# Specify both velocity and pressure at inlet boundary

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 March 1, 2021, 20:30 Specify both velocity and pressure at inlet boundary #1 New Member   Niall Join Date: Mar 2021 Location: Ireland Posts: 2 Rep Power: 0 With the standard set of boundary conditions supplied in Star CCM+ I cannot manage to define both pressure and velocity together at the inlet. Question: Is there a way to specify both velocity and pressure at inlet using a particular inlet boundary type or by any other means in StarCCM+? Background info: I am modelling a compressor and nozzle in StarCCM+. I am using a fully developed interface approach where at each Interface (representing each stage of the compressor) there is a ‘pressure jump’ where values can be input into the StarCCM+ simulation. Using boundary type Stagnation Inlet at inlet with Pressure Outlet at outlet, I obtain results which illustrate the pressure rise through the compressor very well and the decrease in the nozzle. However, the initial velocity is not correct and therefore the final velocity is incorrect also. (Initial conditions velocity value is input = 20m/s) Using boundary type - Velocity inlet at inlet with Pressure outlet at outlet instead, results for velocity are similar to my theoretical calculations and the initial velocity is as input into the boundary conditions which is great and as expected. However, the pressure results show large negative pressures at the inlet and a gradual increase through the compressor to positive values. There shouldn’t be negative values since compressor stage pressures should be above gauge pressure = 0Pa. Options considered: I ensured the correct selection of faces when creating interfaces to avoid negative pressure jumps. I thought the issue may be the turbulence model selection but using laminar provides the same results. I switched from constant density to ideal gas in the physics model but it didn’t work. At this stage I am just guessing. Physics Model – Steady, 3D, Constant Density, K-Omega Turbulence, Segregated flow I am relatively new to CFD (StarCCM+)

 March 2, 2021, 05:54 #2 New Member   Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 8 Hi, what variable is the solver supposed to solve for in a constant density, most likely isothermal fluid flow if you specify pressure and velocity? Short answer is no, you can't specify both pressure and velocity since they are coupled (even with the use of the segregated solver they are coupled in the sence of the flow field). You have to leave one variable for the solver to calculate.

 March 2, 2021, 15:02 #3 New Member   Niall Join Date: Mar 2021 Location: Ireland Posts: 2 Rep Power: 0 Thank you Chris for answering my question! Much appreciated.

 March 2, 2021, 16:33 #4 New Member   Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 8 Sure, you're welcome!

 March 7, 2021, 06:36 #5 Member   Yogesh Nalam Join Date: Sep 2012 Location: München, Germany Posts: 54 Rep Power: 12 (As pressure is used as a Neumann boundary condition.) You can try using the target mass flow option available with static pressure. Hope it helps.

 Tags boundary conditions, inlet bc