The flow around a cylinder is shown in Figure 1.5 as an example of results from the same publication. In pipe and jet flows with Re≤2300, the flow generally turns out to be laminar flow.
Jet Excitation
It is possible to realize large amplitude oscillations, which can be independent of the beam velocity and excitation frequency. Using Pitot tubes to measure jet velocities, they further found that the excitation, combined with the increased jet spread, led to a motion at the virtual origin of the jet stream in the axial direction.
Literature Review – Mechanically Oscillated Planar Jets
A number of studies have been carried out using arrangements where an airfoil is placed in the centerline of the nozzle exit (Figure 1.19c). Collinset first investigated the effects of a small excitation amplitude on the average velocity decay, the jet spreading velocity and the entrainment of the flow. This may be a direct result of the angular deflection imposed on the jet stream by the rotation of the airfoil.
This figure is included on page 31 of the printed copy of the thesis in the University of Adelaide library. In the first region, the jet stream exhibits a fluttering motion, synchronized with the throw of the facility's reciprocating lips. Due to the proximity of the vanes to the nozzle exit, the entire jet stream is forced through the excitation mechanism.
They found that only at excitation strokes of S/h≥1 there is a noticeable enhancement of the flow structure near the nozzle exit. This figure is included on page 39 of the printed copy of the thesis in the University of Adelaide library.
Motivation for the Present Work
Once again the author also claims that xf is independent of the oscillation amplitude, but does not present any substantial data to verify this. The slightly lower value of Stb in this study than Stb=67×10−3 reported in most investigations by the same authors is most likely due to the difference in experimental setup and excitation method when compared to the discussed studies. before. No systematic attempt has been made to relate large-scale flow structures to the initial conditions of a MOPJ.
Only a few studies have examined the change in flow characteristics in a discrete range of Strouhal numbers and reported the large-scale coherent structures found in the jet flow. Furthermore, no studies have been reported on the systematic variation of S/handSth for a single well-defined nozzle configuration. The large variety of different types of nozzles and devices and the absence of a well-defined fundamental study prevent the evaluation of the variation of flow characteristics and coherent structures with changes in flow conditions from the available literature.
Thesis Aim & Scope
Thesis outline
The first part of the chapter examines the formation and near-field behavior of the MOPJs using phase-locked imaging and flow visualization. Data obtained using PIV are then used to demonstrate the congruence of the coherent structures and flow regimes found from flow visualization experiments. It is further extended to report how time-averaged eddy data can be used to undertake flow regime classifications.
Returning to the flow visualization results, this section shows examples of cycle-averaged flow and finds that there are actually three distinct flow regions for two of the three flow regimes identified, largely in agreement with observations documented in previous studies. The vortex formation distance for the flow cases for which quantitative data were obtained is then taken into account and compared with that from previous research. This leads to the suggestion of a relationship between the vortex formation distance and other important flow variables.
Following the PIV results, a small section of Chapter 3 is devoted to the observation of large-scale low-frequency fluctuations in the flow that came to light during the data analysis and, although not part of the scope of this study, warrants a mention for completeness. The fourth chapter is devoted to a case study of an example from each flow regime.
Fundamental Fluid Dynamics
While there is a random velocity component in turbulent flows, Hinze (1959) shows that the velocity distribution at a given point in the flow, if the sampling is long enough, has a periodic or pseudo-periodic pattern and therefore statistical methods can be applied to flows. Actual flows can be seen as a superposition of different characteristic size scales, ranging from macroscales of the size of the nozzle height or diameter in the case of a jet, to microscales in the micrometer-sized region of energy dissipation. Hinze (1959) also reports that while macroscale or large-scale eddies are apparatus dependent, microscales are mainly determined by flow viscosity, so the smallest eddies in a flow are not turbulent but viscous and dominated by molecular effects.
Since the flow is a superposition of eddies of different sizes, certain values of kinetic energy are related to the sizes of the individual eddies and determined by the vorticity and the intensity of the velocity fluctuation at the corresponding frequencies (Hinze, 1959). This issue is included on page 48 of a hard copy of the thesis held by the University of Adelaide Library.
