SIMULATION ON SHOCK TUBE AND PREDICTING OUT THE CHANGE IN PHYSICAL QUANTITY IN TUBE ON SHOCK
OBJECTIVE:-
(I) TO SETUP TRANSIENT SHOCK TUBE SIMULATION .
(II) TO PLOT PRESSURE AND TEMPERATURE HISTORY IN ENTIRE DOMAIN .
(III) PLOT THE CELL COUNT AS A FUNCTION OF TIME .
THEORY :-
THE SHOCK TUBE IS AN INSTRUMENT USED TO REPLICATE AND DIRECT BLAST WAVES AT A SENSOR OR A MODEL IN ORDER TO SIMULATE ACTUAL EXPLOSION AND THEIR EFFECTS, USUALLY ON A SMALLER SCALE . SHOCK TUBES CAN ALSO BE USED TO STUDY AERODYANMIC FLOW. SHOCK FLOW ARE ALSO USED TO INVESTIGATE COMPRESSIBLE FLOW PHENOMENA AND GAS PHASE COMBUSTION REACTIONS .
A SHOCK WAVE INSIDE A SHOCK TUBE MAY BE GENERATED USING BY A SMALL EXPLOSION OR BY THE BUILDUP OF HIGH PRESSURES WHICH CAUSE DIAPHRAGM TO BURST AND A SHOCK WAVE TO PROPOGATE DOWN THE SHOCK TUBE . A SIMPLE SHOCK TUBE IS A TUBE, RECTANGULAR OR CIRCULAR IN CROSS-SECTION, USUALLY CONSTRUCTED OF METAL, IN WHICH A GAS AT LOW PRESSURE AND A GAS AT HIGH PRESSURE ARE SEPARATED BY USING DIAPHRAGM . THE DIAPHRAGM SUDDENLY BURSTS OPEN UNDER PREDETERMINED CONDITIONS TO PRODUCE A WAVE PROPAGATING THROUGH THE LOW PRESSURE SECTION . THE SHOCK THAT EVENTUALLY FORMS INCREASES THE TEMPERATURE AND PRESSURE OF THE TEST GAS AND INDUCES A SHOCK WAVE IN DIRECTION OF FLOW WAVE .
THE LOWER PRESSURE GAS, REFFERED TO AS THE DRIVEN GAS , IS SUBJECTED TO THE SHOCK WAVE. THE HIGH PRESSURE GAS IS KNOWN AS DRIVER GAS. THE CORRESPONDING SECTIONS OF TUBE ARE LIKEWISE CALLED THE DRIVER AND THE DRIVEN SECTION. THE DRIVER GAS IS CHOSEN TO BE HAVE A MOLECULAR WEIGHT. TO OBTAIN THE STRONGEST SHOCKS THE PRESSURE OF THE DRIVEN GAS IS WELL BELOW ATMOSPHERIC PRESSURE.
THE SHOCK FLOW SIMULATION STARTS WITH THE BREAKING OF DIAPHRAGM WHICH BLOWS AWAY WHEN AN STRONG FORCE STRIKES ON THE DIAPHRAGM FROM THE DRIVER SECTION IN THE FORM OF GAS . AS THE DIAPHRAGM BLOWS AWAY, THE GAS FROM DRIVER SECTION ENTERS THE DRIVEN SECTION AND PRODUCES A SUDDEN SHOCK IN DRIVEN SECTION OF THE TUBE AND STRIKES END WALL OF DRIVEN SECTION WHICH CAUSES A REFLECTION BACK IN THE PRESSURIZED GAS SECTION OF TUBE . THIS SIMULATION HELP US IN UNDERSTANDING THE CONCEPT BEHIND THE SHOCK IN TUBE WHICH HELPS US IN STUDYING FLOW UNDER A WIDE RANGE OF TEMPERATURES AND PRESSURES THAT ARE DIFFICULT TO OBTAIN IN OTHER TYPES OF TESTING FACILITIES.
