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The effects of acceleration in jets: kinematics of the near field vortices

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Abstract

Direct and large-eddy simulations (DNS/LES) of accelerating round jets are used to analyze the effects of acceleration on the kinematics of vortex rings in the near field of the jet (x/D < 12). The acceleration is obtained by increasing the nozzle jet velocity with time, in a previously established (steady) jet, and ends once the inlet jet velocity is equal to twice its initial value. Several acceleration rates (α = 0.02–0.6) and Reynolds numbers (Re D = 500–20000) were simulated. Acceleration maps were used to make a detailed study of the kinematics of vortex rings in accelerating jets. One of the effects of the acceleration is to cause a number of new primary and secondary vortex merging events that are absent from steady jets. As the acceleration rate α increases, both the number of primary merging events between rings and the axial position where these take place decreases. The statistics for the speed of the starting ring that forms at the start of the acceleration phase for each simulation, agree well with the statistics for the “front” speed observed by Zhang and Johari (Phys Fluids 8:2185–2195, 1996). Acceleration maps and flow visualizations show that during the acceleration phase the near field coherent vortices become smaller and are formed at an higher frequency than in the steady jet, and their (mean) shedding frequency increases linearly with the acceleration rate. Finally, it was observed that the acceleration decreases the spreading rate of the jet, in agreement with previous experimental works.

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Authors and Affiliations

  1. IDMEC/IST, Mecânica I, 1° andar/LASEF, Av. Rovisco Pais, 1049-001, Lisbon, Portugal

    Carlos B. da Silva, Pedro Neto & José C. F. Pereira

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  1. Carlos B. da Silva
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  2. Pedro Neto
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  3. José C. F. Pereira
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Correspondence to Carlos B. da Silva.

Additional information

Communicated by M.Y. Hussaini

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da Silva, C.B., Neto, P. & Pereira, J.C.F. The effects of acceleration in jets: kinematics of the near field vortices. Theor. Comput. Fluid Dyn. 23, 287–296 (2009). https://doi.org/10.1007/s00162-009-0122-9

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  • DOI: https://doi.org/10.1007/s00162-009-0122-9

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