Swarmlet: a GPU-resident reformulated vortex particle method for coupled multirotor aerodynamics and flight dynamics
We present Swarmlet, a GPU solver for the unsteady aerodynamics and flight dynamics of one or many multirotor vehicles. The wake is a meshless, evolving-core vortex particle field in the reformulated (rVPM) large-eddy formulation, coupled to rotor models of three fidelities, six-degree-of-freedom vehicles and flight control. Particles carry a smooth core of compact support whose field outside four core radii is exactly the singular Biot-Savart field, so the multipole far field of the treecode carries no regularization error. The subfilter closure is a dynamic eddy viscosity on a fixed filter width, applied by core spreading with a Lagrangian exchange of its local excess under an explicit stability bound, and the strengths are relaxed by projection; global corrections make both the exchange and the relaxation conserve total vorticity and linear impulse. All rotor models shed connected bound circulation, the disks along the arcs swept in each step however long, so the wake receives the lift impulse. The solver agrees with direct summation to $5\times 10^{-7}$ and converges at third order in time. Without calibration, an actuator-line model that includes its own near sheet predicts the hover thrust of a DJI 9443 propeller within 2%. With calibrated section polars it reproduces a Crazyflie propeller's thrust and torque within 3.2% at three speeds and the near wake of a hovering quadrotor with relative $L^2$ differences of 0.08-0.21 against particle image velocimetry, while the wake keeps the delivered impulse. A closed-loop formation of 100 quadrotors, run at a coarser setting on one GPU, illustrates the coupled method at swarm scale.