scholarly journals On mixing enhancement by secondary baroclinic vorticity in a shock–bubble interaction

2021 ◽  
Vol 931 ◽  
Author(s):  
Hong Liu ◽  
Bin Yu ◽  
Bin Zhang ◽  
Yang Xiang
Keyword(s):  
Mixing Time ◽  
Streamwise Vortex ◽  
Time Estimation ◽  
Ambient Air ◽  
Azimuthal Velocity ◽  
Passive Scalar ◽  
Mixing Enhancement ◽  
Time Model ◽  
Advection Diffusion Equation ◽  
Bubble Interaction

To investigate the intrinsic mechanism for mixing enhancement by variable-density (VD) behaviour, a canonical VD mixing extracted from a supersonic streamwise vortex protocol, a shock–bubble interaction (SBI), is numerically studied and compared with a counterpart of passive-scalar (PS) mixing. It is meaningful to observe that the maximum concentration decays much faster in a VD SBI than in a PS SBI regardless of the shock Mach number ( $Ma=1.22 - 4$ ). The quasi-Lamb–Oseen-type velocity distribution in the PS SBI is found by analysing the azimuthal velocity that stretches the bubble. Meanwhile, for the VD SBI, an additional stretching enhanced by the secondary baroclinic vorticity (SBV) production contributes to the faster-mixing decay. The underlying mechanism of the SBV-enhanced stretching is further revealed through the density and velocity difference between the light shocked bubble and the heavy ambient air. By combining the SBV-accelerated stretching model and the initial shock compression, a novel mixing time estimation for VD SBI is theoretically proposed by solving the advection–diffusion equation under a deformation field of an axisymmetric vortex with the additional SBV-induced azimuthal velocity. Based on the mixing time model, a mixing enhancement number, defined by the ratio of VD and PS mixing time further, reveals the contribution from the VD effect, which implies a better control of the density distribution for mixing enhancement in a supersonic streamwise vortex.

Diesel engine performance mapping using a parametrized mixing time model

2017 ◽  
Vol 19 (2) ◽  
pp. 202-213 ◽  
Author(s):  
Michal Pasternak ◽  
Fabian Mauss ◽  
Christian Klauer ◽  
Andrea Matrisciano
Keyword(s):  
Diesel Engine ◽  
Fuel Injection ◽  
Mixing Time ◽  
Combustion Process ◽  
Engine Performance ◽  
Injection Timing ◽  
Engine Control ◽  
Reactor Model ◽  
Time Model ◽  
Tabulated Chemistry

A numerical platform is presented for diesel engine performance mapping. The platform employs a zero-dimensional stochastic reactor model for the simulation of engine in-cylinder processes. n-Heptane is used as diesel surrogate for the modeling of fuel oxidation and emission formation. The overall simulation process is carried out in an automated manner using a genetic algorithm. The probability density function formulation of the stochastic reactor model enables an insight into the locality of turbulence–chemistry interactions that characterize the combustion process in diesel engines. The interactions are accounted for by the modeling of representative mixing time. The mixing time is parametrized with known engine operating parameters such as load, speed and fuel injection strategy. The detailed chemistry consideration and mixing time parametrization enable the extrapolation of engine performance parameters beyond the operating points used for model training. The results show that the model responds correctly to the changes of engine control parameters such as fuel injection timing and exhaust gas recirculation rate. It is demonstrated that the method developed can be applied to the prediction of engine load–speed maps for exhaust NOx, indicated mean effective pressure and fuel consumption. The maps can be derived from the limited experimental data available for model calibration. Significant speedup of the simulations process can be achieved using tabulated chemistry. Overall, the method presented can be considered as a bridge between the experimental works and the development of mean value engine models for engine control applications.

Mixing Enhancement by a Micro-Rotor in Step Microchannel

2008 ◽  
Author(s):  
Thien Xuan Dinh ◽  
Yoshifumi Ogami
Keyword(s):  
Strouhal Number ◽  
Passive Scalar ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Exit Channel ◽  
Mean Flow ◽  
Two Fluids ◽  
Convective Mixing ◽  
Active Mixer ◽  
And Performance

This paper describes numerically the design and performance of an active mixer which aims to exploit the chaos generated by a micro-rotor in continuous flow. The mixer consists of a step contraction-expansion microchannel and a micro-rotor placed at the step. The micro-rotor can be set into motion by laser power or magnetic field, inducing 3D motion in the surrounding fluid. By tracking streaklines from the inlet, we observed that fluids from the inlet can penetrate into the space between the paddles of the rotor, and then are mixed here. The streaklines also show that two fluids are twisted 90 degrees after passing the rotor region. It implies that mixing in the exit channel occurs in the height instead in the width of the exit channel. It makes the mixer applicable for channels with high aspect ratio of the cross section. The effectiveness mixing of the mixer is measured by the homogeneity distribution of a passive scalar on the outlet of the mixer. The results show that effectiveness of convective mixing induced by the rotor depends on Strouhal number, which is defined as the ratio of tip paddle velocity to mean flow in the channel. Mixing efficiency increases with increasing Strouhal number.

