“…Firstly, the snapshot matrix becomes very large for problems with many time steps and parameter samples, leading to expensive singular-value decomposition (SVD). For this issue, methods such as the randomized SVD algorithm [31], fast adaptive cross-approximation [32], and local bases solutions method [33] have been used for large-scale problems. Beyond this, Wang et al [34] applied POD twice to reduce the cost of global spatial bases for unsteady flow problems in the parameter domain.…”