In fluid mechanics and computational fluid dynamics (CFD), the fluctuating velocity components u' v' w'′ represent the turbulent fluctuations in the velocity field. These components are used to model the turbulent behavior of fluid flow. The prime notation (u' v' w') denotes the deviation of the velocity from its mean value (U,V,W).
For laminar flow, turbulence is generally not considered, and the flow is assumed to be smooth and ordered. In this case, the fluctuating velocity components (u' v' w') are essentially zero.
In CFD simulations using software like FLUENT, you typically specify the turbulence model and relevant parameters to simulate turbulent flows. Common turbulence models include the k-epsilon model, the k-omega model, and the Reynolds stress model. These models provide equations for the turbulent kinetic energy (k) and the turbulent dissipation rate (ϵ), from which the fluctuating velocity components (u' v' w') can be derived.
In FLUENT, you will need to set up your simulation by defining the geometry, boundary conditions, and fluid properties. Additionally, you'll need to specify the turbulence model and provide initial conditions for the turbulence variables. The software will then solve the governing equations, including the RANS equations and turbulence model equations, to obtain the mean flow field and turbulence quantities, including the fluctuating velocity components.
The specific steps may vary depending on the version of FLUENT and the turbulence model chosen, so it's recommended to refer to the FLUENT documentation or user guide for detailed instructions based on your simulation setup.
You can find very good description of fluctuating velocity components (u' v' w') in the corresponding chapter of the treatise of Hermann Schlichting. This is classical description. Best wishes, Oleh Shvydkyi.
We will get the value of turbulent kinetic energy (k) from fluent or cfd post. this value can be substituted into 𝑘=0.5(𝑢′2+𝑣′2+𝑤′2).
with isotropic turbulence condition 𝑢′= 𝑣′ = 𝑤′
hence the initial equation turns out to be 𝑢′ = sqrt (2k/3)
This way, you can calculate the fluctuations. If you want to know the fluctuations 𝑣′ and 𝑤′, one must use rsm turbulence model.
For better understanding, check the "Turbulence Modelling for CFD" by Wilcox.
You should get fluctuation related info using post processing if not you may need to write a short UDF code (but all depends on the turbulence model you are using for some such info may not be available at all)
In Fluent the fluctuating velocity components u' v' w''′ represent the turbulent fluctuations in the velocity field. These are calculated “automatically by the software”. If you are familiar enough with the software, what you can do is create a report in which the velocity fluctuations were recorded on a graph, since when using the RANS technique these are essentially zero. If this is not possible, you can add all the Data quantity variables and recover them in post-processing either with paraview, tecplot, or similar programs.
Desta,
I am puzzled - you seem to have answered your own question.
Are you honestly looking for help?
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