Producing precursor shock waves in laboratory - 16340590

8/14/2019 Producing precursor shock waves in laboratory - 16340590

1/8

72.S n.)35-S..apotAL. 3 6 - 9 7

0 - 4 , 4 4 4 4 *P 7 ).. )i ,(2-54) 1871 1 1 1 1 1 1 1 1 1 1i Sc7M-115-55-51METHODS OF PRODUCING PRECURSORS IN TEE LABORATORY7t % -,.,,'N,,, r" ,,' 'L.A... , VILAIie.:! , it.:',tf-77 - j S-yrDC.7770".Cmi, hwrArcEl.1 l' KevituDe:1 8. tixrl f .,tzlia; ic!: ..:e.. 0 ,, ,'. !.4,2171 AD C r4 "DC C , e c a . c , . . . . . N s . 7 . . ,. - . . - . f lmotrwi II -Some ,

A w e .. .AN ADDr'l lerieu Dart6 Conve ..:ad r.4_________ I_ . s 7 - - - ( /COMM' . UCAr,O f f K teN 33 . T o d d .ABSTRACT

Six methods of producing precursor-type shock waves in thelaboratory are discussed. Two of these methods have beensucces.fully applied at Sandia to spherical shocks. Theattainment of precursor-type waves from spherical shocksis considered as evidence substantiating the thermal layershock theory.

Case No. 407.02une 15, 1955N EM 9 11110111 1.1MIUNCLASSIFIED


2/8

DISTRIBUTION:W. A. MacNair, 5000K. W. Erickson, 5130G. E. Hansche, 5140E. F. Cox, 5110M. L. Merritt, 5111 (2 copies)J. D. Shreve, Jr., 5172Walker Bleakney, PrincetonC. W. Lampson, BRLG. K. Hartmann, NOH. C. Hottel,C. J. Aronson, NOLHerbert Scoville, Jr., AFSWP Mars, Washington (2 copies)R. L. Corsbie, DiM/AEC, WashingtonE. Wang, Air Materiel Cammand, Wright-Patterson AFB, Dayton, Ohio (2 copies)R. K. Smeltzer, 1922-2M. K. Linn, 1923 ( 6 )


3/8

1878- 3 -

METHODS OF PRODUCING THE PRECURSOR IN THE LABORATORY

IntroductionWeapons effects tests in Nevada have indicated that for certaincombinations of height of burst and yield a precursor (a wave travellingahead of the expected main shock) is formed. Speculation guided byearly German reports and an experiment on shock waves impinging on aboundary layer of carbon tetrachloride and water led to the theory thatthe precursor is caused by a layer of hot air along the ground. E videncefor this theory is found in the good agreement between the predictionsby the theory of occurrence of the precursor with occurrence in the field.*Additional evidence would be production of a three-dimensional precursorin the laboratory by forming a layer of hot air along a surface.Walker Bleakney, and subsequently Cassen, produced one-dimensional pre-cursors in shock tubes, but there still remained the problem of producingprecursors from spherical shocks on a laboratory scale.** Severalattempts on this scale were made at Sandia to induce precursors fromspherical shock waves, two of w hich were successful. Sandia's experimentsand the work of other investigators in this field are reviewed here.

Proposed Methods of Producing PrecursorsOne method of producing the precursor.is by creating layers ofgas with differing sound speeds due primarily to changes in the natureof the gas. At Princeton, Walker Bleakney has produced a layer ofhelium by permitting diffusion through a porous plate, but no experi-ments have been made with spherical shock waves on such a gas layer.The second method, that of producing a sharply defined boundarybetween layers of hot and cold air, can be achieved in several ways:Actual Popcorning of Sand -- Thermal equipment needed to producepopcorning of sand over a sizable area is not often found in the laboratory,but if a sufficient area could be irradiated it would offer several ad-vantages:1 . It would be another method of producing the phenomenon;

2 . It would correspond more nearly than any of the othermethods to the proposed Nevada model; and3. It would offer the possibility of studying the precursorin dust-laden air.

* Shelton, F. H., The Precursor, Its Formation, Prediction, and Effects,Sandia Corporation Report SC-2850(TR) July 27, 1953** Griffith, Wayland C., and Bleakney, Walker, Diffusion Analogy toInteraction of a Shock Wave with a Thermal Boundary Layer, PU-TR-11-17,Jan. 3, 1955 and Cassen, (unpublished).


4/8

1879-4- T I

Separating Hot and Cold Layers by Thin Films (eg g Collodion orSoap bubble) -- Both collodion and soap bubble are used in shock tubessuccessfully. Attempts at using film separation for spherical shocksat Sandia Cor t:oration have not been successful because of the thickerfilms necessary.

