Granular Systems in Microgravity Michael L. Wilson The University of Tulsa Supported by Research Corporation

Design goals  Measure properties of a diffuse granular system in microgravity  Look for clustering  Measure ball impacts & pressures  Measure ball speeds outside cluster  Determine dependence of dynamics on system density

Microgravity Approaches  KC-135 low-gravity simulation aircraft  Flight dates July 23 & 24, 2002  ~40 low gravity periods of ~30 sec. each  NASA-GAS program on Space Shuttle  Flight after ~2004  ~1 hour microgravity data  limited by video data storage.

Experimental System

 3 digital video cameras are used to record video and pressure data sets.  Pressure data saved on digital audio tracks.  Mirrors are used to increase effective optical path length in GAS enclosure.  Energy input to system via mechanical shaker arm.  All components microprocessor controlled.  On board power supplied by D-cell battery pack.

Optical System

 JVC DVM80 and JVCDVM90 cameras  Mini DV digital video & audio format  520 x 480 video 30 fps  2 channel 16 bit 48 kHz  400 mm f.l. achromat added for short focus  3” square front surface mirror to lengthen optical path  Sapphire walled box containing balls  Blue high brightness LED lights

Image Analysis  Cluster and individual ball positions determined using codes written in IDL.  Balls tracked from frame to frame to determine velocities.  Velocities determined only in diffuse regions of cells due to image overlap.

Sample Cells

 8 sapphire walled sample cells  23.4 x 25.4 x 22.5 mm interior volume  Each has an independent piezoelectric sensor on one face  Each camera sees four cells  7 cells are visible by at least one camera  These 7 are filled with brass balls  Eighth is left empty as a sensor control

Experiments  0.50 mm and/or 1.00 mm grade 200 brass  Mean free path (mfp) ~Vol./(N  d 2 )  %Oc.Vl. = % of volume occupied by balls

Piezoelectric Impact Sensor

 Piezoelectric sensor measures high frequency signal due to individual ball impacts  Recorded data high-pass filtered by D/A converter on camera  No direct signal from wall oscillation  Ball impacts cluster when sensor moves into occupied volume  Wall period determined from fft of sensor data

Battery Power System

 Final output is 8 9 volts DC (regulated) for two hours  8 D-dell batteries in series  6 strings in parallel  All diode isolated and fused  PVC shroud provides pressure seal necessary for GAS program

Mechanical Shaker

 Amplitude adjusted by changing the effective length of the swing arm.  Amplitudes from 0.22 mm to 1.2 mm  Frequency adjusted directly in DC motor  Computer control of PWM 9V supply  Internal reduction gearing 10:1  Frequency range from 1 to 30 Hz

Design Shaking Values  Typically low-g passes per flight

Goals  Determine the driving conditions necessary for stability of a free-floating granular cluster  Determine information about internal ball speeds.  Examine energy balance in a real 3-D system  Study dynamics of segregation in 3-D  Flight test system for later Space Shuttle flight opportunity