Numerical Modelling of Rotorcraft Brownout
Brownout dust cloud simulations were conducted for rotorcraft undergoing representative landing maneuvers, primarily to examine the effects of different rotor placement and rotor/airframe configurations. The flow field generated by a helicopter rotor in ground effect operations was modeled by using an inviscid, incompressible, time-accurate Lagrangian free-vortex method, coupled to a semi-empirical approximation for the boundary layer flow near the ground. A surface singularity method was employed to represent the aerodynamic influence of a fuselage. A rigorous coupling strategy for the free-vortex method was developed to include the effects of rotors operating at different rotational speeds, such as a tail rotor. For the dispersed phase of the flow, particle tracking was used to model the dust cloud based on solutions to a decoupled form of the Basset–Boussinesq–Oseen equations appropriate to dilute gas particle suspensions of low Reynolds number Stokes flow.
Fig. 1: A helicopter encountering brownout conditions during a landing maneuver.
(Image courtesy: US Army AFDD)
Fig. 3: Schematic depicting the different mathematical models used to predict rotorcraft brownout.
Fig. 2: Schematic of the different aerodynamic and sediment mobilization mechanisms.
Brownout is a phenomenon that often occurs when rotorcraft operate over ground surfaces covered with mobile sediment material, such as loose soil, gravel or sand. Through a series of complex fluid dynamic uplift and sediment mobilization mechanisms, the rotor wake stirs up a dust cloud that can quickly engulf the rotorcraft. Photographs of a rotorcraft encountering brownout conditions during a landing in the desert are shown in Fig. 1, with a schematic shown in Fig. 2. The equivalent phenomenon under snowy conditions is referred to as whiteout.
The main concern during brownout occurrences is that the pilot loses visibility of the take- off or landing zone and may also experience spurious sensory cues from the relative motion of the dust cloud. From a broader perspective, brownout is an example of operating in a Degraded Visual Environment (DVE) wherein the dust cloud can develop to sufficient concentrations such that it degrades or completely eliminates the optical cues, thereby posing a safety of flight issue. In the absence of these visual cues, the relative motion between the rotorcraft and the motion of the dust cloud can produce vection illusions, which can lead to the pilot experiencing spatial disorientation and leading to mishaps.
Various mathematical models and numerical techniques are coupled to represent the various aspects of the brownout simulation. Figure 3 shows the various components of the overall methodology used in the present work. The methodology is are broadly classified into the techniques required to: 1. Model the aerodynamic environment (blue), 2. Mobilize and convect the sediment particles (yellow), and 3. Expedite computations (orange). The sequence of steps in the simulation is as follows:
The aerodynamic environment of both the main and tail rotor was modelled using a Lagrangian free-vortex method (FVM). A three-dimensional source panel method was used to model the fuselage.
A sediment tracking and mobilization algorithm (which included modelling of key elements such as particle bombardment) was used to mobilize, eject ant track the particles in flight.
Graphics proceesing units (GPUs) were used to expedite computations to reduce wall clock time.
Presented here are a small sample set of the results from the different aspects of the numerical models.
Figure 4 - Wake behind multiple rotor rotorcraft configurations in forward flight.
Figure 5 - Differences in the wake structure of single main rotor with and without a tail rotor in hover.
Figure 6 - Typical development of a dust cloud as a rotorcraft lands highlighting the key features of the cloud.
Figure 7 - Schematic of a approach-to-touchdown maneuver.
Figure 8 - Complex interacting wake system and the resulting dust cloud of a side-by-side rotor configuration.
Fig. 4: Rotor wakes of different rotor configurations in forward flight.
Fig. 5: Structure of the wake as obtained from a free-vortex method with and without the presence of a tail rotor out of ground effect.
Fig. 7: Schematic of the approach to touchdown maneuver.
Fig. 6: Development of the dust cloud as a helicopter (main rotor and fuselage) performs an approach to landing maneuver.
Fig. 8: Development of the dust cloud as the ground vortex of a side-by-side rotor impacts the sediment bed.
Govindarajan, B., and Leishman, J. G., "Prediction of Rotor and Rotor/Airframe Configurational Effects on Brownout Dust Clouds," Journal of Aircraft (Accepted by yet to be published. This paper when presented at the 70th Annual Forum of the American Helicopter Society was adjudged the best paper in the Operations session).
Syal, M., Govindarajan, B., and Leishman, J. G., "Mesoscale Sediment Tracking Methodology to Analyze Brownout Cloud Developments," Proceedings of the 66th Annual American Helicopter Society Forum, May 10-13, Phoenix, AZ, 2011.
Govindarajan, B., "Contributions Towards Understanding the Effects of Rotor and Airframe Configurations on Brownout Dust Clouds," PhD Dissertation, Department of Aerospace Engineering, University of Maryland, College Park, MD, 2014.