Combustion of a Layer of Gasoline -- This method has been triedwith 1/2-ounce charges of Comp B, but the system sets up so much turbu-lence that results are meaningless. Larger scale tests (250 lbs ofTNT) were partially successful in that the shock wave was found to bedistorted. The photographic instrumentation was not adequate to givecomplete details about the phenomenon. The game of the not layeris undoubtedly modified by the inclusion of products of combustion (CO 2 ,CO, etc).Grazing Incidence of Streams of Hot and Cold Air -- Although it isnot yet possible to remove turbulence completely, this method, which

utilizes a spark for a shock source, h a s yielded promising r e s u l t s , F i g .1. Unfortunately, the cold air side was cooled by passing it over dryice. The CO2 included in the gas stream changed the gamma of the colda i r . A lieuid nitrogen heat exchanger is being built to remove thisobjection. In the Sandia Corporation version of this method the hotlayer is heated by passing air over nichrome ribbon heaters while thecold air is cooled by passing it over a heat exchanger. In addition,the possibility of using boiling liquid nitrogen as a source for thecold air becoMes evident when the index refraction of sour! betweenhot and cold layers is considered:.c 1 / T 1N '00 UNT) \ 1/21 1 .c 2,300\iMUchl more is gained by reducing the cold air temperature by 100 C thanby heating the warm air by 100C. However, strong heating will be neces-sary to reach the same index of refraction encountered in the field.'Hot Plate Techniques -- Successful use of the hot plate techniquein the shock tube was first accomplished by Bleakney at Princeton, andsubsequent studies have been made at UCLA. Robert Hess at LangleyField has developed theoretical considerations to classify, and insome cases to qualitatively describe, the flow. Hot plates have beenused with approximations to spherical shock waves by Sandia Corporationand NOL. At Sandia Corporation, shock waves were generated by 5-gramcharges of Comp C initiated by No. 6 caps. The shock'produced bythis system is not entirely spherical but it gives a good approximationin the region of interest. The NOL data are from a spark gap in awell, a method which will also produce a distorted spherical shock wave.

* Hess, R. V., Interaction of a Shock Wave with a Thermal Boundary Layer,to be published.


5/8

18 8-5 -

Shadowgraphs of Sandia Corporation experiments are shown in Figs.2a-b. The temperature of the hot plate was about 200 C and the ambientair about 20 C. The surface of the hot plate is about 1/2-inch insidethe demarcation line between the hot and cold layers.One limitation of this and the grazing incidence technique is thatthere is no radial dependence of temperature. This means that in thelaboratory the thermal layer is much broader in extent than it is inthe field so that the precursor wave does not die out in the same manner.

General LimitationUnfortunately, only the optical methods of shadowgraph, schlieren,and interferometry are available for experimental measurements. Tomeasure adequately the shock parameters for such brief transients, along and laborious instrumentation program which would develop equip-ment with sufficient time resolution and'temperature compensation

would have to be undertaken.C o n c l u s i o n s

1. Corroboratory evidence for the thermal l a y e r t 6 . 7 m r y ' -r p r e -ursor',formation-is,ibund.:in'pradudtton:of.4rectirsoM from :raphdtiddls h o c k , w a v e s , .2. Two methods of producing definite layers of hot and cold aircapable of producing precursors are available.3 . Realization of the full potential from these techniques isdependent upon a successful instrumentation program.


6/8

LSA

Fig. 1 Precursor-type wave generated by grazing incidence flow technique.Note: Time increases downward; travel is from right to left.

1 " )


7/8

1 1 1 1 1 1 1 . 1 1 1 . 1 1 1 1 1 .-7-

1882

Fig. 2a Successive shadowgraphs of a shock wave from a 5-gram chargeas it passes over a hot plate. The surface of the hot plateis below the visible boundary which is caused by the hotlayer. The hot plate was about an inch thick. Note: timeincreases downward.

IED


8/8

-g-

F i g . 2 b S u c c e s s i v e s h a d o w g r a p hs o f a s h o c k w a v e f r o m a 5 - g r a mc h a r g e a s i t p a s s e s o v e r a h o t p l a t e . N o t e t h e d o u b l es h o c k w a v e j u s t a b o v e t h e h e a t e d l a y e r . T h e s u r f a c eo f t h e h o t p l a t e i s b e l o w t h e v i s i b l e b o u n d a r y w h i c hi s c a u s e d b y t h e h o t l a y e r . T h e h o t p l a t e w a s a b o u ta n i n c h t h i c k . T i m e i n c r e a s e s d o w n w a r d .

Documents

Producing precursor shock waves in laboratory - 16340590