Unsteady Gas Research Articles

Unsteady fluid–solid interaction by a kernel‐based particle method

AbstractA Lagrangian stabilized particle method is adopted for the simulation of unsteady fluid flow and gas dynamics. The velocity field smoothing technique, which was successfully implemented for reducing zero energy modes and tensile instabilities in solid problems, is used for the stabilization of the fluid phase. In this method a smoothing technique has been applied and the field values in particles are prevented from excessive jumps due to instability. This approach can replace traditional stabilization methods such as the artificial viscosity approach. In this paper, the corrective smoothed particle method has been implemented for solving governing partial differential equations of fluid. Also a modification in basic governing equations is performed on boundary interfaces to conserve mass and energy quantities. The method is implemented and verified in one‐ and two‐dimensional compressible flow problems. Copyright © 2009 John Wiley & Sons, Ltd.

Numerical method for computing two-dimensional unsteady rarefied gas flows in arbitrarily shaped domains

A high-order accurate method for analyzing two-dimensional rarefied gas flows is proposed on the basis of a nonstationary kinetic equation in arbitrarily shaped regions. The basic idea behind the method is the use of hybrid unstructured meshes in physical space. Special attention is given to the performance of the method in a wide range of Knudsen numbers and to accurate approximations of boundary conditions. Examples calculations are provided.

Development and tending to steady state of subsonic gas condensation on a plane condensed phase

The kinetic equation for a monatomic gas with a model collision operator (S-model) is used to study the development and tending to steady state of one-dimensional unsteady half-space gas condensation on a plane condensed phase. Initially, the gas is at rest and in equilibrium with the body’s surface and, then, the body temperature suddenly drops to a constant value. The problem is solved using an implicit second-order accurate quasi-monotone scheme. The process of reaching a steady flow regime is of primary interest. The effect of the evaporation (condensation) coefficient on the flow pattern is analyzed.

CFD Simulation of Bubble Columns: Modeling of Nonuniform Gas Distribution at Sparger

Most laboratory bubble columns are equipped with sieve plate spargers. The sieve plate spargers are known to lead to nonuniform gas distribution. It is important to account for such nonuniform gas distribution at the sparger in the computational model before experimental data collected from such columns are used to fit the model parameters. In this article, such an attempt is made. A detailed, 3D CFD model was developed to simulate unsteady gas−liquid flows in bubble columns with sieve plate spargers. The sensitivity of the nonuniformity of gas distribution at the sparger with sparger resistance was examined. The model predictions were compared with the experimental data. The developed model and presented results will be useful for simulating industrial bubble columns.

Simulation and testing of pollutant dispersion during preventive maintenance in a cleanroom

Sulfur hexafluoride (SF 6) of >99.9% purity was artificially released to simulate the emission sources in the etching-thin film area of a working cleanroom in a semiconductor fab at the rate of 492 g/h. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF 6 concentrations at different locations of the cleanroom. A three-dimensional numerical model was also used to predict the unsteady gas concentration distribution and the results were compared with the experimental data. Due to high dilution of the pollutant in the cleanroom, it is found that the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and to ensure good product yield. After comparison with the validated numerical results, the well-mixed model is found to predict the peak pollutant concentrations within a reasonable range which is 0.34–1.33 times the experimental values except when the monitored distance is very close to the release point. The well-mixed model is shown to be capable of predicting a reasonable attainable maximum concentration once a pollutant leaks in the cleanroom.

Capabilities of a quasi-gasdynamic algorithm as applied to inviscid gas flow simulation

Ten well-known one-dimensional test problems reflecting the characteristic features of unsteady inviscid gas flows are successfully solved by a unified numerical algorithm based on the quasigasdynamic system of equations. In all the cases, the numerical solution converges to a self-similar one as the spatial grid is refined.

