Experiments

Commonly, experimental approaches to determine the effect of microstructures on surfaces involve tests in wind tunnels or fluid channels. This often restricts the range of the tests, or requires rescaling of the structures for fluid channels. Additionally, the measurements have to be very exact to detect the influence of the structure, so this part of research is very time consuming. Therefore a faster and conveniently more compact approach was taken with utilizing a Taylor-Couette system as a testing environment. Its theoretical background lies in the flow of an incompressible viscous fluid between two coaxial cylinders. In this test chamber, the rotation of the outer cylinder transfers a force through the fluid on the inner stationary cylinder, which leads to the measured torque. Due to the two variables gap between the cylinders and number of revolutions, the drag of different surfaces can be calculated depending on the measured torque. The gap width is slightly variable due to foil thickness and mostly close to 5mm. The number of revolutions changes within one measurement and is predetermined from the measurement engineer. Up to 4000 rpm can be reached, which equals a velocity of more than 30 m/s on the surface. Additionally, the temperature of the fluid has to be known, as the viscosity of the testing fluid is temperature dependent. The microstructures of interest can be evaluated via applying them to the surface of the cylinders, and the drag reduction is calculated from a comparison with measurements of smooth foils. The measurements are fully automated and therefore easily replicable, which is also the reason this test bench is certified to be used for measurements for the aviation industry.

Figure 1: Riblet Testbench

One important aspect of those measurements is that they should take place in the turbulent region, as for laminar flow analytical solutions are faster and cheaper. To ensure this behaviour even in low numbers of revolution, a tripping wire is introduced on both rotor and stator, which provokes vortex formation. This can also be seen in the CFD-simulations of the test chambers in the secondary flows present.

Figure 2: Fluid velocity inside the test bench as simulated with CFD