Simple Harmonic Motion
The vertical motion of the nozzle is periodic and the present study uses a triple decomposition scheme proposed by Reynolds and Hussain (1972) to investigate oscillatory flow. This decomposition scheme has been successfully used by a number of authors such as Kelso (1991) and Iioet al. (2008).
Experimental Facility
Also, the work section floor and ceiling, respectively, were closer (50 hours) than many of those other studies. A submerged false ceiling was used to achieve symmetrical boundary conditions at the top and bottom of the work section (Figures 2.4 & 2.5). The facility is a closed-loop system that uses a Venturi meter to measure flow and an electric pump connected to a variable frequency drive to control volumetric flow.
The flow conditioning section contained a number of polyethylene mesh screens (22GG950) and a flow straightening section made of 5 mm diameter drinking straws and a perforated stainless steel plate to ensure uniform flow distribution upstream of the oscillating plate as shown in Figure 2.6. During the start-up process, an upstream investigation was conducted as part of a final year undergraduate thesis project (Paice, 2005a,b). The objectives of this investigation were to identify the flow structures upstream of the oscillating nozzle and downstream of the flow conditioning section and discharge into the nozzle from different positions.
Notes: 1 = thin cover plate with knife edge in front of oscillating plate (top and bottom); 2 = swing plate; 3 = Perforated metal plate, mesh screens and flow equalization element for flow conditioning. For all investigated cases, it was shown that the liquid and dye input into the nozzle is continuous, smooth and symmetrical, and the flow near the walls is continuous.
Dye Flow Visualisation
The visual access port to the upstream part of the plant can be seen through the black engine frame. To achieve neutral buoyancy of the dye in water, the solution was mixed with small amounts of alcohol. In the present study, methanol was used, and with the help of a hydrometer, the solution density was adapted to the working fluid in the experimental plant (Figure 2.11).
During experiments it was found that the dye density deviation of only 1 kg per cubic meter from the working fluid was sufficient to change the visualized structures in the manner described by Perry and Lim (1978). As a result, great care was taken during the preparation of the dye solution to match the densities as best as possible. One tube was aligned with the oscillation centerline and one each with the nozzle center at both the TDC and BDC of the oscillation stroke (Figure 2.12).
The position of the nozzle exits was changed during the experiments to accommodate different stroke heights. This image is included on page 59 of a hard copy of the thesis held by the University of Adelaide Library.
Phase Locked Imaging & Pseudo-Video
We will touch on the theoretical background only briefly, along with an overview of the experimental equipment and setup used in this study. A more detailed explanation of the PIV arrangement for this study can be found in Appendix A. In this case, the linear transfer function can be found by complex conjugate multiplication of Fourier transforms as
A more detailed description of the physical arrangement of the PIV setup is provided in Appendix A. PIV image acquisition was synchronized with the position of the oscillating plate using a custom-designed control system. During the analysis of the PIV data, it was noticed that a constant overestimation of the jet velocities was obtained along the centerline of the plant in the flow direction at a distance of x/h=3.3±0.2 and atx/h=5±0, 5, as shown. in figure 2.23.
Figures 2.24 and 2.25 show the normalized velocity profiles of the steady jets from the nozzle exit to a distance of x/h=8. Inspection of the individual velocity profiles in the two figures shows the overestimation of the velocities at x/h=3 & 5, especially for the cases with higher Reynolds numbers, as discussed in the previous section (Figure 2.23), providing further evidence for the claim that the velocity errors are inherent to the experimental facility and not the result of faulty data analysis. Examination of the experimental conditions used by the cited reference studies shows that the present study is the only one that is narrowly confined in both the spanwise and transverse directions (Table 2.2).
Husseinet judge (1994), Deo (2005) and Deo et al. (2008) extensively discussed the loss of jet momentum due to induced recirculation around the jet arising from the proximity of physical object boundaries to the exit of the jet nozzle. ).
