Vibrations and Acoustics

Today there are strong environmental pressures to reduce the aerodynamic noise that is generated by helicopters, a subject that has already been introduced in Section 8.19. The sources of noise on a helicopter are numerous, but a significant contribution is the

localized impulsive loading of the rotor blades as a result of wake-related В Vis (see Sec­tions 8.16.4 and 10.4.2). There are also strong marketing pressures to improve ride comfort for crews and passengers alike and to reduce maintenance costs. This has driven much re­search to reduce the vibration levels of current helicopter designs. From a modeling point of view, these problems are similar in that they require very high-resolution predictions of the |

unsteady loads on the rotor blades and careful consideration of the structural dynamics of the complete helicopter as a system. In many situations a significant source of noise and vi – ?

bration originates in the aerodynamic coupling between the various rotors of the helicopter. ‘

The situation is particularly severe when one rotor operates directly within the wake of 5

another (for instance, in a helicopter with conventional configuration, the tail rotor might be 1

operating within the wake generated by the main rotor – see Fig. 11.26). Because the wake from the one rotor usually takes a significant amount of time (in terms of rotor revolutions) ]

to convect to the position of the other rotor (see, for instance, Section 11.4), the acoustic and vibration effects induced by rotor-rotor interactions are particularly challenging from a CFD point of view. This is because even a small level of contamination of the solution jj

by numerical viscosity will distort the evolution of the wake and reduce the strength of the

resultant В Vis. This has the effect of downgrading the accuracy of the calculated acoustic radiation or high frequency vibration dynamics of the rotor(s). 1

Figure 14.16 shows the results of a coupled rotor-fuselage calculation of the fidelity 1

necessary to form an input to a helicopter acoustic prediction. The tandem rotor case shown |

in this case is particularly challenging from an acoustic prediction perspective because each f

rotor is partially immersed in the wake of the other, and the equal size of both rotors makes і

the calculation particularly sensitive to the detailed geometry of the wake generated by j

the entire system. The need for acoustic fidelity means that the aerodynamic interactions j

between the rotors must be fully represented, and the interaction-induced distortion to both I

U I

wakes can be clearly seen. Notice though that to reduce the cost of the computation to

tractable levels, the fuselage can be represented using a surface singularity method such as j

Figure 14.16 Fully unsteady VTM calculation showing the aerodynamic interactions be­tween the fuselage, forward and aft rotors of a generic tandem helicopter as required for acoustic or vibrations prediction. Image is shown looking upward at the helicopter from below. Source: Richard Brown and Imperial College, University of London.

described in Section 14.7 compared to a direct RANS or other CFD type of calculation. This is a good example of the careful blending or “fusion” of two types of computational tools to achieve aerodynamic fidelity while still retaining a practical level of computational cost for engineering purposes.