Mixing Enhancement Research Articles

Seamount-induced mixing revealed through idealized experiments and its parameterization in an Oceanic General Circulation Model

Though hidden under hundreds if not thousands of meters of seawater, seamounts and the vertical mixing they induce are crucial components of mixing in the global ocean. To gain a deeper understanding of the influence of different seamount topographies on vertical mixing, and distributional characteristics and causes of seamount-induced vertical mixing in different grid scales, this study builds a three-dimensional idealized seamount in a regional ocean model. Results illustrate that, given a constant westward inflow, seamount-induced vertical mixing is mainly distributed on the north and south sides of the summit, the foot of the north and east sides, and downstream from the seamount. Mixing on the north and south sides of the seamount summit, and at its foot, is caused by density fronts induced by friction between ambient currents and the seamount. By contrast, mixing downstream of the seamount is due to convective instability, lee waves, and seamount wake eddies. Vertical mixing enhancement at subgrid scales mainly occurs on the southern and eastern sides of the seamount summit, at its foot, and at the seabed. Here, vertical mixing is primarily caused by friction in the bottom boundary layer. Sensitivity experiments reveal that mixing intensity is affected by seamount topographic roughness. As the seamount height and slope increases, vertical mixing at the seamount summit is enhanced, and mixing on its slope gradually shifts to the seamount base. Due to limitations of global ocean model resolutions, seamount-induced vertical mixing is often ignored. Consequently, a global background mixing coefficient (BMC) relationship is built from an Argo dataset-estimated BMC combined with seafloor topography roughness, and then applied to a global Oceanic General Circulation Model (OGCM). Results show that simulations of deep-sea water temperature and salinity improved after modifying the BMC in seamount-rich areas, with a simultaneous reduction of root-mean-square error by 17% and 12%, respectively.

Biochemical Reaction Acceleration by Electrokinetic Mixing in a Microfluidic Chip.

In the past two decades, a large number of micromixers have been studied and reported to solve the low mass transfer in microchannels. However, there is still a lack of research on how much biochemical reaction efficiency or detection sensitivity can be improved by microfluidic rapid mixing. In this study, using our previously developed ultrafast microelectrokinetic turbulent (μEKT) mixing method, taking glucose oxidation reaction as the research object, we investigated the effect of increasing microfluidic mass transfer efficiency on the efficiency and sensitivity of biochemical reactions. The results showed that fast mixing could improve the enzymatic reaction efficiency by improving the mass transfer efficiency. Further detection of glucose in glucose solutions and blood samples showed that the fast mixing could significantly improve the sensitivity of detection. These results indicate that mixing enhancement can be a significant step of microfluidics-based applications such as POCT, liquid biopsy, food safety, and so on.

A circulation prediction model for ramp and vortex generator in supersonic flow: A numerical study

The circulation growth of supersonic streamwise vortex (SSV) formed by a ramp is complicated and essential to estimate scramjet mixing behavior, while a predictive model for circulation growth is still lacking. Through well-validated RANS simulations, it is observed that the circulation growth of streamwise vortex for a ramp is similar to one for vortex generator, which determines three important geometric parameters for SSV growth, i.e., ramp angle, swept angle, and length of the leading edge. The results of different geometries with these three parameters further show that the SSV circulation growth is proportional to the downwash speed influenced by oblique shock intensity. By building the relation between downwash speed and geometric parameters, an SSV circulation prediction model is proposed and validated by the circulation curves of all cases under a wide range of free stream Mach numbers. The novel circulation model has the potential for the preliminary mixing enhancement design for a scramjet ramp injector.

Numerical study of multi-jet with upstream divergent ramp at supersonic cross flow

The efficiency of supersonic air-breathing propulsion is highly associated with fuel mixing enhancement in a tank. Current study performed all-inclusive simulations to scrutinize the impacts of the upstream inclined ramp on the fuel mixing mechanism of four multi-jet configurations at supersonic crossflow. The presence of divergent ramp in two jet spaces of 3 Dj and 10 Dj is fully analyzed via three-dimensional modeling of supersonic flow with four cross jets. Circulation factor and mixing performance are studied to obtain an efficient configuration for the injection system. Our results show that the fluctuation and strength of the circulation factor are more in low jet space. The assessment of fuel mixing productivity indicates that fuel mixing of low jet space is approximately 15% more than high jet space far downstream. Our findings show that a strong unite vortex produced in low jet space is more effective on fuel mixing than low multi vortices.

