# A Deposition Model: Riemann Problem and Flux-Function Limits of Solutions

Authors: Hongjun Cheng; Shiwei Li
Journal: Abstract and Applied Analysis
 (2018)

---

## Abstract

The Riemann solutions of a deposition model are shown. A singular flux-function limit of the obtained Riemann solutions is considered. As a result, it is shown that the Riemann solutions of the deposition model just converge to the Riemann solutions of the limit system, the scalar conservation law with a linear flux function involving discontinuous coefficient. Especially, for some initial data, the two-shock Riemann solution of the deposition model tends to the delta-shock Riemann solution of the limit system; by contrast, for some initial data, the two-rarefaction-wave Riemann solution of the deposition model tends to the vacuum Riemann solution of the limit system. Some numerical results exhibiting the formation processes of delta-shocks and vacuum states are presented.


---

## Body

## 1. Introduction

Consider the following deposition model of conservation laws:(1)vt+uvx=0,ut+ϵvx=0,where v⩾0 denotes the density of the population performing the deposition, u=-∂xh(x,t) with h(x,t) being the deposition height, and ϵ is a positive parameter. The first equation describes the conservation of total population. The second one is derived from the rules governing the time evolution of the deposition system: the deposition rate is proportional to the density of the population, and the population is driven by a velocity field proportional to the negative gradient of height. It was also derived as a decent hydrodynamic limit of some systems of interacting particles with two conserved quantities [1, 2]. This system can also describe the macroscopic behaviors of some (so-called chemotactic) bacterial populations, which are attracted by a chemical substrate [3–5].The first task of this paper is to solve the Riemann problem, one of the fundamental problems associated with nonlinear hyperbolic conservation laws, for (1) with initial data(2)u,vx,t=0=u±,v±,±x>0,where v±>0. System (1) is nonstrictly hyperbolic, and both characteristic fields are genuinely nonlinear. The elementary waves include shocks and rarefaction waves. By the analysis method in phase plane, the unique global Riemann solution is constructed with five different kinds of structures containing shock(s) and/or rarefaction wave(s).Asϵ→0+, system (1) formally becomes(3)ut=0,vt+uvx=0,which can be rewritten in the form of scalar conservation law with a linear flux function involving discontinuous coefficient(4)vt+uxvx=0.The scalar hyperbolic conservation laws with the discontinuous flux functions arise in many areas, such as the continuous sedimentation of solid particles in a liquid, the two-phase flow in porous media, and the traffic flow theory. System (3) has been studied very extensively; for example, see [6–8] and the references cited therein. It has been shown that delta-shocks and vacuum states do occur in the Riemann solutions of (3).Let us recall some knowledge with respect to delta-shocks and vacuum states. Delta-shocks are an important kind of nonclassical wave for systems of conservation laws. Mathematically, they are characterized by the delta functions appearing in the state variables. Physically, they can describe the concentration phenomenon. As for delta-shocks, see [9–18]. The other extreme situation is the vacuum state. It describes the cavitation phenomenon. Recently, the phenomena of concentration and cavitation and the formation of delta-shock and vacuum state have attracted wide attention from researchers. For example, Li [19] and Chen and Liu [20, 21] discussed this topic by considering the vanishing pressure limits of solutions of the isentropic and nonisentropic Euler equations. With respect to this topic, also see [22–24].The second task of this paper is to study the behaviors of solutions of system (1) as the flux ϵv vanishes (i.e., ϵ→0+) by the Riemann problem. We are especially concerned with the phenomena of concentration and cavitation and the formation of delta-shocks and vacuum states in the limit. As a result, we rigorously show that as ϵ→0+, the Riemann solutions of (1) just converge to the Riemann solutions of (3) with the same initial data. Especially, when u+⩽0⩽u-, the two-shock solution of (1) and (2) tends to the delta-shock solution of (3) and (2), where the intermediate density between the two shocks tends to a weighted δ-measure which forms the delta-shock; by contrast, when u+⩾0⩾u-, the two-rarefaction-wave solution of (1) and (2) tends to the two-contact-discontinuity solution of (3) and (2), in which the nonvacuum intermediate state between the two rarefaction waves tends to a vacuum state. It can also be seen that such a flux-function limit may be very singular: the limit functions of solutions are no longer in the spaces of functions BV or L∞, and the space of Radon measures is a natural space in order to deal with such a limit.The rest of the paper is organized as follows. In Section2, we recall the Riemann problem for system (3). In Section 3, we solve the Riemann problem for (1) by the analysis method in phase plane. Sections 4 and 5 are devoted to the studies of the limits of solutions of the Riemann problem for (1) as ϵ→0+. In Section 6, we present some numerical results to examine the formation processes of delta-shocks and vacuum states as ϵ decreases. Finally, we give the conclusions in Section 7.


