Radiative Transfer Research Articles

Detailed chemical composition analysis of the Soi crater region on Titan

The Soi crater region (0° to 60°N, 180°W to −110°W), which includes the well-preserved Soi crater in its center, spans a region from Titan's aeolian-dominated equatorial regions to fluvially-dominated high northern latitudes. This provides a rich diversity of landscapes, one that is also representative of the diversity encountered across Titan. Schoenfeld et al. (2023) mapped this region at 1:800,000 scale and produced a geomorphological map showing that the area consists of 22 types of geomorphological units. The Visual and Infrared Mapping Spectrometer (VIMS) coverage of the region enabled the detailed analysis of spectra of 261 different locations using a radiative transfer technique and a mixing model, yielding compositional constraints on Titan's optical surface layer. Additional constraints on composition on the near-surface substrate were obtained from microwave emissivity. We have derived combinations of top surface materials between dark materials, tholins, water-ice, and methane suggesting that dark mobile organic material at equatorial and high latitudes indicates “young” terrains and compositions, while tholin/water-ice mixtures that dominate areas around latitude 35°N show a material that is older plains deposits that we interpret to be the end stage of aeolian and fluvial transport and deposition. We found no spectral evidence of CO2, HC3N, and NH3 ice. We use the stratigraphic relations between the various mapping units and the relation between the geomorphology and the composition of the surface layers to build hypotheses on the origin and evolution of the regional geology. We suggest that sedimentary deposits, likely aeolian, are dominant in the region with fluvial activity and leaching changing the nature of the top surfaces of the midlatitude areas of the Soi crater region.

An impressionist view of V Hydrae

Context. Asymptotic giant branch (AGB) stars enrich the interstellar medium through their mass loss. The mechanism(s) shaping the circumstellar environment of mass-losing stars is not clearly understood so far. Aims. Our purpose is to study the effect of binary companions located within the first 10 stellar radii from the primary AGB star. In this work, we target the mass-losing carbon star V Hydrae (V Hya) and search for signatures of its companion in the dust-forming region of the atmosphere. Methods. The star was observed in the L and N bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of the photosphere and surroundings of V Hya using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used the 1D radiative transfer code DUSTY to model the spectral energy distribution. Results. The star is dominated by dust emission in the L- and N-bands. The MATISSE reconstructed images show asymmetric and elongated structures in both infrared bands. In the L band, we detected an elongated shape of approximately 15 mas that likely is of photospheric origin. In the N band, we found a 20 mas extension northeast from the star and perpendicular to the L-band elongated axis. The position angle and the size of the N-band extension match the prediction of the companion position at the MATISSE epoch. By comparing MATISSE N-band with MIDI data, we deduce that the elongation axis in the N-band has rotated since the previous interferometric measurements 13 yr ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. Conclusions. The MATISSE images unveil the presence of a dust enhancement at the position of the companion. This opens new doors for further analyses of the binary interaction with an AGB component.

Validation of aerosol chemical composition and optical properties provided by Copernicus Atmosphere Monitoring Service using ground-based global data.

Monitoring particulate matter (PM) air pollution in terms of both concentration and composition, is very important due to its effects on human health and climate. In the PRIMARY project we aim at retrieving the aerosol composition from space using the hyperspectral observations from the Italian Space Agency’s PRISMA mission. To this end, we are developing a machine learning algorithm trained with synthetic top-of-atmosphere reflectances and underlying aerosol fields. As part of this process, we plan to use the global forecasts from the Copernicus Atmosphere Monitoring Service (CAMS) as the core to generate this synthetic dataset. However, to proceed in this direction, a preliminary assessment of the reliability of this model-based dataset when compared to observations is necessary, also to bias correct the output if needed. With this aim, we assess the representation of the aerosol chemical composition and the related optical properties at selected globally distributed sites in CAMS, comparing the simulations with near-surface aerosol chemical analyses from the SPARTAN network and column sun-photometer observations from the AERONET network. We found that CAMS forecasts skills changed over time due to updates in the modelling system, with the latter two version cycles (46 and 47) being similar. Generally, they reproduce the aerosol composition within a factor of 2. We found a substantial overestimation of organic matter (OM) by a factor of 3. Applying a correcting factor to OM (constant at the global level) warrants a much more realistic representation of PM2.5 total mass and relative fraction of single species in CAMS. From the so derived CAMS aerosol-speciated profiles, we calculate aerosol optical properties, needed for subsequent use in a radiative transfer model. Comparison against AERONET indeed shows that OM bias correction resulted in improvements in Extinction Ångstrom Exponent (). Aerosol Optical Depth (AOD), Single Scattering Albedo (SSA) and Asymmetry Parameter (g) simulations resulted slightly degraded, confirming the possibility of using CAMS as the base for a synthetic retrieval training dataset.

