated circular jet in crossflow mainly generates smaller drop sizes compared to the aerated elliptical jet.

Impact of assuming a circular orifice on flow error through elliptical regurgitant orifices: computational fluid dynamics and in vitro analysis of proximal flow convergence.

Grounded in hydrodynamic theory, proximal isovelocity surface area (PISA) is a simplistic and practical technique widely used to quantify valvular regurgitation flow. PISA provides a relatively reasonable, though slightly underestimated flow rate for circular orifices. However, for elliptical orifices frequently seen in functional mitral regurgitation, PISA underestimates the flow rate. Based on data obtained with computational fluid dynamics (CFD) and in vitro experiments using systematically varied orifice parameters, we hypothesized that flow rate underestimation for elliptical orifices by PISA is predictable and within a clinically acceptable range. We performed 45 CFD simulations with varying orifice areas 0.1, 0.3 and 0.5 cm2, orifice aspect ratios 1:1, 2:1, 3:1, 5:1, and 10:1, and peak velocities (Vmax) 400, 500 and 600cm/s. The ratio of computed effective regurgitant orifice area to true effective area (EROAC/EROA) against the ratio of aliasing velocity to peak velocity (VA/Vmax) was analyzed for orifice shape impact. Validation was conducted with in vitro imaging in round and 3:1 elliptical orifices. Plotting EROAC/EROA against VA/Vmax revealed marginal flow underestimation with 2:1 and 3:1 elliptical axis ratios against a circular orifice (< 10% for 8% VA/Vmax), rising to ≤ 35% for 10:1 ratio. In vitro modeling confirmed CFD findings; there was a 8.3% elliptical EROA underestimation compared to the circular orifice estimate. PISA quantification for regurgitant flow through elliptical orifices produces predictable, but generally small, underestimation deemed clinically acceptable for most regurgitant orifices.

Links between language, fluid dynamics, and the airflows that transport pathogens

Growing evidence shows that the airflows that accompany speech during social interactions contribute to the transport of pathogens, such as the SARS-CoV-2 virus. However, it is still elusive how the manners of articulation during speaking, such as time-varying airflow rate, and instantaneous teeth and lip movements, may affect the transport features of airflow exiting the mouth. We combine experimental and numerical approaches to investigate the flow patterns produced by representative vowels, such as /a/, /o/, and /i/, and consonants, such as /p/, /k/, /s/, and /h/, that have distinct articulatory characteristics. A 3D vocal tract is modeled with a temporally varying exit that captures key morphologic and kinematic features of the human vocal tract, including teeth and lips, during speaking. An incompressible flow solver based on a sharp-interface immersed-boundary-method (IBM) is employed to compute the resultant airflow. By comparing representative utterance pairs, like /apa/ and /aha/, we are able to isolate the effect of articulatory features and show significant differences in spatial-temporal patterns of airflow under the influences of a time-varying orifice, different orifice aspect ratios, and different relative lip-teeth positions. This work helps bring insight into the understanding of articulatory phonetics, and the links to different languages, from a fluid dynamics perspective.

Bicuspid aortic valve morphology and hemodynamics by same-day echocardiography and cardiac MRI.

This study investigated the impact of bicuspid aortic valve (BAV) on valve morphology and motion as well as proximal and aortic hemodynamics using a same-day echocardiography and cardiac MRI. Transthoracic echocardiography, two-dimensional cine MRI of the aortic valve, and aortic 4D flow MRI were performed on the same day in 9 normofunctional BAV patients (age = 41 ± 12, 3 female), 4 BAV with moderate to severe aortic stenosis (AS) (age = 63 ± 5, 1 female), and 36 healthy tricuspid aortic valve controls (age = 52 ± 10, 21 female). Valve opening and closing timings and transvalvular peak velocity were measured using B-mode and Doppler echocardiogram, respectively. Valve orifice morphology at a fully-opened state was characterized using cine MRI. Ascending aortic (AAo) wall shear stress (WSS) was measured using 4D flow MRI data. Valve motion timings were similar between BAV and controls. BAV was associated with an increased orifice aspect ratio (1.44 ± 0.11 vs. 1.10 ± 0.13, P < 0.001), transvalvular peak velocity (1.5 ± 0.3 vs. 1.2 ± 0.2m/s, P < 0.001) and maximum AAo WSS (1.62 ± 0.31 vs. 0.91 ± 0.24Pa, P < 0.001). The increased orifice aspect ratio was associated with the increase in transvalvular peak velocity (r = 0.80, P < 0.0001) and maximum AAo WSS (r = 0.83, P < 0.0001). Transvalvular peak velocity was also positively correlated with maximum AAo WSS (r = 0.83, P < 0.0001). A same-day echo and MRI imaging allows for a comprehensive assessment of the impact of aortic valve disease on valve function and hemodynamics. In this pilot application to BAV, we found increased orifice aspect ratio may be responsible for increased transvalvular peak velocity and maximum AAo WSS.

