# Mach number too low

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July 1, 2014, 09:52
Mach number too low
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Dear CFD-Online community,

for my diploma work I'm simulating the flow through an aircraft engine core compressor. A company supplied me with geometry and boundary conditions, and the flow field I simulated with that looks reasonable. There's only the major problem that my Mach number is drastically lower than theirs.

All my research how that could be turned up nothing, I'm all out of ideas, you would really help me if you have some ideas of your own for a Mach number discrepancy.

I'll explain my calculation in detail. Attached sketches can also help you visualize it. My results are compared to a through flow calculation.

It's a 2D simulation on a plane with constant radius, containing 3 rows. To not simulate the whole ring, only one passage per row is gridded and periodic boundaries on the borders in circumferential direction were added. To keep mass flow constant, the pitch of every row has to be constant. Two of the rows were scaled to accomplish that. See attachment_1.

As CFX can't calculate 2D, the 2D mesh was extrused in the radial direction, equally in the direction of the hub and the direction of the shroud. Those boundaries are set to a free slip wall. A throughflow calculation showed stream tube contraction. To account for that, the height of the extrusion narrows with increasing axial coordinate. See attachment_2.

The grid is an O-grid around the airfoils with a boundary layer resolution suitable for a wall function. The freestream is gridded with an H-grid.

The inlet is defined by total pressure, total temperature and a flow direction. The outlet by a static pressure.

If there's anything more you want to know, I'm happy to oblige!
Attached Images
 attachment_1.jpg (22.1 KB, 10 views) attachment_2.png (31.7 KB, 8 views)

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