Computational study of periodically unsteady interaction between a wall jet and an offset jet for various velocity ratios

• Unsteady flow characteristics are investigated for various values of the velocity ratio Vr.
• Near-field demonstrates von Karman-like vortices when Vr lies in the range 0.78–1.34.
• When 0.77 ≤ Vr or Vr ≥ 1.35, the flow field remains steady.
• Within the range 0.78 ≤ Vr ≤ 1.34, Strouhal number increases with increase in Vr.

The interaction between a turbulent wall jet and a turbulent offset jet for various velocity ratios is numerically investigated using two-dimensional unsteady RANS equations. Throughout the study, the ratio of jets separation distance (d) and jet width (w  ) is kept at d/w=1,d/w=1, and the Reynolds number based on jet width is considered as Re = 10,000. The velocity ratio, Vr = uw/uo, is varied by varying the inlet velocity of the wall jet (uw) consistently while the inlet velocity of the offset jet (uo) is retained as a reference velocity. Computational results reveal that when the velocity ratio lies in the range 0.78 ≤ Vr ≤ 1.34, a periodic vortex shedding phenomenon occurs close to the nozzle plate similar to what would be expected in the region behind a two-dimensional bluff body immersed in a fluid. On the contrary, this periodic phenomenon ceases if Vr ≤ 0.77 and Vr ≥ 1.35. Unsteady time-marching simulations show that for Vr = 0.77 and 1.35, two stable counter rotating vortices form close to the nozzle plate. The velocity signals, within the range for which the periodic vortex shedding occurs, exhibit a trend of pure sinusoidal oscillation. Fast Fourier transform (FFT) of the velocity signals provides the vortex shedding frequency corresponding to the Strouhal number that gradually increases with the progressive increase in velocity ratio.