shocks / reflection in a duct

D.Stratmann

hello,

i`m an undergraduated of mechanical engineers and started working with fluent last year.

case :

i want to simulate the internal flow through very thin duct. it consists of an nozzle and a diffusor. indeed it is a model for an air gap in a compressor with a gap depth of 0.4 mm (nozzle > gap > diffusor. inlet pressure is 2 bar and outlet pressure 1 bar

experience data :

as we detected from experience there is a transient flow. as the flow-speed increases and reaches the gap, there appears a shock that is refleceted at the walls several times

questions :

- is it possible to simulate such small geometries with fluent ?

- i got flow-speed over 1 Ma (sound-velo), but shocks does not appear very sharp and are not reflected at the walls (i`m using viscoud, turbulent models, a very fine mesh with about 50.000 cells, steadz simulation). during the simulation shocks are formed but dissapear again... is unsteady simulation necessary ?

- as i see in cfd-online, shocks are not easy to handle. does fluent5.3 uses density-based solution-progress ? should i use other boundary-condition ?

- do i have to consider any other aspects ? (hint, tips ...)

many thanks for helping

dirk (university of dortmund, germany)