In this paper, we consider the evolution of decaying homogeneous anisotropic turbulence without mean velocity gradients, where only the slow pressure rate of strain is nonzero. A higher degree nonlinear return-to-isotropy model has been developed for the slow pressure–strain correlation, considering anisotropies in Reynolds stress, dissipation rate, and length scale tensor. Assumption of single length scale across the flow is not sufficient, from which stems the introduction of length scale anisotropy tensor, which has been assumed to be a linear function of Reynolds stress and dissipation tensor. The present model with anisotropy in length scale shows better agreement with well-accepted experimental results and an improvement over the Sarkar and Speziale (SS) quadratic model.

Autonomous underwater vehicles play an essential role in geophysical data collection, deep water mining, seafloor mapping, ocean exploration, and in many other related activities starting from military to scientific applications. A detailed understanding of hydrodynamic characteristics will lead to better design, better control, and optimal path planning of autonomous underwater vehicles in the deepest corner of oceans. This article will provide a detailed review of the hydrodynamic characteristics of autonomous underwater vehicles, starting from different experimental techniques used in the analysis of hydrodynamic parameters, methods used for fixing the autonomous underwater vehicles in towing tank, instruments used for measurement of the hydrodynamic parameters. Furthermore, numerical methods employed in performing computational analysis, hydrodynamics-based shape optimization, studies on drag reduction, and finally a detailed list of turbulence models used in the computational fluid dynamics–related numerical simulations. The hydrodynamics-based optimal shape of the autonomous underwater vehicles, the best technique to predict the hydrodynamic parameters, and the best turbulence model for the computational fluid dynamics–based prediction of hydrodynamic parameters will be recommended. At last, the hydrodynamic characteristics of different bio-inspired autonomous underwater vehicles are discussed.

This article presents experimental and numerical studies on the effect of free stream turbulence on evolution of flow over an autonomous underwater vehicle (AUV) hull form at three Reynolds numbers with different submergence depths and angles of attack. The experiments were conducted in a recirculating water tank and the instantaneous velocity profiles were recorded along the AUV using Acoustic Doppler Velocimetry (ADV). The experimental results of stream-wise mean velocity, turbulent kinetic energy(TKE) and Reynolds stresses were used to validate the predictive capability of a Reynolds stress model (RSM) with the wall reflection term of the pressure strain correlation. From the high fidelity RSM based simulations it is observed that in presence of free stream turbulence, the pressure, skin friction, drag and lift coefficients decrease on the AUV hull.The variation of the hydrodynamic coefficients were also plotted along the AUV hull for different values of submergence depth and angle of attack with different levels of free stream turbulence. The conclusions from this experimental and numerical investigation give guidance for improved design paradigms for the design of AUVs.

This paper presents experimental and numerical analysis of grid generated turbulence with and without the effects of applied mean strain. We conduct a series of experiments on decaying grid generated turbulence and grid turbulence with mean strain. Experimental data of turbulence statistics including Reynolds stress anisotropies is collected, analyzed and then compared to the predictions of Reynolds Stress Models to assess their accuracy. The experimental data is used to evaluate the variability in the coefficients of the rate of dissipation model and the pressure strain correlation models used in Reynolds Stress Modeling.For both models we recommend optimal values of coefficients that should be used for experimental studies of grid generated turbulence.

Improved designs for Autonomous Underwater Vehicles (AUV) are becoming increasingly important due to their utility in academic and industrial applications. However, a majority of such testing and design is carried out under conditions that may not reflect the operating environment of shallow water AUVs. This may lead to imprecise estimations of the AUV's performance and sub-optimal designs. This article presents experimental and numerical studies carried out in conjunction, to investigate the hydrodynamic characteristics of AUV hulls at different Reynolds numbers over sloped channel-beds. We carry out experiments to measure the velocity field and turbulent statistics around the AUV with quantified uncertainty. These are contrasted against corresponding flat bed experiments to gauge the effect of test bed topography on AUV performance.The experimental data was used to validate Reynolds Stress Model predictions. Hydrodynamic parameters such as drag, pressure and skin friction coefficients were predicted from the RSM simulations at different test bed slopes, angles of attack and drift angles of the AUV hull, to analyze the hydrodynamic performance of the AUV. The results presented in this article offer avenues for design improvement of AUVs operating in shallow environments, such as the continental slope and estuaries.The exploration of ocean and to interpret its underwater behavior is of importance in todays world. To examine the ocean bed, convenient accessories are required to minimize the presence of human operators underneath of ocean. Autonomous Underwater Vehicles (AUV) are automated vehicles that are capable of underwater locomotion. These have applications in a variety of fields. For example, in the commercial field, oil and sub-sea drilling companies use AUVs for the purpose of checking the appropriate oceanic area where drilling may be optimally beneficial. For research and exploration, AUVs are used to track reefs and other life-forms that exist underwater. Additionally, AUVs have found applications in military and academic fields as well, besides others[1].The initial AUV designs were just modifications on existing designs of submersible torpedoes [2]. However, with the new found applications of these vehicles, focused investigation into their optimal design have become more critical. For instance, there is a demand for AUVs that can execute missions of the order of weeks and months. This requires that the AUV design be as optimal as possible, necessitating a careful analysis of the hydrodynamic performance of the AUV structure. Several experimental and numerical studies are available in the literature to analyze the hydrodynamic parameters around AUVs [3,4,5,6,7,8,9,10,11,12,13,14,15].Mansoorzadeh and Javanmard [3] have studied the effect of free surface on drag and lift coefficients of AUV at different submergence depths and compared the experimental data with the computational fluid dynamics (CFD) simulations. It was observed that hydrodynamic coefficients were very much responsive to the submergence...