Hi all,

Can anybody help me with a problem of a turbulent jet into the atmosphere with a Re number of 10^6. I will simulate this with a commercial cfd code (fluent)and don´t know the area witch is needed that the jet is far away from the boundaries.I have one inlet boundary. The other boundaries are solid borders. Can I do 10 times the biggest geometrical dimension or 100 times. I run an transient simulation.

Dear Henry

I don't understand very well why are you talking about a "jet into the atmosphere" and the boundaries are solids!!!. Can't you use free/open boundary conditions for your case in FLUENT.

What exactly do you want to study? Watch out your distances. 10 or 100 times (assuming the jet's diameter) depends on what you want to study.

Regards

Kike

Hallo Kike,

Thanks for your quick answer. I simulate a temperature driving machine in the atmosphrere. The outlet of this machine produced this turbulent high Re number jet. When i use pressure boundary conditions for the boundaries the machine is then pressure driving and not temperature driving.That is not the same!! You get inflows and outflows on the boundary. But this is not the problem, then the safe distance to the boundary (wall or Pressure boundary) is unknown.The distance changes it between the simulation, because the simulation is unsteady. Is my solution to near the boundary, then i can forget my solution, because it is not boundary independent.

Henry

Dear Henry

I am sorry but I can't reach to understand your problem. I don't know anything about this kind of machines, but I will try to make me a simplified picture.

Thats is why I asked you what you want to obtain from your simulation. For instance, you maybe want to describe the evolution of the flow in the near wake of you trailing geometry (to control the drag), or you maybe want to describe the formation of NOxes from the thermal reaction in your machine to avoid contamination.

Let say, your domain is 2D (x,y) corresponding to the backward of your machine. So, the outlet (outflow parameters) of this machine corresponds to your inlet boundary condition (say the "west" border). The rest of the boundaries (if they corresponds to the atmosphere) are placed somewhere in the "open space".

In the two previous cases I would choose two different length to locate the rest of the boundaries (east, north and south). But in both cases I would use open boundaries, ie. those which allows inflows and outflows.

Those are maybe the most difficult boundary conditions in CFD (personal oppinion) but if this is your choise I send you two papers from which I have extracted those I use in my simulations.

(a) "Résumé and remarks on the open boundary condition minisymposium" Sani and Gresho, Int. Jour. for Num. Meth. in Fluids, Vol 18, 983-1008 (1994)

(b) "Effective downstream boundary conditions for incompressible Navier-Stokes equations" Bruneau and Fabrie, Int. Jour. for Num. Meth. in Fluids, Vol. 19, 693-705, (1994)

Try spacially with the proposal of the second one, it works very well (personal opinion/nessecities).

Regards

Kike

Dear Kike,

Thanks for your answer. The correct boundary far away from my machine is a problem , I know. I will order the literature witch you have advised, thanks.I will write you back,when i have read this.

Do you know references for a "buoyant momentum jet" or forced plume in axissymmetric??

Good bye, Henry

Hi there,

They may interest you:

(1) W. K. George and R. L. Alpert and F. Tamanini, "Turbulence measurements in an axisymmetric buoyant plume", Int. J. Heat Mass Transfer", 1977, vol. 20, 1145--1154.

(2) A. Shabbir and W. K. George, "Experiments on a round turbulent buoyant plume", J. Fluid Mech. 1994 vol. 275, 1--32

(3) X. Zhou, K. H. Luo, and J. J. R. Williams, "Large-eddy Simulation of a Turbulent Forced Plume", European J. Mechanics, B/Fluids, Vol. 20, No.2, pp. 233-254, 2001

Hi, Henry

No, I am sorry. I will check those Mr. Xu Zhou had gave you. Bye

Kike

hello, Could you please give us an idea of the geometry and the fluid of the jet?

hello, Could you please give us an idea of the problem geometry and the fluid and the of the jet?

Hallo Mr. A. Hassaneen,

The outflow of the machine has a diameter of 10 meters. The geometrie of the outflow is a simple tube. A full turbulent velocity profile is assumed.The temperature profile is konstant. The outflow shows against the direction of the gravity.The fluid is air.

At steady stage: The mean velosity is 5 m/s. The outflow temperature is 320 K. The atmosphere is at 300 K and in silence.

Thanks for your Response Henry

Henry, I don't think that your problem needs any special treatment. It's an air jet discharging into air, you may need to specify a confinement to the jet and use an outlet boundary at certain distance from the jet (let's say 10 diameter) and see what happen. Be careful when you specify the turbulence model. I think your problem will be mainly with the continuity.