Large Eddy Simulation On Vortex Shedding For A Helical-Twisted Cylinder Subjected To Crossflow
Author
Mohr Peraza, Sergio
Term
4. semester
Education
Publication year
2020
Submitted on
2020-06-15
Pages
69
Abstract
Helically twisted (spiral‑grooved) cylinders are used in safety‑related applications because their not‑perfectly round cross‑section can improve grip and disturb the flow around the surface. This thesis builds a Large Eddy Simulation (LES) turbulence model to predict and compare how such profiles shed vortices—the repeating swirls that form behind a body in a flow and can cause vibrations. A circular cylinder is used as a numerical baseline and is compared with helical‑twisted profiles that vary in groove height (e) and pitch (p). These dimensions are reported as ratios to the cylinder diameter D (e/D and p/D). Six cylinder designs are exposed to an air crossflow at a Reynolds number of 3.7×10^4, and aerodynamic force coefficients (lift and drag) are recorded to evaluate how vortex shedding is reduced. The profile with p/D = 1 and e/D = 0.05 lowers the vortex‑shedding frequency by 16% relative to the smooth circular cylinder. This reduction is linked to damping of fluctuating lift forces and weakening of their spanwise correlation (i.e., vortices are less synchronized along the cylinder), which makes the near‑wall wake more three‑dimensional and less coherent. In contrast, other helical‑twisted configurations are less effective, and deeper or larger cavities can aggravate vortex shedding. Even so, reduced lift‑amplitude fluctuations are observed for p/D = 1, e/D = 0.2; p/D = 2, e/D = 0.1; and p/D = 0.5, e/D = 0.1. Overall, the results identify a specific helical‑groove geometry that best suppresses vortex‑induced vibrations under the tested conditions, while warning that overly deep grooves may have the opposite effect.
Spiralvridne cylindre med riller bruges i sikkerhedsrelaterede sammenhænge, fordi deres ikke‑helt runde tværsnit kan give bedre greb og forstyrre strømningen langs overfladen. Denne afhandling opbygger en LES‑turbulensmodel (Large Eddy Simulation) for at forudsige og sammenligne, hvordan sådanne profiler danner hvirvelafløsning—de gentagne hvirvler, der opstår bag en krop i en strømning og kan fremkalde vibrationer. En cirkulær cylinder fungerer som numerisk reference og sammenlignes med spiralvridne profiler med varierende rilledybde (e) og stigning (p). Disse mål angives som forhold til cylinderens diameter D (e/D og p/D). Seks cylinderdesign udsættes for en tværgående luftstrøm ved et Reynolds‑tal på 3,7×10^4, og aerodynamiske kraftkoefficienter (løft og modstand) registreres for at vurdere reduktionen af hvirvelafløsning. Profilen med p/D = 1 og e/D = 0,05 sænker hvirvelafløsningsfrekvensen med 16% i forhold til den glatte, cirkulære cylinder. Reduktionen hænger sammen med dæmpning af udsving i løftekraften og en svækkelse af deres korrelation langs cylinderens længde, hvilket gør efterstrømmen nær overfladen mere tredimensionel og mindre sammenhængende. Andre spiralvridne konfigurationer er derimod mindre effektive, og dybere eller større hulrum kan forværre hvirvelafløsningen. Ikke desto mindre ses dæmpning af løft‑amplitudeudsving for p/D = 1, e/D = 0,2; p/D = 2, e/D = 0,1; og p/D = 0,5, e/D = 0,1. Samlet peger resultaterne på en bestemt rillegeometri, der bedst dæmper hvirvelinducerede vibrationer under de testede betingelser, samtidig med at for dybe riller kan have den modsatte effekt.
[This apstract has been rewritten with the help of AI based on the project's original abstract]
Keywords