## 2. Solutions of the Riemann Problem for (3)

In this section, we briefly recall the Riemann problem for (3) with initial data (2), for which we also refer the readers to the papers [6, 7]. The characteristic roots of (3) are λ1(u,v)=0 and λ2(u,v)=u, and the corresponding right characteristic vectors are r→1=(u,-v)T and r→2=(0,1)T, respectively. They satisfy ∇λi·r→i≡0(i=1,2), where, in what follows,   ∇=(∂/∂u,∂/∂v) is the gradient operator. Therefore, (3) is nonstrictly hyperbolic because of λ1=λ2 at u=0, and λi(i=1,2) is linearly degenerate.Since the equations and the Riemann initial data are invariant under uniform stretching of coordinates(x,t)→(βx,βt)(β>0), we consider the self-similar solutions (u,v)(x,t)=(u,v)(ξ), where ξ=x/t. Then this Riemann problem turns into(5)-ξuξ=0,-ξvξ+uvξ=0,u,v±∞=u±,v±.This is a two-point boundary value problem of first-order ordinary differential equations with the boundary values in the infinity.Besides the constant states and the vacuum states (v≡0), the self-similar wave (u,v)(ξ)(ξ=x/t) of the first family is a standing wave discontinuity(6)SW:ξ=0,ulvl=urvrand that of the second family is a contact discontinuity(7)J:ξ=ul=ur,where the indices l and r denote the left and right states, respectively.Using the above waves, by the analysis in phase plane, one can construct the solutions of Riemann problem (3) and (2) in the following cases.(1)
Whenu->0,u+>0, the solution is SW+J: (8)u,vξ=u-,v-,ξ<0,u+,u-v-u+,0<ξ<u+,u+,v+,ξ>u+.(2)
Whenu-<0,u+<0, the solution is J+SW: (9)u,vξ=u-,v-,ξ<u-,u-,u+v+u-,u-<ξ<0,u+,v+,ξ>0.(3)
Whenu-⩽0⩽u+, the solution is J+Vac+J: (10)u,vξ=u-,v-,ξ<u-,u-,0,u-<ξ<0,u+,0,0<ξ<u+,u+,v+,ξ>u+.However, for the caseu+⩽0⩽u-, the singularity cannot be a jump with finite amplitude; that is, there is no solution which is piecewise smooth and bounded. Hence a solution containing a weighted δ-measure (i.e., delta-shock) supported on a line should be introduced in order to establish the existence in a space of measures from the mathematical point of view.Denote byBM(R) the space of bounded Borel measures on R, then the definition of a measure solution of (3) in BM(R) can be given as follows.Definition 1.
A pair(u,v)(x,t) is called a measure solution of (3) if it satisfies that(a)
u∈L∞([0,∞),L∞(R))∩C([0,∞),H-s(R))(b)
v∈L∞([0,∞),BM(R))∩C([0,∞),H-s(R)),s>0(c)
u is measurable with respect to v almost for all t≥0.
And (3) is satisfied in the measure and distributional senses; that is,(11)∫0∞∫Ruψtdxdt=0,∫0∞∫Rψt+uψxdvdt=0for all ψ∈C0∞([0,∞)×R).Remark 2.
The continuity conditions inC([0,∞),H-s(R)) may be used to give an interpretation for (u,v)(x,t) to take on initial values [25].Definition 3.
A two-dimensional weighted delta functionw(s)δL supported on a smooth curve L parameterized as x=x(s),t=t(s)(c≤s≤d) is defined by (12)wsδL,ψx,t=∫cdwsψxs,tsdsfor all ψ∈C0∞([0,∞)×R).We propose to find a solution of (3) of the form(13)u,vx,t=u1,v1x,t,x<xt,uδt,wtδx-xt,x=xt,u2,v2x,t,x>xt,where uδ(t),w(t),x(t)∈C1[0,∞), and (u1,v1)(x,t) and (u2,v2)(x,t) are respective bounded smooth solutions of (3).We assert that (13) is a measure solution of (3) if the relation(14)dxtdt=uδt,-uδtu=0,dwtdt=-uδtv+uvis satisfied, where [g]=g¯1-g¯2 is the jump of g across the discontinuity with g¯ being the limit value of g on the discontinuity. The proof is similar to that in [11, 17], and we omit it. System (14) is called the generalized Rankine-Hugoniot relation.In addition, to guarantee uniqueness of solution, we propose the following admissible condition:(15)λ2u¯2,v¯2⩽λ1u¯2,v¯2⩽dxtdt⩽λ1u¯1,v¯1⩽λ2u¯1,v¯1,which means that all characteristics on both sides of discontinuity line are not outgoing.A discontinuity in the form (13) satisfying (14) and (15) will be called a delta-shock, symbolized by δ.Remark 4.
Here the delta-shock is defined as a measure solution just as in [10, 17, 18, 25]. In fact, like [14, 15], it can also be defined as a solution in the sense of distributions, which gives a natural generalization of the classical definition of the weak L∞-solution and specifies the definition of measure solution.Remark 5.
Here in a delta-shock, we assignu to be uδ(t) on the discontinuity line. Physically, for instance, in adhesion particle dynamics, the formation of delta-shock can describe the process of the concentration of particles, and such an assignment can be interpreted as the velocity of colliding particles [17].Now we consider the Riemann problem (3) and (2) for the case u+⩽0⩽u-. At this time, the solution is a delta-shock in the form(16)u,vx,t=u-,v-,x<xt,uδt,wtδx-xt,x=xt,u+,v+,x>xt.Solving the generalized Rankine-Hugoniot relation (14) under the admissible condition (15) gives(17)xt=0,uδt=0,wt=u-v--u+v+t.


