Ralf Unger

Computational Aero-Structural Coupling For Hypersonic Applications

For the coupled thermal and mechanical analysis of spacecraft structures, a simulation environment has been developed within the German IMENS project. The software environment combines existing and validated fluid and structural analysis codes and provides state-of-the-art techniques for a numerical coupling. The numerical concept is based on the weak formulation of the interface conditions on the coupling surface. The approach enables the coupling of non-matching surface grids in this environment. Sequential and iterative staggered schemes are available to handle transient and steady state problems. Aspects of the developed flexible software architecture and some of its implementation details are described. Applications to spacecraft structures demonstrate the environments features.

Experimental and Numerical Fluid-Structure Analysis of Rigid and Flexible Flapping Airfoils

A combined experimental and computational study is presented for an airfoil undergoing a combined pitching and plunging motion at Reynolds number 100,000, where transition takes place along laminar separation bubbles. The numerical simulation approach addresses unsteady Reynolds-averaged Navier―Stokes solutions and covers transition prediction for unsteady mean flows. To study the effect of wing flexibility, the aerodynamic computational method is coupled with a structural solver using a Galerkin method. The numerical simulations are validated using high-resolution, phase-locked stereoscopic particle image velocimetry for one flapping case with a reduced frequency of k = 0.2. Hereby both a rigid and a flexible birdlike airfoil are investigated. The flow reveals strong unsteadiness resulting in moving laminar separation bubbles, both well captured by the numerical simulations performed in this study.

Coupling techniques for computational non-linear transient aeroelasticity

Abstract Some numerical aspects for the coupling process of a discrete non-linear aeroelastic system are presented. The objective of this paper is two-fold: first, a consistent time-integration method of the whole coupled system is developed and the robustness is shown. Second, several data transfer methods — conservative interpolation, Galerkin-based transfer, dual-Lagrange-based transfer, and Sobolev-norm-based transfer — are employed and the importance of an accurate transfer scheme is demonstrated. Numerical results obtained from simulations of an oscillating one-dimensional plate in a transonic flow and a three-dimensional wing example serve as a typical benchmark problem to show the applicability of the presented concepts and the importance on the behaviour of an aeroelastic system.

Structural Design and Aeroelastic Analysis of an Oscillating Airfoil for Flapping Wing Propulsion

To investigate the aeroelastic eects, the design as well as the numerical analysis of a flexible and oscillating airfoil is described in this contribution. Due to the interaction of the fluid flow with the structural system, a multiphysical approach is employed here, were firstly the airfoil shape for low speed range based on a bird’s hand foil is designed and secondly the structural subsystem is developed including the interaction eects of the fluid flow. Further, in this paper the investigation of low-Reynolds-number flows past this flexible and flapping airfoil is presented. Transition takes place along a laminar separation bubble. To predict the point of transition, a linear stability solver fully coupled to an unsteady Reynolds-averaged Navier-Stokes flow analysis code is utilized. Results of the simulation of the airfoil’s flapping motion in air are presented for specific parameters and discussed in detail.

Aerodynamics and Structural Mechanics of Flapping Flight with Elastic and Stiff Wings

The flapping flight mechanism is expected to provide revolutionary operation capabilities for tomorrow’s Micro Air Vehicles (MAV). The unsteady aerodynamics of the flapping flight is vastly different from traditional fixed-wing flyers. Boundary layers with moving laminar-turbulent transition, three-dimensional wake vortices and fluid-structure interaction with anisotropic wing structure are only a few examples for the challenging problems. To get basic understanding of these effects, the authors develop a computational method that is validated with boundary-layer measurements on flexible and inflexible, flapping wings in a wind-tunnel. The computational method solves the unsteady Reynolds-averaged Navier-Stokes equations and is combined with both transition prediction and fluid structure interaction capability. Using generic airfoils shapes inspired by seagulls and hawks, different aerodynamic, structural and kinematic effects are systematically analyzed on their influence on thrust and propulsive efficiency of the flapping flight mechanism. In particular, we demonstrate that a slight forward-gliding motion during the flapping downstroke can increase significantly thrust and efficiency.Wing elasticity however seems to lower the propulsive efficiency in the investigated cruise flight flapping case. Beyond,we show that the wake structure of 3D flapping wings generates an efficiency loss of about 10% compared to equivalent two-dimensional flapping cases.

Deformation Measurement of a Birdlike Airfoil with Optical Flow and Numerical Simulation

A Lucas–Kanade optical flow technique was developed to measure the deformation of a flexible birdlike airfoil undergoing an increasing angle of attack at Reynolds number 100,000. A pyramidal scheme was used to allow large displacements; a nonlinear structure tensor diffusion was introduced to preserve image intensity discontinuities. The accuracy and convergence of this Lucas–Kanade method was studied. Then, this optical flow method was adapted for a wind-tunnel experimental setup to estimate the vertical deformation of a newly designed flexible, birdlike airfoil with an improved wing loading. At last, the proposed optical flow method was compared to image correlation and numerical simulation results that were carried out solving the fluid–structural interaction problem through a well-validated coupling environment with conservative load transfer and a partitioned coupling approach.

Application of Lagrange Multipliers for Computational Aeroelasticity

The prediction of aeroelastic effects is one of the key problems during the design process of an aircraft. One challenging aspect of this goal is to compute space and time-accurate fluid and structural interactions. In the loose coupling approach, well-established CFD and CSD codes are taken and integrated in a flexible software environment.