# Lie Group Analysis of Mixed Convection  Flow with Mass Transfer over a Stretching Surface  with Suction or Injection

Authors: Mudassar Jalil; Saleem Asghar; Muhammad Mushtaq
Journal: Mathematical Problems in Engineering
 (2010)

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## Abstract

The mixed convection flow with mass transfer over a stretching surface with suction or injection is examined. By using Lie group analysis, the symmetries of the equations are calculated. A four-finite parameter and one infinite parameter Lie group transformations are obtained. Two different cases are discussed, one for the scaling symmetry and the other for spiral symmetry. The governing partial differential equations are transformed into ordinary differential equations using these symmetries. It has been noted that the similarity variables and functions available in the literature become special cases of the similarity variables and functions discussed in this paper.


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## Body

## 1. Introduction

The study of continuously stretching sheets has many applications in manufacturing industries. Application of stretching sheets can be found in the areas like paper production, hot rolling, glass blowing, continuous casting of metals, and wire drawing. First of all Sakiadis [1, 2] investigated the boundary layer behavior on stretching surfaces and presented numerical solution for the sheet having constant speed. Extension to this problem where velocity is proportional to the distance from the slit was given by Crane [3]. Flow and heat transfer in the boundary layer on stretching surface was studied by Tsou et al. [4]. Fox et al. [5] presented different methods (analytical or numerical) for solving problems of stretching sheet with suction and injection. Heat and mass transfer on stretched surface with suction and injection was introduced by Erickson et al. [6]. P. S. Gupta and A. S. Gupta [7] studied the same problem for linearly stretching sheet. Heat transfer past a moving continuous plate with variable temperature was studied by Soundalgekar and Murty [8] and Grubka et al. [9]. Ali [10] presented similarity solutions for stretched surface  with suction and injection. Hayat et al. [11] investigated the effect of heat and mass transfer for Soret and Dufour's effect on mixed convection boundary layer flow over a stretching vertical surface in a porous medium filled with a viscoelastic fluid.Lie group analysis is a classical method discovered by Norwegian mathematician Sophus Lie for finding invariant and similarity solutions [12–15]. Yürüsoy and Pakdemirli [16] presented exact solution of boundary layer equations of a special non-Newtonian fluid over a stretching sheet by the method of Lie group analysis. They extended their work to creeping flow of second-grade fluid [17]. Sivasankaran et al. [18, 19] studied the problem of natural convection heat and mass transfer flow past an inclined plate for various parameters using Lie group analysis without and with heat generation.In this paper, Lie group analysis of mixed convection flow with mass transfer over a stretching surface with suction or injection is studied. Here the application of Lie group analysis for the fluid flow problem is discussed, with the help of examples from the literature, in such a way that one can easily understand its importance for finding similarity variables and functions. There are two important features of this work that make it different from the other. First, instead of taking a specific form of the boundary conditions (i.e., stretching velocity of the surface, wall temperature, and concentration), these are taken as arbitrary functions. Second, symmetry operator is applied not only to the equations of the problem but also to the boundary conditions of the problem in such a way that with infinite parameter Lie group transformations those finite parameters from infinitesimals are also discarded under which boundary conditions fail to be invariant. It has been wiliness that the number of symmetries reduce in order to comply with the invariance of boundary conditions. It is shown that mixed convection flow with mass transfer is possible only for two types of vertical stretching, the polynomial stretching and the exponential stretching. From examples it is shown that the similarities of the problems discussed by Ali and Al-Yousef [20], Yih [21], and Chen [22] become special subcases of the case of polynomial stretching and the problem discussed by Partha et al. [23] becomes a special subcase of the exponential stretching case. In the case of polynomial stretching a new example is included.


