Large-scale experiments on the mechanics of pyroclastic flows: Design, engineering, and first...

Large-scale experiments on the mechanics of pyroclastic flows: Design,

engineering, and first results

Pierfrancesco Dellino,1 Bernd Zimanowski,2 Ralf Buttner,2 Luigi La Volpe,1

Daniela Mele,1 and Roberto Sulpizio1

Received 27 January 2006; revised 22 December 2006; accepted 26 December 2006; published 10 April 2007.

[1] A newly designed apparatus for experimental studies of pyroclastic flows consists of acylindrical conduit that is filled with samples of natural volcanic products (tephra).Blowing nozzles in the base plate of the conduit are connected to a volume of highlypressurized gas. Opening of fast solenoid valves results in impulse-like coupling of thereleased gas to the sample. The system was designed so that the range of mechanicalenergy transferred to the particle mass in the conduit reflects the mechanical energyobserved and measured during fragmentation experiments with melts of similarcomposition. Depending on the specific mechanical energy (SME) of the system, whichresults from DPV/m, where DP is gas overpressure (i.e., pressure > atmospheric), V is gasvolume, andm is sample mass, different behaviors are observed. If SME > 2.6 kJ/kg, a diluteplume develops, and particles are sedimented by fallout exclusively. If SME < 1.5 kJ/kg,the exiting column collapses and develops a shear current similar to a pyroclastic flow. TheReynolds number of the shear currents is >106, implying that flows are fully turbulentand that particle coupling to gas turbulence of natural pyroclastic flows is replicated by theexperiments. The measured shear current velocities are proportional to the impact massflow rate, i.e., the product of mixture density and impact velocity. Experimental data andgrain-size analysis of the produced particle deposits suggest that the scale of the experimentis large enough to reproduce the transport dynamics of natural pyroclastic flows.

Citation: Dellino, P., B. Zimanowski, R. Buttner, L. La Volpe, D. Mele, and R. Sulpizio (2007), Large-scale experiments on the

mechanics of pyroclastic flows: Design, engineering, and first results, J. Geophys. Res., 112, B04202, doi:10.1029/2006JB004313.

1. Introduction

[2] Pyroclastic flows, also known as pyroclastic densitycurrents, are the most hazardous events of explosive volca-nism [Cas and Wright, 1987; Carey, 1991; Branney andKokelaar, 2002]. They form when, after eruption from thecrater, a mixture of gas and solid particles collapses back tothe ground under the effect of gravity (or during gravity-driven collapse of domes). Other mechanisms of pyroclasticflow generation include the lateral expansion, by overpres-sured jets, of the gas-particle mixture issuing from thecrater. Both triggering mechanisms generate a current thatmoves at high velocity across the ground, usually along thevolcano slopes [Burgisser and Bergantz, 2002; Branney andKokelaar, 2002].[3] Pyroclastic flow deposits are abundant in the geolog-

ical record of explosive volcanoes. The high devastationpotential of these flows has been demonstrated by historiceruptions, and they threaten densely populated areas sur-rounding active volcanoes.

[4] Pyroclastic flows are subdivided into two categories:concentrated and dilute ones [Cas and Wright, 1987;Wohletz, 1998]. Such categories can be generically consi-dered as analogous to the concentrated and disperse flows ofmultiphase physics [Crowe et al., 1998]. Recently, however,many researchers have pointed out that this schematicdistinction is not strictly applicable to the natural case. Inthe time-space evolution of a pyroclastic flow, the twocategories can coexist, frequently leading to the formationof a thin concentrated current near the crater, which grad-ually transforms with runout distance into a thick, fullyturbulent, dilute flow [Burgisser and Bergantz, 2002;Dellinoet al., 2004a; Vazquez and Ort, 2006]. Other researchers haveinferred instead that currents become less concentrateddowncurrent [Sparks, 1976; Wilson, 1980].[5] Because of the very hostile nature of pyroclastic flows,

the details of their mechanical behavior cannot be capturedduring actual eruptions, and our models are based either onthe deposits of past eruptions or on numerical simulations[Valentine and Wohletz, 1989; Neri and Macedonio, 1996;Neri et al., 2003a; Scott et al., 1996; Wadge et al., 1998;Miyabuchi, 1999; Nield and Woods, 2003; Dellino et al.,2004b; Saucedo et al., 2004; Cole et al., 2005; Sulpizio et al.,2007]. To date, these models are difficult to experimentallyvalidate because the very few laboratory experiments per-formed on pyroclastic flows do not reach a scale comparableto the natural phenomenon [Burgisser et al., 2005].

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B04202, doi:10.1029/2006JB004313, 2007

1Centro Interdipartimentale di Ricerca sul rischio sismico e vulcanico,Dipartimento Geomineralogico Universita di Bari, Bari, Italy.

2Physikalisch Vulkanologisches Labor, Universitat Wurzburg, Wurzburg,Germany.

