Analysis of flow control by boundary-layer manipulation using 2D frequency response (original) (raw)
Related papers
Turbulence control in wall-bounded flows by spanwise oscillations
Applied Scientific Research, 1993
The feasibility of control of wall turbulence by high frequency spanwise oscillations is investigated by direct numerical simulations of a planar turbulent channel flow subjected either to an oscillatory spanwise crossflow or to the spanwise oscillatory motion of one of the channel walls. Periods of oscillation, T+c. = Tos~.u~/u, ranging from 25 to 500 were studied. For 25 < T+c. < 200 production of turbulence is suppressed. The most effective suppression of turbulence occurs at T+~ = 100, for which the overall turbulence production is reduced by 62% compared to the unperturbed channel and sustained turbulent drag reductions of 40% are obtained. The suppression of turbulence is due to a continual shift of the near wall streamwise vortices relative to the wall layer streaks, which in turn leads to a widening, merging and weakening of the wall layer streaks and an overall reduction in the turbulence production. The turbulence suppression mechanism observed in these studies opens up new possibilities for effective control of turbulence in wall-bounded flows.
Turbulent boundary layer flow subject to streamwise oscillation of spanwise wall-velocity
Direct numerical simulations have been performed to study the effect of a stationary distribution of spanwise wall-velocity that oscillates in the streamwise direction on a turbulent boundary layer. For the first time, a spatially developing flow with this type of forcing is studied. The part of the boundary layer which flows over the alternating wall-velocity section is greatly affected with a drag reduction close to 50% which exhibits an oscillatory distribution with a wavenumber which is twice that of the imposed wall-velocity. The maximum in drag reduction occurs where the wall velocity is at its maximum (or minimum) and the minimum occurs where the wall velocity is zero. Comparisons of the mean spanwise velocity profiles with the analytical solution to the laminar Navier-Stokes equations show very good agreement. The streamwise velocity profile indicates a thickening of the viscous sub-layer when scaled with the local friction velocity and an upward shifting of the logarithmic region when scaled with the reference (unmanipulated) friction velocity. An estimation of the idealized power consumption shows that-with the present wall forcing magnitude-more energy is required for the spatial oscillation than what is saved by drag reduction.
The delay of turbulent boundary layer separation by oscillatory active control
1989
The aims were to develop an active method to control turbulent boundary layer separation, to study its efficiency, and to study the flow regime after its activation. In a subsonic open-ended wind tunnel, a sharp angle in a flat plate created a local discontinuity and a strong downstream positive pressure gradient, causing boundary layer separation from the plate. A vibrating flap at the discontinuity constituted the active means of separation control. A hot-wire probe was used to measure the velocity field along the length of the plate. The data were processed to obtain information on the fluid behavior, averaged over frequency and time. The similarity of the velocity profile of the upper part of the separated flow to the average velocity profile in a 2-dimensional mixing layer in which a vibrating flap increased the mixing among different velocities, led to the suggestion that, in the present case of a separated boundary layer, a vibrating flap would enhance the mixing of the energy-rich upper part of the flow with the energy-poor lower part, leading to reattachment of the flow. Reattachment occurred, characterized by 3 regions: a small region of large 2-dimensional vortices; a region in which the vortices decayed, whose location depended on vibration frequency, and in which the boundary layer was characteristic of a vibrating fluid moving against a positive pressure gradient; and a region where the decaying large vortices no longer affected the flow near the plate surface, and with a classic turbulent boundary layer. The principle conclusion was that a vibrating flap provides an effective active control means for a turbulent boundary layer, and can prevent flow separation.
Streamwise oscillation of spanwise velocity at the wall of a channel for turbulent drag reduction
Physics of Fluids, 2009
Steady forcing at the wall of a channel flow is studied via DNS to assess its ability of yielding reductions of turbulent friction drag. The wall forcing consists of a stationary distribution of spanwise velocity that alternates in the streamwise direction. The idea behind the forcing builds upon the existing technique of the spanwise wall oscillation, and exploits the convective nature of the flow to achieve an unsteady interaction with turbulence.
