MRV Challenge 3—Overview and Initial Findings

The third installment of a global measurement challenge for pushing the state-of-the-art capability in magnetic resonance imaging (MRI)–based measurement technology in turbulent flows is presented. A fully turbulent water flow with Reynolds number of 15,000 based on the channel hydraulic diameter passes over building-like roughness elements in a square cross section water channel. The full elements align in the channel centerplane, with partial elements along both side walls at two alternating heights. The flow is steady and includes a second inlet along the floor in the wake of the third building element, which merges with the flow with an initial velocity that matches the bulk free stream. MRI-based measurement techniques include a comparison of three components of time-averaged velocities as well as a scalar—either concentration or temperature for all challenge participants. In this case, concentration measurements are presented from a combined team of researchers from Stanford University, the US Military Academy at West Point, and Oak Ridge National Laboratory. The results depict the neutrally buoyant isothermal flow as it mixes turbulently with the free stream and impacts the local velocity field. In addition, the concentration of a dilute copper sulfate solution from an injection port is tracked downstream as it interacts with the clean free stream flow and is entrained in wakes of the roughness elements, depicting a highly 3D flow field. Because the flow is nonreacting, it represents a passive scalar that can be matched with temperature-based measurements in flows with modest differences in temperature, forming the basis of an intriguing comparative activity. The outcome of the measurement campaign has utility in turbulent mixing and represents an additional capability of MRI-based measurement systems in turbulent channel flows.