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Ulrich Rist - One of the best experts on this subject based on the ideXlab platform.
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DNS AND LES OF THE TRANSITION PROCESS IN A Laminar Separation BUBBLE
Direct and Large-Eddy Simulation V, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:A region of strong local adverse pressure gradient acting on a Laminar flat-plate boundary layer can produce a closed fully Laminar Separation bubble for sufficiently small pressure rise and Reynolds number. However, such a flow field is hydrodynamically highly unstable and transition will occur in the region of adverse pressure gradient. Due to an interaction with the potential flow, the transition process may even suppress Laminar Separation completely.
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Active Control of a Laminar Separation Bubble
Aerodynamic Drag Reduction Technologies, 2020Co-Authors: Kai Augustin, Ulrich Rist, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of Laminar Separation bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Numerical Simulation of Laminar Separation-Bubble Control
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: Ulrich Rist, Kai Augustin, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of Laminar Separation bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Direct Numerical Simulation of Non-Linear Transitional Stages in an Experimentally Investigated Laminar Separation Bubble
High Performance Computing in Science and Engineering’ 05, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:This paper details a joint numerical and experimental effort to investigate a transition process in a Laminar Separation bubble, with the emphasis being put on the numerical contribution. A Laminar Separation bubble is formed if a Laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reattached turbulent boundary layer. Development of disturbances during the transition process in such a Separation bubble is studied by means of direct numerical simulation with controlled disturbance input. Focus is put on the stage of non-linear development of these perturbations, for which a detailed comparison between numerical and experimental results is given. Beside physical phenomena like shear-layer roll-up and vortex shedding, computational aspects such as the performance of the numerical code on supercomputers are treated.
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Experimental and Numerical Investigations on Transition in a Laminar Separation Bubble
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A Laminar boundary layer separates in a region of adverse pressure gradient on a flat plate, undergoes transition, and finally the turbulent boundary layer reattaches. Laminar-turbulent transition within this Laminar Separation bubble (LSB) is investigated by means of measurements with a Laser-Doppler-Anemometer (LDA), flow visualization in water and direct numerical simulation (DNS). The role of unsteady disturbances with and without controlled spanwise variation in the occuring mechanism of transition are examined in detail.
Hermann F. Fasel - One of the best experts on this subject based on the ideXlab platform.
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Laminar-turbulent Transition in a Laminar Separation Bubble in the Presence of Free-stream Turbulence
Procedia IUTAM, 2020Co-Authors: Shirzad Hosseinverdi, Hermann F. FaselAbstract:Abstract Direct numerical simulations (DNS) are employed to investigate the hydrodynamic instability mechanisms and transition to turbulence in Laminar Separation bubbles (LSBs) on a flat plate. A set of numerical simulat2013-0264ions has been carried out to investigate the transition process, and in particular to shed light on the development of the large coherent structures, which arise during transition. Particular focus is directed towards understanding and identifying the relevant physical mechanisms governing the interaction of Separation and transition in Laminar Separation bubbles in the presence of free-stream turbulence (FST). For the natural flow. i.e. zero FST, the transition mechanism involves a Kelvin-Helmholtz instability and a growth of three-dimensional very low-frequency disturbances of the shear layer. With the inclusion of FST, transition is accelerated. For the Separation bubbles investigated, the transition process is the result of two different mechanisms: i) Strong amplification of high-frequency (order of the shedding frequency), essentially two-dimensional or weakly oblique fluctuating disturbances and ii) low-frequency, three- dimensional Klebanoff perturbations caused by FST. Depending on the intensity of the FST, one of these mechanisms would dominate the transition process, or both mechanisms are blended together and contribute simultaneously.
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Active Control of Laminar Separation: Simulations, Wind Tunnel, and Free-Flight Experiments
Aerospace, 2018Co-Authors: Andreas Gross, Hermann F. FaselAbstract:When a Laminar boundary layer is subjected to an adverse pressure gradient, Laminar Separation bubbles can occur. At low Reynolds numbers, the bubble size can be substantial, and the aerodynamic performance can be reduced considerably. At higher Reynolds numbers, the bubble bursting can determine the stall characteristics. For either setting, an active control that suppresses or delays Laminar Separation is desirable. A combined numerical and experimental approach was taken for investigating active flow control and its interplay with Separation and transition for Laminar Separation bubbles for chord-based Reynolds numbers of Re ≈ 64,200–320,000. Experiments were carried out both in the wind tunnel and in free flight using an instrumented 1:5 scale model of the Aeromot 200S, which has a modified NACA 643-618 airfoil. The same airfoil was also used in the simulations and wind tunnel experiments. For a wide angle of attack range below stall, the flow separates Laminar from the suction surface. Separation control via a dielectric barrier discharge plasma actuator and unsteady blowing through holes were investigated. For a properly chosen actuation amplitude and frequency, the Kelvin–Helmholtz instability results in strong disturbance amplification and a “roll-up” of the separated shear layer. As a result, an efficient and effective Laminar Separation control is realized.