Copyright 2007 by the American Geophysical Union.0148-0227/07/2006JB004313$09.00

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[6] In order to cover this gap, we designed and engineeredan experimental facility for the generation of large-scale,multiphase, gravity-driven currents. The currents generatedduring the experiments are large enough to allow both scalingof the main macroscopic fluid-dynamic parameters of naturalpyroclastic flows and sampling of their deposits.[7] The material used for experiments was collected from

pyroclastic flow deposits exposed at Somma-Vesuviusvolcano in order to allow a quantitative comparison betweenthe material deposited by experiments and natural pyroclasticflows.[8] One of the main objectives of the experimental facility

is to study the role of turbulence in the transportation of solidparticles in the current. For this reason, the experiment wasdesigned with the idea of producing flows reaching the fullyturbulent regime, i.e., a Reynolds number in excess of 104.On the other hand, we wanted the input conditions, especiallythe mechanical energy applied to the particle mass prior toeruption, to be adjusted in such way that they reflect theresults of fragmentation experiments of melts with similarcompositions [Zimanowski et al., 1997; Buttner et al., 2002,2006]. In these studies, the mechanical energy of the exitingparticle system is connected to the fragmentation energyconsumed during the production of the particles.[9] There is increasing evidence, both from the strati-

graphic analysis of past eruptions and from observation ofrecent explosive eruptions [Christiansen and Peterson,1981; Rowley et al., 1981; Freundt and Schmincke, 1986;Cough and Sohn, 1990; Scott et al., 1996; Cole et al., 1998;Sulpizio et al., 2005, 2007], that pyroclastic flows arefrequently generated during discrete impulsive events andnot only by the continuous fountaining of a Plinian column.The experiments here described are particularly devoted tounderstanding the inception and development of pyroclasticflows under such impulsive conditions by means of the fastapplication of a finite stress to a finite mass of pyroclasticparticles via expansion of compressed gas.[10] In the experiments, gas and particles are at room

temperature. The effect of thermal convection by heat transferfrom the gas-particle mixture to surrounding atmosphere,which is an important process for the generation of thermalplumes in explosive volcanic eruptions, is therefore not consi-dered here. For the first experiments, we wanted to check therole of the mechanical coupling of the gas flow to the particles.In the future, the role of the additional thermal effect will beverified by adding a heating system to the base of the conduit.[11] The first experimental runs were performed with the

primary objective of testing the functionality of the expe-rimental facility and to check whether the design was able toproduce currents of scales and characteristics that wouldprovide insight into the mechanics of natural flows. To oursatisfaction, additional valuable information on the condi-tion of inception of pyroclastic flows versus formation ofvertical plumes was also gained.

2. Design and Engineering

[12] The experimental apparatus (Figure 1) consists of (1)a pack of 16 interconnected bottles of compressed gas (gasstorage compartment, Figure 2a); (2) a high-pressure sectionconsisting of nine steel-reinforced rubber hoses each 30 mlong, with 8-mm internal diameter; (3) a rapid-compression

section consisting of nine steel-reinforced rubber hoses each1.5 m long, with 8-mm internal diameter; and (4) a low-pressure section consisting of a 2.2-m-long stainless-steelconduit, which is made up of a stack of four connectedsectors, each 0.55 m high with a 0.6-m internal diameter,mounted on a massive base plate.[13] The gas bottles are coupled to the high-pressure stage

via two valves and a hub, in line with manometers (Figure 2a)that control the reservoir pressure and the pressure in thehigh-pressure section. High-speed solenoid valves connectthe high-pressure section via a second hub to the rapid-compression section, where the driving pressure of thesystem is monitored by a transducer (Kistler14073A100).The pressure transducer is mounted at the exit of the secondhub after the solenoid valves. Finally, the short hoses areconnected to nine blow nozzles in the base plate of the low-pressure section. Blow nozzles cover 1% of the total area ofthe base plate. The two hubs and the conduit are eachmounted on steel standings and anchored to the ground.[14] Because of the large dimension of the design and

also for safety reasons, the experiment must be performedoutdoors with an ample free space around the conduit. Theexperimental facility was therefore engineered and built ina flat area of a clay quarry, which serves a brick factorylocated near Bari (Italy). Machinery operating in the quarrywas used for (1) landscaping the ground and digging thebunker where the computers and the gas-storage compart-ment were located; (2) digging the pit where the secondhub, the rapid-compression section, and the standing of theconduit were located (Figure 2c); and (3) preparing thetunnel through which the long hoses of the high-pressuresection and all the wiring were channeled.[15] The pyroclastic material was placed into the conduit

and rested directly on the base plate. The experiment wasfully computerized: the opening of each individual solenoid,the trigger signal to the data acquisition systems, the start-upand shut down of high-speed cameras, and optical andacoustic warning signals were controlled with a reprodu-cibility of 200 ms. For the initial experiments, a four-channeldata-recording system was used at a resolution of 16 bit andat 1-kHz sampling rate, capturing the trigger signal from thecontrol computer (giving the opening time of the solenoids),the driving pressure, and the signals of two geophonesmounted at distances of 1 and 20 m from the conduit.The geophones measured the occurrence and timing of theimpact of the material collapsing from the eruptive flow,which is the process that triggers the pyroclastic flows.Three digital video cameras, positioned at different distan-ces and viewing angles, were used to capture video sequen-ces of the experiments. The ground was rendered horizontaland flat, and a set of blue plastic sheets was positioned allaround the conduit (Figure 2d) to allow sampling of thepyroclastic material deposited after the passage of theexperimental flows. Preliminary tests performed with com-pressed air, oxygen, and nitrogen yielded identical results,so we decided to use nitrogen because its inert nature shouldinhibit oxidation of the metallic parts of the apparatus.

3. Material Characteristics

[16] The material used in the experiments comes directlyfrom pyroclastic deposits of Somma-Vesuvius. In particular,

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it was sampled (Figure 3) from a proximal layer of ash andlapilli deposited from a pyroclastic flow of the Mercatoeruption [Santacroce, 1987]. The grain size of the materialis shown in Figure 4a. Components include highly vesicular

pumice, dense juvenile particles, lithics, and loose crystals.Only the coarse fraction (>2 cm), which represents a minorcoarse tail of the bulk deposit, was discarded in the field byhand sieving.

