H.N. Abramson - The dynamic behavior of liquids in moving containers. With applications to space vehicle technology (798543), страница 104

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The Bondnumber will be maintained in a bench test ona Xooth-scale model. If the general size of theprototype is of the order of 10 meters, themodel size is sufficiently large so as to bemanageable.Hydrostatic configurations at low and zero gcan be bench tested, using a second liquid inplace of the gas. In hydrostatics, the densitydifference is really what appears in the Bondnumber, and this difference can be adjusted soas to give the proper modeling. However, thetechnique should not be used for dynamicmodeling, for the greater inertia of the secondliquid will not properly model the behavior ofa light gas.Bench testing is by far the easiest andcheapest means for simulating a low-g environment, and should be considered wheneverfeasible.Free-Fall SimulationFree-fall capsules provide the simplest meansfor simulating low-g environments.

The timeavailable for a zero-g experiment is approximately given byt=-12.2f i seconds(11.120)where h is the height in meters. A 5-meterdrop therefore allows roughly 1 second. Adrop of 20 meters is required to double thetest time to 2 seconds. With greater falling-I:-&.-.--.-.-UID~~~LII~C~J,,1IIi.C&St&ii~C?XCCXBS& SC'.'CrCproblem. A double-capsule system, in whichthe outer capsule acts as a drag shield, ispreferable even for short-fall systems. Withthe long drop come severe arresting problemsand longer setup time.With a drag shield and a partially evacuatedcarrier capsule, environments of the orderlo-' .go can be simulated.

The addition of435counterweights can provide simulation in therange 0.02-1 go. The difficult range to obtainin a controlled way is 10-2-10-7 go. Thecounterweight system is awkward because theeffective mass of even a rising cable is apprcciable. If the inner capsule must be securedto the drag shield to get the counterweighteffect, then the velocity-dependent drag forceswill cause the effective g to vary during thedrop.

Very weak springs attached betweenthe capsules offer an interesting possibilityfor getting into the moderately low-g range.We used thin rubberbands for this purpose insome early experiments.The designer of simulation experiments usingfree fall in a drop tower should make sure thatthe response time of the phenomena in themodel is less than the drop time. The datagiven earlier should be useful in making theseestimates.With free-fall times of the order of 1 second,the experiments must necessarily be of smallscale if meaningful low-g results are to beobtained. Referring to figure 11.6, we seethat characteristic lengths of the order of1 or 2 centimeters would be required for afluid having a kinematic surface tension of theorder of 50 cm3/sec2. Since, for a givenlength variesresponse time the ~haracte~isticas only the one-third power of the kinematicsurface tension, we can estimate that for mostfluids 1-centimeter experiments would be practicable.

This limitation on the ~ermissiblesize in free-fall experiments is the chief drawback of this relatively inexpensive low-g simulation technique.Aircraft SimulationA variety of low-g experiments have beenperformed in aircraft executing a Keplerian32 seconds of near~ P JP- - - -I- ~ ~J P ~ --rr-----~A nnrn~irnn.t.~lyF V ,zero g can be obtained with a jet transport.Free-floating capsules are used to isolate theexperiment from the airframe.Although experiments of larger scale thanin free-fall towers are possible, and the simulation time is longer, the aircraft experimentationhas some serious drawbacks.

I t is difficult forthe pilot to accurately maneuver the aircraft436THE DYNAMIC BEHAVIOR OF LIQUIDSaround the floating capsule, and collisionsbetween the experiment and the padded wallsof the aircraft are frequent. Stress loadingson the aircrnf t are high, resulting in limitedlifetime and therefore high cost per flight.Initial roll imparted to the experiment uponrelease of the capsule from the airframe ismaintained during the floating period, and isdifficult to control. ;?;evertheless, aircraft experiments have providcri much vnlrlablc information on system performance during briefperiods of low gravity.Orbital and Suborbital SystemsThe use of suborbital systems further increases the experimentation time to the orderof several minutes. Drawbacks are the largeexpense involved, the high launch acceleration,and the need to either recover the capsule ortelemeter the data.

The esperiments aresubject to the motions of the vehicle, and controlof the<b>Текст обрезан, так как является слишком большим</b>.

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