Jean Rabault

Observations of wave dispersion and attenuation in landfast ice

Observations of wave propagation in landfast ice were obtained in Tempelfjorden, Svalbard during March 2015. Wave motion was measured near the ice edge using inertial motion units and consisted of a combination of swell from the North Atlantic and wind-generated waves. The waves were observed to be unidirectional in the ice with comparable magnitudes in the vertical and horizontal displacements. The dispersion relation was calculated from the measured phase difference between two adjacent sensors separated by a distance of approximately 60 m. Deviations from the gravity wave dispersion relation were observed during the growth phase of the waves and were consistent with the presence of flexural waves. This period of wave growth was accompanied by significant wave attenuation in the high frequency portion of the wave spectrum which persisted for 3–5 h.

The dynamics of a capsule in a wall-bounded oscillating shear flow

The motion of an initially spherical capsule in a wall-bounded oscillating shear flow is investigated via an accelerated boundary integral implementation. The neo-Hookean model is used as the constitutive law of the capsule membrane. The maximum wall-normal migration is observed when the oscillation period of the imposed shear is of the order of the relaxation time of the elastic membrane; hence, the optimal capillary number scales with the inverse of the oscillation frequency and the ratio agrees well with the theoretical prediction in the limit of high-frequency oscillation. The migration velocity decreases monotonically with the frequency of the applied shear and the capsule-wall distance. We report a significant correlation between the capsule lateral migration and the normal stress difference induced in the flow. The periodic variation of the capsule deformation is roughly in phase with that of the migration velocity and normal stress difference, with twice the frequency of the imposed shear. The maximum deformation increases linearly with the membrane elasticity before reaching a plateau at higher capillary numbers when the deformation is limited by the time over which shear is applied in the same direction and not by the membrane deformability. The maximum membrane deformation scales as the distance to the wall to the power 1/3 as observed for capsules and droplets in near-wall steady shear flows.

Measurements of Waves in Landfast Ice Using Inertial Motion Units

Inertial motion units (IMUs) are used to perform measurements of waves in water covered by landfast ice close to the ice edge in Svalbard. The effective noise level of the instruments is assessed in controlled wave tank experiments. A set of measurements collected in Tempelfjorden, Svalbard in March 2015 is presented, and the ability of the sensors to operate in the field is validated. Several characteristics of the recorded signals, including correlation between the different sensors, are analyzed. Horizontal and vertical motions are of the same order of magnitude. A clear transition in the signal properties is observed in relation with changes in incoming wave field and the development of cracks in the ice layer. We show that complex physics takes place when waves propagate in landfast ice and that the use of times series containing information on the full three-dimensional linear acceleration, rather than spectra, is required to capture the underlying phenomena.

A Method to Estimate Reflection and Directional Spread Using Rotary Spectra from Accelerometers on Large Ice Floes

AbstractThe directional wave spectra in sea ice are an important aspect of wave evolution and can provide insights into the dominant components of wave dissipation, that is, dissipation due to scattering or dissipation due to viscous processes under the ice. A robust method for the measurement of directional wave spectra parameters in sea ice from a three-axis accelerometer—or a heave, pitch, and roll sensor—is proposed. The method takes advantage of certain aspects of sea ice and makes use of rotary spectra techniques to provide model-free estimates for the mean wave direction, directional spread, and reflection coefficient. The method is ideally suited for large ice floes—that is, where the ice floe length scale is much greater than the wavelength—but a framework is provided to expand the parameter space where the method may be effective.