Figure 1. Sketch design of the various parts of the experimental facility.

Figure 2. Photos showing the various parts of the experimental facility. (a) General photograph of thebunker where the gas-storage compartment, the manometers box, the first hub, and computers arelocated. (b) General photograph of the pit where the second hub, solenoid valves, and the conduit arelocated. The base plate of the conduit, with tennis balls positioned on top of nozzles for scale, is alsoshown. (c) General view of the channel where the steel-reinforced rubber hoses and the wires connectingthe pit and the bunker are positioned. (e) General view of the conduit. The blue plastic sheets forsampling and the near-field geophone are visible.


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[17] Yellow tennis balls were added on top of the pyro-clastic material filling the experimental conduit. Theymimic the behavior of coarse pumice pyroclasts and areeasier than natural coarse particles to track individuallyduring analysis of the video footage. They allow thereforethe observation of the decoupling of higher-inertia particlesfrom the main front of the gas-particle mixture exiting theconduit.

4. Working Phases of the Experiments

[18] The experiment starts by opening the valves thatconnect the gas-storage compartment to the high-pressuresection. By means of manometers, the pressure of the gas inthe high-pressure section is controlled and regulated up tothe desired operational value. When this value is reached,the valves are closed, and a charge of 14 liters of com-pressed gas is loaded into the high-pressure section.[19] After closure of the valves, the countdown routine of

the control computer is started, the acquisition computerstarts recording signals from the sensors, and the camerasstart recording the video footage. Upon firing of the trigger,the solenoid valves open, and a gas flow is established over1 ms between the high-pressure section and the compressionsection, which results in mechanical coupling of the pres-surized gas to the pyroclastic material filling the conduit.[20] The pressure quickly reaches a peak value in the

compression section, then it stays quite constant for a briefperiod of time before a slow decompression phase is regis-tered. In this phase, the pyroclastic material starts moving asa granular mass while it is accelerated inside the conduit, thenmixes with gas and overlying air, and eventually is expelledas a two-phase mixture out of the conduit.[21] Depending on the balance between the operational

pressure of the compressed-gas volume in the high-pressure

section and the mass of pyroclastic material, differentprocesses that closely resemble actual eruptive processesare observed at the conduit exit. The ratio of DPV/m, whereDP is the initial gas overpressure (i.e., pressure > atmo-spheric) in the high-pressure section, V is the volume of thegas charge in the high-pressure section, and m is the massof pyroclastic material, can be expressed as the specificmechanical energy (SME) of the system. It represents thepotential of the gas to move the pyroclastic material. WhenSME is lower than 1.5 kJ/kg, the ‘‘eruptive’’ flow evolvesas a dense (high particle concentration) column. A massivecollapse of a substantial part of the gas-particle columnoccurs (Figure 5a), and on impact with the ground, thecollapsing material produces a shear flow (a fluid flowmoving normally to a solid boundary [Furbish, 1997]),similar to a pyroclastic flow. When the SME is higher thanabout 2.6 kJ/kg, the eruptive flow evolves as a low-density(low particle concentration) vertical plume (Figure 5b), nomassive collapse occurs, and the pyroclastic particles settleindividually from the plume, reflecting a process similar toa volcanic plume, even though no heat transfer is involvedin the experiment. When SME is between 1.5 and 2.6 kJ/kg,the gas-particle mixture develops a transitional column(Figure 5c). From the upper and least dense part of thecolumn, fallout occurs, and from the basal, more concen-trated, part of the column, a partial collapse and a smallshear current develop. This transitional style is similar toeruption dynamics reported in the recent volcanologicalliterature [Fierstein et al., 1997; Rosi et al., 2001; Sableet al., 2006] and accounts for the contemporaneous forma-tion of fallout and pyroclastic flow deposits in eruptivestratigraphic sequences. This process, which seems quitecommon in explosive eruptions, is capable of formingdeposits in which there is an interplay, at sedimentation,between particles settling from the column and the material

Figure 3. Photo showing the pyroclastic flow deposit, enclosed by solid lines, from which thepyroclastic material was sampled. The layer is 2 m thick.


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Figure 4. Grain-size histograms and statistics of the pyroclastic flow deposit sampled in the field and ofthe material deposited from the passage of the shear current of a collapsing column run. Grain-sizedistributions were obtained by combining sieve data of the material coarser than 3f with Coultermultisizer data of finer sizes. (a) Bulk material sampled in the field. Materials were collected at thefollowing distances from the conduit base: (b) at 0.5 m, (c) at 1 m, (d) at 1.5 m, (e) at 2 m, (f) at 2.5 m, (g) at3 m, (h) at 4 m, (i) at 5 m, and (l) at 5.5 m. In the insets, velocity, thickness, and time of passage ofshear current over the samples’ location are reported. F = �log2 d, where d is particle diameter in mm;Mdf = median size; sf = sorting; Dist. = distance from the conduit; Fl. Thick. = flow thickness;Fl. Vel. = flow velocity; Time = time after triggering.


