Algebraic Type Approximation to the Blasius Velocity Profile

The so-called Blasius function f(x) describes the stream on the boundary layer over a flat plate. There are lots of analytical approximation methods to the Blasius function f(x) such as the variational iteration method [1–6], the Adomian decomposition method [7–9], and the homotopy analysis method [10–12]. Recently, spectral methods based on orthogonal functions have been applied in approximation of solutions for the nonlinear boundary value problems like the Blasius problem [13–17]. In addition, numerical solutions of the nonlinear differential equations for the boundary layer problems such as Falkner-Skan equations, including the Blasius equation as a special case, have been studied by many researchers [18–25]. Concerning the streamwise velocity profilef(x), we note an approximate analytical solution proposed in the literature [26] of the form

Concerning the streamwise velocity profile   (), we note an approximate analytical solution proposed in the literature [26] of the form where  and  are determined by the known properties of the Blasius function () at the wall  = 0 and far from the wall, respectively.The parameter  is chosen by minimizing the residual function Recently, Savas ¸ [27] introduced another approximate analytical solution for the streamwise velocity profile as for the constants (, ) = (0.33206, 3/2) or (0.33245, 5/3).
In the next section, motivated by the analytical solutions (3) and ( 5), we propose another algebraic type approximate analytical solution for the velocity profile as given by ( 6) and explore its properties with a method to determine the parameters therein.In Section 3, by using an appropriate weight function, we introduce a correction method to improve the accuracy of the presented approximation.Moreover, for further improvement we employ an auxiliary term which appropriately reflects the error of the presented approximation.Some numerical experiments are performed to demonstrate the efficiency of the presented method.

Approximation to the Velocity Profile
To approximate the velocity profile   () directly we suggest an algebraic type analytical function as Journal of Applied Mathematics for a constant  > 0 and an exponent  > 1.We note that   , satisfies the boundary conditions   , (0) = 0 and   , (∞) = 1 given in (2), and its derivative is Since   , (0) = , we may set which is a well-known Blasius constant [28].In addition, the velocity profile  =   , () has an inversion of a simple form as The related approximation  , to the Blasius stream function () can be obtained by the formula In fact, using the symbolic computational software Mathematica (version 9), one can find the analytical form of  , as where (, , ; ) is the hypergeometric function [29] whose series expansion is and ()  is the shifted factorial defined by with () 0 = 1.
For an appropriate parameter  in   , () we may choose a value  =  * at which the  2 -norm of the residual function  , , is minimized.To find  * one can use a package, Mathematica, for example, and we will obtain the local minimum in ‖ , ‖ 2 at the value  * ≈ 4.216.
Figure 1 shows the errors of the presented approximate velocity profile,   , (), with integers  = 4 and  = 5 near the value  * ≈ 4.216.The error means difference between   , () and the numerical solution for the velocity profile   () which is regarded as an exact solution.By numerical experiments for various values of , we can see that the accuracy of   , () becomes better far from the wall  = 0 as  goes large while it becomes better near  = 0 as  goes small.

Improvement by a Weighted Average
In order to improve the accuracy of the proposed approximate velocity profile over the whole region, we introduce a weighted average for 1 <  < , where () is a weight function defined as for  > 0 and  > 0. It follows that 0 ≤ () < 1 for 0 ≤  < ∞ with () = 1/2.Moreover, it should be noticed that for a sufficiently large   () ∼ { 0, for  < , 1, for  >  (17) and thus f ,, () ∼ {   , () , for  < ,   , () , for  > . ( This implies that the point  =  plays the role of a threshold between two approximate velocity profiles   , and   , .On the other hand, the related approximate stream function f,, can be obtained by numerical integration in the equation Referring to Figure 1 for the cases of  = 4 and  = 5, we may take  = 3.5134 in (16) which is a center of the points  = 4.4033 and 2.6234 at which  ,4 () and  ,5 (), respectively, have the maximum absolute errors.Thick lines in Figure 2 indicate errors (i.e., differences from the numerical solution) of the corrected approximate stream function f,4,5 () and the velocity profile f ,4,5 () with  = 3.5134 and  = 6 in the weight function () = (, ; ).We can see that the maximum error is about 0.01 in the velocity profile and   2(a) and the velocity profiles   ,4 () and   ,5 () in Figure 2(b), one can find distinct improvement of the corrected velocity profile f ,4,5 () defined in (15).In practice, by numerical experiments, we can find better case of parameters like (, ) = (3.8,5.8), for example, which results in more accurate approximation with the maximum errors about 0.003 and 0.005 in the velocity profile and the stream function, respectively.However, this choice of the parameters looks rather ambiguous.Thus, for development of plausible further improvement, we refer to the correction method proposed in the literature [30] which uses an auxiliary term reflecting the error of the presented approximation.First, observing the behavior of the error () =   () − f ,, () given in Figure 2, for example, we can have the numerical values of the critical points ( 1 ,  1 ) = ( 1 , ( 1 )) and ( 2 ,  2 ) = ( 2 , ( 2 )) of ().Then, to approximate () appropriately, we suggest a function  , () ≈ () of the form  , () =  ( − )  −(−) 2 + , where The value of  in (20) can be determined by the condition   , ( 1 ) =   , ( 2 ) = 0 which implies that