Flow Control, Active and Passive Applications (original) (raw)

Flow Control Applied to the Front Rounded Edge of a Bluff Body

An experimental study was performed on a 2D square prism with one rounded front edge with the purpose of reducing the aerodynamic drag by means of active flow control of boundary layer separation. Highly efficient suction and oscillatory blowing (SaOB) actuators were installed at the front curved edge, aimed to inhibit the separation phenomenon. The experimental results show significant decrease in the form-drag, narrowing of the wake and correspondingly an increase in the base pressure in the presence of the SaOB actuation. The mechanism of drag reduction is elimination of the separation from the front-cured edge of the model.

Analysis of flow control by boundary-layer manipulation using 2D frequency response

Asia-Pacific Journal of Chemical Engineering, 2015

Flow control is the manipulation of fluid flow into a desired behaviour with the purpose to either enhance mixing or suppress turbulence. However, because all flow systems are essentially distributed parameter systems, which are often described as partial differential equations, systematic analysis of the effectiveness and control design of the flow control is often difficult to perform. Oscillatory flows have the potential to enhance mixing by providing high-amplitude shear oscillations. For example, oscillating electro-osmotic flow caused by an externally induced electric field can be applied to membrane systems to reduce polarisation or fouling. Therefore, an approach that can model and analyse the relationship between oscillatory flows and the resulting mixing enhancement is needed. This paper proposes an approach that systematically analyses the effects of external inputs, which vary in both spatial and temporal directions, on the mixing enhancement in channel flows. A two-dimensional (2D) frequency response is obtained from the reduced-order model of the Navier-Stokes equations. The 2D frequency response describes the system behaviour at different frequencies and wavenumber. The analysis results are useful for system and control design, e.g. in identifying the locations to install actuators and sensors, which is often very difficult for distributed parameter systems.

Active Control of Instabilities in Laminar Boundary Layers—Overview and Concept Validation

Journal of Fluids Engineering, 1996

This paper (the first in a series) focuses on using active-control methods to maintain laminar flow in a region of the flow in which the natural instabilities, if left unattended, lead to turbulent flow. The authors review previous studies that examine wave cancellation (currently the most prominent method) and solve the unsteady, nonlinear Navier-Stokes equations to evaluate this method of controlling instabilities. It is definitively shown that instabilities are controlled by the linear summation of waves (i.e., wave cancellation). Although a mathematically complete method for controlling arbitrary instabilities has been developed, the review, duplication, and physical explanation of previous studies are important steps for providing an independent verification of those studies, for establishing a framework for the work which will involve automated transition control, and for detailing the phenomena by-which the automated studies can be used to expand knowledge of flow control.

The Study of Influence of Active and Passive Methods of Boundary Layer Control

The paper deals with influence of the passive and active method of control on the flow field. It is well known fact that boundary layer separation has a fundamental effect on aircraft airfoil performance, compressors and turbines performance and other bodies, subject to study of internal and external aerodynamics. By the boundary layer control, it is possible to obtain both operational economy and extension of efficient operational region. Traditional methods of active boundary layer control are very complex and their application needs complex facilities. Modem methods based on the control by synthetic jets or plasma actuator are simpler, but their design has to be carried out at rather well experience and detailed knowledge about boundary layer and its interaction with the actuator.

Turbulence management by active wall for fully developed two-dimensional boundary layer

19th AIAA, Fluid Dynamics, Plasma Dynamics, and Lasers Conference, 1987

Fully developed two-dimensional incompressible turbulent boundary lbvers in the presence of pressure gradient over moving wavy surfaces for the case of driven wall motion is studied numerically. The c m p u t ted solution indicates strong interaction between the unsteady wave-induced f l o w and the time averaged rean flow at a certajn discrete wavenumber for a given phase speed of the driven interface. Both favourable and adverse pressure gradient cases indicate skin friction reduction. However, f o r the adverse pressure gradient case, the interaction becomes stroncer, tending to drastically reduce the s k i n friction and in certain rases, converged results are not obtained for high wavenumbers irrespective of their amplitudes. However, the present approach indicates a possible management of viscous drag in a qualitative way.

