Applications of Cattaneo–Christov fluxes on modelling the boundary value problem of Prandtl fluid comprising variable properties

Stretched flows have numerous applications in different industrial, biomedical and engineering processes. Current research is conducted to examine the flow phenomenon of Prandtl fluid model over a moveable surface. The phenomenon of mass and thermal transportation is based on generalized theory of Cattaneo–Christov which considers the involvement of relaxation times. In addition to these, variable characteristics of thermal conductivity and diffusion coefficient are considered as a function of temperature. The physical problem in Cartesian coordinate system is modeled via boundary layer theory which yields a coupled system of partial differential equations. Group scaling transportation is applied to model these PDEs system. The converted equations have been approximated via optimal homotopic scheme. The efficiency and validity of used approach has been shown by computing the error analysis and establishing a comparative study. It is noted that the enhancement in magnetic parameter plays a controlling role for velocity field and it augment the concentration and temperature fields. Furthermore, increase in thermal relaxation parameter and Prandtl number maintains the fluid temperature.

The rheology of non-Newtonian fluids has complex nature and it cannot be exactly described by the stress-strain relatively as proposed by Newton. Non-Newtonian materials appear frequently, and it has applications in different mechanisms. One of the important non-Newtonian fluid is Prandtl fluid model 1 which is describes the following constitutive relation where " τ " denotes the stress tensor for Prandtl model, ""A and C "" are material parameters. The classification of fluids (non-Newtonian) is based on their behaviors and shear thinning category of fluids is considered as important in fluids. The model related to Prandtl fluid is known as a shear thinning liquid. Human blood, paint, blood, industrial and polymers products are examples of Prandtl fluid. This kind of liquid performs like a non-Newtonian fluid. Several applications of such non-Newtonion fluid in sugar production, cement industry, making shampoos and drilling muds etc. Several researchers have worked on this model by considering the different physical effects under different circumstances and assumptions. For instance, Hamid et al. 1  www.nature.com/scientificreports/ radiation and magnetohydrodynamic influences. They studied the mass and thermal transport on Prandtl model by applying the boundary layer theory on the associated conservation laws. They obtained the solution via numerical method namely "Crank-Nicolson" coded in MATLAB symbolic computational package. They recorded that augmenting volume of unsteadiness parameter upsurges the momentum, thermal and concentration fields. Rajesh and Rajasekhara Gowd 2 discussed the rheology of Prandtl fluid during the transport of solute particles and thermal energy past a porous vertical channel. They also considered thermal radiation phenomena along with magnetic field. Later on, the governing model is transformed into PDEs by appropriate substitution and then solved analytically. They found a rise in velocity field against gravitational parameters. Eldabe et al. 3 focused on peristaltic motion of Prandtl liquid considering chemical reaction and electrical conductivity (variable) obeying the mixed convection in symmetric channel. They used differential transform procedure to tackle the resulting equation. They illustrated the solutions through graphs. They noticed the decline in pressure profile against chemical reaction parameters. Flow of Prandtl model over a rotating slippery sheet with convective condition, radiation and concentration dependent diffusion coefficient was investigated by Sajid et al. 4 They recorded the amplification in thermal profile for radiation parameter and decline in velocity field against rotating parameter. Reddy et al. 5 modeled the compartment of modified heat flux. They handled the resulting equations as a numerically. They reported that velocity profile escalates against elastics and Prandtl fluid parameters Transient numerical solution for Prandtl model in rotating surface was discussed by Le et al. 6 . They reported the phenomenon of bifurcation. Transportation of heat energy as well as mass has significant application in different field of applied sciences. Ahmad et al. 7 used non-Fourier's model to study the heating effects on micropolar fluid over a hot surface. They assumed the slip conditions, thermal radiation and magneto-hydrodynamic effect in their research. Numerically they handled the resulting modeled expression. They studied the entropy analysis and recorded that mounting value of material parameter and Reynolds number entrances the entropy generation. Tassadiq 8 studied the homotopic solution for micropolar hybrid nanofluid model with dissipation and Joule heating effects and they solved the modeled equations with the help of numerical approach. Through graphical solution, they predicted the decline in velocity filed against