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Turbulence intensity

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Definition

The turbulence intensity, also often refered to as turbulence level, is defined as:

I \equiv \frac{u'}{U}

Where u' is the root-mean-square of the turbulent velocity fluctuations and U is the mean velocity (Reynolds averaged).

If the turbulent energy, k, is known u' can be computed as:

u' \equiv \sqrt{\frac{1}{3} \, ( u_x'^2 + u_y'^2 + u_z'^2 )} = \sqrt{\frac{2}{3}\, k}

U can be computed from the three mean velocity components U_x, U_y and U_z as:

U \equiv \sqrt{U_x^2 + U_y^2 + U_z^2}

Estimating the turbulence intensity

When setting boundary conditions for a CFD simulation it is often necessary to estimate the turbulence intensity on the inlets. To do this accurately it is good to have some form of measurements or previous experince to base the estimate on. Here are a few examples of common estimations of the incoming turbulence intensity:

  1. High-turbulence case: High-speed flow inside complex geometries like heat-exchangers and flow inside rotating machinery (turbines and compressors). Typically the turbulence intensity is between 5% and 20%
  2. Medium-turbulence case: Flow in not-so-complex devices like large pipes, ventilation flows etc. or low speed flows (low Reynolds number). Typically the turbulence intensity is between 1% and 5%
  3. Low-turbulence case: Flow originating from a fluid that stands still, like external flow across cars, submarines and aircrafts. Very high-quality wind-tunnels can also reach really low turbulence levels. Typically the turbulence intensity is very low, well below 1%.

Fully developed pipe flow

For fully developed pipe flow the turbulence intensity at the core can be estimated as:

I = 0.16 \; Re_{d_h}^{-\frac{1}{8}}

Where Re_{d_h} is the Reynolds number based on the pipe hydraulic diameter d_h.

The above equation is from the ANSYS Fluent User's Guide (Release 18.0, Eq. (6.62)); however, no reference is provided. Russo and Basse published a paper [1] where they derive turbulence intensity scaling laws based on CFD simulations and Princeton Superpipe measurements. The turbulence intensity over the pipe area is defined as an arithmetic mean (AM). The measurement-based scaling laws are:

I_{\rm Smooth~pipe~axis} = 0.0550 \; Re^{-0.0407}
I_{\rm Smooth~pipe~area,~AM} = 0.227 \; Re^{-0.100}

Scaling of the area-averaged turbulence intensity has been calculated in [2]:

I_{\rm Smooth~pipe~area} = 0.317 \; Re^{-0.110}

References

[1] Russo, F. and Basse, N.T. (2016), "Scaling of turbulence intensity for low-speed flow in smooth pipes", Flow Meas. Instrum., vol. 52, pp. 101–114.

[2] Basse, N.T. (2017), "Turbulence intensity and the friction factor for smooth- and rough-wall pipe flow", [1].

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