Mathematical model of unsteady gas to solid particles heat transfer in fluidized bed

The mathematical model of unsteady one-dimensional gas to particles heat transfer for non-isothermal fluidized bed with periodic heating of solid particles has been described. The method of numerical solution of governing differential equations, the algorithm and the computer program, have been presented. By using mathematical model and computer program, the temperature profiles for interstitial gas, gas in bubbles, and solid particles along the height of fluidized bed in function of time, have been determined. The results obtained on the basis of prediction method are compared to the experimental results of the authors; the satisfactory agreement has been found for interstitial gas temperature and solid particle temperature. On the basis of this comparison, the mathematical model has been verified.

Unsteady Gas Flow Monitoring and Control

Unsteady Gas Flow Monitoring and Control

Internal combustion engine exhaust pipe flow simulation. Part I: theoretical aspects

Unsteady gas flow theory can be used for simulating a spark ignition internal combustion engine’s exhaust system, using pressure waves. The method explained here is based on the discretization of interpolated spaces (called meshes) which are located throughout the whole length of the exhaust pipe, irrespective of its form or size. The most important aspects of this theory are theoretically explored, such as pressure wave movement and shock and their application to cases found in real engines’ exhaust pipes. This work also considers how the simulation must be made, based on the previous exploration. The results (presented as equations in this first paper) show the great influence exerted by pressure wave movement on flow through the engine and therefore on its final performance.

Influence of the lift force closures on the numerical simulation of bubble plumes in a rectangular bubble column

Numerical Eulerian–Eulerian simulations of the unsteady gas–liquid flow in a centrally aerated two-dimensional bubble column were carried out in order to understand the effect of different formulations of the lift force coefficient ( C L ) on the computational results. Three different values of the superficial gas velocity ( U G =2.4, 12.0 and 21.3 mm s −1) that ensure the existence of different flow regimes were experimentally and computationally studied. The validation of the simulated results was based on visual observations and measurements of the global gas hold-up ( ε G ) and the plume oscillation period (POP). The results presented reveal that, at U G =12.0 and 21.3 mm s −1, using C L <0 results in under- and over-estimation of the ε G and POP, respectively. On the other hand, taking C L >0 does not affect the POP while it leads to increasingly higher ε G values, which are different from those experimentally reported. At U G =2.4 mm s −1, the effect of the lift force is not so evident, although it slightly improves the prediction of experimental values. Particularly interesting is the case of C L >0.4 at U G =21.3 mm s −1, producing a non-symmetric bubble plume oscillation. Since using Tomiyama's lift coefficient correlation does not improve the results, including the lift force into the simulation of bubble plumes is not recommended.

Control of the aerodynamic characteristics of a profile oscillating with respect to the incidence angle in subsonic viscous flow

The results of a numerical simulation of the unsteady subsonic viscous gas flow around a two-dimensional profile oscillating with respect to the incidence angle are presented and the possibility of controlling the nonstationary aerodynamic characteristics is considered. The hysteresis phenomena typical of oscillatory profile motions are investigated, the dependence of the lift force and drag is found for various laws of periodic variation of the incidence angle with time, and the effect of the frequency and amplitude of the angular profile oscillations on the shape of the hysteresis curves is studied. The calculations were based on the numerical solution of the nonstationary Navier-Stokes equations averaged in the Reynolds sense (Reynolds equations) which were closed using the k-ω turbulence model with modeling of the laminar/turbulent transition.

Importance of lateral momentum equation to the modeling of sandwich propellant combustion

This paper reports the results of numerical studies carried out for a periodic sandwich propellant geometry with two-dimensional unsteady gas and condensed phase. A non-planar regressing surface along with a kinetic model of two reaction steps in the gas phase was used. This paper discusses the importance of lateral momentum equation to the combustion of sandwich propellants. It demonstrates that the instabilities in sandwich combustion reported in literature are due to neglect of lateral momentum equation and incorrect boundary conditions at the regressing surface. It demonstrates that neglect of momentum equations will lead to a different result from the one obtained considering the momentum equations for sandwich propellants.

