Air Layer Drag Reduction Research Articles

Research and Development of Inland Green and Smart Ship Technologies in China

Nowadays, the development of green and smart ships has become a trend in the global shipping industry. Some countries such as Japan and Korea, as well as several European countries, have already made some progress and advantages. In recent years, China has issued a series of policies to encourage and support the development of green and smart ships. Beyond that, the demonstration of green and smart ships has been promoted along with the trend of larger-sized and standardized inland vessels. The Chinese status and plans of green technologies are summarized including air layer drag reduction, efficient propulsion, DC networking, and clean energy. Some smart technologies have also been illustrated, for instance, intelligent driving, intelligent engine rooms, energy efficiency, hulls, cargo control, and information integration. Smart ship technologies need to be fully validated in order to improve industrialization applications. In the shipping industry, the development and application of smart ship technology need international and inter-regional cooperation, so as to achieve much higher energy savings, ensure environmental protection, achieve collaborative intelligence, and ensure safety and reliability. In turn, this will aid economic advances worldwide.

Numerical Study on the Influence of Water Depth on Air Layer Drag Reduction

Over the years, air lubrication technology has been widely applied to maritime vessels, demonstrating its significant energy-saving and emission-reducing effects. However, the application of this technology in inland waterway transportation faces unique challenges due to the shallower water depths, particularly during low water periods. Under such conditions, the formation of the air layer and its associated drag-reduction effects may undergo alterations. Conducting research on air lubrication technology in shallow water conditions holds great practical significance for promoting its application in inland waterway vessels. Therefore, a numerical study is undertaken to examine the impact of water depth on Air layer Drag Reduction (ALDR) to promote the use of ALDR technology on inland canal boats with shallow water depths. The object was a specific river-sea direct ship model, and a groove was created at the bottom of the model with air injection. At two distinct speeds, numerical simulations were run for four different depths: deep water, moderate water, shallow water, and ultra-shallow water. A comparative examination of the air layer morphology on the ship bottom and drag reduction was conducted to investigate the impact of water depth on ALDR and confirm the viability of using ALDR technology on shallow-water navigation boats. The results indicate that due to the change in the velocity and pressure fields at the bottom of the ship, the efficiency of drag reduction and the form of the air layer on the ship’s bottom are significantly impacted by variations in water depth in restricted waters. However, the total resistance can still be significantly reduced by setting grooves on the hull with air injected in shallow waterways. Reduced frictional resistance no longer predominates the overall resistance reduction in shallow water; the proportion of the decrease in viscous pressure resistance rises and can reach up to 4.8 times the decrease in frictional resistance. The research confirms the application prospects of this technology on inland waterway transport ships.

Effect of exit geometry of blowing air on friction drag of an underwater plate

Air layer drag reduction (ALDR) is one of the important techniques for reducing the frictional resistance of ships and underwater vehicles. The air injection configuration directly influences air layer coverage on the surface and hence drag reduction. Several shapes of air exit have been investigated in the past, including narrow slots, regular-shaped holes, porous media, and arrays/combinations of them. A new exit shape, i.e. modified Nekomimi, is proposed and investigated numerically for ALDR of an underwater flat plate. The work is inspired by air film cooling technology where it is desirable to have an air layer segregating wall from main gas flow. The Reynolds number examined is 6970–22890, based on local turbulent boundary layer thickness, for the incoming water flow or 700–5550, based on the equivalent exit diameter of blowing air. Results show that the Nekomimi exit produces a larger and more even air layer coverage, as indicated by the air volume fraction on the plate, accounting for a more pronounced drag reduction and net energy saving than a circular exit with the same area or spanwise dimension. Multi-Nekomimi-exit arrangement is also investigated to obtain a larger air-layer coverage on the plate, showing good promise in engineering applications.

