In the present paper, we study the Aw-Rascle (AR) macroscopic model of traffic flow
where is the density, is the velocity, P is the velocity offset which is called as the “pressure” inspired from gas dynamics. The derivation process of the above AR model and the application can be discovered in  - .
In , Aw and Rascle studied the limit behavior and found that the pressure term is active. In , Shen and Sun investigated the limit behavior without the constraint of the maximal density.
In  , M. N. Sun studied the following model
In  , they studied the elementary wave interactions and obtained the stability of the Riemann solutions under such a perturbation on the initial data.
In , G. D. Wang investigated the Riemann problem of (1) and
where , . This is the so-called generalized Chaplygin gas for (1).
In the present paper, we investigate the elementary wave interactions for (1) and (3). In our paper , we study the wave interactions containing no delta shock, so we just consider the wave interactions for (1) containing delta shock wave with the three piecewise constants
where the perturbation parameter is sufficiently small. (4) can be regarded as a local perturbation on the initial values
This paper is arranged as follows. In Section 2, we give curtly the Riemann problem for the model (1) (3) and (5) for the convenience of the readers. In Section 3, we investigate the elementary wave interactions by the characteristic analysis method. In Section 4, we summarize our main conclusion.
We give briefly the Riemann problem for (1) (3) and (5) .
The characteristic roots of (1) are , which shows that (1) is strictly hyperbolic. The corresponding right characteristic vector of and is respectively given by
If , we get
which indicates that is genuinely nonlinear and the associated wave is either shock wave or rarefaction wave, is always linearly degenerate and the associated wave is the contact discontinuity, where denotes the gradient.
We construct the self-similar solution , . The Riemann problem (1) (5) becomes the following boundary value problem of the ordinary differential equations
and . For smooth solutions, let , (8) becomes
Besides the constant state solution , (9) has a rarefaction wave solution. For the given left state , the rarefaction wave curve is given by
For a bounded discontinuity at , it holds the Rankine-Hugoniot conditions
where , , , etc.
For the given left state , the shock wave is given by
Since is linearly degenerate, from (9) or (11) we know the contact discontinuity
All the above rarefaction waves R, shock waves S, and contact discontinuities J are the elementary waves for (1). Notice the shock curves coincide with the rarefaction curves in the phase plane . It is very important because it can simplify the process of the elementary wave interactions.
According to the right state in the different region (Figure 1), we obtain the unique Riemann solution. When or II, the unique Riemann solution is , when or IV, the unique Riemann solution is , when , i.e., , we should construct the delta shock wave solution as follows.
Consider a piecewise smooth solution of (1) with the form
-measure solutions of (1) (3) and (5) is given
here , is the Heaviside function.
The measure solution (14) and (15) satisfies the generalized Rankine-Hugoniot condition
where is the jump of u across the discontinuity , etc.
Figure 1. Wave curves in (u, ρ).
The -entropy condition is
We know that all the characteristics on both sides of the -shock wave curve are incoming.
In order to consider the interaction of elementary waves containing delta shock wave, we briefly recall the concept of left- and right-hand side delta functions as follows.
Let be divided into two disjoint open sets and with a piecewise smooth boundary curve L, which satisfies and . Let and be the space of bounded and continuous real-valued functions equipped with the -norm and the space of measures on , respectively. Let us assume that and , then the product of and is defined as an element , where can be defined as the usual product of a continuous function and a measure. Thus, it is obvious that the above-defined product makes sense.
We view the measure on as a measure on with support in . Then the mapping can be obtained by taking . In a similar way, we have . The solution concept used in our paper when we consider the delta shock can be described as follows: carry out the multiplication and composition in the space and take the mapping before differentiation in the space of distributions.
Based on the above analysis, we have the following conclusion.
Theorem 2.1 The Riemann solution of the initial value problem (1) (3) and (5) is unique.
3. Interactions of Elementary Waves Containing Delta Shock Wave
Now we study the elementary wave interactions for (1) (3) with (4). (4) is regarded as the perturbation on the Riemann initial values (5). In order to cover all the cases containing delta shock completely, we have three possibilities according to the different combinations from and as follows: and , and , and .
Case 1: and .
We consider the interaction of a delta shock wave emanating from and a delta shock wave emanating from . When t is small enough, the solution of the initial value problem (1) (3) and (4) is expressed as (Figure 2).
Figure 2. Interaction of δS1 and δS2.
