An alternative to CFD: piecewise linear models for frequency spectra of flow induced drag in hard disk drives

Abstract

A method is proposed to estimate the flow-induced drag on the actuator arm inside a hard disk drive. Typically, drag forces and moments on the actuator are computed as part of a computational fluid dynamics (CFD) solution of the flow field in the entire drive. Unidirectional coupling from the flow to the structure is then imposed to determine the structural response of the arm to the flow induced forcing. The methodology proposed here aims to reduce the simulation time associated with the flow calculations by directly estimating the forcing functions. The approach involves fitting a piecewise linear model (PLM) to the forcing frequency spectrum and interpolating or extrapolating the model to provide estimates of the spectrum at different points in the parameter space. A simple guideline for the formulation of such models is the conservation of energy between the CFD and PLM spectrum. Numerical experiments show that the linear models predict the behavior of arm to within 3% accuracy of the full CFD solution. The proposed technique is applied to two parameters: the disk RPM and the radial position of the arm. Clear trends are manifested for both parameters, making it possible to use this method to estimate forcing functions for a range of disk speeds and radial positions of the arm. This technique opens up the possibility of flow related design or optimization, which was previously thought to be prohibitively expensive.