The influence of hull boundary layers on waterjet intake performance

The influence of hull boundary layers on typical waterjet propulsion system intakes used in the fast catamaran ferry industry has been experimentally and computationally studied. Particular attention was paid to the effect of boundary layer thickness on flow quality at the pump impeller face. A scale waterjet intake model mounted on the side wall of a wind tunnel enabled performance to be determined experimentally. Surface mini-tufts, ingestion streamtube determination, skin friction measurements and velocity and thermal anemometry traverses were used to detail the flow. The most significant observation was the presence of a separation zone on the intake roof. Two complete sets of results were obtained: one with the wind tunnel's naturally thin boundary layer and another with an artificially thickened boundary layer. The latter showed more extensive flow separation. Computational work was conducted using the commercial finite element package FIDAP. Grid independence required a surprisingly large number of nodes (approximately 300,000). However, good agreement with experimental results was obtained with about 120,000 nodes. Waterjet intakes installed on fast catamaran ferries ingest thick hull boundary layers (hull boundary layer thickness is comparable with the intake diameter). Both experimental and computational results show that a thick hull boundary layer adversely effects the performance of such intakes. Therefore, inclusion of thick hull boundary layers in the modelling/ designing of waterjet systems is essential for realistic performance predictions. The intake performance under manoeuvring conditions (zero vessel speed) was also studied both experimentally and computationally. It was found that cut-water flow separation choked the inlet and severely limited the available manoeuvring thrust. Computational results for an increased cut-water radius indicated that substantial improvement is possible with no detrimental effect on cruise performance.