Flow, Turbulence and Combustion最新文献

Mixing models for multiple mapping conditioning (MMC) methods are revisited as some details of their implementation have not yet been assessed. We use simulations of scalar mixing in non-reacting homogeneous isotropic decaying turbulence (HIT) such that (1) key modelling parameters can be taken from the direct numerical simulations without incurring additional modelling uncertainties and (2) direct validation is possible. Variants of Curl’s model are studied and direct comparison is sought with the variants’ performances in the context of standard (intensive) and sparse (such as MMC) particle approaches for the modelling of the probability density function (PDF). The second aim is to show the relative importance of micro-mixing and spatial diffusion in the presence of differential diffusion. The results demonstrate that MMC approximates the correct relaxation towards Gaussian independent of the mixing model’s variant. This is different from the standard PDF approach that requires a clear spatial localization given by the computational mesh to achieve a similar outcome. This spatial localization is not needed in MMC as the MMC mixing model already employs a localization in reference space. Differential diffusion effects can, however, only be accurately predicted if not only mixing but also spatial transport accounts for the differences in the molecular diffusion term. It is insufficient to adjust the mixing time scales only and future MMC models may require adjustments for accurate prediction capabilities.

Based on a (synthetic) turbulent signal which obeys a Gaussian probability density function (PDF) together with some form of prescribed two-point statistics (i.e. integral length or time scales or turbulent energy spectrum), a simple algorithm is proposed to transform the original signal, such that it follows a new target PDF. It is shown that for many practical applications the transformation does not change the integral length or time scale more than a few per cent. The algorithm can be combined with any turbulence generator. It has applications for prescribing boundary or initial conditions of non-Gaussian signals in scale resolving simulations of turbulent flows, such as passive scalars like temperature, bounded passive scalars occurring in reactive flows or velocity signals close to walls.

Direct numerical simulations (DNS) of plane Couette flows over thermally heterogeneous surfaces at bulk Reynolds number (Re=10^4) and bulk Richardson number (Ri=0.25) are performed. The focus of the present study (that extends previous work by the authors) is the effect of surface heterogeneity orientation on boundary-layer structure. The temperature of the upper and lower walls is either homogeneous or varies sinusoidally, where the temperature-wave crests are either normal or parallel to the mean flow (HETx and HETy cases, respectively). Importantly, the horizontal-mean surface temperature is the same in all simulations. The stratification is strong enough to quench turbulence over a homogeneous surface, but turbulence survives over heterogeneous surfaces. In all heterogeneous cases, both molecular diffusion and turbulence transfer momentum down the gradient of mean velocity. The total (turbulent plus diffusive) heat flux is down-gradient, but quasi-organized eddy motions generated by the surface thermal heterogeneity induce heat transfer up the gradient of the mean temperature. Comparative analysis of HETx and HETy cases shows that the configuration with the spanwise heterogeneity is more turbulent and more efficient in transporting momentum and heat vertically than its counterpart with the streamwise heterogeneity. Vertical profiles of mean fields and turbulence moments differ considerably between the HETx and HETy cases, e.g., the streamwise heat flux differs not only in magnitude but also in sign. A close examination of the second-order turbulence moments, vertical-velocity and temperature skewness, and the flow eddy structure helps explain the observed differences between the HETx and HETy cases. The implications of our DNS findings for modelling turbulence in stably-stratified environmental and industrial flows with surface heterogeneity are discussed.

Longstanding design and reproducibility challenges in inertial confinement fusion (ICF) capsule implosion experiments involve recognizing the need for appropriately characterized and modeled three-dimensional initial conditions and high-fidelity simulation capabilities to predict transitional flow approaching turbulence, material mixing characteristics, and late-time quantities of interest—e.g., fusion yield. We build on previous coarse graining simulations of the indirect-drive national ignition facility (NIF) cryogenic capsule N170601 experiment-a precursor of N221205 which resulted in net energy gain. We apply effectively combined initialization aspects and multiphysics coupling in conjunction with newly available hydrodynamics simulation methods, including directional unsplit algorithms and low Mach-number correction-key advances enabling high fidelity coarse grained simulations of radiation-hydrodynamics driven transition. Our presentation includes discussion of the capsule initialization and implosion dynamics, analysis of the vorticity production budget, transition signatures, quantities of interest—late-time ion temperature and fusion-neutron yield, numerical uncertainty quantification, and comparisons with NIF data.