Evaluating Numerical Methods to Investigate Spectral Solar Radiative Transfer in Plant Canopies

Evaluating Numerical Methods to Investigate Spectral Solar Radiative Transfer in Plant Canopies

Lyα emission as a sensitive probe of feedback-regulated LyC escape from dwarf galaxies

Abstract Lyα emission is an exceptionally informative tracer of the life cycle of evolving galaxies and the escape of ionising photons. However, theoretical studies of Lyα emission are often limited by insufficient numerical resolution, incomplete sets of physical models, and poor line-of-sight (LOS) statistics. To overcome such limitations, we utilize here the novel PANDORA suite of high-resolution dwarf galaxy simulations that include a comprehensive set of state-of-the-art physical models for ionizing radiation, magnetic fields, supernova feedback and cosmic rays. We post-process the simulations with the radiative transfer code rascas to generate synthetic observations and compare to observed properties of Lyα emitters. Our simulated Lyα haloes are more extended than the spatial region from which the intrinsic emission emanates and our spatially resolved maps of spectral parameters of the Lyα emission are very sensitive to the underlying spatial distribution and kinematics of neutral hydrogen. Lyα and LyC emission display strongly varying signatures along different LOS depending on how each LOS intersects low-density channels generated by stellar feedback. Comparing galaxies simulated with different physics, we find the Lyα signatures to exhibit systematic offsets determined by the different levels of feedback strength and the clumpiness of the neutral gas. Despite this variance, and regardless of the different physics included in each model, we find universal correlations between Lyα observables and LyC escape fraction, demonstrating a robust connection between Lyα and LyC emission. Lyα observations from a large sample of dwarf galaxies should thus give strong constraints on their stellar feedback-regulated LyC escape and confirm their important role for the reionization of the Universe.

Cumulative Dose from Recurrent CT Scans: Exploring the DNA Damage Response in Human Non-Transformed Cells.

Recurrent computed tomography (CT) examination has become a common diagnostic procedure for several diseases and injuries. Though each singular CT scan exposes individuals at low doses of low linear energy transfer (LET) radiation, the cumulative dose received from recurrent CT scans poses an increasing concern for potential health risks. Here, we evaluated the biological effects of recurrent CT scans on the DNA damage response (DDR) in human fibroblasts and retinal pigment epithelial cells maintained in culture for five months and subjected to four CT scans, one every four weeks. DDR kinetics and eventual accumulation of persistent-radiation-induced foci (P-RIF) were assessed by combined immunofluorescence for γH2AX and 53BP1, i.e., γH2AX/53BP1 foci. We found that CT scan repetitions significantly increased both the number and size of γH2AX/53BP1 foci. In particular, after the third CT scan, we observed the appearance of giant foci that might result from the overlapping of individual small foci and that do not associate with irreversible growth arrest, as shown by DNA replication in the foci-carrying cells. Whether these giant foci represent coalescence of unrepaired DNA damage as reported following single exposition to high doses of high LET radiation is still unclear. However, morphologically, these giant foci resemble the recently described compartmentalization of damaged DNA that should facilitate the repair of DNA double-strand breaks but also increase the risk of chromosomal translocations. Overall, these results indicate that for a correct evaluation of the damage following recurrent CT examinations, it is necessary to consider the size and composition of the foci in addition to their number.

