The direct simulation Monte Carlo method is applied in this paper to simulate the micro Rayleigh-Bénard convection for the Rayleigh number of 10,159 and the Knudsen number of 0.01 in a time-dependent manner. A monatomic gas is enclosed between two infinite, parallel plates with the bottom plate at a higher temperature. Cases of three different computational domain sizes in the horizontal directions are simulated. Evolutions of the convective flow unsteady disturbances patterns and the wall heat transfer are studied in detail. Three stages of distinct flow characteristics can be identified as the flows develop from an initially uniform state. In the first stage, the heat is transferred mainly by conduction. The onset of the vortical flow structures marks the beginning of the second stage. Significant differences in the flow and the heat transfer characteristics are observed in the third stage of the three simulated flows. It is found that the simulated microflows develop vortex rolls that advect along the plates at uniform speeds, which has not been reported previously.

Mathematical modeling of the cardiovascular system is useful in quantifying system-level responses to external and internal perturbations, which are challenging to achieve in clinical studies. Most existing cardiovascular models are closed-loop systems and do not take into account the fluid and electrolyte exchanges and the body-fluid homeostasis. We propose a new system-level simulation model that modifies and couples an open-source cardiovascular model with a renal system model. The new model has the capability to simulate both the short- and long-term responses of the cardiovascular system and the body-fluid homeostasis due to changes in the amount of fluid intake. In the present study, we apply the model to investigate the short- and long-term changes in the human cardiovascular and renal systems, in responses to excessive fluid intake. For the short-term simulations, an instant but mild increase in mean arterial pressure is found, while the heart rate decreases slightly, after an ingestion of 500 mL of water. The results agree reasonably well with data from clinical studies. Simulation results also show that a prolonged elevation of daily fluid intake leads to chronic adaptations of system parameters, such as the blood pressure and hormonal levels. These adaptations cause an increase in the left ventricular stroke work during each cardiac cycle, which may have implications on the chronic remodeling of the myocardial structures and its mechanical functions. These chronic overload remodeling may eventually lead to pathological cardiovascular conditions, such as cardiac arrhythmias and congestive heart failure.

This paper describes the simplest hybrid configuration, called an aeroship, that integrates an airship module and a wing to meet demanding requirements in takeoff/landing, low-speed flight, and control/maneuver for certain challenging flight missions. A flight performance analysis of an aeroship is given, particularly on the optimum ratio between the aerostatic lift and the total weight to achieve the maximum lift-to-drag ratio. To critically examine the conceptual design and performance analysis, a remote-controllable model aeroship is built and tested in flight. The results obtained by using a stereo-videogrammetric system in flight testing are consistent with the performance analysis.

The direct simulation Monte Carlo (DSMC) method is used in a time‐dependent manner to simulate three‐dimensional micro Couette flows. An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the averaged properties and disturbances, are studied. The computations have been performed using a parallel computer cluster. The results presented include those with various channel heights, plate speeds, and the forcing level. These changes also result in changes in the flow Reynolds number and Knudsen number. Spatially coherent flow patterns can be identified in the averaged flow and the disturbance flow fields. The results indicate that the discrete approach can capture unsteady, three‐dimensional vortical flow structures. In cases with strong forcing, the disturbance energy spectra show significant content above statistical scatter.

Three-dimensional unsteady flows between two infinite walls are simulated by using the direct simulation Monte Carlo (DSMC) method. An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the long time average and the disturbance components, are studied. The computations have been preformed using parallel computer clusters. The results presented are for two different channel heights with various values for the forcing coefficient. The change in the channel height, which also results in changes in the flow Knudsen number and Reynolds number, affects the development of both the mean flows and the disturbances. Spatially coherent mean flow patterns, which are dominated by a hierarchy of harmonic modes, can be identified in the DSMC solutions. Temporally, the evolution of the Fourier amplitudes of the harmonic modes shows that these modes grow in a sequential manner. Disturbances with energy spectra that are significantly higher than the statistical noises are resolved. Their pathline patterns indicate that the disturbance flow fields are three dimensional and spatially coherent. These results suggest that the discrete DSMC approach is capable of capturing unsteady, three-dimensional flow disturbances that evolve around a stationary mean flow.

