Summary of Flow Modulation and Fluid-Structure Interaction Findings

In the present paper the development of vortex wake starting from the vortex sheet at the trailing edge of a transport aircraft wing up to the far field over 60 spans downstream is investigated. Different configurations of a half model were investigated in wind and water tunnels as well as in a towing tank by hot-wire anemometry and particle image velocimetry. In addition to an understanding of the development of the wake, means for the attenuation of the impact on following aircraft were investigated. A fin was installed on the suction side of the wing to investigate the impact on the vortex system downstream. The possibility of exploiting short-wave cooperative instabilities to accelerate vortex decay were investigated as well. This included active excitation of instabilities via ailerons oscillated in antiphase.

In this paper the development of the new adaptive solver QUADFLOW is described. It is based on an integral concept and consists of a flow solver, a grid generation tool using parametric mapping based on B-splines, and local grid adaptation based on multiscale analysis. QUADFLOW has been designed to obtain a solution method for multidisciplinary problems in the field of aerospace engineering. The most important applications are the simulation of high-lift configurations and elastic wings in cruise configuration. A partitioned field approach is used for the solution to aeroelastic problems. In this article the finite volume solver and its inclusion into an aeroelastic solver are outlined. The results of the validation study and a new matrix-free Newton-Krylov method, offering potential to accelerate convergence, are presented.

We report on recent work on adaptive timestep control for weakly instationary gas flows [16, 17, 18] carried out within SFB 401, TPA3. The method which we implement and extend is a space-time splitting of adjoint error representations for target functionals due to Süli [19] and Hartmann [10]. In this paper, we first review the method for scalar, 1D, conservation laws. We design a test problem for weakly instationary solutions and show numerical experiments which clearly show the possible benefits of the method. Then we extend the approach to the 2D Euler equations of gas dynamics. New ingredients are (i) a conservative formulation of the adjoint problem which makes its solution robust and efficient, (ii) the derivation of boundary conditions for this new formulation of the adjoint problem and (iii) the coupling of the adaptive time-stepping with the multiscale spatial adaptation due to Müller [3, 12], also developed within SFB 401. The combined space-time adaptive method provides an efficient choice of timesteps for implicit computations of weakly instationary flows. The timestep will be very large in regions of stationary flow, and becomes small when a perturbation enters the flow field. The efficiency of the Euler solver is investigated by means of an unsteady inviscid 2D flow over a bump.

The concept of the new fully adaptive flow solver Quadflow has been developed within the SFB 401 over the past 12 years. Its primary novelty lies in the integration of new and advanced mathematical tools in a unified environment. This means that the core ingredients of the finite volume solver, the grid adaptation and grid generation are adapted to each others needs rather than putting them together as independent black boxes. In this paper we shall present recent developments and demonstrate their efficiency by numerical experiments for some representative basic configurations.

The interaction of wing-tip vortices and jets in the extended wake is experimentally and numerically investigated. The measurements focus on the unsteady wake of a swept-wing half-model equipped with an engine jet and on the analysis of meandering vortex. The aircraft engine is modeled by a cold jet driven by pressurized air. To investigate the influence of the location of the engine jet on the vortex wake, it is mounted in two different positions under the wing model. The spatial development of a vortex wake behind a wing is simulated up to the extended near field. The measurements are used as inflow distribution for a large-eddy simulation (LES) of the wake region. To better capture the motion of the wake vortices a method for hexahedral block structured adaptive mesh refinement with vertex-centered fluxes is introduced. The numerical simulations of the wake are able to predict trajectories and instabilities of the vortex core. The closer the engine is located near the root of the wing, the smaller is the deflection of the vortex and the fewer wave modes of the vortex are excited. The meandering motion of the vortex core is triggered by the engine jet.

A summary of experimental and numerical results concerning the phenomenon of upstream moving pressure waves in the transonic flow regime is presented. As experimental and numerical time-resolved shadowgraphs show, such waves initiate near the sharp trailing edge of a supercritical airfoil and in the wake, propagate upstream in the subsonic region of the flow, and strengthen before becoming apparently weaker and almost disappear near the leading edge. Using high order numerical simulation several mechanisms of wave generation based on vortex dynamics as well as vortex/trailing edge interactions and wake fluctuations are distinguished. These waves on the upper side of the airfoil are also captured with pressure transducers mounted in the airfoil model as pressure oscillations of dominant frequencies ranging between 1 to 2 kHz. Furthermore, the statistical analysis of the pressure histories allowed for the determination of wave propagation direction, strength and speed.

In computational science and engineering, the role of computer science includes the mechanical generation of programs for the fast computation of accurate derivatives and the efficient utilization of parallel computer architectures. Automatic differentiation and parallel computing are two technologies enabling robust high-performance simulations in various scientific and engineering disciplines. This article gives a survey of selected results of an interdisciplinary research project where automatic differentiation, parallel computing, and their interplay are investigated in the context of two computational fluid dynamics packages developed at RWTH Aachen University.

Results presented here were obtained within a project which was part of the Collaborative Research Centre SFB 401 “Flow Modulation and Fluid-Structure Interaction at Airplane Wings” funding from the Deutsche Forschungsgemeinschaft (DFG). The goal of this project was to gain better understanding of aircraft wake vortices in order to investigate possibilities to mitigate the hazard posed by these to following aircraft. To this end wind tunnel testing was undertaken in which the vortex wakes of various wings were measured using hot wire anemometry. It was shown that in the near field, the rolling moment induced on a following aircraft can be significantly reduced by introducing additional turbulence into the wake. Another focus point was the investigation into excitation of short wave instability mechanisms in the vortex wake and their effects in the far field. For these purposes experiments with various models and oscillating control surfaces were conducted in water towing tanks in which the vortex wakes were measured using particle image velocimetry. The results show that for an appropriate multi-vortex system, inherent instabilities can be excited leading to a more rapid vortex decay within the first 30 span lengths behind the model. The effects of these mechanisms further out in the far field are, however, minimal.

