To better understand the flow physics associated with shedding from marine hydrokinetic (MHK) foils, Sandia conducted an experimental study in the Penn State University Applied Research Laboratory. Our team tested MHK foil shapes in a free-stream flow to investigate their unsteady lift and trailing-edge shedding characteristics.
The experimental apparatus was designed so the force gauges (along with the soft beryllium copper attachment screws) were the only attachment points connecting the airfoil to the water tunnel frame to prevent any shorting paths that would bias the force measurements. To understand the details of the flow near the trailing edge of the foil, our team also performed particle shadow velocimetry (PSV) measurements.
The water tunnel was operated over six speeds between 3.0 m/s and 9.6 m/s. The inflow turbulence was very low due to the design of the nozzle design typical of water tunnels.* The unsteady lift was measured for both hyrofoils and was made nondimensional by normalizing the unsteady lift spectrum level and frequency by the chord length, water density, and free-stream velocity. If done properly, the nondimensional spectra at the various speeds should collapse into a single curve. Our results showed the good collapse that occurred for each foil.
Additionally, we see that MHK (Sandia foil) spectra is a higher level than the NACA 65 series foil. To understand why this is the case, the details of the flow field near the trailing edge will have to be examined. Perhaps the thickness or loading on the Sandia foil has an impact. The PSV results should add some insight here.
* A small boundary layer builds up on the tunnel wall near the ends of the foil. The boundary layer’s extent on each wall is ~5% of the span. Fillets on the end walls were investigated to evaluate their impact on the unsteady lift. This assessment is ongoing.