Could any one please guide me on how to calculate the turbulent intensity..While doing k-epsilon turbulent modeling I chose Turbulent Intensity and Hydraulic Diameter Option.Please Help me in this regard.Thanks in Advance

hi prem It is there in Fluent manual. turbulance Intensity = 0.16*{(Re)power(-1/8)} hydraulic dia. for circular section it is equal to diameter of section and for any other section it is sqrt(4*A/pie), where A is area if section . it will be better if u have a look of manual.

HI balmiki thanks a lot for your guidance

[champshof]

Originally Posted by balmiki
;128512

hi prem It is there in Fluent manual. turbulance Intensity = 0.16*{(Re)power(-1/8)} hydraulic dia. for circular section it is equal to diameter of section and for any other section it is sqrt(4*A/pie), where A is area if section . it will be better if u have a look of manual.

I know this is an old topic...

But why is the intensity irreversible proportional to the Reynolds number? I would expect that the intensity increases for a higher Reynolds number

[nawar]

Originally Posted by balmiki
;128512

hi prem It is there in Fluent manual. turbulance Intensity = 0.16*{(Re)power(-1/8)} hydraulic dia. for circular section it is equal to diameter of section and for any other section it is sqrt(4*A/pie), where A is area if section . it will be better if u have a look of manual.

Hi
i want to ask is there any formula for calculating the turbulent intensity of an internal flow in a pipe in the case of non-fully developed flow

i do know how to use the formula of I=0.16〖(Re)〗^(-1/8), but this formula,as i think, can be used only for fully developed flow.

thanks
nawar

[Philipov]

Fluent manual point two methods for calculating: one for internal flows and one for external....

[Technoyoungman]

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:

1. High-turbulence case: High-speed flow inside complex geometries like heat-exchangers and flow inside rotating machinery (turbines and compressors). Typically the turbulence intensity is between 5% and 20%
2. Medium-turbulence case: Flow in not-so-complex devices like large pipes, ventilation flows etc. or low speed flows (low Reynolds number). Typically the turbulence intensity is between 1% and 5%
3. 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 the turbulence intensity is very low, well below 1%.

[Marli]

For 3D external flow around a 3D wheel, bounded by 'parallel' boundaries, what would the boundary conditions for the velocity inlet be in this case? The velocity inlet is 70m/s in my case. (actually simulating the wheel moving at 70m/s, but instead the wheel stationary and the air moving over it). Using the equation previously stated, 'I' comes out at 0.0224 (so 2.24%?). Regarding the second boundary condition, I thought Hydraulic Diameter was not appropriate because the eddies wouldn't be restricted by any boundaries in external flow, so would I used 'Length Scale' as have it has the wheel diameter?
I'm a bit confused as the literature doesn't really define a case like mine.

[Technoyoungman]

Originally Posted by Marli

For 3D external flow around a 3D wheel, bounded by 'parallel' boundaries, what would the boundary conditions for the velocity inlet be in this case? The velocity inlet is 70m/s in my case. (actually simulating the wheel moving at 70m/s, but instead the wheel stationary and the air moving over it). Using the equation previously stated, 'I' comes out at 0.0224 (so 2.24%?). Regarding the second boundary condition, I thought Hydraulic Diameter was not appropriate because the eddies wouldn't be restricted by any boundaries in external flow, so would I used 'Length Scale' as have it has the wheel diameter?
I'm a bit confused as the literature doesn't really define a case like mine.

Yes, there is no literature about turbulent length scale for external flow!!!

[seifelyak]

[seifelyak]

A turbulence intensity of 1% or less is generally considered low and turbulence intensities greater than 10% are considered high. Ideally, you will have a good estimate of the turbulence intensity at the inlet boundary from external, measured data. For example, if you are simulating a wind-tunnel experiment, the turbulence intensity in the free stream is usually available from the tunnel characteristics. In modern low-turbulence wind tunnels, the free-stream turbulence intensity may be as low as 0.05%.

For internal flows, the turbulence intensity at the inlets is totally dependent on the upstream history of the flow. If the flow upstream is under-developed and undisturbed, you can use a low turbulence intensity. If the flow is fully developed, the turbulence intensity may be as high as a few percent. The turbulence intensity at the core of a fully-developed duct flow can be estimated from the following formula derived from an empirical correlation for pipe flows:



At a Reynolds number of 50,000, for example, the turbulence intensity will be 4%, according to this formula.