Rocket engines and high-power new generations of gas-turbine jet engines and diesel engines oftentimes involve the injection of one or more reactants at subcritical temperatures into combustor ...environments at high pressures, and more particularly at pressures higher than those corresponding to the critical points of the individual components of the mixture, which typically range from 13 to 50 bars for most propellants. This class of trajectories in the thermodynamic space has been traditionally referred to as transcritical. However, the fundamental understanding of fuel atomization, vaporization, mixing, and combustion processes at such high pressures remains elusive. In particular, whereas fuel sprays are relatively well characterized at normal pressures, analyses of dispersion of fuel in high-pressure combustors are hindered by the limited experimental diagnostics and theoretical formulations available. The description of the thermodynamics of hydrocarbon-fueled mixtures employed in chemical propulsion systems is complex and involves mixing-induced phenomena, including an elevation of the critical point whereby the coexistence region of the mixture extends up to pressures much larger than the critical pressures of the individual components. As a result, interfaces subject to surface-tension forces may persist in multicomponent systems despite the high pressures, and may give rise to unexpected spray-like atomization dynamics that are otherwise absent in monocomponent systems above their critical point. In this article, the current understanding of this phenomenon is reviewed within the context of propulsion systems fueled by heavy hydrocarbons. Emphasis is made on analytical descriptions at mesoscopic scales of interest for computational fluid dynamics. In particular, a set of modifications of the constitutive laws in the Navier–Stokes equations for multicomponent flows, supplemented with a high-pressure equation of state and appropriate redefinitions of the thermodynamic potentials, are introduced in this work based on an extended version of the diffuse-interface theory of van der Waals. The resulting formulation involves revisited forms of the stress tensor and transport fluxes of heat and species, and enables a description of the mesoscopic volumetric effects induced by transcritical interfaces consistently with the thermodynamic phase diagram of the mixture at high pressures. Applications of the theory are illustrated in canonical problems, including dodecane/nitrogen transcritical interfaces in non-isothermal systems. The results indicate that a transcritical interface is formed between the propellant streams that persists downstream of the injection orifice over distances of the same order as the characteristic thermal-entrance length of the fuel stream. The transcritical interface vanishes at an edge that gives rise to a fully supercritical mixing layer.
Microduct flows are known for their inherent laminar regimes resulting from the characteristic small dimensions and low velocities. In this regard, direct numerical simulations are employed to ...investigate an innovative approach that harnesses the unique thermophysical properties of high-pressure transcritical fluids to achieve significantly higher rates of mixing and heat transfer in microduct geometries. The strategy is based on the sizeable changes in properties that supercritical fluids, at pressures and temperatures exceeding their critical value, undergo across the pseudo-boiling region. To this end, four different cases are considered, and systematically analyzed, in which the bulk pressure and temperature difference between walls are varied. The results obtained indicate that laminar flow prevails at low-pressure conditions, while flow regimes with turbulent characteristics can be achieved when operating at high-pressure conditions with a transversal temperature difference. The transition to the turbulence-like regime is assessed by quantifying variations in velocity and temperature profiles, accompanied by the observation of secondary flow motions. As a result, substantial increases in the Nusselt number of roughly 20×, indicative of enhanced heat transfer, are obtained at the hot wall in comparison to cases with same temperature differences at low pressure.
•Microconfined laminar and turbulent regimes in square duct flow.•Analysis of the effects of drastic changes in thermophysical properties.•Systematic analysis of first- and second-order flow statistics.•Enhancement of secondary flow motions and wall Nusselt number.
•Data-driven methodology for extracting important dimensionless groups.•Buckingham’s π theorem augmented with active subspaces and ridge functions.•Methodology applied to study the physics of ...irradiated particle-laden turbulence.•Most of the system’s response is dominated by two dimensionless groups.•Results can be leveraged to effectively reduce the dimensionality of the problem.