Control of a boundary layer separation

PAMM, 2007

The results of experimental study on a boundary layer separation control are given in the paper. The boundary layer on a flat wall is subjected to adverse pressure gradient. The active control strategy evolving a synthetic jet has been applied. The separation process is investigated using TR‐PIV method. Dynamical aspects of the phenomenon are analyzed in details. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Feedback shear layer control for bluff body drag reduction

Drag reduction strategies for the turbulent flow around a D-shaped body are examined experimentally and theoretically. A reduced-order vortex model describes the interaction between the shear layer and wake dynamics and guides a path to an efficient feedback control design. The derived feedback controller desynchronizes shear-layer and wake dynamics, thus postponing vortex formation. This actuation is tested in a wind tunnel. The Reynolds number based on the height of the body ranges from 23 000 to 70 000. We achieve a 40% increase in base pressure associated with a 15% drag reduction employing zero-net-mass-flux actuation. Our controller outperforms other approaches based on open-loop forcing and extremum-seeking feedback strategies in terms of drag reduction, adaptivity, and the required actuation energy. † e.g. Volvo, Great Dane Trailers, Georgia Tech.

2006

The effects of blowing and suction on the steady compressible boundary-layer flow with adverse pressure gradient and heat transfer over a wedge are numerically examined. The fluid is considered to be a compressible, viscous and Newtonian ideal gas (air) and it is subjected to a constant velocity of suction/injection applied globally to the wedge or locally to specific slots on the surface. The Reynolds-Averaged Boundary-Layer (RABL) equations and their boundary conditions are transformed using the compressible Falkner-Skan transformation. The resulting coupled and nonlinear system of PDEs is solved using the Keller box method. For the eddy-kinematic viscosity the turbulent models of Cebeci-Smith and Baldwin-Lomax are employed. For the turbulent Prandtl number the extended model of Kays-Crawford is used. Numerical calculations are carried out for the case of an adiabatic, cooled or heated wall and for different values of the dimensionless pressure-gradient parameter (m). The obtained...

A Nonlinear Control Strategy for Finite-amplitude Perturbations in a Boundary-layer Flow

Energy Procedia, 2015

The present work describes an optimal control strategy, based on the full Navier-Stokes equations, aiming at hampering the rapid growth of unsteady finite-amplitude perturbations in the Blasius boundary-layer flow. An optimization strategy is used to find the blowing and suction control law at the wall providing the maximum damping of the perturbation energy at a given target time. Two optimally-growing finite-amplitude initial perturbations have been employed to initialize the flow. The nonlinear control procedure can drive such perturbations back to the laminar state, provided that the target time of the minimisation and the region in which the blowing and suction is applied have been suitably chosen. On the other hand, an equivalent control procedure based on the linearized Navier-Stokes equations is much less effective, being not able to lead the flow to the laminar state when finiteamplitude disturbances are considered.

Boundary layer receptivity mechanisms relevant to laminar flow control [microform] /

Ph D Thesis Arizona Univ Tucson, 1990

Receptivity processes by which free-stream acoustic waves generate instability waves in boundary layers are investigated. Concentration is placed on mechanisms associated with local regions of short scale variation in wall suction or admittance distribution. These mechanisms are relevant to laminar flow control technology, in which suction is utilized to control the growth of boundary layer instabilities. The receptivity process requires a transfer of energy from the long wavelength of the free-stream disturbance to the short wavelength of the instability wave. Time harmonic, 2-D and 3-D interactions are analyzed using the asymptotic, high Reynolds number, triple deck structure. The acoustic wave orientation and the geometry of the wall suction or admittance distribution are found to significantly influence the amplitude of the generated instability wave.