## 3. Solutions of the Riemann Problem for (1)

In this section, we solve the Riemann problem for system (1) with initial data (2) and examine the dependence of the Riemann solutions on the parameter ϵ>0. For (1), the characteristic roots and corresponding right characteristic vectors are (18)λ1ϵ=u-u2+4ϵv2,λ2ϵ=u+u2+4ϵv2,r→1ϵ=1,u-u2+4ϵv2ϵT,r→2ϵ=1,u+u2+4ϵv2ϵT.It is easy to calculate that(19)∇λiϵ·r→iϵ=1+-1iuu2+4ϵv≠0i=1,2.So (1) is nonstrictly hyperbolic because of λ1ϵ=λ2ϵ as u=v=0, and both characteristic fields are genuinely nonlinear in virtue of (19).As usual, we seek the self-similar solutions(u,v)(x,t)=(u,v)(ξ), where ξ=x/t. Then the Riemann problem turns into the two-point boundary value problem(20)-ξuξ+ϵvξ=0,-ξvξ+uvξ=0,(21)u,v±∞=u±,v±.For any smooth solution, (20) becomes(22)-ξϵvu-ξuvξ=0.Besides the constant states, the smooth solutions are composed of the backward rarefaction waves(23)ξ=λ1ϵ=u-u2+4ϵv2,dvdu=u-u2+4ϵv2ϵ,and the forward rarefaction waves(24)ξ=λ2ϵ=u+u2+4ϵv2,dvdu=u+u2+4ϵv2ϵ.For them, we have(25)dλiϵdu=∂λiϵ∂u+∂λiϵ∂vdvdu=1+-1iuu2+4ϵv>0,i=1,2.Let(ul,vl) and (ur,vr) denote the states connected by a rarefaction wave on the left and right sides, respectively. Then the conditions λ1ϵ(ur,vr)>λ1ϵ(ul,vl) and λ2ϵ(ur,vr)>λ2ϵ(ul,vl) are required for the forward and backward rarefaction waves, respectively. From (25), we have that both forward and backward rarefaction wave should satisfy(26)ur>ul.By solving the differential equations in (23) and (24), we further have the backward and forward rarefaction waves as follows:(27)R←:ξ=λ1ϵu,v=u-u2+4ϵv2,ur2+4ϵvr-urur2+4ϵvr+2ur=ul2+4ϵvl-ulul2+4ϵvl+2ul,ur>ul,R→:ξ=λ2ϵu,v=u+u2+4ϵv2,ur2+4ϵvr+urur2+4ϵvr-2ur=ul2+4ϵvl+ulul2+4ϵvl-2ul,ur>ul.For a given state(ul,vl), all possible states which can connect to (ul,vl) on the right by a backward rarefaction wave must be located on the curve (28)R←ul,vl:u2+4ϵv-uu2+4ϵv+2u=ul2+4ϵvl-ulul2+4ϵvl+2ul,u>ul.For a given state (ur,vr), all possible states which can connect to (ur,vr) on the left by a forward rarefaction wave must be located on the curve(29)R→ur,vr:u2+4ϵv+uu2+4ϵv-2u=ur2+4ϵvr+urur2+4ϵvr-2ur,u<ur.It is easy to check that the backward shock curve R←(ul,vl) is monotonously decreasing and convex, and the forward shock curve R→(ur,vr) is monotonously increasing and convex. Furthermore, R←(ul,vl) has the u-axis as the asymptote as Kl>0 and interacts with the u-axis at u=-Kl2/23 as Kl⩽0, where (30)Kl=ul2+4ϵvl-ulul2+4ϵvl+2ul;R→(ur,vr) has the u-axis as the asymptote as Mr>0 and interacts with the u-axis at u=Mr2/23 as Mr⩽0, where (31)Mr=ur2+4ϵvr+urur2+4ϵvr-2ur.Let us turn to the discontinuous solutions. For a bounded discontinuity atx=x(t), the Rankine-Hugoniot relation reads(32)-σu+ϵv=0,-σv+uv=0,where σ=dx/dt, [u]=ul-ur with ul=u(x(t)-0,t) and ur=u(x(t)+0,t), and so forth.From (32), one easily obtains(33)ϵvu2-ϵuvu=0.By noticing (34)uvu=vl+urvu=vr+ulvu,we solve (33) to obtain (35)ϵvu=ur±ur2+4vl2=ul±ul2+4ϵvr2.Then we obtain two kinds of discontinuities(36)σ1=ur-ur2+4ϵvl2,ul-ul2+4ϵvr=ur-ur2+4ϵvl,(37)σ2=ur+ur2+4ϵvl2,ul+ul2+4ϵvr=ur+ur2+4ϵvl.Notice that the second equations in (36) and (37) are equivalent to(38)vr-vl=ur-ur2+4ϵvl2ϵur-ul=ul-ul2+4ϵvr2ϵur-ul,(39)vr-vl=ur+ur2+4ϵvl2ϵur-ul=ul+ul2+4ϵvr2ϵur-ul,respectively.In order to identity the admissible solutions, the discontinuity (36) associating with λ1 should satisfy (40)σ1<λ1ϵul,vl<λ2ϵul,vl,λ1ϵur,vr<σ1<λ2ϵur,vr,while the discontinuity (37) associating with λ2 should satisfy(41)λ1ϵul,vl<σ2<λ2ϵul,vl,λ1ϵur,vr<λ2ϵur,vr<σ2.Then one can check that both inequalities (40) and (41) are equivalent to(42)ur<ul.The discontinuity (36) with (42) is called a backward shock and symbolized by S←, and (37) with (42) is called a forward shock and symbolized by S→.For a given state(ul,vl), all possible states which can connect to (ul,vl) on the right by a backward shock must be located on the curve(43)S←ul,vl:ul-ul2+4ϵv=u-u2+4ϵvl,u<ul.For a given state (ur,vr), all possible states which can connect to (ur,vr) on the left by a forward shock must be located on the curve(44)S→ur,vr:u+u2+4ϵvr=ur+ur2+4ϵv,u>ur.One can check that the backward shock curve S←(ul,vl) is monotonously decreasing, convex, and limu→-∞v=+∞, and the forward shock curve S→(ur,vr) is monotonously increasing, convex, and limu→+∞v=+∞.DenoteW←(ul,vl)=R←(ul,vl)∪S←(ul,vl) and W→(ur,vr)=R→(ur,vr)∪S→(ur,vr). Then the curve W←(ul,vl) is monotonously decreasing, convex, and limu→-∞v=+∞; besides, it has the u-axis as the asymptote as Kl>0 and interacts with the u-axis at u=-Kl2/23 as Kl⩽0. The curve W→(ur,vr) is monotonously increasing, convex, and limu→+∞v=+∞; besides, it has the u-axis as the asymptote as Mr>0 and interacts with the u-axis at u=Mr2/23 as Mr⩽0.We can construct the solutions of the Riemann problem by using the standard analysis method in the phase plane [26, 27]. Draw the backward wave curve W←(u-,v-) passing the left state (u-,v-) and the forward wave curve W→(u+,v+) passing the right state (u+,v+). According to the different locations of the intersection point of W←(u-,v-) and W→(u+,v+), one can construct the unique global Riemann solution with five different structures as follows:(i) a backward rarefaction wave + nonvacuum intermediate state + a forward rarefaction wave,(ii) a backward rarefaction wave + nonvacuum intermediate state + a forward shock,(iii) a backward shock + nonvacuum intermediate state + a forward rarefaction wave,(iv) a backward shock + nonvacuum intermediate state + a forward shock,(v) a backward rarefaction wave + vacuum intermediate state (v≡0) + a forward rarefaction wave.The conclusion can be stated in the following theorem.Theorem 6.
The Riemann problem for (1) with initial data (2) has a unique piecewise smooth solution consisting of waves of constant states, vacuums, shocks, and rarefaction waves.


