Moustf Es ,Mostf S.Amin ,Ahmed Hssn ,*

a Eng.Mechanics Dept.,Military Technical College,Kobry Elkobbah,Cairo,Egypt

b Civil Eng.Dept.,Military Technical College,Kobry Elkobbah,Cairo,Egypt

Keywords:Blast mitigation Blast barriers Protection system Mitigation percent Protection evaluation

ABSTRACT Recent researches focused on developing robust blast load mitigation systems due to the threats of terrorist attacks.One of the main embraced strategies is the structural systems that use mitigation techniques.They are developed from a combination of structural elements and described herein as conventional systems.Among the promising techniques is that redirect the waves propagation through hollow tubes.The blast wave propagation through tubes provides an efficient system since it combines many blast wave phenomena,such as reflection,diffraction,and interaction.In this research,a novel blast load mitigation system,employed as a protection fence,is developed using a technique similar to the technique of the bent tube in manipulating the shock-wave.The relative performance of the novel system to the conventional system is evaluated based on mitigation percent criteria.Performances of both systems are calculated through numerical simulation.The proposed novel system proved to satisfy high performance in mitigating the generated blast waves from charges weight up to 500 kg TNT at relatively small standoff distances(5 m and 8 m).It mitigates at least 94% of the blast waves,which means that only 6% of that blast impulse is considered as the applied load on the targeted structure.

1.Introduction

The rise of the terrorist attacks brought the attention of the researchers to focus on studying the effect of blast loads on structures and developing and/or modifying new protection techniques to satisfy relative protection for important buildings.The researchers focused on studying the interaction between the blast load and structure response,considering two strategies for structural protection against the blast wave.The first strategy is to structurally dimension the main supporting elements to carry the expected blast load.The second strategy is meant to design and employ the blast wave mitigation system placed in front of structures to attenuate the generated blast wave and consequently,reduce its effect significantly.In the following subsections,part of the researches carried out in this area is reviewed.

Numerous studies introduced computation equations,charts and empirical formulas that can guide researchers to compute or predict the pressure-time history.This step is followed by the measure of the structural response.Accordingly,blast-resistant design of structures subjected to blast loads,and various mitigation strategies can be achieved[1-3].

The design of blast load mitigation systems should include the study of the attenuation mechanism and its influence on mitigation efficiency.Berger et al.[4,5]investigated numerically and experimentally pressures induced on the end-wall of a shock tube by a shock wave passes through single and multiple obstacles.Considerable conclusions reached related to the dependency between the blast wave and the barriers sizes and shapes and the effect of the geometry which became dominant when the blockage ratio is large.Foglar et al.[6-10]studied the effects of the barrier surface on the shock wave mitigation.Also,they examined numerically and experimentally the effect of different shaped rigid barriers on blast wave propagation.Barriers found to have some effect on lowering the peak overpressure,but only in the area directly behind the barrier.Soni et al.[11]carried out numerous simulations to investigate the shock reflection phenomenon over the complex geometry of the concave cylindrical double wedge reflectors as well as the changes in the incident shock wave Mach number.The results reveal several interesting shock reflection behaviours.It is found that the shock reflection pattern varies notably with the incident shock wave Mach number,as well as with the initial wedge angle.

The fence type mitigation system is a blast wall that consists of structural columns of suitable materials placed at strategic arrangements as wave obstacles.These arrangements sustained to bring about wave reflection,diffraction,and interaction to result in the self-cancellation of wave energy.Asprone et al.[12]described the results of detailed numerical analysis,compared to experimental results,simulating blast tests conducted on a discontinuous GFRP barrier used as a protection system.Some results are described in Ref.[13]for the blast test campaign conducted on fullsize specimens of the proposed barrier shown in Fig.1 and discussed their effect in mitigating blast waves.

Hadjadj and Sadot[14]studied wave phenomena generated by large scale explosions in complex environments that pass through complex media.Chaudhuri et al.[15]analyzed numerically shockwave propagation through different arrays of solid obstacles and its attenuation.They concluded that the use of a combination of staggered matrices of reversed triangular prisms obstacles is found to be very effective in blast wave mitigation.

