The generation of negative entropy changes in the wake of a cylinder embedded in a sheared velocity flow

The Air Force Aeropropulsion Laboratory (AFAPL) found that in an instrumented advanced turbofan compressor, local adiabatic efficiency exceeded 100 percent near the hub section indicating negative entropy changes on the lee side of the compressor blade. Tests were conducted at a University of Tennessee Space Institute (UTSI) wind tunnel to determine the mechanism of these changes.

A cylinder embedded in a sheared velocity profile was used as a simple model to simulate the rotating compressor blade. By using variously instrumented cylinders, negative entropy changes were found in the lee side of the cylinder representing the downstream hub section of the compressor blade. By following trace gases, it was found that there existed a spanwise transport next to the cylinder on the lee side, and that this transport was due to the Von Kármán vortices shed from the cylinder.

The results show that the simple model of a cylinder in a sheared flow simulates the compressor blade very well. Analysis of experimental data shows that there exists a pressure gradient on the lee side of the cylinder which drives a spanwise flow. In addition, there exists a region of negative entropy changes in the area of the cylinder corresponding to the hub region of the AFAPL compressor blade which is due solely to the spanwise transport of energy.