Quantitative criteria for designing thrust gates to mitigate jet-flap interaction noise

Abstract

Thrust gates have been reported as a promising concept to reduce jet-flap interaction noise. This manuscript proposes two quantitative criteria for the design of thrust gates based on time-averaged flow parameters that can be extracted from steady-state simulations. Computations were carried out by solving the explicit, transient, compressible lattice Boltzmann equation, while the acoustic far-field was obtained via the Ffowcs-Williams and Hawkings analogy. After being validated based on the truncation error and comparisons with measurements for the far-field acoustics, the simulation model was used in a parametric analysis of thrust gates, varying the width (W/DJ = 1,2,3,4) for a fixed depth B/CF = 1, where DJ is the nozzle exit diameter and CF is the flap chord. Also, two acoustic Mach numbers (Ma = 0.5 and Ma = 0.7) and two flap deflection angles (α = 7° and α = 30°) were considered. The results show that thrust gates with W/DJ > 1 and W/DJ > 2 do not reduce significantly the far-field noise in weak (α = 7°) and strong (α = 30°) jet-flap interaction configurations for both Mach numbers. In this context, the jet-spreading angle and pressure-load are proposed as criteria for designing thrust gates that are effective for noise reduction but with minimum cutout on the flap. Thrust gates designed according to the pressure-load criterion were quieter by around 0.89 dB (α = 7°) and 4.25 dB (α = 30°) for Ma = 0.5, and 0.94 dB (α = 7°) and 3.53 dB (α = 30°) for Ma = 0.7 in comparison with those designed following the jet-spreading criterion. Additionally, the pressure-load criterion was also shown to be effective in reducing the cutout on the flap.