## 2. Equations of Motion

Consider a two-dimensional laminar flow of a steady incompressible viscous fluid over a permeable surface stretching with velocityuw(x̃). The temperature distribution and concentration at the surface are Tw(x̃) and Cw(x̃), respectively. The surface is moving through a quiescent ambient fluid with constant temperature T̃∞ and constant concentration C̃∞. vw(x̃) is the suction or injection velocity through the surface. Here we do not assume a specific form of velocity, temperature, and concentration variations for the stretching surface. Using Boussinesq approximations, the governing equations for the boundary layer flows over a stretching surface with suction or injection are expressed as(2.1)∂ũ∂x̃+∂ṽ∂ỹ=0,ũ∂ũ∂x̃+ṽ∂ũ∂ỹ=ν∂2ũ∂ỹ2+gβ(T̃-T̃∞)+gβ*(C̃-C̃∞),ũ∂T̃∂x̃+ṽ∂T̃∂ỹ=α∂2T̃∂ỹ2,ũ∂C̃∂x̃+ṽ∂C̃∂ỹ=D∂2C̃∂ỹ2,
along with the following boundary conditions:(2.2)ũ=U0uw(x̃),T̃-T̃∞=T0Tw(x̃),ṽ=V0vw(x̃),C̃-C̃∞=C0Cw(x̃)atỹ=0,ũ→0,T̃→T̃∞,C̃→C̃∞asỹ→∞,
where ũ and ṽ are the components of velocity in x̃ and ỹ directions, ν is the fluid kinematic viscosity, g is the gravitational acceleration, β is the coefficient of thermal expansion, β* is the coefficient of concentration expansion, α is the thermal diffusivity, and D is the mass diffusivity.U0 and V0 are the reference velocities, whereas T0 and C0 are the reference temperature and mass concentration, respectively.Introducing the nondimensional parameters(2.3)x=x̃L,y=ỹL(U0Lν)1/2,u=ũU0,v=ṽU0(U0Lν)1/2,T=(T̃-T̃∞)T0,C=(C̃-C̃∞)C0,
in (2.1), one gets(2.4)∂u∂x+∂v∂y=0,(2.5)u∂u∂x+v∂u∂y=∂2u∂y2+gβT0LU02T+gβ*C0LU02C,(2.6)u∂T∂x+v∂T∂y=1Pr∂2T∂y2,(2.7)u∂C∂x+v∂C∂y=1Sc∂2C∂y2,
where Pr=ν/α is the Prandtl number and Sc=ν/D is the Schmidt number. Boundary conditions for the above equations are(2.8)y=0;u=uw(x),v=V0vw(x)U0(U0Lν)1/2,T=Tw(x),C=Cw(x),y→∞;u=0,T=0,C=0.


## 3. Symmetries of the Problem

By applying Lie group method [12] to (2.4)–(2.7), the infinitesimal generator for the problem can be written as(3.1)X=ξ1∂∂x+ξ2∂∂y+ϕ1∂∂u+ϕ2∂∂v+ϕ3∂∂T+ϕ4∂∂C.One requires that the equations remain invariant under the infinitesimal  Lie point transformations(3.2)x*=x+ϵξ1(x,y,u,v,T,C)+O(ϵ2),y*=y+ϵξ2(x,y,u,v,T,C)+O(ϵ2),u*=u+ϵϕ1(x,y,u,v,T,C)+O(ϵ2),v*=v+ϵϕ2(x,y,u,v,T,C)+O(ϵ2),T*=T+ϵϕ3(x,y,u,v,T,C)+O(ϵ2),C*=C+ϵϕ4(x,y,u,v,T,C)+O(ϵ2).By employing a tedious and straightforward algebra, the form of the infinitesimals is(3.3)ξ1=c+dx,ξ2=by+γ(x),ϕ1=(-2b+d)u,ϕ2=-bv+uγ′(x),ϕ3=-e+(-4b+d)T,ϕ4=βT0β*C0e+(-4b+d)C.Therefore, the equations admit four finite parameter Lie group transformations.γ(x) is infinite parameter Lie group transformation. Parameter c corresponds to the translation in the variable x, and parameter e corresponds to the translation in the variables T and C. Parameter b corresponds to scaling in the variables y, u, v, T, and C. Parameter d corresponds to the scaling in the variables x, u, T, and C. In the following sections, similarity variables and functions corresponding to the above symmetries will be derived.Before finding similarity variables and functions for all of the symmetries of the problem, the symmetry operatorX will be applied to the boundary conditions and then only those symmetries will be discussed that leave the boundary conditions invariant.


