Development of computational methods .O. ABSTRACT …

Development of computational methods

and conduct of experimental tests for blast

loading analysis

J.E. Slater," D.V. Ritzel" & P.A. Thibault&

"Defence Research Establishment Suffield,

f .O.

Avenue S.E., Medicine Hat, Alberta,

Canada, TM (9B5

ABSTRACT

Sample results from an extensive series of experimentalair-blast loading tests are presented to illustrate loadenhancement due to shock interaction and focussing, whichcan result in unexpectedly higher peak blast pressure andimpulse loading on exposed structures. An advanced computa-tional fluid dynamics (CFD) method that has been developedand integrated with a Graphical User Interface (GUI) on aUNIX-based workstation for blast analysis is discussed. Toillustrate the capabilities of the Blast Analysis Worksta-tion, results are presented and correlated with test data.

INTRODUCTION

Determination of air-blast loading, whether from weaponor accidental explosion, is of major interest to the mili-tary, civil defence, and transportation and petro-chemicalindustries. Ultimately the damage to structures and injuryto personnel by the blast is of paramount concern. To main-tain structural integrity and protect personnel and equip-ment, the structures must be designed to withstand specifiedlevels of blast pressure and fragmentation loading from anexplosion [1-5] . Enhanced pressure loading, however, fromshock interaction and focussing can occur in confined areasfor certain configurations and loading conditions whichresult in unexpectedly high load and damage levels [6-8] .

Extensive free-field and shock-tube tests have beenconducted to measure air-blast loading on full-size andmodel-scale structures of various configurations (see Figure1) . The results have provided a database for determiningthe load enhancement. Considerable effort has also beenapplied to develop a range of computational methods forpredicting the blast pressure- 'loading. Originally,empirical relationships were established to estimate theloading on relatively simple 2D and 3D structural configura-tions [1-3,9-11]. These methods cannot, however, be used to

Transactions on the Built Environment vol 8, © 1994 WIT Press, www.witpress.com, ISSN 1743-3509

384 Structures under Shock and Impact

BLAST

BoxTarget

1

2

3

4

5

6

7

8

9

H

L

L

L

L

2L

2L

2L

2L

L'

D

L

L

L

L

L

L

2L

L

L'

W

L

2L

4L

00

L

2L

L00

2L'where L = 254 mm

L' = 2438 mm

BLAST

CornerTarget

1

2

3

4

5

0(deg)

90

80

100

140

90

H

L

L

L

L

L'

D

L

L

L

L

2L'where L = 254 mm

L' = 2438 mm

Figure 1: Diagram of rectangular box and re-entrant cornerconfigurations for air-blast loading studies.

determine the detailed blast loading on complex structures,such as in re-entrant corners and over multiple 3D objectswhich cause complex shock reflections, diffraction andinteractions leading to unexpected amplification of the peakblast pressure and impulse.

This paper will focus on describing the recentdevelopment and capabilities of an advanced CFD method andintegrated GUI workstation [12] for blast analysis. Also asummary of test results are presented on blast pressure andimpulse amplification in 2D and 3D re-entrant corners whichillustrate the significant increase in the load occurring inconfined areas.

COMPUTATIONAL METHODS

Besides the analytical and empirical methods which givepredictions of approximate blast - loading on relativelysimple shapes, an advanced CFD method has been developedrecently that can now analyze the detailed blast loading forcomplex structures and loading conditions [13]. The CFDmethod has been further extended and interfaced with


Structures under Shock and Impact 385

advanced finite element structural modelling, processing andanalysis programs PATRAN and ADINA to determine the blast-structure interaction and dynamic response for damageassessment. ^ The loading and structural computationalmethods have^been integrated onto a UNIX-based workstationwith convenient graphical user interfaces and database foruse by both expert and casual users [12].

The workstation has been used to analyze the blastloading from internal and external explosions for various 2Dand 3D targets to illustrate the blast enhancement thatoccurs in confined areas. Sample results of analyses for a2D rectangular section (Figure 1, box target #4) and a 90°re-entrant corner configuration (corner target #5) arepresented in Figures 2 and 3, respectively. The velocityvector and overpressure profile plots in Figure 2 show thedetailed shock reflection and diffraction over therectangular section with the occurrence of transientvortices on the top and back of the target. The results inFigure 3 for a planar shock entering symmetrically into a 2D90 corner show the spatial distribution of the complexshock development at various times during the shockinteractions with the two adjacent vertical walls and at thecorner apex. The labels on the diagrams indicate the originof the Shockwave interactions. The nature of the complexblast loading is clearly demonstrated; particularly theenhanced pressure in the corner, on the two vertical wallsand over the horizontal structure or ground surface.

