Analysis of Pressure Drop Data in Channel Flows Over Foul-Control Coatings

The two-dimensional channel flow is of great interest for experimental as well as numerical studies. From the experimental perspective test in channel equipment is preferred, because it is simple, practical and offers a favorable economic running cost. Especially with the growing interest in marine coating research, it is critical that coating testing equipment delivers realistic flow conditions, is simple to cut the experimental time and effort and yet accurate. In this sense, the flow channel facility offers more advantages than classical cases for experimental investigations. Whereas from the numerical investigations point of view, channel flow exhibits favorable boundary conditions to save computational effort, while providing a deep insight into details of the flow structures. An initiative is taken at Chalmers university to develop the channel flow (or a flowcell) experimental facility to cater for the need of studies on coatings. Therefore, the current paper, in the first step, describes the design and manufacture of the flowcell. Secondly, it presents the thoroughly conducted verification study of the smooth reference test section to demonstrate that experimental facility holds the measurement expectations. Subsequently, skin friction data for selected foul control coatings obtained from the pressure drop measurements in the flowcell are presented

An experimental Investigation into the surface and hydrodynamic characteristics of marine coatings with mimicked hull roughness ranges

There are limited scientific data on contributors to the added drag of in-service ships, represented by modern-day coating roughness and biofouling, either separately or combined. This study aimed to gain an insight into roughness and hydrodynamic performance of typical coatings under in-service conditions of roughened ships’ hull surfaces. Comprehensive and systematic experimental data on the boundary layer and drag characteristics of antifouling coating systems with different finishes are presented. The coating types investigated were linear-polishing polymers, foul-release and controlled-depletion polymers. The data were collected through state-of the-art equipment, including a 2-D laser Doppler velocimetry (LDV) system for hydrodynamic data in a large circulating water tunnel. Three coating systems were first applied on flat test panels with ‘normal’ finishes in the first test campaign to represent coating applications under idealised laboratory conditions. In order to address more realistic roughness conditions, as typicallyobserved on ships’ hulls, ‘low’ and ‘high’ roughness densities were introduced into the same types of coating, in the second test campaign. The data collected from the first test campaign served as the baseline to demonstrate the effect on the surface roughness and hydrodynamic drag characteristics of these coating types as a result of ‘in-service’ or ‘severely flawed’ coating application scenarios. Data collected on coatings with a range of in-service surface conditions provided a basis to establish correlation between the surface roughness characteristics and hydrodynamic performance (roughness function). The findings of the study indicate that the estimations of drag penalties based on well-applied, relatively smooth coating conditions underestimate the importance of hull roughness, which although undesirable, is commonplace in the world’s commercial fleet

Experimental Quantification of Drag Change of Commercial Coatings Under the Effect of Surface Roughness and Soft Fouling

The performance of ships will be adversely affected by the excessive hull roughness to an extent where financial penalties will be incurred. It is beyond the bounds of possibility to achieve a perfectly smooth hull, as the ship building process and paint application will leave their “fingerprint” on the surface. The roughness of the hull may vary from fine to coarse according to the substrate finish, the coating application methods used and further driven due to exposure to aggressive sea environment. The drag performance data of newly-applied and clean coatings is not sufficient to fully reflect the drag characteristics and efficacy of marine coatings over a typical period between dry-docking. Usually during this period, the increase in surface roughness and development of different fouling stages on marine coatings occur. Therefore, the study focuses on comparison of drag characteristics of hull coatings with relatively smooth, coarse roughness finishes and fouling conditions using time- and cost-efficient approach.\ua0 The study describes experimental tests carried out to quantify the drag change of commercial coatings due to the presence of physical and biological roughness. Firstly, biocidal and non-biocidal coatings with relatively smooth and coarse roughness finishes are tested. Secondly, mentioned coating types and roughness ranges are exposed to fouling growth to explore the extend of algae fouling and its effect on drag characteristics. The results of the study may be useful to estimate the added drag and overall fuel penalty for ships with various coating roughness ranges and soft fouling

Review and Comparison of Methods to Model Ship Hull Roughness

There is a large body of research available focusing on how ship hull conditions, including various hull coatings, coating defects, and biofouling, influence the boundary layer, and hence resistance and wake field of a ship. Despite this there seems to be little consensus or established best practice within the ship design community on how to model hull roughness for ship-scale CFD. This study reviews and compares proposed methods to model hull roughness, to support its use in the ship design community. The impact of various types of roughness on additional resistance and wake fields are computed and presented for the well-established test case KVLCC2. The surfaces included in the review are divided into three groups: 1) high quality, newly painted surfaces, 2) surfaces with different extent of poor paint application and/or hull coating damages; and 3) surfaces covered with light slime layers. The review shows the use of a variety of roughness functions, both Colebrook-type and inflectional with three distinct flow regimes, as well as a variety of strategies to obtain the roughness length scales. We do not observe any convergence within the research community towards specific roughness functions or methods to obtain the roughness length scales. The comparison using KVLCC2 clearly illustrates that disparities in surface texture cause large differences in additional resistance, and consequently no strong correlation to a single parameter, e.g. AHR (Average Hull Roughness). This implies that, to be able to select a suitable hull roughness model for a CFD-setup, more details of the surface characteristics are required, such as hydrodynamic characterization of hull coating and expected fouling

