Turbulence intensity inversion induced by the mass-reducing hole in an air or helium filled hard disk drive

Since its introduction, the mass-reducing hole in the middle of an actuator arm has been investigated as a source of turbulent flow in the space between a pair of co-rotating disks in an air-filled Hard Disk Drive (HDD). The present study investigates the effect of the mass-reducing hole by performing Particle Image Velocimetry (PIV) and numerical calculation in both air and helium. Helium was selected as an alternative medium due to its high kinematic viscosity which is expected to stabilize the turbulent flow around the actuator arm. A double-scale experimental apparatus was built to simulate a commercial drive. The same model was simulated numerically. The investigations were performed for two different positions of the actuator arm and two angular speeds of the disk (1,000 and 3,000 rpm; corresponding to 4,000 and 12,000 rpm in a 3.5-inch commercial drive). Experimental data was collected at the inter-disk mid-plane and ensemble-averaged to compute the turbulence intensity. The results show that, as expected, the helium flow induces lower turbulence intensity than the air flow at low speeds of rotation. In particular, the helium flow stabilizes the turbulent flow around the Slider Suspension Unit (SSU) more effectively than the air flow. However, at high speeds of rotation, the helium flow generates a higher level of turbulence intensity immediately behind the mass-reducing hole than the air flow. The physical mechanism of the switch is explained.