[anon_p]

Hey guys,

I am setting up a simulation of the internal flow of a nozzle for a diesel engine. At the moment I have some difficulties with defining the boundary conditions. I already tried to find some references online about values for the inflow and outflow boundary conditions for tke and eps. However, I just find some few, that do not fully satisfy me. I think for eps of the inflow boundary I could use a fraction of the inlet diameter for the length scale. For tke of the inflow boundary and tke and eps of the outflow boundary I am not quite sure which values make sense. Do you have some hints or tips for me how to set these parameters?

Thanks in advance.

Best regards

[jhuang]

The inflow tke can be specified as a turbulence intensity dirichlet condition. At the outflow, the tke and eps would be neumann. However to account for any backflow, a backflow BC is needed for tke and eps. If the outflow boundary dimension is similar to the inflow, then eps boundary condition can be specified using the same length scale. Likewise, a similar intensity value for tke can be specified if we expect similar turbulence intensity (which will likely be the case if both the inflow and outflow are open to the same atmospheric conditions).

We have a SprayA example case you may refer to. At the inflow, tke intensity is 0.001, eps length_scale is 0.0001. At the outflow, we use a specified tke of 0.1 m2/s2 and eps of 0.0001 m2/s3. Please note that your outflow boundary should be placed as far as possible. I've seen before that if this boundary condition is placed too close, pressure wave bouncing back and forth in the computation domain may cause crash.

Hope this helps. Thanks!

[SamWijey]

Hi GNKK,

We have several diesel engine tutorials on our downloads site (www.convergecfd.com/downloads) including a case with ports and valves. For typical engine cases (including diesels) you can set the inflow turbulent intensity to be be 1% - 3%. You can set the length scale to be 2%-5% of the hydraulic diameter of the intake port.

Thanks,