Patrick A. McMurtry

Linear eddy modeling of droplet spectral evolution during entrainment and mixing in cumulus clouds

Abstract The explicit mixing parcel model (EMPM) of Krueger et al. [Krueger, S.K., Su, C.-W., McMurtry, P.A., 1997. Modeling entrainment and fine-scale mixing in cumulus clouds. J. Atmos. Sci. 54, 2697–2712.] is extended to explicitly account for individual droplet growth. The model provides a 1D representation of physical properties within a cloud parcel and incorporates the rising of the cloud parcel, entrainment of environmental air into the parcel, finite-rate turbulent mixing parameterized by Kolmogorov inertial range scalings, and droplet growth. Because the simulation is performed in a 1D domain, all relevant length scales of the mixing process can be resolved. For the typical domain size used (20 m) in this study, the growth of approximately 2000 droplets is explicitly calculated based on their local environments. Comparisons of computed droplet spectra with spectra obtained from airplane penetration exhibit good agreement. Further results showing effects of entrained CCN, droplet sedimentation and mixing rates on droplet spectral evolution are provided.

Modeling Entrainment and Finescale Mixing in Cumulus Clouds

Abstract A model used to study entrainment and mixing of thermodynamic properties in the stratus-topped boundary layer has been extended to represent these processes in cumulus clouds. The new model, called the “explicit mixing parcel model” (EMPM), depicts finescale internal structure of a rising thermal in a cumulus cloud using a 1D domain. The EMPM links the conventional parcel model, which has no internal structure, and multidimensional cloud models, which resolve cloud-scale structure produced by large eddies. In the EMPM, the internal structure evolves as a consequence of a sequence of discrete entrainment events and an explicit representation of turbulent mixing based on Kerstein’s linear eddy model. In this version of the EMPM, subgrid-scale (eddy) diffusion is found to be adequate for representing the effects of the smallest turbulent eddies. In addition, a simple parameterization is used to determine the local condensation or evaporation rates. If the grid size is reduced so that the Kolmogorov ...

Linear eddy simulations of mixing in a homogeneous turbulent flow

The linear eddy mixing model is used to predict the evolution of a decaying scalar field in statistically steady homogeneous turbulent flow over a wide range of Reynolds and Schmidt numbers. Model results at low Reynolds number and order unity Schmidt number are shown to be in good overall agreement with direct numerical simulations. Results at higher Schmidt and Reynolds numbers reproduce conventional scaling properties of the scalar statistics. Predictions of Schmidt number and Reynolds number sensitivity of the evolution of the scalar concentration probability density function are presented and interpreted.

Interfacing continuum and molecular dynamics: An application to lipid bilayers

A new methodology is presented for interfacing atomistic molecular dynamics simulations with continuum dynamics, and the approach is then applied to a model lipid bilayer system. The technique relies on a closed feedback loop in which atomistic level simulations are coupled with continuum level modeling. This approach allows for the examination of the trans-temporal and trans-spatial phenomena that occur in many biological systems, and nonequilibrium molecular dynamics are used to calculate the relevant transport coefficients that are required at the continuum level. It is found that for the membrane system there is significant information transfer across disparate spatial and temporal regimes, resulting in significant nonlinear behavior.

Calculating the Bulk Modulus for a Lipid Bilayer with Nonequilibrium Molecular Dynamics Simulation

Nonequilibrium molecular dynamics (NEMD) computer simulations are used to calculated the bulk modulus for a dimyristoylphosphatidylcholine bilayer. A methodology is developed whereby NEMD can be effectively used to calculate material properties for complex systems that undergo long time-scale conformational changes. It is found that the bulk modulus upon expansion from a zero stress state agrees well with experimental estimates. However, it is also found that the modulus upon contraction from a zero stress state is larger. From a molecular perspective, it is possible to explain this phenomena by examining the molecular origins of the pressure response. The finding that the two moduli are not equal upon compression and expansion is in apparent contradiction to osmotic stress experiments where the area modulus was found to be the same upon expansion and contraction. This issue is addressed.

