Rayleigh Angle Research Articles

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Metamaterial control of the surface acoustic wave streaming jet

The phenomenon of surface acoustic wave (SAW) streaming, where a streaming jet is created, occurs when an SAW propagating on the surface of a solid interacts with water, and underpins the increasingly important area of SAW microfluidics. A key characteristic of the streaming jet is the Rayleigh angle, i.e. the angle at which the jet is formed relative to the surface normal of the solid, which is determined by the ratio of the velocity of the acoustic wave in the fluid and in the solid. Although the ability to dynamically tune this angle would offer a novel tool for microfluidic control, the SAW velocity is normally fixed by the characteristics of the solid and liquid material properties. In this paper we show, using finite element method modelling, that changing the SAW Rayleigh wave phase velocity by patterning a metamaterial array, consisting of square annular holes, onto the surface of an SAW device can change the acoustic streaming Rayleigh angle by approximately a factor of two, in good agreement with calculations based on the change in velocity.

A set of generic correlations for high-Rayleigh-number thermal convection in inclined rectangular enclosures

A detailed review of the high-Rayleigh-number thermal convection literature for inclined enclosures indicates that correlations proposed thus far for the response variable, Nusselt number, have been either for a particular angle of inclination or fail to include all control parameters for the problem. The present study proposes a set of empirical correlations to predict Nusselt number for turbulent thermal convection in differentially-heated, tilted, rectangular enclosures of different aspect ratios (1, 3, 6, and 10) and angles of inclination (0°–150°) for a given range of Rayleigh numbers (1.85×106≤Ra≤1.04×1011). This study utilizes previously published experimental data to devise a set of generic correlations for Nusselt number in terms of Rayleigh number (Ra), aspect ratio (AR), and angle of inclination (θ) of the rectangular enclosure for any given Prandtl number (Pr). Predicted Nusselt numbers obtained from these generic correlations yield a mean absolute percentage error of ∼5% when compared to the corresponding experimental results. Additionally, the predicted values are also compared with literature-reported values for studies pertaining to control parameters outside the scope of the present study and found to be in good agreement, with a maximum deviation of ∼10%.

Rayleigh angle incident ultrasonic beam shape design influence on reflected beam

Rayleigh angle incident ultrasonic beam shape design influence on reflected beam

Residue-free acoustofluidic manipulation of microparticles via removal of microchannel anechoic corner

Surface acoustic wave (SAW)-based acoustofluidics has shown significant promise to manipulate micro/nanoscale objects for biomedical applications, e.g. cell separation, enrichment, and sorting. A majority of the acoustofluidic devices utilize microchannels with rectangular cross-section where the acoustic waves propagate in the direction perpendicular to the sample flow. A region with weak acoustic wave intensity, termed microchannel anechoic corner (MAC), is formed inside a rectangular microchannel of the acoustofluidic devices where the ultrasonic waves refract into the fluid at the Rayleigh angle with respect to the normal to the substrate. Due to the absence of a strong acoustic field within the MAC, the microparticles flowing adjacent to the microchannel wall remain unaffected by a direct SAW-induced acoustic radiation force (ARF). Moreover, an acoustic streaming flow (ASF) vortex produced within the MAC pulls the particles further into the corner and away from the direct ARF influence. Therefore, a residue of particles continues to flow past the SAWs without intended deflection, causing a decrease in microparticle manipulation efficiency. In this work, we introduce a cross-type acoustofluidic device composed of a half-circular microchannel, fabricated through a thermal reflow of a positive photoresist mold, to overcome the limitations associated with rectangular microchannels, prone to the MAC formation. We investigated the effects of different microchannel cross-sectional shapes with varying contact angles on the microparticle deflection in a continuous flow and found three distinct regimes of particle deflection. By systematically removing the MAC out of the microchannel cross-section, we achieved residue-free acoustofluidic microparticle manipulation via SAW-induced ARF inside a half-circular microchannel. The proposed method was applied to efficient fluorescent coating of the microparticles in a size-selective manner without any residue particles left undeflected in the MAC.

