ESI was commissioned to investigate the effect of the frequency of the vortical structures within the shear layer and how these frequencies evolve as the flow passes downstream. The experimental study was conducted in the near field of an axisymmetric jet exiting from a circular pipe supplied by a fully developed turbulent flow at a Reynolds number of 7,500 based on exit pipe diameter. Data was acquired in the near field of the jet using both miniature hot-wire anemometry, Figure 1a, and laser light Mie scattering, Figure 1b and later used to validate a CFD model.
Passing Frequency of Vortical Structures in the Near Field of an Axisymmetric Turbulent Jet
The turbulent flow emitting from a round orifice has many industrial and military applications ranging from thermal management to vector steering. The flow field emanating from a round jet is characterized by a central potential core region and an expanding shear layer, Figure 1a.
Figure 1a: Diagrammatic Flow From a Circular Orifice
Including a Hot-Wire Probe
Figure 1b: Diagram of Laser Light Mie Scattering Apparatus
Figure 2: FFT of the Hot-Wire Signal Acquired 2D
Downstream of the Jet Orifice
The visual results indicate a decrease in passing frequency with increasing distance downstream of the jet exit. The results were compared to the ‘preferred mode’ of an axisymmetric jet found in previous studies.
Table 1: Comparison Between Hot-wire and
Flow Visualization Techniques
It is, therefore, concluded that there is not a universal ‘preferred mode’ for an axisymmetric jet of various sizes and speeds due to the merging of the vortices. However, there seems to be a range of ‘preferred modes’ as displayed in previous published literature in which the current experimental study is in very good agreement, Figure 4.
In fact, this variation in the literature appears to be related to the measurement uncertainty within each of the studies.
The passing frequency of the vortical structures was determined at four streamwise locations downstream of the jet exit (i.e. 1D, 2D, 3D, and 4D). The acquired anemometry data was processed and analyzed with an FFT, with Figure 2 displaying the data acquired 2D downstream of the tube exit.
Similarly, optical data shows that as the jet fluid travels downstream, it entrains surrounding fluid causing the spacing between vortices to increase. In addition, the vortex may undergo a pairing process and merge with another vortex causing a decrease in the passing frequency as it travels downstream as shown in Figure 3.
Figure 3: Laser Illuminated Flow Field of the Near Field of the
Jet Showing Pairing of the Vortices
Comparison of the non-dimensional frequency in terms of Strouhal number (St = fD/U) for the hot-wire (HW) and laser flow visualization (FV) results are provided in the following Table 1.
Figure 4: Near-field Strouhal Number Comparison