Turbulent flow is a type fluid flow, characterized by recirculation, eddies and apparent randomness, in which the fluid undergoes irregular fluctuations(chaotic), in contrast to laminar flow, in which the fluid moves in smooth paths or parallel layers. Usually, the Reynolds number (the ratio of inertial forces to viscous forces) is the key parameter for predicting the flow type.

In an unsteady laminar flow, the velocity may changes. however, it is different from velocity fluctuation of turbulent flow.

Turbulent flow will have following three syndromes :

vorticity,

mixing, and

Disorder.

However, the velocity fluctuation can be directional and/ or temporal. The temporal fluctuation (u-prime) of velocity must be present in turbulent flow along with the above three syndromes.

The laminar and turbulent flow is a phenomenon in the boundary layer, but the turbulence scale maybe exist in the free stream flow. So, when the flow in the boundary layer is laminar, maybe there is also turbulence level in the free stream region. For this case, should we name the flow is turbulent or laminar?

The short answer is no. Every (real) freestream has a nonzero turbulent intensity. This is a measure of how much of the energy of the flow is causing movement which is different to the bulk flow velocity. Generally for flight in the atmosphere the turbulent intensity is low (Tu

Dear Anthony D. Gardner

Please check when you make an experiment in the wind tunnel. The honey comb structure (with the porosity depends on the WT design) will produce turbulence intensity. in my WT, the turbulence level is around 1%. According to the average velocity and diameter of the object tested (cross flow), the Reynolds number is less than 100,000. So, the flow is laminar (less then 5.105 ) but there was turbulence level of 1%.

What about this?

The turbulence intensity, , is defined as the ratio of the root-mean-square of the velocity fluctuations, , to the mean flow velocity, . A turbulence intensity of 1% or less is generally considered low and turbulence intensities greater than 10% are considered high.

A turbulent flow is the state where a flow is turbulent. If a flow is completely uniform(no velocity difference), turbulence does not occur. In reality, a completely uniform flow does not exist and may sound oddly for you.However it is theoretically true .When velocity difference occurs due to some factor, turbulence occurs. So it is not true to consider turbulent flow as turbulence intensity.

As Dr. Abdelmaeed mentioned turbulence intensity is ratio of RMS value of velocity fluctuations to the mean flow velocity which is inherently present in any flow. Less than 0.05 percent is suitable for Boundary layer wind tunnels specifically for transitional flow research. But once you say about turbulent flow that means you intentionally triggered upstream flow to study nature of this kind of flow . In this case turbulence intensity may be 0.1 percent and above.

Thanks Mohsen Jahanmiri

Does anyone know about what turbulence level value that can be neglected?

Free stream turbulence has non zero intensity. Therefore, some length scale of turbulence is available so we can calculate turbulence level at the free surface

Yes Santosh Kumar Singh

But, could we assume that the flow is laminar? Cause, the Reynolds number was below 100,000. Normally, criteria of laminar flow is Re< 5.105

Yes you are correct. But the flow may be taken as laminar even at 5.10^5

Turbulence intensity alone is not a good indicator as to whether a flow is turbulent or not. Within a flow, turbulence can be convected, generated and dissipated. In all flows turbulence is convected and dissipated, but only in turbulent flows is it generated. Turbulence is most commonly generated through shearing of the mean flow (e.g. in boundary layers), but shearing will not produce turbulence if the dissipation is sufficiently high (laminar boundary layers).

In this case, turbulence is generated by the wind tunnel mesh and honeycomb boundary layers and wakes. A short distance downstream, once the wake shear layers have dissipated, turbulence generation will cease, but the turbulence generated will continue to be convected downstream and will only dissipate slowly. The freestream therefore contains only 'old' turbulence and can therefore be considered as laminar.

You quote a Re=100,000 for the object in the wind tunnel and so the flow around the object is most likely laminar, but this will depend on the geometry of that object. Your quoted a critical Re=500,000 value but this is a general figure for flat plate boundary layers and the value for an object (e.g. a cylinder) is lower at about Re=200,000, because of the adverse pressure gradient in the cylinder boundary layers. The critical values are also greatly influenced by the level of freestream turbulence, but only levels above 1% are likely to have a significant effect, so with Re=100,000 your flow is most likely laminar.

Thanks for the explanations Prof. Johnson. Your explanation is very helpful....

Dear Prof. Mark Wyatt Johnson

Could you tell me how to calculate theoretically that the turbulence behind the honeycomb structure will dissipated and finally disappear in laminar flow, please?

If you have a reference it will be valuable for me.

Thank you very much.

Dear Researchers,

Have a nice weekend with your family....

Dear Nazaruddin Sinaga : Take a RANS solver of your choice best suited to your geometry (a pipe?), and compute the eddy viscosity with the help of the attached theory for asymptotic high-Re turbulence. If turbulence production by wall shear exceeds dissipation, then you will see soon the decay of turbulence along the axis. Then you may decide the onset of laminarization by experiences of other authors, preferentially from practical studies. Onset of laminarization is a tricky problem, very specific as it depends on the microscopic setup of the fluid in question.

I use

Roach, P.E. 'The generation of nearly isotropic turbulence by means of grids', Int. Jnl. of heat and fluid flow, vol. 8, no.2, 1987

to determine the turbulence intensity and length scale which will be produced by a grid with a specified geometry. The paper also contains formulae to determine how the turbulence intensity and length scale decay with distance downstream.

Thanks Mark Wyatt Johnson

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