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# Response and Recovery of Turbulent Pipeflow Past Wall Changes Targeting Distinct Azimuthal Fourier Modes

• Author / Creator
Masoumifar, Mehran
• This study focuses on numerical evaluation of the response and recovery of turbulent pipe flow to three-dimensional perturbed wall changes at a range of Reynolds numbers. These perturbations (pipe inserts) were designed based on distinct azimuthal Fourier modes corresponding to $m=3$ (Case I), $m=15$ (Case II), and their superposition at $m\in 3+15$ (Case III). A thorough validation and verification analysis enabled creating a benchmark study in simulating perturbed wall-bounded turbulent flows using turbulence models. The long-lasting response of the flow and the recovery was examined by characterizing both the mean and turbulent fields in the wake of pipe inserts for each Reynolds number.
Although a fast turbulence decay was observed immediately past the perturbation for all cases at higher Reynolds numbers, Case I showed a faster overall recovery compared to the other two cases with higher Fourier Modes. The flow response for Case I depicted a monotonic response, while the other two cases presented a non-monotonic second-order response characteristic along with a delayed recovery trend. Dominant flow structures were further identified in the downstream wake of each wall shape, which enabled a qualitative description of potential mechanisms behind the observed recovery trends. This study was then extended to the implications of Reynolds number. Increasing the Reynolds number was identified to prolong the flow recovery until it approached an asymptotic range at $Re\geq 7.5\times10^4$. The recovery trend was scaled with $Re^4$ for both the mean velocity and turbulence kinetic energy. In addition, the mean velocity along the wake centerline showed two peaks, where the location of peaks followed a power-law trend in the form of $L_{p}/D\propto Re^{4/3}$. The long-lasting flow response impeded the return to the fully-relaxed state at a distance of $20D$, even for the lowest Reynolds number considered here. Overall, the recovery exhibited a second-order response. This study provides a thorough characterization of targeted turbulent pipe flow response and recovery with applications in flow manipulation in pipelines for reduced drag, improved efficiency, lower rates of erosion and corrosion, as well as lower greenhouse gas emission for energy transportation.

• Subjects / Keywords
Fall 2021
• Type of Item
Thesis
• Degree
Master of Science
• DOI
https://doi.org/10.7939/r3-v3qx-d449