Grams, JonasJonasGramsLe Borne, SabineSabineLe Borne2026-02-112026-02-112026-01-30International Journal for Numerical Methods in Fluids (in Press): (2026)https://hdl.handle.net/11420/61432Fluid flow problems can be modelled by the Navier-Stokes or, after linearization, by the Oseen equations. Their discretization results in discrete saddle point problems. These systems of equations are typically very large and need to be solved iteratively. Standard (block-) preconditioning techniques for saddle point problems rely on an approximation of the Schur complement. Such an approximation can be obtained by a hierarchical ((Formula presented.) -) matrix LU-decomposition which first approximates the Schur complement explicitly. The computational complexity of this computation depends, among other things, on the hierarchical block structure of the involved hierarchical matrices. However, widely used techniques do not consider the connection between the discretization grids for the velocity field and the pressure, respectively. Here we present a hierarchical block structure for the finite element discretization of the gradient operator which is improved by considering the connection between the two involved grids. We prove a logarithmic depth estimate for cluster trees generated with the coupled clustering. Numerical results will show that the improved block structure allows for a faster computation of the Schur complement which is the bottleneck for the set-up of the (Formula presented.) -matrix LU-decomposition. The presented coupled clustering is also applicable to other types of mixed (finite element) problems.en1097-03632026115Wileyhttps://creativecommons.org/licenses/by/4.0/clusteringhierarchical matrixOseen problempreconditioningNatural Sciences and Mathematics::530: Physics::530.4: States of Matter::530.42: Fluid PhysicsNatural Sciences and Mathematics::518: Numerical AnalysisJournal Articlehttps://doi.org/10.15480/882.1665010.1002/fld.7006310.15480/882.16650Journal Article