Scalar Dissipation Research Articles

Impact of a discretization scheme on an autoignition time in LES of a reacting droplet-laden mixing layer

We analyse an autoignition process in a two-phase flow in a temporally evolving mixing layer formed between streams of a cold liquid fuel (heptane at 300 K) and a hot oxidizer (air at 1000 K) flowing in opposite directions. We focus on the influence of a discretization method on the prediction of the autoignition time and evolution of the flame in its early development phase. We use a high-order code based on the 6th order compact difference method for the Navier–Stokes and continuity equations combined with the 2nd order Total Variation Diminishing (TVD) and 5th order Weighted Essentially Non-Oscillatory (WENO) schemes applied for the discretization of the advection terms in the scalar transport equations. The obtained results show that the autoignition time is more dependent on the discretization method than on the flow initial conditions, i.e., the Reynolds number and the initial turbulence intensity. In terms of mean values, the autoignition occurs approximately 15% earlier when the TVD scheme is used. In this case, the ignition phase characterizes a sharp peak in the temporal evolution of the maximum temperature. The observed differences are attributed to a more dissipative character of the TVD scheme. Its usage leads to a higher mean level of the fuel in the gaseous form and a smoother distribution of species resulting in a lower level of the scalar dissipation rate, which facilitates the autoignition process.

Scalar structures around the mixing interface of a thermal mixing layer in multiscale-generated decaying turbulent flows visualized using DNS database

This study shows the results for the visualization of a scalar mixing layer in decaying turbulence produced by a multiscale-generated turbulence grid. For this purpose, a thermal mixing layer in the air is used as the scalar mixing layer. A fractal generating grid proposed in the previous study is used for the generation of multiscale-generated turbulence. In addition, a parameter for determining the grid shape is varied. The scalar field is then visualized using a database given by direct numerical simulations. The instantaneous scalar field, the instantaneous scalar fluctuation field, the instantaneous scalar variance field, and the scalar dissipation field are visualized, and the visualization results are discussed using the wake interaction length scale.

Experimental investigation of solute transport in variably saturated porous media using x-ray computed tomography

Solute transport through variably saturated porous media is ubiquitous in multiple subsurface flows, piquing the geoscience community's interest. This study adopts a novel experimental approach using microfocus x-ray computed tomography for real-time imaging of a three-dimensional NaI tracer plume in a partially saturated packing column. A stabilized two-phase flow field is achievable through continuous co-injection of two-phase fluids: NaCl solvent and pump oil. Thus, the critical role of the NaCl saturation Sw and Péclet number on dispersion can be fully studied by controlling the NaCl fractional flow rate and the total flow rate from the Buckley–Leverett theory. Furthermore, we study solute transport behavior based on statistical moments, the dispersion coefficient, the dilution index, and the mean scalar dissipation rate. Experimental results indicate that the solute transport is Fickian for high Sw ≥ 0.34. In contrast, anomalous transport behavior is found for Sw < 0.34, where the concentration distribution is initially left-tailed and leptokurtic before reaching a well-dispersed regime. The dispersion coefficient is 2–10 times larger for partially saturated cases compared with the fully saturated case and shows a non-monotonical dependency on Sw. Finally, the analysis of the dilution index indicates that the overall mixing strength increases when Sw decreases, whereas the mean scalar dissipation rate reveals that the time scaling of transverse mixing is the largest at an intermediate Sw. The results can be used to elucidate the solute transport behavior in a two-phase system.

Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz

Abstract Observations of the Galactic Center supermassive black hole Sagittarius A* (Sgr A*) with very long-baseline interferometry (VLBI) are affected by interstellar scattering along our line of sight. At long radio observing wavelengths (≲1 cm), the scattering heavily dominates image morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in 2017 provided new constraints on the intrinsic and scattering properties of Sgr A*, but the stochastic nature of the scattering Requires multiple observing epochs to reliably estimate its statistical properties. We present new observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian structure in the scattered image seen in 2017. In particular, the ALMA–GBT baseline shows more flux density than expected for an anistropic Gaussian model, providing a tight constraint on the source size and an upper limit on the dissipation scale of interstellar turbulence. We find an intrinsic source extent along the minor axis of ∼100 μas both via extrapolation of longer wavelength scattering constraints and direct modeling of the 3.5 mm observations. Simultaneously fitting for the scattering parameters, we find an at-most modestly asymmetrical (major-to-minor axis ratio of 1.5 ± 0.2) intrinsic source morphology for Sgr A*.

