U-Net–LSTM with incremental time-stepping for robust long-horizon unsteady flow prediction

Transient computational fluid dynamics (CFD) remains expensive when long horizons and multi-scale turbulence are involved. Data-driven surrogates promise relief, yet many degrade over multiple steps or drift from physical behavior. This work advances a hybrid path: an incremental time-stepping U Net LSTM model that forecasts unsteady dynamics by predicting field updates rather than absolute states. A U-Net encoder decoder extracts multi-scale spatial structures, LSTM layers carry temporal dependencies, and the network is trained on per-step increments of the physical fields, aligning learning with classical time marching and reducing compounding errors. The model is designed to slot into solvers based on projection methods (such as SIMPLE, PISO, etc), either as an initializer that delivers a sharper first guess for pressure-velocity coupling or as a corrective module that refines provisional fields. Across representative test cases, the approach improves long-term stability (54.53 to 84.21 % reduction of cumulative errors) and preserves engineering metrics, integral and averaged quantities, more reliably than standard learning baselines. These properties make it a plausible component of hybrid CFD-ML pipelines designed to accelerate unsteady simulations without compromising quantitative fidelity.