Hartmann number and micropolar parameters. Ahmad et al. 9 reported the phenomenon of shallow wave dispersive equation arising in mathematical physics. They presented the graphical analysis of two methods. Tulu and Ibrahim 10 studied the use of CNT-Ethylene Glycol to enhance the thermal transport over a rotating stretchable disc with modified heat flux model. They solved the nonlinear transport equation numerically. Through graphs they recorded the behavior of solution against numerous emerging parameters. They noticed that radial stretching of the disc improves the cooling effect which is beneficial to control the thermal stability of the system. Numerical solution of Jeffrey liquid model is captured by Mabood et al. 11 with heat transport phenomenon. Graphically they plotted the behavior of parameters for the assisting and opposing flow situations. They mentioned the decline in temperature field against Deborah number. Finite element analysis is applied to study the double diffusion theories on Casson model by Ali et al. 12 . They noticed the decrease in primary and secondary velocity against magnetic parameter and fluid parameter, where, rise in concentration and thermal profile is seen. Khan et al. 13 presented the theoretical analysis on thermal transport with the inclusion of different nanoparticles. They noticed the depreciation in thermal field for Prandtl number. Heat transport in Oldroyd-B model over elastic surface in inspected by Ramana et al. 14 numerically. Jakeer et al. 15 considered finite volume approach to study the thermal transport in lid-driven porous cavity. They plotted the streamlines against flow controlling parameters. Nadeem et al. 16 modeled the transport phenomenon of thermal energy in Newtonian liquid in the attendance of non-Fourier's theory. They investigated this transport phenomenon past heated surface via Optimal HAM approach. Ahmad et al. 17 scrutinized theory related to the non-Fourier in micropolar fluid considering the presence of heat absorption. They used the role nanoparticles in micropolar fluid and they also simulated the entropy generation mechanism. Ahmad et al. 18 simulated the features of thermal energy and solute particles in micropolar involvement of nanoparticles and hybrid nanoparticles in base fluid called water. They used the stretching heated surface formeasurement of considering features in the presence of non-Fourier's theory along with activation energy. Ahmad and Nadeem 19 analyzed the role of non-Fourier's law in the energy equation along with slip conditions (Thomson and Troian). Several recently studied contribution are reported in [20][21][22][23][24][25][26] and references therein. Available literature tells that no study is conducted with variable thermo-physical properties for 3D Prandtl model with double diffusion theories. This model covers all aspects which are missing in published work. The adjustment of present model is considered into following: Literature with detailed description is performed in "Introduction" section, fluid rheology with heat and mass transport phenomenon are listed in "Mathematical formulation of transport problem" section with dimensionless equations, "Numerical method for solution" section contains the methodology, "Discussion and analysis on graphical and tabular results" and "Concluding key points" sections describe the detailed discussion on graphical outcomes with conclusion.

Mathematical formulation of transport problem
Steady incompressible flow of Prandtl fluid heat and mass transport is considered in this project as shown in Fig. 1. The stretching of sheet is responsible to maintain the flow. The sheet is stretched in x-and y-direction respectively and flow region is z ≥ 0. The direction of uniform magnetic field is considered to normal of x-and y-directions as shown in Fig. 1. The characteristics of heat energy and diffusion of fluid particles are reported under the action of timrelaxations. The concept fluxes (heat and mass) including relaxation times are www.nature.com/scientificreports/ Using incompressibility and steady flow assumptions, we get where " Q " is heat flux, " V " represents velocity field, " K(T) " the thermal conductivity, " J " the mass flux, "D(T)" the diffusion coefficient, " α 1 " and " α 2 " are thermal and concentration relaxation times using the boundary layer theory the conservation laws for Prandtl model takes the form Boundary conditions (BCs) are necessary to compute the solution of derived problem and BCs of present model are With the use of following similarity variables governing laws with associated conditions reduces to www.nature.com/scientificreports/ where " β 1 " presents Prandtl fluid parameter, " β 2 " the elastic parameter, " α " ratio parameter, "M" the magnetic parameter, "Pr" the Prandtl number, "Sc" the Schmidt number, " ǫ 1 and ǫ 2 " are small parameters and " δ 1 and δ 2 " denotes thermal and concentration relaxation times.
Physical quantities. To understand the several practical applications, the study of stress, heat and mass transfer rate have significant use. The empirical relations of these quantities are the dimensionless form is