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

A computational method for the prediction of the bursting frequency associated with the coherent streamwise structures in high-speed compressible turbulent boundary layers is presented. The structures are described as wavelike disturbances of the turbulent mean flow. A direct resonance theory is used to determine the frequency of bursting. The resulting hydrodynamic linear stability equations are discretized by using a Chebyshev collocation method. A global numerical method capable of resolving the entire eigenvalue spectrum is used. Realistic turbulent mean velocity and temperature profiles are applied. For all of the compressible turbulent boundary layers calculated, the results show at least one frequency that satisfies the resonance condition. A second frequency can be identified for cases with high Reynolds numbers. An estimate is also made for the profile distribution of the temperature disturbance.

The linear stability of a compressible confluent wake/boundary layer is studied. The base flow model considered is the superposition of a compressible boundary layer and a Gaussian-like wake located above the boundary layer. The linear stability equations have been solved by using a global numerical method. The stability modes of interest have been identified as the boundary-layer modes, the antisymmetric wake mode, and the symmetric wake mode. The effects of wake height on the first Tollmien-Shlichting mode and the second mode associated with the boundary layer are discussed. Results for unstable modes associated with the wake are also presented. At high speeds, in contrast to the incompressible results, a reduced wake height has a strong stabilizing effect on the growth rates of both the first and the second modes associated with the boundary layer. The effects of the reduced wake height on the growth rate of the unstable antisymmetric wake mode vary. For the high-Mach-number case calculated, there exists a frequency for which the growth rate of the unstable antisymmetric mode is neither enhanced nor reduced by the changes in wake height.

The heat transfer and the fluid dynamics characteristics of subsonic gas flows through microchannels are examined using the direct simulation Monte Carlo (DSMC) method. A simple implicit treatment for the low-speed inflow and outflow boundaries for the DSMC of the flows in microelectromechanical systems (MEMS) is used. Micro-Couette flows and micro-Poiseuille flows are simulated with the value of the Knudsen numbers ranging between 0.06 and 0.72. Where appropriate, the calculated velocity slip and temperature distribution are compared with analytical solutions derived from the Navier-Stokes equations with slip-boundary conditions. A patterned microstructure with nonuniform surface temperature is also simulated. The computational results show that the Knudsen number and the geometric complexity have significant effects on the heat transfer as well as the fluid dynamics properties of the microfluid flows studied.

The heat transfer characteristics of supersonic flows in microchannels is studied using direct simulation Monte Carlo (DSMC) method. The velocity components and the spatial coordinates of the simulated particles are calculated and recorded by using a variable-hard-sphere (VHS) collision model. The effects of Knudsen number (Kn) on the heat transfer of the microchannel flows are examined. The results show that the magnitude of the temperature jump at the wall increases with increasing Kn. The heat transfer to the isothermal wall is found to increase significantly with Kn. The possible causes for the increase of wall heat transfer are discussed.

The spatial linear instability of incompressible confluent wake/boundary layers is analyzed. The flow model adopted is a superposition of the Blasius boundary layer and a wake located above the boundary layer. The Orr-Sommerfeld equation is solved using a global numerical method for the resulting eigenvalue problem. The numerical procedure is validated by comparing the present solutions for the instability of the Blasius boundary layer and for the instability of a wake with published results. For the confluent wake/boundary layers, modes associated with the boundary layer and the wake, respectively, are identified. The boundary-layer mode is found to be amplified as the wake approaches the wall. On the other hand, the modes associated with the wake, including a symmetric mode and an antisymmetric mode, are stabilized by the reduced distance between the wall and the wake. An unstable mode switching at low frequency is observed where the antisymmetric mode becomes more unstable than the symmetric mode when the wake velocity defect is high.

Comparisons of the performance of several state-of-the-art low-Reynolds number turbulence models in the prediction of shock wave/turbulent boundary-layer interactions in transonic and supersonic flows are reported in this paper. The models include a realizable k-ε model, a k-ε/k-ω hybrid model, and a one-equation νt model. Several flow experiments focused on the shock wave/turbulent boundary-layer interaction phenomena are chosen for model comparisons. The selected flows represent a range of flow conditions with the turbulent boundary layers being fully attached, incipiently separated, or displaying large region of flow separation. Care has been taken to obtain solutions on sufficiently refined grids in all calculations. The model predictions are compared with experimental data and with the results obtained using Chien’s low-Reynolds number model. The results show that the models tested provide improvements over Chien’s model in the predictions of shock wave/turbulent interactions.

