A Method for Reducing Sonic Boom Strength by Tailoring the Shape of the Propulsive Streamtube

Abstract

Over the past decade, Gulfstream Aerospace Corporation has published on several methods for tailoring the shape of the nacelle for reducing its profile and contribution to the sonic boom signature of an aircraft traveling at supersonic speed. Through computational and experimental campaigns, these design methods have been shown effective at reducing the strength of the nacelle's compression features. However, even heavily attenuated shocks can remain unacceptably strong for aircraft constrained by an ultra-quiet sonic boom design requirement. This particular challenge was the catalyst for Gulfstream's recent development of a concept for removing all remaining significant shock features created by the nacelle, an outcome impossible using traditional design methods. The technique uses purpose-tailored extensions of both the inlet compression and nozzle expansion surfaces to precisely control the shape of the propulsive streamtube fore and aft of the nacelle. Careful concurrent matching of the nacelle geometry permits tangential lofting of the streamtube onto and from the nacelle surface. The method is predictable, adaptable, and extremely effective at nearly eliminating discrete compression and expansion features generated by the propulsion system at the design operating point while being robust against off-design operating conditions. Along with sonic boom strength, the nacelle's profile drag is reduced by as much as 70 percent. Computational fluid dynamics analyses have proven the concept to be an important component of a portfolio of ultra-low sonic boom design technologies. Included in this paper is a general discussion of the benefits of the shaped streamtube methodology applied to an inlet-nacelle-nozzle system sized for a 15,000 lbf thrust class turbofan engine operating at a local Mach number of about 1.65. The inlet in this example employs axisymmetric relaxed isentropic compression, while the nozzle features an axisymmetric expansion plug and dualannular high-flow bypass exhaust. Selected CFD-based solutions are included, and challenges associated with the use of a shaped streamtube propulsion system are discussed. Nomenclature AOA= angle of attack CFD = computational fluid dynamics Cp = pressure coefficient GAC = Gulfstream Aerospace Corporation MFR = mass flow ratio, the ratio of actual intake flow to maximum theoretical flow MOC = method-of-characteristics

Adjoint-Based Mesh Adaptation and Shape Optimization for Simulations with Propulsion

“…Only the minimum-bound geometric constraints prevent the spike from disappearing completely. This spike geometry is very similar to the design presented by Conners et al 1 Thrust is reduced by 2.7% compared to the benchmark case. Furthermore, the shocks emanating from the nozzle lip region are stronger than those in the first design problem, perhaps so much so that they could affect the ground pressure signature despite airframe shielding.…”

“…We consider a notional shrouded supersonic nozzle design to demonstrate shape optimization with mass-flowrate constraints and multiple inflow boundaries. This case is roughly based on the high-flow bypass nacelle design presented by Conners et al 1 and studied by Heath et al 38 to minimize nacelle effects on sonic-boom noise. Figure 16 shows the baseline geometry, which consists of a traditional axisymmetric spike nozzle that is mounted inside a shroud.…”

“…For example, simulations with active inlets and exhaust plumes are required to accurately capture the complex near-body shock system of low-boom supersonic aircraft to predict both aerodynamic performance and ground noise carpets. [1][2][3] Similarly, computational studies of high-bypass fanjet installations must accurately resolve the propulsive stream-tube to capture the changes in the induced drag due to the non-planar lifting system and the interference effects of transonic wave drag. 4,5 Another important example is efficient simulation of helicopters and multi-rotor vehicles, where capturing the rotor interference effects is essential to predicting aerodynamic performance.…”

Viscous Aerodynamic Shape Optimization with Installed Propulsion Effects

“…In addition, conceptual design often omits viscous aerodynamic effects which can have a detrimental impact on installed propulsive performance and vehicle sonic boom loudness levels. For example, prior research [5][6][7] has indicated that adding viscous or propulsion effects into a pre-optimized low-boom airframe signature can greatly compromise sonic boom performance.…”