Zong et al.[16]carried out numerical simulations to investigate the effect of the fence layout with different column geometries,spacing,dimensions,and fence layers on blast load reduction.The proposed fence wall proved to significantly reduce the peak blast pressure and impulse,especially the fence made of a front circular column layer and a second isosceles right triangular column layer as it reduces the pressure and impulse of the blast loads behind the wall by up to 70%.Xiao et al.[17]presented a prediction method for the effectiveness of various blast wall configurations in protecting against air blast.They also conducted numerical investigations validated by experimental data on the shock wave attenuation effect of a new barrier made of square cross-section steel posts with different arrangements located in front of a building[18,19].

Mechanical control is a robust and reliable concept[20].Since the applied load is activating the mechanical mechanism to start reacting in a way that serves the system objective.This concept is employed hereinafter in the design and implementation of the proposed mitigation system.

Fig.1.Prototype of the barrier[13].

Kudo et al.[21]demonstrated experimentally through the implementation of a mechanical control system as the steady-state oblique detonation waves propagated stably through rectangularcross-section bent tubes.The oblique detonation waves were stabilized under the conditions of high initial pressure and a large curvature radius of the inside wall of the bent tube section.They also performed visualization experiments employing rectangular cross-section curved channels to examine the fundamental characteristics of a curved detonation wave propagating stable through an annular channel.Curved channels with different inner radii of curvature were studied to determine the stable propagation condition.

Li et al.[22]employed a detailed reaction model in simulating two-dimensional cellular detonations propagating through smooth pipe bends in a stoichiometric mixture.Also,Li et al.[23]investigated numerically the propagation mechanism of steady cellular detonations in curved channels with a detailed chemical reaction mechanism as shown in Fig.2.Part of the results evinced that,as the radius of curvature decreased,detonation failed near the inner wall due to the strong expansion effect.While increasing radius of curvature caused the detonation front near the inner wall to sustain a dilute detonation[23].

Luo et al.[24]conducted a 3D numerical study of detonation wave propagations in straight/varying angle bending detonation tubes with different inner diameters.Their results show that the higher the bend angle,the smaller the average detonation wave speed when it has passed the bending section.

The present research is carried out aiming to develop a novel blast load mitigation system that can be used as a fence to satisfy acceptable protection for the lower floors of targeted structures since it is the main supporting floor of the building and it is at the level of the possible position of the detonation center of the charge.A technique similar to the technique of the bent tube is used;in which it is advantageous to guide the blast waves into a hollow cylindrical structural element to have the control/guide on the blast wave to let it transfer from the birth departure to the desired destination.The study consists of two stages:The first stage includes the development of the common system of mitigation,based on previous researches,which is the Baseline Conventional Mitigation System(BCMS)through the implementation of the state of the art in the field to use it as a reference system for the current research.The second stage includes the development of the proposed novel Chimney Tube Mitigation System(CTMS),in which its components are integrated to control and direct the blast waves through the planned pattern to cause self-destruction and minimize its influence on the targeted structure.The conceptual design includes the employment of chimney tubes that are subjected to different Fluid-Structure Interaction(FSI)scenarios.The CTMS design procedure included numerous parametric numerical simulations to reach the optimal configuration.The performance of CTMS is numerically measured through comprehensive parametric studies,in which its results are compared to its counterparts of the BCMS.The CTMS proved to significantly reduce the effect of the blast waves on the targeted structure.

2.Description of BCMS

The reference BCMS system is developed based on the extensive literature introduced earlier,in which the barrier systems consist of two rows of staggered reversed triangular prism elements(the concave side is facing the blast wave source)[14,15,25].This configuration is found from literature to be a valuable arrangement that allows mitigation of blast waves and withstands the blast loads based on its stiffness[26,27].The system consists of two staggered rows of 4 m high of steel angles with a cross-section dimension equal to 180×180×8 mm.The lateral spacing between the two adjacent angles equals 78.77 mm.The external longitudinal spacing between the two staggered rows is 1 m.The BCMS weighs 542.6 kg/line meter.The general arrangement and the system configuration are illustrated in Fig.3 and Fig.4.

