Turbulence subject to axisymmetric expansion is experimentally investigated using opposed multiple-jet arrays. For each array, jet interaction generates decaying, nearly homogeneous and isotropic turbulence within a duct. The turbulent flows from the opposed arrays collide and spread radially, forming a mean-flow stagnation point with associated mean strain. Flow properties are examined using particle image velocimetry. The mean velocity gradient tensor, A₈₉ = uᵢ / x\!₉, satisfies Aₗₗ: Aₘₘ: Aₙₙ = -2: 1: 1 with Aₗₗ 0, indicating axisymmetric expansion. Turbulence is strongly influenced by this expansion, becoming increasingly anisotropic towards the stagnation point, suggesting a cumulative effect of mean strain. The ratios of streamwise to transverse root-mean-square velocity fluctuations, uₑ₌ₒ/vₑ₌ₒ, and of their integral scales both increase relative to an isotropic state, consistent with rapid distortion theory (RDT). However, because the strain time scale is comparable to that of large-scale motions, deviations from RDT arise, including larger uₑ₌ₒ/vₑ₌ₒ values and a steeper decay of energy spectra in the inertial subrange than the -5/3 law. The spectral slope change is opposite to that reported for axisymmetric contraction, suggesting a common mechanism for spectral modification in both strain types, since both are described by the same tensor form with opposite signs of A₈₉. Consistently, the scaling exponents of velocity structure functions differ from predictions based on Kolmogorov’s second similarity hypothesis, even for low-order functions. These results confirm that axisymmetric mean strain significantly modifies turbulence properties, some of which are considered universal for other turbulent flows.
Han et al. (Thu,) studied this question.