Experimental and numerical studies on the air-injection drag reduction of the ship model

Experimental and numerical studies on the air-injection drag reduction (AIDR) of the ship model are conducted. For the experiments, the ship model and air-injection device are specially designed and the AIDR of the ship model is investigated considering one draft and three speeds. Then, considering two ship speeds, the AIDR of the ship model is simulated using two numerical models, namely the Euler-Euler two-fluid model and VOF model. It is found that for the surface ship there are still three drag reduction regions. Microbubble drag reduction (MBDR) is dominant for low air flow rates and air layer drag reduction (ALDR) is dominant for high air flow rates, and the transitional region exists for the medium air flow rates. For MBDR, the mechanism is related to the reduction of both effective dynamic viscosity and normal velocity gradient. Bubble size is the key influencing factor. Smaller bubbles are more favorable for drag reduction. Large millimeter bubbles can increase drag, which is owing to the extra turbulence viscosity. For ALDR, the mechanism is mainly owing to the loss of wet surface area and the key influencing factor is the air cover area. In general, ships with higher speed are more favorable for AIDR.

Numerical study of air layer drag reduction of an axisymmetric body in oscillatory motions

Numerical study of air layer drag reduction of an axisymmetric body in oscillatory motions

Air Layer on Superhydrophobic Surface for Frictional Drag Reduction

We examined the feasibility of combining a superhydrophobic surface (SHS) and air layer drag reduction (ALDR) to achieve the frictional drag reduction (DR) shown achievable with traditional ALDR, but at a reduced gas flux to increase the achievable net energy savings. The effect of a commercial SHS coating on the gas flux required to maintain a stable air layer (AL) for DR was investigated and compared with that of a painted non-SHS at Reynolds numbers up to 5.1 X 106. Quantitative electrical impedance measurements and more qualitative image analysis were used to characterize surface coverage and to determine whether a stable AL was formed and maintained over the length of the model. Analysis of video and still images for both the SHS and painted surface gives clear indications that the SHS is able to maintain AL consistency at significantly lower gas flux than required on the non-SHS painted surface. Hydrophobicity of the surfaces was characterized through droplet contact angle measurements, and roughness of all the flow surfaces was measured. The results from these preliminary experiments seem to indicate that for conditions explored (up to Rex = 5.1 X 106), there is a significant decrease in the amount of gas required to establish a uniform AL (and hence presumably achieve ALDR) on the SHS when compared with a hydraulically smooth painted non-SHS.

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

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.

A numerical investigation of the mechanism of air-injection drag reduction

In order to investigate the mechanism of air-injection drag reduction, two different CFD models in OpenFOAM including the Eulerian-Eulerian two-fluid model and VOF model have been applied to study the there-dimensional drag reduction of an axisymmetric body by air injection. The calculated drag reduction rates obtained from the two models are compared against the experimental measurements. It is found that the numerical results from these two models agree well with the experimental data in two kinds of drag reduction regions: microbubble drag reduction (MBDR or BDR) region and air layer drag reduction (ALDR) region. For the transition region between BDR and ALDR, the data-coupling method based on the air flow ratio is proposed to calculate the drag reduction rates and the final coupling drag reduction rates are in well concordance with the experimental data under various air flow ratios. It is indicated that the CFD models presented in this study can be treated as good candidates to investigate the phenomenon of air-injection drag reduction. In addition, the predicted morphology of the mixture flow and air void fraction distributions around the body are also presented to provide valuable insights on this complex physical phenomenon. Based on the numerical results, it is found that in BDR region the air void fraction around the body can be considered as an important factor influencing the drag reduction, and in ALDR region the formation area of the air layer around the body surface will directly influence the drag reduction. For BDR, bubble size distribution and the effects of bubble size on drag reduction have been also investigated. In the end, the mechanism of air-injection drag reduction has been analyzed.

Experimental Study of Air Layer Drag Reduction with Bottom Cavity for A Bulk Carrier Ship Model

Air lubrication by means of a bottom cavity is a promising method for ship drag reduction. The characteristics of the bottom cavity are sensitive to the flow field around the ship hull and the effect of drag reduction, especially the depth of the bottom cavity. In this study, a ship model experiment of a bulk carrier is conducted in a towing tank using the method of air layer drag reduction (ALDR) with different bottom cavity depths. The shape of the air layer is observed, and the changes in resistance are measured. The model experiments produce results of approximately 20% for the total drag reduction at the ship design speed for a 25-mm cavity continuously supplied with air at Cq = 0.224 in calm water, and the air layer covers the whole cavity when the air flow rate is suitable. In a regular head wave, the air layer is easily broken and reduces the drag reduction rate in short waves, particularly when λ/Lw1 is close to one; however, it still has a good drag reduction effect in the long waves.

