Control of turbulent boundary layer flows by sound (original) (raw)
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Unsteady Topology and Control of a Turbulent Boundary Layer Separation over an Airfoil
2020
The subject of interest for this thesis is the detachment of a turbulent boundary layer. Engineers are interested in techniques that delay or suppress flow separation entirely because the performance of many fluidic devices, such as airfoil and diffuser, are hindered by this flow phenomenon. The sensitivity of flow separation to numerous flow-related parameters, and unsteady nature of the Preface All of the experimental work detailed in this thesis, including both the setup and collection of data, presented in this thesis were conducted by me personally with assistance from other lab members as required. Analysis of the collected data were done by me, the author of the thesis. However, some MATLAB codes were obtained from other lab members and then adapted for the data presented here. The contents in chapter 4 of the presented thesis contains the results of a published paper under Journal of Fluid Mechanics. The published paper is given as Ma, A., Gibeau, B., & Ghaemi, S. (2020). Time-resolved topology of turbulent boundary layer separation over the trailing-edge of an airfoil. J. Fluid Mech. (2020). The results in section 4 are presented nearly identical to the published version with slight modifications to include more detail that was not necessary for the journal. I was responsible for designing the experiment, collecting and analyzing all of the data, generating all the figures, and writing almost all of the text. The two co-authors provided guidance and assistance with all those stages whenever required by me. In addition, the co-authors reviewed previous versions of the manuscript and offered suggestions for improvements. They also added or modified existing text
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.
Dynamics of Controlled Boundary Layer Separation
Colloquium FLUID DYNAMICS, 2007
Dynamical aspects of the boundary layer separation on a circular cylinder in cross-flow are studied experimentally. Two cases are compared, the base case and the case with excitation using a synthetic jet actuator. The TR-PIV technique was used to catch both space and time velocity field correlations. Instantaneous velocity fields and skin friction distributions are evaluated. For dynamical structures study, the BOD method was applied based on energetic principle. Dynamical modes connected with vortex shedding process, dynamical excitation by synthetic jet and transition to turbulence are identified.
Control of a boundary layer separation
PAMM, 2007
V příspěvku jsou uvedeny výsledky experimentální studie řízení odtržení mezní vrstvy na rovinné stěně za působení nepříznivého tlakového gradientu. Pro řízení byly vybrány různé strategie: pasivní (drsná stěna, vírový generátor) a aktivní (syntetizovaný paprsek). Proces odtržení je zkoumán pomocí rychlé PIV metody. Při analýze dat je kladen důraz na dynamické aspekty zkoumaných jevů.
Control strategies for boundary layer separation
2007
V příspěvku jsou uvedeny výsledky experimentální studie řízení odtržení mezní vrstvy na rovinné stěně za působení nepříznivého tlakového gradientu. Pro řízení byly vybrány různé strategie: pasivní (drsná stěna, vírový generátor) a aktivní (syntetizovaný paprsek). Proces odtržení je zkoumán pomocí rychlé PIV metody. Při analýze dat je kladen důraz na dynamické aspekty zkoumaných jevů.
On the Lagrangian description of unsteady boundary-layer separation. Part 1. General theory
Journal of Fluid Mechanics, 1990
Although unsteady, high-Reynolds-number, laminar boundary layers have conventionally been studied in terms of Eulerian coordinates, a Lagrangian approach may have significant analytical and computational advantages. I n Lagrangian coordinates the classical boundary-layer equations decouple into a momentum equation for the motion parallel to the boundary, and a hyperbolic continuity equation (essentially a conserved Jacobian) for the motion normal to the boundary. The momentum equations, plus the energy equation if the flow is compressible, can be solved independently of the continuity equation. Unsteady separation occurs when the continuity equation becomes singular as a result of touching characteristics, the condition for which can be expressed in terms of the solution of the momentum equations. The solutions to the momentum and energy equations remain regular. Asymptotic structures for a number of unsteady three-dimensional separating flows follow and depend on the symmetry properties of the flow (e.g. line symmetry, axial symmetry). In the absence of any symmetry, the singularity structure just prior to separation is found to be quasi two-dimensional with a displacement thickness in the form of a crescent-shaped ridge. Physically the singularities can be understood in terms of the behaviour of a fluid element inside the boundary layer which contracts in a direction parallel to the boundary and expands normal to it, thus forcing the fluid above it to be ejected from the boundary layer.
Effective delay of turbulent boundary layer separation could be achieved via closed-loop control. Constructing such a system requires that sensor data be processed, real-time, and fed into the controller to determine the output. Current methods for detection of turbulent boundary layer separation are lacking the capability of localized, fast and reliable identification of the boundary layer state. A method is proposed for short-time FFT processing of time series, measured by hot-film sensors, with the purpose of identifying the alternation of the balance between small and large scales as the boundary layer separates, favoring the large scales. The method has been validated by comparison to other criteria of separation detection and over a range of baseline and controlled flow conditions on a "simplified" high-lift system, incorporating active flow control.
Management and control of unsteady and turbulent flows
1991
Conclusions from a wide range of experiments in transitioning, turbulent, separated and unsteady flow fields include the following highlights: The simultaneous generation of controlled phase-coupled plane TS waves and oblique waves was used to investigate the development of three dimensional disturbances and mechanisms of transition in a Blasius boundary layer. From these experiments, the detuning of the fundamental/subharmonic resonance emerges as a primary candidate for the transition process under natural conditions. Three dimensional mappings of the Reynolds stress producing events in turbulent boundary layers over a range of Reynolds numbers and initial conditions have demonstrated that an integral size of these dynamical motions scales better with outer variables as compared with inner variables. While the wall or inner layer is responsible for their initial, the hierarchy of their scales in the log layer expands with Reynolds number according to this outer scaling. Real-time ...