Fig.2.Pressure and temperature contours during detonation propagation in the curved channel[23].

3.Description of CTMS

The proposed CTMS is a novelty configuration that is benefited from a mitigation technique similar to the technique of the bent tubes in manipulating the shock-wave[22,23,28,29].It is mainly depending on directing the majority of the blast waves away from the targeted structure,in addition to its relatively high stiffness ability to absorb the residual of blast wave strain energy.The CTMS consists of one row of connected side by side 4 m high steel tubes as shown in Fig.3.The tube’s cross-section is 1000×5 mm closed at the top by steel cap.The tubes are configured as shown in Fig.5,in which a lower tapered prismatic trap is attached to direct the blast wave inside the tube;two successive wave holes and finally the upper trap which consists of two vertical 300 mm long windows generated to evacuate and dissipate the concentration of the trapped waves.The tube’s thickness is selected as 5 mm.This selection enables the weight of the meter-long of the CTMS almost equal to or less than that of the BCMS.The CTMS weighs 459.9 kg/line meter which is less than the BCMS weighs.The aforementioned CTMS configurations have been studied in such a way to ensure self-destruction and to direct the majority of the waves away from the targeted structure behind the system.

Fig.3.The general configuration of the two contender systems.

Fig.4.Conventional BCMS system configuration.

4.Finite element modeling

4.1.Numerical simulations for FSI

The commercial software AUTODYN(under ANSYS)[30]is used for the numerical simulations of the current research.The Euler/Lagrange interaction(FSI)model implemented in AUTODYN couples the Eulerian domain to the Lagrangian structural domain.This coupling algorithm in AUTODYN allows the modeling of the interaction between the external faces of the structures and the cells of the Euler grids,whereas the true shell thickness represents the third dimension of the 2D shell element to simulate the external faces of the structures and the coupling algorithm is applied to the Euler cells that are bounded by the effective coupling thickness as shown in Fig.6.FSI occurs with all the Lagrangian elements(Shell elements=Steel)inside the Euler domain(Euler-FCT elements=Air)as shown in Fig.7.

Fig.5.Proposed CTMS configuration.

Fig.6.Equivalent coupling thickness for FSI for thin elements in AUTODYN[30].

Fig.7.Configuration of CTMS numerical model.

4.2.Calculation of blast load

For the current parametric studies,the possible TNT charges are assigned according to Samali et al.[31]as shown in Table 1.Numerical simulations are carried out to calculate the resulted pressure and impulse time-histories.The acquired results are compared with its counterparts calculated using CONWEP with the following menu(Weapons EffectsAir-blastAboveground detonation)[31,32]for 5 m,and 8 m standoff distances to validate the percentage of agreement between the generated impulse and the selected boundary condition of(Hemispherical Surface Burst).Fig.8 illustrates the pressure and impulse time-histories of the selected charges as a comparison between AUTODYN and CONWEP results.

4.3.Materials’properties and inputs of the numerical model

For the current study,a sensitivity analysis is carried out to obtain the reasonable elements’sizes,the fences are modeled by unstructured Shell elements.The discretization of the unstructured elements is constrained by minimum edge length(40 mm).The steel used in mitigation systems is STEEL_4340 with the properties shown in Table 2.The load(Blast-load)is modeled by Euler-FCT(ideal gas)domain(6 m×3.5 m×8.2 m),however,that domain is charged with a Remapping 2D data file corresponding to the case of TNT weight for each study.The cell size of that Air domain is(20 mm×20 mm×20 mm).The air is represented as ideal gas Equation of State(EOS)with reference density 0.001225 g/cm3,TNT is represented as Jones-Wilkins-Lee EOS with reference density 1.63 g/cm3along with auto-convert to an ideal gas(Fig.7).The global settings of the numerical model are:Symmetric about(Y)axis,the global erosion is done by 1.5 geometric strain,the global cutoffs of maximum expansion are 0.1,trajectory interaction among the lagrangian parts,and fully coupled between the Euler sub-grid and the shell elements.

Table 1Assigned TNT charges by transportation means[31].

Fig.8.Overpressure and impulse time-histories(a)Standoff distance Z=5 m,(b)Standoff distance Z=8 m.

