| 1. |
- Dadfar, Reza, et al.
(author)
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Centralised Versus Decentralised Active Control of Boundary Layer Instabilities
- 2014
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In: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 93:4, s. 537-553
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Journal article (peer-reviewed)abstract
- We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n (2) interconnections. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11 % reduction of control performance. We explain how "wide" in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.
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| 2. |
- Fabbiane, Nicolò, 1986-
(author)
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Adaptive and model-based control in laminar boundary-layer flows
- 2014
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Licentiate thesis (other academic/artistic)abstract
- In boundary-layer flows it is possible to reduce the friction drag by breaking the path from laminar to turbulent state. In low turbulence environments, the laminar-to-turbulent transition is dominated by local flow instabilities – Tollmien-Schlichting (TS) waves – that exponentially grows while being con- vected by the flow and, eventually, lead to transition. Hence, by attenuating these disturbances via localised forcing in the flow it is possible to delay farther downstream the onset of turbulence and reduce the friction drag.Reactive control techniques are widely investigated to this end. The aim of this work is to compare model-based and adaptive control techniques and show how the adaptivity is crucial to control TS-waves in real applications. The control design consists in (i) choosing sensors and actuators and (ii) designing the system responsible to process on-line the measurement signals in order to compute an appropriate forcing by the actuators. This system, called compen- sator, can be static or adaptive, depending on the possibility of self-adjusting its response to unmodelled flow dynamics. A Linear Quadratic Gaussian (LQG) regulator is chosen as representative of static controllers. Direct numerical simulations of the flow are performed to provide a model for the compensator design and test its performance. An adaptive Filtered-X Least-Mean-Squares (FXLMS) compensator is also designed for the same flow case and its per- formance is compared to the model-based compensator via simulations and experiments. Although the LQG regulator behaves better at design conditions, it lacks robustness to small flow variations. On the other hand, the FXLMS compensator proved to be able to adapt its response to overcome the varied conditions and perform an adequate control action.It is thus found that an adaptive control technique is more suitable to delay the laminar-to-turbulent transition in situations where an accurate model of the flow is not available.
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| 3. |
- Fabbiane, Nicolò, 1986-, et al.
(author)
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Adaptive and model-based control theory applied to convectively unstable flows
- 2014
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In: Applied Mechanics Review. - : ASME International. - 0003-6900 .- 1088-8535 .- 2379-0407. ; 66:6, s. 060801-
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Research review (peer-reviewed)abstract
- Research on active control for the delay of laminar-turbulent transition in boundary layers has made a significant progress in the last two decades, but the employed strategies have been many and dispersed. Using one framework, we review model-based techniques, such as linear-quadratic regulators, and model-free adaptive methods, such as least-mean square filters. The former are supported by a elegant and powerful theoretical basis, whereas the latter may provide a more practical approach in the presence of complex disturbance envi- ronments, that are difficult to model. We compare the methods with a particu- lar focus on efficiency, practicability and robustness to uncertainties. Each step is exemplified on the one-dimensional linearized Kuramoto-Sivashinsky equa- tion, that shows many similarities with the initial linear stages of the transition process of the flow over a flat plate. Also, the source code for the examples are provided.
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| 4. |
- Fabbiane, Nicolò, et al.
(author)
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Adaptive control of finite-amplitude 3D disturbances in 2D boundary-layer flows
- 2015
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In: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015. - : TSFP-9.
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Conference paper (peer-reviewed)abstract
- Friction drag is reduced in a two-dimensional (2D) boundary-layer flow by delaying the laminar-to-turbulence transition. A localised forcing in the wall region is used to attenuate the growing 3D disturbances that eventually trigger the turbulent regime farther downstream. An adaptive filtered-X least-mean-squared (FXLMS) algorithm is used to process the information of the flow gathered from two rows of surface hot-wires sensors and compute the forcing, performed by a row of plasma actuators. LES simulations are used to evaluate and analyze the performance of the described control strategy: in particular, a study on the streamwise position of the sensor and an actual transition delay scenario are presented.
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| 5. |
- Fabbiane, Nicolo, et al.
(author)
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Energy efficiency and performance limitations of linear adaptive control for transition delay
- 2017
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In: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 810, s. 60-81
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Journal article (peer-reviewed)abstract
- A reactive control technique with localised actuators and sensors is used to delay the transition to turbulence in a flat-plate boundary-layer flow. Through extensive direct numerical simulations, it is shown that an adaptive technique, which computes the control law on-line, is able to significantly reduce skin-friction drag in the presence of random three-dimensional perturbation fields with linear and weakly nonlinear behaviour. An energy budget analysis is performed in order to assess the net energy saving capabilities of the linear control approach. When considering a model of the dielectric-barrier-discharge (DBD) plasma actuator, the energy spent to create appropriate actuation force inside the boundary layer is of the same order as the energy gained from reducing skin-friction drag. With a model of an ideal actuator a net energy gain of three orders of magnitude can be achieved by efficiently damping small-amplitude disturbances upstream. The energy analysis in this study thus provides an upper limit for what we can expect in terms of drag-reduction efficiency for linear control of transition as a means for drag reduction.
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| 6. |
- Fabbiane, Nicolo, et al.
(author)
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Energy efficiency and performance limitations of linear adaptive control for transition delay
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In: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645.