Measurements of trajectories and spatial distributions of diatoms (Coscinodiscus spp.) at dissipation scales of turbulence

Measurements of trajectories and spatial distributions of diatoms (Coscinodiscus spp.) at dissipation scales of turbulence

Weakened energy cascade in elastoinertial turbulence.

Newtonian turbulence is characterized by interscale transport of energy from the forcing scales to the dissipation scales. In elastoinertial turbulence, this interscale energy flux is weakened. Here, we explain this phenomenon by numerically showing that elastoinertial energy is predominantly dissipated through polymer chain relaxation. As opposed to Newtonian dissipation, chain relaxation is neither restricted to small nor to large scales but instead it is effective on all the scales. Chain relaxation does not therefore require interscale transport of elastoinertial energy from the forcing scales to the dissipation scales.

Spectrum of kinetic plasma turbulence at 0.3-0.9 astronomical units from the Sun.

We investigate spectral properties of turbulence in the solar wind that is a weakly collisional astrophysical plasma, accessible to in situ observations. Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence. At such small scales, we show that magnetic power spectra between 0.3 and 0.9 AU from the Sun have a generic shape ∼f^{-8/3}exp(-f/f_{d}), where the dissipation frequency f_{d} is correlated with the Doppler shifted frequency f_{ρe} of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with f_{d}=f_{ρe}/1.8. Our results indicate that the electron gyroradius plays the role of the dissipation scale and marks the end of the electromagnetic cascade in the solar wind.

Experimental signature of quantum turbulence in velocity spectra?

Velocity measurements in turbulent superfluid helium between co-rotating propellers are reported. The parameters are chosen such that the flow is fully turbulent, and its dissipative scales are partly resolved by the velocity sensors. This allows for the first experimental comparison of spectra in quantum versus classical turbulence where dissipative scales are resolved. In some specific conditions, differences are observed, with an excess of energy at small scales in the quantum case compared to the classical one. This difference is consistent with the prediction of a pileup of superfluid kinetic energy at the bottom of the inertial cascade of turbulence due to a specific dissipation mechanism.

Development of an extended reactor configuration to analyze preferential segregation impact on spray autoignition

The chemical reactor concept, usually based on a homogeneous mixture, is extended to non-homogeneous two-phase flows to study the impact of preferential segregation on the autoignition of an n-heptane spray in air. To introduce inhomogeneities, two-dimensional reactors are resolved thanks to direct numerical simulations (DNS) with Eulerian/Lagrangian description to follow the evolution of the carrier phase and the dispersed evaporating droplets, respectively. Two-way coupling is considered through exchange of mass, momentum, and energy between the carrier-gas phase and the dispersed phase. A chemistry mechanism with 29 species and 52 reactions was chosen to describe the chemical reaction paths. Several simulations were performed with various initial gas temperatures (i.e. low, intermediate and high) and various geometrical and physical characteristics of the preferential segregation. Results confirmed that evaporative cooling, vapor mass quantity and turbulence mixing play important roles in the two-phase flow autoignition. We discussed the ignition delay as well as the location of the first hot spots according to the initial gas temperatures. The dependence of most reactive mixture fraction on initial carrier-gas temperature is non-monotonic, thus there is a strong correlation between the first autoignition location and low scalar dissipation rate. On the other hand, the autoignition can start whatever the vorticity magnitude is.

Nonequilibrium dissipation scaling in large Reynolds number turbulence generated by rectangular fractal grids

We present experimental observations of the turbulence field generated by a rectangular fractal grid. For engineering tests in wind tunnels, the grid can be used to produce turbulent flows with high turbulence intensities and large integral length scales. Our results also add to previous knowledge of nonequilibrium dissipation scaling, which is observed in decaying turbulence, even though homogeneity and isotropy are not strictly satisfied.

CFD analysis of combustion and emission characteristics of primary reference fuels: from transient Diesel spray to heavy-duty engine

The pre-blending of low- and high-reactivity fuels for a single direct injection system has been proven to be an effective way to control the reactivity of mixtures in compression ignition engines, having the potential to simultaneously reduce fuel consumption and pollutant emissions. However, there is not much knowledge about the complex physical–chemical phenomena in the turbulent sprays with fuels having widely different auto-ignition qualities, although this information is critical for the design and development of cleaner combustion systems based on this concept. For this reason, a computational analysis of ignition behavior, flame structure, and soot production for reacting sprays with five primary reference fuels (PRFs), from PRF0 (n–heptane) to PRF80 (20% n–heptane, 80% iso-octane) with 20% increment in iso-octane mass fraction, was first performed using the Tabulated Flamelet Progress Variable (TFPV) approach based on the tabulation of diffusion flamelets for different scalar dissipation rates. The temporal and spatial features of the flame structure and soot formation for different fuels were investigated with the so-called intensity-axial distance-time (IXT) plots. Then, ten PRFs, from RRF0 to PRF90 with 10% increment in iso-octane mass fraction, were investigated and compared in a heavy-duty Diesel engine operating at the conventional high-temperature, short-ignition delay (HTSID) condition. The injection timing was altered from −5 to −13 ° ATDC to optimize the combustion phase and engine performance for different fuels. The results showed that PRF70 exhibited the best performance at the tested condition, which reduced the soot mass to 5% of the baseline value without sacrificing fuel efficiency.

