Dynamic mode decomposition-based modal regulation and unsteady wake mechanism of the main wing for ground-effect flying cars

To accurately characterize the nonlinear unsteady aerodynamic characteristics of the main wing induced by ground effect during near-ground takeoff and landing of flying cars, this paper employs time-resolved particle image velocimetry (TR-PIV) and dynamic mode decomposition (DMD) to conduct flow-field measurements, reduced-order modal analysis, and aerodynamic performance correlation on the main-wing mid-span section and wingtip-vortex section of a tandem-wing flying car. The dynamic modal regulation mechanism of ground effect on the wake vortex system is systematically revealed. Results show that DMD can accurately decompose the flow field into quasi-static mode, drift modes, and conjugate modes. Ground effect does not change the global stability of the flow field, but significantly reshapes the modal energy distribution and temporal evolution characteristics. For the main-wing mid-span section, the quasi-static mode accounts for more than 44% of the total flow-field energy and is only slightly attenuated by ground effect, ensuring the basis of steady lift. The low-frequency drift mode is significantly suppressed, while the medium- and high-frequency modes are locally enhanced, driving large-scale continuous vortices to break down into multi-scale discrete small vortices. For the wingtip-vortex section, the energy of drift modes decays significantly under ground effect, while the energy of conjugate modes remains basically unchanged. Accordingly, the wingtip vortex core shifts upward with a compressed shape and reduced oscillation amplitude. Flow-field reconstruction verification demonstrates that low-order DMD modes can effectively filter experimental noise and agree well with the original TR-PIV flow field in spatial distribution, showing reliable reduced-order modeling capability. Combined with force measurement data, ground effect increases the wing lift coefficient by 12% and reduces the drag coefficient by 15.9% at an angle of attack α = 10°. This study confirms that ground effect achieves lift enhancement and drag reduction through the synergistic mechanism of stabilizing the mean flow, suppressing unsteady dissipation, and reconstructing the multi-scale wake vortex structure. The dual-section DMD analysis framework established in this paper can provide data-driven theoretical support for the aerodynamic design, flow control, and nonlinear aerodynamic prediction of the main wing of flying cars under ground effect.