The effect of nozzle lip shape on screech tone is investigated by a computational aeroacoustic approach. First, the scattered patterns of screech tone are examined without a jet mean flow, where plane waves interact with a nozzle lip wall. Then, an axisymmetric jet screech is simulated in the Mach number range from 1.07 to 1.2, where the axisymmetric mode is a dominant screech mode. The conservative form of the axisymmetric Euler equations written in generalized coordinates are used to treat the complex nozzle lip geometry for the scattering problem as well as the Reynolds Averaged Navier-Stokes equations with the modified Spalart-Allmaras turbulence model. The computed shock-cell structure, screech tone frequency, and sound pressure levels in the near field are in good agreement with existing experimental data.
To investigate the feasibility of a highly efficient flapping system capable of avian maneuvers, such as rapid takeoff, hover and gliding, a full scale bird-like (ornithopter) flapping-wing micro aerial vehicle (MAV) shaped and patterned after a typical pigeon (Columba livia) has been designed and constructed. Both numerical and experimental methods have been used in the development of this vehicle. This flapping-wing micro aerial vehicle utilizes both the flapping and feathering motions of an avian wing by employing a novel flapping-feathering mechanism, which has been synthesized and constructed so as to best describe the properly coordinated flapping and feathering wing motions at phase angle difference of 90° in a horizontal steady level flight condition. This design allows high flapping and feathering amplitudes and is configurable for asymmetric wing motions which are desirable in high-speed flapping flight and maneuvering. The preliminary results indicate its viability as a practical and an efficient flapping-wing micro aerial vehicle.
We obtain a set of pointed wet delay observations at a total of 81 stations at 9 separate time epochs by ray tracing through the output fields of a Japanese numerical weather model with a grid spacing of 10 km. For each station-epoch, we invert the simulated data set, consisting of 4000 pointed delays, using an isotropic and an anisotropic delay model. The isotropic model has only one parameter-the zenith wet delay (ZWD). The anisotropic delay model of Chen and Herring has two additional lateral gradient parameters. We find that the anisotropic model leads to a much better fit between the delay model and the delay observations. On the other hand, the ZWD parameters estimated using the differing delay models are almost identical. This implies the ZWD estimates are not greatly perturbed by lateral heterogeneity of the atmosphere.
The long-term evolution, over 54 years, of a sample of objects released in geostationary orbit with area-to-mass ratios (A⁄M) up to 50 m2/kg was analyzed, taking into account geopotential harmonics (8×8), luni-solar perturbations, direct solar radiation pressure with eclipses and, when applicable, air drag. The results indicate that objects with A⁄M up to 25 m2/kg might explain the recently discovered debris population with mean motions of about one revolution per day and orbital eccentricities as high as 0.6. At so large area-to-mass ratios, the orbital evolution was mainly driven by solar radiation pressure. Although the general behavior observed was the same in all studied cases, the details of the evolution depended on the initial conditions. The simulated objects with A⁄M≤37 m2/kg were characterized by an orbital lifetime > 54 years, while for A⁄M>40–45 m2/kg, the exact value again depending on the initial conditions, the lifetime dropped rapidly to a few months with increasing values of the area-to-mass ratio. A growth of A⁄M had, as a consequence, a larger amplitude of the yearly oscillations that dominate the eccentricity evolution, in addition to a faster and wider orbit pole clockwise precession.
A simulation method for full helicopter configuration is constructed by combining an unsteady Euler code and an aero-acoustic code based on the Ffowcs-Williams and Hawkings formulation. The flow field and helicopter noise are calculated using a moving overlapped grid system, and the mutual effect of main rotor and tail rotor are studied for the helicopter in hover or forward flight. In the hovering flight calculation, the tip vortex of the tail rotor is dragged by the induced flow of the main rotor, and the detailed phenomena of the flow pattern are captured well. In the forward-flight calculation, noises from the main rotor and tail rotor are predicted to show tail rotor noise for both self noise and the interaction noise with the main-rotor wake. Comparison of noise magnitude shows the relative importance of tail rotor noise according to flight conditions.
To improve the fidelity of measured aerodynamic characteristics at high angle of attack for modern jet fighters, this paper examines the model wake blockage effect. The wake blockage effect in a 2.2×3.1 m low-speed wind tunnel is investigated by analyzing drag and wall pressure measurements. Circular flat plates of different sizes are used to simulate a test model at high angles of attack. The present analysis results in simple formulas for corrections of model wake blockage effect. To verify the present correction formula, the NASA TP-1803 model is force-tested in the tunnel. The corrected test data agree well with the NASA TP-1803 data.
This paper deals with a nonlinear optimal control approach to helicopter inverse simulation. The reference trajectory is prescribed in prior, and the integral deviation from this trajectory is treated as an additional penalty cost to convert the system optimality to an unconstrained optimal control problem. The resultant two-point boundary value problem has been solved by a multiple-shooting algorithm. The nonlinear helicopter model in this study includes main rotor flap dynamics and a dynamic inflow model. The applications cover the inverse simulation for bob up, turn, and slalom maneuvers. This paper focuses on resolving the convergence issue using the indirect method, the main root causes of which are related to the inherent system instability of the helicopter and with poor initial guesses on state and costate variables. For this reason we will investigate the effect of the shooting node number on convergence and use a hybrid-model approach, where the optimal state and costate variables, calculated using the linear model, are used as initial guesses for those using the nonlinear model. The analyses show good convergence history and capability of tracking the prescribed trajectory. So the results in this paper can provide a valuable motivation for applying indirect methods to nonlinear helicopter flight mechanic analysis.
This paper is based on the concepts of lagrangian Reynolds transport equation (LRTE), momentum, and the generalized total kinetic power of a rocket. The wood rocket model is used as a vivid model to explain propulsive efficiency of a rocket. We prove that the novel U-turn launch mode can convert total kinetic power and gravitational potential energy into rocket propulsive power. Furthermore, we use the first stage rocket engine of the Saturn V as an example to simulate and analyze its propulsive efficiency computationally with Matlab software. In conclusion, this revolutionary launch mode has the potential to dramatically improve the propulsive efficiency of launch vehicles.