## 4. Limits of Solution of (1) and (2) for u->u+,u-v->u+v+

In this section, we study the limits of solution of (1) and (2) as ϵ→0+ for the case u->u+,u-v->u+v+. We especially pay more attention on the phenomenon of concentration and the formation of delta-shocks in the limit.Lemma 7.
Ifu->u+,u-v->u+v+, then there exists ϵ0>0 such that (u+,v+)∈S←S→(u-,v-) for any ϵ<ϵ0.Proof.
All states(u,v) connected with (u-,v-) by S1 or S2 satisfy (45)S←u-,v-:v-v-=u-u2+4ϵv-2ϵu-u-,u<u-,or (46)S→u-,v-:v-v-=u+u2+4ϵv-2ϵu-u-,u<u-.If v+=v-, ϵ0 may be taken as any real positive number. If v+≠v-, we have the conclusion by taking (47)ϵ0=u+-u-u+v+-u-v-v+-v-2.The proof is finished.For fixedϵ<ϵ0, let Uϵ(ξ) denote the two-shock Riemann solution for (1) and (2):(48)Uϵξ=uϵ,vϵξ=u-,v-,ξ<σ1ϵ,u∗ϵ,v∗ϵ,σ1ϵ<ξ<σ2ϵ,u+,v+,ξ>σ2ϵ,where (u-,v-) and (u∗ϵ,v∗ϵ) are connected by a shock S← with speed σ1ϵ, and (u∗ϵ,v∗ϵ) and (u+,v+) are connected by a shock S→ with speed σ2ϵ:(49)S←:σ1ϵ=u--u-2+4ϵv∗ϵ2,v∗ϵ-v-=u--u-2+4ϵv∗ϵ2ϵu∗ϵ-u-,u∗ϵ<u-,v∗ϵ>v-,(50)S→:σ2ϵ=u++u+2+4ϵv∗ϵ2,v+-v∗ϵ=u++u+2+4ϵv∗ϵ2ϵu+-u∗ϵ,u∗ϵ>u+,v∗ϵ>v+.The following lemmas describe the limit behaviors of the intermediate state(u∗ϵ,v∗ϵ) and the speeds of two shocks as ϵ→0+.Lemma 8.
v ∗ ϵ is monotonously decreasing with respect to ϵ.Proof.
From (49) and (50), we have the equation with respect to v∗ϵ(51)u--u+=u+v+-u-v-v∗ϵ+u-2+4ϵv∗ϵ·1-v-v∗ϵ+u+2+4ϵv∗ϵ·1-v+v∗ϵ≔Hv∗ϵ.For ϵ1>ϵ2, assume v∗ϵ1⩾v∗ϵ2; then we have H(v∗ϵ1)>H(v∗ϵ2), which contradicts with H(v∗ϵ1)=H(v∗ϵ2)=u--u+. Therefore, for ϵ1>ϵ2, we must have v∗ϵ1<v∗ϵ2, which gives the conclusion. The proof is complete.Lemma 9.
ϵ v ∗ ϵ is monotonously increasing with respect to ϵ.Proof.
Forϵ1>ϵ2, assume ϵ1v∗ϵ1⩽ϵ2v∗ϵ2; then we have H(v∗ϵ1)<H(v∗ϵ2), which contradicts with H(v∗ϵ1)=H(v∗ϵ2)=u--u+. Therefore, for ϵ1>ϵ2, we must have ϵ1v∗ϵ1>ϵ2v∗ϵ2.Lemma 10.
One has(52)limϵ→0+v∗ϵ=u-v-u+,foru->u+>0,u-v->u+v+,u+v+u-,for0>u->u+,u-v->u+v+,+∞,foru-⩾0⩾u+.Proof.
Whenu->u+>0,u-v->u+v+ or 0>u->u+,u-v->u+v+, assume limϵ→0+v∗ϵ=+∞; then from (51), we have (53)u--u+=u-2+4limϵ→0+ϵv∗ϵ+u+2+4limϵ→0+ϵv∗ϵ,which is impossible. So when u->u+>0,u-v->u+v+ or 0>u->u+,u-v->u+v+, we have limϵ→0+v∗ϵ=K≠+∞. Then from (51), we can deduce (54)K=u+v+-u-v--u-v--u+v+u--u+-u--u+,which gives the desired conclusions. When u-⩾0⩾u+, similarly, there must be limϵ→0+v∗ϵ=+∞.Lemma 11.
One has(55)limϵ→0+⁡ϵv∗ϵ=0.Proof.
Letlimϵ→0+ϵv∗ϵ=M<+∞. When u->u+>0,u-v->u+v+ or 0>u->u+,u-v->u+v+, in virtue of Lemma 10, M=0 is obvious. When u-⩾0⩾u+, from (51), one has (56)Mu--u+2=0,when M=0.Lemma 12.
One has(57)limϵ→0+u∗ϵ=u+,foru->u+>0,u-v->u+v+,u-,for0>u->u+,u-v->u+v+,0,foru-⩾0⩾u+.Proof.
From the second equation in (50), we have (58)u∗ϵ=u++12u+-u+2+4ϵv∗ϵv+v∗ϵ-1.Then the conclusions can be easily obtained by virtue of Lemmas 10 and 11.Lemma 13.
One has(59)limϵ→0+σ1ϵ,σ2ϵ=0,u+,foru->u+>0,u-v->u+v+,u-,0,for0>u->u+,u-v->u+v+,0,0,foru-⩾0⩾u+.Proof.
