Rapid mixing is crucial for the efficient and environment-friendly operation of many industrial and propulsion devices involving jet flows. In this dissertation, two methodologies, self-excited nozzles and radially lobed nozzles, are studied and presented in order to enhance mixing in the near field of coflowing, subsonic, turbulent, free jet flows. The characteristics of the concentration field and the mixing performance are examined, mainly in quantitative manner. Two new parameters, mixing index and mixing efficiency index, are defined for free jets, allowing quantitative analysis of the mixing performance and efficiency. The flow fields are studied with hot wire anemometry, and with CFD simulation for some of the radially lobed nozzles. Due to the large vectoring angle of the jet flows from these nozzles, a new definition for the entrainment ratio is also adopted in order to take the large radial velocity component into consideration. Self-excited nozzles, rectangular and square shaped, are examined at Reynolds numbers of 17,000 and 31,000. The self-excited square jet has fastest mixing and highest mixing efficiency, with 400% higher mixing index at 4 diameters downstream than the unexcited square jet. The mixing is improved as the excitation frequency or coflow velocity increases. The study of flow field shows the presence of one pair of periodic, coherent array of large-scale, streamwise, counter-rotating inviscid vortices shedding from each of the two flaps which dominate the mean flow and the mixing process. The coflow is primarily entrained into the jet in the minor plane while the jet fluid vectors in the major plane. Significant increase in turbulent kinetic energy immediately downstream the nozzle exit improves small-scale mixing. Radially lobed nozzles, a cross-shaped and a clover-shaped with four lobes each, are analyzed in comparison to a conical nozzle. In addition, a few modified radially lobe nozzles, including a 6-lobe nozzle and an 8-lobe nozzle, two type of fully penetrating nozzles, and a cross-shaped nozzle with centerbody, are examined in order to achieve better mixing than the cross-shaped nozzle. At 4 diameters downstream, the mixing index of the cross-shaped nozzle is 650% higher than that of the conical nozzle. The cross-shaped nozzle with centerbody, the 6- lobe and 8-lobe nozzles have slower mixing and lower efficiency than the cross-shaped nozzle,but the fully-penetrating nozzles are generally better than the cross-shaped nozzle, especially at low coflow velocities and in the far field. The flow field study shows that parallel lobe walls and deep penetration of the coflow are importance factors responsible for the observed mixing enhancement. / Ph. D.

The objective of this work is to study the basic mechanism of the active control of free shear flows and look for new concepts in actuation methodologies. The possibility of controlling the evolution of a triangular jet is discussed in Chapter 2. The piezoceramic actuators were mounted on the flat sides of the nozzle. The flow was fully sheared near the nozzle. Single mode excitation with frequency as the varying parameter was found to be ineffective for controlling the far field evolution. In contrast, excitation of the jet with non-integer and counter propagating azimuthal modes yielded marked changes in the jet evolution. A new method employing dynamic spatial modes to control the far field of a circular jet is examined in Chapter 3. The basic mechanism that governs the far field control was found to be the non-linear interaction of the instabilities of the standing waves. This observation agrees with conclusions of former investigators but with a new understanding that the non-linear interaction mechanism and the jet expansion are not to become effective until the potential core ends, after which the jet demonstrates large, directional expansion. It was shown in the experiment that this expansion could be easily predicted and controlled. These results point out a better control mechanism -- the dynamic mode control method. Three schemes were proposed: 1) the phase modulation, 2) the switching modulation, and 3) the spatial mode modulation, where the first two were implemented with great success in controlling the evolution of the jet flow. Finally, a triangular jet with a top hat initial velocity profile is examined in Chapter 4. The results of excited and unexcited jets were compared with those studies where the initial flow conditions were different. It was found that the initial flow condition affected the flow field in two ways: the axis switching and the jet expansion. The combination of an initial top-hat velocity profile and a non-symmetric nozzle geometry was proved to be the necessary condition to create axis switching. Proper combination of initial velocity profile, nozzle geometry and spatial mode could greatly enhance the jet expansion. / Ph. D.

Hydraulic energy is one of the most important sources in the world for electricity production. Nowadays the trend is to limit the production of the electricity from fossil fuels and to protect the environment. The main idea is to use more renewable energy sources such as wind and solar energy. Unfortunately, these alternative sources are strongly dependent on current weather conditions, which causes the instability of the electrical grid. Luckily pumped storage and hydraulic power plants provide the solution. However, it requires an extension of the operating range of the hydraulic machines. For that reason, the water turbines now operate over and extended range of regimes, that can be quite far from the best efficiency point. Hence two types of unstable two-phase flows in the Francis turbine draft tube occur: part load overload. This diploma’s thesis is focused on the Francis turbine operating at the part load. During part load conditions the helical vortex rope is being developed in the draft tube, it causes pressure pulsation and it can lead to the hydro-acoustic resonance, which damages the elements of the power plant. The aim is to eliminate the pulsation by jet control of the swirling flow in the draft tube. In the diploma’s thesis, various approaches to jet control of the flow in conical diffuser are tested using the computational fluid dynamics.