RIASSUNTO
ABSTRACT
A ship moving in water experiences resistances, the major component of which is frictional resistance arising from water viscosity. Therefore, drag-reduction has been an active research field in marine transportation over the last decades. A variety of innovative drag reduction techniques have been proposed, among which are riblets, microgrooves, compliant coatings, addition of polymers, air injection method, to name a few. Among them, air-injection drag reduction technique has achieved attraction increasingly in recent years owing to its overwhelming advantages, such as considerable potential drag reduction, easy operations, environmental friendliness and low costs.
There are basically two types of air-injection drag reductions: Microbubble Drag Reduction (MBDR) and Air-Layer Drag Reduction (ALDR). It is interesting that they can be modeled by Eulerian-Eulerian two-fluid model and VOF (Volume of Fluid), respectively in OpenFOAM. In this paper, we show how to numerically model the two types of air-injection techniques. After that, a comparison with a recently experimental data is made for an axisymmetric body. It is surprising that such comparison reveals that MBDR and ALDR are all present in the experimental test. MBDR is dominant for low flux rate while ALDR is dominant for high flux rate. Their transition is restricted to a very narrow region, similar to a phase transition characterized by a sharp jump from one mechanism to another. Based on such observations we propose a combined numerical model based on the data-coupling method to handle the case that both MBDR and ALDR are present with a sharp transition. The combined model is a linear one in the framework of interpolation. It works well with satisfactory results obtained.
Numerical results show that in the MBDR-dominant region the air void fraction around the body is the determinant factor influencing the drag reduction. In the ALDR-dominant region the formation area of the air layer around the body surface determines the amount of the drag reduction. The above results are useful for designing an effective air-injection drag reduction method.