## 4. Invariance of the Boundary Conditions

By applying the symmetry operatorX, that is defined in (3.1), on boundary conditions, we get differential equations(4.1)∂uw∂x-(-2b+d)(c+dx)uw=0,∂vw∂x+b(c+dx)vw=0,∂Tw∂x-(-4b+d)(c+dx)Tw=0,∂Cw∂x-(-4b+d)(c+dx)Cw=0,
with some restrictions e=γ(x)=0.Therefore, the possible boundary conditions for the current problem are as follows: ford≠0,(4.2)uw(x)=A1(c+dx)(-2b+d)/d,vw(x)=A2(c+dx)-b/d,Tw(x)=A3(c+dx)(-4b+d)/d,Cw(x)=A4(c+dx)(-4b+d)/d.
For d=0 and c≠0,(4.3)uw(x)=A5exp(-2bcx),vw(x)=A6exp(-bcx),Tw(x)=A7exp(-4bcx),Cw(x)=A8exp(-4bcx),
where Ai for i=1,2,3,…,8 are constants of integration.


## 5. Determining Similarity Variables and Functions

From (4.2) and (4.3) it is shown that only two types of vertical stretching are possible: polynomial stretching for d≠0 and exponential stretching for d=0. In the following work each case will be described with examples.


### 5.1. Case  1: Ford≠0

In this case, the parametersb, c, and d are taken as arbitrary and e=γ(x)=0 is considered. This case is combination of scaling and translational symmetry. For c=0 it becomes a scaling symmetry.The characteristic equations for obtaining the similarity transformations would then be(5.1)dx(c+dx)=dyby=du(-2b+d)u=dv-bv=dT(-4b+d)T=dC(-4b+d)C.
The resulting similarity variable is(5.2)η=yK11/2(c+dx)b/d,
and the similarity functions are(5.3)u=(c+dx)(-2b+d)/dF′(η),v=(c+dx)-b/dH(η),T=(c+dx)(-4b+d)/dθ(η),C=(c+dx)(-4b+d)/dφ(η).
From (4.2) and (5.3), we have for Ai=1  (i=1,2,3,4)(5.4)u=uw(x)F′(η),v=-vw(x)H(η),T=Tw(x)θ(η),C=Cw(x)φ(η).Using the continuity equation we can define stream functionψ such that(5.5)u=∂ψ∂y,v=-∂ψ∂x.
Integrating (5.5) by using (5.3), we obtain(5.6)ψ=K11/2(c+dx)(-b+d)/dF(η).
By putting value of  ψ from (5.6) into (5.5), then comparing with (5.3) the form of H(η) becomes(5.7)H(η)=K11/2[(-b+dd)F-bηF′].Substituting the above new variables in (2.5)–(2.7), we get the following system of ordinary differential equations:(5.8)F′′′+K1(-b+d)dFF′′-K1(-2b+d)dF'2+K1GrRe2θ+K1GcRe2φ=0,(5.9)1Prθ′′+K1(-b+d)dFθ′-K1(-4b+d)dF′θ=0,(5.10)1Scφ′′+K1(-b+d)dFφ′-K1(-4b+d)dF′φ=0,
and boundary conditions are transformed to(5.11)F′(0)=1,F(0)=-d*1K11/2((-b+d)/d),θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where Gr=gβT0Tw(x)L3(c+dx)3/ν2 is the thermal Grashof number, Re=U0uw(x)L(c+dx)/νis the Reynold number, and Gc=gβT0Tw(x)L3(c+dx)3/ν2 is the solutal Grashof number.In  (5.11)(5.12)d*=vw(x)U0uw(x)(U0uw(x)L(c+dx)ν)1/2.
By using values of uw(x) and vw(x) in (5.12), we get(5.13)d*=V0U0(U0Lν)1/2,
which is a nondimensional constant.


