Wing in Ground Effect
The thesis presents a two pronged approach for predicting aerodynamics of air- foils/wings in the vicinity of the ground. The ﬁrst approach is eﬀectively a model for ground eﬀect studies, employing an inexpensive Discrete Vortex Method for the 2D pre- dictions and the well known Numerical lifting line theory for the 3D predictions. The second one pertains to the dynamic ground eﬀect analysis which employs the state of the art moving mesh methodology based time accurate CFD. In that sense, the thesis deals with two ends of spectrum in the ground eﬀect analysis; one, a model to be used in the concept design phase and the other an advanced CFD tool for analysis. The proposed model for ground eﬀect studies is based on the well known Discrete Vortex Method (DVM). An important aspect of this method is that it employs what is referred to as the Generalized Kutta Joukowski Theorem (GKJ), meant for interaction problems with multiple vortices, for predicting the lift (and drag) within a potential ﬂow framework. After ascertaining the correctness of using the GKJ theorem for lift prediction for airfoils in ground eﬀect, a modiﬁed DVM is presented as a model for ground eﬀect predictions. As per this model, knowing the free stream lift and drag (either from an ex- periment or from a RANS computation) the aerodynamics of the section in ground eﬀect can be predicted. The model is eﬀectively built by constraining the DVM to produce the reference lift/drag in the free stream. The accuracy of the model, particularly for the more relevant high lift sections used during take-oﬀ and landing, is systematically estab- lished for a number of test cases. Knowing the sectional ground eﬀect, the extension to 3D analysis is very simple and this is achieved through the well known Numerical Lifting Line theory. The eﬃcacy of the proposed method for the 3D applications is demonstrated using a high lift wing in ground eﬀect. It is worth noting that the proposed model predicts the lift and drag very accurately, practically at no computational cost as compared to modern RANS based CFD tools requiring over 40 or 50 million volumes at a high computational cost and intense human intervention for generating the grids for every ground clearance. The other aspect of the thesis pertains to what is referred to as the Dynamic Ground Eﬀect. Normally the CFD computations mimic the ground eﬀect experiments in simulat- ing the ground eﬀect. These simulations do not maintain geometric similarity with the actual landing or take-oﬀ sequence of the aircrafts and this can only be achieved when the simulations are dynamic. Dynamics is also important in case of combat aircrafts (particularly their naval versions) with an aggressive landing and take-oﬀ. The dynamic ground eﬀect simulations also provides a framework for simulating varied gust conditions. This dynamic simulation of the ground eﬀect is accomplished using a novel sinking grid methodology, which allows the grids to sink in the ground as the aircraft approaches the ground along the glide path. These simulations make use of the state of the art, time accurate moving grid methods and therefore can be computationally expensive. Never- theless, the utility of such computations in terms of their ability to produce continuous data has been highlighted in the thesis. In that sense, these dynamic computations will be cheaper as compared to the static simulations to produce data at the same level of resolution.
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