Yasutaka Nagano

An Improved k-ε Model for Boundary Layer Flows

An improvement of the k-e model has been made in conjunction with an accurate prediction of the near-wall limiting behaviour of turbulence and the final period of the decay law of free turbulence. The present improved k-e model has been extended to predict the effects of adverse pressure gradients on shear layers, which most previously proposed models failed to do correctly. The proposed model was tested by application to a turbulent pipe flow, a flat plate boundary layer, a relaminarising flow and a diffuser flow with a strong adverse pressure gradient. Agreement with the experiments was generally very satisfactory.

Characteristics of a turbulent natural convection boundary layer along a vertical flat plate

Abstract A detailed hot-wire measurement of a turbulent natural convection boundary layer is made paying close attention to the characteristics of the near-wall region which has not been clarified quantitatively. Heat transfer rate and wall shear stress are determined from mean temperature and mean velocity profiles on theoretical grounds. These measurements yield important results concerning the availability of the conventional analogy between heat and momentum transfer and the concept of the viscous sublayer for natural convection. Also, the streamwise development of the turbulent boundary layer is systematically investigated for mean velocity, mean temperature and both velocity and temperature fluctuations.

Effects of Adverse Pressure Gradients on Mean Flows and Turbulence Statistics in a Boundary Layer

Measurements in boundary layers with ‘moderate’ to ‘strong’ adverse pressure gradients are presented and discussed. With increasing adverse pressure gradients, the velocity profile in \( {\overline U ^ + } \sim {y^ + }\) coordinates lies below the standard log law, thus indicating a reduction in the thickness of the sublayer. Correspondingly, the turbulence energy components as well as the Reynolds shear stress peak in the outer region of the boundary layer. Higher-order moments of velocity fluctuations are also seriously affected by the adverse pressure gradient. In strong adverse-pressure-gradient flows, the triple products of velocity have completely opposite signs to those in zero-pressure-gradient flows over most of the boundary layer.

Computational study of laminar heat transfer downstream of a backward-facing step

Abstract Laminar heat transfer in a separating and reattaching flow was numerically studied by simulating the flow and heat transfer downstream of a backward-facing step. A series of computations was conducted in which three principal parameters governing the heat transfer in this geometry (i.e. channel expansion ratio ER, Reynolds number Re and Prandtl number Pr) were systematically changed. As a result, detailed relations between these parameters and the fundamental heat transfer characteristics have been elucidated. Some important findings are: (1) the distribution of the local Nusselt number depends strongly on all of these parameters; (2) the peak of the local Nusselt number does not necessarily locate at or very near the point of flow reattachment, in contrast to the common belief; and (3) if the Prandtl number is considerably low, the peak itself does not even appear and hence the heat transfer enhancement, usually assumed around the flow reattachment point, can never be expected in such cases.

STATISTICAL CHARACTERISTICS OF WALL TURBULENCE WITH A PASSIVE SCALAR

Various types of moments of velocity and scalar fluctuations of the first to the fourth order have been measured and analysed. First, an orthogonal series expansion for the 3-D joint probability density function (pdf) is developed using the emulants and Hermite polynomials. This pdf is found to provide satisfactory predictions for the statistical characteristics,including triple products, of turbulent momentum and scalar transfer. Next, the conditional sampling and averaging technique is employed to investigate the statistical characteristics of coherent turbulent transfer processes of momentum and scalar. Conditional pdfs are developed for various moments of velocity and scalar up to the third order. It is shown that the present pdfs can predict the detailed role of coherent motions in the dynamics of wall turbulent shear flows and in the relevant process of scalar transport by turbulence. In particular, the importance of coherent motions in the turbulent diffusion process of Reynolds-stress components and scalar fluxes is demonstrated for the first time by the present theory.

Structure of turbulent boundary layer subjected to adverse pressure gradient

Abstract A turbulent boundary layer subjected to a sustained adverse pressure gradient is experimentally investigated. Waveforms of fluctuating velocity components in the boundary layer, especially in the near-wall region, are remarkably elongated in time in comparison with those in zero-pressure-gradient flows, and thus time scales increase with an increasing pressure gradient parameter

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

P^+

Turbulence measurements in a natural convection boundary layer along a vertical flat plate

. The increase in time scales is not in proportion to the corresponding increase in the conventional viscous time scale

Mixed convection with flow reversal in the thermal entrance region of horizontal and vertical pipes

\nu/u_\tau^2

On Reynolds-stress expressions and near-wall scaling parameters for predicting wall and homogeneous turbulent shear flows

. It is found that the Taylor time scale is most appropriate to describe the essential characteristics of non-equilibrium adverse pressure gradient flows. Even the near wall-limiting behavior of streamwise velocity fluctuations for different