Roughness Effects of a Model Seabed on Flow Around an Underwater Vehicle

In this work, we report an experimental investigation of the roughness effects of a model seabed on the flow around an underwater vehicle. The seabed is modeled by the bottom wall of a test flume with or without an array of roughness elements. These roughness elements consist of an array of hemispheres with an approximate pitch-to-height ratio of 3. The underwater vehicle is also modeled by the hull of a scaled DARPA submarine. The vehicle is mounted on the model seabed and subjected to an open channel turbulent flow of Reynolds number 2.7 x 10⁴ (based on the free stream velocity and the body length). Particle image velocimetry is used to obtain detailed velocity measurements of the flow upstream of the vehicle, over the vehicle, and downstream of the vehicle with or without roughness elements on the model seabed.  Mean velocities and higher order moment turbulent statistics are assessed to determine the effect of the roughness elements. The results indicate significant increments in mass flux, momentum flux and streamwise turbulence intensities due to the presence of the wall roughness.  The flow is also marked by anisotropic Reynolds stresses with shear components that are muted behind the vehicle.  Overall, these results have implications on the kind of turbulence models suitable for simulating the flow, as well as the quantified drag associated with flows with such wall roughness effects.