Data from a 1152×760×1280 direct numerical simulation (DNS) [N. J. Mueschke and O. Schilling, “Investigation of Rayleigh–Taylor turbulence and mixing using direct numerical simulation with experimentally measured initial conditions. I. Comparison to experimental data,” Phys. Fluids 21, 014106 (2009)] of a transitional Rayleigh–Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipation and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh–Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. These results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier–Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh–Taylor instability-generated flow.
Hypersonic flow field can experience significant thermochemical nonequilibrium effects. However, very little ground test data exists for nonequilibrium hypersonic wake flows. Simulating air chemistry in nonequilibrium is highly challenging due to the lack of validation data. To address a lack of validation data, ballistic free flight tests were conducted examining nonequilibrium chemistry effects in the wake of 25 mm spheres at Mach 10. A laser-based diagnostic system was used to measure naturally-produced nitric oxide concentrations. Measurements were made up to 53 body diameters behind the sphere. NO concentrations in the wake peaked between 4.1 +/- 0.8% and 6.8 +/- 1.3%.
Abstract A computational modeling effort was undertaken to combine finite element analysis (FEA) and computational fluid dynamics (CFD) methods to simulate the closing of the blind shear rams of a blowout preventer (BOP) under flowing conditions. The objective of this effort was to develop a high-fidelity fluid-structure interaction (FSI) simulation methodology that reliably assesses the combined mechanical and hydrodynamic forces acting on BOP shear rams that could potentially impact the rams’ ability to safely shut-in a well. BOP shear ram designers typically consider the material properties and geometry of the drill pipe to be sheared and the hydrostatic flowing pressure during ram closure. Fluid hydrodynamic effects on the rams are difficult to simulate and currently cannot be produced in a laboratory setting due to complexity and personnel safety in conducting such tests under high transient conditions, and thus are often neglected. To determine the best computational approach in terms of complexity and accuracy, a novel simulation methodology was developed by coupling the fluid interaction with the BOP as it shears a drill pipe using LS-DYNA® and ANSYS® Fluent®. The conditions and assumptions made for this analysis are presented herein, and initial simulation cases are compared against validation data to confirm model accuracy. For a high-pressure, high-flow well scenario in the Gulf of Mexico (GOM), a one-way coupling of the FEA and CFD simulations was determined to be the best approach for modeling the closing of blind shear rams under flowing conditions. This investigation also confirms that as long as the fluid is single phase, the ram forces due to fluid flow effects are small in comparison to the mechanical shearing force. It is noted, that highly dynamic flow events such as slugging flow or the potential erosive effects of sands or solid particles present additional risks, and the analysis methodology described here can serve as the basis for additional investigations into more complicated flow scenarios.
A 1152×760×1280 direct numerical simulation (DNS) using initial conditions, geometry, and physical parameters chosen to approximate those of a transitional, small Atwood number Rayleigh–Taylor mixing experiment [Mueschke et al., J. Fluid Mech. 567, 27 (2006)] is presented. In particular, the Atwood number is 7.5×10−4, and temperature diffusion is modeled by mass diffusion with an equivalent Schmidt number of 7. The density and velocity fluctuations measured just off of the splitter plate in this buoyantly unstable water channel experiment were parametrized to provide physically realistic, anisotropic initial conditions for the DNS. The methodology for parametrizing the measured data and numerically implementing the resulting perturbation spectra in the simulation is discussed in detail. The DNS is then validated by comparing quantities from the simulation to experimental measurements. In particular, large-scale quantities (such as the bubble front penetration hb and the mixing layer growth parameter αb), higher-order statistics (such as velocity variances and the molecular mixing parameter θ on the center plane), and vertical velocity and density variance spectra from the DNS are shown to be in favorable agreement with the experimental data. The DNS slightly underestimates the growth of the bubble front hb but predicts αb≈0.07 at the latest time, in excellent agreement with the experimental measurement. While the molecular mixing parameter θ is also slightly underestimated by the DNS during the nonlinear and weakly turbulent growth phases, the late-time value θ≈0.55 compares favorably with the value θ≈0.6 measured in the experiment. The one-dimensional density and vertical velocity variance spectra are in excellent agreement between the DNS and experimental measurements. Differences between the quantities obtained from the DNS and from experimental measurements are related to limitations in the dynamic range of scales resolved in the DNS and other idealizations of the simulation. Specifically, the statistical convergence of the DNS results and confidence interval bounds are discussed. This work demonstrates that a parametrization of experimentally measured initial conditions can yield simulation data that quantitatively agrees well with experimentally measured low- and higher-order statistics in a Rayleigh–Taylor mixing layer. This study also provides resolution and initial conditions implementation requirements needed to simulate a physical Rayleigh–Taylor mixing experiment. In Paper II [Mueschke and Schilling, Phys. Fluids 21, 014107 (2009)], other quantities not measured in the experiment are obtained from the DNS and discussed, such as the integral- and Taylor-scale Reynolds numbers, Reynolds stress and dissipation anisotropy, two-dimensional density and velocity variance spectra, hypothetical chemical product formation measures, other local and global mixing parameters, and the statistical composition of mixed fluid. These quantities are valuable for assessing the predictions of Reynolds-averaged Navier–Stokes and large-eddy simulation models of Rayleigh–Taylor turbulent mixing.
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