Junction Flows Horseshoe Vortex Dynamics
When a boundary layer flow (either laminar or turbulent) encounters an
obstacle projecting from that surface, some distance upstream of the
obstacle the boundary layer separates as a result of the adverse
pressure gradient, and rolls up to form three-dimensional complex
vortices. These vortices have the characteristic shape of a horseshoe,
such that the legs of the vortices extend downstream, to both sides of
the body. Figure below is obtained through hydrogen bubble
visualization and shows the shape of the horseshoe vortex forming
upstream of a cylinder-plate juction.
Figure 1: Laminar horseshoe vortex forming upstream of the cylinder-plate junction. Visualization is achieved through hydrogen bubble technique in our water channel.
Horseshoe vortices form in many real scenarios, such as at wing-body junctions in airplanes, turbine blade-hub junctions, cooling flow past computer chips, and so on. Horseshoe vortices often have large effects on flow properties in the junction region, such as the local heat transfer, skin friction, and noise. For example, at the junction of an airplane wing with the fuselage, formation of horseshoe vortices and associated complex flow can dramatically impair the performance of the aircraft. It can result in an increase in local skin friction, noise, and drag. The drag induced by the wing-fuselage junction is estimated to be 10% of the total drag in modern civil airplanes. Hence, the design of wing-body junctions or empennage-fuselage junctions is important for aircraft makers. In the case of a bridge or pier suport, for instance, owing to the horseshoe vortices, local skin friction can cause severe erosion at the junction. Another situation, where horseshoe vortices have dramatic effects, is a turbine blade-hub junction. In this case, horseshoe vortices can affect increase the local heat transfer rates and thereby cause thermal gradients in the blade.
The present investigation consists of two phases:
In the first phase, the project investigates how the characteristics of the oncoming bounday layer affects the unsteady dynamics of horseshoe vortex systems. To this end, an experimental investigation is conducted involving cylinder-channel floor and cylinder-end plate junctions. By changing the leading-edge geometry of the endplates and the cylinder position from the tip of the endplate, effects both laminar and turbulent aproach flows with various characteristics are studied. Below, in Figure 2, one excerpt from many of our interesting findings is given. A journal publications is at the submission stage on this phase of the investigation. Interested fellows: stay tuned for further information.
Figure 2: Iso-contours of vorticity: Snapshots of the junction flow at the upstream of the symmetry plane are shown for cylinder-endplate junction in the first three columns and for the cylinder-channel floor junction in the last column. An increase in the thickness of the laminar boundary layer results in a gradual decrease in the formation frequency of horseshoe vortices. This effect can be distinguished visually by the increasing distance between the primary and secondary horseshoe vortex in this figure.
In the second phase, to reduce the adverse effects of the unsteady horseshoe vortex flows, modifications to the geometry of the wing/body will be examined. The project will specifically investigate the effects of adding simple flow control devices, such as fillets, leading edge adjustments, and fairings, on idealized wing junctions.