Flow Enhancement using Surface Protrusions

Small surface protrusions aligned with the direction of flow, often called Riblets, are one of the few techniques that have been successfully applied to the reduction of the skin friction in turbulent boundary layers. Different geometries of Riblets including triangular, notched-peak, sinusoidal and U-shaped Riblets, have been tested in wind tunnels and have shown that drag reductions over flat plates could be obtained of the order of 10%. Although more often used for aerodynamic purposes, small surface protrusions have also been used for a number of other industrial applications, such as in oil channels where are larger Riblet dimensions could be used for improved control of the geometry, with extensive tests being conducted on blade-shaped and trapezoidal-grooved Riblets.

ESI has recently been involved in the determination of how improvements to the flow quality may be obtained with three dimensional surface protrusions, with the studies carried out in ESI’s Tabulator Evaluation Facility (TEF tunnel). The TEF test section is 4 ft x 4 ft and 11 ft long with excellent optical access. Three geometrically different surface protrusions were constructed on large plates and installed in the floor of the TEF and evaluated using both thermal anemometry and particle image velocimetry (PIV). Figure 1 shows the three types of surface geometries that were investigated for aerodynamic drag improvement and flow control. 

Three different test surfaces with three-dimensional protrusions

Figure 1: Three Different Test Surfaces with Three-Dimensional Protrusions

The drag of these types of surfaces were determined from an analysis of the boundary layer, law-of-the-wall, measurements using both a single and crossed hot wire probe (a single hot wire probe is shown in Figure 1). In addition to thermal anemometry, a Laser light sheet PIV system, Figure 2, was used, along with 5 µm seed particles to determine the effect of the outer boundary layer along the length of the surface roughened plates, some 7ft downstream of the leading edge.

Diagrammatic sketch of PIV system

Figure 2: Diagrammatic Sketch of PIV System

Typical particle distribution along the surface roughened plates

A typical single image of the particle distribution, obtained with this PIV system and a high-speed video camera, is shown in Figure 3. 

In-house propriety software was used to evaluate the location of the particles and velocity vectors determined from a sequential image, with the flow results of each plate configuration being compared to that of a smooth flat plate.

Figure 3: Typical Particle Distribution Along

the Surface Roughened Plates