The propagation speed of and satisfy respectively the -entropy conditions
and we know that
where and are respectively the propagating speed of and , and are respectively the strength of and .
It easy to see that which shows that will overtake at a point which is determined by
At the intersection point , a new initial data is formed as follows
where is the sum of the strengths of the incoming delta shock wave and . A new delta shock wave will generate after interaction and we denote it by , which is given by
where H is the Heaviside function and is a split delta function. All of them are supported by the line , is the propagating speed of . Although they are supported by the same line, is the delta measure on the set and is the delta measure on the set respectively.
From (23), we obtain
Substituting (24) and (25) into the first equation of (1), we obtain
From (15), we get
Case 2: and .
In this case, a shock wave followed by a contact discontinuity emits from and a delta shock wave emits from (Figure 3). The propagating speed of the contact discontinuity is , and the propagating speed of the delta shock wave satisfies the -entropy condition
. It easy to see that J will overtake at which given by
From (26), we get
The delta shock wave will pass through J with the same speed as before but the strength changes due to the difference between and . We still denote it with after the time , and .
From the -entropy condition
Figure 3. Interaction of S + J and δS.
and the shock entropy condition
we know that S will overtake at which satisfies
The new initial data will be formulated at as follows
denotes the strength of at the time and from and we can determine the value of .
A new delta shock wave will be generated after the interaction of S and , denoted by here. It satisfies that
The Heaviside function H and the split delta function are supported by the line , is the propagating speed of . From (31), similarly with the above case we obtain that the strength of after the interaction of S and is
As , the delta shock wave will propagate with the invariant speed which is given by (15) with and as its right and left state. Furthermore, from the condition we know the -entropy condition for the new delta shock wave holds which shows is an overcompresive wave.
Case 3: and .
When t is small enough, the solution of the initial value problem (1) (3) and (5) can be described by (Figure 4)
Similar with the above case, the contact discontinuityJ will overtake the delta shock wave at the point given by (27). The delta shock wave will pass through J with the same speed as before but the strength changes due to the difference between and . We still denote it with after the time , and . Since the propagation speed of wave front in the rarefaction wave is and that of the delta shock wave satisfies the -entropy condition
it is shown that R will interact with at which is determined by
it follows that . The strength of at can be calculated by . At the same time, a new delta shock wave with varying speed is generated. Here we use to express the curve of and it is given in the following form
Figure 4. Interaction of R +J and δS1.
where is a split delta function on the new delta shock, and is the strength of the new delta shock at the time t.
When , the delta shock wave cannot penetrate the rarefaction wave; when , the delta shock wave can penetrate the rarefaction wave completely.
Now we construct the unique solution of the elementary wave interactions and get the following main conclusion. Using the characteristic analysis method, i.e., by analyzing the elementary wave curves in the phrase plane, we get the unique solution of the initial problem (1) with the state equation (3) and the initial values (4). We observe that the elementary wave interactions have a much simpler structure for Temple class than general systems of conservation laws since the wave interaction of the same family does not generate wave of other families for Temple systems. It is important to study the elementary waves interactions for (1) not only because of their significance in practical applications in the traffic flow system for the generalized Chaplygin gas, but also because of their basic role as building blocks for the general mathematical theory of the traffic flow system.
Theorem 4.1 The Riemann solutions of the initial value problem (1) (3) with the initial data (4) are constructed which are stable under such small perturbation on the initial data.
Supported by the Foundation for Young Scholars of Shandong University of Technology (No. 115024).
 Berthelin, F., Degond, P., Delitata, M. and Rascle, M. (2008) A Model for the Formation and Evolution of Traffic Jams. Archive for Rational Mechanics and Analysis, 187, 185-220.
 Shen, C. and Sun, M.N. (2010) Formation of Delta-Shocks and Vacuum States in the Vanishing Pressure Limit of Solutions to the Aw-Rascle Model. Journal of Differential Equations, 249, 3024-3051.
 Sun, M.N. (2011) A Note on the Interactions of Elementary Waves for the AR Traffic Flow Model without Vacuum. Acta Mathematica Scientia, 31B, 1503-1512.
 Liu, Y.J. and Sun, W.H. (2018) Wave Interactions and Stability of Riemann Solutions of the Aw? CRascle Model for Generalized Chaplygin Gas. Acta Applicandae Mathematicae, 154, 95-109.