The Effect of Reynolds Number and Laminar Separation on Axial Cascade Performance

Abstract

Testing over a range of Reynolds numbers was done for three NACA 65 Profiles in cascade. The testing was carried out in the VKI C-1 Low Speed Cascade Wind Tunnel; blade chord Reynolds number was varied from 250,000 to 40,000. A semiempirical theory is developed which will predict the behavior of the shear layer across a laminar separation bubble. The method is proposed for two-dimensional incompressible flow and is applicable down to short bubble bursting. The method can be used to predict the length of the laminar bubble, the bursting Reynolds number, and the development of the shear layer through the separated region. As such it is a practical method for calculating the profile losses of axial compressor and turbine cascades in the presence of laminar separation bubbles. It can also be used to predict the abrupt leading edge stall associated with thin airfoil sections. The predictions made by the method are compared with the available experimental data. The agreement could be considered good. The method was also used to predict regions of laminar separation in converging flows through axial compressor cascades (exterior to the corner vortices) with good results. For Reynolds numbers below bursting the semiempirical theory no longer applies. For this situation the performance of an axial compressor cascade can be computed using an empirical correlation proposed by the author. Comparison of performance prediction with experiment shows satisfactory agreement. Finally, a tentative correlation, based on the NACA Diffusion Factor, is presented that allows a rapid estimation of the bursting Reynolds number of an axial compressor cascade.

Effects of free-stream turbulence intensity on laminar separation bubbles

“…Laminar separation bubbles formed over airfoils have been studied experimentally [9,18,31,32,40,41] as well as numerically [17,[42][43][44][45]. Separation bubbles can also be induced on a flat plate with imposed adverse pressure gradients both experimentally [7,46,47] and numerically [30,43,48,49].…”

Turbulence Amplification With Incidence at the Leading Edge of a Compressor Cascade

“…A short laminar bubble "bursts" into a long bubble when reattachment is not possible in the short state, see Fig. 3 (for a description of the flow regimes possible with varying Reynolds number see, Roberts, 1975). As can be seen from Fig.…”

“…5 where the turbulence required to match the data is indicated: Tu=8.5% for ,81=40°, Tu=9.0% for ,81 = 43.3° and Tu = 11.0% for ,81 = 46.4°. Not only does this indicate that at medium to high incidence the leading edge turbulence is amplified, but that (Roberts, 1975).…”

“…An inviscid-viscous scheme was used for the subsonic calculations (Martensen, 1959;LeFoll, 1965;Roberts, 1975). It was found that during the calculation the inviscid-viscous method predicted a "burst" laminar separation bubble very near the leading edge for /31 ~ 38°, while the data of Sanger and Shreeve (1986) indicated the presence of a short bubble.…”

“…The Reynolds number and freestream turbulence intensity (Tu = 1.5%) were taken from upstream test conditions as given by Elazar and Shreeve (1989). The inviscid-viscous method was calibrated by Roberts (1975) for short and long mid-chord laminar separation bubbles, and for freestream turbulence intensities between ",0% and 5%. The method has been successfully applied to short bubbles found near the leading edge of a large chord wing model (",1.2m) of an NACA 66 13-018 section, which was .experimentally measured by Gault (1955) in a low turbulence wind tunnel.…”