I have the instantaneous velocity data in the near field of a round turbulent jet. Can someone help me with the procedure of removing the high frequency noise to estimate the dissipation rate and Kolmogorov scale. Currently I am using the procedure by J Mi et al. (Meas. Sci. Technol 22 (2011) ). I use the same technique to estimate the Taylor's scale also. But my Taylor's scale decreases with distance from 1D to 7D (D being inlet Diameter of the jet) then increases afterwards. While the dissipation rate increases from 1D to 7D. This seems quite absurd. I don't have any near field (1D to 8D) Taylor's scale measurements or the dissipation rates for round turbulent jet. Can any one suggest a better procedure or data so that I can check the correctness of my data.
Hello,
I am not sure that your results are absurd as you said... Since your measurements are made in the very near field of the jet (sometimes referred to as the production region), it is not surprising to observe the energy dissipation rate increasing in this particular region of the flow. A careful analysis of figs 7 to 9 of Mi et al, MST 2011 reveals that epsilon increases from 5D to 7D and then starts decaying. Consequently, the Taylor and Kolmogorov length scales decrease before increasing beyond 7D (see figs 8(b) and 9).
Did you try to compare your results to that of Mi et al (Meas. Sci. Technol 2011), the latter being in the range 5 \leq x/D \leq 30. The measurements of Mi et al in the round jet were carried out at a Reynolds number of 20100, I hope you made yours at a similar Reynolds number. Otherwise, have a look to the following publication by the same authors (http://dx.doi.org/10.1063/1.4811403), even though the range of x/D is slightly larger (>10D).
Note also that, independently of the electronic noise contamination, the relevance of your measurements depends on the validity of the local isotropy hypothesis as I guess that you use the 1D surrogate of epsilon (\epsilon = 15 \nu (du_dx)^2), or perhaps a more sophisticated surrogate. Anyway, this hypothesis is unlikely to apply in the near field.
Finally, apart from the hypothesis of local isotropy, the ratio L/\eta (L being the length of the hot wire) may be also critical for a reliable estimate of epsilon to be inferred...
The best would be to compare your results to a DNS. I am not sure but perhaps BJ Boersma reported some profiles of epsilon in the near field of a round jet ...
Good luck
Fabien
Hello,
I am not sure that your results are absurd as you said... Since your measurements are made in the very near field of the jet (sometimes referred to as the production region), it is not surprising to observe the energy dissipation rate increasing in this particular region of the flow. A careful analysis of figs 7 to 9 of Mi et al, MST 2011 reveals that epsilon increases from 5D to 7D and then starts decaying. Consequently, the Taylor and Kolmogorov length scales decrease before increasing beyond 7D (see figs 8(b) and 9).
Did you try to compare your results to that of Mi et al (Meas. Sci. Technol 2011), the latter being in the range 5 \leq x/D \leq 30. The measurements of Mi et al in the round jet were carried out at a Reynolds number of 20100, I hope you made yours at a similar Reynolds number. Otherwise, have a look to the following publication by the same authors (http://dx.doi.org/10.1063/1.4811403), even though the range of x/D is slightly larger (>10D).
Note also that, independently of the electronic noise contamination, the relevance of your measurements depends on the validity of the local isotropy hypothesis as I guess that you use the 1D surrogate of epsilon (\epsilon = 15 \nu (du_dx)^2), or perhaps a more sophisticated surrogate. Anyway, this hypothesis is unlikely to apply in the near field.
Finally, apart from the hypothesis of local isotropy, the ratio L/\eta (L being the length of the hot wire) may be also critical for a reliable estimate of epsilon to be inferred...
The best would be to compare your results to a DNS. I am not sure but perhaps BJ Boersma reported some profiles of epsilon in the near field of a round jet ...
Good luck
Fabien
Hello Fabien,
Thanks for your answer. As you said I compared the results between 5D to 8D it is agreeing well. I did not consider the effect of wire length. I will try to include it.
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