Numerical method for solution
In this section, comprehensive literature survey on the considered topic is presented and "Mathematical formulation of transport problem" section comprises the mathematical form of the considered model-along with associated conditions. The current approach has ability to simulate complex flow problems. Several analytical and numerical schemes exist for the solution of differential equations. Here, optimal homotopy scheme is proposed for the solution. This procedure requires the linear operators and initial guess for the start of algorithm. These are The operators in Eq. (20) obey 1 Pr 1 , U w = ax, V = by,  Table 1 is prepared to notice the error analysis against higher order approximations.

Discussion and analysis on graphical and tabular results
The      www.nature.com/scientificreports/ Figure 8 plots the role α on g ′ . It is estimated that from outcome sketch flow of fluid particles in y-and x-directions become enhanced by large values of α . Moreover, viscosity of fluid becomes decrease by applying the increment of α . So, less viscosity is generated for large values of α and secondary and primary velocities are enhanced. The ratio number is considered as ratio of horizontal movement and vertical movement. The motion of fluid particles is acerbated due to motion of elastic heated surface. Meanwhile, ration parameter plays a vital impact to maximize the motion in fluid particles.

Simulations of heat energy versus physical parameters.
The simulations of thermal energy are captured by variation of parameters called Prandtl (Pr) , very small (ǫ 1 ) and thermal relaxation (δ 1 ) by Figs. 9, 10 and 11. Figure 9 captures the variation in thermal energy with applying the large values of ǫ 1 . It is estimated that ǫ 1 is appeared due to role of changing of thermal conductivity with respect to temperature in dimensionless energy equation while the appearance of ǫ 1 decides the role of constant and variable thermal conductivities. From the Fig. 9, it can be clearly seen that enhancement in thermal energy is investigated versus large values of ǫ 1 . This enhancement in thermal energy is happened because ǫ 1 has direct relation versus temperature in temperature dependent thermal conductivity model. So, this direct relation generates maximum heat energy via large values of ǫ 1 . The parameter related to ǫ 1 is occurred due to variable thermal conductivity in energy equation (dimensionless). This parameter treats as linear function in thermal energy and an enhancement in ǫ 1 results increase the random motion of fluid particles due to temperature gradient. An increment in random motion of fluid particles correspondences increment in heat energy of fluid particles is happened. Hence, temperature enhances. Figure 10 indicates the characterization of thermal energy with higher values of Pr . As, Pr discusses the dual behavior of boundary layers regarding moment and thermal layers during the flow of fluid at boundary of the sheet. Actually, inverse relation between thermal layer and Pr . So, layers regarding thermal energy becomes declined versus the large values of Pr . Hence, increment in Pr results the reduction in thermal energy. Physically, Prandtl is considered as a fractional between the momentum boundary and thermal boundary layers. By the definition of Pr , layers regarding thermal are based on variation of Pr whereas thickness of thermal layers are increased versus the enlargement in Pr. The term regarding thermal relaxation parameter is developed using concept of non-Fourier's law in energy equation as well as in concentration equation. The appearance of δ 1 decides the role of non-Fourier's and Fourier's laws in present flow model while the appearance of δ 1 is simulated by Fig. 11. From this figure, layers regarding thermal energy attain less temperature. Consequently, temperature decreases with respect to large values of δ 1 . In physical view, the term related to relaxation means reappearance of system into equilibrium condition and every process of relaxation are considered as a relaxation time. The large values of δ 1 reveals an enhancement in capability of Prandtl fluid to return the equilibrium state while this happening brings the minimizing variation in thermal state of Prandtl fluid. Figures 12, 13 and 14 illustrate the influences of concentration with respect to variation of Sc, ǫ 2 and δ 2 . Figure 12 conducts the role of Sc on the curves of concentration. The division of mass and momentum diffusion coefficients is knows as Sc while diffusion of mass particles becomes slow down. This decline in diffusion of particles is happened because of inverse relation between Sc and mass diffusion coefficient. Physically, the solute particles are declined due the definition of Sc. This decline in solute particles becomes slow down due to inclement in diffusion of particles. Hence, diffusion of fluid particles becomes slow down versus large values of Sc. The variation in concentration field is investigated by δ 2 while outcomes regarding concentration versus δ 2 is displayed by Fig. 13. It is noticed that δ 2 in dimensionless concentration equation is modeled due to the influence of non-Fourier's law. The diffusion of solute particles for the case of (appearance of Cattaneo-Christov law) is higher than diffusion of solute particles for the case of (disappearance of Cattaneo-Christov law). So, the performance of concentration in the absence of δ 2 is much better than for the case of non-Fourier's law. So, higher values of δ 2 make the reduction in profiles of concentration. It is noticed that δ 2 has analog versus the Deborah number and the fluid has ability to return original state elated to concentration in the fluid. Figure 14 captures the character of ǫ 2 on diffusion of fluid particles. The graph between the diffusion of fluid particles and ǫ 2 is increasing. In this relation, diffusion of fluid particles becomes fast while ǫ 2 is appeared due to variable concentration. Further, this increment in diffusion of fluid particles because ǫ 2 exists as a direct relation versus concentration. So, this direct relation brings increment in diffusion of fluid particles (Figs. 2,3,4,5,6,7,8,9,10,11,12,13,14).

