Computation of the Flow of a Dual-Stream Jet with External Solid and Perforated Wedge Deflectors for Noise Reduction

Abstract

We present a methodology for the computation of flow fields involving complex perforated surfaces in propulsion applications. Of particular interest is the treatment of perforated flaps used as wedge-shaped fan flow deflectors for reducing jet noise of a supersonic turbofan engine. A three-dimensional Reynolds-Averaged Navier-Stokes solver is used to compute the flow field of the external jet plume with solid and perforated deflector flaps, the latter with 50% porosity. Flow computation for perforated flaps presents a particular challenge and is handled by use of a localized body force model in the momentum equation. The study is conducted at two operating conditions: a cold condition at which mean velocity surveys were conducted and against which the computational code is validated; and a hot condition that corresponds to the takeoff engine cycle. The code predictions replicate adequately the mean velocity fields of the cold experimental flows. The code is then extended to the conditions of the actual engine cycle to study the impacts of the deflectors on the turbulent kinetic energy (TKE) distributions in the jet plume and on the aerodynamic performance of the nozzle. Consistent with findings of past studies, the solid flaps decrease the TKE on the underside of the jet and increase it at the top of the jet. The perforated flaps mitigate the TKE increase on the top of the jet, thus reducing the potential of excess noise from that region. The thrust loss of the perforated flaps is approximately one half that of the solid flaps.

Aerodynamic and Acoustic Optimization for Fan Flow Deflection

“…3 During noise-sensitive segments of the flight, the flaps could be deployed and the vane deflectors actuated. Computational predictions of specific-thrust loss for the entire engine (including the core stream) are 0.39% from the optimal vane arrangement and 0.53% from the wedge, 10 for a total thrust loss of 0.92%. During portions of the flight when noise reduction is unnecessary, such as cruise, the wedge flaps could be retracted and the vanes positioned at α = 0 o .…”

Body Force Model for the Aerodynamics of Inclined Perforated Surfaces

“…5. Past computations on solid FFD wedges have shown that the flow angularity at the measurement locations was typically less than 20 deg [16]. At these shallow angles, the error in the pitot measurement due to misalignment is less than 1% [18].…”

“…To facilitate the interpretation of the results of this study, it is useful to have an overall understanding of the aerodynamics of the FFD wedge, focusing initially on the solid case. These observations are based on past experiments and computations of the FFD wedge flow [15,16], and the experiments of Von Tanner on the drag of 2-D wedge cylinders [17]. We note that the FFD wedge acts on a flow bounded by a free surface: the shear layer between the fan stream and the ambient.…”

“…The net lateral force is zero, so the wake closes and forms a recirculation region. Because of the weak base suction of the FFD wedge, its drag coefficient is about 75% lower than that of the wedge cylinder [15,16].…”

Modeling of Wedge-Shaped Porous Flaps for Jet Noise Reduction

“…For a porous plate normal to a flow, the inertial resistance coefficient is primarily a function of porosity. 10 As a first approximation, the inertial resistance coefficient was obtained by calibrating the model against experimental measurements of drag coefficient for a perforated plate inclined normal to a freestream. 11 The coefficient of drag is plotted versus porosity in Fig.…”