Prediction of Noise from Low Reynolds Number Rotors with Different Number of Blades using a Non-Linear Vortex Lattice Method

Abstract

The demand of micro air vehicles (MAV) with multiple rotors is increasing in both military and civil applications because of their versatility on various missions. However, the use of MAVs for some missions still has limited success because of their noise pollution. One of the main noise sources is aeroacoustic sound produced by the low Reynolds number flows around the rotors. There have been many previous studies about small-scale rotor systems of MAVs during the past decades, but they mainly focused on investigations of the aerodynamics rather than the acoustics. Several studies considering the acoustics have started recently. However, only steady loading forces computed by using Blade Element Momentum Theory (BEMT) were considered in the previous studies, and the noise from unsteady flow phenomena were not taken into account. The main objective of the current study is to investigate the noise mechanisms and further to find ways to reduce the noise levels in unsteady low Reynolds number flows. A Non-linear Vortex Lattice Method (NVLM) is used to simulate the unsteady low Reynolds number flow and model the corresponding noise sources. The tonal components of far-field noise is predicted by using an acoustic analogy based on Ffowcs Williams-Hawkings (FW-H) equations. These numerical methods are applied to low Reynolds number propeller and rotor cases, and validated upon experimental data. Then, they are used to investigate the influence of the number of blades on both aerodynamic and aeroacoustic performance and provide further insight into prominent sources of noise.

A Hybrid Non-Linear Unsteady Vortex Lattice-Vortex Particle Method for Rotor Blades Aerodynamic Simulations

“…The Γ method [22] is less robust since it strongly relies on the under-relaxation to achieve convergence and the method fails in the post-stall region. There has been some development of a non-linear vortex lattice method for rotor blades based only on the Γ method [49], later extended to propellers [70] and wind turbines [71]. Additionally, these methods use an arbitrary point at which the induced velocities are computed instead of using the fact that a blade section is a 2D incompressible thin profile with a lift coefficient slope ∂C l ∂α = 2π.…”

“…The UVLM has successfully been used to model a hovering rotor in a hover out of ground effect at a high Reynolds and low Mach condition [8], and it has also been applied to a smaller rotor [48] where the thrust coefficient was overestimated by 12%. Shortly after, non-linear UVLM (NL UVLM) with circulation coupling was used for the prediction of the aeroacoustics of low Reynolds number rotors [49]. Recently, Morency [25] applied the NL UVLM with alpha coupling to compute the heat transfer coefficient, but the hover simulations close to the ground were more challenging.…”

Fast and accurate quasi-3D aerodynamic methods for aircraft conceptual design studies

“…Recently, it was shown in ref. [48] than nonlinear VLM results can be successfully used to predict the base flow field with sufficient accuracy for aero-acoustic noise calculations for micro air vehicle rotors. The method used an iterative correction of the sectional circulation values based on a lookup table of aerofoil viscous XFOIL or CFD results, the wake being modelled as vortex particles rather than rings to reduce the computational effort.…”

Characterisation of boundary layer transition over a low Reynolds number rotor

“…The purpose of estimating the boundary layer thickness was to compare it to the height of the roughness, and to estimate U k the undisturbed velocity at the roughness height, to calculate the roughness Reynolds number, Re k . The pressure over the surface of the rotor was calculated using XFOIL [27] for the local Reynolds number based on local radial speed and the effective aerodynamic angle of attack (α eff ) for the corresponding radial position and rotation speed was calculated using a Non Linear Vortex Lattice Method (NLVLM) which should take into account some of the 3D effects present, the method is explained in detail in Jo et al [28]. Using the pressure obtained from XFOIL (calculated with an N-Factor of 7), the δ was calculated using ONERA's in house boundary layer code 3C3D, which solves Prandtl's boundary layer equations for three-dimensional boundary layers using a method of characteristics along local streamlines.…”