Taking(60)σ1ϵ=u--u-2+4ϵv∗ϵ2,σ2ϵ=u++u+2+4ϵv∗ϵ2and the Lemma 11 into account, one can easily get the conclusions.LetU0(ξ)=limϵ→0+Uϵ(ξ). Then when u->u+>0,u-v->u+v+, (61)U0ξ=u-,v-,ξ<σ1,u+,v∗,σ1<ξ<σ2,u+,v+,ξ>σ2,where σ1=0,σ2=u+ and v∗=u-v-/u+. When 0>u->u+,u-v->u+v+, (62)U0ξ=u-,v-,ξ<σ1,u-,v∗,σ1<ξ<σ2,u+,v+,ξ>σ2,σ1=u-,σ2=0, and v∗=u+v+/u-. It can be seen that U0(ξ) coincides with the Riemann solution for (3) with the same initial data.For the caseu-⩾0⩾u+, it has been shown that two shocks will coincide at ξ=0 as ϵ→0+. Furthermore, for the component uϵ(ξ), it has been proven that (63)limϵ→0+uϵξ=u-,ξ<0,0,ξ=0,u+,ξ>0.For the component vϵ(ξ), we have proven that the intermediate state v∗ϵ becomes infinity as ϵ→0+. Next let us study in more detail the limit behaviors of vϵ(ξ).Lemma 14.
One has(64)limϵ→0+σ2ϵ-σ1ϵv∗ϵ=u-v--u+v+.Proof.
With (49)–(51), it follows that (65)limϵ→0+σ2ϵ-σ1ϵv∗ϵ=limϵ→0+12·u+-u-v∗ϵ+u+2+4ϵv∗ϵ+u-2+4ϵv∗ϵv∗ϵ=limϵ→0+12·u-v--u+v++v+u+2+4ϵv∗ϵ+v-u-2+4ϵv∗ϵ=u-v--u+v+.The proof is finished.Takeϕ(ξ)∈C0∞(-∞,+∞) such that ϕ(ξ)≡ϕ(0) for ξ in a neighborhood Ω of ξ=0 (ϕ is called a sloping test function). Assume when ϵ<M0 it holds that σ1ϵ∈Ω and σ2ϵ∈Ω. It is well known that solution (48) satisfies the weak formula(66)-∫-∞+∞vϵuϵ-ξϕ′dξ+∫-∞+∞vϵϕdξ=0.Since (67)∫-∞+∞vϵuϵ-ξϕ′dξ=∫-∞σ1ϵ+∫σ2ϵ+∞vϵuϵ-ξϕ′dξ,we have (68)limϵ→0+∫-∞+∞vϵuϵ-ξϕ′dξ=limϵ→0+∫-∞σ1ϵv-u--ξϕ′dξ+limϵ→0+∫σ2ϵ+∞v+u+-ξϕ′dξ=u-v--u+v+ϕ0+∫-∞+∞Hξϕdξ,where (69)Hξ=v-,ξ<0,v+,ξ>0.Returning to (66), we get(70)limϵ→0+∫-∞+∞vϵ-Hξϕdξ=u-v+-u+v-ϕ0for all sloping test functions ϕ(ξ)∈C0∞(-∞,+∞).For an arbitraryφ(ξ)∈C0∞(-∞,+∞), we take a sloping function ϕ(ξ) such that ϕ(0)=φ(0) and (71)maxξ∈-∞,+∞ϕ-φ<μ.We have (72)limϵ→0+∫-∞+∞vϵ-Hξφdξ=limϵ→0+∫-∞+∞vϵ-Hξϕdξ+limϵ→0+∫-∞+∞vϵ-Hξφ-ϕdξ.For the first limit on the right side, it follows that (73)limϵ→0+∫-∞+∞vϵ-Hξϕdξ=u-v--u+v+ϕ0=u-v--u+v+φ0.For the second limit on the right side, it follows that(74)∫-∞+∞vϵ-Hξφ-ϕdξ=∫σ1ϵσ2ϵvϵ-Hξφ-ϕdξ=∫σ1ϵσ2ϵvϵφ-ϕdξ+∫σ1ϵσ2ϵHξφ-ϕdξ,which converges to 0 by sending μ→0 and recalling Lemma 14. Thus we have that(75)limϵ→0+∫-∞+∞vϵ-Hξφdξ=u-v+-u+v-φ0for all test functions φ(ξ)∈C0∞(-∞,+∞).Letψ(x,t)∈C0∞((-∞,+∞)×[0,+∞)) be a smooth test function, and let ψ~(ξ,t)≔ψ(ξt,t). Then it follows that(76)limϵ→0+∫0+∞∫-∞+∞vϵxtψx,tdxdt=limϵ→0+∫0+∞∫-∞+∞vϵξψξt,tdξtdt=limϵ→0+∫0+∞t∫-∞+∞vϵξψ~ξ,tdξdtand from (75) (77)limϵ→0+∫-∞+∞vϵξψ~ξ,tdξ=∫-∞+∞Hξψ~ξ,tdξ+u-v--u+v+ψ~0,t=t-1∫-∞+∞Hxψx,tdx+u-v--u+v+ψ0,t.Combining the two relations above yields (78)limϵ→0+∫0+∞∫-∞+∞vϵxtψx,tdxdt=∫0+∞∫-∞+∞Hxψx,tdxdt+∫0+∞u-v--u+v+tψ0,tdt.The last term, by the definition, (79)∫0+∞u-v--u+v+tψ0,tdt=wtδx=0,ψx,twith (80)wt=u-v--u+v+t.Thus we have the following result.Theorem 15.
Letu-⩾0⩾u+, and (uϵ(x,t),vϵ(x,t)) is the two-shock solution of (1) and (2). Then (uϵ(x,t),vϵ(x,t)) converges in the sense of distributions. Denote the limit function by U0(x,t); then (81)U0x,t=u-,v-,x<0,0,wtδx,x=0,u+,v+,x>0,where w(t)=(u-v--u+v+)t, which is just the delta-shock Riemann solution of (3).


