Session: 10.3.2 - Turbulent Flows II

Paper Number: 157806

157806 - Impact of Smooth Perturbations on Turbulent Pipe Flow Manipulation Using Targeted Wall Shapes 

Abstract: 

Turbulent pipeflow disturbances caused by abrupt changes, such as an orifice or roughness element, introduce large scale features in the flow that initiate complex flow response and recovery. Smooth perturbations, however, lack flow separation and recirculatory dynamics associated with abrupt disturbances. Thus, they induce more subtle manipulations that decelerates the flow at the wall and generates local mixing, potentially lowering skin friction drag and heat losses to the wall. The current study explores how the size and length of smooth perturbations impact the flow response and recovery dynamics. This research motivates the development of a new technology for improving the efficiency of geothermal energy extraction. By implementing controlled smooth perturbations into geothermal well casing, in the form of targeted wall-shapes, we can alter wall-bounded turbulence and achieve lower drag.

Flow dynamics are explored numerically at Reynolds number of 25,000 and 40,000, representative of geothermal systems, using various techniques, including Reynolds-Averaged Navier-Stokes (RANS) k- SST and k- models, Large-Eddy-Simulations (LES) and Improved-Delayed-Detached-Eddy-Simulations (IDDES). Initial modeling of the mean and turbulent flow characteristics across various pipe-insert geometries were completed using k- SST and k- models. Perturbation amplitude (0.05, 0.1, and 0.15) and length (2D, 4D, and 6D, where D is the pipe diameter) were systematically varied to create a thorough study of how perturbation length and size impact the bulk flow response and recovery. Preliminary RANS results revealed a direct relationship between pipe insert length and bulk flow recovery length. Perturbations generated a secondary peak in the axial turbulent kinetic energy profile, with the secondary peak shifting further downstream and increasing in magnitude with perturbation length (2D to 6D). Similarly, radial profiles of mean axial velocity gradient revealed the longest pipe insert (6D) experienced the longest flow recovery length, associated with a delay in the flow characteristics returning to their fully developed state. These findings suggest that shorter inserts introduce a more abrupt and localized disturbance to the flow, while longer perturbations sustain turbulence by targeting specific energy modes (m=15) over an extended length, prolonging the recovery process. This is also demonstrated with the longest insert (6D) experiencing the high concentration of axial vorticity near the wall. This suggests significant disturbances of coherent structures, contributing to a prolonged recovery to its fully developed state. The pipe-inserts induce peaks in turbulent kinetic energy and mean axial velocity, corresponding to a deceleration near the wall that leads to increased flow rates and reduced skin friction drag.

Three configurations with most prominent flow dynamics are further studied using LES and IDDES. The high-resolution results provide us with deeper insights into the mechanisms associated with the flow response and recovery due to smooth perturbations or wall-shapes. Preliminary LES results revealed that resolved radial and azimuthal flow structures are formed near the center of the pipe, which were not captured by the RANS models. To better understand the flow dynamics, proper orthogonal decomposition (POD) analysis examined the evolution of boundary layer structures generated by targeting specific Fourier modes (m = 15). These studies revealed the fundamentals of how we can design an optimized pipe systems (e.g., well casing) for increasing the efficiency of energy systems. To this end, future studies will include investigations of fluid-structure interactions and thermal effects for the proposed pipe-inserts.

Presenting Author: Yaren Dincoglu University of Alberta

Presenting Author Biography: