4 research outputs found
A sűrűsĂ©ghajtotta Ăłceáni vĂzkörzĂ©s Ă©s mĂ©lysĂ©gi konvekciĂł numerikus Ă©s laboratĂłriumi modellezĂ©se
BolygĂłnk hĹ‘transzportjának körĂĽlbelĂĽl felĂ©t az Ăłceánok vĂ©gzik. A VilágĂłceán kĂĽlönbözĹ‘ medencĂ©inek Ă©s rĂ©tegeinek vĂzcserĂ©jĂ©t az Ăşgynevezett nagy Ăłceáni szállĂtĂłszalag (Great Ocean Conveyor – GOC) biztosĂtja, mely összekapcsolja a felszĂni, jellemzĹ‘en meridionális hĹ‘fluxuskĂĽlönbsĂ©g-hajtotta áramlásokat az ellenkezĹ‘ irányĂş aljzati visszaáramlással.
A GOC hajtĂłmotorja a lesĂĽllyedĂ©s. A felszĂni Ă©s a mĂ©lyĂłceán között csak akkor alakulhat ki vĂzcsere (Deep Water Formation, DWF), ha az Ăłceáni medencĂ©nek van olyan terĂĽlete, ahol a felszĂni vĂznek lehetĹ‘sĂ©ge van nagyobb sűrűsĂ©get elĂ©rni, mint a legalul lĂ©vĹ‘ vĂztömegĂ©. Ez a helyzet csak magas földrajzi szĂ©lessĂ©geken állhat elĹ‘, Ă©s csak az Atlanti-Ăłceán Ă©szaki Ă©s dĂ©li vĂ©gĂ©n valĂłsul meg tĂ©nylegesen.
A dolgozatban bemutatott kutatás során azt a kĂ©rdĂ©st tettĂĽk fel, hogy ezeken a pontokon milyen felszĂni hĹ‘fluxus-peremfeltĂ©telek mellett sĂĽllyedhet le az aljzatig az Ăłceáni vĂztömeg. Továbbá prĂłbára tettĂĽnk egy Ă©rdekes, Ăşjszerű feltevĂ©st. Azon Ă©szrevĂ©telĂĽnk alapján, hogy a lesĂĽllyedĂ©si pontok olyan terĂĽleteken helyezkednek el, melyek alatt az aljzati hőáram magasabb Ă©rtĂ©kei fedezhetĹ‘k fel (hot spotok, szubdukciĂłs zĂłnák mögött kialakulĂł vulkáni tevĂ©kenysĂ©g), megvizsgáltuk, hogy ez mennyiben befolyásolhatja a mĂ©lysĂ©gi átkeveredĂ©st.
Numerikus szimuláciĂłk segĂtsĂ©gĂ©vel a lesĂĽllyedĂ©si pontok alapvetĹ‘ fizikáját modelleztĂĽk. Ennek során egy kádat vizsgáltunk, melynek felszĂnĂ©n Ă©s oldalfalain kĂĽlönbözĹ‘ hĹ‘mĂ©rsĂ©klet-eloszlásokhoz relaxáltattuk a rendszert. SikerĂĽlt igazolnunk, hogy a hideg Ă©s meleg oldal (a termĂ©szeti analĂłgiában a pĂłlus Ă©s az EgyenlĂtĹ‘) közötti hĹ‘mĂ©rsĂ©kletkĂĽlönbsĂ©gre a rendszer rendĂvĂĽl Ă©rzĂ©keny. Ezek után aljzati hĹ‘forrás mĂłdosĂtĂł hatásait vettĂĽk szemĂĽgyre. Megmutattuk, hogy a magasabb hĹ‘mĂ©rsĂ©klettel rendelkezĹ‘ aljzati pontok fölött lĂ©trejövĹ‘ turbulens keverĂ©s segĂtheti a vĂz teljes lesĂĽllyedĂ©sĂ©t, vagyis már kisebb meridionális hĹ‘mĂ©rsĂ©kletkĂĽlönbsĂ©g mellett lĂ©trejöhet a teljes mĂ©lysĂ©gi átkeveredĂ©s.
KövetkeztetĂ©seink alapján laboratĂłriumi kĂsĂ©rletekben megfigyeltĂĽk a kĂ©t konvekciĂłs állapotot, ezzel kvalitatĂve ellenĹ‘rizve a numerikus modell egyes eredmĂ©nyeit
The effect of a localized geothermal heat source on deep water formation.
In a simplified two-dimensional model of a
buoyancy-driven overturning circulation, we numerically
study the response of the flow to a small localized heat source
at the bottom. The flow is driven by differential thermal
forcing
applied along the top surface boundary. We evaluate the
steady state solutions versus the temperature difference between
the two ends of the water surface in terms of different
characteristic parameters that properly describe the transition
from a weak upper-layer convection state to a robust full depth
deep convection. We conclude that a small additional
bottom heat flux underneath the “cold” end of the basin is
able to initiate full-depth convection even when the surface
heat forcing alone is not sufficient to maintain this state
An experimental study of the Atlantic variability on interdecadal timescales
A series of laboratory experiments has been carried out to model the basic dynamics of the multidecadal variability observed in North Atlantic sea surface temperature (SST) records. According to the minimal numerical sector model introduced by te Raa and Dijkstra (2002), the three key components to excite such a low-frequency variability are rotation, meridional temperature gradient and additive thermal noise in the surface heat forcing. If these components are present, periodic perturbations of the overturning background flow are excited, leading to thermal Rossby mode like propagation of anomalous patches in the SST field. Our tabletop scale setup was built to capture this phenomenon, and to test whether the aforementioned three components are indeed sufficient to generate a low-frequency variability in the system. The results are compared to those of the numerical models, as well as to oceanic SST reanalysis records. To the best of our knowledge, the experiment described here is the very first to investigate the dynamics of the North Atlantic multidecadal variability in a laboratory-scale setup
Post-test examinations on Zr-1%Nb claddings after ballooning and burst, high-temperature oxidation and secondary hydriding
The objective of the present study was to provide further data on E110G cladding behaviour. The results presented here are from new post-test examinations (PTEs) carried out on samples of secondary hydriding experiments conducted earlier in MTA EK.
The as-received Zr-1%Nb cladding samples were pressurised at high temperature to balloon and burst and then oxidised in steam atmosphere. The post-test investigation was focusing on geometric change in the cladding, ductility, oxidation and hydrogen absorption.
Outer and inner oxide layers were formed on the samples, with increasing thickness near the thermal centre. The results include radial and axial distribution of oxygen in the cladding after oxidation. The hydrogen uptake of the alloy shows the expected characteristic axial distribution. Mechanical testing of the oxidised and non-oxidised samples confirmed the results of the previous mechanical tests that after ballooning the samples still had notable flexural strength, whereas after oxidation this decreased.
The results were evaluated against those obtained through simulations, making it possible to estimate the level of oxidation, and to develop better models through further simulation