#### 5.1.1. Example  1

Forc=0, b=(1-m)/2, d=1, K1=2/(m+1), and -4b+d=n, the similarity variable and similarity function are(5.14)η=y(m+12)1/2(x)(m-1)/2,ψ=(2m+1)1/2(x)(m+1)/2F(η).
Then uw(x), Tw(x), and Cw(x) take the following form:(5.15)uw(x)=xm,Tw(x)=xn,Cw(x)=xn.
For these variables (5.8)–(5.10) become(5.16)F′′′+FF′′-2mm+1F+2λm+1θ+2λ*m+1φ=0,1Prθ′′+Fθ′-2nm+1F′θ=0,1Scφ′′+Fφ′-2nm+1F′φ=0,
subject to boundary conditions(5.17)F′(0)=1,F(0)=-d*2m+1,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2. In this case from (5.3) and (5.7) we have(5.18)v=-(2m+1)1/2x(m-1)/2[(m+12)F+(m-12)ηF′].
This is all the same as that taken by Ali and Al-Yousef [20] for the problem of mixed convection flow over a stretching surface without considering mass transfer.


#### 5.1.2. Example  2

Consider the case forc=0, b=0, d=1, and K1=1. In this case the form of similarity variable and similarity function is(5.19)η=y,ψ=xF(η).
Therefore, we have(5.20)uw(x)=x,Tw(x)=x,Cw(x)=x,
in consequence of these, (5.8)–(5.10) can be written as(5.21)F′′′+FF′′-F'2+λθ+λ*φ=0,1Prθ′′+Fθ′-F′θ=0,1Scφ′′+Fφ′-F′φ=0,
subject to boundary conditions(5.22)F′(0)=1,F(0)=Fw,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where Fw=-d*.This problem exactly match to the work done by Yih [21] if we neglect MHD effects in his work.


#### 5.1.3. Example  3

In the absence of suction or injection(5.23)vw(x̃)=0atỹ=0.
In such case for c=0, b=(1-m)/2, d=1, and K1=1, the similarity variable and similarity function are obtained as(5.24)η=y(x)(m-1)/2,ψ=(x)(m+1)/2F(η).
We then have(5.25)uw(x)=xm,Tw(x)=x2m-1,Cw(x)=x2m-1,
and (5.8)–(5.10) take the form(5.26)F′′′+(m+12)FF′′-mF'2+λθ+λ*φ=0,1Prθ′′+(m+12)Fθ′-(2m-1)F′θ=0,1Scφ′′+(m+12)Fφ′-(2m-1)F′φ=0,
together with the boundary conditions(5.27)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0.
This in fact is the problem considered by Chen [22] neglecting mass transfer.


#### 5.1.4. Example  4

Forb=-2, c=1, d=2, and K1=2. The physical quantities uw(x), Tw(x), and Cw(x) take the following form(5.28)uw(x)=(1+2x)3,Tw(x)=(1+2x)5,Cw(x)=(1+2x)5,
and the similarity variable and functions are(5.29)η=y(1+2x)2,ψ=2(1+2x)2F(η),T=Tw(x)θ(η),C=Cw(x)φ(η),
as a result (5.8)–(5.10) become(5.30)F′′′+4FF′′-6F'2+2λθ+2λ*φ=0,1Prθ′′+4Fθ′-10F′θ=0,1Scφ′′+4Fφ′-10F′φ=0,
subject to the boundary conditions(5.31)F′(0)=1,F(0)=-d*,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where d* is the same as given in (5.13).Such types of stretching velocity, wall temperature, and concentration have not been discussed so far.


