In this paper, we investigate inertially forced isothermal quantum turbulence (the coflow of normal and superfluid components) at temperatures of 1.6 and 2 K. The experiments are carried out in a large optical cryostat, where quasi-isotropic, homogeneous turbulence is generated using a double oscillating grid. Turbulence intensity is tuned by varying the grid stroke and frequency. The flow is probed via two-dimensional (2D) and three-dimensional (3D) reconstructions of quasi-isodense microsphere trajectories, from which we extract the large-scale properties of the flow in the fully turbulent regime for different Reynolds numbers: the turbulent velocity fluctuations, the energy transfer rate, and the integral length scale. Additionally, we determine the mean vortex line density $\mathcal{L}$ via attenuation of a second sound standing wave across the entire measurement volume. Our results confirm with an improved accuracy that the intervortex spacing $\ell =1/\sqrt{\mathcal{L}}$ scales with the Reynolds number ${\text{Re}}_{\kappa }$ (based on the quantum of circulation $\kappa$) as $\ell \propto {\text{Re}}_{\kappa }^{-3/4}$, with a well-defined numerical prefactor and no observed temperature dependence. This scaling recalls that of the dissipative length scale in classical Kolmogorov (K41) turbulence and it lead previous authors to interpret $\ell$ as an effective dissipation length scale. However, in our temperature range, this interpretation is not consistent with the apparent temperature independence of the prefactor. Based on those arguments, we propose an alternative interpretation that suggests that the intervortex distance in coflow turbulence is the consequence of the quantum-restricted depth of the superfluid component energy cascade.

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• Energy cascade
• Isotropic turbulence
• Vortex dynamics