Near field buoyant plume modelling with ambient flow: Cross comparison of experiments, CFD and CorJet
Translated title
Modellering af udledningsfaner med baggrundsstrømning: Sammenligning af forsøg, CFD og CorJet
Author
Dyhr, Aske Dahl
Term
4. term
Publication year
2026
Submitted on
2026-06-04
Abstract
This thesis examines how a buoyant discharge behaves close to its source (the so‑called near field) when released into moving water. The focus is on three aspects of the resulting plume: how much it is diluted, how far it is transported downstream, and how stable the flow is. Three approaches are compared: laboratory experiments, computer simulations using Computational Fluid Dynamics (CFD), and the widely used integral model CorJet, which is designed to describe such discharge plumes. The aim is to assess how well these methods capture plume behavior under controlled conditions as the background current is gradually increased. The data set is based on basin experiments at three different flow levels. In these experiments, the centerline position of the plume and its dilution were measured at fixed sampling points. The same geometry and boundary conditions were then recreated in a CFD model, and the results were compared with CorJet calculations and selected analytical model estimates for the corresponding scenarios. The results show that all three approaches reproduce the same overall trend: increasing background flow pushes the plume further downstream and leads to higher dilution. The main differences lie in the absolute values of dilution and displacement. CorJet overestimates dilution in all three cases, and this deviation grows with increasing flow. In contrast, CFD underestimates dilution compared with the experiments, with the largest difference at the highest flow level. CorJet also predicts a shorter downstream displacement than both the experiments and the CFD simulations. Overall, the thesis indicates that CorJet, although widely used in practice, may systematically overestimate dilution for this type of configuration, and that this should be investigated further. Conversely, CFD models may underestimate dilution if their setup and model settings are not carefully configured and calibrated.
Dette speciale undersøger, hvordan en opadstigende udledning (en opdriftsdomineret udsivning) opfører sig i et bassin med strømmende vand tæt ved udledningspunktet (det såkaldte nærfelt). Fokus er på tre ting: hvor meget udledningen fortyndes, hvor langt den flyttes nedstrøms, og hvor stabil strømningen er. Der sammenlignes tre metoder: laboratorieforsøg, computersimuleringer (CFD – Computational Fluid Dynamics) og et udbredt beregningsværktøj, CorJet, som beskriver udledningsfaner ved hjælp af en såkaldt integreret model. Målet er at vurdere, hvor godt de tre metoder kan beskrive udledningsfanens adfærd, når baggrundsstrømmen i vandet gradvist øges under kontrollerede forhold. Datagrundlaget er bassinforsøg ved tre forskellige strømningsniveauer. Her blev udledningens centerlinje og fortynding målt i faste punkter i fanen. Den samme geometri og de samme randbetingelser blev derefter genskabt i CFD-modellen, og resultaterne blev sammenlignet med både CorJet-beregninger og udvalgte analytiske modelestimater for de samme situationer. Resultaterne viser, at alle tre tilgange giver samme overordnede billede: Når baggrundsstrømmen øges, flyttes fanen længere nedstrøms, og fortyndingen stiger. Forskellene ligger i størrelsen af fortynding og nedstrøms forskydning. CorJet overvurderer fortyndingen i alle tre scenarier, og afvigelsen vokser med stigende strømning. CFD undervurderer derimod fortyndingen i forhold til laboratorieforsøgene, med den største forskel ved det højeste strømningsniveau. CorJet forudsiger også en kortere nedstrøms forskydning end både forsøg og CFD. Samlet peger specialet på, at CorJet – selv om værktøjet er meget udbredt – kan have en tendens til systematisk at overvurdere fortyndingen i denne type opsætning, og at dette bør undersøges nærmere. Omvendt kan CFD-modeller komme til at undervurdere fortyndingen, hvis opsætning og modelparametre ikke er omhyggeligt valgt og kalibreret.
[This abstract has been rewritten with the help of AI based on the project's original abstract]
Keywords