Adaptive Test Section Research Articles

Experiments on transonic limit-cycle-flutter of a flexible swept wing

The aeroelastic behavior of wing models is nonlinear particularly in the transonic speed range. The interaction between aerodynamic and structural forces can lead to the occurrence of Limit-Cycle Oscillations (LCOs). If in addition the wing model is flexible and backward swept, the kinematic coupling between bending and torsion makes the situation even more complex.In the research project “Aerostabil” such a wing was investigated, which was equipped with pressure transducers in three sections and accelerometers. The experiments were performed in the adaptive test section of the transonic wind tunnel TWG in Göttingen. Already Dietz et al. (2003) have reported about experimental details and preliminary results. Based on these data Bendiksen (2008) studied numerically LCO-flutter behavior using a very similar, theoretical model (G-wing) and Stickan et al. (2014) used the original data as a LCO flutter test case. The influence of flexibility on the steady aerodynamics of the wing was described in Schewe & Mai (2018). In this paper now the flutter experiments with the same flexible model were analyzed systematically in the transonic range 0.84 <Ma <0.89 and for six angles of attack from 1.46°to 2.7°. Maps of stability, LCO amplitudes and instantaneous pressure distributions are presented. It was found that unstable regions are islands, whose extent depends on the angle of attack. A LCO test case, already treated in the literature is examined in more detail. The analysis of the time functions showed that during LCO-flutter the motion induced aerodynamic sectional lift forces particularly in the outer wing are asymmetric and thus acting as amplitude limiter. The reason for the asymmetry lies in the shock/boundary layer interaction. The test case, containing the stages of built-up and the transition to the limit cycle provides an excellent opportunity for improving our knowledge about LCOs and for code validation purposes.

Influence of flexibility on the steady aeroelastic behavior of a swept wing in transonic flow

The development in the last decades generally shows that large aircraft wings have become more light and flexible, thus the investigation of the effects of elasticity is suggested. If for example the flexible wing is also backward swept, then the situation becomes even more complex — the kinematic coupling between bending and torsion leads to a structural washout effect.In order to investigate the influence of flexibility, in the project “Aerostabil” an aeroelastically scaled half-model was tested compared to its rigid equivalent. The flexible model was equipped with pressure transducers in three wing sections and accelerometers, while the rigid model had a reduced number of sensors. The experiments were performed in the adaptive test section of a transonic wind tunnel. Steady and unsteady pressure-, and force measurements were conducted for fixed and oscillating wings. Already Dietz et al. (2003) have reported about the special features of the wing models, their structural properties and preliminary results. The present paper is focused on the analysis of the global forces and pressure distributions for the range 0.5 ≤Ma≤ 0.88. The angle of attack was varied from −4° to 4° and also the quasistatic aeroelastic derivatives for lift and moment were obtained.When the model is rigid in the transonic regime and at moderate angles of incidence the pressure distribution exhibits a single shock system, in contrast for the flexible wing there is a double shock system. For the flexible wing up to about Ma=0.82 the curves of global lift, moment and their derivatives are rather smooth and are remaining on nearly the same level. Beyond there are moderate deviations up to the end of the transonic regime. However examining the corresponding curves of the rigid wing the changes are drastic, particularly in the transonic range. Obviously the structural wash-out, particularly of the outer wing leads to an attenuation of the transonic effects.

Experiments on heave/pitch limit-cycle oscillations of a supercritical airfoil close to the transonic dip

Recent results from flutter experiments of the supercritical airfoil NLR 7301 at flow conditions near the transonic dip are presented. The airfoil was mounted with two degrees of freedom in an adaptive solid-wall wind tunnel, and boundary-layer transition was tripped. Two limit-cycle oscillation (LCO) test cases obtained in an adaptive test-section are proposed for comparison with numerical simulations. The results link the global aerodynamic force behavior to the observed LCOs and the identified transonic dip. The time lag of the lift response to the pitching motion of the airfoil appears to be responsible for the characteristic shape of the transonic dip. The amplitude limitation of the LCOs results from a slightly nonlinear dependency of lift and moment on the amplitude of the airfoil motion. LCOs can be controlled by relatively small forces, but amplitudes strongly depend on the damping of the aeroelastic system.

Numerical and experimental investigations on the reduction of wind tunnel wall interference by means of adaptive slots

AbstractThe flow in many wind tunnel experiments is affected by the presence of test section walls. The resulting interference can be minimised by correcting the measured model pressures, or by influencing the model flow directly with the use of ventilated or adaptive test section walls. The objective behind the latter technique is to guide the flow in the test section to achieve low interference (i.e. free flow) condition at the model. The most successful technique of flexible, adaptive walls is still restricted to small research wind tunnels due to its mechanical complexity. However, a very promising alternative is the use of adaptive slots in the test section walls. This concept combines the method of passive slotted walls, as they are already implemented in many large wind tunnels, and flexible walls. Additionally, this technique presents the opportunity of full 3D adaptations because the slots can be situated in all four test section walls.This paper presents preliminary experimental results and the latest numerical calculations on the effectiveness of adaptive slots. The experiments were conducted under high subsonic flow conditions in the new slotted test section of the transonic wind tunnel at TU Berlin’s Aeronautical Institute (ILR).The numerical results presented are focussed on the 2D slot adaptation of a 2D-model (CAST7 aerofoil) and the 3D slot adaptation of a body of revolution (3D-ETB). In addition, basic studies were made of the flows associated with a single slot on one wall and a bump on the other.The numerical and the first experimental investigations have shown the potential of adaptive slots to reduce wall interferences effectively. The adaptation accuracy of the investigated slot configurations deviated not more than 3% from the reference case (2D-wall adaptation).

Operational experience and test results in the adaptive test section of the DLR transonic tunnel

AbstractThe transonic facility of DLR Göttingen (TWG) has been modernised with respect to an improved flow quality and flow simulation, as well as to the logistics of exchangeable test sections, operational reliability and productivity. It is presently equipped with a Laval nozzle for supersonic flow measurements, a perforated test section (6% open, 60° slanted holes) for transonic measurements, and a two-dimensional adaptive test section for subsonic measurements. The latter can perform two-dimensional adaptive tests on wing profiles, using the Cauchy integral formula for wall adaptation, as well as two-dimensional wall adaptation for three-dimensional models, utilising the Wedemeyer/Lamarche procedure, also known as the VKI method. For both cases, it is shown that the wall adaptation was successful. The 3D force results compare quite well to test results from the perforated test section, as well as to nominally interference-free results. The pressure distribution from the wing profile tests compare quite well to theoretical calculations. In the course of windtunnel modernisation, the transonic facility of DLR Göttingen has been equipped with the new software system DeAs for data acquisition and control of experimental facilities. In this environment the wall adaptation programs had to be implemented. Before adjusting the computed wall shape, it has to be controlled in the loop for sufficient accuracy and tolerable bending stresses. A simplified finite element method — also taking into account the pressure loads at the wall — is used for this purpose. The simplified approach has been checked against more detailed computations using ANSYS and NASTRAN.