Numerical Study of Inclination Effect of the Floating Solar Still Fitted with a Baffle in 3D Double Diffusive Natural Convection

A three-dimensional computational study of double-diffusive natural convection was conducted to explore the impact of tilt on the thermal and solutal performance of a floating pyramidal solar still filled with an air-steam mixture. In the present work, the still is cooled from the upper walls and is maintained at a low vapor concentration. The bottom wall of the still is maintained at a hot temperature and high concentration. Four different models of baffles placed in the upper region of the solar still have been studied. The mathematical formulation of the equations governing the problem is based on the vector current potential -vorticity formalism. The numerical method of finite volumes is used. The effect of Rayleigh and tilt angle of the floating solar still on the flow structure, iso-temperatures, iso-concentrations, and heat and mass transfer rates were examined. The most relevant results of this study are (i) an uncooled air-vapor mixture outlet was observed during tilting for the solar still equipped with a small, flat baffle, (ii) triangular and curvilinear baffle configurations are suitable to guide the air-vapor mixture towards the cold walls during the tilting of the solar still floating on the sea, and (iii) the triangular baffle configuration is the most relevant design, enhancing thermal and solutal performance by 20%.

NDE Characterization of Surface Defects on Piston Rods in Shock Absorbers Using Rayleigh Waves

In general, shock absorbers are components that can absorb shock and vibration energy caused by wheel behavior, and they provide handling stability. As a piston rod is an important component in shock absorbers, multiple processes are performed in order to guarantee its quality during manufacturing. Micro-defects can be generated on the surfaces of piston rods after processing. Because these defects can degrade the function of shock absorbers, proper non-destructive techniques are necessary to monitor the surfaces of piston rods. In this study, micro-defects were artificially machined on the surfaces of piston rods. In particular, a Rayleigh wave technique was adopted to detect defects on the surfaces of the piston rods, and Rayleigh wave behaviors were analyzed to establish beam profiles. In terms of the experimental method, defects were fabricated on the piston rods, and the optimal Rayleigh angle was determined using the pulse-echo method with ultrasonic transducers in a water tank. This was performed to evaluate the characteristics of the Rayleigh waves. In testing, regardless of the types of micro-defects on the surfaces of the pistons, it was found that the optimal inspection condition could be in the range of 5–10 mm, where ultrasonic signals were received with a high resolution. Moreover, the behaviors of the transmitted Rayleigh waves were simulated, and reflection, transmission, and scattering occurred due to defects at the interface between the water and steel. Thus, the propagation of Rayleigh waves and the optimal test conditions were implemented through FEM simulation to generate effective Rayleigh waves.

Heat transfer enhanced by angle-optimized fan-shaped porous medium in phase change thermal energy storage system at pore scale

High-thermal-conductivity porous medium can effectively improve the heat transfer rate of solid-liquid phase change in thermal energy storage system, enhancing the usage efficiency of renewable energy. In this paper, the fan-shaped porous medium was applied to accelerate heat transfer rate of phase change material in thermal energy storage, and the corresponding heat transfer performance affected by Rayleigh number, porosity and angle of porous medium during melting and solidification process was investigated. The results showed that the porous medium can reduce heat accumulation and improve heat transfer rate. Compared with the pure PCM, the complete melting time was shortened by 51.2% after inserting 150° porous medium with porosity of 0.5, when the Rayleigh number was 105. In addition, there was an optimal angle according to the melting rate and energy storage density. When the Rayleigh number was 5 × 105, the optimal angle of porous medium is 180°. Besides, the heat transfer effect was significantly improved by the porous medium with larger angle during solidification process.

