IUTAM Symposium on Flow Control and MEMS

The materials and structures issues associated with high power density Microelectromechanical Systems (MEMS) are addressed. An overview of projects conducted at MIT and the key physical challenges affecting their performance goals is provided. These goals are translated into requirements for the materials and structural design. Particular emphasis is provided on the approaches taken to designing with brittle materials at low temperatures and designing with ductile creeping materials at high temperature. A novel approach to structural reinforcement using SiC in combination with Si is introduced.

An introduction of MEMS in general is made. Then the paper provides an overview of essential MEMS devices already elaborated for different problems of flow control in aeronautics. In the last part, attention is focused on solutions based on Micro-Magneto-Mechanical Systems (MMMS).

Synthetic jet actuators are used in various applications, such as separation control, mixing enhancement, and thermal management. Each of these requires the design to be optimized to meet specific performance requirements. This paper presents a design and scaling analysis of an electrodynamic synthetic jet actuator using a lumped element modeling approach. Various performance parameters, such as the resonant frequency, output volumetric flow rate and velocity, and jet formation, are studied as a function of device scale and current density. The viability and potential advantages of microscale synthetic jets are discussed.

Enhancing the ability to control flows in different configurations and flow conditions can lead to improved systems. Certain active flow control (AFC) actuators are efficient at low Mach numbers but the momentum and vorticity they provide limits the utilization to low speeds. At higher Mach numbers, robust, unsteady, efficient and practical fluidic actuators are a critical, largely missing, enabling technology in any AFC system. A new actuator concept, based on the combination of steady suction and oscillatory-blowing (SaOB) is presented. The actuator can achieve near-sonic speeds at about 1 kHz. It has no moving parts and therefore is expected to have superior efficiency and reliability. The operation principle of the SaOB actuator is presented along with two predictive computational models and their experimental validation.

A micro-valve pulsed-jet vortex-generator driven by piezoelectric actuation was successfully modelled numerically to determine the feasibility and response characteristics of such a design. This includes: modelling the dynamic motion of a unimorph cantilever and the fluid-structure interaction occuring between the unimorph and the fluid flowing over such a structure; the unsteady developing channel flow that would occur through the outlet orifice was also modelled. The response time of the actuator was found to be governed by the micro-valve opening rather than the time taken to establish the jet. However, the resistance of the pulsed-jet actuator was shown to be governed by the outlet orifice.

In this paper, we present the first characterization of new pulsed microjets. The specificity of these new actuators is their large nozzle for MEMS device: the total exit cross-section on a single actuators can reach up to 3×3.28 mm

2

⋍ 10 mm

2

. Two different geometries are presented: one with a single slot and one with three slots on a single MEMS device. We measure the jet velocities as a function of the inlet pressure. Maximum jet velocity can reach up to 28 m/s with a pulsation frequency around 100 Hz. Other phenomena are discussed, like the deviation of the jet axis from the vertical and the wrong evaluation of the jet velocity close to the nozzle.We finally apply the three slots actuators to a backward-facing step flow and show a small reduction of the recirculation length when the pulsation frequency is equal to the shear layer frequency.

This paper describes the design, fabrication and characterization of silicon based, high flow rate microvalves for the active control of separated air flows. The fabricated system provides pulsed microjets with an outlet speed reaching 150 m/s at an actuation frequency ranging from static actuation to 2.2 kHz, using either electromagnetic actuation or a self oscillating mode. After a brief introduction, the microvalve dimensioning and fabrication process are presented. The actuation techniques used are then described and discussed.

This paper reviews existing microelectromechanical systems-based shear stress sensors in the context of their suitability for various flow control situations. The advantages and limitations of existing devices for use in flow control systems are discussed. Unresolved technical issues are summarized and recommendations provided for future sensor development.

The present paper discusses the formation and evolution of finite span synthetic jets and their application for performance enhancement of fluid/thermal systems. PIV measurements revealed that the synthetic jet field has a unique flow pattern, where along its slit, the flow is two-dimensional near the orifice, while farther downstream the vortex pair lines develop secondary counter-rotating 3-D structures. Moreover, the streamwise and spanwise spacing between these structures vary with stroke length and formation frequency. Next, a couple of examples of the implementation of the synthetic jets to improve system performance are presented, including a scaled Cessna 182 model, and spray cooling. Using synthetic-jet-based flow control for flight control showed comparable effects to those of conventional ailerons at moderate deflection angles. For heat transfer, synthetic jets were shown to alter the global and detailed characteristics of a water spray and thus augment its cooling performance.

