Fluid motion for microgravity simulations in a random positioning machine

BACKGROUND To understand the role of gravity in biological systems one may decrease inertial acceleration by going into free-fall conditions such as available on various platforms. These experiments are cumbersome and expensive. Thus, alternative techniques like Random Positioning Machines (RPM) are now widely used to simulate the micro-gravity environment (Yuge et al., 2003; Borst and van Loon, 2009; Pardo et al., 2005). These instruments generate random movements so that cumulative gravitational effects cancel out over time. However, comparative studies performed with the RPM and culture cells were unable to reproduce the spaceflight results (Hoson et al., 1997). These differences may be explained by stresses acting on the culture cells in an RPM whereas these stresses are not present in microgravity conditions. They may be caused by internal fluid motion, originating from instationary rotation. The aim of this study is to quantify fluid flow behavior and wall shear stresses (as they are relevant to cells cultured at the flask wall), and internal shear stresses as they are relevant to suspended (free-floating) cells in an RPM container. We do this both experimentally using Particle Image Velocimetry (PIV) and numerically using 3D Direct Numerical Simulation (DNS) of the flow.