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  • 1.
    Borglund, Dan
    KTH, Superseded Departments, Aeronautical Engineering.
    Active nozzle control and integrated design optimization of a beam subject to fluid-dynamic forces.1999In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 13, no 2, p. 269-287Article in journal (Refereed)
    Abstract [en]

    Active nozzle control is used to improve the stability of a beam subject to forces induced by fluid flow through attached pipes. The control system has a significant effect on the structural stability, making both flutter and divergence type of instabilities possible. The stability analysis is carried out using a state-variable approach based on a finite element formulation of the structural dynamics. The simultaneous design of the control system and the beam shape minimizing structural mass is performed using numerical optimization. The inclusion of the control system in the optimization gives a considerable reduction of the structural mass but results in an optimal design which is very sensitive to imperfections. Using a simple model of the control system uncertainties, a more robust design is obtained by solving a modified optimization problem. Throughout the study, the theoretical findings are verified by experiments.

  • 2.
    Borglund, Dan
    KTH, Superseded Departments, Aeronautical Engineering.
    On the optimal design of pipes conveying fluid.1998In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 12, no 3, p. 353-365Article in journal (Refereed)
    Abstract [en]

    The stability and optimal design of a beam subject to forces induced by fluid flow through attached pipes is investigated. The structure is assumed to have the same dynamics as a fluid-conveying pipe, and the dynamic stability is analysed using a finite element formulation of the linear equation of motion. The optimal design problem of minimizing the structural mass at fixed critical flow speed is solved. The numerical results are compared to experiments with satisfactory agreement, provided that the lower bounds of the beam dimensions are properly chosen. The influence of structural damping on the critical flow speed is significant, and is found to be strongly design-dependent.

  • 3.
    Borglund, Dan
    et al.
    KTH, Superseded Departments, Aeronautical Engineering.
    Kuttenkeuler, Jakob
    KTH, Superseded Departments, Aeronautical Engineering.
    Active wing flutter suppression using a trailing edge flap2002In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 16, no 3, p. 271-294Article in journal (Refereed)
    Abstract [en]

    The aeroservoelastic behaviour of a thin rectangular wing with a controllable trailing edge flap is investigated. A rather high aspect ratio motivates a numerical model based on linear beam theory for the structural dynamics and strip theory for the unsteady aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis, and the impact of the trailing edge flap on the dynamics is verified by open-loop testing. The problem of stabilizing the wing utilizing the trailing edge flap is posed, and the design of a fixed-structure feedback controller is performed using numerical optimization. The problem of maximizing closed-loop modal damping with constraints on actuator performance is solved for a sequence of flow speeds and the obtained controller is synthesized using gain scheduling. The fairly large predicted increase in critical speed is experimentally verified with satisfactory accuracy.

  • 4.
    Finnveden, Svante
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Birgersson, F.
    Ross, Urmas
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Kremer, Tobias
    A model of wall pressure correlation for prediction of turbulence-induced vibration2005In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 20, no 8, p. 1127-1143Article in journal (Refereed)
    Abstract [en]

    The vibration response of a structure excited by a turbulent boundary layer is investigated experimentally and numerically. First, the wall pressure in a high speed acoustic wind tunnel is characterized and the cross-spectral density is approximated using a Corcos model with frequency dependent correlation lengths and a modified Chase model. Both models agree quite well with the measured cross spectrum. Second, based on these turbulence models, the vibration response is predicted and compared to measurements. At lower frequencies both models perform well. In a higher frequency region, however, the vibration response is greatest for length scales that are much longer than the one given by the convection velocity of the turbulence, and in this frequency region only the modified Chase model works effectively.

  • 5.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Konieczny, P
    Alfredsson, Per Henrik
    KTH, Superseded Departments, Mechanics.
    Flow around a porous cylinder subject to continuous suction or blowing2004In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 19, no 8, p. 1031-1048Article in journal (Refereed)
    Abstract [en]

