text: Wallis and Dobson (1973) estimate their criterion with transition observations that they call “Slugging” and note that empirically the stability limit is described by 
  Here  and  where H is channel height and U, h and ρ denote the mean velocity, holdup and density respectively. The subscripts G and L stand for gas and liquid and g denotes Gravity.
  Taitel and Dukler (1976) [TD] expanded the (Kelvin and helmholtz) KH analysis first to the case of a finite wave on a flat liquid sheet in horizontal channel flow and then to finite waves on stratified liquid in an Inclined pipe. In order to apply this criterion they need to provide the equilibrium liquid level hL (or liquid holdup). They calculate  through momentum balances in the gas and liquid phases (two fluid models) in which shear stresses are examine and assessed using conventional friction factors definitions. In two fluid models, the pipe geometry is taken into consideration through wetted perimeters by the gas and liquid phases, including the gas-liquid interface. This states that the wall resistance of the liquid is similar to that for open-channel flow and that of the gas to close-duct flow. This geometry analysis is general and could be applied not only to round pipes, but to any other possible shape. In this method, each pair of superficial gas and liquid velocity relates to a distinctive value of .
  According to [TD], a finite wave will grow in a horizontal rectangular channel of height H, when  or  for inclined pipe. D is the pipe diameter and A is the cross section area. Note that . If , , and this is compatible with the result of Wallis and Dobson(1973) The [TD] overall procedure result to a weak dependence on viscosity, through the calculation of .
  [TD] also identify two kinds of stratified flow: stratified smooth (SS) and stratified wavy (SW). These waves, as they say, “are produced by the gas flow under conditions where the velocity of gas is enough to cause waves to form, but slower than that needed for the quick wave growth which leads transition to intermittent or annular flow.” [TD] suggest a standard to predict the transition from stratified smooth to stratified wavy flow, based on Jeffreys’ (1925, 1926) ideas.