A Prediction Model for Separated-Flow Transition

Abstract

The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers which are laminar at separation and those which are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation - short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation - long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number data. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

Numerical investigations of separation-induced transition on high-lift low-pressure turbine using RANS and LES methods

“…He also tried to identify critical parameters for the bursting onset and showed that for the longer bubbles and low freestream turbulence, much of the flow within the bubble is laminar and Tollmien-Schlichting instabilities have been observed. The phenomenon of transition triggered by separation bubble was also described by Hatman and Wang [5] [6][7] [8]. For constant freestream conditions (turbulence intensity and adverse pressure gradient) when lowering the Reynolds number the bubble size increases.…”

Effectiveness of micro synthetic jet actuator enhanced by flow instability in controlling laminar separation caused by adverse pressure gradient

“…It was believed that the T-S waves, triggered by the synthetic jet and enhanced by the natural instability of the base flow, effectively stopped the flow separation [12]. In the model development for boundary layer separation, Hatman and Wang assumed that the transition to turbulence in separated boundary layer be a result of the superposition of the effects of K-H instability and T-S instability [10]. The fluctuating velocity profiles in the separation zone in Figure 3 As found in our earlier studies [11], when a synthetic jet was injected in a quiescent condition (without a cross flow), the jet velocity and momentum were maximum at a forcing frequency which was equal to the resonant frequency of the membrane material of the actuator.…”

“…SeeTable 1for measurement positions in x direction.The effectiveness of the synthetic jet on resisting flow separation can also be identified with the displacement thickness. According to the identification methods developed by Hatman and Wang[10], in laminar separation, the maximum displacement of the shear layer occurs at the onset of the transition, and the maximum turbulence level occurs at the first reattachment point. The boundary layer condition in the current study belongs to the laminar separation-short bubble.Figure 4(a) shows the comparison of displacement thicknesses in the conditions with the synthetic jet on and off.…”

Controlling secondary flows in very highly-loaded low-pressure turbine cascades

“…This is noteworthy, as it will be shown in Section 7.4.3 that PAKD-FP has a small suction surface separation bubble while PAKD-F does not. Previous work has shown that the existence and location of a separation bubble is dependent on the pressure gradient history of the boundary layer (Hatman and Wang, 1999;Yaras, 2001;Houtermans et al, 2004).…”