This review article is concerned with the design of linear reduced-order models and control laws for closed-loop control of instabilities in transitional flows. For oscillator flows, such as open-cavity flows, we suggest the use of optimal control techniques with Galerkin models based on unstable global modes and balanced modes. Particular attention has to be paid to stability-robustness properties of the control law. Specifically, we show that large delays and strong amplification between the control input and the estimation sensor may be detrimental both to performance and robustness. For amplifier flows, such as backward-facing step flow, the requirement to account for the upstream disturbance environment rules out Galerkin models. In this case, an upstream sensor is introduced to detect incoming perturbations, and identification methods are used to fit a model structure to available input-output data. Control laws, obtained by direct inversion of the input-output relations, are found to be robust when applied to the large-scale numerical simulation. All the concepts are presented in a step-by-step manner, and numerical codes are provided for the interested reader.
Optimal feedback design for mixing enhancement in boundary layers of membrane systems
This paper proposes a scheme for improving the mixing in the boundary layer of pressure-driven membrane systems such as reverse osmosis and ultrafiltration. Through application of an external electric field, a flow of ions in the vicinity of the membrane surface will be generated, creating simultaneous electro-osmotic flow that should reduce the concentration polarization on the membrane surface. The objective of feedback design for this system is to determine the voltage (and waveform) required to produce an electric field that can effectively increase mixing in the vicinity of the membrane wall. This paper uses a mixing index in terms of a measure of spatial gradients of the perturbation velocities, which describes the mixing caused by both length stretching and vortices. An optimal control problem is defined and a control strategy is developed to achieve mixing enhancement and improve energy efficiency. The efficacy of the feedback scheme is validated by Computational Fluid Dynamics (CFD) simulations. The control law presented in this paper shows the desired waveforms for such applications.
Control study on mixing enhancement in boundary layers of membrane systems
Journal of Process Control, 2013
This paper proposes an activation scheme for improving the mixing in the boundary layer of pressuredriven membrane systems such as reverse osmosis and ultrafiltration. Through the application of an external electric field, a flow of ions in the vicinity of the membrane surface is generated, creating an electro-osmotic flow that should reduce the extent of concentration polarization. An optimal control problem is formulated and solved to determine the waveform of the control action required to produce an electric field that can effectively increase mixing in the vicinity of the membrane surface with improved energy efficiency. This paper uses a mixing index in terms of a measure of spatial gradients of the perturbation velocities, which describes the mixing caused by both length stretching and vortices. The efficacy of the proposed control is validated by Computational Fluid Dynamics (CFD) simulations.
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.
Control of wall turbulence by high frequency spanwise oscillations
3rd Shear Flow Conference, 1993
A new technique for the control of turbulence in wall-bounded flows is discussed. Turbulence control is achieved by disrupting the spatial coherence of turbulence structures in the near-wall region using spanwise forcing. The feasibility of the control scheme has been demonstrated by direct numerical simulations of a turbulent channel flow which is subjected either to an oscillatory spanwise crossflow or to the spanwise oscillatory motion of one of the channel walls. In either case, oscillations at TAC, = T o 3 c. u~/ v = 100 are seen t o result in a 60% reduction in overall turbulence production and a 40% reduction in the turbulent drag. Alternative methods for the implementation of these concepts in practical applications are discussed.
Laminar and turbulent comparisons for channel flow and flow control
Journal of Fluid Mechanics, 2007
A formula is derived that shows exactly how much the discrepancy between the volume flux in laminar and in turbulent flow at the same pressure gradient increases as the pressure gradient is increased. We compare laminar and turbulent flows in channels with and without flow control. For the related problem of a fixed bulk-Reynolds-number flow, we seek the theoretical lowest bound for skin-friction drag for control schemes that use surface blowing and suction with zero-net volume-flux addition. For one such case, using a crossflow approach, we show that sustained drag below that of the laminar-Poiseuille-flow case is not possible. For more general control strategies we derive a criterion for achieving sublaminar drag and use this to consider the implications for control strategy design and the limitations at high Reynolds numbers.