An efficient method for numerical modeling of thin air layer drag reduction on flat plate and prediction of flow instabilities

Experimental researches on flat plates and ship models show that among many different methods of drag reduction, Air Layer Drag Reduction (ALDR) seems to be one of the most efficient methods. The physics behind different phenomena in this method is not completely clear and numerical modeling can help resolving this manner. There is a lack of exact numerical modeling of this flow because of its complexity and huge numerical cost due to the two-phase instabilities in case of DNS. Also common CFD simulations using URANS models involve significant errors. In this paper an efficient method for numerical modeling of ALDR is introduced which is a combination of linear stability and URANS modeling of the flowfield. The base flow is extracted from CFD which is used in linear stability analysis as the basis which results the frequency of the most unstable mode. The final CFD simulation is then carried out plus an additional perturbation term with that specific frequency. Results show that the implementation of this strategy significantly improves the results of the numerical modeling compared to the experimental results. The predicted flowfields also show some new aspects of physics of ALDR that had not been previously shown in experimental tests.

The characteristics and mechanism of microbubble drag reduction on the axisymmetric body

To investigate the characteristics and mechanism of microbubble drag reduction on the axisymmetric body, both experiments and the numerical simulation of microbubble drag reduction have been conducted in this paper. Based on the experiments, the morphology of microbubble flow and the characteristics of microbubble drag reduction were analyzed. The size distribution of microbubbles and the influence of microbubble size on the drag reduction efficiency were quantitatively investigated. Based on the numerical simulation, the influences of microbubbles on the turbulence intensity and vorticity were analyzed to investigate the mechanism of microbubble drag reduction. The results indicate that the diameters of microbubbles basically obey the normal distribution at various conditions, and the microbubble flow presents the uniform microbubble, roll-up and cavity. As the air injection rate increases, the drag reduction respectively shows microbubble drag reduction, mixture drag reduction and air-layer drag reduction. Besides, the drag reduction ratio correspondingly presents the increasing stage, rapidly increasing stage and stable stage. At the same void ratio, the smaller microbubbles show the higher efficiency in drag reduction. The microbubbles injected into the boundary layer can reduce the turbulence intensity and the frequency of the bursting events in the flow field, which results in the drag reduction.

Comparison of Microbubble and Air Layer Injection with Porous Media for Drag Reduction on a Self-propelled Barge Ship Model

Ship resistance issues are related to fuel economy, speed, and cost efficiency. Air lubrication is a promising technique for lowering hull frictional resistance as it is supposed to modify the energy in the turbulent boundary layer and thereby reduce hull friction. In this paper, the objective is to identify the optimum type of air lubrication using microbubble drag reduction (MBDR) and air layer drag reduction (ALDR) techniques to reduce the resistance of a 56-m Indonesian self-propelled barge (SPB). A model with the following dimensions was constructed: length L = 2000 mm, breadth B = 521.60 mm, and draft T = 52.50 mm. The ship model was towed using standard towing tank experimental parameters. The speed was varied over the Froude number range 0.11–0.31. The air layer flow rate was varied at 80, 85, and 90 standard liters per minute (SLPM) and the microbubble injection coefficient over the range 0.20–0.60. The results show that the ship model using the air layer had the highest drag reduction up to a maximum of 90%. Based on the characteristics of the SPB, which operates at low speed, the optimum air lubrication type to reduce resistance in this instance is ALDR.

