Turbulence intensity
From CFD-Wiki
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The turbulence intensity is defined as: | The turbulence intensity is defined as: | ||
| - | :<math> | + | :<math>I \equiv \frac{u'}{U}</math> |
Where <math>u'</math> is the root-mean-square of the turbulent velocity fluctuations and <math>U</math> is the mean velocity ([[Reynolds averaging|Reynolds averaged]]). | Where <math>u'</math> is the root-mean-square of the turbulent velocity fluctuations and <math>U</math> is the mean velocity ([[Reynolds averaging|Reynolds averaged]]). | ||
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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: | 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: | ||
| - | #'''High-turbulence case''': High-speed flow inside complex geometries like heat-exchangers and flow inside rotating machinery (turbines and compressors). Typically | + | #'''High-turbulence case''': High-speed flow inside complex geometries like heat-exchangers and flow inside rotating machinery (turbines and compressors). Typically I is between 5% and 20% |
| - | #'''Medium-turbulence case''': Flow in not-so-complex devices like large pipes, ventilation flows etc. or low speed flows (low [[Reynolds number]]). Typically | + | #'''Medium-turbulence case''': Flow in not-so-complex devices like large pipes, ventilation flows etc. or low speed flows (low [[Reynolds number]]). Typically I is between 1% and 5% |
| - | #'''Low-turbulence case''': Flow originating from a fluid that stands still, like | + | #'''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 I is very low, well below 1%. |
===Fully developed pipe flow=== | ===Fully developed pipe flow=== | ||
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For fully developed pipe flow the turbulence intensity at the core can be estimated as: | For fully developed pipe flow the turbulence intensity at the core can be estimated as: | ||
| - | :<math> | + | :<math>I = 0.16 \; Re_{d_h}^{-\frac{1}{8}}</math> |
Where <math>Re_{d_h}</math> is the [[Reynolds number]] based on the pipe [[hydraulic diameter]] <math>d_h</math>. | Where <math>Re_{d_h}</math> is the [[Reynolds number]] based on the pipe [[hydraulic diameter]] <math>d_h</math>. | ||
Revision as of 22:15, 18 April 2006
Definition
The turbulence intensity is defined as:
Where 
is the root-mean-square of the turbulent velocity fluctuations and 
is the mean velocity (Reynolds averaged).
If the turbulent energy, 
, is known can be computed as:
can be computed from the three mean velocity components 
, 
and 
as:
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:
- High-turbulence case: High-speed flow inside complex geometries like heat-exchangers and flow inside rotating machinery (turbines and compressors). Typically I is between 5% and 20%
- Medium-turbulence case: Flow in not-so-complex devices like large pipes, ventilation flows etc. or low speed flows (low Reynolds number). Typically I is between 1% and 5%
- 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 I 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:
Where 
is the Reynolds number based on the pipe hydraulic diameter 
.
