Extension of the KDO turbulence/transition model to account for roughness

Wall roughness significantly influences both laminar-turbulent transition process and fully developed turbulence. A wall roughness extension for the KDO turbulence/transition model is developed. The roughness effect is introduced via the modification of the k and νt boundary conditions. The wall is considered to be lifted to a higher position. The difference between the original position and the higher position, named as equivalent roughness height, is linked to the actual roughness height. The ratio between the two heights is determined by reasoning. With such a roughness extension, the predictions of the KDO RANS model agree well with the measurements of turbulent boundary layer with a sand grain surface, while the KDO transition model yields accurate cross-flow transition predictions of flow past a 6:1 spheroid.

Enhancing turbulence: a potential strategy for drag reduction based on ground transportation systems (GTS) model

Purpose The purpose of this paper is to investigate the mechanism and performance of a potential strategy, which is to enhance turbulence to carry out drag reduction for heavy trucks. Design/methodology/approach Enhancing turbulence deflector (ETD) was placed on the roof surface of an ground transportation system (GTS) to investigate the drag reduction mechanism of enhancing turbulence. Transition shear-stress transport improved delay detach eddy simulation model was adopted to simulate the unsteady small-scale flow around the ETD. Findings By optimizing the three influencing factors, diameter, streamwise length and streamwise position, the optimized ETD has achieved a maximum drag reduction of 7.04%. The analysis of flow field results shows that enhancing turbulence can effectively suppress flow separation and reduce the negative pressure intensity in the wake region of GTS. Originality/value The present work provides another potential possibility for the improvement of the aerodynamic performance of heavy trucks.

Extension of the KDO Turbulence/Transition Model to Account for Roughness

Wall roughness significantly influences both laminar-turbulent transition process and fully developed turbulence. This work has developed a wall roughness extension for the KDO turbulence/transition model. The roughness effect is introduced via the modification of the k and νt boundary conditions, i.e., the wall is considered to be raised at an extra height. The equivalent roughness height is linked to the actual roughness height, and the ratio between them is determined by reasoning. With such a roughness extension, the predictions of the KDO RANS model agree well with the measurements of turbulent boundary layer with a sand grain surface, while the KDO transition model yields accurate cross-flow transition predictions of flow past a 6:1 spheroid.

Symmetry breaking Paradigm In Typical Laminar-Turbulence Transition System

A stationary cylindrical vessel containing a rotating plate near the bottle surface is partially filled with liquid. With the bottom rotating, the shape of the liquid surface would become polygon-like. This polygon vortex phenomenon is a ideal system to demonstrate the Laminar-Turbulent transition process. Within the framework of equilibrium statistical mechanics, a profound comparison with Landau's phase transition theory was applied in symmetry breaking aspect to derive the evolution equation of this system phenomenologically. Comparison between theoretical prediction and experimental data is carried out. We concluded a considerably highly matched result, while some exceptions are viewed as the natural result that the experiment break through the up-limit of using equilibrium mechanics as a effective theory, namely breaking through the Arnold Tongue. Some extremely complex Non-equilibrium approaches was desired to solve this problem thoroughly in the future. So our method could be viewed as a linear approximation of this theoretical framework.

Quantitative analysis method for laminar-turbulence transition process of mechanical seal by a catastrophic model

Fluid flow regime has an important influence on the lubrication performance of mechanical seal. A novel method is proposed to analyze the flow regime transition quantitatively based on a cusp catastrophic model with dimensionless analysis. Furthermore, from this method, a turbulence transition factor is derived strictly to describe the turbulent transition process of mechanical seal, which is verified by the published results. Then this factor is applied to hydrodynamic lubrication mechanical seal to analyze the influence of flow regime on sealing performance. The results indicate that the performance parameters of mechanical seal are greatly influenced by the transition of fluid flow regime, and the stability of mechanical seal decreases in the transition to turbulent flow. The novel method of flow regime transition analysis provides a new insight into the turbulence model.