Showing papers in "International Journal of Flow Control in 2009"

Active Combustion Control Using a Fluidic Oscillator for Asymmetric Fuel Flow Modulation

Abstract: Fluidic oscillators are of special interest for fuel-based active control schemes featuring high frequency fuel flow modulation, as they are much more durable then conventional valves due to the absence of fast moving parts. In this work the performance of a fluidic oscillator in an active combustion control scheme is demonstrated. The oscillation frequency was controlled by varying the inlet mass flow of the oscillator. High speed camera recordings and hot wire measurements were performed to investigate the fluidics' oscillation characteristics. The oscillator was then incorporated into a bluff body burner, where it modulated parts of the fuel flow blended with nitrogen. Pressure and heat release fluctuations in the combustor were recorded and images of the flame were taken. At stable combustion the spectra of the heat release signals showed a clear peak corresponding to the fluidics' oscillation frequency, thus validating the ability of the oscillator to influence the combustion process. When operating ...

Simple Models of Zero-Net Mass-Flux Jets for Flow Control Simulations

Abstract: Although computational fluid dynamics is well suited for modeling the dynamics of zero-net mass-flux (ZNMF) actuators, the computational costs associated with large-scale flow control simulations necessitate the use of simple models for these devices. A new model based on only the slot of ZNMF jets in grazing flows is proposed. A study of the dimensionless parameters governing the ZNMF jet in grazing flow is conducted, and the performance of the model is assessed in terms of the vortex dynamics and the mean integral quantities that define characteristics of the ZNMF jet. A comparison with full cavity simulations as well as the often-used sinusoidal, plug-flow model indicates that the new model provides a good prediction of the jet outflow. In addition, the fidelity of the model has also been explored for a canonical separated flow. Results show that the model is able to predict the effect of the jet on the separation bubble much more accurately than the conventional plug-flow model.

Active vibration control of a wind turbine blade using synthetic jets

Abstract: Active vibration control via an array of synthetic jet actuators was investigated experimentally in a wind tunnel. Using synthetic jets the flow over a small scale S809 finite wind turbine blade was controlled, resulting in reduction of the blade's structural vibrations. The effectiveness of the synthetic jets was explored for a range of post-stall angles of attack at Reynolds numbers between 7.1 × 104 and 2.38 × 105. The blade vibrations were measured and quantified using a pair of strain gauges mounted at the root of the model. Using flow control, significant vibration reduction was observed for some post-stall angles of attack. A correlation between vibration reduction and the degree of flow reattachment, measured using Particle Image Velocimetry, was found.

Experimental and Numerical Investigation of Active Control of Inlet Ducts

Abstract: The effect of steady and unsteady control jets on the performance of a very aggressive (length to exit diameter ratio, L/D, of 1.5) inlet duct was investigated. Experiments were performed for a range of inlet Mach numbers from 0.2 to 0.45 and compared with numerical simulations for the baseline flow case at an inlet Mach number of 0.45. A brand new facility was designed and built to enable various actuation methodologies as well as multiple measurement techniques. In the present work, a pair of control jets was placed in streamwise locations where flow was expected to separate. Steady and unsteady static pressure measurements, along the upper and lower walls of the duct, were performed for various combinations of actuation. The parameters that were tested include the control jets momentum coefficient, the blowing ratio, the actuation frequency, as well as different combinations of jets. It was shown that using mass flux ratio as a criterion to define flow control is inappropriate, and one needs to provide...

Combustion Powered Actuator with Integrated High Frequency Oscillator

Abstract: A new actuation concept for flow control has been tested combining the high jet strength of chemically-based, combustion powered actuation with the high frequency of fluidic oscillators. The device exhausts the jet from a combustion powered actuator through a fluidic oscillator and embeds a high frequency [∼ O(1-10) kHz] oscillation into the lower frequency [∼ O(10-100) Hz] of the repetitively ignited combustion bursts. The present work includes characterization of the fluidic oscillator in steady operation for the properties which vary over the combustion burst (pressure, temperature, and gas composition) and results from test firing of the fully integrated device successfully indicating high frequency oscillation in the high speed actuation jet while maintaining a no moving parts design.

Using Linear Stability Analysis as a Tool to Evaluate Jet and Cavity Flow Control Situations

Abstract: Flow control has the potential to improve the efficiency of aerospace systems. The design of a flow control system requires knowledge of flow phenomena including information on the evolution of naturally occurring disturbances in the flow. Natural flow disturbances can evolve at various frequencies and growth rates and can assume various mode shapes. Knowledge of the behavior of natural flow instabilities could aid actuator selection and strategy for flow control. In this paper we use linear stability analysis as a tool to analyze jet and cavity flow control situations. We present results from a linear stability study of the resonant acoustic mechanisms in subsonic impinging jets and subsonic cavity flows using the compressible Rayleigh equation. In an impinging jet, the subsonic instabilities are known to be responsible for the acoustic feedback, which is an essential link in setting up the resonances associated with this flow. The instabilities of the helical type are also relevant from the standpoint o...

Flow Control of Jet Flow by Passive Nozzle Configuration Changes

Abstract: Nozzle configuration may offer the possibility of a passive control technique to provide high mixing and heat transfer rates. Thus, numbers of investigations have been carried out to reveal the influence of nozzle configuration on the flow characteristics. However, very few papers in the scientific literature have focused on orifice jets. Despite the evidence indicating that orifice jets improve the mixing and heat transfer characteristics, we were unable to find a systematic study of them that focused on the contraction area ratio. The effects of varying the contraction area ratio CR from 1.00 to 0.11 on the flow characteristics of a free jet issuing from an orifice nozzle were examined. The large vortex structure of a submerged orifice water jet was visualized by the tracer method, which indicated the coherent vortex structure was highly affected by the CR value. The mean and fluctuating velocities of the orifice air jet were also measured using a single hot-wire and the effects of CR on them were demon...

