No description available.

The goal of this thesis was to both theoretically and experimentally show the effect of a plasma actuator for flow separation control.  In the theoretical part a solver was implemented in MATLAB code, to solve the governing equations describing the plasma actuator.  The experimental part included PIV (Particle Image Velocimetry) measurements of the velocity field induced by the plasma actuator, visualization of the effect in a wind tunnel and the development of a simple model of the plasma actuator based on the empirical result whose purpose is to be used in CFD (Computational Fluid Dynamics). The PIV measurements were performed with an acceptable result even though a lot of disturbance occurred in and near the plasma region.  The empirical result was used to develop the empirical plasma actuator model for CFD, which showed some interesting result.  The model implies that the induced force by the plasma actuator grows exponential with the applied peak-to-peak voltage.   The model was also used to predict airfoil performance with plasma actuators which showed an increase of the lift coefficient on a NACA0012 with a chord length of 0.1m.  Simulations were done for free-stream velocities up to 20m/s with three different configurations, without plasma actuator for comparison, with one actuator at the quarter-chord and one with three actuators on the airfoil.  With three actuators the increase of the lift coefficient was 108 percent at 5m/s and 14 percent at 20m/s. The simulations with one actuator were only performed up to 10m/s were the effect of the actuator still could be seen but for higher velocities the effect would probably be minor. The wind tunnel experiment clearly showed the effect and the advantages of utilizing plasma actuators for flow separation control.  The experiment showed that a single plasma actuator placed at the quarter chord of a fully stalled NACA0012 airfoil with a chord length of 0.1m, at approximately 20 degrees angle of attack and with a free-stream velocity of 1.5m/s, was able to reattach the flow behind the actuator. The result of the theoretical part was inconclusive, the code could not run with the appropriate voltage and frequency of the plasma actuator.  Some result was however obtained, implying that the time-average force induced by the plasma actuator was in the expected direction.  The theoretical model is however considered to have potential, the major problems concern the code which requires further development.

The stringing regulation on greenhouse gases emissions coupled with the rising fuel price and the growth in aviation transportation have imposed increasing demands on the aircraft industry to develop revolutionary technologies to meet such challenges. Methods of delaying flow separation on aircraft high lift systems have been sought which can lead to an increase in the aircraft performance and ultimately a reduction in aircraft operational costs and its impact on the environment. Synthetic jet actuators are a promising method of delivering flow control for aircraft applications due to their ability to inject momentum to an external flow without net mass flux and their potential in being integrated in MEMS through micro-fabrication with relative ease. It has been demonstrated in many laboratory experiments that synthetic jets are capable of delaying flow separation on aerodynamic bodies of various shapes. However, currently the operating conditions of synthetic jets are mostly chosen by trial-and-error, and thus the flow control effectiveness varies from one experiment to another. In order to deliver an effective flow separation control which achieves a desired control effect at minimum energy expenditure, a better understanding of the fluid mechanics of the behaviour of synthetic jets and the interaction between synthetic jets and a boundary layer are required. The aims of the present research were to achieve such a goal through a series of purposely designed numerical simulations. Firstly, synthetic jets issued from a circular orifice into quiescent air were studied to understand the effect of dimensionless parameters on the formation and the extent of roll-up of vortex rings. The computational results confirmed that the Stokes number determines the strength of vortex roll-up of a synthetic jet. Based on the computational results, a parameter map was produced in which three different operational regimes of synthetic jets were indentified and a criterion for vortex roll-up was also established. A circular synthetic jet issued into a zero-pressure-gradient laminar boundary layer was then investigated. The capability of FLUENT in modelling the key characteristics of synthetic jets was validated using experimental data. The formation and evolution of coherent structures produced by the interaction between synthetic jets and a boundary layer, as well as their near-wall effect in terms of the wall shear stress, were examined. A parameter map illustrating how the appearance of the vortical structures and their corresponding shear stress patterns vary as the synthetic jet operating condition changes was established. In addition, the increase in the wall shear stress relative to the jet-off case was calculated to evaluate their potential separation control effect.Finally, the study moved one step forward to investigate the flow separation control effect of an array of three circular synthetic jets issued into a laminar boundary layer which separates downstream on an inclined plate. The impact of synthetic jets on the boundary layer prior to separation and the extent of flow separation delay on the flap, at a range of synthetic jet operating conditions, were examined and the correlation between them was investigated. Furthermore, the optimal operating conditions for this synthetic jet array in the current study were identified by considering both the flow control effect and the actuator power consumption. The characteristics of the corresponding vortical structures were also examined.The findings from this work have produced some further insights of the behaviour and the interaction between synthetic jets and a boundary layer, which will be useful for ensuring an effective application of synthetic jets in practical settings.

No description available.

Many flow cases in fluid dynamics face undesirable flow separation due to ad-verse pressure gradients on wall boundaries. This occurs, for example, due togeometrical reasons as in a highly curved turbine-inlet duct or on flow-controlsurfaces such as wing trailing-edge flaps within a certain angle-of-attack range.Here, flow-control devices are often used in order to enhance the flow and delayor even totally eliminate flow separation. Flow control can e.g. be achieved byusing passive or active vortex generators (VGs) for momentum mixing in theboundary layer of such flows. This thesis focusses on such passive and activeVGs and their modelling for computational fluid dynamics investigations. First, a statistical VG model approach for passive vane vortex genera-tors (VVGs), developed at the Royal Institute of Technology Stockholm andthe Swedish Defence Research Agency, was evaluated and further improvedby means of experimental data and three-dimensional fully-resolved computa-tions. This statistical VVG model approach models those statistical vortexstresses that are generated at the VG by the detaching streamwise vortices.This is established by means of the Lamb-Oseen vortex model and the Prandtllifting-line theory for the determination of the vortex strength. Moreover, thisansatz adds the additional vortex stresses to the turbulence of a Reynolds-stresstransport model. Therefore, it removes the need to build fully-resolved three-dimensional geometries of VVGs in a computational fluid dynamics mesh. Usu-ally, the generation of these fully-resolved geometries is rather costly in termsof preprocessing and computations. By applying VVG models, the costs arereduced to that of computations without VVGs. The original and an improvedcalibrated passive VVG model show sensitivity for parameter variations suchas the modelled VVG geometry and the VVG model location on a flat plate inzero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil withits high-lift system extracted. It could be shown that the passive VG modelqualitatively and partly quantitatively describes correct trends and tendenciesfor these different applications. In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow atTechnische Universitä̈t Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. Dependencies of the generated vortices and their characteristics on the VGJsetup parameters could be identified and quantified. These dependencies wereused as a basis for the development of a new statistical VGJ model. This modeluses the ansatz of the passive VVG model in terms of the vortex model, theadditional vortex-stress tensor, and its summation to the Reynolds stress ten-sor. Yet, it does not use the Prandtl lifting-line theory for the determinationof the circulation but an ansatz for the balance of the momentum impact thatthe VGJ has on the mean flow. This model is currently under developmentand first results have been evaluated against experimental and fully-resolvedcomputational results of a flat plate without pressure gradient. / <p>QC 20120511</p>