Using artificial neural network to optimize the flow and natural heat transfer of a magnetic nanofluid in a square enclosure with a fin on its vertical wall: a lattice Boltzmann simulation

The heat transfer of a nanofluid in a square enclosure is numerically simulated using FORTRAN software in this paper, by considering the radiation (Rad) effect. The enclosure is under a magnetic field (MaF) and can also deviate from the horizontal axis. The enclosure has insulated top and bottom walls, as well as hot and cold right and left walls. There is a fin (0.1 × 0.4) with constant thermal conductivity on its hot wall. Entropy generation (EnG) is investigated as one of the important results in this paper. The lattice Boltzmann method is used for the simulations. A sensitivity analysis is performed on three parameters; including Rayleigh (Ra), Hartmann (Ha), and enclosure angle are introduced as the most effective parameters. Finally, based on these parameters, the performance of the cavity was simulated using an artificial neural network. The results show that the Nusselt (Nu) and EnG increase with the Ra and decrease with the Ha. The parameters mentioned above are subjected to optimization, indicating that the maximum Nu which is equal to 22.24 corresponds to the Ra of 992,409, the Ha of 0.004, and the inclination angle of 34.74°.

Thermal convection in an inclined cavity under the influence of partial magnetic field

This work is focused on the implication of a partial magnetic field on a square cavity heated isothermally at the bottom wall and cooled at the sidewalls. The top wall is insulated. The enclosure is filled with electrically conducting fluid. The coupled transport equations are solved numerically using a written computing code adopting FVM and SIMPLE algorithm applying specific boundary conditions. The analysis is carried out for different involved parameters like Rayleigh number (Ra), Hartmann number (Ha), and angle of inclination (γ). The results are illustrated using contours of streamline, isotherms, and heat transfer characteristics. It is found that the flow structure, the temperature distribution is severely affected by the above-mentioned parameters. The rate of heat transfer is significantly reduced with the increasing Ha.

Droplet Ejection at Controlled Angles via Acoustofluidic Jetting.

We study the nozzle-free ejection of liquid droplets at controlled angles from a sessile drop actuated from two, mutually opposed directions by focused surface acoustic waves with dissimilar parameters. Previous researchers assumed that jets formed in this way are limited by the Rayleigh angle. However, when we carefully account for surface tension in addition to the driving force, acoustic streaming, we find a quantitative model that reduces to the Rayleigh angle only when inertia is dominant, and suggests larger ejection angles are possible in many practical situations. We confirm this in demonstrating ejection at more than double the Rayleigh angle. Our model explains the effects of both fluid and input parameters on experiments with a range of liquids. We extract, from this model, a dimensionless number that serves as an analog for the typical Weber number for predicting single droplet events.

Standing surface acoustic waves, and the mechanics of acoustic tweezer manipulation of eukaryotic cells

Manipulation by focused ultrasound is an emerging technology with much promise for non-contact handling of microscale objects. A particularly promising approach for achieving this with living cells involves incorporating standing surface acoustic waves (SSAWs) into a microfluidic device. SSAWs must be tuned to provide the necessary range of acoustic radiation force (ARF), but models enabling this tuning have neglected the mechanics of the cells themselves, treating cells as rigid or homogenous spheres, and have also neglected energy transfer from the substrate to the fluid at the Rayleigh angle. We therefore applied Mie scattering theory to develop a model of the ARF arising from a SSAW impacting an idealized eukaryotic cell in an inviscid fluid. The cell was treated as a three-layered body with a nucleus, cytoplasm, and cortical layer. Results showed strong dependence on cell structures and the Rayleigh angle that can be harnessed to develop novel applications for cell manipulation and sorting. ARF can be tuned using the new model to both push away and pull back a cell towards the sound source. The proposed analytical model provides a foundation for design of microfluidic systems that manipulate and sort cells based upon their mechanical properties.