Time Estimation and Total Subjective Time

1977 ◽  
Vol 44 (2) ◽  
pp. 527-532 ◽  
Author(s):  
James L. Walker
Keyword(s):  
Life Span ◽  
Time Estimation ◽  
Chronological Age ◽  
Subjective Time ◽  
Square Root ◽  
Time Model ◽  
Subjective Duration ◽  
Retrospective Reports ◽  
Age Model ◽  
Perceived Duration

Techniques for estimation of magnitude were used in a questionnaire given to 100 university students to test the hypothesis that the subjective duration of an interval of actual time decreases in proportion to total subjective time rather than total chronological age. The results supported the subjective time hypothesis for retrospective reports of perceived duration of a year at both one-half and one-quarter of the subject's present age. In both cases the subjective time hypothesis provided a better fit to the data than the chronological age model. The hypothesis of the subjective time model that subjective life-span is equal to the square root of the statistically expected life-span was also tested but was not confirmed.

scholarly journals Sub-Bottom Sediment Classification Using Reliable Instantaneous Frequency Calculation and Relaxation Time Estimation

2021 ◽  
Vol 13 (23) ◽  
pp. 4809
Author(s):  
Shaobo Li ◽  
Jianhu Zhao ◽  
Hongmei Zhang ◽  
Siheng Qu
Keyword(s):  
Relaxation Time ◽  
Instantaneous Frequency ◽  
Relaxation Times ◽  
Time Estimation ◽  
Variational Mode Decomposition ◽  
Time Data ◽  
Time Model ◽  
Sediment Types ◽  
Mode Decomposition ◽  
Time Map

The shift in IF (instantaneous frequency) series and the corresponding relaxation time have the potential to characterize sediment properties. However, these attributes derived from SBP (sub-bottom profiler) data are seldom used for offshore site investigations because of the unsoundness in attribute calculation. To overcome this problem, a new reliable method combining VMD (variational mode decomposition) and WVD (Wigner–Ville distribution), as well as relaxation time, is presented. Since the number of modes in classical VMD should be provided in advance, a modified VMD algorithm, MVMD (modified variational mode decomposition), is proposed here, where the distribution of the frequency domain of modes is taken into account to automatically determine the number of modes. Through the relaxation time model, the IF data of a series of pings calculated through MVMD-WVD are transformed into a relaxation time map. A robust estimation algorithm is applied to the relaxation time map to reduce the effects of interferences and obtain robust relaxation times. The final relaxation time data are used to determine the sediment types. Real data from SBP experiments, as well as borehole sampling and geotechnical analysis results, verified the good performance of the proposed method.

Spatially varying mixing of a passive scalar in a buoyancy-driven turbulent flow

2014 ◽  
Vol 742 ◽  
pp. 701-719
Author(s):  
Daan D. J. A. van Sommeren ◽  
C. P. Caulfield ◽  
Andrew W. Woods
Keyword(s):  
Diffusion Coefficient ◽  
Turbulent Flow ◽  
Turbulent Diffusion ◽  
Saline Solution ◽  
Passive Scalar ◽  
Eddy Diffusion ◽  
Turbulent Diffusion Coefficient ◽  
Advection Diffusion Equation ◽  
Eddy Diffusion Coefficient ◽  
Spatially Varying

