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The deformation and breakup of water droplets impacted by a shock wave has been largely attributed to surface mechanisms. This study investigates the possibility of cavitation-induced droplet breakup. Shock waves of Mach 4 are used in this study to impact groups of droplets, both groups of degassed droplets and a group of non-degassed droplets. Distilled water droplets on the order of 1-3 mm in diameter are introduced into the shock tube. High speed images and deformation plots are used to explore the existence of cavitation in the droplets, as well as how they deform comparatively. 

This paper investigates the aeroacoustic interactions of small hovering rotors, using both experiments and computations. The experiments were conducted in an anechoic chamber with arrays of microphones setup to evaluate the azimuthal and polar directivity. The computational methodology consists of high fidelity detached eddy simulations coupled to the Ffowcs-Williams and Hawkings equation, supplemented by a trailing edge broadband noise code. The aerodynamics and aeroacoustics of a single rotor are investigated first. The simulations capture a Reynolds number effect seen in the performance parameters that results in the coefficient of thrust changing with the RPM. The acoustic analysis enables the identification of self-induced noise sources. Next, dual side-by-side rotors are studied in both counter-rotating and co-rotating configurations to quantify the impact of their interactions. Higher harmonics appear due to the interactions and it is verified that the counter-rotating case leads to more noise and a less uniform azimuthal directivity. Difficulties that arise when trying to validate small rotor calculations against experiments are discussed. Comparisons of computational and experimental results yield further insight into the noise mechanisms that are captured by each methodology. 

A multirotor trim module is developed for the HPCMP CREATETM-AV Helios rotorcraft simulation code. Trimmed free-flight simulation results are presented for two multirotor configurations, using rotor frequencies and aircraft attitudes as the control variables. The loose-coupling procedure is used to achieve trim, where aerodynamic loading on the rotor blades and fuselage are computed using a simplified aerodynamic model, and modified at each coupling iteration using the airloads computed by the higher fidelity CFD based aerodynamics. Two different optimization methods are tested: a least-square regression algorithm, with the norm of the loads at the center of gravity as the objective function, and a nonlinear constrained optimization code, with the total power as the objective function, and with constraints applied to satisfy trim. First, a commercial small-scale UAV is simulated in forward flight. A reference model for midscale UAM applications is then trimmed in hover to demonstrate the module’s ability to model and trim a complex configuration. 

Computational fluid dynamics (CFD) simulations of a small quadrotor were conducted using CREATETM-AV Helios. Two near-body CFD solvers and multiple turbulence models including transition models available in Helios were tested. The DJI Phantom 3 was chosen as a representative configuration because it has been studied extensively and is typical of commercial unmanned aerial vehicles. The airfoil at three-quarters span of the rotor geometry was extracted to perform both two-dimensional (2D) airfoil and three-dimensional (3D) wing studies in order to determine appropriate grid spacings for use with the various models. Isolated rotor simulations for DJI Phantom 3 rotor in hover utilizing appropriate grids were completed for fully turbulent and turbulence transition models. The predicted thrust from all of the methods lie within experimental uncertainty. The Spalart Allmaras model gave consistent results across the two CFD solvers and was most computationally efficient. As such it was chosen for the simulations of the full quadrotor performance in hover. The results indicate that a transition model is not required in order to obtain satisfactory thrust predictions as compared to experiment for a small quadrotor in hover using the Helios package. However, the figure of merit is underpredicted by both fully turbulent and transition models. Therefore, the effect of transition modeling on torque prediction needs further investigation. 

The acoustic far-field pressure is determined for one-dimensional finite-chord panels with uniform porosity in a single-sided uniform flow. The unsteady, non-circulatory pressure on the panel is computed using a previously established analysis method. The acoustic field is computed using the Green’s method. Results from this acoustic analysis identify the sensitivity of the far-field pressure magnitude and directivity to changes in flow Mach number, the reduced frequency of the panel vibration, and the panel porosity level characterized by a Darcy-type porosity boundary condition.