Pressure Node Research Articles

Study on the Susceptibility of Steel Arches with Letting Pressure Nodes Based on Incremental Dynamic Analysis

When soft rock tunnels pass through fractured fault zones, they are particularly susceptible to extrusion and large-scale deformations, especially during seismic events. To address these challenges, this study introduces an innovative yield-support steel arch design featuring a circumferential letting pressure node at its core. This design delivers incremental support resistance within the deformation zone and a susceptibility curve is applied to evaluate the damage probability of the steel arch with a letting pressure node under seismic loading conditions. Measurements of the surrounding rock pressure and structural forces on the steel arch with the letting pressure node were conducted at the Baoshan Jewel Mountain Tunnel in China. The field experiment results revealed a 23% reduction in the surrounding rock pressure and an 11% decrease in the internal forces of the support structure. These findings demonstrate the successful application of the letting pressure node-supported steel arch in mitigating large deformations in soft rock environments. Additionally, using finite element software ANSYS 2022, a seismic time-history analysis was conducted, employing the relative deformation rate of the letting pressure node steel arch as the damage index and the peak ground acceleration (PGA) as the strength parameter to generate the incremental dynamic analysis (IDA) curve. According to the susceptibility curve derived from the incremental dynamic analysis, at the design ground motion level of 8 degrees, the letting pressure node steel arch has a 94% probability of exceeding its normal service life limit and experiencing damage. The findings of this study offer a novel approach to addressing large deformations in soft rock tunnels. The proposed susceptibility curves for steel arches with letting pressure nodes provide a robust foundation for predicting the damage probability of yielding support structures under seismic conditions.

Formation of cell spheroids using Standing Surface Acoustic Wave (SSAW).

3D bioprinting becomes one of the popular approaches in the tissue engineering. In this emerging application, bioink is crucial for fabrication and functionality of constructed tissue. The use of cell spheroids as bioink can enhance the cell-cell interaction and subsequently the growth and differentiation of cells in the 3D printed construct with the minimum amount of other biomaterials. However, the conventional methods of preparing the cell spheroids have several limitations, such as long culture time, low-throughput, and medium modification. In this study, the formation of cell spheroids by SSAW was evaluated both numerically and experimentally in order to overcome the aforementioned limitations. The effects of excitation frequencies on the cell accumulation time, diameter of the formed cell spheroids, and subsequently, the growth and viability of cell spheroids in the culture medium over time were studied. Using the high-frequency (23.8 MHz) excitation, cell accumulation time to the pressure nodes could be reduced in comparison to that of the low-frequency (10.4 MHz) excitation, but in a smaller spheroid size. SSAW excitation at both frequencies does not affect the cell viability up to 7 days, > 90% with no statistical difference compared with the control group. In summary, SSAW can effectively prepare the cell spheroids as bioink for the future 3D bioprinting and various biotechnology applications (e.g., pharmaceutical drug screening and tissue engineering).

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.

Acoustic Assembly and Scanning of Superlens Arrays for High-Resolution and Large Field-of-View Bioimaging.

High-resolution and dynamic bioimaging is essential in life sciences and biomedical applications. In recent years, microspheres combined with optical microscopes have offered a low cost but promising solution for super-resolution imaging, by breaking the diffraction barrier. However, challenges still exist in precisely and parallelly superlens controlling using a noncontact manner, to meet the demands of large-area scanning imaging for desired targets. This study proposes an acoustic wavefield-based strategy for assembling and manipulating micrometer-scale superlens arrays, in addition to achieving on-demand scanning imaging through phase modulation. In experiments, acoustic pressure nodes are designed to be comparable in size to microspheres, allowing spatially dispersed microspheres to be arranged into arrays with one unit per node. Droplet microlenses with various diameters can be adapted in the array, allowing for a wide range of spacing periods by applying different frequencies. In addition, through the continuous phase shifting in the x and y directions, this acoustic superlens array achieves on-demand moving for the parallel high-resolution virtual image capturing and scanning of nanostructures and biological cell samples. As a comparison, this noncontact and cost-effective acoustic manner can obtain more than ∼100 times the acquisition efficiency of a single lens, holding promise in advancing super-resolution microscopy and subcellular-level bioimaging.

