Daisuke Sasaki

Adaptive mesh refinement and load balancing based on multi-level block-structured Cartesian mesh

ABSTRACT We developed a framework for a distributed-memory parallel computer that enables dynamic data management for adaptive mesh refinement and load balancing. We employed simple data structure of the building cube method (BCM) where a computational domain is divided into multi-level cubic domains and each cube has the same number of grid points inside, realising a multi-level block-structured Cartesian mesh. Solution adaptive mesh refinement, which works efficiently with the help of the dynamic load balancing, was implemented by dividing cubes based on mesh refinement criteria. The framework was investigated with the Laplace equation in terms of adaptive mesh refinement, load balancing and the parallel efficiency. It was then applied to the incompressible Navier–Stokes equations to simulate a turbulent flow around a sphere. We considered wall-adaptive cube refinement where a non-dimensional wall distance y+ near the sphere is used for a criterion of mesh refinement. The result showed the load imbalance due to y+ adaptive mesh refinement was corrected by the present approach. To utilise the BCM framework more effectively, we also tested a cube-wise algorithm switching where an explicit and implicit time integration schemes are switched depending on the local Courant-Friedrichs-Lewy (CFL) condition in each cube.

Reduction of the head-up pitching moment of small quad-rotor unmanned aerial vehicles in uniform flow:

Multi-rotor unmanned aerial vehicles are unstable in headwind because of nose-up pitching moment generation and thrust degradation. Reducing the pitching moment generation could enhance the tolerance of the unmanned aerial vehicles to wind and improve cruise flight speed. In this study, a canted rotor configuration was proposed to reduce the moment in uniform flow and examined. The objective of the study is to evaluate the effect of moment reduction by canted rotors. First, flow interactions between rotors of the quad-rotor were visualized using a smoke wire method. Second, the moment reduction by canted rotors was estimated based on isolated rotor performance. Third, the moment of the quad-rotor varying the body pitching angle was examined using a wind tunnel. At an 8u2009m/s wind, when the body pitching angle is horizontal, slanting the rotor by 20° to outside the body degraded the moment by 26% compared with the parallel rotor configuration.

Aerodynamic Analysis of NASA Common Research Model by Block-Structured Cartesian Mesh

In this study, the aerodynamic analysis of NASA Common Research Model (wing-body configuration) was conducted by a block-structured Cartesian mesh method named BuildingCube method. The building-cube method (BCM) has the advantages of the quick and robust mesh generation and efficient parallel computing, and it is easy to run on a large-scale computing system. However, the BCM suffers a restriction on the resolution of the turbulent boundary layer. The aim of this study is to improve the resolution issue of the turbulent boundary layer at high Reynolds number flows using the immersed boundary method. By utilizing the advantages of the BCM, we challenge to resolve the boundary layer by mesh subdivision. In this paper, the results of the validation before subdivision mesh were shown by using a coarse mesh. The computational results were compared with the transonic wind tunnel test and the results of another flow solver. Although the results showed a similar tendency with the results of another flow solver, the decomposed aerodynamic coefficient appeared with some discrepancy.

Towards Aerodynamic Characteristics Investigation Based on Cartesian Methods for Low-Reynolds Number Flow Simulation

Micro Aerial Vehicles (MAVs) are recently focused for various usage such as monitoring, photographing, and filming. One of the issues of MAVs is the limitation of operation time. An efficient configuration is required for MAVs, however, complex low-Reynolds number flows causes the difficulty. In this research, Cartesian-based CFD approach is applied to a flat plate, a NACA0012 airfoil, and a circular arc at low-Reynolds number flows to investigate the aerodynamic characteristics. Block-structured Cartesian mesh solver, Building-Cube Method, was capable to investigate the complicated flowfields at lower angles of attacks.

Robust airfoil optimization of human-powered aircraft against manufacturing variations

Robust design optimizations of an airfoil are conducted to increase the aerodynamic performance and to reduce the performance deterioration due to the manufacturing variations of a human-powered aircraft. Randomly-generated perturbed designs are evaluated for the robustness index. Maximum performance difference is used for robustness definition to prevent the worst performance deterioration. The approach can design less-sensitive airfoil against the manufacturing variations while improving the performance compared to initial airfoil. In addition, the essential parameter to control performance and robustness is discussed.

Design Optimization of a Mild-Stall Airfoil/Wing for UAV and PAV Applications

This paper describes a design methodology of a fixed-slat airfoil to achieve mild-stall characteristics as well as high cruising aerodynamic performance for Personal Air Vehicle (PAV) and Unmanned Aerial Vehicle (UAV). In our previous research, we demonstrated that a wing composed of NACA4415 airfoil and fixed-slat airfoil whose outline was designed based on NACA4415 can achieve preferable mild-stall characteristics. This unique characteristic is derived from the difference of the stall features of the two airfoils. This characteristic is desirable for PAV to prevent accidents due to stall. However, these wings cause higher drag because of the fixed slat. The objective of the study is to design a fixed-slat airfoil having high L/D at cruise condition while maintaining mild-stall characteristics. FreeForm Deformation is applied to modify both the slat shape and the airfoil. A fixed-slat airfoil was designed through Multi-Objective Evolutionary Algorithms coupled with Kriging approximation model. The fixed-slat airfoil was finally extended to 3D wing, and the performance was analyzed.