### 5.2. Case  2: Ford=0

In this section, the parametersb and c are taken to be arbitrary and e=d=γ(x)=0 is considered. This becomes a spiral symmetry where we have considered translation in the variable x and scaling in the variables y, u, v,T, and C. Setting b=0 results in a translational symmetry.The characteristic equations then are(5.32)dxc=dyby=du-2bu=dv-bv=dT-4bT=dC-4bC.
The similarity variable is (5.33)η=yK21/2exp((b/c)x),
and the similarity functions are(5.34)u=exp(-2bcx)F′(η),v=exp(-bcx)H(η),T=exp(-4bcx)θ(η),C=exp(-4bcx)φ(η).
From (4.3) and (5.34) we have for Ai=1(i=5,6,7,8)(5.35)u=uw(x)F′(η),v=-vw(x)H(η),T=Tw(x)θ(η),C=Cw(x)φ(η).
Following the same procedure as done in case 1, we have(5.36)ψ=K21/2exp(-bcx)F(η).
Note that the variables will be balanced if(5.37)H(η)=bcK21/2[F+ηF′].
In this case, (2.5)–(2.7) will take the following form(5.38)F′′′-K2bcFF′′+K2bcF'2+K2GrRe2θ+K2GcRe2φ=0,1Prθ′′-K2bcFθ′+4K2bcF′θ=0,1Scφ′′-K2bcFφ′+4K2bcF′φ=0,
subject to the boundary conditions(5.39)F′(0)=1,F(0)=-d*1K21/2(b/c),θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where the nondimensional physical parameters are(5.40)Gr=gβT0Tw(x)L3ν2,Re=U0uw(x)Lν,Gc=gβ*T0Tw(x)L3ν2,(5.41)d*=vw(x)U0uw(x)(U0uw(x)Lν)1/2.
By incorporating (5.34) in (5.41), we get(5.42)d*=V0U0(U0Lν)1/2.
Again d* is a nondimensional constant.


#### 5.2.1. Example  1

In the case of rigid surface(5.43)vw(x̃)=0atỹ=0.
For b/c=-1/2 and K2=2 the similarity variable and function are(5.44)η=y[exp(x)2]1/2,ψ=[2exp(x)]1/2F(η),
and the surface velocity, temperature, and mass concentration are(5.45)uw(x)=exp(x),Tw(x)=exp(2x),Cw(x)=exp(2x).
With the above functions (5.38) become(5.46)F′′′+FF′′-2F'2+2λθ+2λ*φ=0,1Prθ′′+Fθ′-4F′θ=0,1Scφ′′+Fφ′-4F′φ=0,
and boundary conditions are (5.47)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2.Which is the same as that taken by Partha et al. [23] for mixed convection flow along exponentially stretching surface without considering the mass transfer. In that problem effects of viscous dissipation were examined that are not considered in this paper.


## 5.1. Case  1: Ford≠0

In this case, the parametersb, c, and d are taken as arbitrary and e=γ(x)=0 is considered. This case is combination of scaling and translational symmetry. For c=0 it becomes a scaling symmetry.The characteristic equations for obtaining the similarity transformations would then be(5.1)dx(c+dx)=dyby=du(-2b+d)u=dv-bv=dT(-4b+d)T=dC(-4b+d)C.
The resulting similarity variable is(5.2)η=yK11/2(c+dx)b/d,
and the similarity functions are(5.3)u=(c+dx)(-2b+d)/dF′(η),v=(c+dx)-b/dH(η),T=(c+dx)(-4b+d)/dθ(η),C=(c+dx)(-4b+d)/dφ(η).
From (4.2) and (5.3), we have for Ai=1  (i=1,2,3,4)(5.4)u=uw(x)F′(η),v=-vw(x)H(η),T=Tw(x)θ(η),C=Cw(x)φ(η).Using the continuity equation we can define stream functionψ such that(5.5)u=∂ψ∂y,v=-∂ψ∂x.
Integrating (5.5) by using (5.3), we obtain(5.6)ψ=K11/2(c+dx)(-b+d)/dF(η).
By putting value of  ψ from (5.6) into (5.5), then comparing with (5.3) the form of H(η) becomes(5.7)H(η)=K11/2[(-b+dd)F-bηF′].Substituting the above new variables in (2.5)–(2.7), we get the following system of ordinary differential equations:(5.8)F′′′+K1(-b+d)dFF′′-K1(-2b+d)dF'2+K1GrRe2θ+K1GcRe2φ=0,(5.9)1Prθ′′+K1(-b+d)dFθ′-K1(-4b+d)dF′θ=0,(5.10)1Scφ′′+K1(-b+d)dFφ′-K1(-4b+d)dF′φ=0,
and boundary conditions are transformed to(5.11)F′(0)=1,F(0)=-d*1K11/2((-b+d)/d),θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where Gr=gβT0Tw(x)L3(c+dx)3/ν2 is the thermal Grashof number, Re=U0uw(x)L(c+dx)/νis the Reynold number, and Gc=gβT0Tw(x)L3(c+dx)3/ν2 is the solutal Grashof number.In  (5.11)(5.12)d*=vw(x)U0uw(x)(U0uw(x)L(c+dx)ν)1/2.
By using values of uw(x) and vw(x) in (5.12), we get(5.13)d*=V0U0(U0Lν)1/2,
which is a nondimensional constant.


