The influence of the convergent geometry at the parietal flow behavior in a water tunnel test section

Resumo

Water tunnels are generic equipment designed to promote controlled flow conditions in a test section. The controlled flow conditions include low turbulent intensity and a regular and uniform velocity profile. In addition, if the study concerns parietal flow, it is important an uniform flow at the wall. The present work aims to numerically evaluate the flow behavior at the walls of a water tunnel test section according to the convergent geometry. The convergent geometry before the test section plays a fundamental role in the equipment performance. The numerical model consists of the representation of a quarter of a square section convergent. The CFD tool employed was the OpenFOAM, v11. The hydrodynamic model consists of imposed velocity at the inlet and zero pressure gradient at the outlet. At the walls, the no-slip boundary condition and the symmetry boundary condition at the symmetry planes. The thermal model is represented by the inlet flow at the ambient temperature ( 25 °C) and the test section walls are evaluated at the imposed temperature of 100 °C. The PISO pressure-velocity coupling algorithm and the k-omega SST turbulence model were used in this model. For the advective terms of the Navier-Stokes and for the time integration, second order algorithms were perspectively used: upwind and Euler. For the geometry representation, the convergent was modeled using a polynomial function of order 3. The slopes at the beginning and at the end of the convergent shall be zero in order to avoid recirculation zones. The polynomial coefficients are derived from these premises and the overall geometrical dimensions, that is, the convergent length, inlet hydraulic diameter and the outlet hydraulic diameter. The ratio between the inlet and the outlet areas is 9. The proposed geometry promoted a low head loss, uniform velocity profile and consistent hydrodynamic and thermal boundary layers. Thus, the proposed geometry should be adequate not only for a uniform flow behavior at the center of the test section, but also at the walls.