Three-dimensional generalization for W modification of a Godunov method

A high-accuracy modification of Godunov’s method for three-dimensional unsteady ideal gas flows is proposed. For the linear advection equation, a fully three-dimensional second-order accurate monotone scheme is designed with corrections computed on a variable stencil whose orientation depends on the signs of the equation coefficients. For the linear scalar advection equation, the scheme is proved to possess the positive approximation property. The method is tested by computing the flow in a three-dimensional Ludwieg tube with a square cross section.

Impulse Generation by an Open Shock Tube

We perform experimental and numerical studies of a shock tube with an open end. The purpose is to investigate the impulse due to the exhaust of gases through the open end of the tube as a model for a partially filled detonation tube as used in pulse detonation engine testing. We study the effects of the pressure ratio (varied from 3 to 9.2) and the volume ratio (expressed as fill fractions) between the driver and driven section. Two different driver gases, helium and nitrogen, and fill fractions between 5 and 100% are studied; the driven section is filled with air. For both driver gases, increasing the pressure ratio leads to larger specific impulses. The specific impulse increases for a decreasing fill fraction for the helium driver, but the impulse is almost independent of the fill fraction for the nitrogen driver. Two-dimensional (axisymmetric) numerical simulations are carried out for both driver gases. The simulation results show reasonable agreement with experimental measurements at high pressure ratios or small fill fractions, but there are substantial discrepancies for the smallest pressure ratios studied. Empirical models for the impulse in the limits of large and small fill fractions are also compared with the data. Reasonable agreement is found for the trends with fill fractions using the Gurney or Sato model at large fill fractions, but only Cooper’s bubble model is able to predict the small fill fraction limit. Computations of acoustic impedance and numerical simulations of unsteady gas dynamics indicate that the interaction of waves with the driver-driven gas interface and the propagation of waves in the driven gas play an essential role in the partial-fill effect.

Development of Local Gas Temperature Measurement Technique inside a Combustion Chamber Using Two-Wire Thermocouple

Thermocouples are widely used to measure the local gas temperature due to its accuracy and convenience. However, it is difficult to use thermocouples in a transient phenomenon such as reacting fields. In this study, the unsteady gas temperature inside a combustion chamber was measured by using an improved two-wire thermocouple technique. Based on previous two-wire methods, some modifications were examined. Firstly, numerical analysis of heat transfer between transient flow and thermocouple was performed to see what kind of modification was required. Secondly, a correction term was added to the basic equation, which was validated by experiments using a Rapid Compression and Expansion Machine. Finally, an improved two-wire thermocouple technique was evaluated by measuring the transient gas temperature inside a combustion chamber and compared to the estimated temperature using measured pressure data and assumptions of chemical equilibrium.

1810 呼吸を模擬可能な非定常流量発生装置の開発(S24-2 流量計測の新たな展開(2),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This paper deals with the development of unsteady gas flow generator for breathing reproduction. At first, we measure the breathing flow using a laminar flow meter with high speed response. And we gave these measured results as the target value of the unsteady gas flow generator and tried the breathing reproduction. As a result, we confirmed that the unsteady gas flow generator succeeded in breathing reproduction.

Oscillatory Gas Flow in a Circular Nanotube

Gas flow through nanoscale conduits shows distinctly different physics compared to flow at the macroscale. One of the first manifestations of this is the appearance of wall slip. Here we consider the oscillatory flow of an incom- pressible Newtonian gas through a circular nanotube, with first order wall slip due to rarefaction. It was found that the ma- jor effect of wall slip at lower oscillation frequencies is to alter the instantaneous velocity profiles from Poiseuille-like to more plug-like, with an overall enhancement to the fluid velocity magnitude. However, at higher frequencies the en- hancement to velocity magnitude due to wall slip is greatly diminished, and rarefaction introduces a region of backflow near the centerline of the nanotube. These flow characteristics may have important implications for the design of various practical applications of unsteady gas flow through nanoscale conduits, from fuel cell membranes to gas-powered nanomachines constructed from carbon nanotubes.