Cumulative Dose from Recurrent CT Scans: Exploring the DNA Damage Response in Human Non-Transformed Cells

Recurrent computed tomography (CT) examination has become a common diagnostic procedure for several diseases and injuries. Though each singular CT scan exposes individuals at low doses of low linear energy transfer (LET) radiation, the cumulative dose received from recurrent CT scans poses an increasing concern for potential health risks. Here, we evaluated the biological effects of recurrent CT scans on the DNA damage response (DDR) in human fibroblasts and retinal pigment epithelial cells maintained in culture for five months and subjected to four CT scans, one every four weeks. DDR kinetics and eventual accumulation of persistent-radiation-induced foci (P-RIF) were assessed by combined immunofluorescence for γH2AX and 53BP1, i.e., γH2AX/53BP1 foci. We found that CT scan repetitions significantly increased both the number and size of γH2AX/53BP1 foci. In particular, after the third CT scan, we observed the appearance of giant foci that might result from the overlapping of individual small foci and that do not associate with irreversible growth arrest, as shown by DNA replication in the foci-carrying cells. Whether these giant foci represent coalescence of unrepaired DNA damage as reported following single exposition to high doses of high LET radiation is still unclear. However, morphologically, these giant foci resemble the recently described compartmentalization of damaged DNA that should facilitate the repair of DNA double-strand breaks but also increase the risk of chromosomal translocations. Overall, these results indicate that for a correct evaluation of the damage following recurrent CT examinations, it is necessary to consider the size and composition of the foci in addition to their number.

Constraining the physical properties of gas in high-z galaxies with far-infrared and submillimetre line ratios

Optical emission line diagnostics, which are a common tool for constraining the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows that are accessible for ground-based submillimetre facilities, could provide ISM diagnostics alternative to optical emission lines. We investigated FIR line ratios involving CII OIII OIII NII and NIII using synthetic emission lines applied to a high-resolution gas $= 883.4 M$_ odot $) cosmological zoom-in simulation, including radiative transfer post-processing with the code at z = 6.5. We find that the CII NII 122 ratio is sensitive to the temperature and density of photodissociation regions. It might therefore be a useful tool for tracing the properties of this gas phase in galaxies. We also find that NII NIII is a good tracer of the temperature and that OIII OIII 88 is a good tracer of the gas density of HII regions. Emission line ratios containing the OIII line are sensitive to high-velocity outflowing gas.

Inner walls or vortices? Crescent-shaped asymmetries in ALMA observations of protoplanetary discs

ABSTRACT Crescent-shaped asymmetries are common in millimetre observations of protoplanetary discs and are usually attributed to vortices or dust overdensities. However, they often appear on a single side of the major axis and roughly symmetric about the minor axis, suggesting a geometric origin. In this work, we interpret such asymmetries as emission from the exposed inner cavity walls of inclined discs and use them to characterize their vertical extent. Here we focus on the discs around CIDA 9 and RY Tau, first modelling their observations in visibility space with a simple geometric prescription for the walls, and then exploring more detailed radiative transfer models. Accounting for the wall emission yields significantly better residuals than purely axisymmetric models, and we estimate the dust scale height of these systems to be 0.4 au at 37 au for CIDA 9 and 0.2 au at 12 au for RY Tau. Finally, we identify crescent-shaped asymmetries in twelve discs, nine of which have constraints on their orientation – in all cases, the asymmetry appears on the far-side of the disc, lending support to the hypothesis that they are due to their inner rims. Modelling this effect in larger samples of discs will help to build a statistical view of their vertical structure.

