Drag Production of Filamentous Biofilm

Abstract

The presence of significant hard and soft biofouling on the hull of a marine vessel can produce increased resistance potentially limiting system performance and leading to added fuel consumption. It has been estimated that the presence of biofouling has led to roughly $2.9 billion in additional operating costs and the resulting excess fuel consumed each year is expected to introduce about 23 million tons of carbon dioxide into the atmosphere. Therefore, it is important to understand how biofouling leads to increased friction drag for flows over marine surfaces with the ultimate aim of reducing their deleterious effects.

While a body of research exploring the consequences and means of drag production for hard fouling exists which informs the efforts of ship designers and operators, the effects and drag production mechanisms of soft fouling remain relatively unexplored. The few studies performed on soft biofilm layers show drag increases ranging from ~10—300% in lab scale experiments; ship trials showed that after removal of a biofilm layer from its hull, a frigate required 18% less shaft power to maintain a cruising speed of 25 knots. That such large drag penalties can be produced by low-form, `slimy' surfaces is a surprising result which researchers suggest may be due to additional physical mechanisms different from those of hard biofouling.

The primary objective of this dissertation was to disassociate the added drag produced by filamentous biofilm layers into contributions from roughness and compliance effects. Biofilm layers comprised of diatoms and filamentous green algae were grown on smooth acrylic surfaces for nominal incubation times of three, five, and ten weeks in a specially built flow loop. During hydrodynamics trials, biofouled surfaces were installed in a high-aspect ratio, fully developed channel flow facility and exposed to flows ranging from ReH ≈ 5 000 -- 30 000. Measurements of the pressure drop along each fouled panel revealed drag increases spanning ΔCf ≈ 14--364%. The wide range in drag penalty was linked to variations in flow speed, average thickness of the biofilm layers, and the percentage of each surface covered by fouling. These results support a previously proposed scaling correlation which also relates frictional performance of biofilm layers to their thickness and coverage. Empirical formulations were produced that characterized the added drag of stable biofilm layers within ±10% of their measured values.

Seven rigid surfaces replicating six biofilm layers were manufactured using highly resolved laser scans of the time-averaged, spatially filtered biofilm surface profiles. Drag penalties of ΔCf ≈ 57--193% were measured for these rigid surfaces and shown to scale with the average trough-to-peak roughness height and downstream spacing between large streamers. An empirical formula characterized the added drag on rigid replicas to within ±15% of measurements.

Finally, a simple model was proposed which decomposes the added drag experienced by a biofilm layer into contributions from roughness and compliance effects. Once applied to this model, resistance data show that about half of the added drag experienced by the biofilm layers was due to rough effects.

Particle Image Velocimetry measurements captured the mean flow along the surfaces. Results reveal that flow above biofilm layers and rigid replicas largely resemble one another in the outer flow region suggesting that mechanisms underlying the drag increase are confined to a region of flow in the immediate vicinity of the biofilm layers.