A streamlined form of underwater towed body is considered here for the experimental and CFD studies to verify its stability when towed in submerged condition. Its physical characteristics lead to different behavior in the hydrodynamic tow conditions. Influence of lifting fins and tow arrangements on the operational depth and stability of the tow body are studied. The main variants associated with the body, and which affect the tow characteristics, are identified as basic body shape, tow point location, fin size for stable tow and dynamic trim due to forward speed. In order not to disturb the flow around the sensor location, the fin is placed further away towards the lower aft end. Additionally, the stable tow is validated and demonstrated in the towing tank at IIT Madras using the scaled model. The drag and lift force values obtained from model tests are extrapolated to the prototype and compared with CFD results.
The wavenumber–frequency spectra (WFS) of turbulent wall pressure fluctuations are used to design underwater acoustic systems, and a method is presented to determine them for flat plate flow using large-eddy simulations (LES). First, the Reynolds averaged Navier–Stokes (RANS) analysis is carried out for flow over the plate using k–ω shear-stress transport model. Flow parameters are extracted from the output of this analysis and used to impose boundary conditions for large-eddy simulations using a smaller domain with finer mesh. Mesh convergence studies are done to establish the adequacy of the proposed meshing scheme for estimating the turbulent boundary layer wall pressures. The stream-wise velocity profiles, turbulence intensities, and power spectra obtained using LES are compared with other computational and experimental results. The time history of fluctuating pressure on the wall at various stream-wise locations is used to estimate the WFS. Estimations are made of the convective ridge as well as sub-convective and low wavenumber portions of the spectrum. The computations are performed at momentum thickness-based Reynolds numbers (Rθ) of 5761, 7988, and 7709. The effect of downstream distance on the WFS is studied by computing it at three downstream locations and shown that the downstream distance has little effect on the WFS once the flow has become fully turbulent. The use of a desktop workstation for the estimation of WFS has not been reported earlier. The results show that the WFS of turbulent pressure due to flow over more complex geometries can be estimated using a similar method.
The underwater towed system described here consists of tow cables, a towed body, an acoustic module and tail rope towed behind a surface ship. The required depth at a particular speed of the towing ship is obtained by paying out specified length of cable from the winch. However the excessive drag forces on the various components of the towed system results in impractically large values of cable length, especially at higher speeds. A hydrodynamic depressor is designed to improve the depth performance. The design is evolved based on numerical analysis and towing tank tests. Estimation of depth attained is carried out based on steady state theory of tow cables. Validation of the numerical analysis results is carried out through field evaluation of depressor performance during sea trial of the towed system.
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