High Velocity Gradients Research Articles

A novel technique to resolve directional ambiguity for Particle Streak Velocimetry

In this study, a novel streak direction-resolving algorithm is introduced to determine the direction of the flow field for a single image frame with multiple streaks. The streak direction was resolved by varying its intensity from 100% to 50% of the total intensity of the streak image. This technique was applied to two main types of flows parallel and Hill’s vortex flows, parallel flows were divided into further two types constant velocity parallel flow and accelerating parallel flow. The purpose of using different types of flows was to test the robustness of the algorithm with easy and complex flows. The performance of the algorithm was checked by the angular deviation between the true and least-square fitted velocity vectors. The number of correct synthetic streak directions was measured with the success rate in percentage. A high success rate means low angular deviation and a high number of velocity vectors with correct direction was obtained. In this research, four different types of image formats were considered DP (double precision), 16-bit (without noise), 16-bit (1.0% noise), 16-bit (5.0% noise), and 8-bit (without noise) and the best results were obtained for DP and 16-bit image formats. The results of the parallel flows indicated a 100% streak direction success rate, Hill’s vortex was a type of complex flow therefore, the algorithm hard to resolve some streak directions due to very low velocities (less than 0.1 px (pixel)/interval) and very high-velocity gradients (greater than 52 px/interval). The observation shows that Hill’s vortex synthetic streak images were resolved with a success rate of 92.76% which means that the majority of the synthetic streaks were resolved. Experimental analysis was also done by using the PDMS microchannel setup. Intensity variation streaks were recorded with long camera exposure time and for maintaining the intensity variation, as the LED switched on maximum illumination was started, and continuous decrement in illumination was set by switching off at 50% of the total illumination intensity. The best results were achieved with a 94% success rate. Therefore, the proposed novel approach can be used with less expensive hardware for image processing with a single image frame and is useful for multiple applications.

Reliability of Moment Tensor Inversion for Different Seismic Networks

AbstractThe article investigates the reliability of moment tensor (MT) inversion in time domain with use of first P-wave amplitude, a method used to determine the source mechanisms of earthquakes, across four different seismic networks. The study compares the synthetic tests results of MT inversion for two underground mining and two artificial reservoir monitoring seismic networks. The analysis was performed to assesses how consistency and accuracy of the results depend on different factors like: network configuration, events depth, velocity model, focal mechanism of event and applied noise. The findings highlight the impact of network configuration compared to other variables and data quality on the reliability of moment tensor inversion and provide insights into different factors which have to be considered to enhance MT accuracy. The significance of events depth in P-wave amplitude MT inversion and the necessity to consider velocity model influence, especially presence of high velocity gradient, is highlighted by the presented results.

Experimental and simulation study of flow field characteristics of a disturbing rotary centrifugal air classifier

In the present study, a new disturbing rotary centrifugal classifier was designed and a method was proposed to characterize its pressure signals. The models for evaluating the classification effects were established based on multi-zones. It was found that the flow field, processing parameters, and material properties significantly affected on the particle force and classification performance. The pressure with high amplitude and energy analyzed using Hilbert-Huang Transform (HHT) concentrated within 200–400 Hz, and the screening cage facilitated energy transfer to lower frequency bands. Correlation analysis demonstrated the correlation coefficients (A5 and A6) between adjacent measurement points increased with frequency, further enhanced by the screening cage. The flow field of simulation was characterized by a low-pressure, high-velocity gradient at the center and a high-pressure, low-velocity gradient at the sidewalls and the gas velocity and flow for experiment achieved the maximum at z = 0.50 m, while the magnitude of pressure change was significant at the inlet end and discharge end, where the velocity gradient and pressure drop had the same trend and small errors for experiments and simulation. The flow field affected the competing relationship between centrifugal and drag forces on the particles with different kinematic behavior during the classification process. The errors between the predicted values of the theoretical, simulated, and experimental cut particle size models and the true values were all within 10 %, based on which the multi-zone classification efficiency model applicable to the disturbing centrifugal classifier was established, and it was found that the models all had high prediction accuracy and suitability. The article delves into the correlation between pressure signals, flow field characteristics and particle behavior, aiming to provide an important reference for the internal flow field and classification performance of air classifiers.