Associated pressure time histories for the inflowcondition and at the corner apex are presented in Figure 4for a test configuration of a 90° corner oriented 7° CCWrotation from the symmetric shock approach. Comparison oftwo analyses, assuming either a flat shock or a decayinginflow shock profile corresponding to the test condition,are shown with the measured pressure traces. Theamplification ratio of the peak corner pressure to theinitial inflow shock level is considerable in magnitudeamounting to a value of approximately 14. This numericalresult is compared with the extensive experimental data inFigure 5 (at PI/PO = 4); which also shows that the impulsewas amplified as well by a factor of approximately 5 duringthe initial shock clearance time (taken as 10ms). Thussubstantial enhancement of both the peak pressure andimpulse loading will occur from compound shock interactionsin a corner formed by the adjacent walls, floor or ceiling.

The computational results are invaluable forvisualizing and understanding the complex blast loading inconfined areas and on multiple configurations. The GUI andCFD capabilities of the workstation make it very convenientto set-up a numerical model, conduct an analysis and plotthe detailed results.

EXPERIMENTAL RESULTS

As mentioned above, the data from the extensive seriesof blast loading tests on 2D re-entrant corners at various


VELOCITY VECTOR PLOTS

sere50'0 50̂ 0OVERPRESSURE PROFILES

Figure 2: CFD computation of shock reflection and diffraction over a rectangular sectionshowing the spatial distributions of particle velocity vectors arid pressure profiles.

I00=ro


t=3.4 ms t=5.8 ms

(m)

Figure 3: Computed shock reflection process in a 90° re-entrant corner configuration due tothe compound shock interactions. (I = incident shock, Rl = initial wall reflection,R2 = Mach stem, R3 = collision of wall reflections, R4 = focus at corner apex).

C/32



OVERPRESSuRE

(MPa)

2.0

1.6

1.2 -

0.8 -

0.4 -

0.0

345 kPa INFLOW SHOCK CONDITION

•FLAT SHOCK PREDICTION-DECAYING SHOCK ANALYSIS•MEASURED PRESSURE

6.0

5.0 -

4.0 -

3.0 -

2.0 -

1.0 -

0 5 10 15

CORNER SHOCK PRESSURE LOADING

20

0.0

-5 5 10TIME (ms)

15 20

Figure 4: Comparison of numerical predictions based on flator decaying inflow shock condition with measuredpressure-time records.

orientations and with various apex angles are summarized inFigures 5 and 6, respectively. Similarly, results for aplanar shock entering three-dimensionally into the variouscorner configurations are given in Figure 7. The pressuredata have been reduced to the ratios of peak pressure toincident shock pressure, and initial peak impulse toincident shock impulse in the corner to illustrate thesignificant load enhancement relative to the incident shocklevel or the normal reflected shock loading (lowest curvesshown in Figures 5 and 6).

As an example, for a 2D 90° corner subjected to asymmetric shock (either Figure 5 or 6), the peak pressureand impulse amplification factors -range from approximately6 to 25, and 2.75 to 5.5, respectively, for inflow shocklevels from 0.2 to 4 (nondimensionalized by the ambientpressure PQ) . For a 3D orientation (from Figure 7), the peakpressure and impulse amplification factors increase by



40-

20-

PEAKPRESSURE 10-

AMPLIFICATION 8"FACTOR 6-

PEAKIMPULSE 4-

AMPLIFICATIONFACTOR 3-

I/Il

0.4 0.6 1 2 4 6

NONDIMENSIONAL INCIDENTPRESSURE

Figure 5:

10

2D reflected blast pressure and impulseamplification factors for 90° re-entrant cornerat various orientations.

approximately 5 and 2, respectively. Thus the blast loadingin a re-entrant corner is substantially higher than theincident shock or the normal reflected shock loadinggenerally used in current design methods.

CONCLUDING REMARKS

Target geometries invariably include corners or otherconfining configurations which are now shown to cause severeblast load enhancement due to compound shock reflections.There is not only an amplification of the peak pressureloading, but there is also an increase in the initial shockimpulse loading. Often, this severe load amplificationcombines with the inherent weakness of corners, due tostress concentration or joint details, to become the sourceof the failure mechanism for many blast/target scenarios.



40-

20-

PEAKPRESSURE 10-

AMPLIFICATIONFACTORP/Pl

PEAKIMPULSE 4 -|

AMPLIFICATIONFACTOR

0.2 0.4 0.6 1 2 4 6 10NONDIMENSIONAL INCIDENT

PRESSURE

Figure 6: 2D reflected blast pressure and impulseamplification factors for oblique wall andre-entrant corners with various apex angles.