Numerical-simulation studies are undertaken to investigate how Helmholtz resonance affects the interaction of nominally inactive micro-jet actuators with a laminar boundary layer. Two sets of numerical simulations are carried out. The first set models the response of an actuator in ambient conditions to a small jump in its internal pressure. These results verify our theoretical criterion for Helmholtz resonance. In the second set of simulations, two-dimensional Tollmien-Schlichting waves, with frequency comparable with, but not particularly close to, the Helmholtz resonant frequency, are incident on a nominally inactive micro-jet actuator. The simulations show that under these circumstances the actuator acts as a strong source of 3D Tollmien-Schlichting waves. It is surmised that in the real-life aeronautical applications with turbulent boundary layers, broadband disturbances of the pressure field would cause nominally inactive actuators to act as strong disturbance sources. Should this be true, it would probably be disastrous for engineering applications of such massless microjet actuators for flow control.

An experimental investigation on passive scalar mixing due to the interaction of a synthetic jet with a thermal boundary layer is presented. From velocity measurements, performed by particle image velocimetry, two jet behaviours were identified. For jet to crossflow velocity ratios less than 1.2, the velocity fluctuations due to the jet/crossflow interaction stayed close to the wall. At higher ratios, the fluctuations moved away from the wall. The thermal mixing was examined using laser induced fluorescence. During expulsion, the thickness of the downstream thermal boundary layer increased whilst the thermal boundary layer was annihilated immediately downstream of the jet during entrainment.

The global evolution of the aerospace market is driving flow control research towards full industrial scale applications. In this approach, technologies need to demonstrate effectiveness, as well as compliance with the aircraft performance constraints. The design of a synthetic-jet-based system for a civil transport aircraft would provide an early understanding on the viability and potential applications of the technology. This study characterises experimentally an optimised piezoelectric-based synthetic jet actuator (SJA). Three full scale systems were developed for an A321: a flap, a slat and a cruise, both for separation and shock control. The systems were designed based both on experimental results and on an extensive hardware research. The laboratory optimisation of SJAs has led to the achievement of peak velocities of 130 m/s and peak conversion efficiencies of around 15%. All systems presented power and weight requirements within the aircraft performance budget, where the flap system resulted in the lowest values. The question that still remains is whether or not the performance benefits will outweigh the system costs.

PIV measurements along the centerline of a synthetic jet embedded in a flat plate boundary layer were conducted for three types of jet vortex structures identified by the authors in previous flow visualization studies, namely hairpin vortices, stretched vortex rings and tilted vortex rings. The primary purpose of this work was to quantify the near wall effect of these structures in terms of their manipulation of the boundary layer velocity profile. In the near field region, synthetic jets composed of stretched vortex rings, which remain within the boundary layer and tilted vortex rings, which rapidly penetrate the boundary layer produced fuller velocity profiles in comparison to the jet off case. Further downstream, only the velocity profiles manipulated by the hairpin vortices and stretched vortex rings continued to fill out close to the wall, thus suggesting that these embedded structures may offer potential as an optimal configuration for flow separation control.

Large-Eddy Simulations (LES) are used to investigate the physical processes involved in the injection of a synthetic (zero-net-flux) jet into a zero-pressure-gradient turbulent boundary layer at conditions corresponding to experimental data obtained by others. The boundary layer ahead of the jet is generated by a separate precursor simulation at a momentum-thickness Reynolds number of

Re

θ

=920, providing the main simulation with a full and accurate description of the unsteady conditions. Phase-averaged results and time-averaged integral quantities are presented and compared against experimental data. This main study is preceded by a simulation of a synthetic-jet injected into stagnant surroundings, mainly in order to verify the computational framework, but also to gain insight into the behaviour of the vortex rings injected through a square orifice, in accord with corresponding experimental conditions.

In synthetic jet actuators with an orifice diameter typically less than 1 mm (termed as small-scale synthetic jet actuators), the compressibility and viscosity effects often become significant causing them behave differently from large-scale synthetic jets. In this paper, a numerical study of the synthetic jet from an orifice diameter of 0.5 mm is undertaken and the results are compared with the synthetic jet from an orifice diameter of 5 mm. The results reveal that, given the same dimensionless parameters

L

and

Re

L

, the appearance and circulation of vortex rings produced from synthetic jets of different scales are identical in the near field. It is also found that although the linear relationships between (

L

and

Re

L

) and actuator operating conditions observed for large-scale synthetic jets are no longer valid for small-scale synthetic jets, the linear relationships between the dimensionless jet performance parameters and (

L

and

Re

L

) still exist. This finding is very useful to support the development of low-dimensional predictions models.