SHOCK TUBES HAVE BEEN DEVELOPED IN WIDE RANGE OF SIZES. THE SIZE AND METHOD OF PRODUCING THE SHOCK WAVE DETERMINE THE PEAK AND DURATION OF THE PRESSURE WAVE IT PRODUCES. RESULTS FROM SHOCK TUBE SIMULATION EXPERIMENTS CAN BE USED TO DEVELOP AND VALIDATE NUMERICAL MODEL OF THE RESPONSE OF A MATERIAL OR OBJECT TO AN AMBIENT BLAST WAVE WITHOUT SHARPNEL . SHOCK FLOW SIMULATION HELPS US IN DETERMINING WHICH MATERIALS AND DESIGNS WOULD BE BEST SUITED TO THE JOB OF ATTENUATING AMBIENT BLAST WAVES WITHOUT SHARPNEL .
APPLICATION :-
1. SHOCK TUBES HAVE BEEN USED TO MEASURE DISSOCIATION ENERGIES AND MOLECULAR RELAXATION RATES FOR AERODYNAMIC TESTS .
2. THE FLUID FLOW IN DRIVEN GAS CAN BE USED AS WIND TUNNEL, ALLOWING HIGHER TEMPERATURES AND PRESSURES THEREIN REPLICATING CONDITIONS IN THE TURBINE SECTIONS OF JET ENGINES WHERE TESTS TIMES ARE LIMITED TO FEW MILLISECONDS.
3. THEY HAVE BEEN FURTHER DEVELOPED INTO SHOCK TUNNELS, WITH AN ADDED NOZZLE AND DUMP TANK. THE RESULTANT HIGH TEMPERATURE HYPERSONIC FLOW CAN BE USED TO SIMULATE ATMOSPHERIC RE-ENTRY OF SPACECRAFT WITH LIMITED TESTED TIME.
MODEL SETUP :-
HERE FOLLOWING ASSUMPTIONS WERE CONSIDERED FOR SHOCK FLOW SIMULATION .
(A) TYPE OF FLUID USED :- NORMAL AIR (MIXTURE OF BOTH NITROGEN AND OXYGEN).
(B) TYPE OF MODEL:-
IT IS BASICALLY CONSIST OF TUBE WHCIH IS IN THE SHAPE OF RECTANGULAR PROFILE IN CROSS-SECTION .
(C) DIMENSION OF CHANNEL:-
- LENGTH OF TUBE:- 0.2 m 2. WIDTH OF PIPE:- 0.01 m 3. HEIGHT OF PIPE:- 0.01 m
(G) BOUNDARY CONDITION :-
- WALL FROM LEFT AND RIGHT SIDE OF TUBE (WALL FOR INLET AND OUTLET ).
- WALL FROM TOP AND BOTTOM SIDE OF TUBE.
- 2D FROM FRONT AND BACK SIDE OF TUBE.
(D) SIZE OF GRID :-
FOR CASE 1 :- dx=0.0015 m ; dy=0.0015 m ; dz=0.0015 m .
FOR CASE 2 :- dx=0.001 m ; dy=0.001 m ; dz=0.001 m
FOR CASE 3 :- dx=0.0005 m ; dy=0.0005 ; dz=0.0005 m.
NOTE :- HERE GRID SIZE HAVE BEEN DETERMINED FOR THREE CASES.
(E) INTIAL CONDITION :-
THE DOMAIN OF TUBE HAS BEEN DIVIDED IN TWO REGIONS :-
REGION 1 CONSIST OF ONLY NITROGEN SPECIE WHEREAS REGION 2 CONSIST OF ONLY OXYGEN SPECIE . HERE BOTH REGION HAS BEEN SEPARATED BY A DIAPHRAGM WHICH IS ARTIFICIALLY CREATED THROUGH EVENTS.
REGION 1 CONSIST OF HIGH PRESSURE WHEREAS REGION 2 IS HAVING LOW PRESSURIC ZONE.
FOR REGION 1:-
PRESSURE:- 1.25 BAR
VELOCITY:- 0 m/sec
TEMPERATURE:- 300 K
FOR REGION 2:-
PRESSURE:- 6.0 BAR
VELOCITY:- 0 m/sec
TEMPERATURE:- 300 K
(F) PARAMETER FOR SIMULATION TIME :-
1. INTIAL TIME STEP :- 1E-6 SECOND .
2. TIME PERIOD FOR SIMULATION :- 0.05 SECONDS .
(H) SOLVER TYPE USED:- TRANSIENT BASED SIMULATION .