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transported in suspension by the lateral current [Dellino et al.,2004a].[22] Two end-member situations are proposed in the

following sections, based on two typical experiments thatformed a collapsing column and a plume. Transitionalcolumns simply represent intermediate situations and willnot be described in further detail. We analyze data from thesensors and geophone signals and combine them withquantitative analysis of the video footage.[23] Velocity was determined by calculation of the dis-

tance traveled over time by the flow front. Data wereobtained, on a frame-by-frame basis, by quantitative analy-sis of the video sequences shot by digital video cameras.The precision of distance measurements is related to frameresolution and pixel’s physical dimension, which in our caseis of the order of 1 cm, so the absolute error of distance is ofthe order of 1 cm. Time has a precision related to the internaldigital clock of the video cameras, which error is negligible

when compared to that of distance. So velocity measurementshave an absolute error of the order of 1 cm/s.[24] Density data were obtained by the ratio of particle

weight and flow volume. For flow volume measurements,synchronous frames shot by three video cameras, positionedat different viewing angles, were considered. For eachframe, the flow outline was traced, the three outlines fromthe synchronous frames were crossed, and a solid model ofthe flow was generated by three-dimensional modelingsoftware. Volume of the generated solid was finally calcu-lated. For testing precision, the same procedure was repeatedon natural organic objects with shapes similar to the flow(pears, mushrooms), of which the volume was measured bydisplacement in a liquid. Relative error was of the order of1%. Weight of the pyroclastic material was measured by aprecision balance, which has a negligible error compared tothat of volume. So flow density data have a relative error ofthe order of 1%.

Figure 5. Photographs showing the various types of columns developed in the experimental runs.(a) Collapsing column. Frame shot 2.1 s after triggering. For collapse evolution, see Figure 6. (b) Plume.Frame shot 1.1 s after triggering. For plume evolution, see Figure 8. (c) Transitional column. Frame shot1.2 s after triggering. Wind from left.

Table 1. Main Parameters of the Experimental Runsa

Type ofColumn Run

OpPr,bars

Load,kg

SME,kJ/kg

PkPr,bars

PkFc,kN

NgFc,kN

DF,kN

ExVel,m/s

ExDen,kg/m3

PMFR,kg/m2s

ColHt,m

DMH,kg/m3

Collapsing 1 130 120 1.5 92 4.16 1.18 2.98 7.4 177 1300 5.2 30.4Collapsing 2 165 180 1.3 114 5.16 1.76 3.40 9.4 245 2300 8.2 22.8Collapsing 3 160 190 1.2 108 4.88 1.86 3.02 9 269 2400 7.6 17Transitional 4 175 130 1.9 131 5.92 1.27 4.65 12 136 1600 9.2 4.8Transitional 5 165 130 1.8 120 5.42 1.27 4.15 8.7 172 1400 6.7 6.3Transitional 6 158 100 2.2 107 4.84 0.98 3.86 11.4 143 1600 7.4 4Plume 7 150 80 2.6 105 4.75 0.78 3.97 10.4 109 1100 9.2 3.5Plume 8 150 50 4.2 103 4.65 0.49 4.16 11.2 61 683 10 3

aRun, Run number; OpPr, operational pressure of the compressed gas charge; Load, weight of the pyroclastic material filling the conduit; SME, specificwork of the system; PkPr, peak pressure reached in the compression stage; PkFc, peak force applied to the pyroclastic material at the base of the conduit;NgFc, negative force exerted by gravity on the pyroclastic material at the conduit base;DF, force differential between peak force and negative force; ExVel,peak exit velocity of the gas-particle mixture; ExDen, average density of the column at conduit exit; PMFR, peak mass flow rate of the gas-particle mixtureat conduit exit; ColHt, maximum height reached by the column; DMH, average density of the column at maximum column height.


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Table 2. Additional Parameters of Collapsing Column Runsa

Run CollDen, kg/m3 BHMC, m ImpVel, m/s IFR, kg/m2s PVSF, m/s SFMT, m

1 30.4 2.2 4 122 4.5 22 22.8 5 10.6 242 6 2.83 17 2.2 4.8 82 3.2 2.1aRun, Run number; CollDen, average column density at collapse; BHMC, base height of the main collapse; ImpVel, velocity

of the collapsing mixture at impact with the ground; IFR, impact mass flow rate; PVSF, peak velocity of the shear flow; SFMT,maximum thickness of the shear flow.

Figure 6. Evolution with time of a typical collapsing column run (run 2 of Tables 1 and 2). (a) Diagramshowing data recorded by sensors during a typical collapsing column run. One volt corresponds to100 bars in the pressure sensor data curve. (b) Formation of a vertical gas-particle mixture column.Frame shot 0.4 s after triggering. (c) Expansion of a column over the vent. Frame shot 0.9 s after triggering.(d) Column collapse. Frame shot 2.3 s after triggering. (e) Formation of a shear current. Frame shot 2.8 safter triggering. (f) Lateral development of a shear current and lobe formation. Frame shot 4.1 s aftertriggering.


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[25] Pressure was measured by high-precision manometersand sensors, which have an absolute error less than 1 bar.[26] The main data of the various runs are reported in

Tables 1 and 2.