### 5.1.1. Example  1

Forc=0, b=(1-m)/2, d=1, K1=2/(m+1), and -4b+d=n, the similarity variable and similarity function are(5.14)η=y(m+12)1/2(x)(m-1)/2,ψ=(2m+1)1/2(x)(m+1)/2F(η).
Then uw(x), Tw(x), and Cw(x) take the following form:(5.15)uw(x)=xm,Tw(x)=xn,Cw(x)=xn.
For these variables (5.8)–(5.10) become(5.16)F′′′+FF′′-2mm+1F+2λm+1θ+2λ*m+1φ=0,1Prθ′′+Fθ′-2nm+1F′θ=0,1Scφ′′+Fφ′-2nm+1F′φ=0,
subject to boundary conditions(5.17)F′(0)=1,F(0)=-d*2m+1,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2. In this case from (5.3) and (5.7) we have(5.18)v=-(2m+1)1/2x(m-1)/2[(m+12)F+(m-12)ηF′].
This is all the same as that taken by Ali and Al-Yousef [20] for the problem of mixed convection flow over a stretching surface without considering mass transfer.


### 5.1.2. Example  2

Consider the case forc=0, b=0, d=1, and K1=1. In this case the form of similarity variable and similarity function is(5.19)η=y,ψ=xF(η).
Therefore, we have(5.20)uw(x)=x,Tw(x)=x,Cw(x)=x,
in consequence of these, (5.8)–(5.10) can be written as(5.21)F′′′+FF′′-F'2+λθ+λ*φ=0,1Prθ′′+Fθ′-F′θ=0,1Scφ′′+Fφ′-F′φ=0,
subject to boundary conditions(5.22)F′(0)=1,F(0)=Fw,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where Fw=-d*.This problem exactly match to the work done by Yih [21] if we neglect MHD effects in his work.


### 5.1.3. Example  3

In the absence of suction or injection(5.23)vw(x̃)=0atỹ=0.
In such case for c=0, b=(1-m)/2, d=1, and K1=1, the similarity variable and similarity function are obtained as(5.24)η=y(x)(m-1)/2,ψ=(x)(m+1)/2F(η).
We then have(5.25)uw(x)=xm,Tw(x)=x2m-1,Cw(x)=x2m-1,
and (5.8)–(5.10) take the form(5.26)F′′′+(m+12)FF′′-mF'2+λθ+λ*φ=0,1Prθ′′+(m+12)Fθ′-(2m-1)F′θ=0,1Scφ′′+(m+12)Fφ′-(2m-1)F′φ=0,
together with the boundary conditions(5.27)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0.
This in fact is the problem considered by Chen [22] neglecting mass transfer.


### 5.1.4. Example  4

Forb=-2, c=1, d=2, and K1=2. The physical quantities uw(x), Tw(x), and Cw(x) take the following form(5.28)uw(x)=(1+2x)3,Tw(x)=(1+2x)5,Cw(x)=(1+2x)5,
and the similarity variable and functions are(5.29)η=y(1+2x)2,ψ=2(1+2x)2F(η),T=Tw(x)θ(η),C=Cw(x)φ(η),
as a result (5.8)–(5.10) become(5.30)F′′′+4FF′′-6F'2+2λθ+2λ*φ=0,1Prθ′′+4Fθ′-10F′θ=0,1Scφ′′+4Fφ′-10F′φ=0,
subject to the boundary conditions(5.31)F′(0)=1,F(0)=-d*,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where d* is the same as given in (5.13).Such types of stretching velocity, wall temperature, and concentration have not been discussed so far.


