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The residual ratios used by AcuSolve are a non-dimensional measure of how "out-of-balance" the governing equations are. The residual ratios are recomputed at each timestep to give an indication of how well the solution is converging. The solution ratio that is computed by AcuSolve represents the ratio of the change in the solution between timesteps to the solution at the previous timestep. This value indicates the level of unsteadiness present in the solution. Note that the solution ratios and residual ratios are computed using the contribution of each node in the model. The single value is obtained by taking the appropriate norm of the contribution from each node. The details of this procedure are described in the attached document.
AcuSolve uses the Finite Element Method to discretize the governing equations. By default, AcuSolve uses linear shape functions and Gaussian quadrature to perform the necessary integrations. This results in a numerical method that is second order accurate in space. Although there is no direct comparison to the common diferencing schemes found when using the finite volume method, this approach most resembles central differencing with a small amount of upwinding blended locally when necessary for stability. There is no way to reduce the spatial accuracy of AcuSolve below second order, however, quadratic tetrahedra elements are supported to increase the spatial accuracy above second order.