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Mechanics, Germany

Bansmer S.E.,TU Braunschweig | Bansmer S.E.,Institute of Fluid Mechanics | Radespiel R.,TU Braunschweig
AIAA Journal

A combined experimental and computational study is presented for a wing segment undergoing a combined pitching, plunging, and rolling motion at Reynolds number of 100,000, where transition takes place along laminar separation bubbles. The numerical simulation approach addresses unsteady Reynolds-averaged Navier-Stokes solutions and covers three-dimensional transition prediction for unsteady mean flows. The numerical simulations are validated using high-resolution, phase-locked stereoscopic particle image velocimetry for a three-dimensional flapping case with a reduced frequency of k = 0:25. The flow reveals strong unsteadiness resulting in moving laminar separation bubbles, whose spatial extensions are varying in the spanwise direction. The experimental results are well captured by the numerical simulations performed in this study. Because of the vortex structure in the wake of the wing segment, the three-dimensional aerodynamics cannot be reproduced as a spanwise sequence of two-dimensional results. Copyright © 2011. Source

Scheit C.,Institute of Process Machinery and Systems Engineering | Karic B.,Institute of Fluid Mechanics | Becker S.,Institute of Process Machinery and Systems Engineering
Journal of Sound and Vibration

Radial impellers have several technical applications. Regarding their aerodynamic performance, they are well optimized nowadays, but this is in general not true regarding acoustics. This work was therefore concerned with analyzing the flow structures inside isolated radial impellers together with the far-field sound radiated from them in order to optimize the aerodynamic and acoustic performance. Both numerical and experimental techniques were applied in order to study the effect of varying wrap angle and otherwise identical geometric configuration on aerodynamics and acoustics of the radial impellers. The results give a detailed insight into the processes leading to sound generation in radial impellers. Measurements were performed using laser Doppler anemometry for the flow field and microphone measurements to analyze the radiated noise. In addition, unsteady aerodynamic simulations were carried out to calculate the compressible flow field. An acoustic analogy was employed to compute far-field noise. Finally, the phenomena responsible for tonal noise and the role of the wrap angle could be identified. Using this knowledge, design guidelines are given to optimize the impeller with respect to the radiated noise. This work shows that improved aerodynamic efficiency for isolated impellers does not automatically lead to a smaller flow-induced sound radiation. © 2011 Elsevier Ltd. All rights reserved. Source

Schutte A.,German Aerospace Center | Schutte A.,German Institute of Aerodynamics and Flow Technology | Hummel D.,TU Braunschweig | Hummel D.,Institute of Fluid Mechanics | Hitzel S.M.,Airbus
Journal of Aircraft

Within the NATO Research and Technology Organisation Applied Vehicle Technology (AVT)-161 task group, titled "Assessment of Stability and Control Predictions for NATO Air and Sea Vehicles," a 53 swept and twisted lambda wing with rounded leading edges is considered. In a first step, the symmetric flow conditions are analyzed in this paper in order to understand the corresponding flow physics. Experiments by the task group are used to develop proper numerical simulation tools for further applications in the design process of unmanned combat aerial vehicles as a part of future air-combat systems. The philosophy of the configuration under consideration is explained. The vortical flowfield is simulated using the DLR, German Aerospace Center TAU-Code applied with different turbulence models on various computational grids. Finally, a best practice is evaluated for medium and large angles of attack. A combination of these numerical results and experimental data lead to a proper understanding of the complex flow structure. Furthermore, this paper addresses the necessity for the predictability and understanding of controlled and uncontrolled flow separation, together with the interaction of the corresponding vortex systems in order to estimate stability and control issues for the entire flight envelope. Source

Masood R.M.A.,Institute of Fluid Mechanics | Khalid Y.,King Saud University | Delgado A.,Institute of Fluid Mechanics
Chemical Engineering Journal

Numerical investigations using scale adaptive simulation (SAS) in 3D transient Euler-Euler framework for bubble columns have been carried out and all the simulation results are compared with available experimental data. The performance of SAS model was also compared with re-normalization group (RNG), large eddy simulation (LES) and k-. ε models. Numerical results are in good agreement with experiments in predicting time averaged turbulent kinetic energy and axial velocity profiles. The superior flow predicting ability of SAS model is highlighted and it has shown that SAS turbulence model can capture large scale turbulence behavior and can be used efficiently for simulating flow field and turbulent quantities in case of bubble column flows. © 2014 Elsevier B.V. Source

Probst A.,TU Braunschweig | Probst A.,Institute of Fluid Mechanics | Radespiel R.,TU Braunschweig | Radespiel R.,Institute of Fluid Mechanics | And 2 more authors.
AIAA Journal

An extended eN-based modeling approach for Tollmien-Schlichting-type transition in aerodynamic flow simulations with the low-Re "h-Reynolds- stress model is presented. Instead of simply activating the turbulence production terms at the transition location (point-transition approach), the method incorporates the otherwise neglected Reynolds-stress contributions by the fluctuations of the Tollmien-Schlichting waves and provides them as local input for the turbulence model at the transition point. The shapes of the Reynolds-stress profiles are derived from linear stability analysis within the eN method, whereas their absolute magnitudes are calibrated with the aid of direct numerical simulation data of a transitional boundary layer with adverse-pressure gradient. The dissipationrate input is adjusted to theoretically match the amplification rate of the fluctuations but requires a correction to account for the low-Re damping in the "h-Reynolds-stress model. The paper describes the general modeling ideas and the implementation in the flow solver. Aspects of the numerical discretization and a verification for threedimensional flows are addressed as well. Besides a basic validation for the adverse-pressure-gradient boundary layer, simulations of the SD7003 airfoil flow comprising a laminar separation bubble are presented, which yield very good agreement with measurements. Results of a transitional flat-plate flow are, however, impaired by the lack of intermittency modeling. Finally, the method is applied to a flowthrough nacelle near stall conditions in order to prove its ability to compute consistent transitional behavior in complex three-dimensional flows. Copyright © 2011 by the authors. Source

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