Candidate: Fengjun Liu

Degree of: Doctor of Philosophy

Department:

Title:

Date:

Committee:

Dr. William W. Liou, Chair

Dr. Chris Cho

Dr. Iskender Sahin

Dr. Ronald D. Joslin

Abstract:

The transition of flows around a multi-element airfoil has been numerically studied using a k-e two-equation transition model, LST and DNS. The transition model uses and effective eddy-viscosity by coupling an intermittence-like correction to a turbulence eddy-viscosity that can be obtained via solving a parent k-e turbulence model. The transition model is truly predictive in that it is able to predict transition onset locations without having to specify prior knowledge of the targeted transition process. The predicted transition onset locations for all the cases studied were compared with the measured data. The results suggest that a better understanding of the confluent wake/boundary layers occurring near the main-element or flap leading edge is important to consistent predictions of such complex flows.

To better understand the confluent wake/boundary layer flow, a linear analysis of the confluent wake/boundary has been performed using a global numerical solution method. The modes associated with the boundary layer of he wake, respectively, have been identified. The modes associated with the wake, including a symmetric mode and an anti-symmetric mode, are stabilized by the reduced distance between the wall and the wake. On the other hand, the boundary-layer mode has been found being amplified as the wake approaches the wall. The important effects of the wake modes on

the disturbance growth of the boundary layer have been confirmed using a DNS approach. The initial conditions of for the DNS were provided by the linear analysis. The mean flow velocity profile was obtained using a time-averaged Navier-Stokes equation solver. The DNS results show that the disturbances in the wake region grow rapidly and promote a growth of the disturbances in the boundary layer. The effects of different disturbance forcing, such as amplitude and frequency, on the numerical simulations have been discussed.

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