k-epsilon
 

hello, I was wondering if someone could tell me why the K-epsilon model is the most broadly used model for turbulence ?

any downsides, upsides ?

thanks in advance

Re: k-epsilon
 

it's a asimple one, very easy to solve, mostly validated by various researchers more or less all kind sof applications. Moreover, it resembles real situations to the good extent.

Re: k-epsilon
 

hi i want to compressible flow through nozzle, my inlet pressure and temperatures are 125 atm and 3600 k. which model( turbulence) i have to select for modelling this? i want to calulate the temperature of gas at the exit of the nozzle, and also machnumber at exit? can any body help me in this regard. thanks in advance.

Re: k-epsilon
 

well, the k-epsilon model is the one used by almost all cfd codes, so anyways you should use that one. I think your gas is gonna be under-expanded and sonic, so you must work out the equivalent diameter + concentration of gas already mixed with air to model the leak.

Re: k-epsilon
 

3600 Kelvin ?

Re: k-epsilon
 

ya,inlet temperature at inlet of nozzle is 3600 k and pressure at inlet is 125 atm. i modelled the geometry of the nozzle as axi-symmetric and try to obtain the initial solution using k-epsilon model, but initially itself divergence is coming the error is coming as divergence detected in AMG solver. when i initialize the solution and ask it to compute from inlet , the message coming in solver pannel is PRESSURE IS LIMITED TO 5E6 IN ALL CELLS. but my actual pressure is only 125 atm, why this message is coming , do u know the answer? waiting for your reply

Re: k-epsilon
 

can you give me the molar mass of your gas ?

Re: k-epsilon
 

and your nozzle diameter please, thanks

Re: k-epsilon
 

hi my nozzle diameter at inlet is 124 mm and at throat is 73 mm, divergent i,e exit is 145 mm. the molar mass of the gas ( molecular weight of gas at inlet is 23.291 kg/kmol, at exit is 24.601 kg/kmol) density at inlet is 9.4196 kg/m3, at outlet is 6.6765 kg/m3. ok

thanks already for reply.

Re: k-epsilon
 

For a 145 mm diameter, a molar mass of 24.601 Kg/Kmol. my program gives : gas exits at 822 m/s (sonic release then) with a massflow of 312 kg/sec at 1630 K temperature.

If you want to model it, then you would have to use a pseudo diameter cause it's under expanded. programm gives an equivalent diameter of 1.5 metres.

If you use that pseudo diameter,the gas has already mixed with air so you need the right concentration.

for your divergence problem, check the boundary conditions but you should post a new thread cause i am not an expert at all.

good luck.

Re: k-epsilon
 

hi thanks for your reply, but the temperature which you have given by your program is at exit only. but same configuration may exit is at 2600 k. how can it be possible. thanks already for reply

Re: k-epsilon
 

I am not sure i know what you meant here. please be more concise.