Investigation into the Vortex Formation Threshold and Infrasound Generation in a Jet Engine Test Cell
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis details an in investigation of two problems arising during the testing of a jet engine in a test cell, namely the formation and ingestion of vortices and the generation and propagation of infrasound. Investigation involved the use of computational fluid dynamic as well as analytical tools. The author extended the work of previous researchers by investigating the effect when a suction inlet in surrounded by four walls, (as it is in a test cell). A previously suspected but not documented small region of unsteady vortex was discovered to lie between the steady vortex and no vortex regions. The preferential attachment of the vortex, when formed, to a particular surface was investigated and a low velocity region near that surface has been proven as a possible cause. A cell bypass ratio > 90% was found to be necessary to avoid the formation of vortices in typical situations. Parametric studies (conducted cetaris paribus) on four different geometries and flow parameters were also conducted to determine how they affected the vortex formation threshold. Boundary layer thickness on the vortex attachment surface, upstream vorticity, size of suction inlet was found to have a direct relationship with probability of vortex formation whereas Reynolds number of flow was found to have an inverse relationship. Three hypotheses regarding the generation and propagation of infrasound in test cells were analysed. The first hypothesis states that the fluctuating of flow within the test cell led to a periodic fluctuation of pressure. The second hypothesis predicts a change in flow conditions can leads to a change in the acoustic reflection characteristics of the blast basket perforates. The final hypothesis proposes that changing engine location and size of augmenter, can lead to a reduction in the slip velocity between the engine exhaust jet and the cell bypass flow thus reducing the engine jet noise. The first hypothesis has been disproved using CFD techniques, although the results are as yet inconclusive. The second and third hypotheses have been proven to be potentially feasible techniques to be employed in the future. The changes proposed in the final hypothesis are shown to reduce the engine jet noise by up to 5 dB.