Label-free separation of peripheral blood mononuclear cells from whole blood by gradient acoustic focusing

Efficient techniques for separating target cells from undiluted blood are necessary for various diagnostic and research applications. This paper presents acoustic focusing in dense media containing iodixanol to purify peripheral blood mononuclear cells (PBMCs) from whole blood in a label-free and flow-through format. If the blood is laminated or mixed with iodixanol solutions while passing through the resonant microchannel, all the components (fluids and cells) rearrange according to their acoustic impedances. Red blood cells (RBCs) have higher effective acoustic impedance than PBMCs. Therefore, they relocate to the pressure node despite the dense medium, while PBMCs stay near the channel walls due to their negative contrast factor relative to their surrounding medium. By modifying the medium and thus tuning the contrast factor of the cells, we enriched PBMCs relative to RBCs by a factor of 3600 to 11,000 and with a separation efficiency of 85%. That level of RBC depletion is higher than most other microfluidic methods and similar to that of density gradient centrifugation. The current acoustophoretic chip runs up to 20 µl/min undiluted whole blood and can be integrated with downstream analysis.

Ultrasonic-assisted dewatering of crude oil under various transient flow regimes: an experimental and simulation study

This paper aims to separate a water-in-oil emulsion using ultrasonic standing waves generated by a piezoelectric transducer inside a channel under the transient flow regimes. In the proposed method, the acoustophoretic force at pressure nodes accumulates the water droplets pulled down due to gravity as they agglomerate and get heavy enough. Furthermore, a simulation was conducted to clarify and explain the fundamentals of the procedure. This approach was used at various flow rates (18, 26, and 34 ml/s) and different percentages of water-in-oil (20%, 40%, and 60%). According to the design of experiments based on response surface methodology (RSM), these parameters and their interaction are significant. Results indicated that the water separation was achieved successfully in a noticeable number of the conducted experiments which could confirm the proficiency of the proposed technique. The maximum separation percentage of 80 was obtained when the flow rate and water content were 18 ml/s and 20%, respectively.

Study on unsteady evacuation characteristics of multi-layer insulation during launch/in orbit of the cryogenic propellant tanks

The MLI + foam is used for the thermal protection of the cryogenic propellant. However, the MLI generates unsteady evacuation when the cryogenic rocket is launched and traveled in space. The insulation performance of the MLI is determined by the evacuation level of the interstitial gas. Aiming at the unsteady evacuation of the MLI, the interlayer pressure node algorithm was developed. The transient evacuation characteristics of the interlayer pressure were predicted. The effect of layer number, slit rate, and layer density on evacuation was investigated. The evacuation and insulation performance of MLI were optimized. The results show that the innermost pressure of the 60 mm thickness (th60) MLI dropped to less than 0.5 Pa when the evacuation was conducted for more than 25 h. The pressure of the innermost layer and 10th layer is greater than 1 order of magnitude higher than that of the outermost layer. The evacuation rate of th20 is faster than that of th40 and th60. The innermost layer pressure value of the th20 is 0.168 Pa, which is 10.47 times higher than that of the outermost layer. The innermost layer pressure of the th20 is 48.3 % and 64.8 % lower than that of th40 and th60, respectively. The gas conduction decreased by 61.22 % when the opening pattern was optimized. These results can provide an important reference for the thermal protection design of cryogenic propellants.

On the behavior of prolate spheroids in a standing surface acoustic wave field

The active manipulation of particle and cell trajectories in fluids by high-frequency standing surface acoustic waves (sSAW) allows to separate particles and cells systematically depending on their size and acoustic contrast. However, process technologies and biomedical applications usually operate with non-spherical particles, for which the prediction of acoustic forces is highly challenging and remains a subject of ongoing research. In this study, the dynamical behavior of prolate spheroids exposed to a three-dimensional acoustic field with multiple pressure nodes along the channel width is examined. Optical measurements reveal an alignment of the particles orthogonal to the pressure nodes of the sSAW, which has not been reported in literature so far. The dynamical behavior of the particles is analyzed under controlled initial conditions for various motion patterns by imposing a phase shift on the sSAW. To gain detailed understanding of the particle dynamics, a three-dimensional numerical model is developed to predict the acoustic force and torque acting on a prolate spheroid. Considering the acoustically induced streaming around the particle, the numerical results are in excellent agreement with experimental findings. Using the proposed numerical model, a dependence of the acoustic force on the particle shape is found in relation to the acoustic impedance of the channel ceiling. Hence, the numerical model presented herein promises high progress for the design of separation devices utilizing sSAW, exploiting an additional separation criterion based on the particle shape.