Biofouling Control Solutions for Reduced Energy Consumption in Vessels

Without effective fouling control coatings, the accumulation of marine growth on the hull will be slowing down the ship due to increased drag and hence increasing the fuel consumption and carbon footprints. The latter is an important endeavour to limit the global temperature rise according to international agreements (IMO,2009).Therefore there is a huge commitment on ship operations with particular interest in obtaining efficient hull coatings with optimal hydrodynamic performance not only as newly applied but also under “in-service” or real-life conditions. \ua0Within this context testing of new generation marine coatings necessitates the use of reliable experimental facilities to assess the hydrodynamic performance of marine coatings under the realistic testing environments.\ua0\ua0\ua0\ua0\ua0 This study overviews the available facilities which are integral part of experimental research into coating’s roughness and hydrodynamic characteristics, as well as stating the research plan for 1st phase of newly initiated project

Boundary layer and roughness characteristics of hull coatings

This study presents an experimental investigation conducted at the Emerson Cavitation Tunnel (ECT) of Newcastle University to investigate the boundary layer and surface roughness characteristics of three commercially available hull coatings: a tin-free SPC, a new generation Foul Release coating and a novel nanostructured coating. In addition, two coatings that have been artificially roughened by mixing in sand grit during application were included in the study. These coating were roughened in order to mimic either hull surfaces that have been a while in service or poor quality coating applications. In order to examine the boundary layer characteristics of the coatings, a large flat plane model with interchangeable test sections was used. A two-dimensional DANTEC Laser Doppler Velocimetry (LDV) system was used to collect the boundary layer data of each coating. The measurements provided critical parameters including local skin friction coefficients and roughness functions. The surface roughness of the tested coatings was analysed using both a non-contact laser profilometer and a stylus instrument. The tests and subsequent analysis allows to compare the hydrodynamic performance of these antifoulings. The measured boundary layer data were analysed by using different analysis methods to predict and compare the skin friction characteristics of these coatings

Frictional drag measurements of large-scale plates in an enhanced plane channel flowcell

This paper describes the design of an enhanced, plane channel, flowcell and its use for testing large-scale coated plates (0.6 m 0.22 m) in fully developed flow, over a wide range of Reynolds numbers, with low uncertainty. Two identical, hydraulically smooth plates were experimentally tested. Uniform biofilms were grown on clean surfaces to test skin friction changes resulting from different biofilm thickness and densities. A velocity survey of the flowcell measurement section, using laser Doppler anemometry, showed a consistent velocity profile and low turbulence intensity in the central flow channel. The skin friction coefficient was experimentally determined using a pressure drop method. Results correlate closely to previously published regression data, particularly at higher speeds. Repeated measurements indicated very low uncertainty. This study demonstrates this flowcell’s applicability for representing consistent frictional drag of ship hull surfaces, enabling comparability of hydrodynamic drag caused by surface roughness to the reference surface measurements

A multi-aspect study of commercial coatings under the effect of surface roughness and fouling

The paper presents an experimental study of surface and hydrodynamic characteristics for biocidal, non-biocidal antifouling coatings and hard coatings. In the study both smooth panels (Rt(50)≈40-90 μm) and panels with roughness commonly found on ship hulls (Rt(50)≈110-125 μm) were used. Measurements of drag were obtained from torque measurements in a rotating disk rig and the biocide content (Cu) of the coatings was measured using X-Ray Fluorescence instrument. The results show that Cu concentrations in coatings were sufficient to deter hard fouling such as by barnacles. The results from the drag measurements show that over the range of investigated test velocities, poorly applied antifouling biocidal coating with an induced roughness (Rt(50) = 116 μm) caused on average 13% higher moment coefficient () when compared to the same type of biocidal coating but with a smooth finish (Rt(50) = 43 μm). This paper demonstrates primarily how field-grown fouling affects the antifouling efficacy and drag performances of both biocidal and non-biocidal coatings. In addition, the paper also presents a correlation of drag and surface characteristics using a new scaling based on fouling pattern, fouling % cover and surface free energy

Review and Historical Overview of Experimental Facilities Used in Hull Coating Hydrodynamic Tests

The prediction of hydrodynamic performance of hull coatings with different surface conditions is a challenging task. Moreover, with the emergence of new prototype coatings that are relatively smooth in terms of roughness characteristics, the accurate estimation of their drag is particularly important, as this will enable a good grading of drag reducing benefits of coatings. In the context of coating studies, the experimental methods are considered as the backbone and results obtained from experimental facilities with the required performance will enable accurate scaling of test results to full-scale ship results. Although numerical simulations like Computational Fluid Dynamics (CFD) has acquired the level of accuracy good enough to replace some of the systematic model testing used for ship design optimisation, it is still not evident if the simulations will be able to replicate the physical reality such as coating type, its roughness and biofilms accurate enough to enable predictions of the power requirements for ships. Therefore, this paper gives insight into various coating hydrodynamic testing facilities and methods that are capable of measuring drag characteristics of coatings.\ua0 The work highlights the details of each method, identifies the concepts and parameters needed to describe, implement and analyse hydrodynamic coating drag measurements. The paper also summarises the merits and demerits of each type of facility based on reports and studies reported in open literature. Finally, the authors propose a recommendation that can be incorporated into the design of the new hydrodynamic facility