Multiscale turbulence

Short description: Class of turbulent flow

Multiscale turbulence is a class of turbulent flows in which the chaotic motion of the fluid is forced at different length and/or time scales.[1][2] This is usually achieved by immersing in a moving fluid a body with a multiscale, often fractal-like, arrangement of length scales. This arrangement of scales can be either passive[3][4] or active[5]

Three examples of multiscale turbulence generators. From left to right, a fractal cross grid, a fractal square grid and a fractal I grid. See on YouTube the manufacturing of a fractal grid.

As turbulent flows contain eddies with a wide range of scales, exciting the turbulence at particular scales (or range of scales) allows one to fine-tune the properties of that flow. Multiscale turbulent flows have been successfully applied in different fields.,[6] such as:

* Reducing acoustic noise from wings by modifying the geometry of spoilers;[7]
* Enhancing heat transfer from impinging jets passing through grids;[8]
* Reducing the vortex shedding intensity of flows past normal plates without changing the shedding frequency;[9]
* Enhancing mixing by energy-efficient stirring;[10][11]
* Improving flow metering and flow conditioning in pipes;[12]
* Improving combustion.[13][14]

Multiscale turbulence has also played an important role into probing the internal structure of turbulence.[15] This sort of turbulence allowed researchers to unveil a novel dissipation law in which the parameter Cϵ in

ε=Cε𝒰3ℒ

is not constant, as required by the Richardson-Kolmogorov energy cascade. This new law[15] can be expressed as Cϵ∝ReImReLn, with m≈1≈n, where ReI and ReL are Reynolds numbers based, respectively, on initial/global conditions (such as free-stream velocity and the object's length scale) and local conditions (such as the rms velocity and integral length scale). This new dissipation law characterises non-equilibrium turbulence apparently universally in various flows (not just multiscale turbulence) and results from non-equilibrium unsteady energy cascade. This imbalance implies that new mean flow scalings exist for free shear turbulent flows, as already observed in axisymmetric wakes[15][16]

## References

1. ↑ Laizet, S.; Vassilicos, J. C. (January 2009). "Multiscale Generation of Turbulence". Journal of Multiscale Modelling 01 (1): 177–196. doi:10.1142/S1756973709000098.
2. ↑ Mazzi, B.; Vassilicos, J. C. (10 March 2004). "Fractal-generated turbulence". Journal of Fluid Mechanics 502: 65–87. doi:10.1017/S0022112003007249. Bibcode: 2004JFM...502...65M.
3. ↑ Hurst, D.; Vassilicos, J. C. (2007). "Scalings and decay of fractal-generated turbulence". Physics of Fluids 19 (3): 035103–035103–31. doi:10.1063/1.2676448. Bibcode: 2007PhFl...19c5103H.
4. ↑ Nagata, K.; Sakai, Y.; Inaba, T.; Suzuki, H.; Terashima, O.; Suzuki, H. (2013). "Turbulence structure and turbulence kinetic energy transport in multiscale/fractal-generated turbulence". Physics of Fluids 25 (6): 065102–065102–26. doi:10.1063/1.4811402. Bibcode: 2013PhFl...25f5102N.
5. ↑ Thormann, A.; Meneveau, C. (February 2014). "Decay of homogeneous, nearly isotropic turbulence behind active fractal grids". Physics of Fluids 26 (2): 025112. doi:10.1063/1.4865232. Bibcode: 2014PhFl...26b5112T.
6. ↑ Laizet, Sylvain; Sakai, Yasuhiko; Christos Vassilicos, J. (1 December 2013). "Special issue of selected papers from the second UK–Japan bilateral Workshop and First ERCOFTAC Workshop on Turbulent Flows Generated/Designed in Multiscale/Fractal Ways, London, March 2012". Fluid Dynamics Research 45 (6). doi:10.1088/0169-5983/45/6/061001. Bibcode: 2013FlDyR..45f1001L.
7. ↑ Nedić, J., B. Ganapathisubramani, J. C. Vassilicos, J. Boree, L. E. Brizzi, A. Spohn. "Aeroacoustic performance of fractal spoilers". AIAA journal 2012.
8. ↑ Cafiero, G.; Discetti, S.; Astarita, T. (August 2014). "Heat transfer enhancement of impinging jets with fractal-generated turbulence". International Journal of Heat and Mass Transfer 75: 173–183. doi:10.1016/j.ijheatmasstransfer.2014.03.049. Bibcode: 2014IJHMT..75..173C.
9. ↑ Nedić, J.; Ganapathisubramani, B.; Vassilicos, J. C. (1 December 2013). "Drag and near wake characteristics of flat plates normal to the flow with fractal edge geometries". Fluid Dynamics Research 45 (6). doi:10.1088/0169-5983/45/6/061406. Bibcode: 2013FlDyR..45f1406N.
10. ↑ Laizet, S.; Vassilicos, J. C. (23 December 2014). "Stirring and scalar transfer by grid-generated turbulence in the presence of a mean scalar gradient". Journal of Fluid Mechanics 764: 52–75. doi:10.1017/jfm.2014.695. Bibcode: 2015JFM...764...52L.
11. ↑ Suzuki, H.; Nagata, K.; Sakai, Y.; Hayase, T. (1 December 2010). "Direct numerical simulation of turbulent mixing in regular and fractal grid turbulence". Physica Scripta T142. doi:10.1088/0031-8949/2010/T142/014065. Bibcode: 2010PhST..142a4065S.
12. ↑ Manshoor, B.; Nicolleau, F. C. G. A.; Beck, S. B. M. (June 2011). "The fractal flow conditioner for orifice plate flow meters". Flow Measurement and Instrumentation 22 (3): 208–214. doi:10.1016/j.flowmeasinst.2011.02.003. Bibcode: 2011FloMI..22..208M.
13. ↑ Verbeek, A. A.; Bouten, T. W. F. M.; Stoffels, G. G. M.; Geurts, B. J.; van der Meer, T. H. (January 2015). "Fractal turbulence enhancing low-swirl combustion". Combustion and Flame 162 (1): 129–143. doi:10.1016/j.combustflame.2014.07.003. Bibcode: 2015CoFl..162..129V. https://research.utwente.nl/en/publications/14515022-6245-4b8c-aa23-0e2ff82120cd.
14. ↑ Goh, K. H. H.; Geipel, P.; Lindstedt, R. P. (September 2014). "Lean premixed opposed jet flames in fractal grid generated multiscale turbulence". Combustion and Flame 161 (9): 2419–2434. doi:10.1016/j.combustflame.2014.03.010. Bibcode: 2014CoFl..161.2419G.
15. ↑ 15.0 15.1 15.2 Vassilicos, J. C. (2015). "Dissipation in Turbulent Flows". Annual Review of Fluid Mechanics 47 (1): 95–114. doi:10.1146/annurev-fluid-010814-014637. Bibcode: 2015AnRFM..47...95V.
16. ↑ Castro, Ian P. (2016). "Dissipative distinctions". Journal of Fluid Mechanics 788: 1–4. doi:10.1017/jfm.2015.630. ISSN 0022-1120. Bibcode: 2016JFM...788....1C.

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