Numerical study on a novel device for hydrogen mixing enhancement in a scramjet engine: Coaxial injector

The mixing process between the fuel and air is very important for the efficient operation of the scramjet engine, and it has attracted an increasing attention all over the world. In this work, the coaxial injector was developed and investigated by means of the three-dimensional Reynolds-averaged Navier-Stokes equations coupled with the SST k-ω turbulence model, and the numerical approaches were validated against the non-dimensional experimental streamwise velocities in four different streamwise positions on the symmetry plane. Further, the influence of the jet-to-crossflow pressure ratio for the hydrogen and air is evaluated numerically as well. The obtained results show that the mixing process induced by the coaxial injector is conducted more quickly, and it is a promising mixing enhancement device for the scramjet engine. The smallest value of the mixing length for the cases studied in the current study is 1.69. It brings nearly no additional total pressure loss when compared with the single injector, and the jet-to-crossflow pressure ratio has only a small impact on the total pressure recovery coefficient.

Non-isothermal flow past a heated circular cylinder in subcritical regime: a numerical investigation based on large-eddy simulation

Compressible flow past a heated circular cylinder at subcritical Reynolds number of 3900 is numerically investigated by using the large-eddy simulation method. Rigorous validations of the numerical model are carefully performed under isothermal conditions on the basis of available experimental data. The calculated mean flow and Reynolds stresses show good agreement with the published experimental data. The effects of temperature difference between the cylinder surface and the freestream on the flow statistics and thermal characteristics are further studied in detail by setting two kinds of wall temperature boundary conditions. It is manifested that increasing the wall temperature leads to the augmentation of skin friction drag, suppression of turbulent intensity, enhancement of flow mixing and extension of recirculation zone. In addition, it is found that the variations of thermo-physical properties pose a slight effect on the wall heat flux before the boundary layer separates from the cylinder surface. It is worth noting that the recirculation bubble length can be used as a distance scaling parameter to weaken the temperature dependence of the flow and thermal statistics. These results provide a more detailed insight into the statistical difference in the wake region of cylinder when the temperature effect is taken into account.

From fundamental to practical aspects of bubble-induced agitation

<h2>Abstract</h2> While the buoyancy-driven rise of bubbles in a liquid seems to be a simple problem, the bubble dynamics, interfacial phenomena, and the interaction between the rising bubbles and surrounding liquid flow turned out to be very complex, which have fascinated the multiphase flow society for a long time. For example, the rise dynamics of a bubble is governed by the wake structure behind the bubble, which is also known to be the origin of bubble-induced agitation (turbulence) on the liquid flow. The understanding of this interaction is critical because some wish to utilize bubbles for various engineering purposes while others to detect and remove them. Here, the fundamental nature of bubble-induced flow or disturbance that we have learned will be first discussed briefly, which will be followed by the discussion of some of our recent efforts to utilize the rising bubbles as a strategy of flow control such as the fluid mixing enhancement and heat transfer augmentation.

Shear Instability and Turbulent Mixing in the Stratified Shear Flow Behind a Topographic Ridge at High Reynolds Number

Observations on the lee of a topographic ridge show that the turbulence kinetic energy (TKE) dissipation rate due to shear instabilities is three orders of magnitude higher than the typical value in the open ocean. Laboratory-scale studies at low Reynolds number suggest that high turbulent dissipation occurs primarily within the core region of shear instabilities. However, field-scale studies indicate that high turbulence is mainly populated along the braids of shear instabilities. In this study, a high-resolution, resolving the Ozmidov-scale, non-hydrostatic model with Large Eddy Simulation (LES) turbulent closure is applied to investigate dominant mechanisms that control the spatial and temporal scales of shear instabilities and resulting mixing in stratified shear flow at high Reynolds number. The simulated density variance dissipation rate is elevated in the cusp-like bands of shear instabilities with a specific period, consistent with the acoustic backscatter taken by shipboard echo sounder. The vertical length scale of each cusp-like band is nearly half of the vertical length scale of the internal lee wave. However, it is consistent with instabilities originating from a shear layer based on linear stability theory. The model results indicate that the length scale and/or the period of shear instabilities are the key parameters to the mixing enhancement that increases with lateral Froude number FrL, i.e. stronger shear and/or steeper ridge.