Table 2Properties of steel material used in the mitigation systems.

4.4.Gauges location

The majority of the researches carried out in the same field,the pressure-time history is recorded in a 2D plane where the surrounding region is not monitored.In the current paper,the blast impulse histories are studied for 3D zone boundaries and numerically recorded through the measurements acquired by six virtual pressure gauges that bound the critical zone behind the mitigation system.Such that the coordinates of the gauges are assigned for the TNT charge position which is placed at the most critical position,adjacent to the mitigation system,and assigned at specific 3D dimensions’origin,O(0,0,0),as indicated in Table 3.Fig.9 shows these locations schematically that are represented in the FE model in AUTODYN.The bare TNT charges are only considered in the analyses while the resulted fragments generated from the car body are not considered in this study.

Table 3Coordinates of gauges location represented in AUTODYN.

Fig.9.TNT charge and virutal pressure gauges coordinates.

The(X)coordinate represents the level from the ground surface which ranges from(0-2)m.The(Y)coordinate represents the exposed width which ranges from(-3 to 3)m from the detonation origin point.The(Z)coordinate represents standoff distance and its range from(5-8)m from the detonation origin point which is close to the surrounding critical area that bounds the targeted structure and possible human guards’existence as shown schematically in Fig.9.

5.Mathematical formulation of the mitigation percent criteria

The minimum pressure,that causes injuries,is found to be 0.034 MPa which is the primary injury threshold of eardrum rupture according to Samali et al.[31].The impulse is defined as the area under the curve for the overpressure-time history.In the current research,the datum for comparisons is defined as the minimum pressure that causes injuries(0.034 MPa).Consequently,the impulse is calculated as the area highlighted by gray color in the following curves.The pressure gauges’reading for no mitigation system is assigned as the reference that has zero percent of mitigation,however,any mitigation percent are taken as a ratio of that references.

Mitigation percent criteria have been developed to evaluate the injury impulse percentage for both systems.The trapezoidal rule is applied to numerically calculate the impulse(area under the overpressure-time curves).Let time subinterval betk∈[0，tN]such thatt0=0?t1?….?tN-1?tN,(tNis the time at which pressure load decay)andΔt=tk+1-tkthen:

where.

Then,the mitigation percent criteria which are used to compare between the BCMS and CTMS systems are calculated using equations(4)and(5)respectively.The outcomes of these equations are plotted in Fig.12,Fig.15,and Fig.18.

6.Results

6.1.Results for 50 kg TNT charge

Fig.10 and Fig.11 represent gauges’readings for the overpressure-time history of the car boot charge(50 kg TNT)for standoff distance 5 m and 8 m respectively.Fig.12 illustrates the percentages of blast impulse mitigation for both BCMS and CTMS.For the BCMS,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=5 m is found to be 63.39%at G#4.However,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=8 m is found to be 53.19% at G#5.

As a comparison,the readings of the pressure gauges’behind the proposed CTMS system do not exceed the datum pressure for both standoff distances.This means that CTMS satisfies 100%mitigation with complete protection for humans and the targeted structure behind,in the critical zone monitored by the assigned pressure gauges.A detailed explanation for such phenomena will follow in the discussion section supported by illustrative diagrams.

6.2.Results for 100 kg TNT charge

Fig.13 and Fig.14 represent gauges’readings for the overpressure-time history of the small utility/pickup charge(100 kg TNT)for standoff distance 5 m and 8 m respectively.Fig.15 illustrates the percentages of blast impulse mitigation for both BCMS and CTMS.For the BCMS,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=5 m is found to be 53.78% at G#4.However,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=8 m is found to be 28.78% at G#5.

As a comparison,the readings of the pressure gauges’behind the proposed CTMS slightly exceed the datum pressure for both standoff distances.This means that CTMS satisfies 99.5%mitigation on average with almost complete protection for humans and the targeted structure behind,in the critical zone monitored by the assigned pressure gauges.A detailed explanation for such phenomena will follow in the discussion section supported by illustrative diagrams.

