943 
TRANSMISSION OF A SHOCKWAVE FROM WATER TO AIR AT 
NORMAL INCIDENCE 
A, J. Harris 
Road Research Laboratory, 
May 1944 
* * * n * * * * * 
Summary. 
A shock wave in water gives rise, whenever it meets the free surface, to a shock wave 
in the air. Tne case of normal incidence was aiscussed elsewhere (1), In the present note the 
calculations ars extendsa tc an aif shock wave oressure of 100 atmoscheres, and ars ussd to 
evaluate the pressure near woz. charges of C.E. from excerimental gata 2), These pressures 
are comoared with theoretical values 3), 
nn ee EE EEE DEINE EEEEEEEEnE 
In a previous investigation (1) the evaluation of the intensity cf the transmittsa air 
shock was not carried above & atmospheres, since the oerfect gas equations there cease to be 
agolicable. The work has now been extended to 100 atmospheres on the basis of the data yiven 
by Penney ana Davies for shock waves in air at N.T.%, In cerfect gases carticle and shock 
wave velocities are prooortional to the square root of tne absolute temoerature of the 
undisturbed gas, Now althougn air at oressures aDove 6 atmospheres deoarts from cerfection the 
discreoancies in oarticle and wave velocities do not exceed 2.5% even when the oressure is 
100 atmos cheres, and in Consequence it has Deen assumed that for shock waves in air at Te 
absolute, the velocities may be found quite accurat2ly from those given by Fenney and Davies 
by multiplying by a 
Table 1 shows the results so derivea for transmission from water at 20°C. to air at 
N.T.°. and to air at 20°C. ana one atmosohere oressure, Differences of water temperature are 
likely to be less imcortant than differences of air temperature since the effects on density ana 
elasticity are much smaller in a liquid tnan in a gase 
Some measurements of tne shock wave and soray velocities above a water surface when 
4 oz. of C.2. were exoloded benesatn it nave been Given 2, and from tnese the cressure in thé 
water shock Wav2 may be Inferred witn the nelo cf Table 1. Both velocities usually fall off 
rather rapidly so that it is difficult to extracolats back to tne initial velocities with much 
accuracy, but on the basis of these 2xtracolated values the ooints in Figure 1 nave be2n obtained, 
There isa difficulty about tne records taken at 2 incn deoth. The otner recoras show the shock 
wave moving ahead cf the soray but at 2 inch death it cannot be distinguished seoarately. It 
is ocssiole that the soray is moving faster tnan the shock wave but there is no evidence of this 
and it scems best tc assume that the two velocities are equal. A Single velocity measurement 
at 2 inch decth thus yields two water pressures; the higher value being obtaineo when the 
measured velocity is intergreted as soray velocity. it is difficult to see how tne water 
surface can be in front of the shock wave unless the scoray travels much fast¢r than tne surface 
from which it was formed or the snock wave is the more radidly aecelerated. 
Penney and Oasgucta 
(3) have jiven a formula for the water shock wave oressure near T,N.T. 
an exolosive about 5% less effective in oroducing oeak pressure than C.£, They have assumed for 
T.N.T. a Cnemical energy release of 800 cals./ym, but suggest that 1000 cals./ym. would be a 
better valuc. Te cotain from their results the oressures near C.£., the cressures for oz, 
T.N.T. were calculated from the formula of (3) and then increased in tne ratio 1.05 x 
The Calculated values are shown in Figure 1 Tnere is reasonable agreement vetween tne 
calculated cressures and those deauced from the velocity measurements at 4 inches and 6 inches 
but at 2 inches there Is a consiazrable divergenc= escecially if the measurea velccity is 
interoreteo as surface and nct shock wave velocity. A oossible source of disagrzement between 
theory eecoe 