We examine the evolution and transition-to-turbulence of synthetic jets into quiescent air by performing three-dimensional numerical investigations using a high-resolution scheme for solving the compressible Navier-Stokes equations in the context of Implicit Large Eddy Simulation (ILES). The computational results show a good comparison to experimental data obtained from the NASA Langley workshop on CFD Validation of Synthetic Jets and Turbulent Separation Control.

The control of a globally unstable boundary-layer flow along a two-dimensional cavity is considered. When perturbed by the worst-case initial condition, the flow exhibits a large transient growth associated with the development of a wave packet along the cavity shear layer followed by a global cycle related to the least stable global eigenmodes. The flow simulation procedure is coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and actuates at the upstream lip. A reduced model for the control optimization is obtained by a projection on the least stable global eigenmodes. The LQG controller is run in parallel to the Navier-Stokes time integration. It is shown that the controller is able to damp out the global oscillations.

This paper presents the wind tunnel test results concerning the effects of deploying steady and synthetic jets on a NACA0015 airfoil and describes the design of a multi-orifice-single-chamber synthetic jet actuator. Three steady jets with different configurations were tested. The orifice diameter, orientation and spacing were the varying parameters. Synthetic jets were deployed through a single row of orifices that were orientated normal to the airfoil surface. A single row of each type was positioned at 30% of chord from the leading edge. These jets exhibited varying degree of control authority over the lift and drag coefficients. The timescales of attachment and separation were estimated for the test cases of angled steady and synthetic jets. In view of controlling the flow separation in a dynamic manner, a multi-orifice-single-chamber actuator with a typical response time smaller than that of the afore-mentioned time scales was designed, fabricated and tested in quiescent condition.

The effect of changing the pitch to diameter ratio (

P/D

) of a row of round, wall-normal, zero-net-mass-flux (ZNMF) jets located at the leading edge of a NACA 0015 airfoil was investigated. A parametric study and particle image velocimetry (PIV) measurements were conducted on a two-dimensional airfoil in a water tunnel at a Reynolds number of 6.56 ×10

4

. Different optimal forcing frequencies and percentage lift increases between the two

P/D

cases were observed. It is possible that differences in jet interaction mechanisms may have caused the differences in control effectiveness between

P/D

cases. Time-averaged streamlines indicate a reduction in the size of a recirculation region over the upper surface of the airfoil may be causing the improved lift.

This paper deals with the control of airflow separation above a NACA 0015 airfoil using a surface plasma actuator. A dieletric barrier discharge plasma is used to bring velocity in the boundary layer, tangentially to the wall. The goal of the actuation is to displace (upstream or downstream) the separation point, in either reattaching a naturally detached airflow or in detaching a naturally attached airflow. The ultimate goal of these experiments is to better understand where one has to act along the profile chord (as a function of the angle of attack) to be the most efficient. These experiments show that the plasma actuator is more efficient when it acts close to the separation point, and that the power consumption can be highly reduced by using a non-stationary actuation.

The need for adaptive control methodologies which involve realistically obtainable information is driving the direction of active flow control research. In this work, time resolved estimates of the velocity field using a mean-square estimation procedure with the unsteady surface pressure as the condition are presented which highlight the relationships between these quantities. Understanding the causal relationships between the velocity field and the surface pressure, an adaptive control strategy solely based on surface pressure can be developed. To this end dynamic stochastic estimation and system identification approaches are shown to accurately predict future surface pressures based on their time histories which can then be used to form the basis of a closed loop control strategy.

Two-dimensional simulations are used to demonstrate the existence of two different amplitude regimes to control laminar separation bubbles with periodic zero-mass-flux wall jets. One is based primarily on a shear-layer instability found using low-amplitude forcing. The minimum bubble length is obtained for a Strouhal number approximately equal to 0.018, based on a properly defined momentum thickness. Higher forcing is found to create large vortices, which are responsible for very effective control. A relation is presented between the forcing parameters and the size of the vortices. These estimates are then used to explain the range of effective frequencies to control the separation bubble.

