Numerical Simulation of the Disturbed Flow Through a Three-Dimensional Building Array
Numerical Simulation of the Disturbed Flow Through a Three-Dimensional Building Array

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Published: 2004

Total Pages: 58

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A study of the neutrally-stratified flow within and over an array of three-dimensional (3-D) buildings (cubes) was undertaken using simple Reynolds-averaged Navier-Stokes (RANS) flow models. These models consist of a general solution of the ensemble-averaged, steady-state, three-dimensional Navier-Stokes equations, where the k-E turbulence model (k is turbulence kinetic energy and E is viscous dissipation rate) has been used to close the system of equations. Two turbulence closure models were tested; namely, the standard and Kato-Launder k-E models. The latter model is a modified k-E model designed specifically to overcome the stagnation point anomaly in flows past a bluff body where the standard k-E model overpredicts the production of turbulence kinetic energy near the stagnation point. Results of a detailed comparison between a wind tunnel experiment and the RANS flow model predictions are presented. More specifically, vertical profiles of the predicted mean streamwise velocity, mean vertical velocity, and turbulence kinetic energy at a number of streamwise locations that extend from the impingement zone upstream of the array, through the array interior, to the exit region downstream of the array are presented and compared to those measured in the wind tunnel experiment. Generally, the numerical predictions show good agreement for the mean flow velocities. The turbulence kinetic energy was underestimated by the two different closure models. After validation, the results of the high-resolution RANS flow model predictions were used to diagnose the dispersive stress, within and above the building array. The importance of dispersive stresses, which arise from point-to-point variations in the mean flow field, relative to the spatially-averaged Reynolds stresses are assessed for the building array.

Numerical Simulation of Flows Near and Through a Two-Dimensional Array of Buildings
Numerical Simulation of Flows Near and Through a Two-Dimensional Array of Buildings

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Published: 2001

Total Pages: 71

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A numerical simulation has been performed of turbulent flow near and through a two-dimensional array of rectangular buildings (bluff bodies) in a neutrally stratified background flow. The model used for the simulation was the steady-state Reynolds-averaged Navier-Stokes equations with an isotropic, linear eddy viscosity formulation for the Reynolds stresses; and, the eddy viscosity was determined using a high-Reynolds number form of the kappa-epsilon turbulence-closure model with the boundary conditions at the wall obtained with the wall-function approach. The resulting system of partial differential equations along with the concomitant boundary conditions for the problem was solved using the SIMPLE algorithm in conjunction with a non-orthogonal, collocated, cell-centered, finite volume procedure. The predictive capabilities of the numerical simulation of urban flow are validated against a very detailed and comprehensive wind tunnel data set. Vertical profiles of the mean streamwise and vertical velocities and the turbulence kinetic energy are presented and compared to those measured in the wind tunnel simulation. In addition to the comparison between modeled and measured results, the numerical simulation is used to aid in the interpretation of certain features of the flow observed in the wind tunnel simulation. it is found that the overall performance of the model is good--most of the qualitative features in the disturbed turbulent flow field in and near the building array are correctly reproduced, and the quantitative agreement is also generally fairly good (especially for the mean velocity field). The underestimation of the turbulence energy levels may arise from the inability of the model to capture the subtle effects of secondary strain, especially those associated with curvature, on the turbulence.

Numerical Study of Three-dimensional Flow Through a Deep Open Channel - Including a Wire-mesh Segment on One Side Wall
Numerical Study of Three-dimensional Flow Through a Deep Open Channel - Including a Wire-mesh Segment on One Side Wall

Author: Chandrima Jana

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Published: 2011

Total Pages: 85

ISBN-13:

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The goal of this study is to numerically investigate the surface shear stress distribution for a deep open channel of aspect ratio 10, including a wire mesh segment on one side wall. This work is motivated by the need to understand the length-based separation process occurring in a deep open channel in the Bauer-McNett Fiber Classifier (BMC), a length-based fiber separator. The shear stress distribution governs the fiber orientation, which in turn, governs the length-based separation process. The flow through the BMC open channel is turbulent, and the flow field is expected to be characterized by turbulence-driven secondary flows. In order to compute the flow variables, the Reynolds-Averaged Navier-Stokes (RANS) equations have been solved. To close the RANS equations, a second-order closure turbulence model known as the Reynolds stress model is employed. The RANS equations are solved using a well known iterative solution procedure known as the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. The commercial CFD solver ANSYS FLUENT 13.1 is capable of solving the RANS equations using the Reynolds stress model to close the system of equations. It is also equipped with the SIMPLE algorithm, and hence, is chosen to study the flow in the BMC open channel. For any numerical study, the numerical solution procedure must be verified. Due to the similarity in its physical configuration to the open channel in the BMC apparatus, the canonical problem of fully developed turbulent flow in an open channel of aspect ratio unity is chosen as the verification case. Simulation of the square open channel case enables the verification of the computational methodology before it is applied to the present problem. Next, the flow in the BMC open channel is studied considering the vertical side boundaries as solid walls. The flow in the BMC open channel is characterized by the presence of multiple secondary velocities. The secondary flows are weaker than the primary flow. The secondary flows near the free surface are responsible for the increase in the shear stress on the side wall of the open channel. The resultant shear stress vectors in the open channel are found to be oriented in the primary flow direction. Finally, the flow in the BMC open channel considering a wire mesh on a segment of the side wall is studied. The wire mesh is represented by a 3x3 aperture array. The size of the apertures and the wire diameter corresponds to mesh 16. The shear stress distribution on the apertures is significantly lower than that on the solid wires. Also, the shear stress vectors of the fluid occupying the apertures indicate that the shear stress on the fluid is oriented at an angle to the plane of the apertures. The fibers in the flow are expected to align with the shear stress. Since the resulting shear stress vectors on the fluid occupying the apertures are oriented at an angle to the plane of the apertures, it suggests that fibers greater than the opening size of the apertures may escape through them.