Simulations of divergent velocities against physical parameters.
The related simulations of divergent velocities versus the variation of Pr and α are calculated by Tables 2 and 3. The improvement in divergent velocities in y-and x-directions versus the increment in Prandtl number is investigated. Hence, α makes great impact for the enhancement of shear stresses in y-and x-directions. It is also investigated that validation of numerical simulations is done through considered values 0.0, 0.25, 0.5, 0.75 and 1.0 of α with already published work of Wang 27 , Kumar et al. 28 and Hayat et al. 29 . The remarkable comparison is found between present work and published results. Temperature gradient versus the various values of Pr is captured by Table 3. From this table, temperature gradient is enhanced with respect to large values of Pr whereas comparison of results are estimated by published work of Khan et al. 30 and Bilal et al. 31 by considering range of Pr ( 0.07 ≤ Pr ≤ 2.0).

Concluding key points
The numerical simulations of mass and heat energy using conservation laws under the action of non-Fourier's law are numerically solved. The concept of variable properties in view of concentration and thermal conductivity in the presence of Prandtl fluid past a hot surface is considered. The outcomes against the physical parameters are concluded which are listed below: www.nature.com/scientificreports/ • The magnetic field generates retardation force in motion of fluid particles and MBL (momentum boundary layers) become thick due to retardation force in y-and x-directions of the surface; • The motion of fluid particles become fast in y-and x-directions of the hot surface by considering the enhancement in Prandtl, ratio and elastic parameters whereas thickness of momentum boundary layers are reduced using large values of Prandtl, ratio and elastic parameters; • The less production of thermal energy is achieved versus variation in Pr and δ 1 . The maximum heat energy is achieved for the case generalized Fourier's law as compared for the case of non-Fourier's law. Prandtl number makes vital impact for adjustment in TBL as well as in MBL;         www.nature.com/scientificreports/

Data availability
The data used to support this study are included in the article.  Hayat et al. 29 Wang 27 Kumar et al. 28 Present results