Mark S. Reibert

Effect of micron-sized roughness on transition in swept-wing flows

Boundary-layer transition-to-turbulence studies are conducted in the Arizona State University Unsteady Wind Tunnel on a 45-degree swept airfoil. The pressure gradient is designed so that the initial stability characteristics are purely crossflow-dominated. Flow visualization and hot-wire measurements show that the development of the crossflow vortices is influenced by roughness near the attachment-line. Comparisons of transition location are made between a painted surface, a machine-polished surface, and a hand-polished surface. Then, isolated 6 micron roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25 micron rms). These elements amplify a centered stationary crossflow vortex and its neighbors, resulting in localized early transition. The diameter, height, and location of these roughness elements are varied in a systematic manner. Spanwise hot-wire measurements are taken behind the roughness element to document the enhanced vortices. These scans are made at several different chord locations to examine vortex growth.

Development of Stationary Crossflow Vortices on a Swept Wing

Stability experiments are conducted in the ASU Unsteady Wind Tunnel on a 45° swept airfoil. The surface of the is polished to 0.25 μm rms. Under these conditions, natural stationary crossflow vortices are not measurable. This state is used to measure roughness-induced stationary crossflow. Spanwise arrays of 70–150 μm roughness elements are introduced near the attachment line. Detailed hot-wire measurements are taken to document the growth of these vortices. The data clearly show that linear stability theory does not accurately predict the growth rates of stationary crossflow waves under these conditions.

Review of Swept-Wing Transition

This paper reviews the important recent progress in three-dimensi onal boundary-layer transition research. The review focuses on the crossfiow instability that leads to transition on swept wings with a favorable pressure gradient. Following a brief overview of swept-wing instability mechanisms and the crossflow problem, a summary of the important findings of the 1990s is given. The discussion is presented from the experimental viewpoint, highlighting the ITAM work of Kachanov and co-workers, the DLR experiments of Bippes and co-workers, and the Arizona State University (ASU) investigations of Saric and co-workers. Where appropriate, relevant comparisons with CFD are drawn. The recent (last 18 months) research conducted by the ASU team is described in more detail in order to underscore the latest developments concerning nonlinear effects and transition control.

Boundary-layer transition detection and structure identification through surface shear-stress measurements

Several surface shear-stress measurements are made using hot-film sheet anemometry technology in a three-dimensional, swept-wing boundary layer. Various measurements in the crossflow and streamwise directions are made in regions on the wing surface upstream, through, and downstream of the transition region from laminar to turbulent flow. Advanced analysis techniques including proper orthogonal decomposition (POD), spectra, and spatial correlations are used to identify the presence of flow structure and spatial evolutions within the measured surface shear-stress fields. The resulting spatial eigenmodes from the POD solution across the transition front presents a completely objective method for identifying the start and finish of the transition process in the swept-wing boundary layer. The crossflow POD solutions reveal certain transitional processes and spatial relationships important in understanding flow transition and in developing future flow control algorithms.

Control of Transition in 3-D Boundary Layers

Transition to turbulence in swept-wing flows has resisted correlation with linear theory because of its sensitivity to freestream conditions and 3-D roughness and because one of the principal instability modes quickly becomes nonlinear. In the face of such a formidable problem, two rather long-term fundamental efforts have been underway at DLR Gottingen and Arizona State University that address swept-wing transition. These efforts have been recently reviewed by Bippes (1997) and Reibert & Saric (1997). The present work is a continuation of studies on swept-wing boundary layers. In particular, we have taken advantage of the sensitivity to 3-D roughness and the modal nature of the instability in order to propose a particular control strategy.

Insight into Swept Wing Boundary-Layer Transition and Turbulent Flow Physics from Multi-Point Measurements

A review and fresh perspective are provided on a series of multi-point surface shear stress (hotfilm) and hotwire velocity measurements obtained within a three-dimensional, swept-wing boundary layer. The multi-point surface shear-stress measurements were performed in both the crossflow and streamwise directions. The measurements were extracted in regions on the wing surface upstream, through, and downstream of the region where the flow transitions from laminar to turbulent flow. Proper orthogonal decomposition (POD), spectra, and spatial correlations were used to identify the presence of flow structures and their spatial evolution. The POD streamwise spatial eigenmodes obtained through transition, track the start and finish of the transition process in the swept-wing boundary layer. The first POD crossflow spatial eigenmode obtained in the laminar region has a wavelength of 12 mm which is consistent with the crossflow vortex spanwise periodicity. The first POD crossflow spatial eigenmode obtained in the turbulent region is not nearly as distinct but a large spatial structure nearly 24mm in span is observed along with some smaller structures as well. This could indicate that through the transition process, two neighboring crossflow vortices are in the process of merging at this streamwise location. The POD was also applied in span to hotwire velocity measurements at 3 mm above the wing in both the laminar and turbulent regimes and through this analysis, the flow structure in the turbulent flow regime has been identified that is very similar to that obtained in the laminar transition zone. This suggests that the initial instability and transition physics may continue to play a key role even after the flow has transitioned to turbulence.