Numerical Study of Passive Control of an Over-Expanded Nozzle for Fighter Aircraft

An experimental and computational study showed that a passive boundary layer control device in the form of a step incorporated just downstream of a nozzle throat can be used to provide good thrust recovery. The nozzle without the step produced an overexpansion of the supersonic °ow. The computational study was performed for a full-scale nozzle under actual engine exhaust and ambient atmospheric conditions. The original baseline nozzle °ow was highly over-expanded, to produce an extensive region of sub-ambient pressure along almost the entire extent of the divergent region. This caused complex shock, boundary layer interactions toward the nozzle exit. When a step was added near the downstream of the nozzle throat, it reduced the over-expansion, thereby yielding a better pressure recovery in the divergent region. The computed value of thrust coe±cient compared well against that of a 12 percent scale model. The data showed a 2.5 percent improvement in performance over the baseline.