Design of a high-speed, low-turbulence water flume with initial application to free surface turbulent wake flow

• Water flumes crucially support a wide range of research topics involving hydrodynamics and free-surface flows. • The design details of a high-speed, low-turbulence water flume are reported. • An initial application reveals suppressed/delayed vortex shedding near the unsteady free-surface. Despite the substantial progress of numerical methods in fluid mechanics, the complex multi-scale, multi-phase nature of turbulent flows continues to necessitate high-fidelity experimental data. As such, the development of a free-surface water tunnel (i.e. flume) capable of delivering high-speed, low-turbulence, free-surface flows is essential for fundamental research for marine applications. We report the design specifications and performance of a water flume recently constructed at the University of Iowa. The flume delivers high-speed (1.6 m/s), low-turbulence (<1.6%) uniform flows in an 8 m test section. The superior performance in delivering and maintaining well-conditioned flows over an extended test section is due to the properly designed upstream and downstream diffusers and the flow conditioners within. An additional high volumetric rate fluorescent dye visualization system was developed and employed for applications that challenge advanced laser-based measurement, particularly near the free-surface. The capability of the flume and associated instrumentation are demonstrated through an on-going investigation of the turbulent wake flow behind a surface-piercing 2D triangular wedge, revealing near free-surface coherent structures such as delayed vortex shedding and vortex-coupled, air-entrained tubes at high Froude numbers. The reported design details, in particular, the extended test section and downstream diffuser, will guide researchers developing similar hydrodynamic facilities for fundamental research.