Due to variations in the global distribution of temperature and salinity in the oceans, the natural stratification of seawater is commonly acknowledged as a pervasive phenomenon worldwide, with regions of rapidly stratified seawater referred to as "thermocline" or "pycnocline". The generation of density gradients provides a prerequisite for the initiation of internal waves. As vessels and submarines navigate through these stratified environments, disturbances to the density-stratified flow field are induced due to the effects of buoyancy, thereby triggering the far-field propagation of internal waves. In recent years, research and discussions on the wakes of underwater vehicles in density-stratified flows have become relatively prevalent. However, analyses and summaries of the interference caused by free liquid surfaces in density-stratified flows remain unexplored. This study employs a two-dimensional cylinder model to investigate the regulatory effects of a free liquid surface on the wake of a two-dimensional cylinder in a stratified flow environment. A comparison is made between the force coefficients and far-field wake of the two-dimensional cylinder in the real thermocline environment and a homogeneous flow environment around the Froude number range of 0.1 – 0.5. The results indicate that the favorable adverse pressure gradient generated in the stratified environment can reduce the drag force on the cylinder. Despite the lower frequency, stratification is still capable of suppressing the vertical movement range of the wake at
The scholarly discourse on the stratified flow generated by the phenomenon of "dead water" has a longstanding history. The origins of this phenomenon can be traced back to the polar expedition led by the renowned explorer Fridtjof Nansen, who navigated the exploration vessel "Fram" (Walker, 1991). In polar regions, the vessel encountered phenomena such as a decrease in speed and loss of operational control. However, the analogous occurrence of the dead water problem is reminiscent of the existence of the thermocline. In contrast to the stratification resulting from the failure of prompt diffusion between low-density and high-density seawater in the dead water phenomenon, the oceanic thermocline arises from the uneven distribution of solar radiation, salinity, and sediment. The presence of the thermocline establishes a prerequisite for the generation of internal waves, which, in turn, attracts attention to the induced internal waves generated by ships or underwater vehicles as a source of excitation (Esmaeilpour et al., 2016).
