Turbulence Intensity in the Exit Boundary Layer of Nozzles at Subsonic Conditions

Characteristics of a nozzle exit boundary layer (BL) are known to impact jet plume development and noise. They are also critical inputs for accurate numerical simulation. In past experiments, hot-wire (HW) measurements were performed just downstream of a variety of nozzles. When the BL was turbulent, e.g., achieved by a BL trip, the turbulence intensity was frequently found to be surprisingly low. Peak intensity (u’max) was found to be in the range of 4%-7% of the mean velocity in the core of the jet. This contrasts with a vast literature on flat plate BL that reports peak intensity in the range of 10%-14%. This study considers possible measurement issues with the nozzle data. Effects of compressibility, measurement location, frequency response of the HW, flow acceleration, etc. are ruled out. Invoking BL results from the literature and through an analysis of the nozzle data it is inferred that spatial averaging due to the length of the HW is the primary reason for the observed suppression of the peak intensity. A complimentary Large Eddy Simulation (LES) of a BL over a flat plate enables a clearer understanding of the spatial averaging effect.