Study on the impact of heat loss on premixed hydrogen/air deflagration in closed pipelines

This study developed a one-step chemical reaction model for hydrogen combustion and investigated the effect of heat loss on premixed hydrogen/air deflagration in closed pipelines using numerical simulations with the CFD code GASFLOW-MPI. The simulation results were compared and verified with experimental data. The numerical simulations accurately predicted the flame front structure, pressure, and flame tip position during the deflagration of premixed hydrogen/air. Furthermore, a comparison of adiabatic simulation and heat transfer simulation results revealed that large-scale vortices within the turbulent flow were closely linked to heat loss during the tulip flame stage. Simultaneously, heat loss reduced pressure and flame tip position by diminishing the flame wrinkling factor caused by thermodiffusive instability. Finally, convective heat transfer identified as the most critical factor contributing to heat loss during the deflagration of premixed hydrogen/air in closed pipelines. Specifically, during the tulip flame stage, heat loss was mainly due to convective heat transfer (73.8%∼76.3%), with radiation playing a minor role (23.7%∼26.2%). Therefore, accurate prediction of characteristic parameters during the deflagration of premixed hydrogen/air requires numerical simulations that account for both convective and radiative heat transfer.

Constraining the stellar masses and origin of the protostellar VLA 1623 system

We present ALMA Band 7 molecular line observations of the protostars within the VLA 1623 system. We detect (3 -- 2) in the circumbinary disk around VLA 1623A and the outflow cavity walls of the collimated outflow. We further detect redshifted and blueshifted velocity gradients in the circumstellar disks around VLA 1623B and VLA 1623W that are consistent with Keplerian rotation. We used the radiative transfer modelling code pdspy and simple flared disk models to measure stellar masses of $0.27 and $0.64 for the VLA 1623A binary, VLA 1623B, and VLA 1623W, respectively. These results represent the strongest constraints yet on stellar mass for both VLA 1623B and VLA 1623W, and the first mass measurement for all stellar components using the same tracer and methodology. We use these masses to discuss the relationship between the young stellar objects (YSOs) in the VLA 1623 system. We find that VLA 1623W is unlikely to be an ejected YSO, as has been previously proposed. While we cannot rule out that VLA 1623W is a unrelated YSO, we propose that it is a true companion star to the VLA 1623A/B system and that these stars formed in situ through turbulent fragmentation and have had only some dynamical interactions since their inception.

Multiline study of the radial extent of SiO, CS, and SiS in asymptotic giant branch envelopes

Circumstellar envelopes around asymptotic giant branch (AGB) stars contain a rich diversity of molecules, whose spatial distribution is regulated by different chemical and physical processes. In the outer circumstellar layers, all molecules are efficiently destroyed due to interactions with interstellar ultraviolet photons. Here we aim to carry out a coherent and uniform characterization of the radial extent of three molecules (SiO, CS, and SiS) in envelopes around O- and C-rich AGB stars, and to study their dependence on mass-loss rate. To this end, we observed a reduced sample of seven M-type and seven C-type AGB envelopes in multiple lines of SiO, CS, and SiS with the Yebes\,40m and IRAM\,30m telescopes. The selected sources cover a wide range of mass-loss rates, from sim $ $M_ odot $ yr$^ $ to a few times 10$^ $ $M_ odot $ yr$^ $, and the observed lines cover a wide range of upper-level energies, from 2 K to 130 K. We carried out excitation and radiative transfer calculations over a wide parameter space in order to characterize the abundance and radial extent of each molecule. A $ analysis indicates that the abundance is usually well constrained while the radial extent is in some cases more difficult to constrain. Our results indicate that the radial extent of the molecules considered here increases with increasing envelope density, in agreement with previous observational findings. At high envelope densities of $ M /v_ infty $ $M_ odot $ yr$^ $ km$^ $ s, SiO, CS, and SiS show a similar radial extent, while at low envelope densities of $ M /v_ infty $ $M_ odot $ yr$^ $ km$^ $ s, differences in radial extent appear among the three molecules, in agreement with theoretical expectations based on destruction due to photodissociation. At low envelope densities, we find a sequence of increasing radial extent, SiS\,rightarrow \,CS\,rightarrow \,SiO. We also find a tentative dependence of the radial extent on the chemical type (O- or C-rich) of the star for SiO and CS. Interferometric observations and further investigation of the photodissociation of SiO, CS, and SiS should provide clarification of the situation in regards to the relative photodissociation radii of SiO, CS, and SiS in AGB envelopes and their dependence on envelope density and C/O ratio.