Effect of AGM waste filament incorporation on the performance of solid waste curing agents

Absorbed Glass Mat (AGM) waste filaments are waste staple fibers from the production of AGM functional paper. Incorporation of AGM waste filaments into solid waste based curing agents, the experimental results showed that the maximum increase in compressive strength and flexural strength of the cured specimens was achieved when 5% mass fraction AGM waste filament was added with compressive strengths of 9.87 MPa and 10.96 MPa at 7 d and 28 d, respectively, representing an increase of 15.57% and 17.34% compared to the unadulterated one. The flexural strength was increased by 53.55% and 49.71% compared with that of the unadulterated AGM waste filament. The strength enhancement effect of mixing different shapes of AGM waste filaments is higher than that of mixing each shape alone, which proves that AGM waste filaments have a mixed enhancement effect.

Enhanced Turbulent Mixing in the Upper Ocean Induced by Super Typhoon Goni (2015)

Based on the satellite-observed sea surface temperature (SST) data, high-resolution Argo observations and hybrid coordinate model (HYCOM) reanalysis results, this study examined the upper ocean response to Super Typhoon Goni in 2015 in the western north Pacific and revealed the significant diapycnal diffusivity enhancement in the upper ocean induced by Goni. Results indicate that the maximum SST cooling caused by Goni was 7.7 °C, which is greater than the SST cooling caused by most typhoons. The severe SST cooling was related to the enhancement of turbulent mixing induced by Goni. To the right of the typhoon track, the diapycnal diffusivity enhancement in the upper ocean caused by Goni could reach three orders of magnitude, from O (10−5 m2/s) to O (10−2 m2/s) and could last at least 9 days after the passage of Goni. In contrast, the diapycnal diffusivity to the left of the typhoon track did not show significant variations. The enhancement of turbulent mixing was found to be consistent with Goni-induced near-inertial kinetic energy calculated from the HYCOM reanalysis results, which suggests that the enhanced turbulent mixing was caused by Goni-induced near-inertial waves.

Effects of circular and non-circular nozzle exit geometries on subsonic and supersonic jet propagations

The mixing enhancement and core length reduction of a jet without significant loss of thrust are essential for reducing infrared radiation, mitigating aeroacoustic noise, improving combustion characteristics, and thrust vectoring. The jet mixing can be improved by manipulating the flow behavior. In subsonic and sonic jets, the flow manipulation may be achieved by utilizing nozzles with non-circular geometries that shed vortices of varying size due to their non-uniform azimuth curvatures. Non-uniform vortices generate differential spreading along the nozzle’s perimeter, causing axis switching and improving entrainment characteristics. Therefore, the present study examines the effects of two non-circular nozzle exit shapes (elliptic and square) on the mixing augmenting efficacy at subsonic and sonic flow conditions. The circular nozzle is tested for comparison. Both quantitative and qualitative analyses evaluate the efficacy of nozzles with non-circular exit geometries. Among the configurations investigated, the elliptic nozzle is superior in shortening the potential core length and enhancing the jet spread. A maximum reduction of 18.75% in core length with rapid jet decay was accomplished with the elliptic nozzle. The measurement of pressure profiles at different streamwise locations reveals that the spread rate is greater for elliptic and square jets than their circular counterpart. The elliptic jet exhibits the highest spread along the minor-axis direction compared to the major-axis direction. The differential jet spread rate in the elliptical jet causes an early axis-switching––direct evidence of mixing augmentation. Shadowgraph images show the asymmetric pattern of shock cell structures and differential spreading in elliptic and square jets.