PRE-PROCESSING STEPS:-
AFTER SETTING UP THE MODEL, WE SENT OUT DATA TO A PARTICULAR LOCATION WHERE ALL THE DETAIL OF CHANNEL IS AVAILABLE REQUIRED BY FURTHER SOFTWARE FOR SIMULATION.
THEN AFTER GENERATING AND STORING THE DATA, WE USED CYGWIN SOFTWARE WHOSE WORK IS TO RUN SIMULATION.
PROCEDURE FOR RUNNING SIMULATION:-
1. OPEN \\\"CYGWIN\\\" IN COMMAND PROMPT.
2. THEN TRACE THE LOCATION WHERE DATA IS STORED FROM CONVERGE STUDIO.
3. NOW TYPE \\\"converge.exe\\\" FOR SERIES SIMULATION OR \\\"mpiexec.exe -n 4 converge\\\" FOR PARALLEL SIMULATION IN THAT FILE LOCATION.
4. AFTER DOING THIS, SIMULATION WILL START WORKING AND AND WILL STOP AFTER GIVEN NUMBER OF CYCLES (BOTH IMPLICITLY AND EXPLICITLY DEPANDING UPON SIMULATION PERIOD).
5. THIS IS HOW THE WORK OF CYGWIN COMPLETES. AFTER THIS USE POST_CONVERT COMMAND IN OUTPUT FILE IS USED WHICH IS GENERATED BY CYGWIN. IT WILL CONVERT THE GENERATED OUT FILE FROM CYGWIN INTO A COMPATIBLE FROM WHICH WILL BE EASILY ACCESSED BY THE PARAFOAM.
NOW WE OPEN THE PARAFOAM SOFTWARE FOR POST PROCESSING TO VISUALIZE THE RESULT AND COMPARE IT.
POST-PROCESSING STEPS :-
1. IN PARAVIEW, CLICK ON OPEN FROM FILE BUTTON LISTED IN MENU BAR WHICH IS PRESENT AT TOP-LEFT OF THE SCREEN.
2. SEARCH FOR FILE AND TRACE THE LOCATION WHERE FILE IS STORED, GENERATED BY POST CONVERT COMMAND. CLICK AND OPEN IT. REMEMBER IT MUST HAVE \\\".vtm\\\" DOMAIN AFTER FILE NAME.
3. NOW FILE MUST BE LISTED (PIPELINE BROWSER) IN PARAVIEW WITH DEFAULT NAME PROVIDED BY YOU IN CYGWIN COMMAND PROMPT. CLICK ON APPLY, GEOMETRY WILL ORIGINATE. AFTER THIS CLICK ON SLICE OPTION TO SECTION THE GEOMETRY. NOW CLICK ON DROP DOWN MENU (SURFACE WITH EDGES) FROM TOP TOOLBAR OPTIONS . THIS WAY THE MESH WILL GENERATE .
4. AFTER IT, CLICK ON DROP-DOWN MENU NOTED WITH VtkBlockColors OPTION TO SEE THE DIFFERNT PHYSICAL QUANTITES AND APPLY IT.
5. IN THIS WAY, THE DESIRED PHYSICAL QUANITY WILL BE APPEAR ON THE SCREEN WITH VARIATION IN VALUE IN THE FORM OF CONTOUR ALONG WITH COLOR MAP (TELLS VARIED COLOR VALUE).