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Journal article (peer-reviewed)abstract
- Reactive-control techniques have been longly investigated to control local flow instabilities in boundary-layer flows – Tollmien-Schlichting waves – that would eventually cause laminar-to-turbulence transition. Several studies have been published about the control of two-dimensional (2D) disturbances supposing a transition delay. In this study, a three-dimensional (3D) disturbance environment is considered in a 2D zero-pressure-gradient boundary-layer flow. A control- law based on a multi-input-multi-output (MIMO) filtered-x least-mean-squares (fxLMS) adaptive algorithm is introduced and its performances are analysed for increasing disturbance amplitude. Transition delay is achieved by the investigated control set-up; moreover, an energy budget is conducted in order to asses the net energy saving capabilities of the investigated control approach. Ideal as well as real actuators models are considered, focusing in particular on dielectric-barrier-discharge (DBD) plasma actuators. To our knowledge, this is the first time that drag-reduction and energy-saving capabilities are studied for reactive transition-delay techniques.
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| 7. |
- Fabbiane, Nicolo, et al.
(author)
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On the role of adaptivity for robust laminar flow control
- 2015
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In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 767, s. R1-R12
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Journal article (peer-reviewed)abstract
- In boundary layer flows, one may reduce skin friction drag by delaying the onset of laminar-to-turbulent transition via the attenuation of small-amplitude Tollmien Schlichting (TS) waves In this work, we use numerical simulations and experiments to compare the robustness of adaptive and model-based techniques for reducing the growth of two-dimensional TS disturbances. In numerical simulations, the optimal linear quadratic Gaussian (LQG) regulator shows the best performance under the conditions it was designed for However, it is found that the performance deteriorates linearly with the drift of the Reynolds number from its nominal value. As a result, an order-of-magnitude loss of performance is observed when applying the computation-based I.QG controller in wind-tunnel experiments In contrast, it is shown that the adaptive filtered-X least-mean-squares (FXLMS) algorithm is able to maintain an essentially constant performance for significant deviations of the nominal values of the disturbance amplitude and Reynolds number.
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| 8. |
- Fabbiane, Nicolò
(author)
-
Transition delay in boundary-layer flows via reactive control
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Transition delay in boundary-layer flows is achieved via reactive control of flow instabilities, i.e. Tollmien-Schlichting (TS) waves. Adaptive and model-based control techniques are investigated by means of direct numerical simulations (DNS) and experiments. The action of actuators localised in the wall region is prescribed based on localised measurement of the disturbance field; in particular, plasma actuators and surface hot-wire sensors are considered.Performances and limitations of this control approach are evaluated both for two-dimensional (2D) and three-dimensional (3D) disturbance scenarios. The focus is on the robustness properties of the investigated control techniques; it is highlighted that static model-based control, such as the linear-quadratic- Gaussian (LQG) regulator, is very sensitive to model-inaccuracies. The reason for this behaviour is found in the feed-forward nature of the adopted sensor/actuator scheme; hence, a second, downstream sensor is introduced and actively used to recover robustness via an adaptive filtered-x least-mean-squares (fxLMS) algorithm.Furthermore, the model of the flow required by the control algorithm is reduced to a time delay. This technique, called delayed-x least-mean-squares (dxLMS) algorithm, allows taking a step towards a self-tuning controller; by introducing a third sensor it is possible to compute on-line the suitable time-delay model with no previous knowledge of the controlled system. This self-tuning approach is successfully tested by in-flight experiments on a motor-glider.Lastly, the transition delay capabilities of the investigated control con- figuration are confirmed in a complex disturbance environment. The flow is perturbed with random localised disturbances inside the boundary layer and the laminar-to-turbulence transition is delayed via a multi-input-multi-output (MIMO) version of the fxLMS algorithm. A positive theoretical net-energy- saving is observed for disturbance amplitudes up to 2% of the free-stream velocity at the actuation location, reaching values around 1000 times the input power for the lower disturbance amplitudes that have been investigated.
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| 9. |
- Sasaki, Kenzo, et al.
(author)
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On the wave-cancelling nature of boundary layer flow control
- 2018
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In: Theoretical and Computational Fluid Dynamics. - : Springer. - 0935-4964 .- 1432-2250. ; 32:5, s. 593-616
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Journal article (peer-reviewed)abstract
- This work deals with the feedforward active control of Tollmien-Schlichting instability waves over incompressible 2D and 3D boundary layers. Through an extensive numerical study, two strategies are evaluated; the optimal linear-quadratic-Gaussian (LQG) controller, designed using the Eigensystem realization algorithm, is compared to a wave-cancellation scheme, which is obtained using the direct inversion of frequency-domain transfer functions of the system. For the evaluated cases, it is shown that LQG leads to a similar control law and presents a comparable performance to the simpler, wave-cancellation scheme, indicating that the former acts via a destructive interference of the incoming wavepacket downstream of actuation. The results allow further insight into the physics behind flow control of convectively unstable flows permitting, for instance, the optimization of the transverse position for actuation. Using concepts of linear stability theory and the derived transfer function, a more efficient actuation for flow control is chosen, leading to similar attenuation of Tollmien-Schlichting waves with only about 10% of the actuation power in the baseline case.
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| 10. |
- Simon, Bernhard, et al.
(author)
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In-flight active-wave-cancelation via delayed-x-LMS control algorithm in a laminar boundary layer
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In: Experiments in Fluids. - 0723-4864 .- 1432-1114.
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Journal article (peer-reviewed)abstract
- This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broad-band Tollmien-Schlichting (TS) disturbances is per- formed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail withexperimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a “black box” system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS control algorithm allows to operate the controller without a model and an adaption based on varying conditions that may occur during flight in atmosphere.
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| 11. |
- Simon, B., et al.
(author)
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In-flight active wave cancelation with delayed-x-LMS control algorithm in a laminar boundary layer
- 2016
-
In: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 57:10
-
Journal article (peer-reviewed)abstract
- This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broadband Tollmien–Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a ‘black-box’ system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere.
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