In this paper, a DC surface corona discharge established on a rounded edge of a dielectric material was studied in atmospheric air. The flow induced by this actuator was measured and experiments on a NACA 0015 were performed in a subsonic wind tunnel. These measurements showed that this discharge modified the fully detached flow on the airfoil up to

Re

=267 000 and 17.5° of angle of attack.

The purpose of this paper is to study experimentally the active control of separation on an ONERA D airfoil using micro-actuators. The configuration is a massively separated flow on the airfoil corresponding to a stalled case at

R

e

=0.5 × 10

6

and α = 16

°

and the control is performed using continuous blowing microjets. Using PIV measurements and a post-processing based on POD, the main characteristics of the control are highlighted, leading to the developement of a MEMS prototype based on synthetic blowing.

Several studies have shown that a surface Dielectric Barrier Discharge (DBD) may be used as an ElectroHydroDynamic (EHD) actuator. This actuator adds momentum inside the boundary layer close to the wall and could be used for airflow control. In this paper, the actuator has been set up on a small axisymmetrical airfoil and the discharge is used to modify the characteristics of the shear-layer in its wake. Results show that the plasma actuator modifies strongly the airflow around the airfoil for velocities up to 20 m/s.

We present numerical results from an idealized model simulation which implements the Least Mean Square (LMS) and the Filtered-X-Least-Mean-Square (FXLMS) control algorithms as applied to adaptive feedforward control of wall-bounded turbulent shear flows. The FXLMS system is found to work extremely well, effectively controlling the model system which includes phase delays, multiple sensor inputs, high levels of noise and nonlinearities in the forward system.

Comparisons are made between laminar and turbulent flows in pipes with and without flow control, and a formula is derived that shows just how much the discrepancy between the volume flux of laminar and turbulent flow at the same pressure gradient increases as the pressure gradient is increased. Related to this, we investigate the lowest bound for skin-friction drag in pipes for flow control schemes that use surface blowing and suction with zero-net volume-flux addition.

The wall turbulence is forced to a predictable state through localized time-periodical blowing. It is shown through experiments and direct numerical simulations that the temporal waveform of the localized blowing plays a crucial role in the response of turbulent wall drag. Imposed unsteadiness increases the capacity of the controllability significantly. Results on the optimum periodical temporal waveform of localized blowing are also discussed. We use the characteristics of the cyclostationnarity to achieve this particular goal.

Opposition control is a simple feedback control method traditionnally used to attenuate near-wall turbulence and reduce drag in wall-bounded turbulent flows. The idea is to impose blowing and suction at the wall to counteract near-wall quasi-streamwise vortical structures. Unfortunately, the efficiency of this method decreases as the Reynolds number increases. The present study proposes a simple but efficient modification of opposition control (OC) to increase its performance at large Reynolds numbers. We demonstrate a 300% improvement when performing a blowing-only opposition control (BOOC), where OC’s suction part has been removed, on a spatially developing turbulent boundary layer at R

e

τ

=920. It is shown that BOOC only applies blowing at the location of high skin friction events, which suppresses the latter without altering the “natural” low skin friction events. As a result, BOOC dramatically changes the probability density profile of wall shear stress but does not weaken turbulence intensity near the wall.

Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction in a numerical channel flow with a constant mass flow rate and low Reynolds number. The originality of this study comes from the computation of the force field from the geometry of the magnet and the electrode. It is shown that the tilt of the wall-normal component of the vorticity in the spanwise direction characterise the drag reduction caused by alternated spanwise forcing.

Astudy was carried out with an aim to better understand the drag reducing mechanisms by spanwise oscillation and spanwise travelling wave via Lorentz forcing flow control. A maximum 47% of drag reduction was achieved with

w

+

≈ 12.2 when the Lorentz forcing spanwise oscillation was applied in a turbulent boundary layer. It was, however, shown that the spanwise travelling wave forcing can reduce or increase the skin friction drag depending on the operating conditions, which offers a flexibility for flow control. A maximum 28.9% of drag reduction and 22.8% of drag increase have been achieved, respectively. Flow visualization indicated that the spanwise displacement of the streaky structures may play an important role in obtaining the drag reduction by spanwise travelling wave actuation.

Flow control may be used to achieve efficient mixing which is important in many applications including in various combustors and chemical reactors. Mixing and its rate can be measured in terms of the concentration variance,

c

′

2

, and its time dependence. Efficient mixing can be achieved if the energy input required can be minimised for values as low as

c

′

2

or as high as possible mixing rates.