Dynamic mode decomposition of elliptical liquid jets in crossflow

The dynamics of round and elliptical liquid jets in subsonic crossflow was studied using high-speed imaging technique. The experiments were performed at constant gaseous Weber number and liquid–gas momentum flux ratios of 6.45 and 17.87, respectively, with orifices of different aspect ratios (AR) having an equivalent diameter of 0.43 mm. All of the cases were carried out inside an open-loop subsonic wind tunnel with a test section of 100 mm × 100 mm × 750 mm. For each case, dynamic modes were generated directly from the snapshots using a variant of the Arnoldi method known as the dynamic mode decomposition (DMD). The DMD results indicate that elliptical liquid jets have more small-scaled patterns with higher frequencies compared with the round liquid jets. As the first attempt to investigate the dynamics of elliptical liquid jets in crossflow, this study captures the dominant spatiotemporal structures. We also found that the orifice AR can significantly alter the jet wavelengths. The data from this study contribute to understanding the behavior of elliptical liquid jets exposed to gas crossflow in an enhanced capillary breakup regime.

Axis switching in low to moderate aspect ratio rectangular orifice synthetic jets

The behavior and performance of synthetic jets is closely tied to the dynamics of vortex rings, which are a dominant feature of the early jet. When a synthetic jet actuator employs a non-axisymmetric orifice, the self-induced deformations of the vortex rings are responsible for changes in the shape, momentum distribution, and entrainment rate of the jet. In this study, axis-switching of vortex rings in finite-span synthetic jets with a range of orifice aspect ratios was experimentally investigated using stereoscopic particle imaging velocimetry, and the results were compared to vortex dynamics simulations based on the Biot-Savart law to gain additional insight.

Asymmetric Bubble Formation at Rectangular Orifices.

Bubble formation in liquids is frequently observed in nature and applied in various industrial processes. These include pool and flow boiling for thermal management systems, where bubbles may form asymmetrically at narrow slits and in convective flows. While previous studies have focused on symmetric bubble formation at circular orifices, the dynamics of asymmetric bubble formation remains poorly understood. Here, we experimentally investigate bubble formation at rectangular orifices and examine the effects of the orifice size and aspect ratio and the gas flow rate on the bubble size. The asymmetric bubble shape evolution at the rectangular orifice is analyzed, and we find that the size of the bubble neck is controlled either by the orifice size or by the capillary length. Based on these findings, we develop a static force balance model to predict the bubble size in the quasi-static regime, where the roles of Bond number and aspect ratio are identified. The bubble size evolution in the dynamic regime is further understood by introducing a Weber number that evaluates the effect of the virtual mass force induced by gas flow. Our study provides physical understanding of the dynamics of asymmetric bubble formation and guidance to predict the bubble size at asymmetric orifices.