Fig.10.The overpressure-time history of 50 kg TNT at standoff distance Z=5 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.11.The overpressure-time history of 50 kg TNT at standoff distance Z=8 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.12.Relative impulse mitigation percentage for 50 kg TNT charge for both BCMS and CTMS.

6.3.Results for 500 kg TNT charge

Fig.16 and Fig.17 represent gauges’readings for the overpressure-time history of the large utility/pickup charge(500 kg TNT)for standoff distance 5 m and 8 m respectively.Fig.18 illustrates the percentages of blast impulse mitigation for both BCMS and CTMS.For the BCMS,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=5 m is found to be 16.29% at G#4.However,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=8 m is found to be 18.82% at G#5.

Fig.13.The overpressure-time history of 100 kg TNT at standoff distance Z=5 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.14.The overpressure-time history of 100 kg TNT at standoff distance Z=8 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.15.Relative impulse mitigation percentage for 100 kg TNT charge for both BCMS and CTMS.

The results of the parametric studies carried out on the BCMS,for the three threat levels(50,100 and 500 kg TNT),showed that there is a significant difference in the blast wave mitigation percentage between the ground and mid-height levels of the barrier at the same standoff distance(8 m),readings of G#2 and G#5 respectively.These readings indicate that the achieved mitigation does not satisfy acceptable protection for the possible human guards’existence at that distance.

On her way out to the plane, which was still refueling, Dana saw the employee who had initially5 ignored the old man. The employee said, You re lucky the plane didn t leave without you.

As a comparison,the readings of the pressure gauges’behind the proposed CTMS are slightly higher than the datum pressure for both standoff distances.The minimum mitigation percentage calculated based on the readings of gauges at standoff distance=5 m is found to be 91.36% at G#4.However,the minimum mitigation percentage calculated based on the readings of gauges at standoff distance=8 m is found to be 92.14%at G#5.Even though,this means that CTMS satisfies high protection for humans and the targeted structure behind,in the critical zone monitored by the assigned pressure gauges.

The results of the parametric studies carried out on the BCMS,for the three threat levels(50,100 and 500 kg TNT),showed that there is a significant difference in the blast wave mitigation percentage between the ground and mid-height levels of the barrier at the same standoff distance(8 m),readings of G#2 and G#5 respectively.These readings indicate that the achieved mitigation does not satisfy acceptable protection for the possible human guards’existence at that distance.A detailed explanation for such phenomena will follow in the discussion section supported by illustrative diagrams.

7.Discussion

The results of the numerical simulation for the charge(100 kg TNT)are selected for the discussion of the interaction between the blast waves and the proposed BCMS and CTMS.Fig.19 illustrates the shock wave propagation of the BCMS and its distribution represented by contour lines chart with an adapted scale to allow relaxed examination.The blast wave starts its propagation from the center of the detonation point towards BCMS and targeted structure.Fig.19a shows the pressure contours at timeT=1.5 ms at which the front wave exceeds the first row and reaches the second row for the BCMS by dissipation throw the gap between the barriers.

The blast waves are divided into incident waves,diffracted waves,and dissipated waves that have succeeded to go throw the fence and wasted behind it with different directions at timeT=2.5 ms andT=3.5 ms as shown in Figs.19b and 19c respectively.The system satisfies an average mitigation percentage equal to 55.83% at 5 m standoff distance(G#1 and G#4),this percentage increases as the gauge position moves to the side(G#3 and G#6).The diffracted and dissipated waves re-accumulated behind the BCMS,at timeT=8 ms,and reach the gauges at 8 m standoff distance,causing a relative considerable decrease of the blast wave mitigation percentage,especially at the system mid-height level,the position of G#5,as shown in Fig.19d.This interaction explains the reason that BCMS doesn’t achieve acceptable protection especially at 8 m standoff distance.

Fig.16.The overpressure-time history of 500 kg TNT at standoff distance Z=5 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.17.The overpressure-time history of 500 kg TNT at standoff distance Z=8 m(a)no mitigation(b)BCMS(c)CTMS.

Fig.18.Relative impulse mitigation percentage for 500 kg TNT charge for both BCMS and CTMS.