We perform Direct Numerical Simulations (DNS) of electromagnetically fractal-forced and Rayleigh-damped two-dimensional flows. Our simulations show broad band power law energy spectra. When the fractal dimension of the magnets’ distribution is

D

f

=0.5 then

p

≈ 2.5 in agreement with previous laboratory experiment. Moreover, when the fractal distribution of magnets is changed,

p

varies linearly with

D

f

, the fractal dimension of the magnet set up. Hence, fractal control of the energy spectrum is possible.

Active suppression of the naturally occurring travelling wave disturbances that amplify in laminar boundary layers and cause the transition from laminar to turbulent flow is considered. Both open-loop and closed-loop schemes are discussed. Numerical predictions, based on linear stability theory, have been used to model the behaviour of the flow disturbances and the controlled waves. Predictions based on these models have shown that the instability waves that occur on a simple flat plate can be stabilized significantly by both types of control.Wind tunnel experiments have so far been used to validate some of the predictions in the open-loop case.

A summary of recent experimental research efforts at Syracuse University aimed at active flow control is presented with emphasis placed on the development of low-dimensional tools to facilitate closed-loop control. Results indicate that the near-field pressure in a Mach 0.85 high Reynolds number jet is low dimensional and it is primarily the azimuthal near-field pressure mode 0 that correlates with the acoustic field. The turbulent velocity field in the high-speed jet can also be estimated from the near-field pressure, and is used herein to predict the far-field acoustics. Tools being developed to improve recent successful, high Reynolds number, closed-loop flow control in a NACA 4412 airfoil are also discussed. Together, these results set the framework for active flow control in the high-speed jet with the goal of reducing jet noise.

We present the system identification and the online optimization of feedback controllers applied to combustion systems using evolutionary algorithms. The algorithmis applied to gas turbine combustors that are susceptible to thermoacoustic instabilities resulting in imperfect combustion and decreased lifetime. In order to mitigate these pressure oscillations, feedback controllers sense the pressure and command secondary fuel injectors. The controllers are optimized online with an extension of the CMA evolution strategy capable of handling noise associated with the uncertainties in the pressure measurements. The presented method is independent of the specific noise distribution and prevents premature convergence of the evolution strategy. The proposed algorithm needs only two additional function evaluations per generation and is therefore particularly suitable for online optimization. The algorithm is experimentally verified on a gas turbine combustor test rig. The results show that the algorithm can improve the performance of controllers online and is able to cope with a variety of time dependent operating conditions.

The present paper reports on active cancellation of natural Tollmien- Schlichting (TS) instabilities on an unswept wing in compressible flows. The research concentrates on closed-loop active wave control (AWC) experiments at Mach numbers ranging from 0.2 up to 0.4. These high velocities result in thin boundary layers and therefore in TS frequencies up to 10 kHz. Therefore, the resolution of the applied sensors as well as the amplitude and frequency domain of the actuators are subject to challenging requirements. Additionally, the velocity of the convective TS waves demands a powerful, optimized control algorithm working in real time. The AWC principle applied here delays TS induced transition by stabilizing the linear disturbance waves initiating the laminar-turbulent transition process. This method is based on the wave superposition principle, i.e. the superposition of artificially generated anti-disturbances and the naturally occurring TS disturbances. The energy consumption with this method is considerably lower than the stabilization achieved by manipulating the local mean velocity profile (e.g. boundary layer suction).

This paper addresses controversial issues fundamental to the optimal control of aerodynamic flows. Aerodynamic flows being external to wings, the significant region of the flow is in the interface region. In assessing the closedloop performance the relevant performance index must therefore be evaluated exclusively on the wing boundary which is the most significant region for the development of both lift and drag.When this is done the controller may be synthesised relatively easily as it can be shown that the associated optimising co-state equations are identical to the adjoints, which can then be solved by the same methods employed for the Navier—Stokes equations. The control laws may then be deduced by comparing the open and closed loop pressure distributions.

This paper presents a numerical algorithm for the linearized flow initial value problem involving complex geometries where analytical solution is impossible. The method centres around the calculation of an eigenvalue problem involving the linearised flow and its spatial adjoint, and yields the flow perturbations that grow the most in a prescribed time, the magnitude of that growth and the perturbations after the growth has occurred. Previous work has shown that classical stability analysis of flow past a low-pressure turbine blade gives only stable eigenvalues, which cannot explain transition to turbulence in this flow. The inital value problem for this fan blade is presented and demonstrates significant perturbation growth, indicating that this growth may be the facilitator for transition in this case.

