Masahide Inagaki

A Mixed-Time-Scale SGS Model With Fixed Model-Parameters for Practical LES

ABSTRACT A new subgrid-scale (SGS) model for practical large eddy simulation (LES) is proposed. The model is constructed with the concept of mixed (or hybrid) time scale, which makes it possible to use consistent model parameters and to dispense with the distance from the wall. The model performance is tested in plane channel flows, and the results show that this model is able to account for near-wall turbulence without an explicit damping function as in the dynamic Smagorinsky model. The model is also applied to the backward-facing step flow examined by Kasagi and Matsunaga (1995) experimentally. The calculated results show good agreement with experimental data, while the results obtained using the dynamic Smagorinsky model show less accuracy and less computational stability. To confirm the validity of the present model in practical applications, the three-dimensional complex flow around a bluff body ( Ahmed, 1984 ) is also calculated with the model. The agreement between the calculated results and the experimental data is quite satisfactory. These results suggest that the present model is a refined SGS model suited for practical LES to compute flows in a complicated geometry.

Numerical prediction of fluid-resonant oscillation at low Mach number

A method (governing equation set and numerical procedure) suited to the numerical simulation of fluid-resonant oscillation at low Mach numbers is constructed. The new equation set has been derived under the assumption that the compressibility effect is weak. Because the derived equations are essentially the same as the incompressible Navier-Stokes equations, except for an additional term, we can apply almost the same numerical procedure developed for incompressible flow equations without difficulty. With application of a pressure-based method that treats the continuity equation as a constraint equation for pressure, the stiffness problem that arises in solving the usual compressible flow equations under low Mach number conditions has been alleviated. To verify the present method, we apply it to the flows over a three-dimensional open cavity

Large eddy simulation analysis of engine steady intake flows using a mixed-time-scale subgrid-scale model

Abstract Large eddy simulation (LES) using a mixed-time-scale (MTS) subgrid-scale (SGS) model is applied to the intake flows in simplified internal combustion engine geometry. A modified colocated grid system is employed to obtain results as precise as possible and to perform calculations in a stable way with a central difference scheme for convective terms. The results are compared with corresponding experimental data and the Reynolds averaged Navier—Stokes (RANS) equation model results obtained using the low-Reynolds-number linear k— In addition, it is made clear that when the QUICK scheme is used in LES for the convective terms instead of the central difference scheme, the result obtained deteriorates owing to the numerical viscosity. The importance of the discretization method in practical LES is also confirmed.

A MIXED-TIME-SCALE SGS MODEL WITH FIXED MODEL-PARAMETERS FOR PRACTICAL LES

A new subgrid-scale (SGS) model for practical large eddy simulation (LES) is proposed. The model is constructed with the concept of mixed (or hybrid) time scale, which makes it possible to use consistent model parameters and to dispense with the distance from the wall. The model performance is tested in plane channel flows, and the results show that this model is able to account for near-wall turbulence without an explicit damping function as in the dynamic Smagorinsky model. The model is also applied to the backward-facing step flow examined by Kasagi and Matsunaga (1995) experimentally. The calculated results show good agreement with experimental data, while the results obtained using the dynamic Smagorinsky model show less accuracy and less computational stability. To confirm the validity of the present model in practical applications, the three-dimensional complex flow around a bluff body (Ahmed, 1984) is also calculated with the model. The agreement between the calculated results and the experimental data is quite satisfactory. These results suggest that the present model is a refined SGS model suited for practical LES to compute flows in a complicated geometry.

LES Analysis of Engine Steady Intake Flows Using a Mixed-Time-Scale SGS Model

Large eddy simulation (LES) using a mixed-time-scale (MTS) SGS model is applied to the intake flows in simplified internal combustion engine geometry. A modified colocated grid system is employed so as to perform calculations stably with a central difference scheme for convection terms. The results are compared with corresponding experimental data and the computational results obtained by using the low-Reynolds-number linear k-e and cubic nonlinear k-e -A2 turbulence models. The LES results show the best agreement with experimental data in three computational cases, not only in the mean velocity profiles but also in the profiles of turbulent energy. These results suggest that LES using the MTS model is an effective method for accurately predicting the performance of a combustion engine involving turbulent diffusion of spray and combustion flame. In addition, it is clarified that the numerical viscosity from the QUICK scheme for the convection term has a great influence on the computational results and decreases the prediction accuracy of LES.Copyright