Packed beds are commonly found in many engineering systems and have been widely studied for decades. A relatively new packed bed system is the Pebble Bed Reactor, a type of generation-IV nuclear reactor. Unlike many of the packed beds encountered in chemical and process engineering applications, Pebble Bed Reactors are larger and operate at significantly higher Reynolds numbers. As a result of these differences, there is a very limited amount of information on the detailed flow physics that exist in these complex geometries. This work seeks to contribute to a growing database of flow data for Pebble Bed Reactor systems by performing Direct Numerical Simulations of the flow in an experimental bed of 67 pebbles for a range of conditions. Simulations are performed at a Prandtl number of 0.66 and Reynolds numbers from 300–600. These Reynolds numbers are chosen to gain additional knowledge on the spatial development of turbulence in these systems. Analysis of the Turbulent Kinetic Energy, turbulence anisotropy, and Turbulent Heat Flux is performed. Results demonstrate significant development of the TKE across the tested range of Reynolds numbers. Examination of both the TKE and THF reveal that development first occurs near the center of the bed and propagates radially as the flow moves further into the bed. Notable regions of negative production of turbulent kinetic energy are observed in regions where flow accelerates around pebble contact points. These regions are found to coincide with regions of 1-component turbulence.Kindly check and confirm, all authors email id is correctly identified.These are correct

The drag-type Savonius rotor, a type of vertical-axis wind turbine, is designed to capture wind energy and convert it into rotational torque. However, their efficiency is limited, which restricts their commercial viability. This inefficiency is primarily due to the negative torque produced by the returning blades, which results in minimal power output. This study examines the effect of the aspect ratio on a new elliptic-shaped deflector using three-dimensional (3D) computational fluid dynamics (CFD) modeling and an optimization approach. The aim of this novel deflector is to enhance the aerodynamic performance of the Savonius turbine by reducing negative torque during blade sweeping on the return side. Although there is extensive literature on elliptic-shaped bodies, there is a notable lack of research on the interaction between airflow over such a body used as a deflector and the Savonius rotor. This research uses an optimization methodology based on the design of experiments to determine the optimal design. Using the Taguchi method and analysis of variance, the number of blades is identified as the most significant factor, accounting for 77% of the rotor performance near the deflector. At a Tip Speed Ratio (λ) of 0.8, the optimal deflector achieves the highest average power coefficient of 0.34, representing a significant 42% improvement compared to the maximum average power coefficient without a deflector.

Mid-century climate neutrality targets for the aviation industry foster the development of ultra-high overall pressure ratio jet engines. Consequently, comprehensive numerical models driving the design process must tackle the severe thermodynamic conditions expected to occur during the various flight operational phases. In the current study, we present a cost-effective framework for addressing droplet vaporization phenomena in jet-engine-relevant conditions, leveraging real-fluid thermophysical modeling and high-pressure vapor-liquid equilibrium interfacial thermodynamics. We evaluate the impact of a non-ideal fluid approach on predicting the evaporation process of a single n-dodecane droplet in air, mimicking operating conditions relevant to aero-engines. For the conditions examined, the numerical results indicate that adopting a real-fluid thermodynamic treatment results in a deviation of the droplet vaporization rate from an ideal-fluid approach, for which we have outlined the thermodynamic states that lead to mixture non-ideality. Notably, we envisage the most impactful model discrepancies in transport property estimation, thus affecting the heat and mass transfer rates. Lastly, we analyze and quantify the role of the detailed phase equilibrium model in the droplet evaporation process, assessing its actual impact for the conditions of interest, and discussing the cost-effectiveness in commonly computational fluid dynamics tools.

