There have been a number of significant power outages caused by unexpected levels of flow-induced vibration of valves in partially-open operating conditions and as such the understanding of this phenomena is critically important. On-site testing is normally expensive and inherently difficult given typical flow rates but while computational fluid dynamics (CFD) offers a feasible alternative, it demands careful use and analysis to capture the highly coupled physical phenomena present. This paper reports on the fluid flow around a spindle in a high-pressure valve and related flow by means of 2D unsteady Reynolds-Averaged Navier-Stokes (URANS) equations and the k-ω-SST turbulence model. Following a mesh sensitivity study, the measured force frequencies and shockwave patterns predicted around the valve head indicate a high level of agreement with experimental reference data. The unsteady flow is then assessed at various stages of valve-opening conditions which are increasingly common in the context of power throttling to balance supply from more variable renewable energy sources. Although, the averaged force on the valve head reduces linearly with the lift of stem, the fluctuating component of the force rises by almost 100% when the valve opening changes from 25% to 50%. This quantity, significant in the identification of violent high-frequency vibration, is then observed to reduce steadily upon further opening. Spectral analysis and flow field data provide the basis for further insight with respect to the formation and evolution of a series of shock-wave/boundary-layer interactions along the valve head.