Mixing enhancement using the aiding and opposing flow effects in curved micro channel

Development of microfluidic devices requires precise control, quantitative measurement and analysis of small volume of fluids. Micromixers are important microfluidic components finding applications in various biomedical, chemical and reactor systems. The secondary flows (Dean vortices) induced in curved channels are scientifically of interest due to their potential contribution to enhance mixing of fluids. In the present work, the aiding and opposing flow effects due to semi-circular wall obstruction location (convex/concave wall) on mixing and fluid flow in a planar curved microchannel is reported. The curvature induced Dean vortices along with locally enhanced converging-diverging convective flow due to the presence of wall obstruction contribute together to the enhancement of mixing but the opposing effect of concave wall obstruction is credited for better mixing performance when compared with flow favoring convex wall obstruction. Qualitative and quantitative assessment of flow patterns and mixing efficiency was performed by carrying out numerical simulations for Reynolds number varying from 20 to 300. The concept was further extended to curved serpentine micro channel with obstructions on the convex or concave walls. In this case, the concave wall obstructions proved to be better at enhancement of mixing at all the Reynolds numbers.

Numerical Investigation of the Flow and Infrared Radiation Characteristics of Nozzles with Transverse Jets of Different Shapes

The hot jet of an aero engine is one of the main radiation sources of infrared detectors in 3–5 microwave bands. Transverse jets were introduced into a hot jet to enhance mixing and reduce the infrared radiation characteristics. This proved to be a high-efficiency and low-resistance infrared suppression technology. The steady-state distribution of temperature data was simulated, which was needed in the thermal radiation calculation. The radiation characteristics were calculated based on the anti-Monte Carlo method in 3–5 microwave bands. The mechanics of enhanced mixing by a rectangular nozzle or transverse jets was investigated with the LES simulation. Compared with an axisymmetric nozzle, a rectangular nozzle induced abundant counter-rotating vortex pairs (CVP), hairpins, shears, and helical vortexes, which resulted in significant mixing enhancement and infrared radiation decrease of the hot jets. Further, circumferential transverse jets of different types were introduced downstream of the nozzle. These jets enhanced the mixing and reduced the infrared radiation in the 3–5 µm band. The mixing characteristics of these different schemes were studied in detail. Large-scale vortices formed on the windward portion of the hot jet boundary under the effect of the transverse jets, which caused strong CVP structures. They also resulted in hairpin vortexes, shear vortexes, and helical vortexes appearing earlier and occurring more frequently than with nozzles without transverse jets. The enhanced mixing caused by the transverse jets led to an increase in temperature decay and a decrease in infrared radiation in the 3–5 µm band. Further, transvers jets of different geometrical shapes (rectangular, cube, and circular schemes) achieved different mixing characteristics, and the rectangular transverse jets allowed the most significant mixing for the largest Q criterion value.

Adiabatic–radiative shock systems in YSO jets and novae outflows

Context. The termination regions of non-relativistic jets in protostars and supersonic outflows in classical novae are non-thermal emitters. This has been confirmed by radio and gamma-ray detection, respectively. A two-shock system is expected to be formed in the termination region where the jet, or the outflow material, and the ambient medium impact. Radiative shocks are expected to form in these systems given their high densities. However, in the presence of high velocities, the formation of adiabatic shocks is also possible. A case of interest is when the two types of shocks occur simultaneously. Adiabatic shocks are more efficient at particle acceleration while radiative shocks strongly compress the gas. Furthermore, a combined adiabatic–radiative shock system is very prone to developing instabilities in the contact discontinuity, leading to mixing, turbulence, and density enhancement. Additionally, these dense non-relativistic jets and outflows are excellent candidates for laboratory experiments as demonstrated by magnetohydrodynamics scaling. Aims. We aim to study the combination of adiabatic and radiative shocks in protostellar jets and novae outflows. We focus on determining the conditions under which this combination is feasible together with its physical implications. Methods. We performed an analytical study of the shocks in both types of sources for a set of parameters by comparing cooling times and propagation velocities. We also estimated the timescales for the growth of instabilities in the contact discontinuity separating both shocks. We studied the hydrodynamical evolution of a jet colliding with an ambient medium with 2D numerical simulations, confirming our initial theoretical estimates. Results. We show that for a wide set of observationally constrained parameters, the combination of an adiabatic and a radiative shock is possible at the working surface of the termination region in jets from young stars and novae outflows. We find that instabilities are developed at the contact discontinuity, mixing the shocked materials. Additionally, we explore the magnetohydrodynamic parameter scaling required for studying protostellar jets and novae outflows using laboratory experiments on laser facilities. Conclusions. The coexistence of an adiabatic and a radiative shock is expected at the termination region of protostellar jets and novae outflows. This scenario is very promising for particle acceleration and gamma-ray emission. The parameters for scaled laboratory experiments are very much in line with plasma conditions achievable in currently operating high-power laser facilities. This provides a new means for studying novae outflows that has never been considered before.