Dissipation Scalings in the Turbulent Boundary Layer at Moderate $$Re_{\theta}$$

We present an experimental and numerical study of the turbulent boundary layer at \(1200< Re_{\theta} <3400\). We propose a combined approach that involves the use of a multi-hole pitot probe, hot-wire anemometry and direct numerical simulations, that allows to characterise the isotropy and the energy dissipation scalings of the flow in the outer layer, in the range \(0.4< y/\delta <0.75\), with y the vertical coordinate and \(\delta\) the boundary layer thickness. We confirm previous results from the literature that show that for low values of \(Re_{\theta}\) (in our case \(Re_{\theta} <2500\)), on the outer layer the dissipation constant \(C_\varepsilon\) is Reynolds number dependent, following a \(Re_\lambda ^{-1}\) law. This dependency seems to be robust, as the value of \(C_\varepsilon\) collapses for previously reported direct numerical simulations and experiments with different incoming velocities. We also show that, while large scales of the flow are strongly anisotropic on the outer layer, the turbulence energy dissipation rate and the dissipation constant can still be characterised assuming local isotropy and homogeneity.

Electrical Signatures of Diffusion-Limited Mixing: Insights from a Milli-fluidic Tracer Experiment

We investigate how diffusion-limited mixing of a layered solute concentration distribution within a porous medium impacts bulk electrical conductivity. To do so, we perform a milli-fluidic tracer test by injecting a fluorescent and electrically conductive tracer in a quasi two-dimensional (2D) water-saturated porous medium. High resolution optical- and geoelectrical monitoring of the tracer is achieved by using a fluorimetry technique and equipping the flow cell with a resistivity meter, respectively. We find that optical and geoelectrical outputs can be related by a temporal re-scaling that accounts for the different diffusion rates of the optical and electrical tracers. Mixing-driven perturbations of the electrical equipotential field lines cause apparent electrical conductivity time-series, measured perpendicularly to the layering, to peak at times that are in agreement with the diffusion transport time-scale associated with the layer width. Numerical simulations highlight high sensitivity of such electrical data to the layers’ degree of mixing and their distance to the injection electrodes. Furthermore, the electrical data correlate well with time-series of two commonly used solute mixing descriptors: the concentration variance and the scalar dissipation rate.

Direct numerical simulation of bubble-induced turbulence

Abstract

Two-dimensional manifold equations for multi-modal turbulent combustion: Nonpremixed combustion limit and scalar dissipation rates

Manifold-based combustion models have traditionally been limited to asymptotically nonpremixed or premixed combustion relying on one-dimensional manifold equations in terms of either mixture fraction or progress variable, respectively. More recently, multiple formulations of two-dimensional manifold equations have been proposed for multi-modal combustion in terms of both mixture fraction and some progress variable. However, none of these formulations simultaneously satisfies three desirable properties: computationally solvable manifold equations, an explicit transport equation for the progress variable, and the recovery of (quasi-)one-dimensional manifold equations for all asymptotic modes of combustion, a notable challenge for asymptotically nonpremixed combustion that requires a spatially homogeneous progress variable. In this work, a formulation for two-dimensional manifold equations in mixture fraction and generalized progress variable is developed that simultaneously satisfies all three of these desirable properties. Utilizing its flexibility, the generalized progress variable is constructed to be exactly spatially homogeneous for asymptotically nonpremixed combustion by explicitly relating its definition to solutions of one-dimensional manifold equations in mixture fraction. This definition of the generalized progress variable leads to trivial simplification of the two-dimensional manifold equations into (quasi-)one-dimensional equations in mixture fraction for asymptotically nonpremixed combustion. Within these two-dimensional manifold equations, three scalar dissipation rates (the mixture fraction dissipation rate, the generalized progress variable dissipation rate, and the cross-dissipation rate) appear as coefficients, and, to close the manifold equations, models are required for the dependence of these dissipation rates on the mixture fraction and generalized progress variable. Analytical model forms for the scalar dissipation rates, the development of which is aided by the new definition for the generalized progress variable, are compared against simulations of a laminar lifted coflow flame, and existing models for the mixture fraction and generalized progress variable dissipation rates are shown to be adequate. For the cross-dissipation rate, a constant normalized cross-dissipation rate is shown to be a poor model, and an improved model is proposed in which the normalized cross-dissipation rate changes sign across stoichiometric mixture fraction such that both lean and rich combustion are either simultaneously “back-supported” or simultaneously “front-supported”. Additional analysis is conducted to determine the sensitivity of the solutions of the one- and two-dimensional manifold equations to variations in these scalar dissipation profiles and indicates a relative insensitivity of the thermochemical state to the scalar dissipation rate profiles, especially compared to the sensitivity to the overall magnitude of the scalar dissipation rate or the inclusion of the often neglected cross-dissipation rate.