Helium reionization from empirical quasar luminosity functions before and after JWST

ABSTRACT Recently, models of the quasar luminosity function (QLF) rooted on large observational compilations have been produced that, unlike their predecessors, feature a smooth evolution with time. This bypasses the need to assume an ionizing emissivity evolution when simulating helium reionization with observations-based QLF, thus yielding more robust constraints. We combine one such QLF with a cosmological hydrodynamical simulation and 3D multifrequency radiative transfer. The simulated reionization history is consistently delayed in comparison to most other models in the literature. The predicted intergalactic medium temperature is larger than the observed one at $z \lesssim 3$. Through forward modelling of the He ii Lyman $\alpha$ forest, we show that our model produces an extended helium reionization and successfully matches the bulk of the observed effective optical depth distribution, although it overionizes the Universe at $z\lesssim 2.8$ as the effect of small-scale Lyman Limit Systems not being resolved. We thoroughly characterize transmission regions and dark gaps in He ii Lyman $\alpha$ forest sightlines. We quantify their sensitivity to the helium reionization, opening a new avenue for further observational studies of this epoch. Finally, we explore the implications for helium reionization of the large number of active galactic nuclei revealed at $z\gtrsim 5$ by JWST. We find that such modifications do not affect any observable at $z\le 4$, except in our most extreme model, indicating that the observed abundance of high-z AGNs does not bear consequences for helium reionization.

Estimating aboveground biomass dynamics of wheat at small spatial scale by integrating crop growth and radiative transfer models with satellite remote sensing data

Aboveground biomass (AGB) of plants is an agroecological indicator that can be used to monitor crop growth status and quantify biomass carbon stock in agricultural ecosystems. Although satellite remote sensing data and crop models are widely employed to estimate AGB dynamics, their application in small-scale research plots is often constrained by the unavailability of freely and timely high-resolution satellite imageries and the challenge of acquiring reliable model inputs like initial soil water and nitrogen content. This study integrated a crop growth model (APSIM-Wheat) and radiative transfer model (PROSAIL) to estimate AGB dynamics at a small plot scale (ca. 5 to 20 m2) within variety trials (ca. 1 to 2 ha) by assimilating trial-level remote sensing data into the hybrid APSIM-PROSAIL model. The APSIM-PROSAIL integration, developed by deriving PROSAIL input parameters from APSIM-Wheat output variables, was verified by evaluating its capacity to reproduce trial-level Sentinel-2 (S2) NDVI dynamics across 30 variety trials in 2020. The comparison of the simulated and observed trial-level S2 NDVI dynamics yielded an overall RRMSE of 18.2% (n = 646 observations), suggesting the potential of using observed S2 NDVI dynamics to constrain APSIM-PROSAIL and estimate environment variables in cropping systems. This constraint was subsequently assessed in 28 variety trials in 2021, where APSIM-PROSAIL inversely estimated unknown trial-specific variables including initial soil water and nitrogen content across the trials. Using the estimated initial soil status and the plot information including genotypes and management as sown, we demonstrated that this method could also facilitate accurate retrievals of plot-level AGB dynamics with an overall RRMSE of 20.9% (n = 976 measurements). The proposed APSIM-PROSAIL demonstrated the capability to accurately retrieve plot-level AGB dynamics and reproduce trial-level S2 NDVI dynamics across a broad and diverse range of variety trials in the Australian Wheatbelt, indicating its potential utility to facilitate biomass carbon stock assessment. It holds the potential as a tool to explore the relationships between canopy spectral information and crop traits in response to genotype-environment-management interactions in agricultural ecosystems. The method enables the examinations of within-field variabilities for crop traits through retrieving them at small spatial scales from coarse remote sensing data.

