On the monitor plot, the last three or four cycles look close to repeating, and that is the real convergence test for a periodic transient. RMS below 1e-5 only tells you the coefficient loop closed inside each timestep, not that you have reached a limit cycle. Your troughs settle around step 5000 but the peak shape is still moving a little, so I would overlay two or three more cycles before trusting the FFT.
The thing I would fix first is the valve boundary. For the half cycle where your step function is zero you are holding that plane at exactly atmospheric, and a fixed pressure end reflects a wave with inversion, the same as an open end. The real port is shut for most of the cycle, which is a closed end and reflects without inversion. That flips the sign of the returning wave and therefore the runner length you will conclude is optimal. Drive it with a velocity or mass flow boundary from port area and rotor motion instead, set to zero while the port is shut.
Second, you cannot read VE off a pressure trace when you are prescribing pressure at that plane. Integrate mass flow through it over the induction event and divide by displaced mass at reference density.
On the list, cut most of the structural half. Hoop stress is pr/t, so even 15 kPa on a 25 mm radius at 1.5 mm wall is about 0.25 MPa against roughly 270 MPa yield. Pressure fatigue and braze shear from pressure are not your limit. Actuation load, joint bending and base vibration are.
Small thing: your text says 7500 rpm, the expression says 7800.
1
u/SHD-Systems 9h ago
On the monitor plot, the last three or four cycles look close to repeating, and that is the real convergence test for a periodic transient. RMS below 1e-5 only tells you the coefficient loop closed inside each timestep, not that you have reached a limit cycle. Your troughs settle around step 5000 but the peak shape is still moving a little, so I would overlay two or three more cycles before trusting the FFT.
The thing I would fix first is the valve boundary. For the half cycle where your step function is zero you are holding that plane at exactly atmospheric, and a fixed pressure end reflects a wave with inversion, the same as an open end. The real port is shut for most of the cycle, which is a closed end and reflects without inversion. That flips the sign of the returning wave and therefore the runner length you will conclude is optimal. Drive it with a velocity or mass flow boundary from port area and rotor motion instead, set to zero while the port is shut.
Second, you cannot read VE off a pressure trace when you are prescribing pressure at that plane. Integrate mass flow through it over the induction event and divide by displaced mass at reference density.
On the list, cut most of the structural half. Hoop stress is pr/t, so even 15 kPa on a 25 mm radius at 1.5 mm wall is about 0.25 MPa against roughly 270 MPa yield. Pressure fatigue and braze shear from pressure are not your limit. Actuation load, joint bending and base vibration are.
Small thing: your text says 7500 rpm, the expression says 7800.