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effect of free stream turbulence on the structure and dynamics of Laminar Separation bubbles
53rd AIAA Aerospace Sciences Meeting 2015, 2015Co-Authors: Shirzad Hosseinverdi, Hermann F. FaselAbstract:Laminar Separation is always associated with considerable unsteadiness. This unsteadiness is caused by large coherent structures that are a consequence of hydrodynamic instability mechanisms of the mean flow. The mean-flow topology and unsteady behavior of Laminar Separation bubbles (LSB) is in fact mainly governed by instability and transition. In this paper, Laminar Separation bubbles, which are generated on a flat plate by imposing a streamwise adverse pressure gradient, are investigated by means of Direct Numerical Simulations (DNS). The streamwise pressure gradient for the DNS is chosen such that the inviscid wall pressure distribution, as reported in the Gaster 1 experimental series I, case IV, is closely matched. This case was classified as a “short” Laminar Separation bubble. The timeaveraged flow field obtained from the DNS with no external disturbances introduced (no freestream turbulence), reveals that the bubble is longer than observed in the experiments. In fact, the bubble obtained in the simulations appeared to be a “long” bubble. This was confirmed by comparing the simulation results with the measurements by Gaster 1 for a long bubble. The discrepancy between the numerical simulations and experiments is possibly due to an earlier onset of transition in the experiments. In the present simulations, instead of forcing with random disturbances to promote transition, isotropic grid turbulence, which was modeled using a superposition of eigenmodes from the continuous spectrum of the Orr-Sommerfeld and Squire operators is introduced at the inflow boundary. It was observed that as the freestream turbulence (FST) intensity was increased, the bubble became smaller. The Separation bubble was in fact shortened from both sides (Separation and reattachment sides) in the presence of free-stream turbulence. Comparing the wall pressure distribution for 0.2% freestream turbulence with Gaster 1 experiment revealed that then the bubble could be classified as a “short” bubble. Based on the simulations performed, FST can change a Separation bubbles form “long” to “short”. In order to investigate bubble “bursting”, the development of bubble, that had became short due to FST, was simulated after the FST was turned-off. The short bubble grew for a short period of time. Surprisingly however, it did not return to the original, state without FST.
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Direct Numerical Simulations of Laminar Separation Bubbles on a Curved Plate: Part 1 — Simulation Setup and Uncontrolled Flow
Volume 6B: Turbomachinery, 2013Co-Authors: Wolfgang Balzer, Hermann F. FaselAbstract:The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by Laminar Separation. For a modern low-pressure turbine (LPT) stage, in particular when designed for high blade loadings, Laminar Separation at cruise conditions can result in significant performance degradation. Understanding of the physical mechanisms and hydrodynamic instabilities that are associated with Laminar Separation and the formation of Laminar Separation bubbles (LSBs) is key for the design and development of effective and efficient active flow control (AFC) devices. For the present work, Laminar Separation (part I) and its control (part II) were investigated numerically by employing highly-resolved, high-order accurate direct numerical simulations (DNS).Copyright © 2013 by ASME
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Experimental Investigation of the Structure and Dynamics of Laminar Separation Bubbles
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012Co-Authors: Hermann F. FaselAbstract:This work is an experimental investigation of the dynamics of the Laminar Separation bubbles, which are typically present on the suction side of lifting surfaces at a large angle of attack. The Separation bubble was generated on a flat plate by an adverse pressure gradient induced by The adverse pressure gradient was generated by using an inverted wing with a NACA 643-618 airfoil mounted above the flat plate. Using Particle Image Velocimetry (PIV), a parametric study of the effect of the upstream flow velocity and the induced pressure gradient on the mean flow topology and the unsteady behavior of the Separation bubble was carried out in the low-speed water tunnel of the Hydrodynamics Laboratory at the University of Arizona. The topology of the Laminar Separation bubble, and in particular the unsteady flow dynamics, were found to be strongly dependent on these parameters. For certain conditions, strong vortex shedding near the reattachment region of the bubble was observed, which is a characterisc behavior of short bubbles. High-resolution spatio-temporal PIV measurements were made to analyze the formation and breakdown of these flow structures. The frequency of vortex shedding was determined from Fourier analysis of the time series of the velocity fluctuations. The non-dimensionalised frequencies were found to be nearly independent of the Reynolds number for the range of Reynolds numbers investigated here.
Olaf Marxen - One of the best experts on this subject based on the ideXlab platform.
