Wake Decay: Effect of Freestream Swirl

A study of the effects of freestream swirl on the decay characteristics of wakes shed from a rotating blade row is presented. The freestream swirl behind the rotor causes the wakes to skew tangentially, stretching the wakes as they are convected from the rotor to the stator. The effect of stretching on wake decay is illustrated using a simplified two-dimensional model. The model is described and the results are compared to 1) measurements from a two-dimensional cascade facility where no stretching or skewing of the wakes occurs; 2) solutions obtained using a three-dimensional, Reynolds-averaged Navier-Stokes solver; and 3) experimental wake measurements taken behind a low hub-to-tip ratio fan. For typical fan geometries with hub-to-tip ratios of approximately 0.5 and rotor-stator spacings of one to two rotor chord lengths, the wake can be stretched by over 50 percent. The stretching increases the mixing rate, which leads to a reduction in the mean wake velocity deficit of approximately 30 percent and a widening of the wake of about 15 percent. These effects account for much of the difference seen between cascade wake measurements and those taken behind rotating fan blade rows. It is therefore important to include the effects of stretching when using cascade data for prediction of fluid mechanic, acoustic, or structural phenomena associated with fan wakes. Finally, the study also suggests a potential for small (less than 3 dB) reductions in tonal noise because of wake-stator interaction through tailoring the fan loading distribution to produce particular spanwise wake decay characteristics. (Author)