This paper describes simulations of feedback control of small disturbances in laminar Poiseuille flow. A polynomial-form spectral state-space model of the linearised flow with wall transpiration actuation and wall shear-stress measurements is generated, optimal controllers are synthesised, and closed-loop simulations are performed using an independent finite-volume Navier—Stokes solver. In addition, actuation via tangential wall transpiration is investigated, and LMI controllers, which minimise an upper bound on the peak transient energy growth, are synthesised and simulated.

We develop a collection of reduced-order models for fluid systems that operate under time-varying flow and actuation parameters. A switched dynamical system is formulated that is a combination of reduced-order models valid for specific parametric subspaces and discrete switching logic. An open-loop simulation of the switched dynamical system on a two-sided driven cavity demonstrates the system’s ability to capture the evolution of the flow and input parameters in the full-order model.

The aim of this paper is to expose two different strategies for the optimal control of a three-dimensional global mode in a two-dimensional recirculation bubble. The formulation of the optimal control problem, that consists to reduce the energy growth of the global mode, depends on the characteristics of the actuation — unsteady and three-dimensional or steady and two-dimensional. A gradient-based optimization procedure is used and the gradient is evaluated using the adjoint of the stability equations in the former case and the adjoint of the stability equations as well as the adjoint of the base flow equations in the latter case.

This paper presents the application of lumped element modeling for the modeling and design of a synthetic jet actuator. The reduced-order model is first reviewed and the basic dynamic behavior discussed. Quantitative design goals for a specific flow control application are then translated into desirable actuator characteristics, and used to solve the optimal design synthesis problem. The actuator built from the specifications given by the model is finally characterized via hot-wire anemometer (HWA) and compared with lumped element modeling (LEM) prediction. Ultimately, the goal of this work is to achieve drag reduction flow control using the synthetic jet actuator embedded in the afterbody of a car vehicle.

The flow around a modified Ahmed body with a curved rear section is studied at

Re

> 2.10

6

and a line of vortex generators (VG) is used as actuators. By varying the angle α of the VG we observe an optimal value of α defining a minimum of aerodynamic drag and correspondingly a maximum base pressure coefficient. As this optimum value is shown to be Reynolds dependent we use an extremum control strategy for the system to find this optimal condition autonomously. It consists of the synchronous detection of the response measured in either the base pressure signal or in the drag and a slow sinusoidal modulation of the angle of the VG. It is finally demonstrated that the closed-loop system is robust and reacts successfully to unpredictable changes in the external flow conditions.

The paper gives a brief overview about the challenges and the concepts for future aeroengine applications based on microjets. Technical requirement and difficulties of the potential applications of Active Flow Control (AFC) in turbomachines are discussed. The importance of an active stabilized turbocompressor for an efficiency increase is described. Future concepts of advanced turbocompressors with the application of active and passive flow control, partially based on MEMS devices are briefly sketched. Finally, the numerical methods necessary to unveil the physical background of AFC are presented.

This paper summarizes the ONERA/LEMAC contribution within the work-packages 2 and 3 of the EU project ADVACT. This activity is dedicated to the evaluation of pulsed jets based on magnetic actuation principle on a generic separated flow. It is supported by some advanced numerical work based on RANS/LES coupling.

The control of the vibration of two side-by-side cylinders in a cross flow has been experimentally studied using micro actuators. Three spacing ratios,

T/d

=1.2, 1.8 and 3.0, are studied, where T is the center-to-center distance, and d is the diameter of the cylinder. The experiments show that the micro excitation can effectively reduce the flow-induced vibration for

T/d

of 1.2 and 3.0, when the excitation frequency and actuator location are optimized.

The behaviour of the near-field region of a vertical rectangular jet of aspect ratio 4:1 controlled by a rotating cylinder placed on the jet major-axis is investigated experimentally using a new design facility. The objective is to investigate flow control strategies of a rectangular jet based on instability manipulation. It is found experimentally that the controlled jet exhibits a similar behaviour to the one described theoretically and numerically by Hammond and Redekopp [

1

,

2

] on bluff-body wakes with higher control efficiency when the global instability mode is suppressed.

