Physics
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.552m&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Physics
Scientific paper
The predictions provided by different Design Analysis Cycles (DAC s) are now converging and give the possibility to be correlated with experimental measurements. The most important utilization of the acceleration data refer to the possibility of validating numerical simulations that relate the acceleration sources to the real effects they produce, so that the Principal Investigator (PI) would be in a position to foresee the real conditions and to properly select suitable conditions for running the specific experiments. Previous numerical studies (Monti and Savino 2001, Savino and Monti 2001) had to rely only on the DAC's predictions (see e.g. Non Isolated Rack Assessment, NIRA) that, sometime, were contradictory and strongly dependent on the assumptions about the number, location and type of the sources of acceleration (with respect to the microgravity experiment position) and on the accuracy of numerical codes (not validated by flight experimental data). The experience build up so far has identified efficient numerical tools to quickly predict the overall disturbances induced by the Microgravity Environment of the ISS on specific fluid dynamic experiments. Extensive numerical simulations proved that the experiment sensitivity to g-jitter can be evaluated by the numerical solutions of the full Navier-Stokes equations with a time-dependent acceleration (direct formulation), taking into account the different accelerations at the different frequencies of the ISS spectrum, or simply computing the time-average velocity field by solving the time-average (thermovibrational) equations. In the latter case the disturbances of the thermofluidynamic field are easily evaluated assigning as input to the numerical code an overall "equivalent" vibrational Rayleigh number (corresponding to a single frequency g-jitter equivalent to the overall g-jitter spectrum). An analysis of the Navier-Stokes equation is performed to identify the terms in the momentum and vorticity equations that are responsible for the generation of convective motions in a fluid cell either in the presence or in the absence of density gradients. A reference study case has been extensively investigated over the last few years at the University of Naples (cubical fluid cell with a homogeneous liquid and two opposite walls at different temperatures) and numerical simulations are carried out to evaluate the effects of the real acceleration data on the thermofluidynamic field. The geometry, the liquid physical properties and the boundary conditions are chosen according to the Fluid Physics experiment scheduled for the UF3 mission of the ISS (2004) on the orientable Italian Facility GLAD. The NASA measurements of the quasi-steady and periodic acceleration (at different ISS locations and for different crew activities, see Jules et al. 2001) are considered and their effects on the fluid-dynamic experiment are computed and discussed. Numerical simulations are carried out to identify the relative importance of linear and pendular accelerations, due to the possible rotations of the payloads around their Center of Mass, caused by the reaction forces of the umbilicals on the passively "isolated" payloads. The effects caused by variable accelerations created by an actively isolation mount that exhibits an attenuation factor not constant within the payload volume at some frequencies are computed and analyzed.
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