Numerical methods for load prediction in acoustic fatigue

Abstract: Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have several limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed to be perfectly in phase over the entire structural element. This assumption limits the accuracy of the response prediction and by extension the fatigue prediction. In order to take the spatial distribution into account it must first be determined. In this dissertation, load prediction in the context of acoustic fatigue by numerical methods using Computational Fluid Dynamics (CFD) is investigated. From the CFD simulations, both the load intensities and the spatial distributions are extracted. CFD simulations are performed on two model problems where the simulation results are compared to existing measurements on the simulated setup. In paper A, a ramped backward-facing step is used as a model problem for acoustic fatigue. The flow over the step induces a load on an aluminium sheet fitted downstream of the step. With the exception of the cut-off, or shedding, frequency being overpredicted, the spectral qualities of the load and the load intensities are well captured. The extracted load is used as force input to a Finite Element (FE) simulation of the response of the exposed aluminium sheet. The response prediction is found to be good when compared to design guidelines and other studies where the spatial distribution of the load is considered. The model problem studied in paper B is flow over an inclined fence at transonic Mach number and realistic Reynolds number for aircraft operation. Load intensities downstream of the fence are well captured. The spatial characteristics in the form of cross-correlations appear to be on the level required for a good response prediction of an aircraft skin surface panel placed downstream of the fence. A sensitivity study of three different geometrical configurations of the solution domain was performed as it was found that the flow is sensitive to what happens upstream of the fence. It is found that the spectral characteristics of the load downstream of the fence is affected by this geometrical sensitivity. Several aspects of the surface load as well as the flow in general are investigated and compared to existing measurements.