The numerical investigation of dynamic responses to atmospheric turbulence is an important task during the aircraft design and certification process. Efficient methods are desirable since large parameter spaces spanned by e.g. Mach number, flight altitude, load case and gust shape need to be covered. Aerodynamic non-linearities such as shocks and boundary layer separation should be included to account for transonic flight conditions. A linearised frequency-domain method is outlined to efficiently obtain gust responses using computational fluid dynamics. The Reynolds-averaged Navier-Stokes equations are linearised around a steady-state solution and solved for discrete frequencies. The resulting large but sparse system of linear equations can then be evaluated significantly faster than its time-domain counterpart. The method is verified analysing sinusoidal gust responses for an aerofoil and a large civil aircraft considering a broad range of reduced frequencies. Derivatives of aerodynamic coefficients and complex-valued surface pressures are compared for time-and frequency-domain approaches. Next, 1-cos gusts are investigated using an incomplete inverse Fourier transform in conjunction with a complex-valued weighting function to discuss time histories of lift coefficients as well as surface pressures. Finally, introduced techniques are applied to conditions arising from certification requirements to demonstrate the technical readiness. The methods discussed present an important step to establish computational fluid dynamics in the routine aircraft loads process.
To achieve high efficiency and low degradation of a polymer electrolyte fuel cell (PEMFC), it is necessary to maintain an appropriate level of humidification in the fuel cell membrane. Thus, membrane humidifiers are typically used in PEMFC systems. Parameter studies are important to evaluate membrane humidifiers under various operating conditions to reduce the amount of physical tests. However, simulative studies are computationally expensive when using detailed models. To reduce the computational cost, surrogate models are set up. In our study, a 3D computational fluid dynamics (CFD) model of a hollow fibre membrane humidifier is presented and validated using measurement data. Based on the results of the validated CFD model, a surrogate model of the humidifier is constructed using proper orthogonal decomposition (POD) in combination with different interpolation methods. To evaluate the surrogate models, their results are compared against reference solutions from the CFD model. Our results show that a Halton design combined with a thin-plate-spline interpolation results in the most accurate surrogate humidifier model. Its normalised mean absolute error for 18 test points when predicting the water mass fraction in the membrane humidifier is 0.58%. Furthermore, it is demonstrated that the solutions of the POD model can be used to initialise CFD calculations and thus accelerate the calculation of steady state CFD solutions.
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