For targets at threshold of failure, accounting for theblast amplification effect will make the difference betweenpredicting minimal damage or catastrophic failure. Hence itis imperative to account for the details of the blast loadamplification in confined areas.

This paper has presented numerical results of blastloading on a rectangular section and in a 90° corner toillustrate the capabilities of an advanced CFD workstationto analyze the detailed blast conditions. It has alsopresented extracts from an extensive series of test data forblast loading on various 2D and- 3D re-entrant cornerconfigurations, in the form of peak pressure and impulseamplification factors at the corner apex, to show thesignificant load enhancement that occurs in confined areas.These extremely high load conditions can result in



120-

PEAKPRESSURE

AMPLIFICATIONFACTORP/Pj 20H

PEAKIMPULSE

AMPLIFICATIONFACTOR

4 6 100.2 0.4 0.6 1 2

NONDIMENSIONAL INCIDENTPRESSURE

Figure 7: 3D reflected blast pressure and impulseamplification factors for re-entrant corners withvarious apex angles.

unexpected damage levels to the structure and injury topersonnel. Comprehensive analyses and tests on other 3Dconfigurations are being conducted and compiled for futurereporting.

REFERENCES

I. "Structures to Resist the Effects of AccidentalExplosions", Technical Manual, Army TM5-1300/Navy NAVFAC P-397/Air Force AFR 88-22, U.S. Department of DefenseExplosive Safety Board, 1990.

2. Baker, W.E., et al, "A Manual for the Prediction of Blastand Fragment Loadings on Structures", Technical Manual,DOE/TIC-11268, U.S. Department of Energy, Amarillo, Texas,1980.



3. "Fundamentals of Protective Design for ConventionalWeapons", Technical Manual, TM5-855-1, U.S. Army Corps ofEngineers, Huntsville, Alabama, 1985.

4. Forsen, R', and Nordstrom, M., "Damage to ReinforcedConcrete Slabs due to the Combination of Blast and FragmentLoading", in SUSI-II (Ed. Bulson, P.S.), pp. 509 - 520,Proceedings of the 2nd Int. Conf. on Structures Under Shockand Impact, Portsmouth, U.K., 1992, Computational MechanicsPublications, Southampton, 1992.

5. Marchand, K.A. and Cox, P.A., "The Apparent Synergism inCombined Blast and Fragment Loadings", in SUSI-I (Ed.Bulson, P.S.), pp. 239 - 252, Proceedings of the 1st Int.Conf. on Structures Under Shock and Impact, Cambridge,Mass., USA, 1989, Elsevier, Amsterdam, 1989.

6. Schmitt, J.A., "Shock Wave Propagation in a Re-entrantCorner", Journal of Physics of Fluids, Vol.24 No.9, pp. 1756- 1758, 1981.

7. Houlston, R., Slater, J.E. and Ritzel, D.V., "DamageAssessment of Naval Steel Panels Subjected to Free-Field andEnhanced Air-Blast Loading", in Marine Structures-2 (Ed.Smith, C.S. and Dow, R.S.), pp. 416 - 433, Proceedings ofthe 2nd Int. Conf. on Advances in Marine Structures,Dunfermline, Scotland, 1991, Elsevier Applied Science,London, 1991.

8. Houlston, R. and Slater, J.E., "Damage Analysis withADINA of Naval Panels Subjected to a Confined Air-BlastWave", Computers & Structures, Vol.47, No.4/5, pp. 629 -639, 1993.

9. Deikhoff, H.J., Linder, A. and Niessen, E., "BlastLoading of Typical Naval Structures", in MABS-10 (Ed.Reichenbach, H. and Ackermann, J.H.), pp. 354 - 373,Proceedings of the 10th Int. Sym. on Military Applicationsof Blast Simulation, Kurhaus - Bad Reichenhall, FR-Germany,1987, Ernst Mach Institute, Freiburg, FR-Germany, 1987.

10. Glasstone, S. and Dolan, P.S., "The Effects of NuclearWeapons", 3 ed., U.S. Departments of Defense and of Energy,1977.

11. Baker, W.E., et al, "Explosive Hazards and Evaluation",Elsevier Science Publishers, Amsterdam, 1983.

12. Thibault, P. and Sulmistras, A., "Fluid Dynamics/BlastPrediction and Structural Response Workstation", User'sManual, Combustion Dynamics Ltd., Medicine Hat, Alberta,Canada, 1993.

13. Otvos, T., "General Two-Dimensional Compressible FlowCalculations using SPLIT2D", User's Manual, Version 1.3,Combustion Dynamics Ltd., Medicine Hat, Alberta, Canada,1986.© Department of National Defence, Canadian Government


Development of computational methods .O. ABSTRACT …

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