The reduction of drag generated by axisymmetrica bluff bodies resulting from the control of the boundary layer with dimpled surfaces was investigated. A central composite design with 3 factors and 5 levels for each factor was used to optimize the parameters of the dimpled surfaces. Wind tunnel tests with a Mach number of 2.51 and a Reynolds number of 1.88 × 10

6

based on the maximum diameter of the model indicate that the dimples on the rearward configuration can reduce the viscous forebody drag by 4.98%, the base drag by 2.69%, and the total drag by 2.98%, respectively. By using the dimpled surface optimized by the quadratic regression equation, the total drag can be reduced by 3.81%.

Porous layers are added on blunt bodies to change the shear forces and consequently to reduce the disorganisation of the flow or to reduce the drag coefficient. Numerical simulations of two dimensional flows around a riser pipe give a drastic increase in the regularity of the flow when a porous sheath is added. Also, setting some porous devices on a simplified ground vehicle geometry can reduce the pressure drag with an appropriate choice of the location. The pressure gradient in the near wake can be reduced by 67% and so a drag reduction of up to 45% can be achieved.

The effects of aspect ratio and end condition on the control of the development of free shear layers and the force coefficients of cylinders for flows around four cylinders at critical spacing ratio

L/D

=3.5 in the in-line square configuration have been investigated numerically. The study demonstrated that the aspect ratio and end condition of the four cylinders produce a strong influence on the free shear layer development from the upstream cylinder and hence the pressure fields and force characteristics of the cylinders. The mean pressure and fluctuating pressure increase towards the mid-span of the cylinders. The development of free shear layers, the mean and fluctuating pressure as well as the force coefficients are critically affected by the cylinder aspect ratio in the range of

H/D

from 15 to 16.

Large eddy simulations of turbulent flow around wavy cylinders are performed at Re=3000. The mean pressure distribution and mean streamwise velocity in the near-wake region are calculated and compared with those of a circular cylinder. The three-dimensional near-wake structures behind wavy cylinders were captured. It was found that due to the long vortex formation length resulting from the three-dimensional vortex sheet of the wavy cylinder, the mean drag coefficients of the wavy cylinders are less than that of a corresponding circular cylinder. A reduction of mean drag coefficient of up to 16% is obtained. Also, the fluctuating lift coefficients of the wavy cylinders are weakened significantly.

Wind tunnel grid-generated turbulence based on a fractal square motif has been studied via hot-wire anemometry, at several free-stream velocities. The aim of the exercise was to see whether a fractal motif could enable multiscale flow control such that the ‘objective functions’ are the nature of the turbulence decay and its non-dimensional turbulence energy dissipation rate,

C

ɛ

. The outcome is that such a grid architecture does provide a unique homogeneous isotropic decaying turbulence field where the decay is exponential and the non-dimensional turbulence energy dissipation rate field evolves inversely with the Taylor-Reynolds number.

The reduction of skin friction in turbulent flows holds considerable promise for energy savings. The present work shows how and why skin friction and the dissipation are interrelated in turbulent channel flows. A hydraulic model formulation is presented for the skin friction reduction that can be obtained with a surface structure recently proposed for flow control. The model predictions are validated with results from direct numerical simulations.

The vortex shedding phenomenon behind a tapered cylinder is sought to be controlled by means of a placement of a smaller cylinder placed outside the wake of the main cylinder. The quenching of vortex shedding is evident from flow visualisation and hotwire anemometry studies and this is attributed to the suppression of global instability modes.

In this communication, we study experimentally the modification of a boundary layer by a line of four cylindrical vortex generators. We show how the base flow modification and, specifically, the first non-linear mode (the zeroth mode) can lead to new parameters characterizing the efficiency of the vortex generators. We finally apply the vortex generators to a smoothly contoured ramp showing a clear delay of the separation when the parameters are properly chosen.

In this paper we consider the various methods employed by birds to generate lift and control it. We focus on three particular aspects, namely the method that a bird employs to compensate the transport lags, the method of rapid lift generation employed and growth of lift to a steady value and finally the angle of attack at which a bird flies to generate maximum lift. Based on the study of these methods we establish mathematical control models for compensating the transport lags, and establish a constraint for the aeroelastic tailoring of a wing to maintain a steady angle of attack even when flexible modes of vibration are present. Finally the unsteady aerodynamic modeling of vortex flows for active control applications is discussed.