Chen, Fan
(2021)
Numerical and experimental investigation of tsunamis generated by iceberg calving.
PhD thesis, University of Nottingham.
Abstract
With climate change as an increasingly important issue, the Greenland and Antarctic ice sheets have suffered rapid ice mass losses contributing to sea level rise. Iceberg calving is one of the main factors for ice mass loss and may also generate large tsunamis when icebergs calve into a water body, threatening the fishing and shipping industries and coastal communities. One of the most impressive tsunamis generated by iceberg calving with an amplitude of 45 to 50 m was observed at the Eqip Sermia glacier in Greenland in 2014. Herein, such tsunamis are called iceberg-tsunamis (IBTs).
This study aims to investigate the generation and propagation of large IBTs. A novel numerical model is developed based on the Immersed Boundary Method (IBM) provided in a toolbox in the open source code Foam-extend 4.0. A new multiphase flow solver is implemented to solve the Reynolds-Averaged Navier-Stokes equations with the support of handling moving immersed boundaries, and it is then coupled with a motion solver to simulate Fluid-Structure Interaction (FSI) to determine the iceberg motion. Further, unique large-scale laboratory experiments were conducted in a 50 m $times$ 50 m large basin. Two rigid blocks with densities of $approx$920 kg/m$^3$ weighting up to 187 kg were used to mimic icebergs and to model the five idealised calving mechanisms: (A) capsizing, (B) gravity-dominated fall, (C) buoyancy-dominated fall, (D) gravity-dominated overturning and (E) buoyancy-dominated overturning.
The analytical solution of a floating heaving sphere case is used to validate the newly implemented flow solver and the numerical radiated wave amplitudes show a maximum deviation of only 10.3% from the corresponding analytical solution. Further, the numerical model is used to simulate the two laboratory experiments from mechanisms B and D generating the largest and most dangerous IBTs. A laminar flow model is selected as the consideration of turbulence does not improve the accuracy. For IBT generation, the maximum vertical displacement in the fall case and the pitch angle in the overturning case are 24.3 and 19.9% larger than the corresponding laboratory measurements. During this stage of the IBT, the pressure force is the dominant force while the viscosity force can be neglected in the overall force balance. For IBT propagation, the leading IBT amplitude and wave height decay are captured with a maximum relative error of -41.2 and -29.3%, respectively. This large deviation is mainly due to the water splash involved in the laboratory experiments, which can not fully be modelled in the numerical simulations. If this splash is artificially removed, then the corresponding relative errors become -17.2 and -9.2%, respectively. Additionally, the numerical IBT train energy shows a good agreement with the laboratory results with a relative error of 1.0% in mechanism B, while the difference in mechanism D is much larger due to the oscillation of the iceberg.
Further, the capability of this numerical model of simulating FSI is confirmed by the agreement of the contact forces of the icebergs acting on the glacier terminus with that in other studies. Smaller and less important IBTs generated in the remaining mechanisms A, C and E are also simulated. However, due to the nature of the IBM making it challenging to fully satisfy the continuity equation and the no-slip boundary condition around the immersed boundary when multiphase flows are involved, more work is required to also simulate these small waves, in the order of 10$^{-3}$ m at laboratory scale, well. Nevertheless, the numerical model is capable of simulating large and dangerous IBTs as further demonstrated by the successfully reproduced 2014 Eqip Sermia case. Overall, this work demonstrates that the proposed numerical model has the potential to support IBT hazard assessment. Future work will focus on the improvement of the immersed boundary representation technique to numerically investigate IBTs involving additional and more complex calving mechanisms and scenarios.
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