PIV Measurement of Flow Velocity Field under Modeled Ice Floe

Peng Lu, Zhijun Li, Qiang Zhang State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology.

Dalian, Liaoning, China

ABSTRACT

Laboratory experimental study was carried out to measure the flow

velocity field under different kinds of floe models with smooth or rough bottom using Particle Image Velocimetry (PIV) technology. The disturbance depth z ́ under the floe model was found to increase linearly with the geometric roughness of floe underside, namely the obliquity θ of serrate fluctuations on the model bottom. Profile analyses revealed that the roughness length z0, determined from the logarithmical profile, also increased linearly with θ. Influences of floe draft and initial flow velocity on z ́and z0 could be both ignored.

KEY WORDS: Sea ice; PIV; boundary layer; flow field; roughness;

parameterization; drag coefficient.

INTRODUCTION

Sea ice drag coefficient is one of the most important parameters involved in sea ice dynamics, representing the momentum exchange

between sea ice and atmospheric, oceanic boundary layers (Lepparanta, 2005). Parameterization of sea ice drag coefficient means to develop the relationship between drag coefficient and ice geometric parameters such as surface roughness, size, concentration and thickness by dividing the total drag force into different terms and combing with the influences from different factors (Lupkes and Birnbaum, 2005). Compared with the traditional calculation methods of drag coefficient (i.e. eddy correlation method, profile method, momentum method) (Ji et al., 2003), more specific physical mechanisms altering the drag force are contained in the parameterization method. Thus, the accuracy and feasibility of the coefficient can be improved, and further the results of

sea ice dynamics numerical modeling can also be highly improved. Although many theoretical studies on the parameterization of sea ice drag coefficient have been reported (Mai et al., 1996; Garbrecht et al., 2002; Birnbaum and Lupkes, 2002; Lupkes and Birnbaum, 2005), in-situ investigation data is severely limited by field environmental conditions and existing measurement technology. Especially, few systemic field observations of the oceanic boundary layer under sea ice have been reported (eg. Shirasawa and Ingram, 1991a,b), in contrast to the abundant observations in the atmospheric boundary layer above ice

(eg. Prinsenberg and Peterson, 2002), resulting in a lack of the understanding of many key processes involving in the dynamic interactions between sea ice and ocean. To meet such deficiency, laboratory experimental study was proposed to study the ice-ocean dynamic interactions and provide new evidences for the parameterization of sea ice drag coefficient, which has obvious advantages of avoiding the influence from environment and controlling the test conditions (Pite et al., 1995; Lu et al., 2007). In the present paper, a series of laboratory experiments were conducted in a water tank and a kind of floating rectangular block with smooth or

rough bottom was used to simulate sea ice floes with different underside morphology. The Particle Image Velocimetry (PIV) technology, which has been widely applied as a powerful fluid visualization tool to obtain instantaneous velocity measurements (Seybert and Evans, 2005; Liu and Zheng, 2006; Lee et al., 2008), was employed to measure flow velocity field. Variations in the flow field around the floe model under different initial flow velocity, floe draft and underside morphology were then systemic investigated.

EXPERIMENTAL SETUP

Experiments were performed in a recirculating open-water channel driven by a centrifugal pump. A setting chamber and a honeycomb

were placed in sequence to ensure flow homogeneity. Fig. 1 shows a schematic of the test section, PIV system and roughness elements on the bottom of floe model. The dimensions of the test section were 45cm (width)×60cm (depth)×100cm (length). An ice floe model made up of organic glass with the size of 35cm (width)×14.5cm (depth)×45cm (length) was placed in the flow and its vertical position could be adjusted to simulate different ice draft. Both smooth and rough floe bottoms were employed in the experiment. For the rough bottom, as shown in Fig. 1, two-dimensional spanwise rods with a triangle cross-section were periodically arranged on the bottom of the floe model to simulate the roughness on the ice underside. The

height of each rod was 2.0 cm, but the tilt angle θ varied.

Proceedings of the Twentieth (2010) International Offshore and Polar Engineering Conference Beijing, China, June 2025, 2010 Copyright © 2010 by The International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-77-7 (Set); ISSN 1098-6189 (Set); www.isope.org

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