Constant density vs ideal gas modell
 

Hi, I am doing a simulation in star ccm+. My modell i chose are k-e turbulence, reacting, complex chemistry, segregated flow and ideal gas. Then i wanted to make the flow in a helical path in the tube. I realized that when i changed to constant density so my flow became helical shown in streamlines plane, but when i changed to ideal gas then the flow shown straight line. Velocity i entered to boundary inlet was 0.1 m/s. Or 1.56E-05 kg/s.
So my quastion is does it gives me wrong result if i change my modell to constant density? How bad can it be if i assume it is incompressible?