A numerical study of an atomizing jet in a resonant acoustic field

By applying state-of-the-art, high-fidelity methods for compressible multiphase flows, this work features a parametric study of a turbulent liquid jet atomizing under the influence of standing acoustic waves with five different amplitude values. We perform these simulations in a semi-periodic domain to isolate interactions between acoustic and atomization processes and to emulate a pure liquid fuel spray placed at the pressure node of a resonant acoustic field. Characterization of the flow includes measurements of the mean dimensions, droplet number, and surface area of the liquid distribution, alongside qualitative depictions of the interface evolution, breakup events, and hydrodynamic fields. As acoustic forcing is provided to the flow, the added energy provokes atomization when shear would otherwise be insufficient for breakup. The magnitude of the forcing determines the rate of droplet production, which accelerates as the evolving liquid surface modifies the induced motion of the gas phase. At the nodal plane, the acoustic field produces an organizing effect on the liquid structures.

Impact of Transverse Acoustic Excitation and Air Swirl on a Confined Hollow Cone Sheet

Abstract This study investigates the effect of imposing symmetric transverse acoustic excitation and counter-rotating air swirl flow on a confined hollow cone spray sheet. We study the impact of these forcing conditions on two unstable modes—sheet flapping and sinuous wave growth modes—and their phasic relationship. We used the shadowgraphy technique, and only the sheet edges were analyzed using the method of snapshot proper orthogonal decomposition (POD). In this study, we used postprocessing techniques to estimate the nondimensional breakup length, phase averaged cone angle, and continuous wavelet transform (CWT) frequency. Results show that imposing acoustic excitation alone causes the sheet edges to flap, axisymmetrically, at the pressure antinode, while at the pressure node and base flow condition, the asymmetric sinuous wave mode is dominant. At the pressure intermediary, we find the possible dominance of both modes. While the dominant effect of air swirl alone was difficult to ascertain, results from CWT analysis show that POD mode 1 and mode 3 have very similar spectral content, with mode 3 showing a flapping mode. Finally, air swirl flow has a more dominant effect than acoustics in causing sheet breakup.

Particle size effects on stable levitation positions in acoustic standing waves.

Schlieren images can show a two-dimensional representation of pressure distributions. Using this method, we have demonstrated that there is a particle size effect for levitating solid elastic Styrofoam spheres in an ultrasonic acoustic standing wave: (1) spheres of density 13.3 kg/m3 and diameters less than 0.58 λ levitate at pressure nodes, and (2) spheres larger than 0.66 λ levitate with their centers at pressure antinodes. Pressure measurements of ultrasonic standing waves made in conjunction with their schlieren images to identify pressure nodes and antinodes are presented. These observations contribute to prior experimental and theoretical research concerning the influence of particle size on levitation.

The effect of microchannel height on the acoustophoretic motion of sub-micron particles

Acoustophoresis is an effective technique for particle manipulation. Acoustic radiation force scales with particle volume, enabling size separation. Yet, isolating sub-micron particles remains a challenge due to the acoustic streaming effect (ASE). While some studies confirmed the focusing ability of ASE, others reported continuous stirring effects. To investigate the parameters that influence ASE-induced particle motion in a microchannel, this study examined the effect of microchannel height and particle size. We employed standing surface acoustic wave (SSAW) to manipulate polystyrene particles suspended in the water-filled microchannel. The results show that ASE can direct particles as small as 0.31 µm in diameter to the centre of the streaming vortices, and increasing the channel height enhances the focusing effect. Smaller particles circulate in the streaming vortices continuously, with no movement towards the centres. We also discovered that when the channel height is at least 0.75 the fluid wavelength, particles transitioning from acoustic radiation-dominated to ASE-dominated share the same equilibrium position, which differs from the pressure nodes and the vortices’ centres. The spatial distance between particles in different categories can lead to particle separation. Therefore, ASE is a potential alternative mechanism for sub-micron particle sorting when the channel height is accurately adjusted.

Programmable-Modulated Ultrasonic Transducer Array for Contactless Detection of Viral RNAs.

The current polymerase chain reactions-based nucleic acid tests for large-scale infectious disease diagnosis are always lab-dependent and generate large amounts of highly infectious plastic waste. Direct non-linear acoustic driven of microdroplets provide an ideal platform for contactless spatial and temporal manipulation of liquid samples. Here, a strategy to programmable-manipulate microdroplets using potential pressure well for contactless trace detection is conceptualized and designed. On such contactless modulation platform, up to seventy-two piezoelectric transducers are precisely self-focusing single-axis arranged and controlled, which can generate dynamic pressure nodes for effectively contact-free manipulating microdroplets without vessel contamination. In addition, the patterned microdroplet array can act as contactless microreactor and allow multiple trace samples (1-5µL) biochemical analysis, and the ultrasonic vortex can also accelerate non-equilibrium chemical reactions such as recombinase polymerase amplification (RPA). The results of fluorescence detection indicated that such programmable modulated microdroplet achieved contactless trace nucleic acid detection with a sensitivity of 0.21copyµL-1 in only 6-14min, which is 30.3-43.3% shorter than the conventional RPA approach. Such a programmable containerless microdroplet platform can be used for toxic, hazardous, or infectious samples sensing, opening up new avenues for developing future fully automated detection systems.

