A Numerical Investigation of the Impact of Biofouling on Turbulent Boundary Layers
Abstract
It is well established today that biofouling on ships increases the hull roughness, causing in-creased frictional resistance and fuel consumption, as well as decreased top speed and range. An-tifouling (AF) coatings have been widely used to control the problem but they contain biocides which are toxic to marine organisms and may impact non-target species. The latter resulted in aworldwide ban on these coatings for all vessels. Their primary replacement, copper-based paints, are not as effective in controlling fouling. As research on non-toxic alternatives is ongoing there is a pressing need to develop predictive tools that can assess the impact of various types of fouling and coverage level on the frictional resistance of naval vessels. This way quantifiable and cost/efficient hull cleaning decisions can be made. As of today most scaling laws and correlations integrated in the models utilized to make cost/efficient hull cleaning decisions have been informed by canonical roughness experiments involving rough surfaces comprising of uniform distributions of cylinders, hemispheres, frustums, pyramids etc. In topologically complex rough surfaces, however, there is a lack of understanding of the underlying physics, and the effects of the particular topography on existing scaling laws and correlations remain uncertain. Recently the advent of 3D scanning and printing enabled us to thoroughly analyze such surfaces and compute: roughness height statistics (average, skewness, kurtosis, etc.); slope-based parameters characterizing the waviness of the surface; surface based quantities (frontal surface area and the windward surface area) originating from the type and distribution of roughness elements, to name a few.A number of experimental studies, although not specific to biofouling, have been focusing on the effects of such parameters on quantities such as the equivalent sand grain roughness, which is defined as the size of sand grains giving the same skin friction as the roughness being evaluated. This measure of roughness is practically a common currency in evaluating rough surfaces and is widely used in closure models for a variety of numerical prediction codes, as well as for many empirical correlations based on experimental results. Computations are less frequent but have tremendouspotential to lead the way in model development. The main challenge in this case, however, is the geometric complexity and non-smoothness of the surface. This has an adverse impact on accuracy and stability of typical boundary-?fitted solvers rendering their use impractical. The advent of immersed-boundary (IB) methods can address these issues. Among others, our group over the past15 years has developed robust IB based tools, applicable to eddy resolving simulations of turbulent flows, making these computations well within reach.The primary goal of the proposed computational work, which will be conducted in close coordination to ongoing experimental work (groups in NSWVCCD Bethesda and Naval Academy, Annapolis), is to identify geometric parameters on surface roughness topologies found on US Navy ships that encapsulate the drag-producing physics. From a practical perspective we would like to be able correlate these parameters to the measurements that pier side divers can obtain (i.e.,) percent coverage and type of biofouling), facilitating reliable predictions of the biofouling drag and its associated energy costs.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 24, 2019
- Source ID
- N000141912107
Entities
People
- Ilias Balaras
Organizations
- George Washington University
- Office of Naval Research
- United States Navy