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Lina Nikolaidou

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Effect of incoming flow conditions on air lubrication regimes

Journal article (2024) - Lina Nikolaidou, A. Laskari, T.J.C. van Terwisga, T.J.C. van Terwisga, C. Poelma

Different air phase regimes are formed by controlled air injection in a spatially developing flat plate turbulent boundary layer (TBL). The air is introduced via a slot type injector without the use of a backward-facing step or cavitator upstream of the air injection position. The effect of different incoming liquid flow characteristics on the different regimes is investigated by varying both the liquid freestream velocity and the incoming TBL thickness. The latter is realized through changing the position of the air injection along the length of the water tunnel facility. That resulted in a downstream distance based Reynolds number from 1 to 5 million. Three different air phase regimes are identified under different air flow rates and freestream velocities: the bubbly regime, the transitional, and the air layer regime. The morphological differences of each one are described and quantitative analysis is performed based on the non-wetted area in each condition. The incoming TBL as well as the flow around the air layer are measured with planar particle image velocimetry. The latter enabled the determination of the air layer thickness. In addition, the ratio of the air layer to the incoming boundary layer thickness t_air/δ is also calculated (≈ 0.04 – 0.5). This is a significant dimensionless parameter for scaling, which indicates the extent to which the air layer is embedded within the incoming TBL. Depending on the incoming flow conditions, a two or three branch air layer is formed. The length of the air layer is found to increase with increasing liquid freestream velocities. A good agreement between the air layer length and a half gravity wave predicted by the dispersion relation is found. An increase of the air layer length is observed with a decreasing incoming TBL thickness. This is attributed to a decrease in the local mean velocity at the air–water interface due to the TBL growth. Finally, increasing the incoming TBL thickness delays the onset of the air layer regime.

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Effect of incoming boundary layer characteristics on an air layer within a liquid turbulent boundary layer

Conference paper (2022) - Lina Nikolaidou, A. Laskari, C. Poelma, T.J.C. van Terwisga

An air layer within a liquid turbulent boundary layer (TBL) is formed by controlled air injection underneath a flat plate. The incoming boundary layer as well as the flow around the air layer were measured with planar particle image velocimetry (PIV). The effect of different i ...

Contributed

Effect of Upstream Obstacles on Air Layer Regimes in a Turbulent Boundary Layer

An experimental study on the influence of obstacles' size and placement within a turbulent boundary layer on air lubrication

Master thesis (2024) - T.S.F. Wolfs, A. Laskari, Lina Nikolaidou, R. Delfos

Hydrodynamic drag significantly influences the fuel efficiency and operational performance of marine vessels. Gas Injection Drag Reduction (GIDR) techniques, such as Bubble Drag Reduction (BDR), Transitional Layer Drag Reduction (TLDR), and Air Layer Drag Reduction (ALDR), have shown promise in reducing skin friction drag by introducing gas into the boundary layer. However, the influence of hull protrusions, such as sacrificial anodes and fouling, on these techniques remains poorly understood. This research addresses the question: How do the placement and dimensions of upstream cylindrical obstacles affect the formation and stability of gas phases (bubbly, transitional, and air layer) in a turbulent boundary layer (TBL)? To investigate this, protrusions are represented as cylindrical objects placed upstream of the gas injector, exploring their effects on the gas injection regimes in a TBL.

The study examines three gas injection regimes: bubbly, transitional, and air layer. In the bubbly regime, the focus is on bubble morphology and velocity distributions. The transitional regime investigates non-wetted areas and air layer morphology. For the air layer regime, the study examines the length and stability of the air layer near the injector. Image processing techniques are used to distinguish the air layer from the surrounding fluid, providing accurate assessments of the properties of the air layer.

The results demonstrate that cylindrical obstacles have a significant effect on the gas phases. In the bubbly regime, they alter bubble morphology and velocity, producing localized velocity peaks. In the transitional regime, obstacles lead to the formation of non-wetted areas and funnel-shaped regions. For the air layer regime, although the length of the air layer remains mostly unaffected, its stability near the injector is disrupted by upstream obstacles due to wake-induced instabilities. These findings highlight the critical role of obstacle placement and dimensions in shaping gas injection regimes.

Future research is needed to enhance understanding in areas such as the integration of velocity measurements (e.g., Particle Image Velocimetry), widening the experimental field of view to capture more comprehensive flow dynamics, and exploring the impact of various obstacle sizes and geometries. These investigations will be crucial for refining gas injection drag reduction strategies and advancing practical marine applications.

Experimental Study of Flat Plate Drag Reduction by Air Lubrication

Master thesis (2024) - S. Pammi, T.J.C. van Terwisga, A. Laskari, Lina Nikolaidou

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.

Experimental investigation of influence of an air cavity on a turbulent boundary layer using PIV

Master thesis (2021) - A. ANAND, C. Poelma, A. Laskari, W.J. Baars, Lina Nikolaidou

The turbulent boundary layer development under the influence of an air cavity is studied experimentally using planar PIV, with the aim of gaining insight and building upon the flow physics typically encountered in the application of air layer drag reduction. A detection technique based on correlation values is implemented to obtain an approximate shape of the air cavity and the location of the air-water interface. The technique was successful in identifying the maximum cavity thickness with sufficient accuracy. The leading and trailing edges of the cavity however, were harder to identify, the former owing to a limitation of the developed technique and the latter due to the dynamic nature of the flow and a slightly limited FOV. The ratio of the initial boundary layer thickness to the maximum thickness of the air cavity is 6.7, and as a consequence the boundary layer did not separate at the leeward side of the air cavity. The turbulent boundary layer is observed to feel the presence of the air cavity up to 8.5-9.5 cm upstream due to an adverse pressure gradient. Alternating streamwise pressure gradients are generated due to the curvature of the air cavity: from an adverse to favourable and back to adverse. Compared to solid bump studies in literature, additional perturbations due to a free-slip boundary condition and the unstable nature of air cavity increase the complexity of the current flow. The mean velocity profile and stresses are able to capture the effects of alternating streamwise pressure gradients and air injection, with variations mostly restricted to the inner region. Effects of streamline curvature in the outer region are found to be minimal, while potential effects of the free-slip condition were much harder to identify separately and further research would be needed to appropriately assess them. The mean velocity profile is found to deviate from the classic logarithmic behaviour at the apex of the air cavity, although the flow does not seem to relaminarise. Quadrant analysis shows differences in Reynolds stress producing events compared to the baseline turbulent boundary layer case hinting at possible alteration to coherent structuring of the turbulent boundary layer developing below the air cavity.