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Title: Crystal alignment in sea ice
Author: Langhorne, Pat
ISNI:       0000 0000 5355 7444
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1982
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The c-axes of grains in a sheet of sea ice not only lie in the plane perpendicular to the direction of heat flow but also may align about a particular axis in that plane. It has been suggested that the existence of horizontal temperature gradients or the current at the ice-water interface are likely driving forces for this ordering. In addition to making relevant field observations ,we have carried out experiments to discover which factor exerts the greatest influence on the resulting distribution of c-axes. Apparatus to grow NaCl ice in the presence of a water current has been constructed in the cold room at the Scott Polar Research Institute. Two lengthy experiments have been performed in saline water (34%,), one with a current of 3cms⁻¹ beneath the growing ice and a corresponding "null" experiment in which there was no forced flow. Two similar experiments were carried out with brackish water (20%). The growth rate of the ice and the shape of the ice-water interface at the completion of the experiment were found in all four cases. Vertical and horizontal temperature gradients were measured during the saline experiments and the vertical gradients correlate well with those predicted by a theory for growth rate. Horizontal gradients for all experiments may then be deduced from a knowledge of the slope of the interface and the vertical temperature gradient. Nocorrelat ion was found between the observed mean c-axis direction and the predicted or measured horizontal temperature gradients. In the presence of a current the mean of the c-axes was found to lie in the direction of the forced flow. In addition ,in the saline experiment the columnar grains were tilted upstream in to the current , a feature which has been described for metals solidifying in a flowing melt. Where there was no fluid motion the c-axis distribution was significantly less aligned. As the growth velocity of the ice decreased during the forced flow experiment with a brackish solution, a cellul ar to planar transition took place at the ice-water interface. The conditions under which this occurred are in reasonable agreement with a theory for interfacial stability in fluid flow. The experimental and field observations follow a simple, empirical equation relating the fraction of c-axes aligned in the direction of the current to the time at which the ice solidified. This yields an alignment relaxation time which has been related to the fluid velocity outside the boundary layer. Thus it is possible to predict the direction and strength of the c-axis alignment from knowledge of the mean current. Alternatively, the effective current can be found from the fabrics of the sea ice.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Oceanography