5. Typical Collapsing Column

[27] In this experiment (see run 2 of Tables 1 and 2), theoperational pressure in the high-pressure section was 165 bars,and the load of pyroclastic material filling the conduit was180 kg. The specific mechanical energy of the system was1.3 kJ/kg. About 0.07 s after the trigger that opened thesolenoid valves, the pressure inside the compression sectionincreased up to a peak of about 114 bars (Figure 6a). Duringthis compression phase, the gas pressure was impulsivelycoupled, through the injectors of the base plate, to thepyroclastic material, which reacted to the applied stress asa solid body. This was caused by the high inertia and lowpermeability of the granular material filling the conduit. Bymultiplying the peak pressure across the total area of thehoses’ section, a value of about 5.16 kN was obtained,which represents the peak force that, in this phase of theexperiment, was applied to the solid material at the base ofthe conduit. This force overcame the negative force thatgravity exerted on the solid load, which was about 1.76 kN,and a positive force directed upward resulted from the forcedifferential, which was about 3.4 kN. Driven by this positiveforce, the pyroclastic material started to accelerate inside theconduit. For about 0.08 s, the pressure stayed quite constantand formed a plateau in the diagram of Figure 6a, withvalues oscillating around an average of 113 bars. In thisequilibrium condition, the granular material moved, allowingfor incipient gas expansion at the base of the conduit, butthe expansion was compensated by the overpressure stillcoming from the high-pressure stage. At the end of theplateau, about 0.14 s after the trigger, the pressure in thecompression section started to decrease, and an expansionphase started in which the gas was free to stream throughthe solid particles and expand the granular material. About0.27 s after triggering, the resulting gas-particle mixture wasexpelled from the vent (Figure 6b) as a coherent flow. Theflowwas as large as the conduit and had an almost planar front,meaning that the flow of gas and particles inside the conduitwas well homogenized and moving as a highly concentratedgranular fluid. The peak exit velocity, as measured in the firstmeter of the column above the vent, was about 9.4 m/s. Theaverage density of the gas-particlemixture, by consideration ofthe total weight of the pyroclastic material and the total volumeof the cylindrical flow, was about 245 kg/m3. These data givea peak mass flow rate of about 2300 kg/m2 s, which whennormalized to the area of an actual volcanic conduit witha radius of around 100 m, as generally used in volcanology[Carey and Sigurdsson, 1989; Papale, 1999; Neri et al.,2003b], gives a figure for a relative mass eruption rate ofabout 7 � 107 kg/s; this compares pretty well with valuesproposed in the literature for Plinian columns [Jaupart andAllegre, 1991; Kaminski and Jaupart, 2001]. After the gas-particle mixture exited the vent, the overpressure at the base ofthe conduit allowed further expansion and ascent of thecolumn over the conduit (Figure 6c). The tennis balls didnot decouple, at this stage, from the gas-particle mixture,suggesting that even the high-inertia particles remainedconfined to the bulk of the highly concentrated flow. About

2 s after triggering, the column stopped its expansion, andthe gas-particle mixture started to collapse under its ownweight (Figure 6d).[28] At the inception of collapse, the average density of

the gas-particle mixture was about 22.8 kg/m3, as calculatedby measuring the total volume of the column and the load ofparticles in the gas-particle mixture. The base-height fromwhich the main collapse occurred was about 5 m above theground. When, about 2.5 s after triggering, the collapsingmixture hit the ground, it had a vertical downward velocityof about 10.6 m/s, which implies an impact mass flow rateof about 242 kg/m2 s. Shear flow development is verysensitive to impact mass flow rate, and when compared withother collapsing runs, it seems proportional to the peaklateral velocity reached by the shear current (see Table 2).[29] Upon hitting the ground, the dense gas-particle mix-

ture produced an impact that was clearly registered by thenear-field geophone (Figure 6a) and was replicated, withlower amplitude, after a short time delay, by the signal of thefar-field geophone. The impact led to a sudden conversion ofthe vertically directed normal stress acting on the flat groundinto a shear stress, which produced a fast horizontal defor-mation of the collapsing mixture and the consequent devel-opment of a lateral shear flow of gas and particles. The shearflow rapidly acquired velocity and turbulence (Figure 6e),and in doing so, it was able to entrain particles and expandwith time and distance. This clearly resembles the develop-ment and establishment of a turbulent boundary layer shearflow as described in classic fluid dynamics [Schlichting, 1979;Furbish, 1997; Cebeci and Cousteix, 1999].[30] Analysis of the footage from the three video cameras

revealed that the shear current expanded in all directionsfrom the base of the conduit and was made up of lobes(Figure 6f) that had different intensities caused by theasymmetries of the impact with respect to the conduit base.Impact asymmetries were randomly distributed among runsand do not seem related to ground morphology or winddirection.[31] The peak lateral velocity was about 6 m/s at a distance

of 1.5–2 m from the conduit base, and the thickness of theshear current was about 1.1 m. At the location of peak velo-city, the material deposited from the current was collectedfrom an area of 0.1 m2, formed by a rectangle 0.5� 0.2 m, ofwhich the large side was positioned perpendicular to flowdirection. The weight of the collected material was 0.3 kg,and the average density of the pyroclastic particles, asmeasured in the laboratory by means of a standard Gay-Lussac picnometer, was 1630 kg/m3. These data indicate anaverage deposit thickness of about 1.9 mm. By assuming thatthe ratio of deposit thickness and flow thickness representsthe average concentration of the solid material transported inthe current and by assuming that the current can be approx-imated as a density-modified flow where the particles arefully held in suspension and increase the mean density of thefluid, the average flow density can be calculated with theformula rf = C(rs � rg) + rg, where rf is flow density, rg isgas density, rs is particle density, and C is particle volumeconcentration. For this run, rf = 4 kg/m3. This clearlyrepresents a rough estimation that does not take into accountpossible flow-thickness fluctuations during the passage of thecurrent. An alternative method for estimating flow density isby simply dividing the total flow volume by the total load of


