Abstract
Large-amplitude excitations need to be considered for gust load analyses of transport aircraft in cruise flight conditions. Nonlinear amplitude effects in transonic flow are, however, only marginally taken into account. The present work aims at closing this gap by means of systematic unsteady Reynolds-averaged Navier-Stokes simulations. The RAE2822 airfoil is analyzed for a variety of sinusoidal gust excitations at different transonic Mach numbers. Responses are evaluated with respect to lift and moment coefficients, their derivatives and the unsteady shock motion. A strong dependency on inflow Mach number and excitation frequency is observed. Generally, amplitude effects decrease with lower Mach numbers or higher excitation frequencies. The unsteady nonlinear simulations predict lower maximum lift values and lower lift and moment derivatives compared to their linear counterparts for lower frequencies in combination with large-amplitude excitations. For the mid-frequency range, trends are not as clear. Additionally, it is shown that the variables of harmonic distortion and maximum shock motion might not be reasonable indicators to predict a nonlinear response.
Highlights
For aircraft design, large numbers of load cases need to be taken into account.Large-amplitude excitations need to be considered especially for gust load computations.Any large-amplitude excitation in transonic flow implies a nonlinear response and requires nonlinear methods for an adequate prediction
This study shows that long ’1-cos’ gusts are more affected by amplitude effects than shorter gusts, which is in agreement with [22]
The dashed vertical lines mark the steady angle of attack α0 = 3 deg, which is used during the gust encounters
Summary
Large numbers of load cases need to be taken into account. Large-amplitude excitations need to be considered especially for gust load computations. It is shown that this boundary corresponds to a maximum motion of the recompression shock of about 5% over the airfoil chord Another central finding of Dowell et al [19] is that nonlinear effects decrease with increasing excitation frequency, in general. After 1983, additional physical insight on transonic unsteady nonlinear effects was gained only in the last decade [9,12,20,21,22,23], with increasing computational power and growing trust in RANS-based methods for separated flows. Similar to Dowell et al [19], physical quantities such as lift and moment coefficients and their derivatives, as well as the unsteady shock motion are assessed for four transonic Mach numbers and two turbulence models.
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