A simple implicit treatment for the low speed inflow and outflow boundary conditions for the direct simulation Monte Carlo (DSMC) of the flows in microelectromechanical systems (MEMS) is proposed. The local mean flow velocity, temperature, and number density near the subsonic boundaries were used to determine the number of molecules entering the computational domain and their corresponding velocities at every sample average step. The proposed boundary conditions were validated against micro-Poiseuille flows and micro-Couette flows. The results were compared with analytical solutions derived from the Navier-Stokes equations using first-order and second order slip-boundary conditions. The results show that the implicit treatment of the subsonic flow boundaries is robust and appropriate for use in the DSMC of the flows in MEMS.

Compressibility effects on the Reynolds stress are modeled using a Markovianized two-scale method. These effects occur twofold. One is the effect on the turbulent viscosity, which is expressed in terms of the ratio of the normalized density variance to the squared turbulent Mach number. Another comes from the deviation of the Reynolds stress from a turbulent-viscosity representation, which is written using the Langrange derivative of the mean velocity and the spatial derivatives of the mean density and internal energy. A simple model with the former compressibility effect incorporated is applied to fully developed free-shear layers and is shown to capture the steep decrease in the growth rate with the increasing convective Mach number.

A new k-ϵ eddy viscosity model, which consists of a new model dissipation rate equation and a new realizable eddy viscosity formulation, is proposed in this paper. The new model dissipation rate equation is based on the dynamic equation of the mean-square vorticity fluctuation at large turbulent Reynolds number. The new eddy viscosity formulation is based on the realizability constraints; the positivity of normal Reynolds stresses and the Schwarz' inequality for turbulent shear stresses. We find that the present model with a set of unified model coefficients can perform well for a variety of flows. The flows that are examined include: (i) rotating homogeneous shear flows; (ii) boundary-free shear flows including a mixing layer, planar and round jets; (iii) a channel flow, and flat plate boundary layers with and without a pressure gradient; and (iv) backward facing step separated flows. The model predictions are compared with available experimental data. The results from the standard k-ϵ eddy viscosity model are also included for comparison. It is shown that the present model is a significant improvement over the standard k-ϵ eddy viscosity model.

A multiple-scale model for compressible turbulent flows is proposed in this paper. It is assumed that turbulent eddy shocklets are formed primarily by large energetic eddies. The extra straining of the large eddy, due to their interactions with shocklets, enhances the energy cascade to smaller eddies. Model transport equations are developed for the turbulent kinetic energies and the energy transfer rates of the different scale. The turbulent eddy viscosity is determined by the total turbulent kinetic energy and the rate of energy transfer from the large scale to the small scale, which is different from the energy dissipation rate. The model coefficients in the modeled turbulent transport equations depend on the ratio of the turbulent kinetic energy of the large scale to that of the small scale, which renders the model more adaptive to the characteristics of individual flow. The model is tested against compressible free shear layers, boundary layers, and a compression ramp flow. The results agree satisfactorily with measurements.

The linear inviscid hydrodynamic stability of slightly curved free mixing layers is studied in this paper. The disturbance equation is solved numerically using a shooting technique. Two mean velocity profiles that represent stably and unstably curved free mixing layers are considered. Results are shown for cases of five curvature Richardson numbers. The stability characteristics of the shear layer are found to vary significantly with the introduction of the curvature effects. The results also indicate that, in a manner similar to the Gortler vortices observed in a boundary layer along a concave wall, instability modes of spatially developing streamwise vortex pairs may appear in centrifugally unstable curved mixing layers.

New closure models for turbulent free shear flows are presented in this paper. They are based on a weakly nonlinear theory with a description of the dominant large-scale structures as instability waves. Two models are presented that describe the evolution of the free shear flows in terms of the time-averaged mean flow and the dominant large-scale turbulent structure. The local characteristics of the large-scale motions are described using linear theory. Their amplitude is determined from an energy integral analysis. The models have been applied to the study of an incompressible mixing layer. For both models, predictions of the mean flow development are made. In the second model, predictions of the time-dependent motion of the large-scale structures in the mixing layer are made. The predictions show good agreement with experimental observations.