62 research outputs found

    Active flow control of a two-dimensional compressible cavity flow using direct output feedback law

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    Acoustic perturbations emitted from a two dimensional compressible cavity flow are analysed and controlled using direct numerical simulations (DNS) and an output feedback law which expresses the actuation (a synthetic jet) velocity as a function of wall pressure fluctuation sensors. To get the feedback control law, a non-linear reduced-order model (ROM) is first built from un-actuated flow fields and flow forced with a large frequency bandwidth actuation. The control law is then determined by a Linear Quadratic Regulator algorithm. The control law implemented in the DNS provides a significant and global noise reduction (up to 10 dB). The actuation law remains efficient for long times, greater than the temporal window of observation used to design the ROM. Actuation of moderate amplitudes generate some non-linear effects but perturbations continue to be damped

    Adjoint-based sensitivity and feedback control of noise emission

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    A LQR control is performed on a reduce order model built from Direct Numerical Simulation of an open cavity flow, for a 2D geometry, and in the aim of controlling noise emission. A -10 dB achievement is demonstrate

    Sensitivity analysis for subsonic jet using adjoint of non local stability equations

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    The first step of the sensitivy analysis of some quadratic quantity related to acoustic waves with respect to any flow or wall disturbance is proposed in the configuration of a low and large Reynolds number jet flow. The generation of noise has been demonstrated to originate from convective instabilities amplifying in the jet stream. Several authors have investigated them throught the Parabolized Stability Equations approach(PSE). The present work aims to developp the adjoint of the PSE to extract from a mathematically well posed problem the sensitivity coefficients which can be understood as gradient. The final aim is to propose some path of possible actuations in order to decrease noise emission in some jet flows. Event is it is too early to produce some final conclusions, some trend can be given. The shape and the location of the maximun of sensitivity are strongly related to the radial and streamwise variation of the base flow. In particuler the maximun of sensitivy is located along the border of the potential cone and seems to be well correlated with the location of the sound generation. In addition the sensitivity to radial momentum forcing is much higher than to a streamwise momentum forcing. Finally the sensitivity increases when the streamwise coordinate decreases

    Non viscous sensitivity analysis of noise generation mechanism in a low Mach number jet

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    The first step of the sensitivity analysis of some quadratic quantity related to acoustic waves with respect to any flow or wall disturbance is proposed in the configuration of subsonic jet flow. The generation of noise has been demonstrated to originate from convective instabilities that amplify in the jet stream. Several authors have investigated them through the Parabolized Stability Equations approach (PSE). The present work aims to develop the adjoint of the PSE to extract from a mathematically well posed problem the sensitivity coefficients which can be understood as gradient. The final objective is to propose some path of possible actuations in order to decrease noise emission in some jet flows. To date some trend can be given. The shape and the location of the maximum of sensitivity are strongly related to the radial and streamwise variation of the base flow. In particular the maximum of sensitivity is located along the border of the potential cone and seems to be well correlated with the location of the sound generation mechanism. In addition the sensitivity to axial momentum forcing is higher than to a radial momentum forcing. Finally the sensitivity increases when the perturbation is near to the exit of the nozzle

    Sensitivity computations by automatic differentiation of a CFD code based on spectral differences

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    We compute flow sensitivities by modifying a CFD code which is spatially-discretized with spectral differences. Our discrete approach relies on algorithmic differentiation. We obtain two transformed codes, one for each differentiation mode : tangent and adjoint. Both codes compute sensitivities in an unsteady test case of two-dimensional incompressible flow inside a periodic cube with an initial double-shear profile. The sensitivities from both codes agree to within machine accuracy, and compare well with those approximated by finite difference computations. We discuss execution times and describe our strategy to automatically differentiate in adjoint mode a parallel code containing MPI instructions

    Sensitivity computations by algorithmic differentiation of a high-­order cfd code based on spectral differences