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DNS AND LES OF THE TRANSITION PROCESS IN A Laminar Separation BUBBLE
Direct and Large-Eddy Simulation V, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:A region of strong local adverse pressure gradient acting on a Laminar flat-plate boundary layer can produce a closed fully Laminar Separation bubble for sufficiently small pressure rise and Reynolds number. However, such a flow field is hydrodynamically highly unstable and transition will occur in the region of adverse pressure gradient. Due to an interaction with the potential flow, the transition process may even suppress Laminar Separation completely.
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Direct Numerical Simulation of a Short Laminar Separation Bubble and Early Stages of the Bursting Process
Notes on Numerical Fluid Mechanics and Multidisciplinary Design (NNFM), 2020Co-Authors: Olaf Marxen, Dan S HenningsonAbstract:Direct numerical simulation of a pressure-induced short Laminar Separation bubble developing on a flat plate has been carried out. Transition in this bubble was triggered by small disturbance input with a fixed frequency and fixed spanwise wave number. The resulting short bubble was shown to be converged in time to a statistically steady state, while possessing essential features of short Laminar Separation bubbles as reported in the literature. In the present case disturbance input is required to maintain a short bubble. Switching off this disturbance input yields a growing Separation bubble. This phenomenon is denoted as bubble bursting, since indication is found that the bubble develops towards a long-bubble state.
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Direct Numerical Simulation of Non-Linear Transitional Stages in an Experimentally Investigated Laminar Separation Bubble
High Performance Computing in Science and Engineering’ 05, 2020Co-Authors: Olaf Marxen, Ulrich RistAbstract:This paper details a joint numerical and experimental effort to investigate a transition process in a Laminar Separation bubble, with the emphasis being put on the numerical contribution. A Laminar Separation bubble is formed if a Laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reattached turbulent boundary layer. Development of disturbances during the transition process in such a Separation bubble is studied by means of direct numerical simulation with controlled disturbance input. Focus is put on the stage of non-linear development of these perturbations, for which a detailed comparison between numerical and experimental results is given. Beside physical phenomena like shear-layer roll-up and vortex shedding, computational aspects such as the performance of the numerical code on supercomputers are treated.
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Experimental and Numerical Investigations on Transition in a Laminar Separation Bubble
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A Laminar boundary layer separates in a region of adverse pressure gradient on a flat plate, undergoes transition, and finally the turbulent boundary layer reattaches. Laminar-turbulent transition within this Laminar Separation bubble (LSB) is investigated by means of measurements with a Laser-Doppler-Anemometer (LDA), flow visualization in water and direct numerical simulation (DNS). The role of unsteady disturbances with and without controlled spanwise variation in the occuring mechanism of transition are examined in detail.
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A Combined Numerical and Experimental Investigation of Transition in a Laminar Separation Bubble
Recent Results in Laminar-Turbulent Transition, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A Laminar boundary layer separates in a region of adverse pressure gradient, undergoes transition, and finally the turbulent boundary layer reattaches, forming a Laminar Separation bubble (LSB). Laminar-turbulent transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (mono PIV and stereoscopic PIV (SPIV)) and direct numerical simulation (DNS).
Rolf Radespiel - One of the best experts on this subject based on the ideXlab platform.
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dynamics of Laminar Separation bubbles at low reynolds number aerofoils
Journal of Fluid Mechanics, 2009Co-Authors: Rainer Hain, Christian J Kahler, Rolf RadespielAbstract:The Laminar Separation bubble on an SD7003 aerofoil at a Reynolds number Re = 66000 was investigated to determine the dominant frequencies of the transition process and the flapping of the bubble. The measurements were performed with a high-resolution time-resolved particle image velocimetry (TR-PIV) system. Contrary to typical measurements performed through conventional PIV, the different modes can be identified by applying TR-PIV. The interaction between the shed vortices is analysed, and their significance for the production of turbulence is presented. In the shear layer above the bubble the generation and amplification of vortices due to Kelvin–Helmholtz instabilities is observed. It is found that these instabilities have a weak coherence in the spanwise direction. In a later stage of transition these vortices lead to a three-dimensional breakdown to turbulence.
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numerical and experimental flow analysis of moving airfoils with Laminar Separation bubbles
AIAA Journal, 2007Co-Authors: Rolf Radespiel, Jan Windte, Ulrich ScholzAbstract:Experimental measurements and unsteady Reynolds-averaged Navier-Stokes simulations of the low-Reynolds-number flow past an SD7003 airfoil with and without plunge motion at Re = 60 k are presented, where transition takes place across Laminar Separation bubbles. The experimental data consist of high-resolution, phase-locked particle image velocimetry measurements in a wind tunnel and a water tunnel. The numerical simulation approach includes transition prediction which is based on linear stability analysis applied to unsteady mean-flow data. The numerical results obtained for steady onflow are validated against particle image velocimetry data and published force measurements. Good agreement is obtained for specific turbulence models. Flows with plunge motion reveal strong effects of flow unsteadiness on transition and the resulting Laminar Separation bubbles which are well captured in the simulations.
