Kazuhiko Suga

Development and application of a cubic eddy-viscosity model of turbulence

Abstract Many quadratic stress-strain relations have been proposed in recent years to extend the applicability of linear eddy-viscosity models at modest computational cost. However, comparison shows that none achieves much greater width of applicability. This paper, therefore, proposes a cubic relation between the strain and vorticity tensor and the stress tensor, which does much better than a conventional eddy-viscosity scheme in capturing effects of streamline curvature over a range of flows. The flows considered range from simple shear at high strain rates and pipe flow, to flows involving strong streamline curvature and stagnation.

Prediction of turbulent transitional phenomena with a nonlinear eddy-viscosity model

Abstract This paper describes a new nonlinear eddy-viscosity model of turbulence designed with a view to predicting flow far from equilibrium, including transition. The scheme follows earlier UMIST practice in adopting a cubic relation between the stress and the strain/ vorticity tensors but broadens the range of flows to which the model applies by including a third transport equation for an anisotropy parameter of the stress field. Applications are shown for transition on a flat plate at different levels of free-stream turbulence, for the normal impingement of a turbulent jet on a flat plate, and for the flow around a turbine blade. The model is shown to generate much more realistic predictions than what is said to be the best of the linear eddy-viscosity schemes.

Numerical simulation of binary liquid droplet collision

A numerical investigation of binary droplet collision has been conducted. The complete process of the collision of two liquid droplets is dynamically simulated by solving the incompressible Navier-Stokes equations coupled with the convective equation of the level set function that captures the interface between the liquid and the gas phases. The simulations cover four major regimes of binary collision: bouncing, coalescence, reflexive separation, and stretching separation. For water droplets in air, the numerical results are compared with the experiments by and Ashgriz and Poo [J. Fluid Mech. 221, 183 (1990)] on collision consequences. For hydrocarbon (C14H30) droplets in nitrogen gas, the simulated results are compared in detail with the time-resolved photographic images of the collision processes obtained by Qian and Law [J. Fluid Mech. 331, 59 (1997)] in every collision regime. The present numerical results suggest that the mechanism of a bouncing collision is governed by the macroscopic dynamics. Howe...

Towards the development of a Reynolds-averaged algebraic turbulent scalar-flux model

Abstract In order to derive a possible direction for developing Reynolds-averaged algebraic turbulent scalar-flux models, a priori explorations are attempted by processing the LES data presently performed for channel flows under several flow-boundary conditions including shear-free boundaries. The present calibration has elucidated that the turbulent scalar-flux vectors obtainable from the simple generalized gradient-diffusion hypothesis (GGDH) hardly align with the simulation results in wall-shear flows at Pr ⩾0.71. However, the GGDH form returns a quite reasonable approximation in shear-free flow regions and/or lower Pr fluid cases. In the former flow cases, it has been found that an introduction of quadratic products of the Reynolds-stress tensor into the gradient diffusion model may improve the predictive performance.

Nonlinear eddy viscosity modelling for turbulence and heat transfer near wall and shear-free boundaries

Abstract New turbulence and turbulent heat flux models are proposed for capturing flow and thermal fields bounded by walls or free surfaces. The models are constructed using locally definable quantities only, without any recourse to topographical parameters. For the flow field, the proposed model is a cubic nonlinear k–e–A three equation eddy viscosity model. It employs dependence on Lumleys stress flatness parameter A, by solving its modelled transport equation as the third variable. Since A vanishes at two-component turbulence boundaries, introducing its dependency enables a turbulence model to capture the structure of turbulence near shear-free surfaces as well as wall boundaries. To close the modelled A equation, an up-to-date second-moment closure is applied. For the thermal field, an explicit algebraic second-moment closure for turbulent heat flux is proposed. The new aspect of this heat flux model is the use of nonlinear Reynolds stress terms in the eddy diffusivity tensor. This model complies with the linearity and independence principles for passive scalar. The proposed models are tested in fully developed plane channel, open channel and plane Couette–Poiseuille flows at several fluid Prandtl numbers. The results show the very encouraging performance of the present proposals in capturing anisotropic turbulence and thermal fields near both wall and shear-free boundaries in the range of 0.025⩽Pr⩽95.

