Turbulence subject to axisymmetric expansion generated by opposed multiple-jet arrays

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, $\mathsf{A}_{\textit{ij}} = \partial \langle u_i \rangle / \partial x_{\!j}$ , satisfies $\mathsf{A}_{\textit{xx}} : \mathsf{A}_{\textit{yy}} : \mathsf{A}_{zz} = -2 : 1 : 1$ with $\mathsf{A}_{\textit{xx}} \lt 0$ , 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, $u_{\textit{rms}}/v_{\textit{rms}}$ , 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 $u_{\textit{rms}}/v_{\textit{rms}}$ 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 $\mathsf{A}_{\textit{ij}}$ . 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.