Experimental Study of Flat Plate Drag Reduction by Air Lubrication

Abstract

Drag reduction in ships is a key approach to decreasing their energy consumption. Besides saving fuel costs, it also decreases greenhouse gas emissions. One of the most promising techniques for drag reduction in ships is air lubrication, which reduces drag due to friction. Despite its great potential, application of air lubrication in the maritime industry is limited, owing to a lack of understanding of its underlying physical phenomena and stability issues.

The present work aims to experimentally study drag reduction on a flat plate resulting from air lubrication at a representative free-stream velocity of 2.5 m/s over a set of increasing air injection rates covering all the three air layer regimes (BDR, TALDR, ALDR) and assess its dependence on air morphology in terms of the plate’s non-wetted area. The experiments were conducted at the Multi-Phase Flow Tunnel located at the Ship Hydromechanics group of TU Delft.

To achieve this, first, a custom force balance comprising a spring system was designed to facilitate the measurement of the total drag acting on the plate by means of a load cell. Drag measurements in single-phase flow were conducted on a conventional flat plate to obtain a reference dataset and validate the drag measurement system. An uncertainty analysis was performed to quantify the measurement accuracy. Then, single-phase and dual-phase experiments were conducted on another plate of similar dimensions fitted with additional parts necessary for the generation of air lubrication. For the quantification of the plate’s non-wetted area in dual phase flow, these experiments were accompanied by image capture with cameras positioned vertically below the plate. The recorded images were processed using a binary approach to distinguish regions under the plate covered with water and air. Finally, the non-wetted area ratio of the plate was computed based on these processed images. Uncertainty in both drag reduction and the non-wetted area ratio was also determined before
analysing the results.

Results from the present work indicate a positive linear correlation between drag reduction and the associated non-wetted area ratio achieved in BDR and ALDR. However, this correlation varies in slope per air layer regime, possibly due to the physical phenomena governing the regime. Thus, further investigation into the underlying physical phenomena governing each regime could shed light on the reason(s) for different slopes in the
correlation. To improve the current work, estimation of the non-wetted area ratio in the TALDR regime may be carried out to increase the resolution of the correlation found. Moreover, the non-wetted area resulting from the thickness of the air layer may also be studied to define the non-wetted area ratio of the plate more accurately.