Computational investigation of micro helicopter near-wall effect

A key challenge faced by micro rotorcraft operating in very confined environments is disturbance effects that occur due to aerodynamic interactions between rotors and adjacent structures. This study uses computational fluid dynamics to model a micro rotor (Retip “ 50,000) hovering near to a flat vertical wall. The results show that a wall adjacent to a hovering micro rotor will induce two predominant wake asymmetry phenomena: Asymmetry in wake shape; and asymmetry in blade-tip vortex circulation strength. These wake asymmetry phenomena induce asymmetry in the flow field at the rotor disk which causes the lift force acting at each blade to vary with rotor azimuth angle. This lift asymmetry generates significant disturbance moments acting about the rotor disk’s lateral and longitudinal axes. Additional simulations have also shown that as the attitude of the rotor varies by ̆5o the disturbance moments vary linearly. The results presented in this study will serve as a fundamental basis for future real-time estimation of aerodynamic disturbances induced by a vertical wall, which will be used to safely control and guide a micro rotorcraft in confined environments. Introduction Micro helicopters are a class of rotary wing aircraft that have developed rapidly over the last 15 years [6]. Owing to their small footprint and Vertical Take-Off and Landing (VTOL) capability, they are particularly well suited for carrying out search and rescue, surveillance and reconnaissance missions within very confined environments. By definition, confined environment operations require micro helicopters to fly in close proximity to structures, such as walls, floors, ceilings, narrow ducts and passageways, etc. This poses a unique challenge, as aerodynamic interactions between rotor wakes and nearby structures can induce undesirable forces and moments at the rotor disk. Because micro helicopters have very small inertia, these forces and moments present a significant disturbance that can adversely affect performance and stability. In order to develop reliable micro helicopter systems, it is vital to understand micro rotor fluid-structure interactions so that adverse stability effects within confined environments can be detected and compensated for appropriately. An earlier study by the authors began to address this issue by modelling a micro rotor hovering near a vertical wall [8]. The results showed that the wall induces significant wake asymmetry which produces large disturbance forces and moments at the rotor disk. As an extension of this, the study outlined in this paper considers the effect of small variations in rotor attitude on fluid-wall interactions. This paper is structured as follows: computational methodology; validation of computational methodology; results; and concluding remarks. Methodology Computational Fluid Dynamics (CFD) has been chosen for modelling the flow effects in this study because it gives good physical insight into flow phenomena that is otherwise difficult to obtain using experimental flow visualisation techniques. The Figure 1. Domain geometry and mesh cross sections at rotor rotor geometry is based on a Blade Nano CPX micro helicopter [2] that has been modified for use as an experimental testbed. It has mass, m, of 36g; rotor radius, R, of 103mm; chord length, c, of 16mm; collective pitch, α, of 7.5o; rotor speed, Ω, of 3,000RPM and a corresponding blade tip Reynolds number, Retip, of 50,000. The rotor is modelled as a rigid body and the helicopter body and tail rotor have been neglected for simplification. The fluid domain geometry is cylindrical in shape with a height of 30R and a radius of 30R. An illustration of the domain geometry is provided in figure 1. The global coordinate system has been define with the x-axis toward the wall, the yaxis parallel to the wall and the z-axis aligned with the rotor axis of rotation. The computational grid has been generated using an unstructured tetra Delaunay method [1, 11] with prism layers at the blade surface and increased resolution in the blade tip, blade surface and wake regions. Cross sections of the mesh through the rotor are provided in figure 1. A moving mesh method has been used to model rotor rotation with a temporal discretisation of 1.8o of rotation per simulation timestep. The far-field boundary conditions have been prescribed using a source-sink method [10]. The flow throughout the fluid domain is solved using a second order implicit solver with a Spalart-Allmaras turbulence model [3, 4]. The presence of a wall within the fluid domain is modelled by forcing the flow velocity to zero within an immersed boundary region [5]. Validation of Computational Methodology A mesh independence study has been conducted to ensure that the results obtained are independent of the mesh resolution. Three simulations with a wall gap of 2c were run using three different meshes with varying resolutions. Based on a Richardson extrapolation estimate of error, it is estimated that the peak error present in the solution due to mesh dependency remains less than 1% throughout the flow.