Imperial College London

DrSylvainLaizet

Faculty of EngineeringDepartment of Aeronautics

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 5045s.laizet Website

 
 
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Location

 

339City and Guilds BuildingSouth Kensington Campus

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Summary

 

Gravity currents

In recent years, there has been considerable interest in the use of dielectric barrier discharges (DBD) plasma actuators for flow control.  Some of the reasons for the popularity of these actuators are their special features that include being fully electronic with no moving parts, a fast time response for unsteady applications, a very low mass which is especially important in applications with high g-loads, a low power consumption, and an efficient conversion of the input power into fluid momentum. Computational studies of plasma flow control have been limited in comparison to the vast number of experimental studies. Numerically most of the problems regarding plasma actuators are usually treated with the help of ad hoc constants and/or empirical models with very poor performance and limited reliability. The aim of this research is to develop a robust and reliable plasma forcing to be applied for internal flows.

PLASMA ACTUATORS FOR INTERNAL FLOWS

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Drag-reducing flow control is a topic of great interest due to its importance in many engineering applications. Despite many decades of extensive research, a practical and affordable method for skin-friction drag reduction is yet to be found and implemented in real-world applications. For air flows, however, the energy expenditure of typical active drag reduction strategies can be very high, often leading to net energy loss even if substantial skin-friction drag reduction is obtained. The focus of this research is on the spatial development of a zero pressure gradient turbulent boundary layer and the resulting wall friction after control has been applied locally using vertical wall-blowing as a drag-reducing strategy. Employing a reliable optimisation method to determine the optimal parameters of a wall-blowing control technique could potentially lead to substantial net-energy saving. Bayesian optimisation (BO) is derivative-free algorithm that works efficiently with expensive non-convex objective functions. BO plays a prominent role in efficiently optimising the parameters of machine learning algorithms, such as Neural Networks, with superior performance
when compared to more standard approaches. BO is yet to be used for fluid flow problems and very few studies combining DNS/LES and BO have been
published to date.

Optimisation techniques based on Machine Learning

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In the last ten years, laboratory and computational works have used multiscale/fractal objects to generate turbulence in wind and water tunnels (either in the laboratory or virtually in the computer) and have shown that complex multiscale boundary/initial conditions can drastically influence the behaviour of a turbulent flow. Fractal geometry is a concept where a given pattern is repeated and split into parts, each being a reduced-copy of the whole. Multiscale objects can be designed to be immersed in any fluid flow where there is a need to passively control and design the turbulence generated by the object. Unlike regular objects (where the turbulence is generated by only one scale), a slight modification of one of the multiscale object's parameters can significantly modify the turbulent flow. Multiscale objects offer the opportunity to discover new complex flow effects that can help understand how to control and/or manage complex fluid flows.

Fractal-generated turbulence

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A large number of studies in the literature have focused on the control of a turbulent jet, mainly with two objectives: the control of the mixing properties of the jet and for noise reduction purposes. The objective of the present numerical work is to help in our understanding of aeroacoustic mechanisms in the context of fluidic control. More precisely, the idea is to propose control solutions of the acoustic sources of a jet. A microjet device is used in order to modify the near-nozzle region of the main jet. Various effects can be obtained by playing with the number of microjets and their arrangement. For this research, we focus on a configuration with microjets organised as pairs of two converging microjets where each pair of converging microjets produces a chevron-like excitation. The main advantage of such a device by comparison to a nozzle with classic chevrons is its flexibility, with the ability to activate/deactivate easily the control. For aeronautic applications, it means that it is possible to suspend the control during flight and only activate the fluidevrons control during landing and take-off when a noise reduction is needed.

Jet control with microjets

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A large number of studies in the literature have focused on the control of a turbulent jet, mainly with two objectives: the control of the mixing properties of the jet and for noise reduction purposes. The objective of the present numerical work is to help in our understanding of aeroacoustic mechanisms in the context of fluidic control. More precisely, the idea is to propose control solutions of the acoustic sources of a jet. A microjet device is used in order to modify the near-nozzle region of the main jet. Various effects can be obtained by playing with the number of microjets and their arrangement. For this research, we focus on a configuration with microjets organised as pairs of two converging microjets where each pair of converging microjets produces a chevron-like excitation. The main advantage of such a device by comparison to a nozzle with classic chevrons is its flexibility, with the ability to activate/deactivate easily the control. For aeronautic applications, it means that it is possible to suspend the control during flight and only activate the fluidevrons control during landing and take-off when a noise reduction is needed.