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validation of the rans simulation of Laminar Separation bubbles on airfoils
Aerospace Science and Technology, 2006Co-Authors: Jan Windte, Ulrich Scholz, Rolf RadespielAbstract:Abstract This paper presents RANS simulations of the low-Reynolds-number flow past an SD7003 airfoil at Re = 6 × 10 4 , where transition takes place across a Laminar Separation bubble. The transition prediction procedure using an approximate envelope method as well as a linear stability solver is discussed. The numerical results are validated against PIV- and force measurements obtained in several wind- and watertunnels and are also compared to XFOIL results. Good agreement is found within the operational range of the airfoil.
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rans simulation and experiments on the stall behaviour of an airfoil with Laminar Separation bubbles
44th AIAA Aerospace Sciences Meeting and Exhibit, 2006Co-Authors: Ralf Wokoeck, Rolf Radespiel, Normann Krimmelbein, Jens Ortmanns, Vlad Ciobaca, Andreas KrumbeinAbstract:Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carried out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving Laminar Separation bubbles. One of the methods computes transition locations across Laminar Separation bubbles whereas the other assumes transition onset where Laminar Separations occur. For validation of the numerical methods an extensive measurement campaign has been carried out. It is shown, that the methodology mentioned first can simulate the size of Laminar Separation bubbles for angles of attack up to where the Separation bubble and the turbulent Separation at the trailing edge are well behaved and steady in the mean. With trailing edge Separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Cases with large 3D flow structures inside the turbulent trailing edge Separations in windtunnel experiments for high angles of attack are compared and analysed along with 2D and 3D steady RANS calculations that model the measurement section of the windtunnel.
Siegfried Wagner - One of the best experts on this subject based on the ideXlab platform.
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Active Control of a Laminar Separation Bubble
Aerodynamic Drag Reduction Technologies, 2020Co-Authors: Kai Augustin, Ulrich Rist, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of Laminar Separation bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Numerical Simulation of Laminar Separation-Bubble Control
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: Ulrich Rist, Kai Augustin, Siegfried WagnerAbstract:In the present paper an active control mechanism for the control of Laminar Separation bubbles on airfoils is investigated by means of direct numerical simulation and linear stability theory. Boundary layer instabilities excited by periodic oscillations are utilized to control the size and length of the Separation bubble and to make it finally disappear when desired. Unlike traditional vortex generators a sensor-actuator system based on this method will be adaptive to the respective flow conditions and will cause no additional undesired drag.
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Experimental and Numerical Investigations on Transition in a Laminar Separation Bubble
New Results in Numerical and Experimental Fluid Mechanics III, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A Laminar boundary layer separates in a region of adverse pressure gradient on a flat plate, undergoes transition, and finally the turbulent boundary layer reattaches. Laminar-turbulent transition within this Laminar Separation bubble (LSB) is investigated by means of measurements with a Laser-Doppler-Anemometer (LDA), flow visualization in water and direct numerical simulation (DNS). The role of unsteady disturbances with and without controlled spanwise variation in the occuring mechanism of transition are examined in detail.
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Secondary Disturbance Amplification and Transition in Laminar Separation Bubbles
Laminar-Turbulent Transition, 2020Co-Authors: Ulrich Maucher, Ulrich Rist, Siegfried WagnerAbstract:In Laminar Separation bubbles a new mechanism of secondary instability is presented which exists for large boundary layer Reynolds numbers at Separation. If a 2D wave is forced, temporal secondary amplification of 3D modes occurs. It is based on instabilities of instantaneously appearing high-shear layers with respect to 3D perturbations left over from the previous TS-period. After the 3D modes gain large amplitudes transition sets on. This phase is again characterized by the entrainment of 3D disturbances by the 2D shear layer in the re-attachment zone. The 3D disturbances pierce the detached shear layer from underneath and destroy it very rapidly, thus leaving spanwise rolls of turbulent flow.
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A Combined Numerical and Experimental Investigation of Transition in a Laminar Separation Bubble
Recent Results in Laminar-Turbulent Transition, 2020Co-Authors: M Lang, Olaf Marxen, Ulrich Rist, Siegfried WagnerAbstract:A Laminar boundary layer separates in a region of adverse pressure gradient, undergoes transition, and finally the turbulent boundary layer reattaches, forming a Laminar Separation bubble (LSB). Laminar-turbulent transition within such a LSB is investigated by means of Laser-Doppler-Anemometry (LDA), Particle Image Velocimetry (mono PIV and stereoscopic PIV (SPIV)) and direct numerical simulation (DNS).
