Shock Formation in Fluids Having Embedded Regions of Negative Nonlinearity
The steepening of one-dimensional finite-amplitude waves in inviscid Van der Waals gases is described. The undisturbed medium is taken to be unbounded, at rest and uniform. The specific heat is taken to be large enough to generate an embedded region of negative nonlinearity in the general neighborhood of the saturated vapor line and thermodynamic critical point. Under these conditions the shock formation process may differ significantly from that predicted by the perfect gas theory. The present study illustrates these differences for both isolated pulses and periodic wave trains. It is further shown that as many as three shocks, two compression and one expansion, may be formed in a single pulse or, in the case of wave trains, repeated element. It is also shown that the convected sound speed may become identical to the thermodynamic sound speed of the undisturbed medium at three distinct values of the density; the first of these corresponds to the density of the undisturbed medium while the other two are related to an integral of the fundamental derivative along an isentrope. The results obtained are expected to hold for any fluid which possesses such an embedded region of negative nonlinearity.