Numerical Modeling of Instability and Breakup of Elliptical Liquid Jets

Numerical simulations are performed to provide an in-depth insight into the effect of instabilities on liquid jets discharging from elliptical orifices. The axis-switching phenomenon and breakup are simulated and characterized under the effect of disturbances imposed at the nozzle exit. The simulations are based on the volume of fluid approach and an adaptive meshing. A range of orifice aspect ratios from 1 to 4 at the Rayleigh breakup regime is considered. The evolution of the jet cross section and axis switching under the influence of disturbances is compared with that of nonperturbed elliptical jets. It is found that the axis-switching repetition and breakup length exponentially decrease with the initial amplitude of disturbances. For a fixed amplitude, the optimum wavenumber responsible for minimum breakup length is observed to be dependent on ellipticity of the orifice. Moreover, it is found that the form of final breakup leads to elimination of the satellite droplets at the optimum wavenumbers, while the droplet size decreases and becomes more uniform.

Comparative study on interfacial oscillations of rectangular and elliptical liquid jets

The instability characteristics and flow structures of water jets injected from rectangular and elliptical nozzles with aspect ratios varying from 2 to 6 were experimentally studied and compared. Shadowgraph technique was employed for flow visualization, and structures on the liquid jet surface were captured using high speed photography. It was found that disturbances originating from the nozzle geometry initially perturbed the liquid column, and then, at high jet velocities, disturbances generated within the flow dominated the jet surface. It was also found that rectangular nozzles introduced more disturbances into the flow than the elliptical ones. The characteristic parameters of axis-switching phenomenon including wavelength, frequency, and amplitude were measured and compared. Axis-switching wavelength was found to increase linearly with Weber number. Also, the wavelengths of rectangular jets were longer than the elliptical jets. Further, the frequency of axis-switching was shown to be reduced with increase of both Weber number and aspect ratio. It was observed that the axis-switching amplitude increased monotonically, reached a peak, and then decreased gradually. It was also found that the axis-switching amplitude varied with Weber number. At lower values of Weber number, the rectangular nozzles had higher amplitude than the elliptical nozzles. However, at higher values of Weber number, this relation was reversed, and the elliptical nozzles had the higher axis-switching amplitudes. This reversal Weber number decreased with the orifice aspect ratio. The reversal Weber number for aspect ratio of 4 was about 289, and it had decreased to 144 for the aspect ratio of 6.

A Numerical Study of the Bifurcation of Rectangular Synthetic Jets

The present paper explores the mechanism of vortex formation and its bifurcation process for a rectangular orifice synthetic jet. The orifice aspect ratio is equal to two (7 x 14 mm2) with hydraulic diameter 9.33 mm and 3 mm thickness. A 3-D numerical simulation using Large-eddy -simulation (LES) model on Fluent software is performed to investigate the vortex formation and its bifurcation process. The diaphragm oscillates at amplitude of 1.6 mm and 2.6mm compromising a volumetric ejection of 1500mm3 and 2500mm3 respectively through orifice at 1 Hz of actuation frequency. The geometry of the cavity and the diaphragm displacement profile as well as the volumetric ejection is kept identical in both the experimental study and the numerical simulations. It has been observed that vortex ring bifurcates in both the cases and the simulation results are in good agreement with the experimental observations. In both the bifurcation cases, the vortex ring undergoes axial switching followed by bifurcation in the minor axis plane. Further the bifurcated rings moves at away from each other in the same plane at different divergence angle depending on the vortex ejection velocity during forward stroke of the diaphragm and its actuation frequency.

Computational investigation of flame extension height for horizontally oriented rectangular source jet fires impinging on a vertical plate

SummaryThis paper investigates the flame extension height induced by a horizontally oriented rectangular source fire impinging on a vertical plate, which has rarely been reported in literature. Focusing on the scenarios with nonimpinging flame lengths that are comparable with or larger than the plate‐source distance, a series of numerical experiments were conducted, in which the exit velocities varied from 27.5 to 125 m/s and the orifice aspect ratios ranged from 1 to 4. Results show that the flame extension height increases with increasing exit velocity and becomes larger with increasing orifice aspect ratio at a given fuel exit velocity. New correlations are proposed to predict the flame extension height based on the unburnt fuel after impinging, assuming a kite flame shape of circular and elliptical cross section. Moreover, the new proposed correlations provide a general and practical basis for estimating the flame extension height of horizontally oriented rectangular source impinging jet fires.