On multi-point gas injection to form an air layer for frictional drag reduction

Air layer drag reduction has been shown to be a feasible drag reducing technique at the laboratory and at full ship scales. In most studies, the air layers have been generated via gas injection from two-dimensional spanwise slots. However, given ship's structural considerations, it would be preferable to use discrete holes. The present study expands on the work on single orifice gas injection to multi-hole injection. When compared with slot injection, multi-point injection lead to a reduced range of gas fluxes that formed an air layer. Gas injected from a series of discrete holes can exhibit complex flow patterns, including roll-up into the core of liquid vortices that form as part of the process of injecting gas into the liquid boundary layer. The finite span and length of the model utilized for the present experiments was modest. It remains to be shown if a larger model with similar scaled up geometry (and with more beanwise holes) would enable the formation of a stable air layer with a gas flux per unit span that is similar to that required for slot injection. Nevertheless, the results presented here illustrate the complexity associated with gas injection through multiple perforations in a hull.

Experiment and simulation on air layer drag reduction of high-speed underwater axisymmetric projectile

Injection of air could lead to creation of a bubbly mixture or air layer near the surface that can significantly adjust the flow within the turbulent boundary layer. In this paper, stress wave propagation techniques and Split Hopkinson Pressure Bar (SHPB) are used in underwater launching experiment. Simulation with volume of fluid (VOF) method and modified renormalization group (RNG) k-ε model is also performed to study the physical process of drag reduction of axisymmetric body. Comparison between numerical and experimental air layer length shows good correlation. Results indicate that air layer has good effect on drag reduction. Friction drag reduction mechanism is analyzed from two aspects due to different air volume fraction α. At area with high α, fluid is heterogeneous and layered. Drag reduction is from decrease of velocity gradient and dynamic viscosity at the wall. At area with small α, the mixture is homogeneous. Empirical equation of turbulent boundary layer shear stress is applied to describe drag reduction mechanism. The unsteady evolution of drag with injection of air is also studied at last.

On the scaling of air layer drag reduction

AbstractAir-induced drag reduction was investigated on a 12.9 m long flat plate test model at a free stream speed of$6. 3~\mathrm{m} ~{\mathrm{s} }^{- 1} $. Measurements of the local skin friction, phase velocity profiles (liquid and gas) and void fraction profiles were acquired at downstream distances to 11.5 m, which yielded downstream-distance-based Reynolds numbers above 80 million. Air was injected within the boundary layer behind a 13 mm backward facing step (BFS) while the incoming boundary layer was perturbed with vortex generators in various configurations immediately upstream of the BFS. Measurements confirmed that air layer drag reduction (ALDR) is sensitive to upstream disturbances, but a clean boundary layer separation line (i.e. the BFS) reduces such sensitivity. Empirical scaling of the experimental data was investigated for: (a) the critical air flux required to establish ALDR; (b) void fraction profiles; and (c) the interfacial velocity profiles. A scaling of the critical air flux for ALDR was developed from balancing shear-induced lift forces and buoyancy forces on a single bubble within a shear flow. The resulting scaling successfully collapses ALDR results from the current and past studies over a range of flow conditions and test model configurations. The interfacial velocity and void fraction profiles were acquired and scaled within the bubble drag reduction (BDR), ALDR and transitional ALDR regimes. The BDR interfacial velocity profile revealed that there was slip between phases. The ALDR results showed that the air layer thickness was nominally three-quarters of the total volumetric flux (per unit span) of air injected divided by the free stream speed. Furthermore, the air layer had an average void fraction of 0.75 and a velocity of approximately 0.2 times the free stream speed. Beyond the air layer was a bubbly mixture that scaled in a similar fashion to the BDR results. Transitional ALDR results indicate that this regime was comprised of intermittent generation and subsequent fragmentation of an air layer, with the resulting drag reduction determined by the fraction of time that an air layer was present.

Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction

To investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and air-layer drag reduction (ALDR).The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms−1. Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 <Y< 5mm) bubble populations were estimated with a bubble imaging system. The instrument suite was used to investigate the scaling of BDR and the requirements necessary to achieve ALDR.Results from the BDR experiments indicate that: significant drag reduction (>25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR.ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3ms−1. These results indicated that there are three distinct regions associated with drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms−1; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5ms−1with the surface fully roughened (though approximately 50% greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10mm height in the experiment) that helps to create a fixed separation line.