URANS

Unsteady Reynolds-averaged Navier–Stokes (URANS) is a computational fluid dynamics approach that solves the Navier–Stokes equations in a time-dependent, Reynolds-averaged form. Like standard RANS, URANS relies on turbulence closure models to represent the Reynolds stresses, but the equations are integrated in time to capture unsteady flow behavior driven by external forcing or intrinsic instabilities. The turbulence models commonly used with URANS include standard k–ε, k–ω, SST, and Spalart–Allmaras, among others.

URANS is capable of predicting unsteady phenomena such as vortex shedding, flow reattachment, and fluctuating forces on bodies, while typically requiring less computational effort than large-eddy simulation (LES) or detached-eddy simulation (DES). Because the turbulence closure remains model-based, the accuracy of URANS depends on the chosen model, the temporal resolution, and the duration of the simulation used to obtain statistics. It can provide time-averaged and phase-resolved information, but it may not resolve all turbulent scales as LES does.

Compared with LES and DES, URANS generally offers lower cost and simpler implementation, making it attractive for engineering design studies where unsteady effects are important but full resolution of turbulence is impractical. However, URANS may underpredict or misrepresent unsteady separation, stall, or transition in some flows if the turbulence model does not capture the relevant physics.

Typical applications include automotive and aerospace aerodynamics, wind- and hydro-dynamics around bluff bodies, turbomachinery and rotorcraft flows, and other engineering problems where unsteady loads and responses are of interest.