Dissipation scaling and structural order in turbulent channel flows

Scaling and structural evolutions are contemplated in a new perspective for turbulent channel flows. The total integrated turbulence kinetic energy and the total dissipation can be viewed as global constraints on the turbulence structure, leading to predictable, ordered scaling for u′2 and v′2 through its first and second gradients, respectively. This self-similarity allows for profile reconstructions at any Reynolds numbers based on a common template through simple multiplicative operations. Using these scaled variables in the Lagrangian transport equation derives the Reynolds shear stress, which in turn computes the mean velocity profile through the Reynolds-averaged Navier–Stokes equation. The dissipation scaling along with the transport equations renders succinct views of the turbulence dynamics and its structural characteristics. In this way, variable profiles can be analytically reconstructed, which bears potential implications toward solvability and computability of turbulent flows in canonical and other geometries.

Quasi-stationarity of scalar turbulent mixing statistics in a non-symmetric case

The existence of an asymptotic shape at large times of the probability density function (PDF) of a non-reacting scalar, mixed by a solenoidal turbulent velocity field and molecular diffusive transport, was investigated by Sinai and Yakhot [Y. G. Sinai and V. Yakhot, “Limiting probability distributions of a passive scalar in a random velocity field,” Phys. Rev. Lett. 63, 1962 (1989)]. The quasi-stationarity of the mixing statistics along the time evolution by Valiño et al. [“Quasistationary probability density functions in the turbulent mixing of a scalar field,” Phys. Rev. Lett. 72, 3518 (1994)] was an extension to symmetric scalar pdfs; analytic solutions for the scalar fluctuation dissipation rates, conditional upon the scalar value, and the pdf were obtained. This manuscript examines the generalization of the latter results to asymmetric scalar pdfs, further scrutinizes underlying mechanisms for a quasi-stationary statistics, and shows a Monte Carlo implementation which allows non-Gaussian relaxations to prescribed values of skweness and kurtosis.

Multiscale Solar Wind Turbulence Properties inside and near Switchbacks Measured by the Parker Solar Probe

Abstract The Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called “switchbacks” (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals—and regions of solar wind plasma measured just before and after each SB—to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of an SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small-scale structures at the SB edges.

A rigorous bound on the scaling of dissipation with velocity amplitude in flow past a sphere

Abstract

Scaling of magnetic dissipation and particle acceleration in ABC fields

Using particle-in-cell numerical simulations with electron–positron pair plasma, we study how the efficiencies of magnetic dissipation and particle acceleration scale with the initial coherence length $\lambda _0$ in relation to the system size $L$ of the two-dimensional ‘Arnold–Beltrami–Childress’ (ABC) magnetic field configurations. Topological constraints on the distribution of magnetic helicity in two-dimensional systems, identified earlier in relativistic force-free simulations, that prevent the high- $(L/\lambda _0)$ configurations from reaching the Taylor state, limit the magnetic dissipation efficiency to about $\epsilon _{\textrm {diss}} \simeq 60\,\%$ . We find that the peak growth time scale of the electric energy $\tau _{E,{\textrm {peak}}}$ scales with the characteristic value of initial Alfvén velocity $\beta _{A,{\textrm {ini}}}$ like $\tau _{E,\textrm {peak}} \propto (\lambda _0/L)\beta _{A,{\textrm {ini}}}^{-3}$ . The particle energy change is decomposed into non-thermal and thermal parts, with non-thermal energy gain dominant only for high initial magnetisation. The most robust description of the non-thermal high-energy part of the particle distribution is that the power-law index is a linear function of the initial magnetic energy fraction.