Comment on the paper “Theoretical & Applied Mechanics Letters 7 (2017)235-242”

Asterios Pantokratoras

School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece

In the above paper the entropy generation is defined as follows (Eq. (25) in Ref. [1])

In Eq. (1) the parameter RD appears. However, the parameter RD is absent from the dimensional equations in Ref. [1]. It is also absent from the transformed Eqs. (18)-(21) in Ref. [1]. It is also absent from the Nomenclature (Rd is a different parameter named as radiation parameter). This is a serious problem. From the first continuity Eq. (1) until Eq. (24) the parameter RD is absent and appears suddenly in Eq. (25). This is irrational. It is not allowed to use a parameter which is absent from the physical problem.

The Grashof number is defined as (page 238 in Ref. [1])

whereg (m·s-2)is the gravity acceleration,isthe thermal expansion coefficient andisthe fluid kinematic viscosity. Thus the units of the Grashof number are m-3. This means that it is dimensional and therefore wrong.