Analysis of Soot Deposition Effects on Exhaust Heat Exchanger for Waste Heat Recovery System

This study investigates the thermal–hydraulic behavior and deposition characteristics of a shell and tube exhaust heat exchanger using a CFD-based predictive model of soot deposition. Firstly, considering the influences of thermophoretic, wall shear stress, and other deposition and removal mechanisms, a predictive model is developed for long-term performance of heat exchangers under soot deposition. Then, the variations in exhaust heat exchanger performance during a 4 h deposition period are simulated based on the model. Subsequently, the variation of deposition distribution and different deposition velocities are also evaluated. Finally, an analysis of the long-term performance of the exhaust heat exchanger under varying gas velocities and temperature gradients is conducted, revealing the performance variations under all engine-operating conditions. Results show that the deterioration in normalized relative j/f1/2 varies from 5.26% to 24.91% under different work conditions, and the exhaust heat exchanger with high gas velocity and low temperature gradient exhibits optimal long-term performance.

Continuous glass fiber‐reinforced polycaprolactone composite produced in a conventional fused filament fabrication equipment: Process modeling and parameters adjustment

AbstractPolymer composites with continuous fibers are expected to exhibit good mechanical performance due to orientation and high aspect ratio of fillers. Fused filament fabrication (FFF) provides an affordable method for processing these materials as products with tunable architecture. By incorporating continuous bioactive fibers coated with biodegradable polymer, the degradation rate of printed scaffolds may vary over time. As proof of concept, macroporous composites were 3D printed using continuous glass fiber‐reinforced polycaprolactone filament. Parametrization and challenges associated with printing on non‐dedicated equipment are discussed. A model describing the melt flow was employed to evaluate the velocity and shear rate profiles. Although the maximum velocity is approximately 18 mm s−1 for both neat and reinforced polymer, the obstruction caused by fibers results in higher shear rate, up to 481 s−1, higher pressure gradient, 1.95 MPa mm−1, and higher velocity gradient, conditions that limit print quality. Additionally, it was also possible to determine the shear stress experienced by the fiber bundle, 300 KPa, and the influence of different processing conditions. This investigation advances the development and understanding of manufacturing of continuous fiber‐reinforced polymers via FFF.

Numerical simulations of a stochastic dynamics leading to cascades and loss of regularity: Applications to fluid turbulence and generation of fractional Gaussian fields

Motivated by the modeling of the spatial structure of the velocity field of three-dimensional turbulent flows, and the phenomenology of cascade phenomena, a linear dynamics was recently proposed that can generate high velocity gradients from a smooth-in-space forcing term. It is based on a linear partial differential equation stirred by an additive random forcing term that is δ-correlated in time. The underlying proposed deterministic mechanism corresponds to a transport in Fourier space that aims to transfer energy injected at large scales towards small scales. The key role of the random forcing is to realize these transfers in a statistically homogeneous way. Whereas at finite times and positive viscosity the solutions are smooth, a loss of regularity is observed for the statistically stationary state in the inviscid limit. We present here simulations, based on finite volume methods in the Fourier domain and a splitting method in time, which are more accurate than the pseudospectral simulations. We show that our algorithm is able to reproduce accurately the expected local and statistical structure of the predicted solutions. We conduct numerical simulations in one, two, and three spatial dimensions, and we display the solutions both in physical and Fourier space. We additionally display key statistical quantities such as second-order structure functions and power spectral densities at various viscosities. Published by the American Physical Society 2024

Particle Image Velocimetry Investigation Of The Aerodynamic Interaction Between Helicopter And An Aircraft Carrier

