Staff publications (SATM)

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Browsing Staff publications (SATM) by Publisher "American Institute of Aeronautics and Astronautics (AIAA)"

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    Generalized time-to-go inversion guidance with impact time and angle constraints
    (American Institute of Aeronautics and Astronautics (AIAA), 2024) Wang, Pengyu; Lee, Chang-Hun; Cho, Namhoon; Wie, Bong
    A major challenge in homing guidance is that accurately predicting the time to go for missiles is generally difficult, which limits the practical application of a certain class of guidance laws with impact-time constraints or cooperative capabilities. To address this issue, a generalized method referred to as the time-to-go inversion guidance is presented, where the time to go is first specified as an explicit function and then the corresponding guidance law is inversely derived. By employing this new method, a variety of guidance laws and their globally accurate time-to-go solutions can be developed, regardless of whether the impact-angle constraint is considered or not. Different from all existing studies, the proposed method addresses the problem of time-to-go estimation without relying on any linearizing approximations or numerical iterations. To enhance the practicality of the proposed method, it is augmented with bias commands to result in either the impact time control guidance law or the impact time and angle control guidance law. Numerical simulation results demonstrate the higher accuracy, larger feasible solution region, and lower computational demand of the proposed guidance laws even for the missiles with nonconstant speeds.
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    Shock-induced fan cowl separation during aeroengine windmilling at diversion from cruise
    (American Institute of Aeronautics and Astronautics (AIAA), 2024-11-13) Sabnis, Kshitij; Babinsky, Holger; Boscagli, Luca; MacManus, David; Sheaf, Christopher
    When a civil aircraft engine is operated at windmill during the cruise flight phase, there is supersonic flow acceleration around the leading edge of the fan cowl toward the external surface. The terminating normal shock wave can separate the turbulent boundary layer developing on this external surface. A series of experiments at a flight-relevant Reynolds number (1.2 million based on lip thickness) are performed in a quasi-two-dimensional wind tunnel rig to investigate the underlying flow physics. At a nominal inflow Mach number of 0.65 and a nacelle incidence angle of 4.5 deg, as the equivalent engine mass-flow rate is reduced, an increase in shock strength results in flow separation when the shock exceeds Mach 1.4. Over a 10% range in the notional engine mass-flow rate, the boundary layer developing on the external fan cowl thickens by a factor of three on the onset of separation. A reduction in the incoming Mach number from 0.65 to 0.60 weakens the shock wave and thus delays separation. An increase in surface roughness has no significant effect in situations where the boundary layer remains attached. However, for separated cases, an increased local roughness height causes a greater separation extent and a thicker boundary layer downstream of the shock wave.
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    Unsteady swirl distortion in a short intake under crosswind conditions
    (American Institute of Aeronautics and Astronautics (AIAA), 2024-12-31) Piovesan, Tommaso; Zachos, Pavlos K; MacManus, David G; Sheaf, Christopher
    Under crosswind operating conditions, the flow field of an aero-engine intake can be characterized by notable unsteady flow distortion. These distortions are typically associated with flow separation within the intake as well as with the ingestion of the ground vortex. This unsteady flow distortion can have a detrimental effect on the intake performance and potentially on the operability of the downstream compression system. Measurements of the unsteady velocity field within a model-scale intake under crosswind conditions were acquired using stereo particle image velocimetry (S-PIV). This work analyzes the S-PIV data to quantify the unsteady flow distortion, as well as the characteristics of the ingested ground vortex, in a short intake under crosswind conditions. The swirl distortion metrics were calculated for a range of crosswind velocities and intake mass flow capture ratios (MFCRs). The conditions at which the intake flow separates depend on crosswind velocity, ground clearance, the design of the intake, and the MFCR. Flow characteristics of both low MFCR diffusion-driven and high MFCR shock-induced separation were identified. The circumferential extent and intensity of the swirl distortion are strongly dependent on the crosswind velocity and mass flow rate. The swirl distortion caused by the diffusion-driven separation is greater than that due to the shock-induced separation. The diffusion-driven separation affects a larger portion of the intake aerodynamic interface plane with greater time-averaged and peak distortion levels compared to shock-induced separation. The ground vortex characterization at the aerodynamic interface plane showed a decreasing level of unsteadiness in vortex meandering with increasing MFCR.