A Multi-Resolution Direct Numerical Simulation of Turbulent Stratified Flows with High Prandtl Numbers

The turbulent structure of stratified open-channel flow subjected to a radiative volumetric heat source modelled by the Beer–Lambert law, for Prandtl numbers (𝑃𝑟) ranging 0.07 to 7, is investigated using direct numerical simulation (DNS). Most prior DNS studies of stratified turbulent open-channel flow have been limited to 𝑃𝑟 ≤ 1 due to computational constraints. To overcome the increased computational resources required to resolve the thermal fields when 𝑃𝑟 > 1, a multi-resolution method is developed to resolve the temperature and momentum fields with different spatio-temporal resolutions. Stable thermal stratification affects turbulent flows in oceans, rivers, lakes, and the atmosphere, limiting vertical mixing and causing ecological harm. Destratification, triggered by the removal of radiative heat, is vital for restoring ecological balance in rivers, as prolonged stratification can deplete oxygen and harm aquatic habitats. The effects of 𝑃𝑟 on stratification and destratification processes in open-channel flows are examined using the mean flow and one-point second-order turbulent statistics. The spatial organisation of turbulence structures is analysed utilising two-point statistics and spanwise energy spectra. Unlike low-𝑃𝑟 cases with the same bulk stability parameter, the flow with 𝑃𝑟 = 7 demonstrates reduced turbulent activity and exhibits near-laminar behaviour. The increased velocity and temperature, along with the laminarisation for 𝑃𝑟 = 7, are confined to the free surface, resulting in a thinner and more distinct near-laminar layer than for 𝑃𝑟 ≤ 1. Higher buoyancy sensitivity is observed at large spanwise wavelengths, as stratification first reduces energy in large spanwise scales at all the wall-normal locations. Although stratification has negligible effect on the mean velocity near the bottom no-slip wall, it alters the larger spanwise wavelength (𝜆+ > 500) structures in the buffer layer, which are larger than the buffer layer streaks. It is demonstrated that the spanwise length scales far from the wall are suppressed with an increase in bulk stability parameter and 𝑃𝑟. When the heat source is removed, the flow with 𝑃𝑟 = 7 destratifies much slower than 𝑃𝑟 = 0.71 for the same stratification level. The destratification process occurs in two distinct phases: an initial phase, which occurred slower for the higher 𝑃𝑟 values, followed by a second phase where all cases exhibit a similar trend.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2025

Author dc:creator

• Jahani, Bahare

Advisors dc:contributor.advisor

• Norris, Stuart
• MacDonald, Michael

• Direct Numerical Simulation
• CFD
• Open-channel flow
• High Prandtl fluids
• Stratification
• Turbulent flows
• Multi-resolution DNS

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/74136

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/74136