Near-wall characteristics of wall-normal jets generated by an annular dielectric-barrier-discharge plasma actuator

An annular dielectric barrier discharge (DBD) plasma actuator has potential flow control applications due to having a simple design, no moving parts, and being light weight and fully electronic with low power consumption. We have experimentally investigated the detailed near-wall flow characteristics generated by the annular DBD plasma actuator. The actuator generates a wall-normal jet on top of a recirculation zone, with characteristics dependent on the actuation parameter i.e., burst frequency and amplitude. Our results can provide guidance for design and process optimization of several flow control applications i.e. lift enhancement, drag reduction, and mixing enhancement etc.

Study of Mixing Enhancement in Helium Jet/Shock Interaction by Laser Pulses

The mechanism of low-density jet mixing enhancement by a pulsed laser plasma is investigated through a combined numerical and experimental method. A helium jet is injected in parallel to a Mach 2.4 supersonic airstream, and interacts with an oblique shock generated by a compression ramp of 20 deg. The helium jet Mach numbers are set to be 1.01 and 1.80 for the sonic and supersonic fuel jets in the scramjet combustor, respectively. For jet mixing enhancement control, the laser pulses at frequencies ranging from 5 to 25 kHz are introduced inside the jet. The experimental results show that laser pulses significantly improve the jet mixing with jet width increased by 30% seen from the averaged schlieren images. Three-dimensional unsteady Reynolds-averaged Navier–Stokes equations are solved, and the numerical results further reveal that the jet mixing enhancement is strongly associated with large-scale vortex rings stimulated by the laser pulses interacting with shock waves. These large-scale vortices prompt the jet cross-sectional area and mixing efficiency two times larger. However, the total pressure recovery coefficient is reduced by 0.16% downstream. Meanwhile, the turbulent kinetic energy of jet flow is significantly increased and favorable for the jet mixing.

Numerical and experimental investigation of mixing enhancement in the passive planar mixer with bent baffles

As an important pre-treatment part of the micro-total analysis system, the micromixer that ensures the uniform mixing of samples and reagents in a short distance is essential for the improvement of the detection accuracy. Here, a type of passive planar micromixer with narrow gaps and obstacles arranged in the mixing units is proposed. The mixer utilizes the micro-jets and bent baffles to split and recombine streams and promote vortex formation, which breaks the laminar flow when the Reynolds number (Re) varies from 0.1 to 50. The performance of the unoptimized mixer decreases first and then increases with the increase of Re. However, a poor mixing quality is found in the range of 0.5 ≤ Re ≤ 15. To surmount the above shortcomings, a comprehensive geometrical study is performed on this micromixer and the effects of various parameters such as the width and length of the gap, the shape of the baffle, and mixing unit on the mixing quality and pressure drop are analyzed. The results show that the reduction of gap width and the increase of the gap length result in the improvement of mixing efficiency. The staggered Z-shaped baffles accelerate the mixing process compared to traditional V-shaped baffles owing to asymmetric flow and vortex, which leads to excellent mixing. The results also show that the octagonal mixing unit increases mixing efficiency without basically changing pressure drop because the progressive and tapered structure enhances the convection. Lastly, a novel micromixer whose mixing performance is close to 0.8 at Re = 0.5 at the short distance of 2800 µm is proposed, fabricated and tested. This work provides a reference for the development of high-efficiency mixers in the pre-processing part of the micro-total analysis system.

