| 1. |
- Svensson, Anders, et al.
(författare)
-
Bipolar volcanic synchronization of abrupt climate change in Greenland and Antarctic ice cores during the last glacial period
- 2020
-
Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 16:4, s. 1565-1580
-
Tidskriftsartikel (refereegranskat)abstract
- The last glacial period is characterized by a number of millennial climate events that have been identified in both Greenland and Antarctic ice cores and that are abrupt in Greenland climate records. The mechanisms governing this climate variability remain a puzzle that requires a precise synchronization of ice cores from the two hemispheres to be resolved. Previously, Greenland and Antarctic ice cores have been synchronized primarily via their common records of gas concentrations or isotopes from the trapped air and via cosmogenic isotopes measured on the ice. In this work, we apply ice core volcanic proxies and annual layer counting to identify large volcanic eruptions that have left a signature in both Greenland and Antarctica. Generally, no tephra is associated with those eruptions in the ice cores, so the source of the eruptions cannot be identified. Instead, we identify and match sequences of volcanic eruptions with bipolar distribution of sulfate, i.e. unique patterns of volcanic events separated by the same number of years at the two poles. Using this approach, we pinpoint 82 large bipolar volcanic eruptions throughout the second half of the last glacial period (12-60ka). This improved ice core synchronization is applied to determine the bipolar phasing of abrupt climate change events at decadal-scale precision. In response to Greenland abrupt climatic transitions, we find a response in the Antarctic water isotope signals (δ18O and deuterium excess) that is both more immediate and more abrupt than that found with previous gas-based interpolar synchronizations, providing additional support for our volcanic framework. On average, the Antarctic bipolar seesaw climate response lags the midpoint of Greenland abrupt δ18O transitions by 122±24 years. The time difference between Antarctic signals in deuterium excess and δ18O, which likewise informs the time needed to propagate the signal as described by the theory of the bipolar seesaw but is less sensitive to synchronization errors, suggests an Antarctic δ18O lag behind Greenland of 152±37 years. These estimates are shorter than the 200 years suggested by earlier gas-based synchronizations. As before, we find variations in the timing and duration between the response at different sites and for different events suggesting an interaction of oceanic and atmospheric teleconnection patterns as well as internal climate variability.
|
|
| 2. |
- Vinther, Frank, et al.
(författare)
-
Modelling of optimal back-shock frequency in hollow-fibre ultrafiltration membranes II : Semi-analytical mathematical model
- 2016
-
Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388. ; 506, s. 137-143
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of cross-flow velocity and transmembrane pressure on the optimal back-shock frequency and normalized net flux during back-shocking has been studied using a semi-analytical mathematical model. The model uses the flux as a function of time without back-shocking together with knowledge of the streamlines and pathlines during the back-shock cycle to predict the optimal normalized net flux as a function of forward filtration time. The model was used to investigate three different transmembrane pressures and three different cross-flow velocities during a back-shock cycle. The net flux was found to increase under all operating conditions when using back-shocking. The greatest increase in normalized net flux was found at the highest cross-flow velocity and the highest transmembrane pressure, and corresponds to an increase of 37% compared to the steady-state flux. The highest cross-flow velocity and the highest transmembrane pressure gave the highest optimal back-shock frequency of 0.21 Hz. The optimal back-shock frequency was found to decrease with increasing pressure and decreasing cross-flow velocity.The model is easy to use in different applications as it is easy to measure flux during forward filtration without back-shocking. Good agreement was found between the semi-analytical model and a model based on computer fluid dynamics in predicting both the value of the optimal normalized net flux and the optimal back-shock frequency.
|
|
| 3. |
- Vinther, Frank, et al.
(författare)
-
Predicting optimal back-shock times in ultrafiltration hollow fiber modules II: Effect of inlet flow and concentration dependent viscosity
- 2015
-
Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388. ; 493, s. 486-495
-
Tidskriftsartikel (refereegranskat)abstract
- This paper concerns mathematical modeling and computational fluid dynamics of back-shocking during hollow fibre ultrafiltration of dextran T500. In this paper we present a mathematical model based on first Principles, i.e., solving the Navier-Stokes equation along with the continuity equation for both the solute and the solvent. We investigate the validity of the estimate On the optimal back-shock time, i.e., the back-shock time needed to achieve the highest permeate flux, published in a previous paper by the authors (Vinther et al., Predicting optimal back-shock times in ultrafiltration hollow fibre membranes, J. Membr. Sci. 470 (2014) 275-293 [33]). Furthermore, the simulations have been performed with two different inlet velocities, i.e., crossflow velocities and are clone with and without a concentration dependent viscosity. This enables us, for the first time, to investigate the effect of different inlet velocities and the effect of a concentration polarization on the observed rejection and the permeate flux, as a function of different back-shock times. In all cases the average permeate flux and the observed rejection during one period of back-shocking were found to be higher than the steady-state values - representing the long time behavior of a similar separation process performed without back-shocking - when using the optimal back-shock time. It is concluded that the estimate of the optimal back-shock time is in good agreement with the optimal time found in the simulations performed in this paper. Furthermore, it is found that the optimal back-shock time increases when the viscosity is allowed to depend on the concentration It is found that this can be explained by a decrease in the velocity tangential to the membrane due to the increase in viscosity where the concentration is high - resulting in a longer time for the concentration polarization to be convected tangentially along the membrane surface. The ratio between the average flux over a back-shock cycle and the steady-state flux is found to increase with increasing inlet velocity. Furthermore, this ratio increases when the viscosity depends on the concentration. This is clue to the relatively lower steady-state value when the viscosity depends on the concentration. Moreover, an increase in observed rejection is found when using back-shocking. The increase in observed rejection is found to be largest when the inlet velocity is high and the viscosity depends on the concentration. (C) 2015 Elsevier B.V. All rights reserved.
|
|
