Makoto Tsubokura

A numerical study on the eddy structures of impinging jets excited at the inlet

Abstract Three-dimensional eddy structures arising in plane and round impinging jets excited at the nozzle inlet were investigated numerically by direct numerical simulation and large eddy simulation. Special attention is focused on how spanwise or azimuthal disturbances imposed at the inlet velocity affect the eddy structures in the transition process. In this respect, three spatial wavelength of λ / πD =1/3, 1/4 and 1/6, where D is the nozzle width or the diameter, were tested in this study. It was found that the round jet showed definite instability at λ / πD =1/6 while the plane jet showed almost equivalent sensitivity to all modes tested here. As regards the eddy structures in the stagnation region, elongated twin vortices along the impinging plate of the plane jet were reproduced and the number of pairs was found to agree exactly with the spanwise wave number imposed at the inlet. In contrast to the plane jet, no organized structures were observed in the stagnation region of the round jet.

Negative Magnus lift on a rotating sphere at around the critical Reynolds number

Negative Magnus lift acting on a sphere rotating about the axis perpendicular to an incoming flow was investigated using large-eddy simulation at three Reynolds numbers of 1.0 × 104, 2.0 × 105, and 1.14 × 106. The numerical methods used were first validated on a non-rotating sphere, and the spatial resolution around the sphere was determined so as to reproduce the laminar separation, reattachment, and turbulent transition of the boundary layer observed in the vicinity of the critical Reynolds number. The rotating sphere exhibited a positive or negative Magnus effect depending on the Reynolds number and the imposed rotating speed. At Reynolds numbers in the subcritical or supercritical regimes, the direction of the Magnus lift force was independent of the rotational speed. In contrast, the lift force was negative in the critical regime when particular rotating speeds were imposed. This negative Magnus effect was investigated in the context of suppression or promotion of boundary layer transition around the...

Modeling of the dynamic subgrid‐scale viscosity in large eddy simulation

The dynamic subgrid‐scale (SGS) viscosity is reexamined on the basis of the production process of the SGS Reynolds stress. A new model for the SGS viscosity is proposed in place of the Smagorinsky‐type model of mixing‐length form. The model is applied to a channel flow, and the comparison with other models is made.

Proper representation of the subgrid-scale eddy viscosity for the dynamic procedure in large eddy simulation using finite difference method

Alternative representation of the dynamic eddy viscosity coefficient to the traditional Smagorinsky model is investigated for the application of the model to the finite difference method. The model is derived by considering the consistency of the numerical error between Lij and Mij in the dynamic procedure. The proposed model is validated in the plane channel flow at the Reτ of up to 590 and is found to be less sensitive to the discretized test filtering operation.

High-Performance Computing and Visualization of Unsteady Turbulent Flows

The history of high-performance computing in turbulent flows is reviewed and their recent topics in industrial use are addressed. Special attention is paid to the validity of the method in flow visualization, and three-dimensional unsteady simulation is focused. Seemingly fundamental CFD technique for 3-D turbulence simulation has been well developed recently, but its practical use as an industrial tool has not yet become popular. An effort to close a wide gap between fundamental and practical use of scientific computer simulation is introduced through the national project promoting computational science and its development in industries of the next generation.

Technology of Automobile and Visualization Studies: In Celebration of the 10th Anniversary of Journal of Visualization

Computer simulation and visualization techniques for automotive studies are reviewed through the history of computer-aided visualization techniques to celebrate the 10th anniversary of Journal of Visualization. Future direction of visualization study is also mentioned through the introduction of up-to-dated numerical visualizations.

