1. Introduction
An Eyring–Powell fluid is a sub-class of a non-Newtonian fluid of interest in applied sciences. To cite some examples, we remark the applications in manufacturing engineering [1,2] and biological technology [3,4].
The Eyring–Powell model has been of interest for the description of magnetohydrodynamics (MHD). As representative of previous studies, Akbar et al. [5] carried out the analysis of solutions in a two-dimensional MHD fluid. Hina [6] considered an Eyring–Powell fluid for MHD purposes to study heat-transfer processes. Afterward, Bhatti et al. [7] proposed an analysis for a stretching surface under MHD physical principles. Similarly, other references can be cited describing analyses of Eyring–Powell fluids, combining analytical and numerical approaches, from purely mathematical principles to applications in different physical scenarios [8,9,10,11,12,13,14,15,16,17,18,19].
It shall be noted that there exists much literature dealing with the existence criteria of solutions when a fluid is formulated with the classical Newtonian viscosity involved in the Navier–Stokes equations; see the remarkable studies [20,21,22,23,24,25,26,27,28,29]. Nonetheless, the specific rheological properties of a fluid may lead to the exploration of other kinds of viscosity formulations. One of these formulations, based on the kinetic theory of liquids, led to the mentioned Eyring–Powell fluid. To the best of our knowledge, there is not a wide range of literature dealing with the existence and regularity of solutions in Eyring–Powell fluids in three-dimensional geometry. Consequently, our main objective is to introduce such an analysis under the most general conditions.
Considering some recent achievements related to the application of advanced analytical tools to non-Newtonian fluids, we can highlight the recent work of Bilal et al. [30], where the expansion method was employed to obtain exact wave solutions to a Dullin–Gottwald–Holm system. In addition, the solutions to a Korteweg–deVries–Zakharov–Kuznetsov equation were explored in Ref. [31]. Based on a modified extended direct algebraic method, these authors found solutions in the form of solitary, shock, singular, shock singular, solitary shock and double singular solitons. In the present manuscript, we are concerned with the regularity and existence of solutions rather than with the specific forms of such solutions. This, however, establishes a basis for future research topics.
The paper layout is as follows. First, we introduce the framework of our study and describe the Eyring–Powell fluid model. Secondly, a set of three theorems is given so that their proofs permit to draw a conclusion on the regularity and existence of solutions to the proposed Eyring–Powell formulation. The introduced theorems are supported by a number of lemmas that are provided for the sake of clarity and by some propositions that are proved. The involved assessments follow a process that can be introduced sequentially as follows:
–. Formulation of the involved equations in integral form.
–. Derivation of a temporal differential equation in terms of spatial distributions in ; the involved integrals are assessed, typically by parts.
–. Introduction of hypotheses in a space of bounded mean oscillations that assure a bounded solution, and obtain the bounding constants.
–. Application of the Gronwall theorem for a bound in the temporal law, and under spatial distributions in
2. Model Formulation
We consider a flow of an electrically conducting Eyring–Powell fluid. The selection of this type of fluid is justified based on the following ideas. Firstly, the rheological properties of an Eyring–Powell fluid are derived based on the kinetic theory of liquids, instead of empirical or quasi-experimental principles; this can be the case of a Darcy–Forchheimer or a power–law fluid. Deducing a rheological law from a well-known theory makes the Eyring–Powell fluid interesting for purely mathematical assessments such that the analytical concepts rely on theoretical and well-proven physical aspects. Secondly, the Eyring–Powell rheological properties can be understood as an expansion of a typical linear fluid rheology. Then, the scope of our analysis contains some mathematical ideas that can be applied for the study of simpler rheological laws; this naturally extends to Newtonian fluids described by the classical continuity and momentum Navier–Stokes equations.
The Cartesian coordinates , with the corresponding velocity components , are chosen such that the origin is located in the plane sheet at . The fluid occupies the region , and flows from the sheet to .
The conservation of mass and momentum are described in a general basis as
(1)
(2)
where refers to the total derivative of the velocity field, is the fluid density, B is the applied longitudinal (along the z-axis) magnetic field of magnitude driving the flow, J is the current charges density, and refers to the Cauchy stress tensor, which is given by where p is the pressure field in the fluid, I is the identity tensor and is the stress tensor typical in Eyring–Powell fluid models.Based on the kinetic theory of liquids [32], a formulation to such stress tensor is
where is the dynamic viscosity, and and are two characteristic constants related to the fluid spatial behavior and its characteristic relaxation frequency, respectively [33]. By considering thenNote that we may consider higher-order terms, denoted by ‘⋯’ in the expression above, when approximating the function, or even other forms of rheological behavior; see the work of Oke [34] for additional insights.
We assume a boundary layer is developed and analyze the velocity profiles in each -plane for which the following Dirichlet boundary conditions apply: at and .
Based on the exposed arguments and taking and as characteristic values for length, time and density, the governing equations written in dimensionless parameters read [32]
(3)
(4)
in the domain , where refers to the cross-sectional aspect ratio, is the Reynolds number, characterizes the rheological behavior of the fluid, and is the dimensionless effective magnetic field inducing the flow. The kinematic Dirichlet boundary conditions now read at and . Note that the own magnetic field generated by the charges motion is assumed to be negligible.Further, the following conditions shall be considered as well:
(5)
where , are the initial velocity distributions in -planes and .3. Previous Definitions and Summary of Results
3.1. Previous Definitions and Results
Consider the Lebesgue norm to define the functional space . In addition, the usual Sobolev functional space of order m is considered as
with the normAs it will be specified later, we will establish the regularity criteria if and are sufficiently small. Note that BMO denotes the homogeneous space of ’bounded mean oscillations’ associated with the norm
For further details on BMO spaces, we refer the reader to Ref. [35].