The study of thermal radiation interacting with particle-laden turbulence is of great importance in a wide range of scientific and engineering applications. The fundamental and applied study of such systems is challenging as a result of the large number of thermo-fluid mechanisms governing the underlying physics. This complexity is significantly reduced by transforming the problem of interest into its scale-free form by means of dimensional analysis techniques. However, the theoretical framework of classical dimensional analysis presents the limitations of not providing a unique set of dimensionless groups, and no support for measuring the relative importance between them. In the interest of addressing these shortfalls for multiphysics turbulent flow applications, we present a semi-empirical dimensional analysis approach to efficiently extract important dimensionless groups from data obtained by means of computational (or laboratory) experiments. The methodology presented is then used to characterize important dimensionless groups in irradiated particle-laden turbulence. The study concludes that two dimensionless groups are responsible for most of the variation in the system’s thermal response, with the absorption of radiation by particles, the radiative energy deposition rate and the turbulent flow mixing the most important thermo-fluid mechanisms. The generality of the results obtained can be leveraged to effectively reduce the dimensionality of irradiated particle-laden turbulent flows in research studies and in the design and optimization of similar systems.
A common approach in aerodynamic design is to optimize a performance function—provided some constraints—defined by a choice of an aerodynamic model at nominal operating conditions. Practical ...experience indicates that such a deterministic approach may result in considerably sub-optimal designs when the adopted aerodynamic model does not lead to accurate predictions, or when the actual operating conditions differ from those considered in the design. One approach to address this shortcoming is to consider an average or robust design, wherein the statistical moments of the performance function, given the uncertainty in the operating conditions and the aerodynamic model, is optimized. However, when the number of uncertain inputs is large or the performance function exhibits significant variability, an accurate evaluation of these moments may require a large number of function evaluations at each optimization iteration, rendering the problem significantly expensive. To tackle this difficulty, we consider a variant of the stochastic gradient descent method where in each iteration, a stochastic approximation of the objective, constraints, and their gradients is generated. This is done via a small number of forward/adjoint solutions corresponding to random selections of the uncertainties. The methodology is applied to the robust optimization of the NACA-0012 airfoil subject to operating condition and turbulence model uncertainty. With a cost that is only a small factor larger than that of the deterministic methodology, the stochastic gradient approach significantly improves the performance of the aerodynamic design for a wide range of operating conditions and turbulence models.
The investigation of the electromagnetic properties of biological particles in microfluidic platforms may enable microwave wireless monitoring and interaction with the functional activity of ...microorganisms. Of high relevance are the action and membrane potentials as they are some of the most important parameters of living cells. In particular, the complex mechanisms of a cell’s action potential are comparable to the dynamics of bacterial membranes, and consequently focusing on the latter provides a simplified framework for advancing the current techniques and knowledge of general bacterial dynamics. In this work, we provide a theoretical analysis and experimental results on the microwave detection of microorganisms within a microfluidic-based platform for sensing the membrane potential of bacteria. The results further advance the state of microwave bacteria sensing and microfluidic control and their implications for measuring and interacting with cells and their membrane potentials, which is of great importance for developing new biotechnologically engineered systems and solutions.
This work utilizes a novel data-driven methodology to reduce the dimensionality of non-buoyant microconfined high-pressure transcritical fluid turbulence. Classical dimensional analysis techniques ...are limited by the non-uniqueness of scale-free groups and the lack of a general strategy for quantifying their importance. Instead, the data-driven approach utilized is based on augmenting Buckingham’s π theorem with ideas from active subspaces to overcome these limitations. Through this methodology, a principal dimensionless group has been identified that efficiently describes the behavior of the system in terms of normalized bulk turbulent kinetic energy. Additionally, a simplified version of the new dimensionless group is proposed, which presents the structure of a Reynolds number augmented with dynamic viscosity, thermal conductivity, or equivalently Prandtl number and isobaric heat capacity, and specific gas constant to account for thermophysical effects. Finally, the results obtained in this study, which is based on a realistic regime inspired by nitrogen at high-pressure microfluidic conditions, can be generalized to other fluids using the principle of corresponding states.
•Microconfined turbulent flow regimes can be achieved using supercritical fluids.•One main dimensionless group governs turbulence mixing in microconfined supercritical fluids.•Dimensionless group similar to Reynolds number augmented with thermophysical parameters.
