This research analyzed the effect of tip clearance on counter-rotating ducted fans for vertical take-off and landing in unmanned aerial vehicles using commercial computational fluid dynamics tools. The computational results were verified using the subsonic wind tunnel at Hanyang University. In this study, the tip leakage flow of the counter-rotating ducted fan was analyzed in order to enhance the fan performance. The thrust coefficient decreases with increasingly larger front rotor tip clearance due to the increase in tip leakage flow. The power coefficient is influenced by viscous and tip leakage losses from the large tip clearances for the rear rotor. Smaller figures of merit are captured at larger tip clearances for the front and rear rotors because the direction of vortex core changes quickly. In conclusion, to improve the aerodynamic performance of counter-rotating ducted fans, it is necessary to reduce the mass flow rate across the tip clearance and tip vortex loss.

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In this paper, uncertainty quantification approaches are compared quantitatively in an aerodynamic uncertainty quantification problem for a 2D supersonic biplane airfoil. Three advanced uncertainty quantification approaches are compared: an inexpensive Monte-Carlo simulation approach using a Kriging response surface model, an intrusive polynomial chaos approach, and a point collocation non-intrusive polynomial chaos approach. Two-dimensional inviscid compressible flow around the supersonic biplane airfoil is considered with an uncertainty of the freestream Mach number as a normal distribution. A choking phenomenon occurs in this problem setting, which gives discontinuous changes in aerodynamic performance with fluctuation of the freestream Mach number. The accuracies and characteristics of the three uncertainty quantification approaches are investigated. The inexpensive Monte-Carlo simulation approach shows the best performance with larger numbers of sample points in this study. The results of the non-intrusive polynomial chaos approach are sensitive to sampling strategies. Although the intrusive polynomial chaos approach is applied only with lower orders of polynomial chaos, it shows comparable accuracy with the other two approaches from the viewpoint of model accuracy when weighted at the center region of the uncertain input space.

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Spatially high-order flow simulations are conducted for high-Reynolds number flows around two-dimensional high-lift devices. The method uses a ‘flux-reconstruction (FR) approach’ that is applicable to unstructured quadrilateral or hexahedral grids. This is the first study focused on solving Reynolds-averaged Navier-Stokes equations coupled with k-ω turbulence model equations using the FR method. The performance of the turbulence model in the high-order scheme is first verified using standard benchmark problems. The flow around the three-element, high-lift airfoil known as NHLP/L1T2 is then tried. Simulations from second-order (solution polynomial degree 1, or p=1) to fourth-order (p=3) accuracy all predicted the surface pressure well, while the total pressure distribution in the wake was captured well by p=2 and p=3 simulations. The effects of new wall boundary conditions and minimum cell size are qualitatively discussed.

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Instability of the flow on a rotating disk is governed by linearized disturbance equations of the partial differential with respect to the radial distance from the rotation axis and the normal distance from the disk surface. Applying uniform suction from the surface brings a small parameter associated with displacement thickness of the circumferential velocity profile into a dimensionless form of the equation system. Two kinds of series solutions expanded by the powers of this parameter are obtained to describe the cross-flow and centrifugal instabilities of the flow having a twisted velocity profile. The leading terms of the series solutions are determined from two eigenvalue problems of slightly different ordinary differential equations, and the superposition of those equations leads to an eigenvalue problem applicable to multiple-instability characteristics of such three-dimensional boundary layers.

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Space rescue is of crucial importance when sudden accidents occur during manned orbital flights. Various rescue plans have been proposed for returning crews to the Earth. However, some plans require several days to get the crew to a specified location, and some can only direct the crew to a vast sea area within a short period of time. This paper presents a rescue scheme using a low lift-to-drag ratio vehicle that can send the spacecraft crew to a designated landing point within 48 hours. The scheme presented combines two measures, namely a return trajectory maneuver and multiple-revolution orbital phasing, to fulfill the requirements of urgent deorbiting. The return trajectory maneuver extends the deorbit window, avoiding possible losses in system reliability induced by delays in space, while the orbital phasing steers the space emergency rescue vehicle to a selected deorbit point at a given time. A numerical simulation is done to verify the rescue scheme proposed. The results show that the scheme requires little re-entry maneuverability, few fuel consumption, and only one on-ground rescue site, which significantly simplifies the rescue system and reduces rescue cost.

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An exact three-dimensional elasticity solution for buckling of a circular cross-section beam with one end fixed and the other end free is obtained. A comparison is made using the approximate solution by Kardomateas and an exact two-dimensional solution by Chattopadhyay and Gu. The buckling loads of both exact solutions are in good agreement with each other, but the buckling load using the approximate solution is slightly higher than the exact solutions. The exact solutions support Engesser's formula rather than Haringx's formula for the buckling load of a beam with transverse shear effect.

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