Structure involved in the flow dynamics

The instantaneous images of the flow field shown in figure (5.2a-c) are obtained by sticking together the images of separate planes, standing for the blowing ratios of M =0.65, 1 and 1.25 in steady blowing (flow direction is from left to right). In the following figures, the absence of tracer particle in the injectant flow allows us to visualize some basic attributes of the injectant jet issuing in the mainstream flow. It is evident that the jet flow in case of M =0.65 stays close to the wall, while for M =1 and 1.25, it moves slightly away due to the increase of jet momentum. It can also be observed that the growing of the coherent vortices is more organized for M=1 than the rest of the cases. On the bottom side of the jet, the interaction of the wake structures with the lower boundary of the jet and the entrainment of mainstream flow make the formation of vertices somewhat different from the one expected

“Experimental aerothermal characterization of a pulsating jet issuing in a crossflow:

Influence of Strouhal number excitation on film cooling” -134- for the case of a free jet issuing from the injection hole at a similar flow rate. The wake effects are appeared to be dominant in case of M =1.25.

(a)

(b)

(c)

Figure (5. 2): Instantaneous images of the flow filed, (a) M =0.65, (b) M =1 and (c) M =1.25.

To visualize some general characteristics of the near-field interaction of the two flows, the vector field obtained from cross-correlation algorithm is plotted in figure (5.3) for a flow configuration of M =1.25 and St=0, where the jet effects prevail larger than the other cases of steady blowing.

Some important features of the “jet-in-crossflow” can surely be identified here, such as; the contribution of the boundary layer vorticity of the oncoming flow rolling up at the shear layer along with the tube vortices to form the large counter rotating vortex pair (CVP), which greatly influences the wall coverage of the injectant fluid. Experimental studies performed by Foucault et al. (1992) and Dorignac et al. (1992) featured the existence of CVP in a crossflow configuration using an oblique jet. The attributes of the flow field resulting due to the immediate interaction of two flows control the dynamics of the CVP. Walters and Leylek (2000) have mentioned a strong dependence of the near-field interaction on the ultimate distribution of the injectant flow in the downstream region.

“Experimental aerothermal characterization of a pulsating jet issuing in a crossflow:

Influence of Strouhal number excitation on film cooling” -135-

(a) (b)

(c) ²

Figure (5. 3): (a) Vector field of instantaneous velocity, (b) Zoom-in region of vector field shown in (a), and (c) Vector field of fluctuating velocities, M =1.25, St=0.

Figure (5.3b) shows the detailed view of the instantaneous velocity field obtained from the near downstream region of the trailing edge of the hole, which shows the initiation of a small wake region. The instantaneous image given here is randomly chosen from the database. The size of the wake region may vary due to the slight unsteady variations in the dynamics of the upstream flows. Walters and Leylek (2000) have reported the existence of small reverse flow region for an oblique jet issuing at 35°.

The fluctuating field shown in figure (5.3c) indicates the formation of vortices at the upstream edge of the jet due to the rolling-up of the shear layer very close to the exit of jet fluid. For a vertical jet issuing into a cross-stream flow, Kelso et al (1996) referred to the rolling of the shear layer as a consequence of the Kelvin-Helmholtz like instability, which consistently produces these vortices at the leading edge. The entrainment of the cross- stream flow towards the jet and to its wake region is evident from the streamlines of the fluctuating velocity components. Moussa et al. (1977) suggested that the two vortex systems generated at the upstream interface of the two flows and the other one bounded to the lee surface of the jet were responsible for most of the entrainment of crossflow into the

“Experimental aerothermal characterization of a pulsating jet issuing in a crossflow:

Influence of Strouhal number excitation on film cooling” -136- deformed jet and its wake. The lateral motion of the mainstream flow occuring in the wake region is expected to increase with the increase of blowing ratio.

A complete scenario for the flow field can be build in the light of a number of interpretations proposed in the open literature (Lee et al. 1994 ; Walters and Leylek 2000;

Kelso et al 1996; Andreopoulos 1984), as well as the prior understanding of the flow behavior implied from the present study. The vertical structure ejecting from the tube comprises of the vorticity generating in the boundary layer of the injection tube. Interaction of these vortices with the vorticity generating in the boundary layer of mainstream flow takes place in the area of jet boundary. With the interaction, a strong shear layer is form in the region of the interface of two flows. The unstable shear layer in this region rolls-up due to the Kelvin-Helmholtz like instability and contributes to the formation of CVP (Counter- Rotating Vortex Pair). Near the trailing edge, the vorticity generated along the circumference of the injection tube tilts and stretches slightly due to the acceleration of the jet flow close to the trailing edge of the hole. In this region, the piling up of these vortices results in a bound vortex, which interacts with the mainstream flow entraining into a relatively low pressure region lying underneath the jet. The mainstream flow entrained below the jet moves towards the central plan of the jet and deforms the bound vortex to the kidney shape vortex system. This kidney shape vortex determines the major attributes of jet-in-crossflow and also influences the ultimate distributions of the jet fluid in the wall region. Flow configurations allowing higher lateral/secondary motions underneath the jet actually result in the lowering of wall coverage.

Figure (5.4a-c) shows the schematic diagram of the flow field for different cases of blowing ratios. Various zones highlighted in the diagram will be referred frequently in the forthcoming discussion in order to describe the flow field behavior for various cases. The most significant regions include the upper and lower shear layer, wake region and the potential core of the jet. The colors codes belonging to the mean vorticity filed are shown to highlight the different zones of the flow field and only give a qualitative description.

(a)

“Experimental aerothermal characterization of a pulsating jet issuing in a crossflow:

Influence of Strouhal number excitation on film cooling” -137- (b)

(c)

Figure (5. 4): (a) Schematic diagram representing the various region of the flow field in steady blowing, (a) M =0.65, M =1 and M=1.25.