The ceiling effect is a significant factor for unmanned aerial vehicle (UAV) missions that require close flight in the proximity of top planar boundaries such as inverted docking, perching, and flight in a confined space. In this study, the ceiling effect on a 5.5-inch propeller of a small UAV and a 9-inch of a medium-sized UAV was modeled and simulated using computational fluid dynamics (CFD) with multiple reference frame (MRF) method. The result was then validated by an experimental method at propeller speeds ranging from 5000 to 9000 RPM. For the near-ceiling region, computational fluid dynamics and experiment setup results for the propeller thrust were compared with the Cheeseman and Bennett model, the Hayden model, and also using the Conyers model. The results showed a close agreement between the computational fluid dynamics and experimental results, as well to the theoretical models, with some errors. Further, the validated CFD results were used to investigate the extreme-near ceiling region, which is not currently covered by the theoretical models. Our contribution in this study wishes to provide more understanding of such effects in the extreme near ceiling on a single-rotor model by using the velocity streamlines, thus possibly expand to multi-rotor unmanned aerial vehicles (UAVs) of different sizes in future research.
The flight of the Stardust descent capsule is examined. The possibility of simplified calculation of the capsule flow is studied using the perfect gas model when varying the specific heat ratio in the NERAT-2D software. This assumption makes it possible to quickly obtain the accurate flow pattern over a certain altitude range, but often it underestimates the heat flux to the capsule surface.
The lifting force exerted on particles in shear flows can significantly affect the admixture distribution in near-wall gas/liquid-particle/droplet flows. For small particle Reynolds numbers based on the relative velocity, shear rate, and particle angular velocity, the expression for the lifting force was first obtained by P. Saffman in 1965 and corrected in 1968. In this study, we use a modified (extended to finite Reynolds numbers) expression for the lifting force proposed by R. Mei in 1992. In the framework of the one-way coupled two-fluid model we consider three examples of laminar gas/liquid-particle flows of boundary layer type with inertial inclusions: the two-phase Blasius problem on a horizontal and a vertical plate, gas-particle flow in a near-wall dusty plane jet, and inertial-particle focusing in an axisymmetric profiled microchannel. In all examples considered the lifting force exerted on the particles plays a crucial role and results in lateral migration and focusing of the dispersed admixture. The calculations of the particle concentration in the Blasius problem are compared with available experimental data. The study is based on the two-phase boundary layer approximation and the Full Lagrangian approach (FLA), proposed in previous publications of the authors. This approach makes it possible to calculate all particle parameters, including the number density, from the solution of systems of ordinary differential equations on chosen particle trajectories. The flow structure in the regions of crossing particle trajectories and the locations of caustics can also be studied by this method. The results obtained may be of interest for the development of technologies which use near-wall dilute two-phase flows and focused particle beams.
A nonlinear stability analysis of thermal convection in an inclined fluid layer with a temperature-dependent internal heat source is performed using the energy method. The effects of the angle of inclination of the fluid layer, the internal heat source parameters, and the Prandtl number on the critical thermal Rayleigh number are investigated. The region in parameter space for which the basic state remains stable to arbitrary perturbations is identified. Comparison of the nonlinear stability boundary with the corresponding linear instability boundary reveals an uncertain stability region, which depends on the angle of inclination of the fluid layer and the internal heat source parameters.
The main objective of this study is to characterize thermal mixing downstream of a T-junction through which two fluids flow at different temperatures inducing cyclical thermal stresses. The generated turbulent mixing is frequently observed in many industrial applications, particularly in nuclear field. The inevitable temperature variations in fluid circuits can induce cyclic thermal stresses in the walls, particularly, at the points where the flows at different temperatures meet and in areas of thermal stratification. This study aims to estimate the predictive performance of the CFD (computational fluid dynamics) code used to model the turbulent mixing phenomenon, with a view to contributing to its validation. The use of numerical simulation makes it possible to analyze the physical mechanisms involved in detail and to extrapolate the results beyond the experimental configurations available. For this purpose, ANSYS-CFX code was used with the LES (large eddy simulation) model. The numerical predictions were compared with reference experimental data from the Vattenfall installation. A good agreement between the two results was found. This work contributes to the understanding of the thermal mixing phenomenon in T-junctions and shows the predictive capability of the CFD codes to predict thermal hydraulic parameters and the responses of the plant performances.