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particles. Also, in this case, the average flow density is about4 kg/m3. So we consider this value as a reasonable approxi-mation ofmixture density.With these data, the flowReynoldsnumber can be roughly calculated using the formula rfud/m,where u is flow velocity and m is gas viscosity. The mixtureviscosity of a gas-particle flow can be quite different fromthat of pure gas, especially for highly concentrated currents.In our case, anyway, particle concentration is low, about0.17%. For such low concentrations, the formula of Einstein[1906], mm = m(1 + 2.5C), where mm is mixture viscosity, canbe used. An alternative formula, which is valid also forhigher concentrations, is that proposed by Papale [2001] asmodified after the work of Ishii and Zuber [1979], mm =m(1 � C)�2.5. In our case, the application of both formulasresults in mixture viscosities that are less than 1% higherthan that of pure gas. By these data, the Reynolds numberof the current is about 1.4 � 106. This clearly demon-strates that the flow was fully turbulent, therefore capableof replicating the coupling of particles to the gas turbulenceof natural pyroclastic flows.[32] After a first rapid increase of flow thickness and

velocity, the shear current reached a maximum velocity valueand then gradually slowed down. At a distance of 5.5 m fromthe conduit, velocity stabilized and remained quite constantfor a distance of several meters and over a time span ofseveral seconds. In this phase of flow establishment, the flowcould be described as homogeneous. Then the flow, whileslowly thickening, started to decelerate.[33] The combined analysis of flow velocity, flow thick-

ness, and grain size of the material deposited by the shearcurrent is very useful for gaining information on the transportprocesses inside the flow. The material used for grain-sizeanalysis was sampled at increasing distance from the conduitbase (Figure 7a) in a direction opposite to the one where thepeak velocity was registered inside a lobe with relatively lessintensity and lower velocity but where sampling along flowdirection was allowed with better continuity.[34] The impact on the ground clearly produced a massive

sedimentation of particles, a fraction of which was suddenlydeposited at the point of impact (Figure 7b) with othersentrained in the shear flow. Some of the tennis balls weredeposited at the point of impact together with the massive

sediment; others followed the shear current and bounced atthe flow front. During flow propagation, it was clear that thesolid material entrained in the current was split into twosubpopulations. The coarse one was transported as bed loadin the more concentrated, basal part of the current, and thefine one was held in continuous suspension in the upper andless concentrated part of the current. The first subpopulationwas deposited just at the passage of the flow front, whereasthe second population slowly settled from the top of thecurrent in the waning stage, on top of the bed load material.This situation was also evidenced by the grain-size analysisof the material deposited from the experimental flow.[35] The material collected 0.5 m from the conduit base

corresponds to deposits at the point of impact. Lateral flowvelocity was 1.7 m/s, and thickness was 0.36 m. The grainsize (Figure 4b) is similar to that of the bulk of thepyroclastic material filling the conduit, with a median sizeabout 0.19f and sorting about 2f, where f = �log2 d and dis particle diameter in mm.[36] This means that the material deposited on impact,

because of the high concentration of the gas-particle mix-ture, did not experience any selection of grain size (sorting)in the time-integrated history spanning acceleration insidethe conduit, expulsion from the conduit, expansion in thecolumn, and final collapse. On impact, the pyroclasticmaterial that was deposited as a structureless layer representsa highly concentrated thin flow similar to what is reported fornatural pyroclastic flows near the crater [Cas and Wright,1987; Branney and Kokelaar 2002; Houghton et al., 2004;Burgisser, 2005; Vazquez and Ort, 2006].[37] The coarse fraction (coarse ash and lapilli) of the

material collected at increasing distance from the ventrepresents the bed load population deposited at the frontof the current, and its grain size is directly proportional tothe velocity of the shear current. At 1-m distance, flowvelocity was 2.8 m/s, and thickness was 0.51 m. The mediansize was about 0.91f (Figure 4c). At 1.5, 2, and 2.5 m, flowvelocity changed from 3.2 to 4 m/s, and thickness changedfrom 0.8 to 1.05 m. The median size increased, from about0.83f to about 0.51f (Figures 4d–4f), following theincrease of velocity at the flow front. At 3 m, the velocitydecreased to 3.4 m/s, and thickness was 1.2 m. The median

Figure 7. Distribution of the pyroclastic material deposited by the passage of the shear current of atypical collapsing column run. (a) General view of the dispersal area. (b) Close-up of the materialdeposited at the impact zone near the base of the conduit.


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size decreased to 0.54f (Figure 4g) accordingly. At 4 and5 m, velocity continued to drop from 2.8 to 2.2 m/s, and flowthickness increased to 1.5 and 2 m. The median deposit grainsize also decreased to about 2f and 3.5f, respectively(Figures 4h and 4i). The most distal sample, collected at5.5 m from the conduit, clearly indicates the high proportionof the continuously suspended fine-ash material. At thatdistance, velocity dropped to 1.9 m/s, and flow thicknessreached 2.3 m. The median deposit grain size decreased to4f (Figure 4l). The bimodality of the samples is quite clearfrom the histograms, and the second mode, representing thefine ash held in continuous suspension that sedimentedduring the waning stage of the current, increased in propor-tion with increasing distance. This means that the current,by expanding and decreasing its density and velocity, lostcompetence to move coarse material as bed load. These dataare in agreement with what was found in the depositsformed by actual pyroclastic density currents [Dellino et al.,2004a, 2004b] and confirm the ability of the experiments toreplicate key aspects of the transport processes of naturalpyroclastic flows.[38] At a distance of 6 m from the base of the conduit,

