Cascades (Fluid dynamics)

The interaction of rotor turbulence with the stator is currently believed to be the predominant mechanism of noise radiation from turbofans in aircraft engines. This thesis presents a general method to compute unsteady 2-D potential flows past a cascade of airfoils. The procedure uses source and vortex distributions on the surface of the airfoils, creation of wakes downstream of the airfoils and non-linear convection of the perturbed flow. These features are designed to satisfy a condition of no-flow through the surface of the airfoils and the Kutta condition at the trailing edge of each of these airfoils. The investigation proves the importance of applying the Kutta condition. It was also shown that an infinite cascade is well approximated by a small number of airfoils and that the non-linear rather than linear convection of vorticity has a large effect on the spectrum of the unsteady lift of an airfoil.

When a boundary-layer flow, either laminar or turbulent, encounters a hemispherical body extending from a surface, a horseshoe-shaped vortex forms at the juncture. In this thesis, we study the evolution of this vortex using a numerical inviscid model and laboratory experiments. The numerical model is based on determining the evolution of the filament using the cut-off method. The assumption is that although the generation of the vortex depends on viscous effects, the dynamic evolution is well described by inviscid equations of motion. It is found that the vortex filament is fairly steady on the upstream side but on the downstream side, travelling waves appear on it which cannot be suppressed through evolution. For a range of Reynolds number, steady horseshoe-shaped vortex was obtained in the experiments, revealing the shape past the hemisphere. This is compared with the numerical results.

The purpose of this research is to study the modification of a turbulent flow as it passes through a cascade of flat plates. The results will then be compared with experimental results obtained in a companion experimental study being conducted at Virginia Tech. In a typical marine propulsor turbulent flow passes through a set of inlet guide vanes (IGVs) and then interacts with the propeller blades: this process creates unwanted vibration and sound. The purpose of this research is to determine if the arrangement of the IGVs can be used to reduce the propulsor noise generation. In this study the incoming flow to the propeller is modeled as homogeneous turbulence and the IGVs are represented by a cascade of flat plates. We will consider the equations, which describe the blade response to an incoming harmonic gust, and we will represent the turbulent flow using a modal description.