Thilo Knacke

Separation Control by Self-Activated Movable Flaps

Separation control is an important issue in the physiology of birdflight. Here, the adaption of the separation control mechanism by bird feathers to the requirements of engineering applications is described in detail. Self-activated movable flaps similar to artificial bird feathers represent a high-lift system for increasing the maximum lift of airfoils. Their effect on the unsteady flow around a two-dimensional airfoil configuration is investigated by a joint numerical and experimental study. First, attention is paid to the automatic opening and closing mechanism of the flap. Following this, its beneficial effect on lift is investigated for varying incidences and flap configurations. In-depth analysis of experimental and numerical results provides a detailed description of the important phenomena and the effect of self-adjusting flaps on the flow around the airfoil. In the second part of this paper, a contribution is made to verification of the applicability of unsteady Reynolds-averaged approaches using statistical turbulence models for unsteady flows with particular attention to turbulent time scales with comparison to the results of a hybrid simulation based on unsteady Reynolds-averaged Navier-Stokes equations and large-eddy simulation. Finally, flight experiments are described using an aircraft with movable flaps fitted on its laminar wing.

Detached-Eddy Simulation of Landing-Gear Noise

In the present paper, compressible IDDES at Mach numbers of 0.115 and 0.23, coupled with FWH integration, were carried out for a rudimentary landing gear configuration. The obtained Mach number scaling of around M confirms the expectation that the contribution of dipoles is dominant. Comparison with experimental far-field sound measurements has also been made. Both the permeable-surface and the solid-surface compressible results agree well with the experimental data to within 2 dB. Incompressible calculations are often applied to complex industrial configurations in the low-Mach regime. To assess the validity of this practice, incompressible calculations were also conducted. Provided that wave reflections were taken into account in the far-field integration, the incompressible results agreed well with the compressible case, being at most 2 dB quieter. Modest savings in computational resources were achieved, which could have been higher with an adjusted grid. However, the applicability of such incompressible calculations is expected to be strongly case-dependent.

Demonstration of Improved DES Methods for Generic and Industrial Applications

This paper presents an overview of the DES methods implemented at the Institute of Fluid Mechanics and Engineering Acoustics (ISTA) at the TU-Berlin during the course of the European DESider project. As well as the validation of these methods on the basis of simplified, academic flow cases presented in the first part, their suitability and necessity for complex industrial applications is demonstrated using results from other research projects. The methods prove robust and reliable for a wide range of applications, ranging from external to internal flows, from bluff bodies with massive separation to the partial resolution of attached boundary layers.

Noise Prediction of a Rudimentary Landing Gear Using Detached-Eddy Simulation

Detached-Eddy Simulation (DES) is a promising method for efficient simulation of broadband noise at minimal computational cost. Here, results from a study of broadband noise simulation using state-of-the-art DES methods are presented for a rudimentary landing gear configuration. The DDES and IDDES variants are compared with experiments in incompressible simulations. IDDES shows mild improvement in agreement and some increase in the resolution of high frequencies. An attempt is made to independently verify published results for far-field sound prediction, using a compressible simulation coupled with Ffowcs-Williams/Hawkings (FWH) integration. In contrast to the published results, our results do not provide evidence of unexpectedly strong roles played by the ceiling or by quadrupoles. Our results furthermore predict much lower far-field noise levels than the published results. Good agreement between solid and permeable FWH surfaces is found as long as the permeable surfaces are open downstream.

Time-Resolved 3D Simulation of an Aircraft Wing with Deployed High-Lift System

The turbulent flow around a high-lift configuration consisting of slat, main element and flap is simulated at a Reynolds number of 1.7 × 106 with an implicit finite-volume based numerical method. The 3D unsteady motion in separated flow regions is resolved on a 25 million volume mesh employing the recent Delayed Detached-Eddy Simulation (DDES) approach [12]. Compressible calculations and the use of non-reflecting boundary conditions enable sound radiation to be captured in the simulation. The presented results cover the first step in a two-step approach towards the prediction of noise emitted into the acoustic farfield and provide insight into the complex flow dynamics in the slat region. The computed pressure distributions, statistics and spectra exhibit good agreement with findings from NASA Langley Research Center (LaRC) [2, 5].

Advanced DES Methods and Their Application to Aeroacoustics

Detached eddy simulation (DES) is shown to be a suitable method for the simulation of the sound generation of turbulent flows, because it provides access to the resolved turbulent scales at minimal computational cost. The near-wall region is solved efficiently by RANS while LES is applied to all regions containing scales important for the noise generation. In addition to the usual LES resolution requirements, for acoustics the smallest of these scales are defined by the highest frequencies of interest. The sound radiation is generally computed by solving an integral over a data surface surrounding the source region outside the turbulent flow. The grid must be fine enough to resolve the sources and the propagation between the sources and the data surface. Examples for the simulation of noise emission problems are presented for the influence of nozzle serrations on jet mixing noise and three airframe noise sources, which are rod-airfoil interaction noise, the noise of high-lift devices, and airfoil self noise.

Prediction of Broadband Noise from Two Square Cylinders in Tandem Arrangement Using a Combined DDES/FWH Approach

A combined CFD/CAA method is used to study the turbulent flow and the flow-induced noise generated by two square cylinders in tandem arrangement. The selected test case activates noise generation mechanisms which are typical of realistic landing gear struts under approach conditions and the employed prediction methods prove capable of capturing all relevant physical effects. The nearfield and farfield results are discussed in close comparison to experimental data.

Joint Experimental and Numerical Study of Gap-Turbulence Interaction

The paper presents the results of numerical simulations of gap – turbulence interaction which can be considered as a generic test case on airframe noise. In order to be able to assess the predictive capabilities of different numerical tools to accurately describe the noise generation mechanisms in the gap, a corresponding physical experiment is conducted. DDES and IDDES hybrid models implemented by higher accuracy numerical algorithms both for multi-block structured and tetrahedral unstructured meshes are used for the simulations. The paper presents the obtained numerical data including mean flow fields, turbulent boundary layer structure and far field acoustic spectra. Based on the comparison of experimental and numerical results, the study reveals physical features of the phenomenon.