A method to rapidly construct 3D canopy scenes for maize and their spectral response evaluation

Three-dimensional (3D) scenes of a maize canopy can be utilized to depict its vertical structure and serve as the foundation for a 3D radiative transfer model. Since canopy structures are typically heterogeneous, evaluating their spectral response is crucial for remote-sensing inversion algorithms. Yet existing methods to model 3D canopy scenes of crops lack the high efficiency required for large-scale breeding applications. Hence, this study aims to develop a method to rapidly construct 3D canopy scenes of maize based on key structural parameters—namely, leaf area, base angle, and inclination angle—across multiple cultivars and growth stages. The accuracy of this canopy scenes modeling method is validated by comparison to a fine-scale 3D model and a multi-growth stage reflectance dataset. The root mean square error (RMSE) and normalized root mean square error (NRMSE) between the fine-scale 3D model and the constructed 3D model are less than 0.021 and 6.4%, respectively. Moreover, the RMSE and NRMSE between the simulated reflectance and measured reflectance are less than 0.042 and 11.1%, respectively. Next, we couple the 3D radiative transfer model with 3D scenes to analyze the spectral response of maize canopy structure. These results indicate that the near-infrared band was affected more by leaf area than leaf base angle, while the opposite is observed for the red-edge band. Furthermore, the middle layer contributes more to the canopy reflectance than either the upper or lower layers. Notably, two maize scenes where two structural parameters simultaneously change can elicit the same canopy spectra. The proposed method here thus offers the dual advantages of rapid modeling and high efficiency, making it highly applicable for maize and other similar crops.

Large eddy simulation investigation of the effect of radiative heat transfer on the ignition progress in a model combustor

Radiation significantly influences turbulent combustion. This paper investigates the impact of radiative heat transfer on the ignition process in a multi-staged model combustor. The ignition process is simulated both without and with consideration of radiation. Large Eddy Simulations based on the Euler-Lagrange description of the liquid spray and the Dynamic Thickened Flame model coupled with a skeletal chemical reaction mechanism are employed. The Weighted Sum of Gray Gases method are employed to solve the spectral properties and the Discrete Ordinates method is employed to solve the radiative transfer equation. The results indicate that radiative heat transfer influences the flame structure during the ignition process. In the flame propagation phase, both the total heat transfer and the exit average temperature are higher, with the radiative heat transfer during ignition being less than 15% of the total heat transfer. During the kernel generation phase, radiative heat transfer leads to a 7.24% increase in the maximum temperature of the flame kernel, a 7.74% expansion of the flame surface area, and outward movement of the position of the maximum heat release rate. Additionally, the ignition delay time extends by 14.91%. In the flame propagation phase, the flame transfers heat to the mixed gas and droplets near the flame surface through radiation, promoting flame spread.

The hydrodynamic response of small-scale structure to reionization drives large IGM temperature fluctuations that persist to z = 4

ABSTRACT The thermal history and structure of the intergalactic medium (IGM) at $z \ge 4$ is an important boundary condition for reionization, and a key input for studies using the Ly $\alpha$ forest to constrain the masses of alternative dark matter candidates. Most such inferences rely on simulations that lack the spatial resolution to fully resolve the hydrodynamic response of IGM filaments and minihaloes to H i reionization heating. In this letter, we use high-resolution hydrodynamic + radiative transfer simulations to study how these affect the IGM thermal structure. We find that the adiabatic heating and cooling driven by the expansion of initially cold gas filaments and minihaloes sources significant small-scale temperature fluctuations. These likely persist in much of the IGM until $z \le 4$. Capturing this effect requires resolving the clumping scale of cold, pre-ionized gas, demanding spatial resolutions of ${\le} 2$ $h^{-1}$kpc. Pre-heating of the IGM by X-rays can slightly reduce the effect. Our preliminary estimate of the effect on the Ly $\alpha$ forest finds that, at $\log (k /[{\rm km^{-1} s}]) = -1.0$, the Ly $\alpha$ forest flux power (at fixed mean flux) can increase ${\approx} 10~{{\ \rm per\ cent}}$ going from 8 and 2 $h^{-1}$kpc resolution at $z = 4{\!-\!}5$ for gas ionized at $z \ \lt\ 7$. These findings motivate more careful analyses of how the effects studied here affect the Ly $\alpha$ forest.

