Blockage effect on the drag of an axisymmetric underwater body in towing tank tests

Towing tanks are regularly used to measure the drag of underwater vehicles. Depending on the ratio of cross-sectional area between the vehicle model and the towing tank, blockage effects can be present that can significantly impact the accuracy of the measurement. To address blockage effects, blockage correctors have been developed over the years for surface ship experiments in towing tanks, and bluff bodies and aircraft in wind tunnels. However, the accuracy of these blockage corrector formulae is yet to be established for underwater vehicles tested in water channels. This paper investigates the blockage effects for an axisymmetric underwater vehicle model tested within a water channel with different blockage parameter ratios using Computational Fluid Dynamics (CFD).

Existing captive-model experimental datasets are used to validate the CFD model. The validated numerical model is then extended to investigate the relationship between the blockage parameter ratio, the velocity of the model, and its predicted drag. Results show that the blockage effect acting on the deeply submerged model is independent of Reynolds number for the speed range considered and the percentage increase in the drag coefficient is linearly proportional to the blockage parameter ratio. CFD predictions are then compared with ITTC and Wind Tunnel blockage correction formulae, and the applicability of modified Tamura’s and the Wind Tunnel formula in correcting the blockage effect are demonstrated. It is intended to extend these findings to fully-appended underwater vehicle models tested in towing tanks.