Comparison of Computed Tomography Features of Gastric and Small Bowel Gastrointestinal Stromal Tumors With Different Risk Grades.

This study aimed to compare the computed tomography (CT) features of gastric and small bowel gastrointestinal stromal tumors (GISTs) and further identify the predictors for risk stratification of them, respectively. According to the modified National Institutes of Health criteria, patients were classified into low-malignant potential group and high-malignant potential group. Two experienced radiologists reviewed the CT features including the difference of CT values between arterial phase and portal venous phase (PVPMAP) by consensus. The CT features of gastric and small bowel GISTs were compared, and the association of CT features with risk grades was analyzed, respectively. Determinant CT features were used to construct corresponding models. Univariate analysis showed that small bowel GISTs tended to present with irregular contour, mixed growth pattern, ill-defined margin, severe necrosis, ulceration, tumor vessels, heterogeneous enhancement, larger size, and marked enhancement compared with gastric GISTs. According to multivariate analysis, tumor size (P < 0.001; odds ratio [OR], 3.279), necrosis (P = 0.008; OR, 2.104) and PVPMAP (P = 0.045; OR, 0.958) were the independent influencing factors for risk stratification of gastric GISTs. In terms of small bowel GISTs, the independent predictors were tumor size (P < 0.001; OR, 3.797) and ulceration (P = 0.031; OR, 4.027). Receiver operating characteristic curve indicated that the CT models for risk stratification of gastric and small bowel GISTs both achieved the best predictive performance. Computed tomography features of gastric and small bowel GISTs are different. Furthermore, the qualitative and quantitative CT features of GISTs may be favorable for preoperative risk stratification.

Mixing enhancement mechanism of combined H2–Water jets in supersonic crossflows in a combustor with an expanded section

To obtain the mixing enhancement mechanism of H 2 –Water combined jets in supersonic crossflows in a combustor with expanded section for rotating detonation ramjet, the flow field shape and spray structure were studied by experimental and numerical methods. The Eulerian–Lagrangian method was used to investigate the diffusion mechanism and H 2 –Water interaction law of combined jets with different sequences. At the same time, high-speed photography and the schlieren technique were used to capture the flow field. The effects of jet pressure drop, orifice diameter, orifice spacing, incoming Mach number, and other parameters on the penetration depth of water jets were studied. The results of experiment and simulation show that using H 2 –Water combined jets, the penetration depth of the jet spray can be greatly increased and the jet mixing effect can be significantly improved, which will contribute to the engine's ignition and stable combustion. In the case of pre-water/post-H 2 , the penetration depth of the hydrogen jet is greater. In the case of pre-H 2 /post-water, the hydrogen jet raises the water spray mainly by protecting the integrity of the water column. • The mechanism of the mixing process of H2-Water combined jets in supersonic crossflows was investigated. • Eulerian-Lagrangian method was used to investigate the H2-Water interaction law in a combustor with an expanded section. • The penetration depth of water jets under different parameters was studied experimentally.

Numerical investigation on mixing enhancement of the cavity with pulsed jets under oblique shock wave interference

In this investigation, the impact of steady jets and pulsed jets at various frequencies on mixing performance is studied to achieve adequate mixing in the cavity-based combustion chamber disturbed by the oblique shock wave. The pulsed ethylene jets are simulated numerically employing the SST k–ω turbulence model and unsteady RANS. The grid-independence verification is first performed, which is in accordance with the experimental data of wall pressure. The flow structures, mixing efficiency and jet penetration of steady jets and pulsed jets are compared and analyzed. The pulsed jets facilitate the mixing of ethylene in contrast to the steady jets, especially in the near field of the jet. Pulsed jets improve the penetration depth of lower jet-to-crossflow pressure ratio steady jets, but the advantage in this regard is limited as the frequency increases. Among the pulsed jets considered in this study, the 200 kHz pulsed jet has the best mixing coefficient, but the 50 kHz pulsed jet is optimal at the penetration depth. Hence, the pulsed jet with appropriate frequency can simultaneously achieve a larger penetration depth and a more desirable mixing efficiency.