Numerical Investigation of Steady Pressure Deficits Induced by Annular Wall Suction in a Circular Tunnel

Wall suction creates a low-pressure field in a tunnel, but its magnitude depends on the extraction rate and the paths supplying air to the slot. Steady, fully turbulent Reynolds-averaged Navier–Stokes simulations of a circular tunnel with an annular suction slot were used to isolate this response. An uncalibrated two-branch Darcy–Weisbach model reproduces the scale of the regional static deficit, with a mean computed-to-predicted ratio of 1.00 (standard deviation of 0.22) over 35 cases spanning a 182-fold range, but underpredicts the local deficit at the slot. At fixed extraction rate, sevenfold changes in slot width alter the regional static deficit by 1.5–3.1% for As/At=2.2–16. Normalising the total-pressure deficit by an effective hydraulic path length, which combines tunnel length and slot position, gives a suction-velocity exponent of 1.735, close to the Blasius 7/4. Static and total pressure separate the low-pressure background from a proxy for its loss-related component. The centre-peaked position trend agrees qualitatively with published train–tunnel fields; an idealised full-scale calculation places the steady central deficit 6–11 times below the published value. These results show how slot location and dimension, left open by the published studies, act on the steady model-scale base state for slots larger than the tunnel cross-section.