Control and promotion of probiotic cells' aggregation and viability using DC electric field and standing acoustic waves

Bacterial cells' long-term stability can be induced through cell aggregation. This study aimed to stimulate probiotic cells' aggregation, suspended in 10 mM KH2PO4 at 18 ± 2 °C, utilizing DC electric fields (10–100 mA) and combined electric field (40 mA) with standing acoustic waves (at a frequency range of 26 kHz to 296 kHz). It was demonstrated that the sole application of acoustic waves on the rod-shaped Bifidobacterium animalis subsp. lactis (BIFIDO) did not trigger cell aggregation. However, the application of DC electric field to BIFIDO, induced polarization and aggregation of the BIFIDO cells, as indicated by spectrophotometric measurements (OD600) and visualized by fluorescence microscopy. The aggregation of BIFIDO cells was accelerated (∼6 times) when applying simultaneously DC electric field and standing acoustic waves, due to the accumulation of the cells at the acoustic pressure nodes, and due to the polarization of the cells by dielectrophoresis (DEP) phenomenon. The synergistic effect of DC electric field and standing acoustic waves not only facilitated the aggregation of the cells, but also considerably enhanced the hydrophobicity of BIFIDO cells, without compromising their viability or altering their surface charges. Enhanced viability and stability of the aggregated probiotic cells, in comparison to the non-aggregated, was also observed. Industrial relevanceThe present study points out the application of DC electric field and Standing Acoustic waves to aggregate probiotic cells. An important attribute of aggregated cells is their greater tolerance to environmental stresses. This technology could be applied for large-scale manipulation and aggregation of cells, as for instance during fermentation, prior to drying or encapsulation of cells, or during other industrial cell treatment technologies.

Power-controlled acoustofluidic manipulation of microparticles

Recently, surface acoustic wave (SAW) based acoustofluidic separation of microparticles and cells has attracted increasing interest due to accuracy and biocompatibility. Precise control of the input power of acoustofluidic devices is essential for generating optimum acoustic radiation force to manipulate microparticles given their various parameters including size, density, compressibility, and moving velocity. In this work, an acoustophoretic system is developed by employing SAW based interdigital electrode devices. Power meters are applied to closely monitor the incident and reflected powers of the SAW device, which are associated with the separation efficiency. There exists a range of input powers to migrate the microparticles to the pressure node due to their random locations when entering the SAW field. Theoretical analysis is performed to predict a proper input power to separate mixtures of polystyrene microspheres, and the end lateral position of microspheres being acoustically separated. The separation efficiency of four sizes of microspheres, including 20 µm, 15 µm, 10 µm, and 5 µm, is calculated and compared with experimental results, which suggest the input power for separating the mixture of these microspheres. The study provides a practical guidance on operating SAW devices for bioparticle separation using the incident power as a control parameter.

Mode shape-dependent thermoacoustic interactions between a lean-premixed primary flame and an axially-staged transverse reacting jet

Axial fuel staging enables elevation of the turbine inlet temperature in advanced heavy-duty gas turbine combustors to ∼2000 K, ultimately to achieve more than 65% combined cycle efficiency, while limiting excessive nitrogen oxide production. While previous investigations have identified transverse jet-flow-dependent nitrogen oxides emission trends and the topological properties of reacting shear layers in vitiated crossflow environments, there exists a critical gap in the understanding of axial-fuel-staging-induced combustion instabilities. The present study therefore investigates the link between first- and second-stage flame dynamics in a lean-premixed combustor. We demonstrate that the second-stage jet-in-vitiated-crossflow flame is preferentially coupled to higher order acoustic modes than the upstream primary flame with the same characteristic nozzle dimension. The first- and second-stage-driven acoustic modes (two different timescales) are mutually incompatible, and the ensuing selective excitation is critically dependent on the position of the secondary injector relative to the mode shape. When the second-stage flame is located near a pressure node, or a velocity antinode, the transverse jet-stabilized flame is influenced by the primary flame-driven crossflow velocity oscillations, and exhibits conspicuous flapping motions in the crossflow direction accompanied by periodic partial extinction at the windward and leeward-side flame elements. If the second-stage flame is situated near a pressure antinode, on the other hand, the secondary jet flame's dynamics are governed by a combination of jet merging-induced flame surface destruction and the growth of non-axisymmetric coherent structures. In this case, high-intensity pressure oscillations are sustained by the local heat release fluctuations generated solely from the transverse reacting jets, whereas the upstream primary flame remains nearly unperturbed, or decoupled from the underlying feedback processes. This experimental observation is remarkable, and reflects previously unknown properties originating from the complex interactions between the two axially-staged lean-premixed flames.

