ANSYS,FLUENT CFD, Turbulent flow

In summary, the conversation discusses an assignment involving a duct vent and turbulent flow. The flow enters the duct with a certain cross-section and goes over a hot plate before reaching a section with a smaller area. The two primary design constraints are the entrance length, which must ensure fully developed turbulent flow before reaching the narrower section, and the exit length, which must not affect the flow around the plate upstream. The hydraulic diameter is used to determine the entrance length, but the formula given in the book is for a circular duct, while the duct in question has a non-circular cross section. The accuracy of using this formula and the potential need for artificial means to stimulate turbulent flow are discussed.
  • #1
sandpants
21
0
I have an assignment involving a duct vent and turbulent flow.

Flow enters a duct vent of a certain cross-section, it then reaches another section with a smaller area, at the beginning of which there is a horizontal plate that is radiating heat.

It's a 2D problem.
Fluid is Air, inflow T=20C and velocity is 30m/s

I have 2 primary design constraints:
1)The entrance length must be sufficiently long enough so that the turbulent flow is fully developed by the time it reaches the second section, so as to have the fully developed flow entering the narrower section and going over the hot plate.
2)The exit length needs to be sufficiently long enough so that it doesn't affect the flow around the plate.

My attempts:
1)White M. Frank describes in his book "Fluid Mechanics" that the entrance length to achieve a fully developed turbulent flow is Le=4.4DRe^(1/6), where D is the hydraulic diameter. Somewhere on the internet I saw an expression for hydraulic diameters in duct vents to be... something, but I worked it out and for a thickness of 0.2m it was still 0.2m.
However, @30m/s and density of 1.2kg/m3 the Re is 400,000 and the matching Le for D=0.2 is ~7.5m.
Which is huge. Is there any other way to check if the flow is fully developed?
Would looking for a velocity profile work, even though the flow is turbulent, and how could I do that in ansys?
What other verification methods can be used?

2)I am not sure how the exit geometry can affect the flow around the plate upstream.

Please advise.
 
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  • #2
The hydraulic diameter Dh of a non-circular duct is given by the formula:

Dh = 4A/P

where A is the cross sectional area and P is the perimeter of the duct. You should be able to find this formula in White, especially when dealing with open-channel flow and Manning's eqn. 'Somewhere on the internet' is incredibly vague and slapdash. IDK why you are using the thickness of anything to determine the Dh.

If you have an Re = 400,000 and a duct with a hydraulic diameter of 0.2 m, then using the relation Le/D = 4.4(Re)1/6 should give an Le of about 7.55 m.
 
  • #3
SteamKing said:
T

If you have an Re = 400,000 and a duct with a hydraulic diameter of 0.2 m, then using the relation Le/D = 4.4(Re)1/6 should give an Le of about 7.55 m.
It does give me exactly that. And that's for a 0.2m duct width.
My question is whether this is waayyy too long or not.
 
  • #4
sandpants said:
It does give me exactly that. And that's for a 0.2m duct width.
My question is whether this is waayyy too long or not.

I think you should be using the hydraulic diameter to determine the entrance length, and not necessarily the the duct width,if the duct has a non-circular cross section.

These relations for calculating entrance length are somewhat imprecise. I think that in order to guarantee that the flow in a pipe or duct has gone fully turbulent, a greater entrance length is required, unless some artificial means to stimulate turbulent flow are employed.
 
  • #5


I would like to suggest a few things to consider in your assignment involving a duct vent and turbulent flow.

Firstly, ANSYS and FLUENT CFD are powerful tools for simulating fluid flows and can provide accurate and detailed results for your problem. These software packages use numerical methods to solve the governing equations of fluid flow, including Navier-Stokes equations, which are essential for turbulent flow simulations.

Now, coming to the specific problem at hand, turbulent flow is a complex phenomenon and it is important to ensure that the flow is fully developed before it reaches the second section of the duct vent. One way to check this is by monitoring the velocity profile at different locations along the duct. In ANSYS and FLUENT, you can plot the velocity profile at different cross-sections of the duct and compare it to the fully developed profile for turbulent flow. If the profiles match, it can be concluded that the flow is fully developed.

Another method to verify fully developed turbulent flow is by looking at the skin friction coefficient along the duct. In fully developed turbulent flow, the skin friction coefficient remains constant along the length of the duct. Therefore, you can monitor the skin friction coefficient at different locations and check for its constancy.

As for the exit length, it is important to ensure that the flow is not affected by the exit geometry as it can have an impact on the flow around the hot plate. In ANSYS and FLUENT, you can use the "inlet/outlet" boundary condition to specify the exit geometry and ensure that the flow remains unaffected. Additionally, you can also use the "far-field" boundary condition to simulate the flow at a distance from the exit, thus eliminating any effects of the exit geometry.

In conclusion, ANSYS and FLUENT CFD are powerful tools that can help you in simulating and analyzing turbulent flows. I would also recommend consulting with your instructor or a CFD expert for specific guidance on your assignment. Good luck!
 

1. What is ANSYS?

ANSYS is a computer-aided engineering software used for simulation and analysis of various physical phenomena such as fluid dynamics, structural mechanics, and electromagnetic fields.

2. What is FLUENT CFD?

FLUENT CFD is a computational fluid dynamics (CFD) software within the ANSYS suite. It is used to simulate and analyze complex fluid flow behavior, including turbulence, heat transfer, and chemical reactions.

3. What is turbulent flow?

Turbulent flow is a type of fluid flow characterized by chaotic, irregular motion of the fluid particles. It is caused by the interaction of various factors such as fluid viscosity, flow velocity, and surface roughness. Turbulent flow is commonly found in many natural and industrial processes.

4. How does ANSYS FLUENT handle turbulent flow simulation?

ANSYS FLUENT uses a variety of turbulence models, such as the Reynolds-averaged Navier-Stokes (RANS) equations, to simulate turbulent flow. These models use mathematical equations to describe the behavior of turbulent flow, and the software solves these equations to predict the flow behavior.

5. What are some applications of ANSYS FLUENT for turbulent flow analysis?

ANSYS FLUENT can be used in a wide range of industries, such as aerospace, automotive, energy, and chemical processing, to simulate and optimize fluid flow behavior. Some specific applications include aerodynamics of aircraft and vehicles, combustion processes, heat exchanger design, and turbulence modeling in industrial processes.

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