Lattice Boltzmann methods for complex micro-flows: applicability and limitations for practical applications

The extensive evaluation studies of the lattice Boltzmann method for micro-scale flows (?-flow LBM) by the authors group are summarized. For the two-dimensional test cases, force-driven Poiseuille flows, Couette flows, a combined nanochannel flow, and flows in a nanochannel with a square- or triangular cylinder are discussed. The three-dimensional (3D) test cases are nano-mesh flows and a flow between 3D bumpy walls. The reference data for the complex test flow geometries are from the molecular dynamics simulations of the Lennard-Jones fluid by the authors group. The focused flows are mainly in the slip and a part of the transitional flow regimes at Kn?<?1. The evaluated schemes of the ?-flow LBMs are the lattice Bhatnagar?Gross?Krook and the multiple-relaxation time LBMs with several boundary conditions and discrete velocity models. The effects of the discrete velocity models, the wall boundary conditions, the near-wall correction models of the molecular mean free path and the regularization process are discussed to confirm the applicability and the limitations of the ?-flow LBMs for complex flow geometries.

A numerical study on the breakup process of laminar liquid jets into a gas

Surface phenomena of liquid jets into still air are numerically investigated. The liquid and air are treated as a single continuum with dynamically evolving interfaces captured by the level-set method. The jets considered are laminar: the jet exit Reynolds number ranges from 480 to 2300, with the Weber number of 3.1–28 000. The liquid/air density ratios are about 103. In comparison with an empirical correlation, the present simulations reasonably predict the breakup lengths of round liquid jets at low Weber numbers. The development of the surface wave is captured and the breakup mechanisms are discussed referring to a conventional classification chart. The breakup phenomenon at high Weber and Ohnesorge numbers is also analyzed. It is then found that large-scale longitudinal-vortex motions, which are initially generated inside the liquid core by relaxation of the axial liquid velocity profile and surface shear, are amplified by surface motions due to instability. They grow up and eventually overcome inerti...

An experimental study of the local heat transfer characteristics in automotive louvered fins

This paper describes experimental devices for measuring the distribution of heat transfer coefficients in multilouvered fins. The devices are composed of louverlike thin elements having a three fold structure—a base metal, an electrical insulator, and an evaporated nickel film as both heating film and resistance thermometer. The locally averaged heat transfer coefficient for the individual louvers in the louver array is obtained by measuring the electric power input for the film and the temperature difference between the element and air. The validity of the method was verified in comparison with an analytical solution for a flat plate, numerical solutions for two-dimensional louver arrays, and another method for measuring the mean heat transfer coefficient. The relation between the distribution of heat transfer coefficients in the louvered fins and the fin geometries was elucidated. The device developed for measuring the heat transfer coefficients for the individual louvers is useful for achieving optimum design of louvered fin geometries.

Predicting turbulence and heat transfer in 3-D curved ducts by near-wall second moment closures

Abstract This paper presents discussions on predicting turbulence and heat transfer in two types of square sectioned U-bend duct flows with mild and strong curvature by recent second moment closures. Batten et al.s [AIAA J. 37 (1999) 785] modified version of Craft and Launders [Int. J. Heat Fluid Flow 17 (1996) 245] two-component-limit (TCL) turbulence model and Shimas [Int. J. Heat Fluid Flow 19 (1998) 549] wall-reflection free model are presently focused on. They are low-Reynolds-number models totally free from geometrical parameters. The former model is realizable and called the TCL model. For turbulent heat flux, a higher order version of the generalized gradient diffusion hypothesis by Suga and Abe [Int. J. Heat Fluid Flow 21 (2000) 37] is applied along with the TCL model. The results suggest that although both second moment closures are generally good enough for predicting flow and heat transfer in the case of mild curvature, only the realizable TCL model is reliable in the strong curvature case.

A D3Q27 multiple-relaxation-time lattice Boltzmann method for turbulent flows

A three-dimensional twenty-seven (D3Q27) discrete velocity multiple-relaxation-time lattice Boltzmann method is developed. The proposed scheme is validated in fully developed turbulent channel, pipe and porous medium flows through direct and large eddy simulations. The direct numerical simulation of the turbulent channel flow confirms that the present scheme is as reliable as the spectrum method for simulating turbulence. Through the large eddy simulations of the pipe and porous medium flows, the present scheme shows its satisfactory accuracy for simulating turbulent flows bounded by circular walls that is failed by the three-dimensional nineteen (D3Q19) model.