Wildfire behavior predictions typically suffer from significant uncertainty. However, wildfire modeling uncertainties remain largely unquantified in the literature, mainly due to computing ...constraints. New multifidelity techniques provide a promising opportunity to overcome these limitations. Therefore, this paper explores the applicability of multifidelity approaches to wildland fire spread prediction problems. Using a canonical simulation scenario, we assessed the performance of control variates Monte-Carlo (MC) and multilevel MC strategies, achieving speedups of up to 100x in comparison to a standard MC method. This improvement was leveraged to quantify aleatoric uncertainties and analyze the sensitivity of the fire rate of spread (RoS) to weather and fuel parameters using a full-physics fire model, namely the Wildland-Urban Interface Fire Dynamics Simulator (WFDS), at an affordable computation cost. The proposed methodology may also be used to analyze uncertainty in other relevant fire behavior metrics such as heat transfer, fuel consumption and smoke production indicators.
•Multifidelity strategies enabled uncertainty quantification in wildfire CFD models.•Multifidelity methods allowed 100x speedups in aleatoric uncertainty quantification.•A multilevel estimator based on 4 fidelity levels provided the best performance.•RoS sensitivity to fuel and wind parameters was quantified using WFDS.
Transcritical turbulent flows are governed by the compressible Navier–Stokes equations along with a real-gas equation of state. Their computation is strongly susceptible to numerical instabilities ...and requires kinetic-energy- and pressure-equilibrium-preserving schemes to yield stable and non-dissipative scale-resolving simulations. Building upon a recently developed kinetic-energy- and pressure-equilibrium-preserving discretization framework based on transporting a pressure equation, the objectives of this paper are to (i) derive a filtered set of equations suitable for large-eddy simulation, and (ii) characterize the properties of the resulting subfilter-scale terms by performing a priori analyses of transcritical wall-bounded turbulence direct numerical simulation data. The filtering operation leads to three unconventional subfilter-scale terms that emerge from the pressure equation and require dedicated modeling. The subfilter-scale stress tensor is dissected in terms of magnitude, shape and orientation based on an eigendecomposition analysis, and compared with existing subfilter-scale models. A priori analyses confirm that models of eddy-viscosity type are favorable for this framework, although the tensor shape is not fully captured. Closure expressions are finally proposed and tested for the novel subfilter terms, showing acceptable performances.
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•Large-eddy simulation framework for wall-bounded transcritical turbulence.•Kinetic-energy- and pressure-equilibrium-preserving scheme.•Scale-similarity-based subfilter-scale closure expressions.•Stable and non-dissipative high-fidelity scale-resolving simulations.
The aggregation of microplastic and biogenic particles in upper-ocean turbulence is studied by means of direct numerical simulations and Lagrangian tracking of point particles. The range of particle ...properties (size and density) and mixture characteristics (turbulence intensity and particle number densities) analyzed correspond to scenarios reminiscent of the problematic of microplastics in marine systems. A density ratio of 1.5 between the biogenic and microplastic particles, and varying diameter size and number density ratios are assumed. In particular, the focus is placed on the flow and mixture conditions encountered in the surface boundary layer (top region) of oceans, in which microplastics and biogenic particles have been experimentally observed to interact under significantly complex behaviors. The analysis consists of three principal parts involving the spatio-temporal distribution of the disperse and continuous phases, the mechanisms and rates of particle collisions, and the composition of the resulting aggregates. The main findings, for the range of parameters considered in this work and under the simplifying assumptions made in the model, are that (i) microplastics can be found in a large fraction of aggregates in scenarios with different average diameter of the mixture and number density ratios between microplastic and biogenic particles, (ii) microplastic-containing aggregates will sink to the deeper ocean layers particularly in situations where the biogenic particles are larger and/or of similar size than microplastics, and (iii) the Stokes numbers of aggregates tend not to be significantly different from the Stokes numbers of the initial individual microplastic and biogenic particles. In addition, by examining the collision mechanisms, a model for the collision rate that reproduces the computational results is proposed.
•The aggregation of microplastics with biogenic particles can be an important mechanism for settling of microplastics.•Microplastics were present in a significant number of aggregates in different scenarios.•The percentage of total microplastics settling increased almost quadratically with the average diameter of the mixture.
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