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Determination of turbulence parameters

When the turbulent flow enters the domain of the problem (the computational zone), it is necessary to determine the values of turbulence parameters at the inlet or outlet boundaries. To do this, there are two methods:

  1. a) Using profiles: If there is a particular interest in the expression of the boundary layers or the developed zone of the turbulent flow at the inlet, it is better to set the disturbance values by generating profiles from experimental values with empirical formulas.
  2. b) Using uniform and constant values: In some cases, when we have an inlet flow, the same values of the flow characteristics of the turbulent flow enters the problem, for example, the inlet fluid to the canal, where the exact values of the profiles Unknown or even fully developed. In most of the turbulent flows, the turbulent intensity occurs within the shear layers, but the less turbulent intensity inters from the boundaries to the domain of the problem, and therefore, the flow calculations are not affected by the intensity values at the inlet boundaries. Non-physical and non-intrinsic values of the turbulence parameters at the inlet also delays convergence and reduces the accuracy of computations. So, instead of using profiles, constant and uniform values of turbulent parameters can be used. The turbulence parameters are:

Turbulence Intensity:

The intensity of the turbulence, as a proportion of the roots of the oscillatory velocity, is defined as the average flow velocity. The intensities smaller than 1% are small and the intensities greater than 10% are big. For example, in advanced wind tunnels, the turbulence intensity may be even less than 0.05%. In internal flows, the intensity of the inlet depends on the upstream flow. If the upstream flow is undeveloped and laminar, the values of the smallest turbulence intensity can be applied, and if the flow is fully developed, the intensity of the turbulence can be larger and can be considered to be about a few percents. The turbulence intensity of a fully developed flow inside a pipe can be calculated from the empirical formula.

Turbulence Length Scale and Hydraulic Diameter:

The length of the Turbulence Length Scale is a physical quantity depends on large eddies, which involves the turbulent flow energy. In the fully developed canal flow, the characteristic length can’t be larger than the canal size, since the eddy can’t be larger than the canal.

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