Temperature Profile of Thermal Impinging Flow Induced by Horizontally Oriented Rectangular Jet Flame Upon an Opposite Plate

The horizontally oriented jet flame induced by rectangular source impinging upon the opposite wall is actually common in the chemical industry, but the related studies are limited. In this paper, the computational fluid dynamics codes are carried out to investigate the temperature profile in thermal impinging flow of the horizontally oriented methane jet flame with rectangular source, which the rectangular orifice is 400 mm2 with three different aspect ratios (L/W = 1, 2, 4); besides, the jet velocities vary from 27.5 m/s to 125 m/s. As the horizontally oriented methane jet flame impinges on the vertical plate in front of the fuel orifice directly, the vertical temperature along the opposite plate is focused on. Results show that the temperature near the impingement point is the same for different jet velocities, but the temperature along the vertical direction is larger with increasing fuel jet velocity. Moreover, the orifice aspect ratio has a significant effect on the temperature, which increases with the aspect ratio at a given position for the momentum-controlled flame. The effective heat release rate on the basis of unburned fuel and ellipse flame shape hypothesis is put forward to correlate the temperature profile. Finally, a new correlation to illustrate the vertical temperature rising along the opposite plate is proposed in light of the orifice aspect ratio and fuel jet velocity, and the predictions obtained by the proposed model agree well with the numerical results, which is applicable for the horizontally oriented flame with rectangular source impinging upon the opposite wall.

Semi-empirical pressure loss model for viscous flow through high aspect ratio rectangular orifices

A predictive model is developed for the pressure loss coefficient for a viscous flow through a rectangular orifice on a pipe-installed thick plate. The model is developed based on the 1-dimensional Navier-Stokes equation and an asymptotic increase in velocity modeled to have a direct relation with the flow convergence in the near-inlet region. Here, the flow velocity increases asymptotically from the steady mean upstream value to the orifice velocity. This phenomenon is represented by a convergence parameter, ϕ, used in the velocity transition model to quantify the length of the convergence zone. The static pressure drop is measured experimentally for varying orifice aspect ratio, AR, at creeping Reynolds numbers (0.01 ≤ Re ≤ 0.1). A significantly wider range of AR is covered (1 ≤ AR ≤ 250), compared to related works in the literature. Results show that the relative dominance of the convergence phenomenon is affected by AR. The maximum length of convergence is for the square orifice (AR = 1), as the flow experiences comparable convergence from all directions, whereas for higher AR, convergence becomes less dominant in one of the two midplanes of investigation. The loss coefficient thus decreases as AR increases. At constant Re, higher AR generally leads to higher pressure drop but lower values of the loss coefficient. The velocity gradient in the convergence zone is also determined as a function of AR and Re which verifies that lower AR takes a longer distance for the velocity transition due to increased convergence.

On the flow physics and vortex behavior of rectangular orifice synthetic jets

Synthetic jet actuators possess a continuous jet like behavior in its far region and have found wide-scale engineering applications since it allows momentum transport to the flow system without any net mass transfer across the flow boundaries. The case of a non-axisymmetric synthetic jet is particularly significant since it is affected by the differential shear layer at the orifice exit, that depends on its aspect ratio. However, despite exhaustive research on both continuous and synthetic jets, very few studies have experimentally investigated the case of rectangular orifice synthetic jets, focusing on the effect of aspect ratio of the orifice as well as the actuation frequency upon the vortex behavior and the flow physics. In particular, the intriguing phenomenon of vortex bifurcation has mostly been reported only for an individual vortex or for a plain jet. Yet, in a train of vortex rings, such as that obtained in a synthetic jet, the occurrence of vortex bifurcation can be expected, although the flow physics in the wake of individual vortex rings is significantly different. The present study experimentally investigates a rectangular orifice synthetic jet at different orifice aspect ratios and actuation frequencies, focusing on exploring the conditions at which vortex bifurcation occurs, through LIF imaging and Hot-film measurements. The primary objective of these experiments is to provide a qualitative physical insight into the synthetic jet ejected from a rectangular orifice (through LIF imaging), as well as to quantitatively explore the experimental conditions that promote different flow structures (through velocity time trace, time-averaged velocity profiles and power spectral density measurements), particularly the bifurcation of vortex rings. Our experiments indicate that the phenomenon of vortex bifurcation is observed during the axial switching of vortex rings, but only in a narrow range of experimental conditions. Further, the velocity measurements have ascertained that the two prominent reasons behind this bifurcation process are a large disparity in the velocities of the vortex core and the center of vortex ring, as well as the time lag in which the separation distance between the counter-rotating vortices decrease gradually to zero.

