The Effect of Gravity Induced Buoyancy on Velocity Measurement in 1-g Environment

The effects of testing cabin ventilation in gravity to meet a requirement for ventilation on orbit were analyzed. Buoyancy is due to the combined presence of a density gradient within the fluid and a body force that is proportional to the fluid density. Since gravity cannot be removed, the test must be conducted with air at as near to constant density as practical in order to remove buoyancy effects. The effects of gravity induced buoyancy force on the velocity field was analyzed by the Richardson number. Computational Fluid Dynamics (CFD) analysis was performed to verify the theoretical methods. The velocity data for a 1-g and a no gravity case were compared. The ratio between local velocity and free stream velocity, u/U_∞ were analyzed for the dimensionless parameter, η (= y ✓ U_∞/νx). There is a relatively sharp rise in the profile near the wall and an overshoot of the velocity beyond its free stream value. For the range of average velocity in the International Space Station (ISS) module, the buoyancy effect is only significant inside the boundary layer or close to the wall surface. This fact showed that the effect of buoyancy on the overall cabin ventilation velocity is minimal for distances greater than 6 inches from the wall. Test conditions to avoid gravity effects on the velocity measurements were suggested.