Helicopters greatly expand operational capabilities during military missions at the sea. The aircraft carriers are capable of accommodating fixed-wing and rotary-wing aircraft operations. They not only have one runway for take-off, but also have different spots for helicopter operations. As they are spread over the deck of the aircraft carrier, the non-aerodynamic geometries of the take-off ramp and the island can generate complex flows, with high velocity gradients and turbulence intensities that can make complex the helicopter landing and take-off maneuvers for pilots. This study analyses the interaction between the aerodynamic patterns generated by the warship and those generated during the operation of the helicopter. Two wind conditions are simulated: headwind and crosswind. The results are provided by a Particle Image Velocimetry (PIV) system installed at the wind tunnel test section of a low-speed wind tunnel. The model of aircraft carrier and helicopter are tested in a reduced scale of 1:100. And the helicopter rotor rotates at sufficient speed to ensure the similarity of the thrust coefficient with the real case. Finally, during the wind tunnel tests, using an automatic positioning system, the helicopter is placed in different positions above the aircraft carrier flight deck in order to obtain PIV images and extract non-dimensional velocity contours with and without the helicopter effect. The results have shown important effects of the aerodynamics generated by the bow, the hull and the aircraft carrier island, with velocity differences up to 70 % depending on the landing spot analyzed.

Impacts of Bar-driven Shear and Shocks on Star Formation

Bars drive gas inflow. As the gas flows inward, shocks and shear occur along the bar dust lanes. Such shocks and shear can affect the star formation (SF) and change the gas properties. For four barred galaxies, we present Hα velocity gradient maps that highlight bar-driven shocks and shear using data from the PHANGS-MUSE and PHANGS-ALMA surveys, which allow us to study bar kinematics in unprecedented detail. Velocity gradients are enhanced along the bar dust lanes, where shocks and shear are shown to occur in numerical simulations. Velocity gradient maps also efficiently pick up H ii regions that are expanding or moving relative to the surroundings. We put pseudo-slits on the regions where velocity gradients are enhanced and find that Hα and CO velocities jump up to ∼170 km s−1, even after removing the effects of circular motions due to the galaxy rotation. Enhanced velocity gradients either coincide with the peak of CO intensity along the bar dust lanes or are slightly offset from CO intensity peaks, depending on the objects. Using the Baldwin–Philips–Terlevich BPT diagnostic, we identify the source of ionization on each spaxel and find that SF is inhibited in the high-velocity gradient regions of the bar, and the majority of those regions are classified as a low-ionization nuclear emission-line region (LINER) or composite. This implies that SF is inhibited where bar-driven shear and shocks are strong. Our results are consistent with the results from the numerical simulations that show SF is inhibited in the bar where the shear force is strong.

Sweeping jet impingement heat transfer on concave surface: Influence of trapped vortex ring revealed by generalized k-ω (GEKO) model

Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k-ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2% increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer.

Insights into turbulent flow structure and energy dissipation in centrifugal pumps: A study utilizing time-resolved particle image velocimetry and proper orthogonal decomposition

The purpose of the study is to investigate the flow structure and energy dissipation mechanism within centrifugal pump impellers. The velocity fields at the mid-sections of the flow passages of 4- and 5-blade impellers were measured under various flow rates using the Time-resolved Particle Image Velocimetry (TR-PIV) technique; the proper orthogonal decomposition (POD) analysis was performed to extract flow structures with different scales; the dissipation rate was estimated using the large-eddy PIV method; the correlation between turbulent kinetic energy (TKE) and dissipation rate was measured using the cross-correlation analysis. The findings are as follows: impeller with more uniform and stable flow, the scale and energy contribution of large-scale structures in lower order modes are smaller, leading to higher impeller efficiency; large-scale structures identified in the low-order modes correspond to locations of high TKE and dissipation areas; the high velocity gradients within the impeller passage and the interactions between large-scale flow structures encourage the breakup of large-scale turbulent structures into dissipative-scale structures; the correlation between TKE and dissipation rate reflects the extent to which large-scale flow structures within the impeller passage break up into dissipative scales.