## 5. Limits of Solutions of (1) and (2) for u+⩾0⩾u-

In this section, we study the limits of solutions of (1) and (2) as ϵ→0+ for the case u+⩾0⩾u- and show the phenomenon of cavitation and the formation of vacuum states in the limit.Lemma 16.
Ifu+⩾0⩾u-, then there exists ϵ1>0 such that (u+,v+)∈R←R→(u-,v-) for any ϵ<ϵ1.Proof.
Ifv+=v-, ϵ1 may be taken as any real positive number. If v+>v-, ϵ1 may be taken as the value of ϵ satisfying (82)u+2+4ϵv++u+u+2+4ϵv+-2u+=u-2+4ϵv-+u-u-2+4ϵv--2u-,u+>u-.If v+<v-, ϵ1 may be taken as the value of ϵ satisfying (83)u+2+4ϵv+-u+u+2+4ϵv++2u+=u-2+4ϵv--u-u-2+4ϵv-+2u-,u+>u-.The proof is complete.Letϵ-=(3u-2)/(4v-) and ϵ+=(3u+2)/(4v+), which mean u-2+4ϵ-v-+2u-=0 and u+2+4ϵ+v+-2u+=0. Denote ϵ2=minϵ-,ϵ+.Whenϵ1>ϵ>ϵ2, the Riemann solution for (1) and (2) contains two rarefaction waves with a nonvacuum intermediate state. This implies that, for a larger ϵ, no vacuum occurs in the solution.However, whenϵ decreases so that ϵ⩽ϵ1, the Riemann solution becomes two rarefaction waves with a vacuum intermediate state as follows:(84)Uϵξ=uϵ,vϵξ=u-,v-,-∞<ξ⩽λ1u-,v-,R←,λ1u-,v-⩽ξ⩽λ1u∗ϵ1,0,u∗ϵ1,0,λ1u∗ϵ1,0⩽ξ<0,u∗ϵ2,0,0<ξ⩽λ2u∗ϵ2,0,R→,λ2u∗ϵ2,0⩽ξ⩽λ2u+,v+,u+,v+,λ2u+,v+⩽ξ<+∞,where(85)R←:ξ=λ1ϵuϵ,vϵ=uϵ-uϵ2+4ϵvϵ2,uϵ2+4ϵvϵ-uϵuϵ2+4ϵvϵ+2uϵ=u-2+4ϵv--u-u-2+4ϵv-+2u-with v-⩾vϵ⩾0,0>u∗ϵ1⩾uϵ⩾u- and(86)R→:ξ=λ2ϵuϵ,vϵ=u+uϵ2+4ϵvϵ2,uϵ2+4ϵvϵ+uϵuϵ2+4ϵvϵ-2uϵ=u+2+4ϵv++u+u+2+4ϵv+-2u+with v+⩾vϵ⩾0,u+⩾uϵ⩾u∗ϵ2>0. The intermediate states (u∗ϵ1,0) and (u∗ϵ2,0) satisfy(87)u∗ϵ1-2u∗ϵ1=u-2+4ϵv--u-u-2+4ϵv-+2u-,-u∗ϵ22u∗ϵ2=u+2+4ϵv++u+u+2+4ϵv+-2u+.Whenϵ→0+, from (87), we have (88)limϵ→0+u∗ϵ1=u-,limϵ→0+u∗ϵ2=u+;then it follows that (89)limϵ→0+λ1ϵu-,v-=limϵ→0+λ1ϵu∗ϵ1,0=u-,limϵ→0+λ2ϵu+,v+=limϵ→0+λ2ϵu∗ϵ2,0=u+.In summary, as ϵ→0+, the limit function U0(ξ)=(u,v)(ξ) of Uϵ(ξ) is (90)U0ξ=u-,v-,-∞<ξ⩽u-,u-,0,u-⩽ξ<0,u+,0,0<ξ⩽u+,u+,v+,u+⩽ξ<+∞,which is just a solution of (3) and (2) containing a vacuum state that fills up the region formed by the two contact discontinuities ξ=u±.In Sections4 and 5, we have proven that when u->u+,u-v->u+v+ or u+⩾0⩾u-, the limits of solutions of the Riemann problem for (1) just are the solutions of the Riemann problem for (3) with the same initial data. The same conclusions are true for the rest of the cases, and we omit the discussions.


