Investigation of the Laminar-Turbulent Transition of Boundary Layers Disturbed by Wakes

Abstract

The boundary layer transition under instationary afflux conditions as present in the stages of turbomachines is investigated. A model for the transition process is introduced by means of time-space distributions of the turbulent spots during transition and schematic drawings of the instantaneous boundary layer thicknesses. To confirm this model, measurements of the transition with zero and favorable pressure gradient are performed.

Effect of Unsteady Wake Passing Frequency on Boundary Layer Transition, Experimental Investigation and Wavelet Analysis

“…Sc, S2 = 5.155, 15 rods) shows a further upstream movement of the transition process which can be attributed to the same factors mentioned above. These experimental results are in accord with those found by Liu and Rodi (1991) but are in disagreement with the earlier statement by Pfeil et al (1983), and Pfeil and Herbst (1979), which investigated the onset of transition on a flat plate utilizing the boundary layer integral quantities, momentum and thickness. They kept the inlet velocity, the wake generator frequency and the position of their flat plate constant, and measured the time averaged boundary layer velocities for seven different rod spacings.…”

“…Close to the plate leading edge, in the range of 0% to 20% of the length, only small variations were distinguishable. Pfeil et al (1983) presented their original idea of a transition model and concluded that, by keeping the flow and rod velocity constant, the variation of spacing (increasing the rod numbers) would show no influence on the p L position of forced transition start xt.^. This conclusion was primarily drawn from the distribution of the time averaged integral quantities such as displacement and momentum thicknesses without the presentation of experimental evidence essential to rectify the conclusion.…”

“…For the experimental investigations they utilized a wake generator with a series of cylindrical rods arranged circumferentially on two parallel rotating disks positioned upstream of the test objects (squirrel cage type wake generator). Pfeil and Herbst (1979), Herbst (1980), Pfeil et al (1983), Schroder (1985 concentrated their efforts on investigating the transition process of unsteady boundary layers. Using a flat plate and varying the pressure gradient and the wake induced unsteady inlet flow condition, by changing the number, diameter, and frequency of the rods, Pfeil et al generated wake induced transition, where intermittent laminar and turbulent states of the boundary layer were observed.…”

“…Based on the results of their experimental investigations, Pfeil and his co-researchers established a generally accepted unsteady boundary layer transition model. The innovative data published by Pfeil and Herbst (1979) along with their model were instrumental in describing the intermittency function (Herbst 1980, Hodson 1990, Mayle 1991.…”

Fluid Mechanics for Engineers

“…This is due to the exchange of mass, momentum and energy between the highly turbulent vortical cores of the wakes and the low energetic fluid within the bubble as shown in Fig. This decrease is due to the calming phenomenon extensively discussed by several researchers ( [53], [54], [55] and [56]). Increasing the frequency to = 3.18 has moved the velocity distribution further away from the separation, as seen in Fig.…”