1. ## Technical passages (2)

Turbulence is flow characterized by recirculation, eddies, and apparent randomness. Flow in which turbulence is not exhibited is called laminar. It should be noted, however, that the presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow is often represented via a Reynolds decomposition, in which the flow is broken down into the sum of an average component and a perturbation component.

It is believed that turbulent flows can be described well through the use of the Navier–Stokes equations. Direct numerical simulation (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.
Most flows of interest have Reynolds numbers much too high for DNS to be a viable option, given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human (L > 3 m), moving faster than 72 km/h (20 m/s) is well beyond the limit of DNS simulation (Re = 4 million). Transport aircraft wings (such as on an Airbus A300or Boeing 747) have Reynolds numbers of 40 million (based on the wing chord). In order to solve these real-life flow problems, turbulence models will be a necessity for the foreseeable future. Reynolds-averaged Navier–Stokes equations (RANS) combined with turbulence modelling provides a model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the Reynolds stresses, although the turbulence also enhances the heat and mass transfer. Another promising methodology islarge eddy simulation (LES), especially in the guise of detached eddy simulation (DES)—which is a combination of RANS turbulence modelling and large eddy simulation.

Which one is TRUE about Reynolds decomposition?
1. It's a physical symbol for turbulence flow.
2. It's a mathematical phenomena presents in laminar flow
3. It's turbulence flow presents in a variable part around a constant value.
4. It's average part of turbulence plus perturbed laminar flow.

2. ## Re: Technical passages (2)

As I understand it, this forum is not here to answer homework questions for you.
In any case, the options you present at 1 - 4 are written in such bad English that they are practically unintelligible.

3. ## Re: Technical passages (2)

As I previously said there stuffs are not my homework. These are adapted from Iranian Entrance exams. We call them Konkors. I post these ones here in order to share our ideas together. If this is antithesis of the general rules of this site, I will never do that again. :)

4. ## Re: Technical passages (2)

Originally Posted by Freeguy
As I previously said, these things are not my homework. These are adapted from Iranian Entrance exams. We call them Konkors. I post these ones here in order to share our ideas together. If this is antithetical of the general rules of this site, I will never do that again. :)
Sorry, Freeguy; I haven't seen your previous relevant posts.
However, I'm still not sure what you expect by way of a response to your Technical Passage. The multiple choice question at the end relates only to one sentence in the whole passage, ie Mathematically, turbulent flow is often represented via a Reynolds decomposition, in which the flow is broken down into the sum of an average component and a perturbation component.

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