## 6. Numerical Simulations

To understand the formation processes of delta-shocks and vacuum states in the Riemann solutions of (1) and (2) as the flux ϵv vanishes, we present some representative numerical results. To discretize the system, we employ the Nessyahu-Tadmor scheme [28] with 500 cells and CFL = 0.475.To simulate the formation process of delta-shocks, we take the initial data as follows:(91)u,vx,t=0=1,1.6,x<0,-1,1,x>0.The numerical simulations for different choices of ϵ are presented in Figures 1–4.Figure 1
Velocity and density forϵ=0.2 at t=0.4.Figure 2
Velocity and density forϵ=0.08 at t=0.4.Figure 3
Velocity and density forϵ=0.006 at t=0.4.Figure 4
Velocity and density forϵ=0.0001 at t=0.4.One can observe clearly from these numerical results that, whenϵ decreases, the location of the two shocks becomes closer and closer, and the density of the intermediate state increases dramatically, while the velocity is closer to a step function. The numerical simulations are in complete agreement with the theoretical analysis in Section 4.To simulate the formation process of vacuum states, we take the initial data as follows:(92)u,vx,t=0=-0.8,1.2,x<0,0.5,1,x>0.The numerical simulations for different choices of ϵ are presented in Figures 5–8.Figure 5
Velocity and density forϵ=0.6 at t=0.3.Figure 6
Velocity and density forϵ=0.1 at t=0.3.Figure 7
Velocity and density forϵ=0.03 at t=0.3.Figure 8
Velocity and density forϵ=0.001 at t=0.3.One can see that whenϵ=0.6, the vacuum does not appear; when ϵ=0.1, the vacuum appears; when ϵ decreases to zero, the two rarefaction waves become two contact discontinuities. The numerical simulations are also in complete agreement with the theoretical analysis in Section 5.


## 7. Conclusions and Discussions

In the paper [2], Toth and Valko proposed the model of conservation laws(93)vt+uvx=0,ut+γu2+ϵvx=0,where γ and ϵ>0 are constants. It can describe a deposition/domain growth mechanism. When the term ϵv vanishes, system (93) becomes(94)vt+uvx=0,ut+γu2x=0.This is a typical model admitting delta-shocks.In this paper, we consider (93) for the case γ=0, that is, model (1), which arose in the context of the true self-repelling motion constructed by Toth and Werner [1]. Firstly, we solve the Riemann problem, which is very useful for the understanding of equations because all properties, such as shocks and rarefaction waves, appear as characteristics in the solution. By the analysis method in phase plane, we obtain five kinds of structures of solutions containing shock(s) and/or rarefaction wave(s). Secondly, we consider the flux-function limits of solutions of system (1). We prove that the Riemann solutions of system (1) just converge to the Riemann solutions of the limit system (3). We especially identify and analyze the formation of delta-shocks and vacuum states in the limit.For (93), the parameter γ is of crucial importance: different values of γ lead to completely different behaviors. One can carry out the investigation similar to that in this paper for some other cases, such as γ=1 (Leroux equation) and γ=1/2 (shallow water equation).



---

*Source: 8569435-2018-05-08.xml*