An Experimental Investigation of an Infinite Wing in Ground Effect
Quantifying the Influence of Ground Proximity on Steady and Unsteady Lift Response
P. Chandel (TU Delft - Aerospace Engineering)
F. Scarano – Mentor (TU Delft - Aerospace Engineering)
A. Daliri Rezagholigheshlaghi – Mentor (TU Delft - Aerospace Engineering)
L.L.M. Veldhuis – Graduation committee member (TU Delft - Aerospace Engineering)
W. Yu – Graduation committee member (TU Delft - Aerospace Engineering)
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Abstract
The aerodynamic interaction between a lifting surface and a ground plane presents a highly non-linear landscape that is critical to the performance and safety of racing vehicles, wing-in-ground (WIG) craft, and aircraft during take-off and landing. While steady-state wing-ground interaction is well-documented, the coupling of unsteady kinematics with ground proximity remains a frontier in aerodynamics, particularly regarding the evolution of the dynamic stall vortex (DSV). This thesis presents a comprehensive experimental investigation into the steady and unsteady aerodynamics of an infinite NACA0015 airfoil operating in ground effect at a Reynolds number of Re≈2×105. By combining high-frequency surface pressure measurements with phase-resolved 3D Particle Tracking Velocimetry (3D−PTV) using the Shake-the-Box algorithm, the study resolves both surface-level load transients and off-body wake and vorticity dynamics.
The investigation initially establishes a steady-state baseline, revealing that ground influence is non-monotonic. At large clearances (H>0.8), the flow is largely unaffected by the presence of the ground. At moderate clearances (0.4<H≤0.8), lift enhancement is driven by the Venturi effect, where geometric confinement accelerates the underbody flow. In extreme ground effect ($H≤0.4$), a viscous-confinement domain is observed. In this state, the interaction between the airfoil and ground-plane boundary layers induces a pressure-side blockage that caps circulation growth, a phenomenon that contradicts classical inviscid predictions.
In the unsteady domain, the airfoil was subjected to sinusoidal pitching across a matrix of reduced frequencies (k=0.05 to 0.20) and ground clearances (H=0.1 to ∞). In attached-flow conditions, ground proximity modulates the convective time lag of the wake. At intermediate heights, the accelerated gap flow reduces the time required for circulation to equilibrate, effectively compressing the lift hysteresis loops. At H=0.2, physical blockage and air-column stiffening significantly widen the hysteresis loops, indicating a substantial increase in phase lag relative to freestream conditions.
The most significant findings emerge in the dynamic stall domain. The presence of the ground plane serves as a powerful stabilising mechanism. By imposing a vertical geometric constraint, the wall suppresses the coherent roll-up and subsequent shedding of the Leading-Edge Vortex (LEV). This suppression of discrete vortex shedding replaces abrupt load collapses characteristic of classical dynamic stall with geometrically constrained separation. This transition results in a marked increase in aerodynamic damping, suggesting that ground proximity inherently mitigates the energy extraction mechanisms that typically trigger instabilities.
This work demonstrates that the non-dimensional height must be considered as a primary kinematic parameter, and reduced frequency alone is insufficient to characterise unsteady stall in ground effect. These insights provide a quantitative foundation for the development of active aerodynamic control systems and offer a physical explanation for the suppression of porpoising in high-downforce racing applications and low-flying vehicles.