    In the present experimental investigation the surface pressure distribution, vortex shedding frequency, and the wake flow behind a porous circular cylinder are studied when continuous suction or blowing is applied through the cylinder walls. It is found that even moderate levels of suction/blowing (less than or similar to 5% of the oncoming streamwise velocity) have a large impact on the flow around the cylinder. Suction delays separation contributing to a narrower wake width, and a corresponding reduction of drag, whereas blowing shows the opposite behaviour. Both uniform suction and blowing display unexpected flow features which are analysed in detail. Suction shows a decrease of the turbulence intensity throughout the whole wake when compared with the natural case, whilst blowing only shows an effect up to five diameters downstream of the cylinder. The drag on the cylinder is shown to increase linearly with the blowing rate, whereas for suction there is a drastic decrease at a specific suction rate. This is shown to be an effect of the separation point moving towards the rear part of the cylinder, similar to what happens when transition to turbulence occurs in the boundary layer on a solid cylinder. The suction/blowing rate can empirically be represented by an effective Reynolds number for the solid cylinder, and an analytical expression for this Reynolds number representation is proposed and verified. Flow visualizations expose the complexity of the flow field in the near wake of the cylinder, and image averaging enables the retrieval of quantitative information, such as the vortex formation length.

  • 6. Holmvall, M.
    et al.
    Uesaka, T.
    Drolet, F.
    Lindström, Stefan B.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Transfer of a microfluid to a stochastic fibre network2011In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 27, no 7, p. 937-946Article in journal (Refereed)
    Abstract [en]

    The transfer of a microscopic fluid droplet from a flat surface to a deformable stochastic fibre network is investigated. Fibre networks are generated with different levels of surface roughness, and a two-dimensional, two-phase fluid-structure model is used to simulate the fluid transfer. In simulations, the Navier-Stokes equations and the Cahn-Hilliard phase-field equations are coupled to explicitly include contact line dynamics and free surface dynamics. The compressing fibre network is modelled as moving immersed boundaries. The simulations show that the amount of transferred fluid is approximately proportional to the contact area between the fluid and the fibre network. However, areas where the fluid bridges and never actually makes contact with the substrate must be subtracted.

  • 7.
    Lokatt, Mikaela
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Flight Dynamics.
    Aeroelastic flutter analysis considering modeling uncertainties2017In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 74, p. 247-262Article in journal (Refereed)
    Abstract [en]

    A method for efficient flutter analysis of aeroelastic systems including modeling uncertainties is presented. The aerodynamic model is approximated by a piece-wise continuous rational polynomial function, allowing the flutter equation to be formulated as a set of piece-wise linear eigenproblems. Feasible sets for eigenvalue variations caused by combinations of modeling uncertainties are computed with an approach based on eigenvalue differentials and Minkowski sums. The method allows a general linear formulation for the nominal system model as well as for the uncertainty description and is thus straightforwardly applicable to linearized aeroelastic models including both structural and aerodynamic uncertainties. It has favorable computational properties and, for a wide range of uncertainty descriptions, feasible sets can be computed in output polynomial time. The method is applied to analyze the flutter characteristics of a delta wing model. It is found that both structural and aerodynamic uncertainties can have a considerable effect on the damping trends of the flutter modes and thus need to be accounted for in order to obtain reliable predictions of the flutter characteristics. This indicates that it can be beneficial to allow a flexible and detailed formulation for both aerodynamic and structural uncertainties, as is possible with the present system formulation.

  • 8.
    Nagib, H. M.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vidal, A.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vorticity fluxes: A tool for three-dimensional and secondary flows in turbulent shear flows2019In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 89, p. 39-48Article in journal (Refereed)
    Abstract [en]

    In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers Re h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference (w'omega(y)') over bar (+) - (v'omega(z)') over bar (+) is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q1 and Q4 events, which essentially tilt momentum towards the peak.

  • 9. Natali, Damiano
    et al.
    Pralits, Jan O.
    Mazzino, Andrea
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stabilizing effect of porosity on a flapping filament2016In: Journal of Fluids and Structures, ISSN 0889-9746, E-ISSN 1095-8622, Vol. 61, p. 362-375Article in journal (Refereed)
    Abstract [en]

    A new way of handling, simultaneously, porosity and bending resistance of a massive filament is proposed. Our strategy extends the previous methods where porosity was taken into account in the absence of bending resistance of the structure and overcomes related numerical issues. The new strategy has been exploited to investigate how porosity affects the stability of slender elastic objects exposed to a uniform stream. To understand under which conditions porosity becomes important, we propose a simple resonance mechanism between a properly defined characteristic porous time-scale and the standard characteristic hydrodynamic time-scale. The resonance condition results in a critical value for the porosity above which porosity is important for the resulting filament flapping regime, otherwise its role can be considered of little importance. Our estimation for the critical value of the porosity is in fairly good agreement with our DNS results. The computations also allow us to quantitatively establish the stabilizing role of porosity in the flapping regimes.

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