Flow-Field Measurement of Embedded Streamwise Vortex Generated from Passive Flow-Control Devices

Abstract: Detailed flow-field measurements were performed downstream of a single vortex generator (VG) using a Stereo Digital Particle Image Velocimetry system. The passive flow-control devices examined consisted of a low-profile VG with a device height, h, approximately equal to 20 percent of the boundary-layer thickness, Δ, and a conventional VG with h ≈ Δ. Flow-field data were taken at twelve cross-flow planes downstream of the VG to document and quantify the evolution of embedded streamwise vortex. Key parameters including vorticity, circulation, trajectory, and half-life radius—describing concentration, strength, path, and size, respectively—of the device-induced streamwise vortex were extracted from the flow-field data. Peak vorticity and circulation for the low-profile VG decayed exponentially to the distance downstream from the device. The device-height normalized vortex trajectories for the low-profile VG, especially in the lateral direction, followed the general trends of the conventional VG.

Subcritical Pipe Flow Transition Control Using Dielectric Barrier Discharge Plasma Actuators

Abstract: This work details the application of dielectric barrier discharge (DBD) actuators to the control of transition in subcritical Reynolds number (250 ≤ Re ≤ 1000) circular pipe flows for the purposes of increasing mixing or momentum and heat transfer. Primary flow measurements were made using a single hot-wire anemometer and these were augmented using smoke filament visualization with high-speed photography. A sensitive balance was used to calibrate the body force generated by the actuator as a function of input power. Several distinct actuator configurations were considered and a downselection indicated that the introduction of swirl produced the largest coherent oscillations and purely turbulent fluctuations. Using the swirl actuator, a detailed parametric study was conducted where operation parameters such as duty cycle, input power, frequency, momentum, etc. were systematically varied. The largest coherent oscillations occurred at a reduced frequency of approximately 0.08 while peak noncoherent disturban...

Evaluation of Jet Mixing Rate Based on DNS Data of Excitation Jets

Abstract: In order to establish a good measure of mixing rate, jet mixing is examined using the DNS (direct numerical simulation) data of controlled jets. In the computation, spatial discretization is performed using a hybrid scheme that adopts a sixth-order compact scheme in the streamwise direction and a Fourier series in the lateral directions. The Reynolds number is 1500. Two measures such as statistical entropy and a mixedness parameter are investigated using a passive scalar concentration. Compared with simple measures, i.e., jet width, centerline velocity, and turbulent kinetic energy, it is found that the statistical entropy is a good measure for representing the mixing state in different controlled jets, and it is possible to use the fluctuating entropy to detect a highly mixed region and determine the correlation of the mixed regions with the vortical structure. In addition to statistical entropy, the mixedness parameter is also found to have useful properties for the estimation of mixing efficiency. Thes...

Flow in Compliant Tubes: Control and Stabilization by Multilayered Coatings

Abstract: Flow instability and flow control in compliant tubes are studied in application to biological systems and technical devices. Common features of the flows are determined by fluid—structure interaction that may lead to absolute and convective instabilities. The stability of a steady flow of an incompressible viscous fluid through a compliant tube composed of three anisotropic viscoelastic layers with different material properties is studied. The eigenvalues of the system are calculated for the no-stress and no-displacement boundary conditions at the outer surface of the tube. It is shown that the flow can be stabilized by a proper choice of the thickness, viscosity and elastic moduli of the layers. The multilayered viscoelastic coatings can be used to control flows in compliant ducts.

Oscillating Jet Flow in Enclosures with Non-circular Cross Section

Abstract: Liquid spray processes possess a typical spreading characteristics in an enclosing cavity. Therefore, the transient jet behaviour in a plenum or chamber with non-circular cross section is studied. The investigation is connected to problems in jet and spray processes where the process configuration of the flow needs to be investigated in order to achieve optimum working conditions and consequently to yield higher performance of the process. For investigation of the flow configuration numerical modeling and simulation has been used.For analysis of the transient jet flow in enclosed areas full 3D geometries with transient simulations yare employed for studying the oscillating jet behaviour in time and space. Aim of the study is the comparison of different chamber geometry variants with non-circular cross sections. Namely, some typical chambers with square and rectangular cross sections with and without streamwise expansion are compared. Two different types of movements are found in the rectangular cavity, na...

Active Yaw Control of a Ducted Fan-Based MAV using Synthetic Jets

Abstract: The feasibility of using active flow control to stabilize a micro ducted fan unmanned aerial vehicle in a yaw motion was investigated experimentally. Flow control was implemented using synthetic jet actuators to manipulate the flow around the vehicle's stator vanes. As a result, this mechanically simplified control approach can be used to generate a yaw moment on the vehicle instead of moving control surfaces and articulated rotor blades. The rotational control of the MAV (165.5mm in diameter and 177.8mm in height) was obtained by activating surface-mounted synthetic jets that were mounted on a set of four fixed stators downstream of the duct. The synthetic jets were located downstream of a deliberately formed local separation to enable controlled flow reattachment. The flow field around the stators and in the wake was studied using particle image velocimetry (PIV) where the geometrical angle of attack of the stators was either 0° or 6° and the propeller rotational speed was either 4,200RPM or 9,000RPM (g...

Flow Control and the Energy Crisis

Abstract: Energy and global warming are two of the most pressing quandaries facing humanity in the twenty-first century. Flow control can help in both fronts. This brief describes how, but is mostly a bird-eye's view of the energy crisis.