Fig.20 shows the interaction between the blast wave and the CTMS which goes through three stages:

Stage I(wave collection):The start of the shock wave propagates omnidirectional towards the front hole of the CTMS that has 3D circular shape to collect the most of the wave different directions while the inlet tapered prism is adapted to be inclined to achieve minimum losses during the entrance of the ground-reflected wave(Fig.5).

Stage II(wave interaction):this stage is influenced by the tube action that forces the collected wave to move vertically up or down through the lower and upper traps(less impeding directions).These traps delay the front wave while the remaining incident waves continue propagating uniformly around the CTMS so that the lower trap is passively blocking the front wave.

Fig.19.The shock wave propagation of the BCMS and its distribution.

Fig.20.Blast waves manipulation scenario through CTMS.

Fig.21 illustrates the shock wave propagation of the CTMS and its distribution represented by contour lines chart with an adapted scale.The shock wave originated from the center of the explosion,propagating uniformly in an omnidirectional way.Fig.21a shows the pressure contours at timeT=1 ms.The first interaction of the shock wave occurs at the inlet tapered prism.It leads the blast wave to the tube atT=2 ms,which gives two possible directions:up or down as shown in Fig.21b.At timeT=5.5 ms,the front waves flow through the lower and upper trap as shown in Fig.21c.The upper trap is allowing the wave to exhaust throughout the evacuation vertical windows minimizing the influence in the top cover of the CTMS.At timeT=8.5 ms,a region of suction pressure is generated surrounding the detonation source that needs to be compensated as shown in Fig.21d.The latest action results in a significant decrease in the blast waves that pass the CTMS and lead to an increased blast wave mitigation percentage.

The pressure magnitude of the diffracted wave flows beyond the CTMS has a small value compared to its counterpart in the BCMS case,since the dissipated wave is superimposed with the diffracted wave as shown in Figs.19d and 21d respectively.

8.Conclusions

This paper is undertaken to propose a novel blast wave mitigation system and evaluate its performance.The baseline conventional mitigation system BCMS is developed through the implementation of the state of the art in this field to work as a reference system.The novel CTMS is developed to work as a protection fence for buildings that may be targeted by possible terrorist attacks.Its components are integrated to control and direct the blast waves through the planned pattern that cause selfdestruction and significantly minimize its influence on the targeted structure and human guards’existence behind.The CTMS conceptual design procedure included numerous parametric studies to reach the optimal configuration that is succeeded to achieve the objective.The relative performance of the proposed CTMS compared to BCMS is measured through numerical simulation.The main conclusion points are summarized below:

1.The proposed novel CTMS satisfies high performance in mitigating the generated blast waves from charges weight up to 100 kg TNT at standoff distances(5 and 8 m).It satisfies 98.7%mitigation with almost complete protection for humans and structure behind,in the critical zone monitored by the assigned pressure gauges.However,in relative comparison,the BCMS satisfies 53.78%mitigation for 5 ms standoff distance and 28.7%for 8 ms standoff distance.

Fig.21.The shock wave propagation of the CTMS and its distribution.

2.The proposed novel CTMS proved to satisfy high performance in mitigating the generated blast waves from charges weight up to 500 kg TNT at standoff distances(5 and 8 m).It mitigates at least 94% of the blast waves,which means that only 6% of that blast impulse is considered as the applied load on the targeted structure.Such success can be attributed to the conceptual design and geometrical configurations of the components.

3.The performance of the BCMS is decaying with the increase of the charge weight.For 500 kg TNT.It mitigates 16.29% at 5 ms standoff distance and 18.82%at 8 ms standoff distance.This poor performance is in contradiction with the CTMS performance for the same charge and standoff distances.

4.The weight/line meter of the CTMS is 18% less than that of the BCMS,although its mitigation performance is much better than that of BCMS.

5.The assignment of pressure gauges distributed in 3D during current research gives a clear picture of the influence of the mitigation systems on blast waves mitigation in regions adjacent to the center of the explosion at its sides(G#3 and G#6).6.The use of the developed mathematical formulation of the mitigation percent criteria succeeded in precisely calculating the impulse percentage for both systems BCMS and CTMS for relative comparisons.