Numerical modelling of the aerodynamic interference between helicopter and ground obstacles

Abstract

Helicopters are frequently operating in confined areas where the complex flow fields that develop in windy conditions may result in dangerous situations. Tools to analyse the interaction between rotorcraft wakes and ground obstacles are therefore essential. This work, carried out within the activity of the GARTEUR Action Group 22 on “Forces on Obstacles in Rotor Wake”, attempts to assess numerical models for this problem. In particular, a helicopter operating in hover above a building as well as in its wake, one main rotor diameter above the ground, has been analysed. Recent tests conducted at Politecnico di Milano provide a basis for comparison with unsteady simulations performed, with and without wind. The helicopter rotor has been modelled using steady and unsteady actuator disk methods, as well as with fully resolved blade simulations. The results identify the most efficient aerodynamic model that captures the wakes interaction, so that real-time coupled simulations can be made possible. Previous studies have already proved that the wake superposition technique cannot guarantee accurate results if the helicopter is close to the obstacle. The validity of that conclusion has been further investigated in this work to determine the minimum distance between helicopter and building at which minimal wake interference occurs

Validation and Comparison of RANS Propeller Modeling Methods for Tip-Mounted Applications

“…Modeling of the propeller in a RANS simulation by sources of momentum and energy instead of resolving the propeller blades, has been shown to reduce the cost of the propeller-wing interaction problem [12]. These sources are added in a time-averaged sense with an actuator-disk (AD) model, but they can also be introduced in a time-accurate sense with an actuator-line (AL) model, as has been applied for wind turbines [13] and helicopter rotors [14]. In an actuator-line model, the propeller blades are replaced with distributions of momentum and energy sources along lines representing the blades.…”

Numerical Capture and Validation of a Massively Separated Bluff-Body Wake

“…This is critical, as flows over obstacles can produce wakes of aerodynamic significance to bodies in proximity. These interactions concern a variety of engineering disciplines; such as the aerodynamics among building clusters due to the convection of natural wind [1], the handling qualities and flight simluation of rotorcraft encountering building or ships wakes [2,3], or even the controls for unmanned aerial vehicles under the influence of wind conditions induced by urban environments [4][5][6]. These examples demonstrate that the effects of a wake encounter emanated from nearby sources can induce unfavorable-or even dangerous-conditions to aerodynamics and affects engineering design.…”

“…This is observed to occur as close as two characteristic lengths downstream [13,14]. As bodies of interest can be located beyond this [2,3], accurate capture of wake physics at these distances may be unsuitable for these techniques. This inadequacy can be further exacerbated under massively separated conditions, despite good accuracy for surface force integrals.…”

Simulation of the aerodynamic interaction between rotor and ground obstacle using vortex method

“…To date, numerical investigations [3][4][5][9][10][11][12], ranging from simple blade element vortex methods to Navier-Stokes-based CFD, have been employed to study the aerodynamic interaction between rotorcraft wake and obstacles. The aim of the numerical simulation is to find the best method that one can use for simulating such flow.…”

“…More recently, within the GARTEUR AG22 group, several methods had so far been assessed. These included pure Eulerian, grid-based methods, ROSITA [10] and HMB [11], coupled with actuator disk and unsteady actuator disk models, that could resolve with good accuracy the loads on the rotor blades and the near-field of the helicopter but required large grids and CPU time to propagate the helicopter wake away from the rotor. Other methods, like pure Langrangian methods, including the unsteady panel method (UPM), that was based on the potential flow equation for representing the blade loads and on free-wake models for resolving the far-wake of the helicopter [12].…”