Simulating accidental fires and explosions

The Utah C-SAFE project is a highly multidisciplinary effort to build an integrated, large scale, high performance simulation framework to simulate accidental fires and explosions involving hydrocarbons, structures, containers, and high-energy materials. C-SAFE will provide a scalable, high performance, problem solving environment (PSE) in which fundamental chemistry and engineering physics are fully coupled with nonlinear solvers, optimization, computational steering, visualization, and experimental data verification. The availability of simulations using this system will help to better evaluate the risks and safety issues associated with fires and explosions. As the article discusses, we validate and document the five-year product, termed Uintah 5.0, for practical application to accidents involving both hydrocarbon and energetic materials.Once upon a time, in the kingdom of Batch, all access to the magical shrine Mainframe was controlled by a guild of wizards called Assemblers. They offered the machine-punched cards with the arcane symbols of their calling. Then, the monks of the benevolent orders Fortran and Cobol settled in the kingdom, and those who learned their language could actually understand what was in columns 7 through 12 of the sacrificial cards. But eventually the knights of Dartmouth, Sirs Kemeny and Kurtz, revolted against the evil card queue and let ordinary citizens submit Basic petitions to the gods through public shrines called Terminals...

Interfacing molecular dynamics and macro-scale simulations for lipid bilayer vesicles.

A continuum-level model for a giant unilamellar vesicle (GUV) is bridged to a corresponding atomistic model of a dimyristoylphosphatidylcholine (DMPC) bilayer at various cholesterol concentrations via computation of the bulk modulus. The bulk modulus and other microscopically determined parameters are passed to a continuum-level model operating in time- and length-scales orders of magnitude beyond that which is accessible by atomistic-level simulation. The continuum-level simulation method used is the material point method (MPM), and the particular variation used here takes advantage of the spherical nature of many GUVs. An osmotic pressure gradient due to a solvent concentration change is incorporated into the continuum-level simulation, resulting in osmotic swelling of the vesicle. The model is then extended to treat mixtures of DMPC and cholesterol, where small domains of different composition are considered.

Mesoscaling of Reynolds Shear Stress in Turbulent Channel and Pipe Flows.

Experimental and numerical data of the Reynolds shear stress in turbulent channel and pipe flows under a mesonormalization are presented. The mesolength scale associated with this normalization is intermediate to the traditional inner and outer lengths. Justification for the mesoscales is provided by a direct analysis of the mean momentum equation. Specifically, the mesonormalization is revealed through a rescaling that appropriately reflects the physics of an internal mesolayer within which a balance breaking, and subsequent balance exchange of terms in the mean momentum equation takes place. Direct numerical simulation and experimental data are examined and shown to be in good agreement with the new scaling, supporting the new theory.

Prediction of NO(x) production in a turbulent hydrogen-air jet flame

A major concern in the numerical study of turbulent nonpremixed flames is the accurate prediction of trace species. The production of pollutants such as NOx during unsteady combustion needs to be understood and predicted accurately so that the design of the next generations combustion systems can meet the forthcoming stricter environmental restrictions. Numerical studies using steady-state methods cannot account for the unsteady phenomena in the mixing region, and therefore, fail to accurately predict the NOx production that could occur. A novel unsteady mixing model is demonstrated here that accounts for all the length scales associated with mixing and molecular diffusion processes. Finite-rate kinetics in the form of a reduced mechanism have been used to study hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparisons with experimental data and pdf calculations show good agreement, thereby, providing validation of the mixing model.

Effects of turbulence length‐scale distribution on scalar mixing in homogeneous turbulent flow

The linear eddy mixing model is used to study effects of the turbulence length‐scale distribution on the transient evolution of a passive scalar in a statistically steady homogeneous turbulent flow. Model simulations are carried out using both wide‐band length‐scale distributions reflecting high‐Reynolds‐number scaling, and narrow‐band (in effect, low‐Reynolds‐number) distributions. The two cases are found to exhibit qualitative differences in mixing behavior. These differences are interpreted mechanistically. The narrow‐band case yields the best agreement with published direct numerical simulation results, suggesting that those results are, in effect, low‐Reynolds‐number results not readily extrapolated to high‐Reynolds‐number mixing.