Analysis of the relationship between near-wall velocity distribution and wall heat flux under diesel spray flame impingement in an RCEM

To clarify the relationship between the near-wall flow and the heat flux through the wall, the velocity distribution of the diesel spray flame and the heat flux at the combustion chamber wall were measured in a rapid compression and expansion machine (RCEM), which has a twodimensional combustion chamber. The velocity distribution of the diesel spray flame near the wall was measured by the particle image velocimetry (PIV), and the heat flux through the wall where the diesel spray flame impinges was measured using a newly developed multi-point heat flux sensor. Furthermore, the velocity gradient tensor was calculated based on the measured velocity distribution results, and the effect of the shear flow characteristics on wall heat transfer was analyzed. As a result, the higher the injection pressure leads the higher the peak value of the heat flux. A fluctuation of the heat flux exhibits the same trend as the fluctuations of the velocity, and a high heat flux appeared in the high-velocity gradient tensor region, which suggests that the characteristics of shear flow near the wall can affect the heat flux.

Experiments on Cavitation Control around a Cylinder Using Biomimetic Riblets

Experimental investigations were conducted to uncover the impact of cavitation control—through the use of biomimetic riblets on cavitating flows around a circular cylinder. First, the dynamics of cavitation in the flow behind a finite cylinder (without riblets) was unveiled by visualizing the cavitation clouds and measuring the lift force fluctuations acting on the cylinder. Second, in a significant step forward, a comprehensive explanation was provided for the cavitation control methods using two bio-inspired riblet morphologies positioned in different orientations and locations on the cylinder. For the first time, the impacts of these tiny formations on the flow dynamics and the associated cavitation process were scrutinized. This showed that scalloped riblets, with their curved design, induced secondary vortices near their tips and distorted primary streamwise vortices, and that high velocity gradients near the jagged pattern peaks of sawtooth riblets delayed flow separation, which affected cavitation.

Energy loss evaluation in radical inflow turbine based on entropy production theory and orthogonal experiment method

The fluctuation of heat source conditions results in off-design operation of the radial inflow turbines (RIT) in the organic Rankine cycle. However, the flow loss characteristics of RIT under off-design conditions have not been completely revealed. The entropy production theory has the advantage of determining the quantity and location of energy dissipation, which is used to evaluate the energy loss of RIT under different conditions. In addition, the order of operating parameters on the RIT energy loss is determined by the orthogonal experimental method. The results show that each entropy production term and the entropy production of different components increase with the increase in the inlet pressure and inlet temperature, while they decrease with the increase in the outlet pressure of the RIT. Under different operating conditions, the turbulent dissipation and wall dissipation are the main cause of RIT energy loss, which are closely related to vortices and high velocity gradients in the flow field. The rotor and diffuser contribute the main energy loss of RIT. However, the volume-average entropy production and area-average entropy production of the stator and rotor are higher than those of other components. In addition, the wall shear is the main cause of the stator energy loss, while the turbulent dissipation dominants the rotor energy loss. The outlet pressure has the greatest impact on the turbulent entropy production and wall dissipation.

Numerical investigation of energy loss distribution in the cavitating wake flow around a cylinder using entropy production method

The wake flow of a circular cylinder is numerically investigated by Large Eddy Simulation (LES) combined with the Schnerr–Sauer cavitation model. By comparing entropy production in the presence or absence of cavitation, the energy loss distribution in the wake flow field of a cylinder is explored, shedding light on the interactions between multiscale vortex systems and cavitation. The comparative results reveal that, under non-cavitating conditions, the energy loss region in the near-wake area is more concentrated and relatively larger. Energy dissipation in the wake flow field occurs in regions characterized by very high velocity gradients, primarily near the upper and lower surfaces of the cylinder near the leading edge. The influence of cavitation bubbles on entropy production is predominantly observed in the trailing-edge region (W1) and the near-wake region (W2). The distribution trends of wall entropy production on the cylinder’s surface are generally consistent in both conditions, with wall entropy production primarily concentrated in regions exhibiting high velocity gradients.