The application of PIV to the study of unsteady gas dynamic flow within pipes and at pipe discontinuities

This paper describes research using a pulse generator that provides discrete waves. The wave motion was recorded using a series of high speed pressure transducers and the ensuing movement of the gas particles within the pipes was recorded using digital PIV. Gas particle velocities of up to 250m/s were a regular occurrence and vortices of high angular velocity were a common problem with associated spin out of seed particles. Although flows were predominantly two dimensional, stereoscopic techniques were used to give optical access to otherwise inaccessible areas of flow. So far these techniques have been applied to the unsteady gas dynamic flow within plain pipes, at sudden expansions and contractions within pipes and at the end of a pipe open to the environment, both plain ended and when fitted with a bellmouth. Examples of all of these are presented in this paper.

Hydrodynamics of Bubble Column Reactors at High Gas Velocity: Experiments and Computational Fluid Dynamics (CFD) Simulations

This paper focuses on the modeling of flow and mixing in a bubble column reactor operated at high gas velocities (up to 0.40 m/s). A dual-tip conductivity probe was used to measure local void properties such as local time-averaged gas holdup, chord length distribution, bubble velocity distribution, and interfacial area. Chord length distribution was converted to bubble size distribution, using the backward transformation method. Liquid-phase mixing time measurements were conducted using a conductivity probe. A computational fluid dynamics (CFD) model was developed to simulate the unsteady gas−liquid flow in a bubble column using commercial code FLUENT 6.2. The time-averaged flow properties predicted by CFD simulations were compared with the experimental data. The role of unsteady flow structures in mixing was studied. The “multiple snapshots” approach was used to simulate the mixing time using CFD. The mixing times that were predicted for all superficial gas velocities compared favorably to the measured values. This study of the hydrodynamic behavior of a bubble column at higher gas velocity provides a basis for understanding and simulating solid suspension (or solid mixing) in slurry bubble column reactors.

Dynamically adapted grids for interacting discontinuous solutions

Further development of the dynamic adaptation method for gas dynamics problems that describe multiple interactions of shock waves, rarefaction waves, and contact discontinuities is consid- ered. Using the Woodward-Colella problem and a nonuniformly accelerating piston as examples, the efficiency of the proposed method is demonstrated for the gas dynamics problems with shock wave and contact discontinuity tracking. The grid points are distributed under the control of the diffusion approx- imation. The choice of the diffusion coefficient for obtaining both quasi-uniform and strongly nonuni- form grids for each subdomain of the solution is validated. The interaction between discontinuities is resolved using the Riemann problem for an arbitrary discontinuity. Application of the dynamic adapta- tion method to the Woodward-Colella problem made it possible to obtain a solution on a grid consisting of 420 cells that is almost identical to the solution obtained using the WENO5m method on a grid con- sisting of 12 800 cells. In the problem for a nonuniformly accelerating piston, a proper choice of the dif- fusion coefficient in the transformation functions makes it possible to generate strongly nonuniform grids, which are used to simulate the interaction of a series of shock waves using shock wave and contact discontinuity tracking. The dynamic adaptation methods (13-21) are less complicated and make it possible to generate grids with a constant (19) or variable (18) number of grid points. In these methods, the control of the distribution of the grid points is achieved by using information about the dynamic behavior of the solution to be found. This makes it possible to concentrate a large number of grid points in the regions where the solution under- goes sharp changes (13). A close relationship between the dynamic behavior of the solution and the location of the grid points requires that the coordinates of the grid points be recalculated at each time layer. This fact places high requirements upon the consistency of the solution dynamics with the grid points motion. For this reason, the algorithms that do not use any fitting parameters have certain advantages (15). These fea- tures are especially pronounced in the generation of adaptive grids for unsteady gas dynamics problems (see (16-22)), which describe rapidly changing processes. The solution of hyperbolic differential equations,