## 5.1.1. Example  1

Forc=0, b=(1-m)/2, d=1, K1=2/(m+1), and -4b+d=n, the similarity variable and similarity function are(5.14)η=y(m+12)1/2(x)(m-1)/2,ψ=(2m+1)1/2(x)(m+1)/2F(η).
Then uw(x), Tw(x), and Cw(x) take the following form:(5.15)uw(x)=xm,Tw(x)=xn,Cw(x)=xn.
For these variables (5.8)–(5.10) become(5.16)F′′′+FF′′-2mm+1F+2λm+1θ+2λ*m+1φ=0,1Prθ′′+Fθ′-2nm+1F′θ=0,1Scφ′′+Fφ′-2nm+1F′φ=0,
subject to boundary conditions(5.17)F′(0)=1,F(0)=-d*2m+1,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2. In this case from (5.3) and (5.7) we have(5.18)v=-(2m+1)1/2x(m-1)/2[(m+12)F+(m-12)ηF′].
This is all the same as that taken by Ali and Al-Yousef [20] for the problem of mixed convection flow over a stretching surface without considering mass transfer.


## 5.1.2. Example  2

Consider the case forc=0, b=0, d=1, and K1=1. In this case the form of similarity variable and similarity function is(5.19)η=y,ψ=xF(η).
Therefore, we have(5.20)uw(x)=x,Tw(x)=x,Cw(x)=x,
in consequence of these, (5.8)–(5.10) can be written as(5.21)F′′′+FF′′-F'2+λθ+λ*φ=0,1Prθ′′+Fθ′-F′θ=0,1Scφ′′+Fφ′-F′φ=0,
subject to boundary conditions(5.22)F′(0)=1,F(0)=Fw,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where Fw=-d*.This problem exactly match to the work done by Yih [21] if we neglect MHD effects in his work.


## 5.1.3. Example  3

In the absence of suction or injection(5.23)vw(x̃)=0atỹ=0.
In such case for c=0, b=(1-m)/2, d=1, and K1=1, the similarity variable and similarity function are obtained as(5.24)η=y(x)(m-1)/2,ψ=(x)(m+1)/2F(η).
We then have(5.25)uw(x)=xm,Tw(x)=x2m-1,Cw(x)=x2m-1,
and (5.8)–(5.10) take the form(5.26)F′′′+(m+12)FF′′-mF'2+λθ+λ*φ=0,1Prθ′′+(m+12)Fθ′-(2m-1)F′θ=0,1Scφ′′+(m+12)Fφ′-(2m-1)F′φ=0,
together with the boundary conditions(5.27)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0.
This in fact is the problem considered by Chen [22] neglecting mass transfer.


## 5.1.4. Example  4

Forb=-2, c=1, d=2, and K1=2. The physical quantities uw(x), Tw(x), and Cw(x) take the following form(5.28)uw(x)=(1+2x)3,Tw(x)=(1+2x)5,Cw(x)=(1+2x)5,
and the similarity variable and functions are(5.29)η=y(1+2x)2,ψ=2(1+2x)2F(η),T=Tw(x)θ(η),C=Cw(x)φ(η),
as a result (5.8)–(5.10) become(5.30)F′′′+4FF′′-6F'2+2λθ+2λ*φ=0,1Prθ′′+4Fθ′-10F′θ=0,1Scφ′′+4Fφ′-10F′φ=0,
subject to the boundary conditions(5.31)F′(0)=1,F(0)=-d*,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where d* is the same as given in (5.13).Such types of stretching velocity, wall temperature, and concentration have not been discussed so far.