Study on the mechanism of hydrodynamic manipulation of floc structure to achieve high-flux of coagulation macro-filtration for the rapid removal of Aphanizomenonflos-aquae

Coagulation-filtration hybrid process has been widely used in algae removal treatment to improve the water quality and alleviate the membrane fouling. However, the intricate interplay of how floc structure induced by coagulation hydraulic shear influences the cake layer and filtration performance in coagulation macro-filtration of microalgae remains unexplored. In this study, a novel coagulation filtration with macro-pore of 75 μm was first adopted for the rapid removal of Aphanizomenonflos-aquae, and the floc structure was artificially manipulated through various velocity gradients to explore the effect mechanism of the coagulation hydrodynamic shear on the macro-filtration performance from the perspectives of floc characteristics (size and fractal dimension) and cake layer structure (thickness and roughness). The results showed that an optimal average flux of 31257.64 ± 2776.28 L·m−2·h−1 can be achieved at a manipulated appropriate velocity gradient (19 s−1), which is 1.1–2.3 times the flux achieved at higher or non-velocity gradients. The flocs formed at low velocity gradient were large and loose, while those formed at high velocity gradient were small and compact. Subsequently, the cake layer stacked by loose flocs was thicker and coarser than that by compact flocs, and exhibited a lower specific cake resistance, yielding a higher average flux. Moreover, a theoretical model for average flux was developed by introducing the velocity gradient, which directly elucidates how hydrodynamic shear affects floc characteristic, cake structure and filtration flux for the coagulation macro-filtration of Aphanizomenonflos-aquae. This study lays a theoretical foundation for the optimization of coagulation macro-filtration performance of microalgae and provides a new solution for the large flux removal of cyanobacteria by artificially regulating the coagulation hydrodynamic shear.

When less is more: Reducing environmental noise from open cycle gas turbines with acoustically transparent stacks

Sound radiation from large open cycle gas turbine power stations is strongly refracted by high temperature and velocity gradients in the near field of the exhaust stack outlet. Recent research by the authors has shown that in the presence of a cross-flow, the exhaust plume bends downwind and in doing so leads to significant increases in far-field sound pressure levels of up to 10dB compared to levels predicted from spherical spreading. This paper is the second in a series that explores a unique “acoustically transparent silencer,” which limits sound refraction consequently reducing downwind SPLs by separating the sound from the bulk exhaust flow. The efficacy of the approach is demonstrated two ways; initial characterization studies conducted in the reverberation chambers at the newly renovated Acoustic and Vibration Laboratory, University of Adelaide, and subsequent trials undertaken on a laboratory scale of 250 kW gas turbine in the field. Numerous designs are explored to quantify the effective insertion loss, where it will be shown as the transmission loss of the stack walls is reduced, the effective insertion loss increases, thus demonstrating that less is more. Reductions in ground-plane SPLs of up to 10 dB are achieved with less material and weight compared to traditional hard-walled stacks.

Physical Insight Into Whoosh Noise in Turbocharger Compressors Using Computational Fluid Dynamics

Abstract Whoosh is typically the primary noise concern for turbocharger centrifugal compressors without ported shroud recirculating casing treatments, utilized for spark-ignition automotive applications. Whoosh is characterized by broadband elevation of noise in approximately the 4–13 kHz range, where the lower frequency boundary is dictated by the cut-on frequency of the first multidimensional acoustic mode. At mid-to-low compressor flow rates, swirling, reversed flow emanates from the leading-edge tip region of the main impeller blades, comprising an annular zone at the inducer plane. High velocity gradients are observed near the shear layer between the bidirectional forward and reverse flow which results in the formation of rotating instability cells. Whoosh noise is generated due to the interaction of these rotating instabilities with the leading edge of main impeller blades. Along a line of constant rotational speed, whoosh noise exhibits a dome-like character, where its maximum value occurs in the mid-to-low flow region and the levels decrease at elevated and reduced mass flow rates. To provide insight into the variation of broadband whoosh noise with flowrate, this work includes experimentally validated computational fluid dynamics predictions for five mass flow rates at a fixed rotational speed. These predictions span the constant speed flow range from just below the peak efficiency to near the surge boundary. Computational predictions capture the physical mechanism responsible for the dome-like character of whoosh noise as a function of flowrate at a given rotational speed.

