Signal-to-noise ratios S∕N of I (above) and II (below) representing the ratio of rms signal to rms numerical noise σ_meas, for analytical model u₁ (which has non-zero invariants) in Eq. (13), sampled by realisations of N = 128³ particles drawn from a uniform spatial random distribution for 8 ≤ n_DTFE ≤ 64. The signal is negligibly small for n_DTFE = 16, 128 for n_x = 8, 64, respectively,since the test functions are sinusoidal and in phase with the box.