NUMERICAL INVESTIGATION OF TURBULENT-DRIVEN SECONDARY FLOW

Open Access
Author:
Talebpour, Mahdad
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 08, 2016
Committee Members:
  • Xiaofeng Liu, Thesis Advisor/Co-Advisor
Keywords:
  • secondary flow
  • k-omega
  • cfd
  • roughness
Abstract:
Secondary flows of second type (also known as turbulent secondary flows) are one of the most important mechanisms responsible for sediment transport processes in fluvial streams. A new two-equation Reynolds-Averaged Navier-Stokes (RANS) based model is investigated in depth in this work, for modelling secondary flows of second type. This thesis incorporates a new k−omega model with nonlinear fourth-order closure terms for modelling Reynolds stresses. The model has k−omega formulation which enables the model to solve for flow equations near the walls without the need for utilizing wall functions. Moreover, k−omega models are capable of applying roughness on boundaries by imposing rough boundary turbulent attributes through omega functions. The model was tested in two main categories on five case-studies to observe its ability in simulating turbulent secondary flow in various configurations. The tests carried out case scenarios identical to experimental case studies. First, the model was used in simulating turbulent secondary current in a simple case study conducted inside a rectangular duct with smooth boundaries. The model performed well in this part, when simulated data such as secondary velocity profiles, shear stresses, and secondary current vectors being compared with experimental data. In the second test, the model was investigated in case scenarios to explore the model’s capacity for carrying out simulation of turbulent secondary currents over rough boundaries. While in case studies with uniform rough boundaries the model functioned well (velocity profiles, and shear stress distribution, being compared to experimental data), in cases with non-uniform roughness distribution the model needed noticeable tweaking, tuning, and calibration for roughness modelling. However, for validation purposes, the model with calibrated function parameter were tested against other non-uniform distribution case scenarios, in which their results showed excellent agreement with experimental data. The numerical simulations were able to produce secondary velocity profiles very close to experimental studies which are difficult to capture. The proposed roughness height values in roughness modelling function for secondary flows over beds, with nonuniformly distributed roughness, are critically discussed and assessed. During the tuning of the model, it was detected that the original approach for computing wall shear stresses, using law of the wall, provided dissimilar results compared to results which calculated shear stresses directly from velocity gradients at the wall. This disagreement was investigated in depth, and it was concluded that the calculation of wall shear stress, using law of the wall, was not accurate. Finally, providing accurate result along with computational efficiency (due to RANS-based formulation), and applicability to rough case scenarios, this model is advantageous in investigation of turbulent secondary current.