Numerical investigation of clocking effect on unsteady fluid flow in vicinity of tongue for centrifugal pump with vaned diffuser

In order to investigate the clocking effect on unsteady flow distortion in the vicinity of the tongue and hydraulic loss in the volute, experiments and numerical simulations were conducted with four different relative angular positions between the vane and tongue. Numerical results were validated against experimental data, which included measurements of pressure pulsation at the tongue and downstream of the tongue. The radial and circumferential velocities near the tongue at the diffuser-volute clearance were depicted by expanding the rotating surface, and comparisons were made under different diffuser positions to reveal the flow distortion affected by the clocking effect. Shear stress rates and total pressure distributions at the volute exit pipe were also depicted to study the flow loss in the volute affected by the flow pattern from the tongue. The findings show that the pressure fluctuation at the tongue and downstream of the tongue is significantly affected by the diffuser mounting position. The amplitude and intensity of pressure pulsation under θ d1 are clearly smaller than those at other diffuser positions. As the vane trailing edge approaches the tongue, the pressure gradient becomes very large, leading to the formation of a vortex at the tongue that promotes crowding effects and reverse flow occurring in the exit pipe. This, in turn, leads to deflection of the main flow in the volute, high velocity gradients, and subsequently large shear stress rates and hydraulic losses. When the diffuser is installed at θ d4, the flow distortion and flow loss are improved.

Influence of the co- and counter-swirl on combustion instability of the centrally staged combustor

The centrally staged combustor is an effective way to reduce NOx emissions from combustors. However, combustion instability caused by the mutual coupling between flames and acoustics during the combustion process is almost unavoidable. To better understand this problem, the effect of the swirl rotational direction is investigated in this paper using two different schemes with co-swirl and counter-swirl configurations. Pressure fluctuations and flame dynamics are investigated under self-excited combustion oscillation conditions. The CH* chemiluminescence distribution captured by a high-speed camera is utilized to characterize the flame macrostructure and heat release fluctuations. Furthermore, non-oscillating reaction velocity fields are acquired using particle image velocimetry (PIV) technology. The results indicate that the amplitude and frequency of the counter-swirl scheme are higher than those of the co-swirl scheme at varying main stage equivalence ratios. Combining the results from dynamic mode decomposition and the local Rayleigh index, it is found that the heat release regions of the counter-swirl scheme are mainly concentrated in the shear layer. Higher velocity gradients, vorticities, and strain rates in the inner shear layer (ISL) and outer shear layer (OSL) for the counter-swirl scheme are verified using PIV technology. The driving sources of thermoacoustic oscillations are located in the regions of the ISL, OSL, and the area where the flame impinges on the sidewall of the liner. Additionally, the counter-swirl scheme exhibits larger vorticities and strain rates in the ISL and OSL, facilitating the development of thermoacoustic oscillations.

Effects of high-pressure coolant on cooling mechanism in high-speed ultrasonic vibration cutting interfaces

Cutting temperature can be effectively reduced by high-pressure coolant (HPC) method. When using HPC on high-speed ultrasonic vibration cutting (HUVC), the precision machining cutting speed on titanium alloys could reach to 400 m/min due to the significant cutting temperature reduction resulted from HPC and ultrasonic vibration. However, when the cutting speed continued to increase, the advantages of HUVC with HPC were vanished rapidly. Therefore, it was necessary to figure out the effect of HPC on the cooling mechanism of HUVC for further improvement in high-speed machining. In this paper, computational fluid dynamics (CFD) method and experiments on tool life were applied. The results demonstrated that even though the effect of ultrasonic vibration (Nusselt Number) was reduced sharply by HPC from 158,368 to 78296, the high coolant inlet velocity and large temperature gradient caused by the thinned stable-thermal boundary layer (S-TBL) in the cases of applied HPC would in one side cover the loss of the benefits of ultrasonic vibration and in the other side furtherly enhance the heat transfer ability from 26,158 to 46830 J/m2 s. Thus, the tool life had been significantly prolongated more than 10 times. After revealing the cooling mechanism from the aspect of HPC transient state within ultrasonic vibration interfaces, the theory foundation of how to further improve the machinability of difficult-to-cut materials by HUVC was developed.