6. IN THIS WAY, OUR POST-PROCESSING IS DONE.
POST-PROCESS RESULTS :-
GEOMETRY OF SHOCK TUBE ALONG WITH MESH :-
FOR GRID SIZE=0.0005
FOR GRID SIZE=0.0010
FOR GRID SIZE=0.0015
CONVERGE PLOT ON PRESSURE :-
FOR GRID SIZE=0.0015 m
FOR GRID SIZE=0.0010 m
FOR GRID SIZE=0.0005 m
CONVERGE PLOT ON TEMPERATURE :-
FOR GRID SIZE=0.0015 m
FOR GRID SIZE=0.0010 m
FOR GRID SIZE=0.0005 m
CONVERGE PLOT ON CELL COUNT AS FUNCTION OF TIME :-
FOR GRID SIZE=0.0015 m
FOR GRID SIZE=0.0010 m
FOR GRID SIZE=0.0005 m
ANIMATION ON SHOCK FLOW SIMULATION :-
OBSERVATION ALONG WITH EXPLAINATION:-
IN PRESSURE AND TEMPERATURE CONVERGE PLOT FOR DIFFERENT GRID SIZE, WE OBSERVED THAT CONVERGENCE WAS OBTAINED IN ALL THE THREE DIFFERENT CASES BUT HOWEVER DIFFERENCE WAS THAT AS WE WERE DECREASING THE GRID SIZE, WE SAW A SLIGHT VARIATION IN CONVERGENCE PLOT FOR PRESSURE AND TEMPERATURE. MOREVER, WE OBSERVED THAT CONVERGENCE WAS HIGHLY STABLE AT LARGER GRID SIZE COMPARING TO SUBSEQUENT GRID SIZE.
IN TERMS OF CONVERGE PLOT FOR CELL COUNT, HERE WE OBSERVED THE CELL COUNT VARIATION AS A FUNCTION OF TIME OR TRANSIENT CASE FOR OVERALL DOMAIN. HERE, WE OBSERVED THAT AT INTIAL PERIOD, THE CELL COUNT WAS AT HIGHER NUMBER AND WAS STABLE FOR SOME FEW MILLISECONS COMPARING TO FURTHER TIME PERIOD FOR SIMULATION . THIS WAS BEACAUSE AT THIS TIME INTERVAL , THE SIMULATION WAS NOT STARTED AS THE DIAPHRAGM WAS NOT BLOWN AWAY DUE TO WHICH WE OBSERVED A STABLITY IN CONVERGENCE PLOT FOR FEW MILLISECONDS.
BUT AS DIAPHRAGM BLOWN AWAY, THE SIMULATION INTIATED AND SHOCK WAS PRODUCED IN THE TUBE WHICH CAUSE A STEP DECREASE IN PLOT BUT AFTER THAT ATTAIN A STABLITY IN PLOT. THIS CAN BE EXPLAIN IN A WAY THAT WHEN SIMULATION WAS NOT STARTED, THE CELL COUNT WAS PREDEFINED BUT AS IT STARTED, IT REDUCED BECAUSE THE CASE WAS TRANSIENT WHICH MEAN THE CELL COUNT WOULD BE DEFINE ON THE BASIS OF CHANGE IN TIME WHERE THE CELL COUNT WOULDE BE ADJUSTED ACCORDING TO CHANGE IN PHYSICAL QUANTITY AND THIS IS THE REASON WHICH CAUSE CELL COUNT TO ADJUST BY DECREASING IT\'S VALUE.
APART FROM IT, WE ALSO OBSERVED THAT THE CELL COUNT ALSO DEPAND ON THE SIZE OF GRID. WHERE THE CELL COUNT INCREASES IN ACCORDANCE TO DECREASE IN GRID SIZE. THIS IS BECAUSE A MORE MINUTE GRID SIZE WILL LEAD TO A MORE NUMBER OF CELLS TO ADJUST IN WHOLE DOMAIN OF SHOCK TUBE.
IN ANIMATION FOR NITROGEN GAS, AS SIMULATION STARTS WE OBSERVE THAT THE NITROGEN GAS MOVES FORWARD IN DIRECTION OF LOW PRESSURE REGION WHERE OXYGEN IS PRESENT WHERE IN LOW PRESSURIC REGION, THE GAS MOVES CONSTANTLY IN FORWARD DIRECTION FOR FEW MILLISECONDS BUT SUDDENLY STOPS AT A PARTICULAR POINT OF TIME AT PARTICULAR POSITION THEN AFTER THAT THE NITROGEN GAS REVERSES BACK IN OPPOSITE DIRECTION. THIS IS BECAUSE OF STAGNATION PRESSURE WHICH GENERALLY ORGINATES WHEN THERE IS A WALL IN FRONT OR NEAR OF IT. THIS STAGNATION PRESSURE CAUSES THE GAS TO REVERSE BACK IN OPPOSITE DIRECTION AFTER FEW MILLISECINDS OF STAGNATION PERIOD. AND THAT IS HOW THE CONCEPT OF GAS MOVEMENT IN SHOCK TUBE LEADS.
