title: Intermittencies in transitional and turbulent channel flow
Creators: name: Agrawal, Rishav
Publisher: name: University of Liverpool Repository
Year: 2020

description: In recent years, events of intermittent low- and high-drag have been observed in turbulent channel flows near transition. During the low-drag events, the wall shear stress, on average, temporarily reduces to about 40% below its time-averaged value and the velocity profile approaches Virk's maximum drag reduction (MDR) asymptote. There are still open questions regarding this phenomenon, for example the characteristics of Reynolds shear stress (RSS) during the events and whether they continue to exist at higher Reynolds numbers (i.e. in the so-called ``fully-turbulent" flow regime). Investigating intermittencies in turbulent flow close to transition requires knowledge of intermittencies during the laminar-turbulent transition process. However, there is currently no time-resolved experimental wall shear stress data available in the transitional flow regime for channel flow. To study the laminar-turbulent intermittency, instantaneous wall shear stress (using hot-film anemometry, HFA) is probed at the transitional Reynolds numbers. Higher order statistics show that with increasing Reynolds numbers (from the laminar flow regime), the laminar-turbulent intermittency firstly grows, then diminishes, and eventually disappears by Reτ ≈ 72, where Reτ =uτ h/ν and uτ, h and ν indicate the friction velocity, channel half-height and kinematic viscosity, respectively. Using multiple hot-film probes, information about the large-scale turbulent structures during transition is inferred. Additionally, a flow visualization has been conducted to observe the large-scale structures, which qualitatively supports the wall shear stress results. Beyond transition, simultaneous measurements of wall shear stress and velocity (using laser Doppler velocimetry, LDV) have been conducted to detect and characterize the low- and high-drag intermittencies for Reτ = 70 - 250. The fraction of time spent in these intermittent events is observed to be independent of Reynolds number when the criteria for minimum time duration is kept constant in ``inner" units. The previously observed spike in the ensemble-averaged wall shear stress before and after the low-drag events is found to be an artefact of the conditional sampling and ensemble-averaging process and is not a physical phenomenon. Conditionally-averaged streamwise velocity profiles get closer to Virk's MDR asymptote, near the wall, for all the Reynolds numbers studied. Simultaneous wall shear stress and RSS measurements are carried out for Reτ = 70 and 85. An increase in the conditionally-averaged RSS is observed during the low-drag events for all the wall-normal locations measured, but is particularly apparent for y+ ≈ 20 - 40, where y+ = y uτ/ ν and y is the wall-normal distance. When using HFA for long-time wall shear stress measurements, minimizing the non-thermal calibration drift is a significant challenge. A new method to minimize recalibration in thermal anemometry using a non-linear regression technique is proposed and investigated. This new method was utilized in the current work and finds potential applications in cases of correcting for non-thermal calibration drifts in long time measurements and also in scenarios where the direct calibration of hot-films is not possible or suffers significant uncertainties.