Wide range of droplet jetting angles by thin-film based surface acoustic waves

Nozzleless jetting of droplets with different jetting angles is a crucial requirement for 2D and 3D printing/bioprinting applications, and Rayleigh mode surface acoustic waves (SAWs) could be a potential technique for achieving this purpose. Currently, it is critical to vary the jetting angles of liquid droplets induced by SAWs and control the liquid jet directions. Generally, the direction of the liquid jet induced by SAWs generated from a bulk piezoelectric substrate such as LiNbO3 is along the theoretical Rayleigh angle of ∼22°. In this study, we designed and manufactured thin-film SAW devices by depositing ZnO films on different substrates (including silicon and aluminium) to realize a wide range of jetting angles from ∼16° to 55° using propagating waves generated from one interdigital transducer. We then systematically investigated different factors affecting the jetting angles, including liquid properties, applied SAW power and SAW device resonant frequency. Finally, we proposed various methods using thin-film SAW devices together with different transducer designs for realizing a wide range of jetting angles within the 3D domain. A nozzleless jetting method is proposed using thin-film based surface acoustic wave devices to achieve a wide range of jetting angles for droplets.

Acoustic mixing in a dome-shaped chamber-based SAW (DC-SAW) device

The use of an open droplet system for surface acoustic wave (SAW)-based applications has been limited by droplet instability at high input power. This study introduces a dome-shaped chamber-based SAW (DC-SAW) device for the first time, which can be fabricated simply using a single adhesive tape and a drop of ultraviolet-curable material without soft lithography processes. The dome-shaped chamber device with a contact angle of 68° enables the maximizing of the effect of SAW transmitted at a refraction angle of roughly 22°, negating the droplet instability. The DC-SAW device was applied to acoustic mixing to estimate its capability. Acoustic mixing of two different fluids (i.e., deionized water and fluorescent particle suspension) was demonstrated in the dome-shaped chamber device. Moreover, the effect of flow rate and applied voltage on mixing performance was estimated. With the decreasing flow rate and increasing applied voltage, mixing performance was enhanced. At an applied voltage of 20 V, mixing indices were higher than 0.9 at a total flow rate of 300 μl min-1.

High-Rayleigh-number thermal convection of compressed gases in inclined rectangular enclosures

High-Rayleigh-number convection is experimentally studied using compressed gases for a wide range of Rayleigh numbers (1.85 × 106 ≤ Ra ≤ 1.04 × 1011) and angles of inclination (θ = 0°, 30°, 60°, 90°, 120°, and 150°) with rectangular enclosures of varied aspect ratios (AR = 1, 3, 6, and 10). Experimental results reveal that the Nusselt number decreases monotonically with increasing angle of inclination. Furthermore, for any angle of inclination and a given Rayleigh number, the Nusselt number is observed to follow a decreasing trend with an increase in aspect ratio, which gradually amplifies as the angle of inclination is increased. Z-type shadowgraph flow visualization experiments, employed to characterize buoyant flow behavior at various angles of inclination, confirm the observed heat transfer trends.

Cell agglomeration using guided surface acoustic waves through a couplant layer

Cell agglomeration has been useful and crucial for tissue engineering, cell culturing, and drug testing, such as for cancer drug development. However, most of the methods require specific and complex experimental setups and the resulting agglomerates are generally slow or of poor quality. We propose a microfluidic device using surface acoustic waves (SAWs), a contact-free means, in the application of cell engineering. It is composed of a piezoelectric substrate, 127.86 deg Y-rotated X-propagating lithium niobate, and a focused interdigital transducer to generate focused surface waves with a resonant frequency at 100 MHz. An aluminum guiding layer is deposited on top of the substrate to overcome the beam steering and lateral diffraction problems due to the anisotropy nature of the piezoelectric material, further trapping the wave to a small region of a few wavelengths. The SAWs generated travel into a thin layer of liquid and diffract at the Rayleigh angle, so that the acoustic radiation is coupled and propagates into the superstrate, which can be a bio-friendly container culturing cells. The localized acoustic streaming induced from the radiation carries and accumulates the cells to the center of the recirculation. Our data show that the agglomeration can form in less than 20 s, and the separation between the living yeast cell agglomeration islands can be as small as △x = 720 μm. This device is proven to be an effective, reliable, and easy-to-handle approach for cell manipulation, independent of the culturing container.

