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The present study aims to identify the dominant coherent structures in the wall jet flow subjected to external pulsation at Reynolds number 2600 (based on average jet exit velocity and nozzle diameter). The forcing frequency is varied between St = 0 and 0.99 (St is the Strouhal number). Quadrant analysis is employed to identify the relative contribution of different quadrant motions to the total Reynolds shear stress. Unlike boundary layer flows and channel flows, two distinct regions (inner shear region and outer shear region) are observed in the wall jet flows, and the characteristics of different quadrant motions change in these regions. About 70% of the total shear stress is contributed from the first and fourth quadrants in the outer shear region. We observe that ejection motion is more energetic than sweep motion in the downstream direction, although less frequent. The ejection motion is observed to be more violent for St = 0.44 than for the other frequencies. A proper orthogonal decomposition (POD) analysis reveals while the modal structures exist in different regions of the wall for different jet pulsation; there are no dominant modes (30 modes are required to recover about 75% of the total energy), and the energy is fairly distributed over a large number of modes. However, the POD analyses are capable of capturing the response of the wall jet to different jet pulsations. The most dominant and strongest modal structures are found nearer to the impingement region of the wall when St = 0.44 and the jet tends to laminarize for St > 0.9.
The present study aims to identify the dominant coherent structures in the wall jet flow subjected to external pulsation at Reynolds number 2600 (based on average jet exit velocity and nozzle diameter). The forcing frequency is varied between St = 0 and 0.99 (St is the Strouhal number). Quadrant analysis is employed to identify the relative contribution of different quadrant motions to the total Reynolds shear stress. Unlike boundary layer flows and channel flows, two distinct regions (inner shear region and outer shear region) are observed in the wall jet flows, and the characteristics of different quadrant motions change in these regions. About 70% of the total shear stress is contributed from the first and fourth quadrants in the outer shear region. We observe that ejection motion is more energetic than sweep motion in the downstream direction, although less frequent. The ejection motion is observed to be more violent for St = 0.44 than for the other frequencies. A proper orthogonal decomposition (POD) analysis reveals while the modal structures exist in different regions of the wall for different jet pulsation; there are no dominant modes (30 modes are required to recover about 75% of the total energy), and the energy is fairly distributed over a large number of modes. However, the POD analyses are capable of capturing the response of the wall jet to different jet pulsations. The most dominant and strongest modal structures are found nearer to the impingement region of the wall when St = 0.44 and the jet tends to laminarize for St > 0.9.
An experimental study to investigate the effect of jet pulsations on the wall jet development in the uphill region of an obliquely inclined round water jet has been performed using particle image velocimetry technique. The study has been performed at a constant nozzle to target wall distance, L/D = 4 (D is the diameter of the nozzle) by varying the jet impingement angle [Formula: see text]), Reynolds numbers (ReD = 1900 and 3280; based on nozzle diameter and average nozzle exit velocity Uavg), and Strouhal number (0 ≤ St ≤ 0.9; [Formula: see text], where f is the frequency of external pulsation). It is observed that the pulsations have no significant effect on the jet in the free jet region when the target plate is kept at a distance less than the potential core length (the potential core extends up to 4D–6D from the nozzle exit toward the impinging plate), and the jet impingement region extends up to 1D from the plate. The location of the stagnation point is observed to depend on all three parameters: the jet pulsation, the Reynolds number, and the jet impingement angle. An increase in Reynolds number creates an adverse pressure gradient toward the downstream direction in the uphill region, resulting in an intrusion of ambient fluid toward the wall jet. The distance between the geometric center and the stagnation point is observed to be minimum for St = 0.44 at both the Reynolds numbers. The wall jet that develops in the uphill region exhibits a maximum velocity decay rate and a jet half width growth rate corresponding to St = 0.44. These parameters are also observed to increase with the increase in the Reynolds number and decrease in the jet impingement angle. The velocity fields reconstructed using proper orthogonal decomposition reveal the dominant modes in the upstream location for St = 0.44 than the other pulsations. Furthermore, we observed that the jet after impingement deviates entirely in the downhill region for [Formula: see text] irrespective of the jet pulsation, suggesting a non-dependence of the critical angle of inclination on jet pulsations for L/D = 4.
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