A Bernoulli pad uses an axial jet to produce radial outflow between the pad and a proximally located parallel surface, which may be either a mobile workpiece or a fixed wall. The flow field produces a force between the surfaces which depends upon their spacing h. The direction of this force is repulsive for large and small h, but two equilibria exist between these limits. The nearer equilibrium point (heq) is stable, and this distance is dependent on the direction and magnitude of the force... Show moreA Bernoulli pad uses an axial jet to produce radial outflow between the pad and a proximally located parallel surface, which may be either a mobile workpiece or a fixed wall. The flow field produces a force between the surfaces which depends upon their spacing h. The direction of this force is repulsive for large and small h, but two equilibria exist between these limits. The nearer equilibrium point (heq) is stable, and this distance is dependent on the direction and magnitude of the force the pad is required to apply. Increasing the flow rate increases the strength of the contactless grip, subject to cavitation or compressibility constraints, depending on the working fluid. Industry has created devices of this type to grip and transport a variety of workpieces without contact. The present research is inspired by the need to keep a submerged ship hull free of biofouling organisms. Preventative maintenance during idle periods of operations can improve efficiency while prolonging the original surface properties of the hull. The Bernoulli pad for this application is significantly larger and uses the surrounding water as the working fluid. In the present work, the flow field was investigated computationally and experimentally. Field tests were also performed to determine the ability of the device to mitigate biofouling. The computational work, which was validated with experimental results found in literature, indicates that a power-law relationship exists between heq and the inlet fluid power required to sustain this equilibrium spacing when each is appropriately scaled. This scaling is derived principally from the wall shear; an additional term incorporating the inlet Reynolds number is used to account for the force applied to the system. The relationship is valid over a range of forces acting on the system, geometric, and material properties. Major and minor geometry alterations provide insight to customizing pressure or wall shear stress profiles.The biofouling removal ability of a shear-based device was field tested on two submerged surface types, Garolite G-10 and AkzoNobel's Intersleek 1100SR. The latter is a fouling-release coating. Each surface was groomed at four frequencies along with a control group during a seven-week grooming study conducted in Narragansett Bay in Rhode Island. An image-processing algorithm was developed and used to assess the effectiveness of the various grooming protocols, along with direct measurements of chlorophyll a per surface area. The image-processing data showed that the grooming resulted in approximately 50% cleanliness on the Garolite at the end of the study whereas the Intersleek was continuously restored to nearly its initial clean state. Chlorophyll a data supported these overall conclusions. These results indicate that surface cleanliness can be maintained effectively on Intersleek using frequent shear-based grooming. The key to success is to match or supersede the critical wall shear stress of settled biofouling organisms whose adhesive strength is exponential in time.Particle Tracking Velocimetry (PTV) measurements were also taken on the flow field. This Lagrangian measurement approach uses an iterative particle reconstruction technique in combination with high seeding density to reconstruct a 4D (x, y, z, t) flow field. This 4D reconstruction allows the pressure field to be reconstructed using the Navier-Stokes equations. Various experiments have been conducted on confined radial outflow but PTV measurements are presented here for the first time. The PTV measurements were compared with computational results and while there is reasonable agreement in the velocity field data, there are discrepancies is the pressure field data. Recommendations are provided for future work that can reconcile these differences. Show less