IN ANIMATION OF VELOCITY PROFILE, WE OBSERVED THAT VELOCITY WAS HIGHER AT BOTH SIDE OF DIAPHRAGM. AS SIMULATION STARTED, THE VELOCITY RELATIVELY INCREASED AT BOTH SIDE BUT MORE IN LOW PRESSURIC REGION. THIS IS BECAUSE WHEN SIMUATION STARTED, THE PRESSURE WAS HIGHER AT THE DIAPHRAGM SURFACE BUT WHEN DIAPHRAGM BLOWED SOME OPPOSITE PRESSURE REGENERATED WHCIH IN TURN PUT SOME FRACTION OF NITROGEN GAS PARTICLES TO MOVE IN OPPOSITE DIRECTION WHICH WAS HIGHER IN MAGNITUDE BUT NOT FOR MUCH DISTANCE. WHILE ON THE OTHER HAND, SOME FRACTION OF NITROGEN GAS MAINTAINED TO MOVE IN FORWARD DIRECTION WITH HIGHER MAGNITUDE WITH MORE DISTANCE COVERAGE. THIS VELOCITY OF NITROGEN GAS THEN REBACKS IT\'S VELOCITY IN OPPOSITE DIRECTION WHEN STRIKES SHOCK TUBE WALL WHICH LEADS TO A DECREASE IN VELOCITY MAGNITUDE AS THERE IS INTERFERENCE OF VELOCITY FROM BOTH THE DIRECTION. HENCE THAT IS HOW THE CONCEPT OF VELOCITY PREVAILS IN SHOCK TUBE.
CONCLUSION:-
FOR TEMPERATURE AND PRESSURE CONVERGENCE PLOT, WE CONCLUDED THAT DECREASE IN GRID SIZE WILL LEAD TO AN INCREASE IN DEVIATION IN PLOT OF CONVERGENCE FROM IT\'S MEAN PATH CONVERGENCE PLOT. HENCE IN THIS WAY ALTHOUGH HIGHER GRID SIZE WILL HAVE GOOD CONVERGENCE BUT STILL IT HAVE SOME DEVIATION FROM EXPERIMENT DATA RESULTS IN TERMS OF ACCURACY WHICH WILL BE HIGHLY ACHIEVABLE IN SMALLER GRID SIZE.
FOR CELL COUNT, WE LEARNT THAT INCREASE IN GRID SIZE WILL LEAD TO DECREASE IN CELL COUNT AND SIMULATION TIME BUT STILL HAVE DEVIATION FROM ACCURATE RESULT. SO WE CONCLUDED THAT SMALLER GRID SIZE WILL HELP US IN ACHIEVING GOOD ACCURATE RESULT IN COMPARISON TO EXPERIMENTAL RESULT.
IN ANIMATION OF NITROGEN MASS FRACTION, WE CONCLUDED THAT IT HELP US IN PREDICTING OUT THE MASS CONCENTRATION IN SHOCK TUBE DOMAIN AND GAVE US INFORMATION THAT HOW IT MOVE FORWARD TO LOW PRESSURIC ZONE THEN STAGNATED AND FINALLY REBACK TO IT\'S POSITION WHICH WAS QUITE VALUABLE FOR KNOWING THE CONCEPT.
IN ANIMATION OF VELOCITY, IT GAVE US INFORMATION ABOUT THE VELOCITY MAGNITUDE OF PARTICLES OF NITROGEN GAS. IT HELPS US IN UNDERSTANDING THE RELATIVE VELOCITY OF NITROGEN GAS THAT WHERE THE VELOCITY IS HAVING HIGHER MAGNITUDE AND WHERE IT IS HAVING LOWER MAGNITUDE. BESIDES THIS IT ALSO HELPS IN UNDERSTANDING RATE OF CHANGE OF MOMENTUM OF NITROGEN GAS PARTICLES IN SHOCK TUBE WHEN SHOCK IS PRODUCED.