Guo Qian Chen

A numerical study of turbulent line puffs via the renormalization group (RNG) k–ϵ model

The time evolution of a line puff, a turbulent non-buoyant element with significant momentum, is studied using the renormalization group (RNG) k–ϵ model. The numerical results show that the puff motion is characterized by a vortex pair flow; the computed flow details and scalar mixing characteristics can be described by self-similar relations beyond a dimensionless time of around 30. The added mass coefficient of the puff motion is found to be approximately unity. The predicted puff flow and mixing rate are substantially similar to those obtained from the standard k–ϵ model and are well supported by experimental data. The computed scalar field reveals significant secondary concentration peaks trailing behind in the wake of the puff. The present results suggest that the overall mixing rate of a puff is primarily determined by the large-scale motion and that streamline curvature probably plays a minor role.

Turbulent lock release gravity current

The time evolution of a turbulent lock release gravity current, formed by a finite volume of homogeneous fluid released instantaneously into another fluid of slightly lower density, was studied by experimental measurements of the density structure via elaborate digital image processing and by a numerical simulation of the flow and mixing using a two-equation turbulence model. The essential fact that the gravity current passes through an initial slumping phase in which the current head advances steadily and a second self-similar phase in which the front velocity decreases like the negative third power of the time after release is satisfactorily presented by the laboratory observation. An overall entrainment ratio proportional to the distance from the release point is found by the numerical simulation. The renormalization group (RNG)k-ε model for Reynolds-stress closure is validated to characterize the gravity current with transitional and localized turbulence.

A formulation on large eddy simulation

Abstract This note presents a formulation on large eddy simulation with the idea motivated by Fourier series representation. A set of filtered equations are obtained.

Turbulent gravity current of lock release type: A numerical study

Abstract The time evolution of a turbulent gravity current of lock release type, formed by a finite volume of homogeneous fluid released instantaneously into another fluid of slightly lower density, is studied numerically via the renormalization group (RNG) κ-ϵ model for Reynolds-stress closure to characterize the flow with transitional and highly localized turbulence. Consistent with previous experimental observations, the numerical results show that the gravity current passes through two distinct phases, an initial slumping phase in which the current head advances steadily, and a second self-similar phase in which the front velocity decreases like the negative third power of the time after release. An overall entrainment ratio proportional to the distance from the release point is found and compares well with available experimental data for the slumping phase.

Advected turbulent line thermal driven by concentration difference

Abstract The spatial evolution of an advected line thermal driven by concentration difference––a turbulent buoyant body of fluid, for which small density difference is caused by a proportional variation in scalar concentration, horizontally introduced at no excess momentum into a horizontal ambient current, is studied using the standard two-equation k – e model with a buoyancy expansion. The numerical results show that the advected line thermal is characterized longitudinally by a flat trajectory with scalar dilution taking place essentially near the jet exit, and transversely by a vortex-pair flow and a kidney-shaped concentration structure with double peak maxima corresponding to stronger buoyancy effect; the computed flow details and scalar mixing characteristics can be described by self-similar relations beyond a dimensionless distance of around 10; the aspect ratio for the kidney-shaped sectional thermal is found to be around 1.2–1.4; the predicted flow feature and mixing rate are well supported by asymptotic dimensional analysis and related experimental data. The analogy between a steady advected line thermal and corresponding time-dependent line thermal is also found reasonable by a special exploration into the horizontal velocity distribution and the significance of horizontal diffusion effect of the advected line thermal. Only about half of the vertical momentum resulting from the buoyancy effect is found contained in the advected line thermal, corresponding to an added virtual mass coefficient of approximately 1 for the sectional thermal.

Numerical simulation of line puff Via RNG ϰ - ϵ model☆

Abstract The time evolution of a line puff is studied using the renormalization group (RNG) κ - ϵ model. The predicted puff flow and mixing rate are substantially similar to those obtained from the standard k -ϵ model, and are well-supported by experimental data. The computed scalar field reveals significant secondary concentration peaks trailing behind in the wake of the puff. The present results suggest that the overall mixing rate of a puff is primarily determined by the large scale motion, and that streamline curvature probably plays a minor role.

Two-dimensional line thermal

Abstract The time evolution of a two-dimensional line thermal — a turbulent flow produced by an initial element with significant buoyancy released in a large water body, is numerically studied using the two-equation k-e model for turbulence closure.

Asymptotic similarity of axisymmetric thermal

Abstract Self-similarity relations governing axisymmetrical thermals are established via an asymptotic dimensional analysis and a computational fluid dynamics study based on the two-equation κ - e model for turbulence closure.