Phase holograms for the three-dimensional patterning of unconstrained microparticles

Acoustic radiation forces can remotely manipulate particles. Forces from a standing wave field align microscale particles along the nodal or anti-nodal locations of the field to form three-dimensional (3D) patterns. These patterns can be used to form 3D microstructures for tissue engineering applications. However, standing wave generation requires more than one transducer or a reflector, which is challenging to implement in vivo. Here, a method is developed and validated to manipulate microspheres using a travelling wave from a single transducer. Diffraction theory and an iterative angular spectrum approach are employed to design phase holograms to shape the acoustic field. The field replicates a standing wave and aligns polyethylene microspheres in water, which are analogous to cells in vivo, at pressure nodes. Using Gor’kov potential to calculate the radiation forces on the microspheres, axial forces are minimized, and transverse forces are maximized to create stable particle patterns. Pressure fields from the phase holograms and resulting particle aggregation patterns match predictions with a feature similarity index > 0.92, where 1 is a perfect match. The resulting radiation forces are comparable to those produced from a standing wave, which suggests opportunities for in vivo implementation of cell patterning toward tissue engineering applications.

On the stability of inhomogeneous fluids under acoustic fields

In this work, we present the stability theory for inhomogeneous fluids subjected to standing acoustic fields. Starting from the first principles, the stability criterion is established for two fluids of different acoustic impedance (product of density and speed of sound of the fluid) separated by a plane interface. Through stability theory and numerical simulations, we show that, in the presence of interfacial tension, the relocation of high-impedance fluid from the pressure anti-node to the pressure node occurs when the acoustic force overcomes the interfacial tension force, which is in agreement with recent acoustic relocation experiments in the microchannel. Furthermore, we establish an acoustic Bond number that characterizes stable ( $Bo_{a}<1$ ) and relocation ( $Bo_{a}>1$ ) regimes. Remarkably, it is found that the critical acoustic energy density required for relocation can be significantly reduced by increasing the height of the channel which could help in designing acoustofluidic devices that handle immiscible fluids.

The behavior of standing waves near the end of an open pipe with low mean flow

The acoustic standing wave near the end of an open pipe is investigated using spectrally analyzed high-speed transmission electronic speckle pattern interferometry. It is shown that the standing wave extends beyond the open end of the pipe and the amplitude decays exponentially with distance from the end. Additionally, a pressure node is observed near the end of the pipe in a position that is not spatially periodic with the other nodes in the standing wave. A sinusoidal fit to the amplitude of the standing wave inside the pipe indicates that the end correction is well predicted by current theory.

Acoustic and Magnetic Stimuli-Based Three-Dimensional Cell Culture Platform for Tissue Engineering.

In a conventional two-dimensional (2D) culture method, cells are attached to the bottom of the culture dish and grow into a monolayer. These 2D culture methods are easy to handle, cost-effective, reproducible, and adaptable to growing many different types of cells. However, monolayer 2D cell culture conditions are far from those of natural tissue, indicating the need for a three-dimensional (3D) culture system. Various methods, such as hanging drop, scaffolds, hydrogels, microfluid systems, and bioreactor systems, have been utilized for 3D cell culture. Recently, external physical stimulation-based 3D cell culture platforms, such as acoustic and magnetic forces, were introduced. Acoustic waves can establish acoustic radiation force, which can induce suspended objects to gather in the pressure node region and aggregate to form clusters. Magnetic targeting consists of two components, a magnetically responsive carrier and a magnetic field gradient source. In a magnetic-based 3D cell culture platform, cells are aggregated by changing the magnetic force. Magnetic fields can manipulate cells through two different methods: positive magnetophoresis and negative magnetophoresis. Positive magnetophoresis is a way of imparting magnetic properties to cells by labeling them with magnetic nanoparticles. Negative magnetophoresis is a label-free principle-based method. 3D cell structures, such as spheroids, 3D network structures, and cell sheets, have been successfully fabricated using this acoustic and magnetic stimuli-based 3D cell culture platform. Additionally, fabricated 3D cell structures showed enhanced cell behavior, such as differentiation potential and tissue regeneration. Therefore, physical stimuli-based 3D cell culture platforms could be promising tools for tissue engineering.