Investigation of different flow parameters on air layer drag reduction (ALDR) performance using a hybrid stability analysis and numerical solution of the two-phase flow equations

Abstract Among different methods of drag reduction, Air Layer Drag Reduction (ALDR) has been shown to be promising, at least in laboratories. Although a few examples of its practical application is available, important questions regarding the physics of ALDR and consequently its design rules and implementation remain to be answered in order to enhance its reliable, efficient and widespread use. Montazeri and Alishahi (2019) introduced a hybrid numerical procedure for such a problem, capable of producing acceptable and feasible results in comparison with experiments. In the present study, this procedure is modified and reapplied to the problem and its results are studied in more depth and some aspects of ALDR that had been overlooked or even misinterpreted in experimental tests, are discussed. The results show that the air layer develops and then disintegrates downstream in most cases. The ruptured air layer results in mild drag reduction for small air fluxes but high drag reduction happens for large flux values. Also, it is shown that the ALDR conduct is insensitive to bumps usually procured during construction. Results show that employing a superhydrophobic coating on a small surface prior to the injection point provides improved efficiency that would seem promising in practice.