This paper outlines the development of the aero-structural dynamics method SOFIA over the duration of the Collaborative Research Center SFB 401. The algorithms SOFIA applies for the spatial and the temporal aero-structural dynamics coupling are presented. It is described in particular how SOFIA’s load and deformation transfer algorithms suitable for non-matching grids at the coupling interface were enhanced towards the application to complete aircraft configurations. The application of SOFIA to various subsonic and transonic aeroelastic test cases is discussed.

In this article we summarize the development of a unified platform for treating the entire range of geometric preprocessing tasks that preceded the wind tunnel readings and the numerical simulations performed in the collaborative research center SFB 401. In particular, this includes the automated generation of the CAD models which were used for manufacturing multi-parted wing-fuselage configurations as well as the generation of the numerical grids for the corresponding, adaptive numerical simulations.

The new flow solver Quadflow, developed within the SFB 401, has been designed for investigating flows around airfoils and simulating the interaction of the structural dynamics and aerodynamics. This article addresses the following issues arising in this context. After identifying proper coupling conditions and settling the well-posedness of the resulting coupled fluid-structure problem, suitable strategies for successively applying flow and structure solvers needed to be developed that give rise to a sufficiently close coupling of both media. Based on these findings the overall efficiency of numerical simulations hinges, for the current choice of structural models, on the efficiency of the flow solver. In addition to the multiscale-based grid adaptation concepts, proper parallelization concepts are needed to realize for such complex problems an acceptable computational performance on parallel architectures. Since the parallelization of dynamically varying adaptive discretizations is by far not straightforward we will mainly concentrate on this issue in connection with the above mentioned multiscale adaptivity concepts. In particular, we outline the way the multiscale library has been parallelized via MPI for distributed memory architectures. To ensure a proper scaling of the computational performance with respect to CPU time and memory, the load of data has to be well-balanced and communication between processors has to be minimized.We point out how to meet these requirements by employing the concept of space-filling curves.

In this paper we treat subjects which are relevant in the context of iterative methods in implicit time integration for compressible flow simulations. We present a novel renumbering technique, some strategies for choosing the time step in the implicit time integration, and a novel implementation of a matrix-free evaluation for matrix-vector products. For the linearized compressible Euler equations, we present various comparative studies within the QUADFLOW package concerning preconditioning techniques, ordering methods, time stepping strategies, and different implementations of the matrix-vector product. The main goal is to improve efficiency and robustness of the iterative method used in the flow solver.

A comprehensive analysis of experimental results from several wind tunnel test campaigns is presented. The experiments were conducted in the subproject B6 of the Collaborative Research Center SFB 401. The main focus of this research is the interaction of unsteady aerodynamic phenomena in transonic flow over a supercritical BAC 3-11/RES/30/21 high aspect ratio swept wing configuration in the context of aeroelastic instabilities occurring at modern transport type wing configurations. The fluid-structure-interaction is simulated using a simplified aeroelastic test setup with harmonic forcing in the bending and torsional wing degree of freedom. The interaction between shock wave and turbulent separation is the most essential feature in the unsteady wing flow leading to a distinct self-induced oscillation of the flow field. Flow cases with incipient separation and with full scale shock induced separation at Mach numbers 0.86 and 0.92 are compared showing the interference effects of self-induced shock oscillations with the harmonically oscillating wind tunnel model. The results demonstrate the self-limiting nature of the unsteady flow with incipient separation and the aeroelastic coupling in the presence of shock induced separation.

In the present contribution concepts of reduced structural models for Computational Aero-Elastic simulation (CAE) on aircraft wings are presented. Here the idealization approach relies on analytical methods with the aim to shorten in comparison to a typical finite element method computational cost and time, by preserving nearly the same accuracy. Prior to more detailed investigations using higher order models, these simplified models allow an earlier access of insight regarding the aeroelastic and structural behavior of the wing at the very beginning of the design process. At first a one-dimensional idealization that extends the Timoshenko beam by taking into account additional effects due to warpings is developed. To better describe the influence of swept, a three dimensional idealization is derived. Both idealizations yield good agreements in results concerning the global static deformation and the modal behavior of the wing.

The elastic wing model, its excitation and comprehensive high frequency measuring equipment for the High Reynolds Number Aero-Structural Dynamics (HIRENASD) tests in the European Transonic Windtunnel (ETW) are shortly described. Some of the stationary polars are presented in terms of wing deformation, as well as aerodynamic coefficients and pressure distributions. Then unsteady processes observed in the measurements of static aerodynamic coefficients, are regarded with focus on small amplitude pressure waves travelling upstream from the trailing edge and triggering periodically break-down and redeployment of the local supersonic domains with transonic shock waves to run upstream and to disappear. Another focus is on stochastic vibrations excitation while moving forward during nominally static experiments. Emphasis is put on measured variations of pressure distribution on the wing surface caused by defined vibration excitation applying internal force couples at the wing root, whereby the exciter frequencies were chosen close to natural frequencies of the wing model. Phase and magnitude of measured local lift fluctuations as well as real and imaginary parts of pressure distributions are presented.