In addition, we recall the following two lemmas.
Let us consider then
where is a constant.
For the proof of Lemma 1, we refer the reader to Ref. [35].
Given the functions , the following anisotropic inequality holds (see Ref. [22]):
where is a suitable constant, .3.2. Statement of Results
The main results obtained in this analysis are stated as follows.
Assume that In addition, assume that and are sufficiently small, then system (3)–(5) has a bounded global solution in .
Assuming that , then the system (3)–(5) has a global and bounded solution in .
Assume that and that , , ,
are sufficiently small, then the system (3)–(5) has a global and bounded solution in .
The proposed theorems are shown in the coming sections.
4. Proof of Theorem 1
The first intention is to show that the two-dimensional velocity profiles are globally bounded when the fluid is flowing through the z-axis. This means that for any level in the z-axis, the fluid flow exhibits a regular behavior. The following proposition is required to support the proof of Theorem 1.
(Global bound of the two-dimensional velocity profile). Given the set of solutions to Equations (3)–(5) with initial distributions and in the assumption of , , the following global bound holds in :
(6)
where and are suitable constants related with the set of parameters involved in Equations (3)–(5).Multiplying Equation (3) by and integrating
which implies that(7)
where Note that we used Equation (1).Developing further the integration on :
Since at then and Equation (7) becomes where we used Lemma 1.Now, provided that is sufficiently small, we can choose
and therefore, the above equation becomes which implies that(8)
Similarly, multiplying Equation (4) by and integrating again, we obtain
(9)
Adding Equations (8) and (9):
In most realistic cases, the rheological parameter is small , and one can apply the Gronwall inequality to obtain
in , where depend on Re, and M and should be upper bounded by . □From a physical point of view, this upper bound only applies to shear-thinning fluids with . This can be understood as a bound for the viscosity reduction that ensures an exponential or sub-exponential decrease in ) as so that the associated integrals remain finite. For shear-thickening fluids with , in contrast, the increase in viscosity with the applied shear ensures such exponential (or sub-exponential) decay and, in practice, removes any condition on . For additional insights about the rheological properties of Eyring–Powell fluids, the reader is referred to Ref. [34].
Note that the Theorem 1 is proved simply using the bound properties shown in Proposition 1.
5. Proof of Theorem 2
First, multiply Equation (3) by to make the following integration:
(10)
whereAs at , then and Equation (10) simplifies to
Since by initial assumption is sufficiently small, we can take
, and choosing
the following holdsThe application of the Gronwall inequality yields
where and refer to the Gronwall constants, compiling and , that depend on the dimensionless parameters of the problem. Again, should obey the same upper bound derived for .Proceeding similarly, multiplying the Equation () by , we obtain
with the same bounds for the involved constants. □We recall the previous discussion about these bounds in the shear-thinning and shear-thickening cases.
6. Proof of Theorem 3
Before showing the Theorem 3, the following proposition is required.
Assume . In addition, consider that are sufficiently small, then the solution to the set of Equations (3)–(5) satisfies
where is a suitable constant related to the dimensionless parameters involved in the set of Equations (3)–(5) and the BMO bound hypothesis.
Take the inner product in Equation (3) with and integrate with regards to the spatial variables to obtain
which implies that whereUsing Equation (1), we have
where .Since is very small, we can choose to obtain
Considering now the Young’s inequality
where we used Lemma 1.Since is sufficiently small for our purposes, we can choose . Therefore, becomes
where .Then, we arrive to
(11)
where .Similarly, multiplying Equation () by and after integration by parts, we have
(12)
Adding Equations (11) and (12),Finally, the Gronwall inequality yields
where depends on the dimensionless parameters of the problem. □Theorem 3 is shown by making use of the results obtained in Propositions 1 and 2.
Compared to the previous results, the unique bound required here is , which can be understood as a bound for the applied magnetic field.
7. Conclusions
In this paper, we developed the global existence of regular solutions for an Eyring–Powell fluid flowing along a semi-infinite conduct with a rectangular cross-section of dimensions , subjected to a constant longitudinal magnetic field of (dimensionless) magnitude . The initial velocity profiles were given in -planes along the z-axis, and the flow developed in the region . The following results were provided. Firstly, for , , a regular global solution was shown to hold. A similar existence result was proved in the case , . Finally, we obtained similar existence criteria for .
The proposed results can be of practical use to support the resolution of the Eyring–Powell fluid with numerical means. Prior to starting any numerical assessment, the regularity of the solutions can be interpreted based on the results outlined in this work. As a future research topic related to the proposed Eyring–Powell fluid, one can consider the possibility of understanding the behavior of the solutions together with their increasing or decreasing rate. A remarkable question to explore is related to the existence of an exponential profile for a special class of solutions known as traveling waves. The fact of having an exponential behavior leads to state the regularity of the solutions, and shall be compliant with the obtained results.
Conceptualization, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; methodology, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; validation, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; formal analysis, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; investigation, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; resources, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; writing—original draft preparation, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; writing—review and editing, S.u.R., J.L.D.P., N.T., P.S.S. and J.R.G.; funding acquisition, J.L.D.P. and J.R.G. All authors have read and agreed to the published version of the manuscript.
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Abstract
The purpose of the present study is to obtain regularity results and existence topics regarding an Eyring–Powell fluid. The geometry under study is given by a semi-infinite conduct with a rectangular cross section of dimensions
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1 Department of Mathematics, COMSATS University Islamabad, Abbottabad Campus, Abbottabad 22060, Pakistan
2 Department of Education, Universidad a Distancia de Madrid, 28400 Madrid, Spain
3 Spanish User Support and Operations Centre, Center for Computational Simulation, Universidad Politécnica de Madrid, 28223 Madrid, Spain