Influence of Orifice Aspect Ratio on Formation and Noise Characteristics of a Cavitation Jet

Influence of Orifice Aspect Ratio on Formation and Noise Characteristics of a Cavitation Jet

Breakup of elliptical liquid jets in gaseous crossflows at low Weber numbers

Breakup of circular and elliptical liquid jets in subsonic gaseous crossflows is experimentally studied using shadowgraph technique. The experiments are performed at gaseous Weber numbers less than 15, the liquid-to-gas momentum flux ratio between 50 and 320, and the orifice aspect ratio between 0.22 and 4.47. It is found that in addition to the momentum flux ratio, the orifice aspect ratio can change the liquid penetration height significantly. For a fixed momentum flux ratio, the penetration of elliptical jets is less than that of circular jet. Moreover, for a given momentum flux ratio and Weber number, the column breakup location of elliptical jets is earlier than that of circular jet. Empirical correlations for the penetration height as well as the column breakup location of circular and elliptical jets are developed in this study.

Virtual and Physical Surface Modifications as Means for Flow Control

This paper reviews recent work on several flow control actuators, as well as a new hybrid actuator, and their interaction with a laminar boundary layer over a flat plate. The focus is on 1) a finite-span synthetic jet, where the effects of the jet orifice aspect ratio and its skew and pitch angles relative to the flow are presented; 2) a finite-span low-aspect-ratio static or dynamic pin (with similar height to the local boundary layer), where the effect of the pin’s aspect ratio is discussed; and 3) a hybrid actuator, which incorporates the previous two actuators. First, the formation and evolution of flow structures associated with a synthetic jet issued into a laminar boundary layer, where the effect of the orifice’s aspect ratio, as well as its pitch and skew angles, are explored. Next, the effects of the pin’s aspect ratio and its dynamic motion are shown. Finally, the interaction of the hybrid actuator with the flow has shown that the combined effects of the synthetic jet and the pin resulted in alteration of the vortical structures with minimal penalty. This suggests that the hybrid actuator might be efficient for controlling different types of flowfields. An example of controlling separation over a flapped airfoil was also included, where an array of passive cylindrical pins yielded delayed or complete mitigation of separation. This was mostly due to the introduction of streamwise vortices, which redirected the flow toward the surface and energized the boundary layer. Moreover, the effect of a single dynamic pin on flow separation was larger than that of a single static pin.

Validating a Reduced-Order Model for Synthetic Jet Actuators Using CFD and Experimental Data

Synthetic jet actuators (SJA) are emerging in various engineering applications, from flow separation and noise control in aviation to thermal management of electronics. A SJA oscillates a flexible membrane inside a cavity connected to a nozzle producing vortices. A complex interaction between the cavity pressure field and the driving electronics can make it difficult to predict performance. A reduced-order model (ROM) has been developed to predict the performance of SJAs. This paper applies this model to a canonical configuration with applications in flow control and electronics cooling, consisting of a single SJA with a rectangular orifice, emanating perpendicular to the surface. The practical implementation of the ROM to estimate the relationship between cavity pressure and jet velocity, jet velocity and diaphragm deflection and applied driving voltage is explained in detail. Unsteady Reynolds-averaged Navier Stokes computational fluid dynamics (CFD) simulations are used to assess the reliability of the reduced-order model. The CFD model itself has been validated with experimental measurements. The effect of orifice aspect ratio on the ROM parameters has been discussed. Findings indicate that the ROM is capable of predicting the SJA performance for a wide range of operating conditions (in terms of frequency and amplitude).