Criterion for Boundary Layer Separation

As we approach a separation point, we expect the flow to deviate from the boundary layer towards the interior of the fluid. In other words, we expect the normal velocity to become comparable with the tangential velocity. However, we have seen that the ratio is of order . [See Equation (8.18).] Hence, an increase of to such a degree that implies an increase by a factor . For sufficiently large Reynolds numbers, we may suppose that effectively increases by an infinite factor. Indeed, if we employ the dimensionless form of the boundary layer equations, (8.23)-(8.27), the situation just described is formally equivalent to an infinite value of the dimensionless normal velocity, , at the separation point.

Let the separation point lie at , and let correspond to the region of the boundary layer upstream of this point. According to the previous discussion,

(8.121) |

at all (except, of course, , where the boundary conditions at the surface of the obstacle require that ). It follows that the deriviative is also infinite at . Hence, the equation of continuity, , implies that , or , if is regarded as a function of and . Let . Close to the point of separation, and are small. Thus, we can expand in powers of (at fixed ). Because , the first term in this expansion vanishes identically, and we are left with

(8.122) |

or

where is some function of . From the equation of continuity,

(8.124) |

Upon integration, the previous expression yields

where

The equation of tangential motion in the boundary layer, (8.111), is written

(8.127) |

As is clear from Equation (8.123), the derivative does not become infinite as . The same is true of the function , which is determined from the flow outside the boundary layer. However, both terms on the left-hand side of the previous expression become infinite as . Hence, in the immediate vicinity of the separation point,

(8.128) |

Because , we can rewrite this equation in the form

(8.129) |

Because does not, in general, vanish at , we conclude that

(8.130) |

In other words, is a function of only. From Equations (8.123) and (8.125),

(8.131) |

Hence, if this ratio is a function of alone then , where is a constant: that is,

Finally, because Equation (8.126) yields , we obtain

The previous two expressions specify and as functions of and near the point of separation. Beyond the point of separation, that is for , the expressions are physically meaningless, because the square roots become imaginary. This implies that the solutions of the boundary layer equations cannot sensibly be continued beyond the separation point.

The standard boundary conditions at the surface of the obstacle require that at . It, therefore, follows from Equations (8.132) and (8.133) that

(8.134) | ||

(8.135) |

Thus, we obtain the important prediction that both the tangential velocity, , and its first derivative, , are zero at the separation point (i.e., and ). This result was originally obtained by Prandtl, although the argument we have used to derive it is due to L.D. Landau (1908-1968) (Landau and Lifshitz 1987).

If the constant in expressions (8.132) and (8.133) happens to be zero then the point and , at which the derivative vanishes, has no particular properties, and is not a point of separation. However, there is no reason, in general, why should take the special value zero. Thus, in practice, a point on the surface of an obstacle at which is always a point of separation.

Incidentally, if there were no separation at the point (i.e., if ) then we would have for . In other words, would become negative as we move away from the surface, being still small. That is, the fluid beyond the point would move tangentially, in the region of the boundary layer immediately adjacent to the surface, in the direction opposite to that of the external flow: that is, there would be ``back-flow'' in this region. In practice, the flow separates from the surface at , and the back-flow migrates into the wake.

The dimensionless boundary layer equations, (8.23)-(8.27), are independent of the Reynolds number of the external flow (assuming that this number is much greater than unity). Thus, it follows that the point on the surface of the obstacle at which is also independent of the Reynolds number. In other words, the location of the separation point is independent of the Reynolds number (as long as this number is large, and the flow in the boundary layer is non-turbulent).

At , the equation of tangential motion in the boundary layer, (8.111), is written

(8.136) |

where is the pressure just outside the layer, and use has been made of Equation (8.6). Because is positive, and increases away from the surface (upstream of the separation point), it follows that at the separation point itself, where . Hence, according to the previous equation,

(8.137) | ||

(8.138) |

In other words, we predict that the external tangential flow is always decelerating at the separation point, whereas the pressure gradient is always adverse (i.e., such as to decelerate the tangential flow), in agreement with experimental observations.