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    We compute flow sensitivities by differentiating a high-­order computational fluid dynamics code. Our fully discrete approach relies on automatic differentiation (AD) of the original source code. We obtain two transformed codes by using the AD tool Tapenade (INRIA), one for each differentiation mode: tangent and adjoint. Both differentiated codes are tested against each other by computing sensitivities in an unsteady test case. The results from both codes agree to within machine accuracy, and compare well with those approximated by finite differences. We compare execution times and discuss the encountered technical difficulties due to 1) the code parallelism and 2) the memory overhead caused by unsteady problems

    Effects of porosity and inertia on the apparent permeability tensor in fibrous media

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    The flow in three-dimensional fibrous porous media is studied in the inertial regime by first simulating for the motion in unit, periodic cells, and then solving successive closure problems leading – after applying an intrinsic averaging procedure – to the components of the apparent permeability tensor. The parameters varied include the orientation of the driving pressure gradient, its magnitude (which permits to define a microscopic Reynolds number), and the porosity of the medium. All cases tested refer to situations for which the microscopic flow is steady. When the driving force is oriented in a direction whichlies on the plane perpendicular to the fibers’ axis, the results found agree with those available the literature. The fact that the medium is composed by bundles of parallel fibers favours a deviation of the mean flow towards the fibers’ axis when the driving pressure gradient has even a small component along it, and this is enhanced by a decreasing porosity; this phenomenon is well quantified by the knowledge of the components of the permeability. Contrary to our initial expectations, for the over one hundred cases which we have simulated, the apparent permeability tensor remains, to a very good approximation, diagonal, a fact mainly related to the transversely isotropic nature of the medium. To obtain a complete,albeit approximate, database of the diagonal components of the apparent permeability tensor we have developed a metamodel, based on kriging interpolation, and carefully calibrated it. The resulting response surfaces can be invaluable in determining the force caused by the presence of inclusions in macroscopic simulations of the flow through bundles of fibers whose orientations and dimensions can vary in space and/or time

    Noise sensitivity in a 2D compressible boundary layer flow past a cavity

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    Une couche limite 2D, affleurant une cavité engendre une émission acoustique via le mécanisme de Rossiter. Nous étudions la sensibilité du bruit à des perturbations quelconques par des DNS et leur adjoint. L'étude de sensibilité conduit à déterminer les positions optimales ainsi que les directions optimales d'actions ou de mesure, ainsi que leur forme spatio-temporelle. On retrouve les modes 2D d'instabilité de la littérature. La sensibilité qui est un gradient, est introduite dans un algorithme de contrôle optimal afin d'aboutir à une réduction du bruit

    On the generation of the mean velocity profile for turbulent boundary layers with pressure gradient under equilibrium conditions.

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    The generation of a fully turbulent boundary layer profile is investigated using analytical and numerical methods over the Reynolds number range 422 ≤ Reθ ≤ 31,000. The numerical method uses a new mixing length blending function. The predictions are validated against reference wind tunnel measurements under zero streamwise pressure gradient. The methods are then tested for low and moderate adverse pressure gradients. Comparison against experiment and DNS data show a good predictive ability under zero pressure gradient and moderate adverse pressure gradient, with both methods providing a complete velocity profile through the viscous sub-layer down to the wall. These methods are useful computational fluid dynamic tools for generating an equilibrium thick turbulent boundary layer at the computational domain inflow

    A bypass transition in the Lamb-Oseen vortex

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    Transient energy growth in the short-time linear dynamics of a Lamb-Oseen monopole is a potential mechanism for nonlinear bypass transition, a phenomenon already observed in both experiments and numerical simulations. In the present study, we investigate this scenario by means of a nonlinear optimal perturbation approach, i.e. by looking for the initial perturbation whose evolution satisfies the fully nonlinear Navier-Stokes equations and maximizes the energy gain at a given time horizon. Preliminary two-dimensional results show that, for small initial amplitudes, the optimal perturbation and growth mechanisms observed in the linear regime are recovered. More particularly, the time evolution of the m = 2 optimal perturbation leads to an elliptical core deformation of the monopole, which suggests a potential bypass scenario driven by the non-linear dynamics. This is confirmed by computations for larger initial perturbation amplitudes: the optimal perturbation is similar to that of the linear regime but a subcritical bifurcation to a quasi-steady, high-energy, rotating tripole is observed
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