Radiant fluence from ray tracing in optical multipass systems

Optical multipass cells are used in photochemical reactors and laser excitation of weak transitions. In these applications, estimation of the radiation dose in a volume of interest allows us to assess the performance and optimize the design of the cell. We adopt radiant fluence as the figure of merit and employ the radiative transfer equation to derive analytical expressions for average radiant fluence in a given volume of interest. These expressions involve quantities that are either obtained with Monte Carlo ray tracing methods or approximated on the grounds of geometry arguments. Furthermore, we compute the spatial distribution of fluence by applying either of the two methods to individual volume elements in a rectangular voxel grid. Ray tracing is performed with Zemax OpticsStudio 18.9.

Experimental investigation of the characteristics of flame geometry and heat transfer from wind-driven pool fires

Wind-driven pool fire is a frequently encountered thermal hazard in cases of liquid fuel leakage accidents. Given the intricate interplay between wind flow and thermal buoyancy, understanding the flame geometry and external heat transfer behavior of pool fires is vital for effective liquid fuel security management. This paper presents a study to investigate these characteristics under various square pool sizes (side length, D = 0.31, 0.38, 0.44, 0.49, and 0.54 m) and wind speeds (u = 0, 0.5, 1, 2, 4, and 6 m/s). Five parameters pertaining to global flame geometry and two parameters concerning external heat transfer were measured. The findings indicate that the change rate of overall flame geometry parameters accelerates at first and then decelerates with wind speed. The ratio of the net flame horizontal length dominated by inertia force (Li) and the net flame horizontal length dominated by buoyancy force (Lb) to the total horizontal flame length (Lh) (Li / Lh and Lb / Lh) present a non-monotonic trend with increasing Fr number. To further comprehend the global flame geometry, a corresponding physical model is developed based on the ratio of Grs number to the 5/2 power of Rem number (Grs / Rem5/2). Due to the effects of flame thickness and flame attachment length, the variation in radiant heat transfer with wind speed can be bifurcated into two categories. The convective heat coefficient increases with wind speed, enhancing the local convection. Finally, the new models of radiant and convective heat transfers are established using dimensionless wind speed (u*, u∗=u(ṁ″g/ρ0)1/3) and Ri number. To ascertain the suitability of these new models, they have been validated against two classic flame models.

Hydrodynamic simulations of cool stellar atmospheres with MANCHA

Three-dimensional time-dependent simulations of stellar atmospheres are essential to study the surface of stars other than the Sun. These simulations require the opacity binning method to reduce the computational cost of solving the radiative transfer equation down to viable limits. The method depends on a series of free parameters, among which the location and number of bins are key to set the accuracy of the resulting opacity. Our aim is to test how different binning strategies previously studied in one-dimensional models perform in three-dimensional radiative hydrodynamic simulations of stellar atmospheres. Realistic box-in-a-star simulations of the near-surface convection and photosphere of three spectral types (G2V, K0V, and M2V) were run with the MANCHA $-bins. These rates were compared with the ones computed with opacity distribution functions. Then, stellar simulations were run with grey, four-bin, and 18-bin opacities to see the impact of the opacity setup on the mean stratification of the temperature and its gradient after time evolution. The simulations of main sequence cool stars with the MANCHA code are consistent with those in the literature. For the three stars, the radiative energy exchange rates computed with 18 bins are remarkably close to the ones computed with the opacity distribution functions. The rates computed with four bins are similar to the rates computed with 18 bins, and present a significant improvement with respect to the rates computed with the Rosseland opacity, especially above the stellar surface. The Rosseland mean can reproduce the proper rates in sub-surface layers, but produces large errors for the atmospheric layers of the G2V and K0V stars. In the case of the M2V star, the Rosseland mean fails even in sub-surface layers, owing to the importance of the contribution from molecular lines in the opacity, underestimated by the harmonic mean. Similar conclusions are reached studying the mean stratification of the temperature and its gradient after time evolution.