Investigation of natural convection of magnetic nanofluid in an enclosure with a porous medium considering Brownian motion

In this work, magnetic nanofluid natural convection in the porous enclosure considering Brownian motion is studied numerically using Control Volume Finite Element Method (CVFEM). The effective viscosity and thermal conductivity of nanofluid are specified by Corcione correlation in which the impact of Brownian motion on the thermal conductivity is considered. The considered porous enclosure is heated from the inner circular side and it cooled from the outer circular side while two other walls are insulated. The impacts of various controlling parameters such as the Rayleigh number, Hartmann number, Darcy number and angle of magnetic field parameter are examined on the streamlines and isotherms. In addition, the average and local Nusselt numbers are obtained and presented via figures. The results state that the intensity of the convective flow has a direct relationship with the Rayleigh number and Darcy number while it has a reverse relationship with the Hartmann number and inclination angle of the magnetic field.

Force on a compressible sphere and the resonance of a bubble in standing surface acoustic waves

In this paper, the theory for acoustic radiation force exerted by standing surface acoustic waves (SSAWs) is extended to a compressible sphere in inviscid fluids. The conventional theory, developed in plane standing waves, fails to predict the radiation force incident on particles in the SSAW. Our extended formulas reveal that, in the direction normal to the piezoelectric substrate, the acoustic radiation force cannot only push the sphere away, but also pull it back towards the substrate. In the direction parallel to the substrate, both the magnitude of the radiation force and the equilibrium positions for particles can be actively tuned by changing the Rayleigh angle. This is exceedingly meaningful for particle positioning by tilted angle SSAWs and SSAWs. Furthermore, the Rayleigh angle can also be used to actively regulate the resonance point and peak for bubbles acted on by SSAWs. The extended theory can thus be used in the design of SSAWs for manipulating particles and bubbles.

The radiation force on a rigid sphere in standing surface acoustic waves

In this paper, the theory of acoustic radiation force exerted by standing surface acoustic waves (SSAWs) on a rigid sphere in ideal fluids is extended to account for the Rayleigh angle. The conventional theory used to calculate the radiation force acting on spheres in SSAWs is developed for plane standing waves (PSWs). PSWs exclude the dependence on the Rayleigh angle, causing significant limitations in the SSAW-based application settings. The radiation force on a cylinder in SSAWs is calculated based on which spherical structure is to be further considered. The Rayleigh angle effects can amplify the force of acoustic radiation in the direction normal to the piezoelectric substrate. In addition, the Rayleigh angle plays a notable role in both the magnitude and sign of the acoustic radiation force in the direction parallel to the substrate, which is significant for manipulating spherical particles in SSAWs. Thus, our formulas have great potential in the SSAW-based applications.

Revised model for the radiation force exerted by standing surface acoustic waves on a rigid cylinder.

In this paper, a model for the radiation force exerted by standing surface acoustic waves (SSAWs) on a rigid cylinder in inviscid fluids is extended to account for the dependence on the Rayleigh angle. The conventional model for the radiation force used in the SSAW-based applications is developed in plane standing waves, which fails to predict the movement of the cylinder in the SSAW. Our revised model reveals that, in the direction normal to the piezoelectric substrate on which the SSAW is generated, acoustic radiation force can be large enough to drive the cylinder even in the long-wavelength limit. Furthermore, the force in this direction can not only push the cylinder away, but also pull it back toward the substrate. In the direction parallel to the substrate, the equilibrium positions for particles can be actively tuned by changing Rayleigh angle. As an example considered in the paper, with the reduction of Rayleigh angle the equilibrium positions for steel cylinders in water change from pressure nodes to pressure antinodes. The model can thus be used in the design of SSAWs for particle manipulations.