flow velocity was still about 1.8 m/s, and it stayed quiteconstant to a distance of 8.5 m, which was reached 5.8 safter triggering, where flow thickness was 2.8 m. In thishomogeneous phase, the bed load layer was so thin as topreclude effective sampling of its deposits or discriminationfrom deposits of the continuously suspended population.[39] About 10 s after triggering, the shear flow slowed

down significantly. The very fine material, still in continu-ous suspension, slowly settled on top of the coarse materialin ‘‘proximal’’ sites. This process lasted for several seconds,and eventually the fine material was diffused away at aconcentration that became so low as to render the gas-particle mixture almost transparent.[40] Other experimental runs that produced collapsing

columns were qualitatively similar to the one just described,with the only difference that the shear flows’ intensities werelower. It is very interesting to note that the peak lateralvelocity of the shear flow, which is very important in termsof the hazard potential of pyroclastic flows [Valentine, 1998],seems to be directly proportional to impact flow rate(see Table 2). Since the latter results from the product ofaverage flow density and velocity at impact, and since theyin turn are related to both force differential and specificmechanical energy, the impact flow rate is a function ofmany variables. It is too speculative, with the few runsperformed, to try to find a precise functional relationshipbetween impact flow rate and other parameters, but it seems,from these first results, that the base height of the collapseplays a major role. If the collapse happens just at the vent,part of the material is recycled inside the conduit, and theimpact with the ground is slow, so producing a smallerimpact flow rate. If the collapse occurs at a higher elevation,subsidence in the conduit is minor, and impact velocity ismuch higher due to the higher potential energy that the gas-particle mixture possesses at the inception of collapse.

6. Typical Plume

[41] In this experiment (see run 8 of Table 1), the opera-tional pressure in the high-pressure section was 150 bars, and

the load of pyroclastic material was 50 kg. The specificmechanical energy was 4.2 kJ/kg. About 0.07 s after trigge-ring, a peak of 103 bars was registered (Figure 8a). Theplateau lasted for 0.08 s. The gas-particle mixture exited theconduit about 0.2 s after the trigger (Figure 8b). The peakforce was about 4.65 kN, the negative force was about0.49 kN, and the force differential was 4.16 kN. The peakvelocity of the plume at the conduit’s exit was about11.2 m/s. Peak velocity and force differential were higherthan those registered for the collapsing column. This suggeststhat the peak exit velocity of the gas-particle mixture isproportional to the force differential. This proportionalityseems to hold well for the other runs, even though someadditional contribution due to friction along the conduit anddrag at the flow front should be included in a more compre-hensive analysis of the balance of forces at the conduit exit.This is beyond the scope of this paper but could helpexplaining some of the scatter in the data particularly fortransitional runs.[42] The average density of the gas-particle mixture was

about 61 kg/m3, which gives a peak mass flow rate of about683 kg/m2 s. This is lower than the mass flow rate registeredfor the collapsing column run and, when normalized to thearea of a conduit with a radius around 100 m, gives a masseruption rate about 2 � 107 kg/s, which compares well withvalues considered reasonable for sub-Plinian eruptions[Carey and Sparks, 1986; Cioni et al., 2000; Bonadonnaand Phillips, 2003]. Centimeter-sized pyroclastic particlesand also the tennis balls mostly decoupled from the bulk ofthe gas-particle mixture, meaning that the lower concentra-tion of the plume allowed coarse particles to behave accor-ding to their inertia.[43] The expansion stage lasted for about 2.2 s before the

plume reached its maximum ascent (Figures 8c and 8d). Themaximum height that the plume reached above ground levelwas about 10 m (Figure 8e), and the average density wasabout 3 kg/m3. Plume density was much lower than that ofthe collapsing column runs, and it is a major factor for theinception of collapse versus plumes.[44] After the plume stopped its ascent, particles settled

individually on the ground, which strongly resembled thefallout process from actual volcanic plumes (Figure 8f ).Tennis balls followed ballistic trajectories and, on hitting theground, bounced off into the quarry. Therefore althoughthermal effects were not included in the experiment, thesimple mechanical coupling of gas flow to the particlesreplicated the fallout and settling from a plume. The geo-phones did not register any massive localized impact on theground but did record only some very low-amplituderepeated signals; these represent the falling of individualparticles onto the ground and are visible on the diagram ofFigure 8a.

7. Final Discussion and Outlook

[45] Our new facility was designed in such a way that theresults of melt fragmentation experiments, carried by theauthors within the last 10 years [Zimanowski et al., 1997;Buttner et al., 2002, 2006], could be used as input conditionsfor the experiments presented in this paper.[46] The mechanical energy released at melt fragmentation,

which is the process that triggers explosive eruptions, is linked


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to the physical characteristics of particles produced at frag-mentation. Recently performed stress-induced brittle fragmen-tation experiments [Buttner et al., 2006] produced particlessimilar to those used for the experiments presented here. Weused therefore the range of specific mechanical energy offragmentation experiments as input conditions. Also, themethod of impulsively coupling the gas flow to the particlesystem was borrowed from that of fragmentation experiments.[47] Our new facility was able to replicate major processes

of actual volcanic eruptions, i.e., pyroclastic flows and

plumes, by using starting conditions that mimic the releaseof mechanical energy at melt fragmentation. We are there-fore confident that our results can be used for linking meltfragmentation with transportation processes of explosivevolcanic eruptions.[48] Burgisser et al. [2005] pointed out that experimental

flows produced to date did not reach the intensity necessaryfor capturing the whole scale of turbulent eddies, which areresponsible for the transportation and sedimentation ofnatural pyroclastic density currents. The comparison of

Figure 8. Evolution with time of a typical plume run (run 8 of Table 1). (a) Data recorded bysensors during a typical plume run. One volt corresponds to 100 bars in the pressure sensor data curve.(b) Formation of the gas-particle plume. Frame shot 0.3 s after triggering. (c) Vertical ascent of thecolumn over the vent. Frame shot 0.7 s after triggering. (d) Further ascent and expansion of the plume.Frame shot 1.5 s after triggering. (e) Maximum height of the plume. Frame shot 2.3 s after triggering.(f) Stop of plume ascent and start of particles fallout. Frame shot 3.2 s after triggering.


