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  • 1.
    Herbst, Astrid
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical Studies of Turbulent and Separeted Flows2006Doctoral thesis, comprehensive summary (Other scientific)
  • 2.
    Herbst, Astrid
    KTH, Superseded Departments, Mechanics.
    Studies of periodic excitation of a turbulent separation bubble2004Licentiate thesis, comprehensive summary (Other scientific)
  • 3.
    Herbst, Astrid H.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Control of a turbulent separation bubble by periodic excitation2005In: Progress in Turbulence / [ed] Peinke, J; Kittel, A; Barth, S; Oberlack, M, 2005, Vol. 101, p. 181-184Conference paper (Refereed)
  • 4.
    Herbst, Astrid
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The effect of the sweep angle on the turbulent separation bubble on a flate plate2006Report (Other academic)
  • 5.
    Herbst, Astrid
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Deubelbeiss, S
    Spehr, Saskia
    Hanifi, Ardeshir
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Instability characteristics of harmonic disturbances in a turbulent separation bubble2005In: Proceedings of XVII Congresso Aimeta di Meccanica Teorica e Applicata, 2005Conference paper (Refereed)
    Abstract [en]

    The instability characteristics of a turbulent flat plate boundary layer separating under a strong adverse pressure gradient are examined. The analysis is based on the data of direct numerical simulation. A theoretical model of harmonic perturbations is considered, including the contribution of the turbulent part of the flow, to investigate the stability characteristics of the flow. The structure of the organized waves is also investigated by means of Proper Orthogonal Decompositions (POD).

  • 6.
    Herbst, Astrid H.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Bombardier Transportation Sweden AB, Västerås.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The effect of the sweep angle on the turbulent separation bubble on a flate plate2007In: Advances in Turbulence XI - Proceedings of the 11th EUROMECH European Turbulence Conference, 2007, p. 230-232Conference paper (Refereed)
  • 7.
    Herbst, Astrid H.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Simulations of turbulent flow in a plane asymmetric diffuser2007In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 79, no 3, p. 275-306Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined wall of the diffuser is chosen as 8.5 degrees, a case for which recent experimental data are available. Reasonable agreement between the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall inclined at a diverging angle of 10 degrees, which has served as a test case for a number of experimental as well as numerical studies in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of the inflowing, fully turbulent channel flow has been studied, in particular, Re-b = 4,500, Re-b = 9,000 and Re-b = 20,000 with Re-b being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour as the main separation region, i.e. the flow is separating less with higher Re-b . Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for larger Re-b . Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow condition (similar to a precursor simulation) for the diffuser is assessed.

  • 8.
    Herbst, Astrid
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    The influence of periodic excitation on a turbulent separation bubble2006In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 76, no 1, p. 1-21Article in journal (Refereed)
    Abstract [en]

    Turbulent separation limits the performance in many engineering applications, for example creating pressure losses in diffuser like flows or stall on aircraft wings. In the present study the turbulent boundary layer flow over a flat plate separating due to an adverse pressure gradient is studied as a model problem and the effect of periodic excitation in both time and space is investigated through direct numerical simulations. Linear stability analysis is used to analyse the sensitivity of the flow with respect to time-periodic excitations. The dependence on position, amplitude and frequency of the forcing is investigated. For a certain frequency range at sufficiently high amplitudes, it is possible to eliminate the separated region. Furthermore, three-dimensional effects are studied by applying a steady spanwise forcing as well as a both time-dependent and spanwise varying forcing. A forcing varying in spanwise direction is shown to be the most effective in eliminating the separated region, whereas two-dimensional time-periodic excitation was not as efficient as it was expected.

  • 9.
    Levin, Ori
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Herbst, Astrid
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Early turbulent evolution of the Blasius wall jet2006In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 7, p. 1-17Article in journal (Refereed)
    Abstract [en]

    The first direct numerical simulation that is sufficiently large to study the self-similar behaviour of a turbulent wall jet is performed. The investigation is an extension of the simulation performed by Levin et al. (2005, A study of the Blasius wall jet. Journal of Fluid Mechanics, 539, 313-347). The same numerical method is used, but a significantly larger computational domain enables us to follow the development of the flow throughout the transition into its early turbulent evolution. Two-dimensional waves and streamwise elongated streaks, matched to measured disturbances, are introduced in the flow to trigger a natural transition mechanism. The Reynolds number is 3090 based on the inlet velocity and the nozzle height. The simulation provides detailed visualisations of the flow structures and statistics of mean flow and turbulent stresses. A weak subharmonic behaviour in the transition region is revealed by animations of the flow. The averaged data are presented in both inner and outer scalings in order to identify self-similar behaviour. Despite the low Reynolds number and the short computational domain, the turbulent flow exhibits a reasonable self-similar behaviour, which is most pronounced with inner scaling in the near-wall region

  • 10.
    Muld, Tomas W.
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Herbst, Astrid H.
    Bombardier Transportation, Sweden.
    Orellano, Alexander
    Bombardier Transportation, Sweden.
    Analysis of flow structures in the wake of a high-speed train2016In: Proceedings aerodynamics of heavy vehicles III, buses, trucks and trains, Springer, 2016, Vol. 79Conference paper (Refereed)
    Abstract [en]

    Slipstream is the flow that a train pulls along due to the viscosity of the fluid. In real life applications, the effect of the slipstream flow is a safety concern for people on platform, tracksideworkers and objects on platforms such as baggage carts and pushchairs. The most important region for slipstream of high-speed passanger trains is the near wake, in which the flow is fully turbulent with a broad range of length and time scales. In this work, the flow around the Aerodynamic Train Model (ATM) is simulated using Detached Eddy Simulation (DES) to model the turbulence. Different grids are used in order to prove grid converged results. In order to compare with the results of experimental work performed at DLR on the ATM, where a trip wire was attached to the model, it turned out to be necessary to model this wire to have comparable results. An attempt to model the effect of the trip wire via volume forces improved the results but we were not successful at reproducing the full velocity profiles. The flow is analyzed by computing the POD and Koopman modes. The structures in the floware found to be associated with two counter rotating vortices. A strong connection between pairs of modes is found, which is related to the propagation of flow structures for the POD modes. Koopman modes and POD modes are similar in the spatial structure and similarities in frequencies of the time evolution of the structures are also found.

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