A simulator that provides more insight into the aerodynamics of Mars could help design rotor blades for the next generation of Mars helicopters.
Following the success of NASA’s Ingenuity Mars Helicopter, engineers are looking for more optimized helicopters that can fly longer distances at higher altitudes, with a heavier payload.
The ambition is unequivocal, but the conditions on Mars are challenging. NASA says flights on Mars were prepared for atmospheric densities between 0.0145 and 0.0185 kg/m3 (equivalent to 1.2-1.5 percent of Earth’s atmospheric density), but that can drop to just 0.012 kg/m3† Decreases in density lead to a decrease in thrust margin, which can be compensated for with a higher rotor speed. Resourcefulness and platforms that follow should prevent rotor speeds from approaching the speed of sound, as that would lead to drag. This combination of atmospheric density and lower speed of sound means that simple modeling strategies will not yield accurate results.
To remedy this, engineers at Imperial College London have created a ‘virtual wind tunnel’ simulator that mimics the atmospheric conditions of Mars to test helicopter blade designs.
“We are combining the virtual wind tunnel capability with genetic algorithms to develop optimized airfoils for Martian rotorcraft,” said Professor Peter Vincent of Imperial’s Department of Aeronautics.
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By comparing their results with those from the real Mars Wind Tunnel at Tohoku University, Japan, they found that their simulations mimicked real Martian conditions with a much higher degree of accuracy than was previously possible. Imperial’s in-house hi-fi solver – PyFR– directly simulated the prevailing equations of motion on Mars and performed high-order direct numerical simulations over a triangular plane at Mach 0.15 and the Reynolds number of 3000. The team’s findings are detailed in AIAA Journal†
Wind tunnel experiments are well established, but have flaws; in this case mounting rotating blades in close proximity to tunnel walls that don’t exist for a real helicopter blade. Physical testing is time-consuming and costly, and Prof. Vincent added that simulation gives researchers access to all flow fields at any time, enabling detailed analysis of flow physics.
Whirlpools generated by the triangular wing. The unstable nature of the flow is evident (Imperial College London via GIPHY)
According to Imperial, the team has gradually increased the realism of their simulations. They showed that only when the full wingspan of the blade plus the wind tunnel walls were fully simulated did the results come close to the Tohoku experiments, in which researchers measured lift, drag and pressure distributions across the surface of a triangular airfoil.
In addition, simulations at Imperial predicted a significant change in blade behavior with angle of attack; at a certain incidence, the lift mechanism of the blade suddenly changes with the formation of a large vortex that acts to suck on the top surface of the blade, increasing lift.
The team has run its simulations on supercomputers and recently received a large allocation of computing resources on the UK’s largest GPU-enabled supercomputer (Cirrus at EPCC) to work on designs for a next-generation Mars helicopter.