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our results with Table 1 and Figures 6, 7, and 8 of Burgisseret al., where data from previous experiments on volcanicplumes and flows are compiled, demonstrates that theexperimental facility introduced here is capable of coveringthis gap. Not only are the Reynolds numbers of ourexperimental flows in the fully turbulent regime but, sincewe used natural pyroclastic particles, the coupling ofturbulent stress to the solid particles is also similar to thatof natural pyroclastic flows. Therefore we have reduced thenonlinear effects that can be encountered when scaling updata from small laboratory experiments, which use tinysynthetic particles that couple in a different way to the fluidstress compared to natural pyroclastic particles [Dellino etal., 2005]. We are therefore confident that the transportationprocesses revealed in these experiments can be rigorouslyextrapolated to natural situations. In terms of particletransportation processes, several similarities emerge betweenour experimental runs and natural pyroclastic flows. Forexample, the experiments replicated both the lobate dis-persion and three major transportation modes (concentratedundercurrent, bed load at the base of turbulent flow, andcontinuous suspension of fine-ash particles) of actualpyroclastic flows. The transition from a thin concentratedcurrent to a dilute one, in which the solid material is splitinto a coarser population transported as bed load and afine-ash one transported in the upper, highly expanded partof the current, is also reminiscent of processes reported orobserved in explosive eruptions [Valentine, 1987; Dellinoet al., 2004b; Sulpizio et al., 2007]. In addition, theproportional relationship between velocity at the flow frontand grain size of the bed load material is very promisingfor linking the characteristics of particles deposited fromthe current with the main fluid-mechanical parameters ofthe flow.[49] We defer a detailed modeling of the flow dynamics

of the shear currents to after having a significant number ofexperiments covering a broader range of parameters. Theanalysis of video footage from these preliminary runs givesinteresting insights into the mechanics of the gas-particleflows and shows that the lateral shear flow strongly resem-bles a turbulent boundary layer shear flow, which is a well-known phenomenon in fluid dynamics. Our modeling willbe implemented, in the future, with turbulent boundary layershear flow behavior as a basis.[50] In addition to the information of particle transport

processes inside the shear current, the experiments giveconstraints on the conditions that lead to column collapseand on impact on the ground; these collapse and impactcharacteristics control the inception, development, andintensity of pyroclastic flows. There is a clear control byspecific mechanical energy on whether the eruptive mix-ture evolves as a collapsing column, transitional column,or plume. One of the main parameters used in volcanologyfor defining the intensity of explosive eruptions and thevariation in plume behavior is the mass eruption rate,which is the mass of material exiting the vent per unit time(kg/s) [Carey and Sparks, 1986; Carey and Sigurdsson,1989; Jaupart and Allegre, 1991]. It results from theproduct of flow velocity and flow density, which givesthe mass flow rate (kg/m2 s), times the base area of theconduit (m2). The peak mass flow rate of our experiments,when normalized to the base area of the conduit of actual

volcanic eruptions, gives mass eruption rates that arecompatible with the natural situation and therefore willallow scaling the parameters that regulate column collapseand pyroclastic flow inception. It is clear that collapsedensity and impact velocity, as expressed by impact flowrate, are both important for the development and velocityof the shear flow. Determination of a functional relation-ship among these parameters, which could be used topredict column collapse and pyroclastic flow velocity fromthe conditions in the conduit and at the vent, requiresfurther systematic experimental runs.[51] We will use the experimental facility in the future to

vary the main parameters that influence the inception andintensity of pyroclastic flows. The modular design of theexperiment easily allows changing the experimental setup toaccomplish this goal, including changes in base plateposition, the timing of gas flow, and addition of a heatingsystem coupled to the base sector of the conduit formonitoring the role of the additional thermal effect in theformation and evolution of pyroclastic flows and plumes.

[52] Acknowledgments. We are very grateful to the owner of thequarry, Laterificio Pugliese S.p.a. Without access to the quarry, theexperimental facility would have never been realized. Plantone S.n.c.company is gratefully acknowledged for the enthusiasm demonstratedduring the construction of the metallic parts of the experiment. J.D.L.White, A.L. Grunder, and an anonymous reviewer greatly helped improvingthe manuscript. Part of the consumable and travel costs were funded byINGV-DPC 2005–2006 subprojects V3_4 and V3_5.

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��R. Buttner and B. Zimanowski, Physikalisch Vulkanologisches Labor,

Universitat Wurzburg, Pleicherwall 1, D-97070, Wurzburg, Germany.P. Dellino, L. La Volpe, D. Mele, and R. Sulpizio, Centro Interdiparti-

mentale di Ricerca sul rischio sismico e vulcanico, DipartimentoGeomineralogico Universita di Bari, Via E. Orabona, 4-70124, Bari, Italy.([email protected])


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