• Source: Multiscale turbulence

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. 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 or active

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., such as:

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

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




C

ϵ




{\displaystyle C_{\epsilon }}

in




ε
=

C

ε







U



3



L





{\displaystyle \varepsilon =C_{\varepsilon }{\frac {{\mathcal {U}}^{3}}{\mathcal {L}}}}


is not constant, as required by the Richardson-Kolmogorov energy cascade. This new law can be expressed as




C

ϵ


∝



R

e

I


m




R

e

L


n







{\displaystyle C_{\epsilon }\propto {\frac {Re_{I}^{m}}{Re_{L}^{n}}}}

, with



m
≈
1
≈
n


{\displaystyle m\approx 1\approx n}

, where



R

e

I




{\displaystyle Re_{I}}

and



R

e

L




{\displaystyle Re_{L}}

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




References

Kata Kunci Pencarian:

• Multiscale turbulence
• Magnetospheric Multiscale Mission
• Lewis Fry Richardson
• HelioSwarm
• Weinan E
• Charles Meneveau
• James A. Glazier
• Variational multiscale method
• Global Environmental Multiscale Model
• Gem (disambiguation)