## 5.2. Case  2: Ford=0

In this section, the parametersb and c are taken to be arbitrary and e=d=γ(x)=0 is considered. This becomes a spiral symmetry where we have considered translation in the variable x and scaling in the variables y, u, v,T, and C. Setting b=0 results in a translational symmetry.The characteristic equations then are(5.32)dxc=dyby=du-2bu=dv-bv=dT-4bT=dC-4bC.
The similarity variable is (5.33)η=yK21/2exp((b/c)x),
and the similarity functions are(5.34)u=exp(-2bcx)F′(η),v=exp(-bcx)H(η),T=exp(-4bcx)θ(η),C=exp(-4bcx)φ(η).
From (4.3) and (5.34) we have for Ai=1(i=5,6,7,8)(5.35)u=uw(x)F′(η),v=-vw(x)H(η),T=Tw(x)θ(η),C=Cw(x)φ(η).
Following the same procedure as done in case 1, we have(5.36)ψ=K21/2exp(-bcx)F(η).
Note that the variables will be balanced if(5.37)H(η)=bcK21/2[F+ηF′].
In this case, (2.5)–(2.7) will take the following form(5.38)F′′′-K2bcFF′′+K2bcF'2+K2GrRe2θ+K2GcRe2φ=0,1Prθ′′-K2bcFθ′+4K2bcF′θ=0,1Scφ′′-K2bcFφ′+4K2bcF′φ=0,
subject to the boundary conditions(5.39)F′(0)=1,F(0)=-d*1K21/2(b/c),θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where the nondimensional physical parameters are(5.40)Gr=gβT0Tw(x)L3ν2,Re=U0uw(x)Lν,Gc=gβ*T0Tw(x)L3ν2,(5.41)d*=vw(x)U0uw(x)(U0uw(x)Lν)1/2.
By incorporating (5.34) in (5.41), we get(5.42)d*=V0U0(U0Lν)1/2.
Again d* is a nondimensional constant.


### 5.2.1. Example  1

In the case of rigid surface(5.43)vw(x̃)=0atỹ=0.
For b/c=-1/2 and K2=2 the similarity variable and function are(5.44)η=y[exp(x)2]1/2,ψ=[2exp(x)]1/2F(η),
and the surface velocity, temperature, and mass concentration are(5.45)uw(x)=exp(x),Tw(x)=exp(2x),Cw(x)=exp(2x).
With the above functions (5.38) become(5.46)F′′′+FF′′-2F'2+2λθ+2λ*φ=0,1Prθ′′+Fθ′-4F′θ=0,1Scφ′′+Fφ′-4F′φ=0,
and boundary conditions are (5.47)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2.Which is the same as that taken by Partha et al. [23] for mixed convection flow along exponentially stretching surface without considering the mass transfer. In that problem effects of viscous dissipation were examined that are not considered in this paper.


## 5.2.1. Example  1

In the case of rigid surface(5.43)vw(x̃)=0atỹ=0.
For b/c=-1/2 and K2=2 the similarity variable and function are(5.44)η=y[exp(x)2]1/2,ψ=[2exp(x)]1/2F(η),
and the surface velocity, temperature, and mass concentration are(5.45)uw(x)=exp(x),Tw(x)=exp(2x),Cw(x)=exp(2x).
With the above functions (5.38) become(5.46)F′′′+FF′′-2F'2+2λθ+2λ*φ=0,1Prθ′′+Fθ′-4F′θ=0,1Scφ′′+Fφ′-4F′φ=0,
and boundary conditions are (5.47)F′(0)=1,F(0)=0,θ(0)=1,φ(0)=1,F′(∞)=0,θ(∞)=0,φ(∞)=0,
where λ=Gr/Re2 and λ*=Gc/Re2.Which is the same as that taken by Partha et al. [23] for mixed convection flow along exponentially stretching surface without considering the mass transfer. In that problem effects of viscous dissipation were examined that are not considered in this paper.


## 6. Conclusion

Mixed convection flow with mass transfer over a stretching surface with suction or injection is studied. Through Lie group analysis symmetries of equations are determined. By applying these similarity variables and functions, original PDEs are converted into ODEs. For specific values of parameters of Lie group transformations, the similarity variables and functions which are available in the literature become special case of our problem. Following the same procedure, now one can easily find the similarity variable and function of his own choice for the particular problem. Specially for stretching problem, use of integral power in boundary conditions is not necessary. Therefore it is a breakthrough in the study of similarity solutions in the sense that only those symmetries that accept restrictions from the boundary conditions are discussed and similarity variables and functions obtained in this way have physical importance. The methodology will help directly to obtain similarity variables and functions of the boundary value problems for any specific boundary value problem at hand.



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*Source: 264901-2010-11-07.xml*