AbstractWe perform experiments to study the mixing of passive scalar by a buoyancy-induced turbulent flow in a long narrow vertical tank. The turbulent flow is associated with the downward mixing of a small flux of dense aqueous saline solution into a relatively large upward flux of fresh water. In steady state, the mixing region is of finite extent, and the intensity of the buoyancy-driven mixing is described by a spatially varying turbulent diffusion coefficient $\kappa _v(z)$ which decreases linearly with distance $z$ from the top of the tank. We release a pulse of passive scalar into either the fresh water at the base of the tank, or the saline solution at the top of the tank, and we measure the subsequent mixing of the passive scalar by the flow using image analysis. In both cases, the mixing of the passive scalar (the dye) is well-described by an advection–diffusion equation, using the same turbulent diffusion coefficient $\kappa _v(z)$ associated with the buoyancy-driven mixing of the dynamic scalar. Using this advection–diffusion equation with spatially varying turbulent diffusion coefficient $\kappa _v(z)$, we calculate the residence time distribution (RTD) of a unit mass of passive scalar released as a pulse at the bottom of the tank. The variance in this RTD is equivalent to that produced by a uniform eddy diffusion coefficient with value $\kappa _e= 0.88 \langle \kappa _v \rangle $, where $\langle \kappa _v \rangle $ is the vertically averaged eddy diffusivity. The structure of the RTD is also qualitatively different from that produced by a flow with uniform eddy diffusion coefficient. The RTD using $\kappa _v$ has a larger peak value and smaller values at early times, associated with the reduced diffusivity at the bottom of the tank, and manifested mathematically by a skewness $\gamma _1\approx 1.60$ and an excess kurtosis $\gamma _2\approx 4.19 $ compared to the skewness and excess kurtosis of $\gamma _1\approx 1.46$, $\gamma _2 \approx 3.50$ of the RTD produced by a constant eddy diffusion coefficient with the same variance.

Mixing Enhancement in a Novel Type of “Split and Recombine” Static Mixer

2018 ◽  
Author(s):  
Charbel Habchi ◽  
Thierry Lemenand ◽  
Fouad Azizi
Keyword(s):  
Concentration Profile ◽  
Mixing Time ◽  
Reynolds Numbers ◽  
Mixing Enhancement ◽  
Mixing Process ◽  
Low Reynolds Numbers ◽  
Static Mixers ◽  
Two Fluids ◽  
Lamination Process ◽  
Time Required

Mixing in laminar flow regimes is crucial for many engineering applications in which highly viscous and fragile fluids are used. Moreover, the compactness of laminar mixers is a great challenge due to the large mixing time required to obtain the desired homogeneity. The “Split And Recombine” (SAR) static mixers are a promising solution for this challenge. This type of mixers consists of a network of separated and then recombined channels in which two fluids are introduced separately and mixed by a multi-lamination process. The SAR static mixers perform a series of baker’s transforms on the concentration profile enhancing thus the mixing process at very low Reynolds numbers. In the present study, numerical simulations are carried out to analyze the mixing process in a new topology of SAR mixer with double separation and recombination in order to increase the lateral gradients and destroy the concentration profile faster. The new geometry proposed here is compared to two SAR configurations widely studied in the open literature namely the Gray and the Chen SAR configurations. The results show a good enhancement of the mixing process in the new double SAR configuration with decrease in the power dissipation.

scholarly journals Motion of Passive Scalar by Elasticity-Induced Instability in Curved Microchannel

2014 ◽  
Vol 6 ◽  
pp. 734175 ◽  
Author(s):  
Xiao-Bin Li ◽  
Hong-Na Zhang ◽  
Yang Cao ◽  
Marie Oshima ◽  
Feng-Chen Li
Keyword(s):  
Flow Field ◽  
Flow Structure ◽  
Function Analysis ◽  
Passive Scalar ◽  
Main Direction ◽  
Mixing Enhancement ◽  
Chaotic Flow ◽  
Wide Range ◽  
Elastic Instabilities ◽  
Passive Mixing

This paper presented a direct numerical simulation (DNS) study on the elasticity-induced irregular flow, passive mixing, and scalar evolution in the curvilinear microchannel. The mixing enhancement was achieved at vanishingly low-Reynolds-number chaotic flow raised by elastic instabilities. Along with the mixing process, the passive scalar transportation carried by the flow was greatly affected by the flow structure and the underlying interaction between microstructures of viscoelastic fluid and flow structure itself. The simulations are conducted for a wide range of viscoelasticity. As the elastic effect exceeds the critical value, the flow tends to a chaotic state, while the evolution of scalar gets strong and fast, showing excellent agreement with experimental results. For the temporal changing of scalar gradients, they vary rapidly in the form of isosurfaces, with the shape of “rolls” in the bulk and evolving into “threads” near the wall. That indicates that the flow fields should be related to the deformation of viscoelastic micromolecules. The probability distribution function analysis between micromolecular deformation and flow field deformation shows that the main direction of molecular stretching is perpendicular to the main direction of flow field deformation. It implies they are weakly correlated, due to the confinement of channel wall.

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