A fractal-based flame propagation model for large eddy simulation

A novel combustion model for large-eddy simulation (LES) for gasoline engines has been developed. Unlike conventional models based on Reynolds-averaged Navier–Stokes (RANS) models, the new model takes a unique approach; it is described by the fractal characteristics of flame front and a universal expression for the subgrid scale (SGS) flame speed. The present fractal combustion model was applied to calculations of a spark ignition engine. Both the 0–10 per cent and 10–90 per cent combustion periods agree well with the experimental data. Because the modelling of the SGS turbulent speed is based on fractal analysis with experimental observations, the SGS combustion model is able to apply a wide range of engine operating conditions. The present model was applied to a multi-cycle simulation of a single-cylinder engine. The fluctuations at the instant when the heat release rate peaked were compared with data that was obtained experimentally. The calculated magnitude of the fluctuations was found to be close to the experimental values. It is thought that the flow variation generated during the intake stroke significantly influences the cyclic variations.

Wall Thermal Conductivity Effects on Nucleation Site Interaction During Boiling: An Experimental Study

The past decades have witnessed the diverse applications of boiling heat transfer enhancement in the removal of high density heat flux released by electronic components or power devices. People have developed many enhanced surfaces to obtain the highest heat transfer coefficient in nucleate boiling or to raise the CHF. In the boiling arena bubbling and nucleation site density play core parts, and hence it is crucial to correlate them quantitatively with surface structure and heat transfer conditions. For example one can determine by that correlation the best arrangement of boiling cavities for a given heat flux. However, the bubbling is highly influenced by inter-bubble actions. It has been found that the interactions can considerably change the bubble’s size, frequency and spatial distribution. The interactions are needed to be taken accounts of for a good correlation. Researchers tried to formulate the interactions as a single function of the inter-site spacing but have obtained contradictory conclusions, as suggests that they depend also on other parameters. In the present study we conducted a saturated boiling heat transfer experiment to investigate the interactions with respects to both the inter-site spacing and the wall thermal conductivity. The test section was fabricated by both copper and stainless steel, whose surface has two cylindrical artificial cavities of 50μm in diameter. It was heated with a uniform heat flux. The results show that both the bubble diameter Db and frequency f are functions of the inter-cavity distance s, but they vary in different manners in the copper and the stainless steel surfaces. In the copper surface, we observed evident enhancement of the boiling heat transfer at 1> S >0.4 and a slight inhibitive effect at 1.6> S >1, where S = s/Db . On the contrary the two nucleate sites in the stainless steel surface interfere with each other giving rise to evident suppression of boiling heat transfer at 1.6> S >0.65 and only slight enhancement at 0.65> S >0.3. Note that the copper’s thermal conductivity is 22 times larger than the stainless steel. Numerical simulation has revealed that the temperature variation beneath the copper cavities is much less than the stainless steel, which partly explains the differences in our experimental results. It is suggested that modeling the bubble interactions should take accounts of not only the distance-to-diameter ratio but also the fluid and wall properties.Copyright

Continuous Fabrication of Monodisperse Ceria–Zirconia–Yttria Composite Oxide Nanoparticles Using a Novel High-Shear Agitation Reactor

Monodisperse ceria–zirconia nanoparticles have attracted much attention as potential high-performance catalysts. Acidic aqueous solutions are generally used for peptizing aggregated precipitates during the fabrication of disperse nanoparticles. However, the peptization process requires multiple hours of aging, which significantly decreases the production efficiency. Hence, various researchers have attempted to eliminate this stage altogether by performing a coprecipitation process under ambient conditions using common salts as the starting materials. In this work, we report a continuous and direct technique for the fabrication of monodisperse composite oxide nanoparticles via coprecipitation inside a novel high-shear agitation reactor without aging. Using this method, monodisperse ceria–zirconia–yttria composite oxide nanoparticles with diameters of 3 nm were successfully synthesized.

LES Analysis of Flows Around a Forced-Oscillating Circular Cylinder

Large eddy simulation (LES) of flows around a forcedoscillating circular cylinder is carried out using the Arbitrary Lagrangian-Eulerian (ALE) method and a central difference scheme for the convection terms, which is a newly proposed discretization scheme that improves the conservation properties of the mass, momentum, and especially kinetic energy. The results are compared with the measurements that are also carried out by authors, in terms of lock-in phenomenon. It is shown that the numerically predicted pressure and velocity distributions are in good agreement with the experimental data, both in the lower and the upper lock-in region, and the phase difference between the cylinder displacement and the vortex shedding is consistent with previous findings. In addition, it is clarified that the frequency range of the lock-in in LES is almost the same in width as that of the experiment, while it becomes wider in the comparative analysis, in which the QUICK scheme is employed for the convection terms. Such discrepancies between two calculations are prominent, especially in the stationary and the lower frequency region.Copyright