Control Of Turbulent Flows Research Articles

K-1509 アクチュエータアレーによる壁乱流の能動制御(J07-2 マイクロ熱流体現象の機能利用デバイス)(J07 マイクロマシン技術と熱流体現象)

Turbulent flow control using an actuator array and related modification of the turbulent spatial structure were experimentally examined using a PIV system. The actuator array consists of six piezo-ceramic elements which can vertically oscillate in spanwise direction. It can also produce spanwise perturbations in the flow near the wall by changing the mode of actuator elements and will create cross-flow locally. In case of "wavy mode", the regularity of streak-like structures tends to decrease at y^+=50 in x-z plane for the frequency of the actuator array more than 12.5 Hz, which is close to the frequency of bursting intervals at Re=7500 using our channel system.

MEMS applications in turbulence and flow control

Manufacturing processes that can create extremely small machines have been developed in recent years. Microelectromechanical systems (MEMS) refer to devices that have characteristic length of less than 1 mm but more than 1 μm, that combine electrical and mechanical components and that are fabricated using integrated circuit batch-processing techniques. Electrostatic, magnetic, pneumatic and thermal actuators, motors, valves, gears and tweezers of less than 100 μm size have been fabricated. These have been used as sensors for pressure, temperature, mass flow, velocity and sound, as actuators for linear and angular motions, and as simple components for complex systems such as micro-heat-engines and micro-heat-pumps. In this paper, we focus on the use of microelectromechanical systems for the diagnosis and control of turbulent shear flows. We survey the status and outlook of microsensors and microactuators as used for those particular applications, and compare the minute devices to their larger cousins. Microsensors can resolve all relevant scales even in high-Reynolds-number turbulent flows. Arrays of microsensors and microactuators make it feasible, for the first time, to achieve effective reactive control targeted toward specific small-scale coherent structures in turbulent wall-bounded flows.

Optimal Control of Turbulent Flow as Measure Solutions

We use the concept of relaxed or measure-valued solutions for control problems of turbulent flow related to Navicr-Stokes equation. Sufficient conditions guaranteeing the existence of measure solutions arc presented. Results on existence of optimal controls for Blow up time and Bolza problems for such systems are also presented. New results on relaxed necessary conditions of optimality are proved. Further it is shown that the relaxed necessary conditions reduce to classical Pontryagin type necessary conditions if measure solutions degenerate into Dirac structure. The paper is concluded with an algorithm based on the new necessary conditions for computing optimal controls.

Suboptimal control of turbulent channel flow for drag reduction

Two simple feedback control laws for drag reduction are derived by applying a suboptimal control theory to a turbulent channel flow. These new feedback control laws require pressure or shear-stress information only at the wall, and when applied to a turbulent channel flow at Reτ=110, they result in 16–22% reduction in the skin-friction drag. More practical control laws requiring only the local distribution of the wall pressure or one component of the wall shear stress are also derived and are shown to work equally well.

CONTROL OF TURBULENCE

▪ Abstract We discuss a few applications of active control of turbulent fluid flow and their implications for the economy and the environment. We outline a conceptual basis for control, sketching sensors, actuators, and the algorithm. The control of turbulence requires an understanding of turbulent flows beyond our present capabilities, but we describe the physical basis for control of the boundary layer: coherent structures and bursts, the connection between burst frequency and friction velocity, the change of burst frequency and drag reduction possible with polymers or active control, and other effects on burst frequency (e.g. streamline curvature, pressure gradients, and extra rates of strain). Given that the state of the flow must be sensed from the surface, and that this information is necessarily incomplete and aliased, sophisticated techniques may be required to interpret the signals. A control strategy, an algorithm, is necessary, and we express the need for a model of the flow as an interpretor and a predictor. Although surface actuators are necessary, the question of their precise effect on the fluid must be considered. We present surprising results of direct numerical simulation (DNS) of a type of actuator. Before controlling the fluid, we tried to control a model. As background, we present results of attempts to control several different models.

Passive control of backstep flow

Abstract The passive control of turbulent flow downstream of a backward-facing step using a permeable reattachment surface is described. An impermeable reattachment surface leads to the upstream convection of rotational fluid and pressure disturbances which is absent when a permeable surface is used. In the case of one permeable configuration, “flapping” (a low frequency unsteadiness) is completely removed as indicated by wall-pressure spectra and the analysis of velocity data obtained from triple-split film probes. The convective mode of feedback found with an impermeable reattachment surface is removed by a 12.5 h length of permeable surface which has the effect of damping the vortical structures that impinge on it and provides steady upstream convection through a sealed plenum beneath the reattachment surface. The question of the stability of such flows is addressed, and it is suggested that, while the flow is likely to be convectively unstable over a large region, the appearance of a sufficiently large region of absolute instability is not a necessary condition for the appearance of flapping because the global unsteadiness is driven by the impingement of large eddies at reattachment. An estimate of the ratio of the instability wavelength to a growth-rate length scale is of order 10, which is much larger than is perhaps generally accepted.

On the nature of the organized structures in turbulent near-wall flows

A physical mechanism of onset of large-scale organized structures in turbulent flows along a plane wall which are the cause of intensification of turbulent fluctuations is formulated. The structures take the form of high-speed and low-speed streaks caused by streamwise vortices, i.e., motions in the plane of the transverse cross-section. The streamwise vortices are excited as a result of instability under the action of the anisotropy of the normal components of the Reynolds stress tensor. A model for describing these vortices that gives characteristics in qualitative and quantitative agreement with the experimental data is proposed. In particular, the most probable and mean distances between neighboring vortices are correctly reproduced. The theory makes it possible to explain certain methods of turbulent flow control for the purpose of drag reduction.

Control of turbulent separated flow over a backward-facing step by local forcing

An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A 0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.

Modern Developments in Flow Control

This brief article reviews the important advances in the field of flow control that took place during the past few years. This broad area of research remains of great interest for its numerous potential benefits for both the military and civilian sectors. Spurred by the recent developments in chaos control, microfabrication and neural networks, reactive control of turbulent flows is now in the realm of the possible for future practical devices. Other less complex control schemes, passive as well as active, are more market ready and are also witnessing resurgence of interest.

Feedback control for unsteady flow and its application to the stochastic Burgers equation

The study applies mathematical methods of control theory to the problem of control of fluid flow with the long-range objective of developing effective methods for the control of turbulent flows. Model problems are employed through the formalism and language of control theory to present the procedure of how to cast the problem of controlling turbulence into a problem in optimal control theory. Methods of calculus of variations through the adjoint state and gradient algorithms are used to present a suboptimal control and feedback procedure for stationary and time-dependent problems. Two types of controls are investigated: distributed and boundary controls. Several cases of both controls are numerically simulated to investigate the performances of the control algorithm. Most cases considered show significant reductions of the costs to be minimized. The dependence of the control algorithm on the time-descretization method is discussed.

Control of Turbulent Open-Channel Flows by Skew-Arrangements of Strip Roughness

Skew arrangements of strip roughness induces a helicoidal motion of flow, and the transverse components near the free surface and near the bottom have different signs, which may restrict the zones where the materials are transported. In this study, the representative profile of the transverse velocity is analytically deduced from a simplified twodimensional model, and field measurement of turbulent flow over skew arrangements of strip roughness is conducted in an irrigation canal, which is often used for a conveyance of removed snow in winter. Furthermore, V and Δ type arrangements of strip roughness are investigated in order to apply such a helicoidal motion of flow to practical problems of effective conveyance of several materials.