Diffusers are devices found in several engineering applications and their performance and design are object of numerous investigations. However, relatively few investigations have been dedicated to diffusers operating at low and moderate Reynolds numbers. In this regime, the flow could be laminar, turbulent or transitional, and the aerodynamic performance of the diffuser becomes highly dependent on the specific value of the Reynolds number and inlet conditions. In particular, the present study focuses on evaluating the role of inlet conditions on the performance and flow behaviour of two-dimensional diffusers on this specific Reynolds number regime ((Re approx 8000)). Furthermore, the diffuser discharges in a stationary chamber and it does not present a tail-pipe configuration, a condition that has not found a clear presence in the existing literature so far. A numerical investigation of two-dimensional plane diffusers was performed at (Re = 8163) for 9 different cases, combined varying the inlet turbulence intensity (0.05, 3, and 10 percent), and the velocity profile, characterised by different blockage factors (0, 0.05 and 0.33). For each case, the divergence angle ranged from 0 to 30 degrees, and several URANS simulations were performed using the (k-omega) Transitional SST model that accounts for the possible transition of the boundary layer. The results show that the design recommendations valid for high Reynolds number diffusers with a thin boundary layer are not always applicable, and extreme caution must be exercised when dealing with operating conditions that do not ensure a sufficiently high turbulence level at the inlet. The divergence angles of the stall regimes are shown, and performance indicators (e.g. pressure-recovery coefficients) are reported. These reveal a strong decrement (up to 60 percent) of the pressure recovery on reducing turbulence intensity from 10 percent to 0.05 percent. The blockage factor of the velocity profile has an important effect on performance as well. In order to simplify the comparison between the different blockage factors, a modified effectiveness was employed to account for the distortion introduced by a non-uniform inlet velocity profile.

In this work, the effects of realistic roughness scales on boundary layer transition are investigated using high-resolution scale-resolving simulations. This is in contrast to most of the roughness-induced transition studies reported in the literature based on ordered and well-defined surface roughness elements. Two highly irregular surface roughness patterns are generated from a given roughness patch by selectively filtering out the higher frequencies. The transitional behavior of a laminar boundary layer developing over these roughness scales is examined at (text {Re}_{delta _{in}^{*}}=360) and 540, defined in terms of the inflow velocity and inlet displacement thickness. The impact on transition is explored by examining the instantaneous and time-averaged flow fields. The results show that the transition onset is sensitive to the roughness scales: the inclusion of finer scales reduces the spacing between roughness features thereby constraining the lateral development of the flow. The streaks are weaker due to the mutual sheltering effect and the finer scales are shown to promote spanwise inhomogeneity of flow in the transition region. This effect is found to be much more prominent at low Reynolds numbers. In contrast, filtering out the finer roughness scales results in an earlier transition onset, caused by strong streaks developing from the horseshoe vortices wrapping around sparsely packed roughness features. Further cases are studied to investigate the spatial features of roughness that are important for transition. A series of high-fidelity simulations using selective retention of roughness features are performed at (text {Re}_{delta _{in}^{*}}=360). The transition onset can be predicted satisfactorily by retaining the dominant scales (20 tallest peaks in this study) from the original rough surface while the valleys and fine-scale features are shown to have minimal effect. In addition, we demonstrate that modifying the roughness patch or Reynolds number during the simulation alters the transition onset, which gets quickly established within (approx 1725k_{rms}/U_{in}). These findings have the potential to reduce the computational cost and further aid in improving the transition correlations employed in low-fidelity simulations.

Improving mixing between two coaxial swirled jets is a subject of interest for the development of next generations of fuel injectors. This is particularly crucial for hydrogen injectors, where the separate introduction of fuel and oxidizer is preferred to mitigate the risk of flashback. Raman scattering is used to measure the mean compositions and to examine how mixing between fuel and air streams evolves along the axial direction in the near-field of the injector outlet. The parameters kept constant include the swirl level (S_e = 0.67) in the annular channel, the injector dimensions, and the composition of the oxidizer stream, which is air. Experiments are carried out in cold flow conditions for different compositions of the central stream, including hydrogen and methane but also helium and argon. Three dimensionless mixing parameters are identified, the velocity ratio (u_e/u_i) between the external stream and internal stream, the density ratio (rho _e/rho _i) between the two fluids, and the inner swirl level (S_i) in the central channel. Adding swirl to the central jet significantly enhances mixing between the two streams very close to the injector outlet. Mixing also increases with higher velocity ratios (u_e/u_i), independently of the inner swirl. Additionally, higher density ratios (rho _e/rho _i) enhance mixing between the two streams only in the case without swirl conferred to the central flow. A model is proposed for coaxial swirled jets, yielding a dimensionless mixing progress parameter that only depends on the velocity ratio (u_e/u_i) and geometrical features of the swirling flow that can be determined by examining the structure of the velocity field. Comparing the model with experiments, it is shown to perform effectively across the entire range of velocity ratios (0.6 le u_e/u_i le 3.8), density ratios (0.7 le rho _e/rho _i le 14.4), and inner swirl levels (0.0 le S_i le 0.9). This law may be used to facilitate the design of coaxial swirled injectors.