Turbulent flows over permissive boundaries and porous walls
Time: Fri 2024-04-12 09.00
Location: F3 (Flodis), Lindstedtsvägen 26 & 28, Stockholm
Language: English
Subject area: Engineering Mechanics
Doctoral student: Seyed Morteza Habibi Khorasani , Strömningsmekanik och Teknisk Akustik, Linné Flow Center, FLOW
Opponent: Professor Alfredo Soldati, Technische Universität Wien
Supervisor: Shervin Bagheri, Linné Flow Center, FLOW, SeRC - Swedish e-Science Research Centre, Strömningsmekanik och Teknisk Akustik; Geert Brethouwer, Linné Flow Center, FLOW, SeRC - Swedish e-Science Research Centre, Teknisk mekanik
QC 240327
Abstract
This thesis investigates how in wall-bounded turbulent flows changes to the wall can induce changes in the flow. To this end, we investigate the use of wall boundary conditions meant to mimic the effect of textured surfaces. We also study the effect of porous walls on their overlying bulk turbulent flow and the consequences of these effects for heat transfer.
Textured surfaces can alter near-wall turbulence in subtle to dramatic ways. Vanishingly small surface textures of the order of the viscous sublayer thickness cause the displacement of the near-wall turbulence-generating flow structures by either restricting or permitting transpiration from taking place close to the surface. The former leads to drag reduction and the latter to its increase. As the textures increase in size and become comparable to the turbulence scales, they alter the near-wall dynamics and cause structural changes to occur in the flow. These effects are emulated using slip and transpiration boundary conditions called the Transpiration-Resistance model (TRM). Its utility in acting as an effective model for wall roughness is assessed. It captures the effect of vanishingly small roughness well, and to a limited extent larger roughness which protrude into the buffer layer. The TRM also helps to shed light on which near-wall structures play an essential role in the near-wall cycle of turbulence.
Porous walls permit the exchange of mass, momentum and energy with the overlying turbulence. Their permeable structure quickly causes the turbulence to depart from its canonical smooth-wall-like structure, and induce a Kelvin-Helmholtz-like instability which leads to the emergence of spanwise rollers. These rollers efficiently redistribute momentum and turbulent kinetic energy into the porous wall. This pronounced interaction between the wall and bulk flow regions is detrimental for drag but beneficial for the transport of heat. The potential of porous walls for enhancing heat transfer exceeds that of other passive wall structures such as roughness.
As an elementary study of the type of fluid-fluid-solid interactions which can take place on the microscale within porous media, the stability of a cylinder-wrapping corner film is investigated. It is shown that linear stability analysis (LSA) can predict the number of primary droplets when the film breaks up. The film morphology, however, exhibits complexities which cannot be predicted using LSA. A disjoining-pressure model (DPM) demonstrates that smaller secondary droplets may emerge during the film breakup process. Additionally, Volume of Fluid (VoF) simulations show that two initially emerging primary droplets may eventually coalesce into one, highlighting the non-linear mechanisms involved in the film morphology evolution.