Subgrid scale modeling for the turbulence in rotating reference frames

It is well known that large- or middle-scale atmospheric turbulence is strongly affected by Coriolis force which produce particular eddy structures in the flow field. In this study, subgrid-scale modeling considering Coriolis force was presented following the method proposed by Yoshizawa et al. [Phys. Fluids 8 (1996) 1254]. The proposed subgrid-scale model was successfully applied to LES of rotating channel flows at Reτ=150 and 590 normalized by the channel half-width and the friction velocity and the effect of rotation at low and moderate Reynolds number was investigated. The dynamic procedure proposed by Germano et al. [Phys. Fluids A 3 (1991) 1760], was adopted to determine the model parameter dynamically depending on the flow field. The way to discretize spatial filtering operation by the finite difference method required for the procedure, to be also presented, is indispensable for the application of the method to engineering and geophysical problems. Dependence of the results on discretized filtering was also examined.

Negative Magnus Effect on a Rotating Sphere at around the Critical Reynolds Number

Negative Magnus lift acting on a sphere rotating about the axis perpendicular to an incoming flow is investigated using large-eddy simulation at three Reynolds numbers of 1.0× 104, 2.0 × 105, and 1.14 × 106. The numerical methods adopted are first validated on a non-rotating sphere and the spatial resolution around the sphere is determined so as to reproduce the laminar separation, reattachment, and turbulent transition of the boundary layer observed at around the critical Reynolds number. In the rotating sphere, positive or negative Magnus effect is observed depending on the Reynolds number and the rotating speed imposed. At the Reynolds number in the subcritical or supercritical region, the direction of the lift force follows the Magnus effect to be independent of the rotational speed tested here. In contrast, negative lift is observed at the Reynolds number at the critical region when particular rotating speeds are imposed. The negative Magnus effect is discussed in the context of the suppression or promotion of boundary layer transition around the separation point.

Coupled Analysis of Unsteady Aerodynamics and Vehicle Motion of a Heavy Duty Truck in Wind Gusts

In the present study, unsteady aerodynamic forces acting on a simplified heavy duty truck in strong wind gust and their effects on the truck’s motion were investigated by using a coupled analysis. Unsteady fluid dynamics simulation was applied to numerically reproduce unsteady aerodynamic forces acting on the truck under sudden crosswind condition. Taking account of vehicle’s motion, moving boundary techniques were introduced. Motions of the truck were simulated by a vehicle dynamics simulation including a driver’s reaction. The equations of motion of the truck in longitudinal, lateral, and yaw-rotational directions were numerically solved. These aerodynamics and vehicle dynamics simulations were coupled by exchanging the aerodynamic forces and the vehicle’s motion. In order to investigate effects of the unsteady vehicle aerodynamics on the vehicle’s motion, conventional analysis of the vehicle’s motion using quasi-steady aerodynamic forces and one-way coupled analysis with fixed vehicle attitude were also conducted. The numerical results of these simulations were compared with each other, and the effects of the two kinds of unsteady aerodynamics were discussed separately and totally. In the sudden crosswind condition, the unsteady aerodynamics effected significantly on the truck’s motion. An effect of transient aerodynamics as the truck ran into a sudden crosswind was greater than an effect of unsteady aerodynamics caused by unsteady vehicle’s motion, while both of the effects showed significance.Copyright

NUMERICAL STUDY ON THE DIFFERENCE OF THE EDDY STRUCTURES BETWEEN PLANE AND ROUND IMPINGING JETS

ABSTRACT Three-dimensional organized structures around the stagnation region of the plane and round impinging jets were investigated by the visualized images obtained by DNS and LES. We especially focused on the effect of the spanwise or azimuthal disturbances imposed at the inlet velocity on the eddy structures. Three wavelength of λ/π D=1/3, 1/4 and 1/6 were tested in this study, and round jets showed definite instability at λ /π D=1/6 while plane jets were found to show almost equivalent sensitivity to all modes tested here. As regards the eddy structures at the stagnation region, famous twin vortices along the impinging plate of plane jets were reproduced and their number of pairs were found to agree well with the spanwise wave number imposed at the inlet. Contrarily to plane jets, we could not find distinct organized structures at the stagnation region of round jets.