Donghun Park

Particle Image Velocimetry Measurement of Laminar Boundary Layer in a Streamwise Corner

Measurement of velocity profiles along a laminar boundary layer in a streamwise corner was carried out by using particle image velocimetry in contrast to conventional hot-wire anemometry. To make the measurement successful, the laser light source for particle image velocimetry was placed at a far-downstream station, and a light sheet was emitted in parallel to the corner intersection line. The platewas tilted at very small angles relative to the freestream to realize the laminar flow starting from the corner leading edge. Discussions on the characteristic features of the velocity profile in the bisector plane are given based on the measurement data. It was found that the measured velocity profiles correspond to the Blasius branch solution of the theoretical corner layer equations.

Stability analysis of a boundary layer over a hump using parabolized stability equations

Parabolized stability equations (PSEs) were used to investigate the stability of boundary layer flows over a small hump. The applicability of PSEs to flows with a small separation bubble was examined by comparing the result with DNS data. It was found that PSEs can efficiently track the disturbance waves with an acceptable accuracy in spite of a small separation bubble. A typical evolution scenario of Tollmien–Schlichting (TS) wave is presented. The adverse pressure gradient and the flow separation due to the hump have a strong effect on the amplification of the disturbances. The effect of hump width and height is also examined. When the width of the hump is reduced, the amplification factor is increased. The height of the hump is found to obviously influence the stability only when it is greater than the critical layer thickness.

Study on Stabilization and Destabilization Effect of a Smooth Hump in Hypersonic Boundary Layers by PSE

Effect of a two-dimensional smooth hump on linear instability of hypersonic boundary layer is studied. Parabolized stability equations (PSE) is employed to analyze the linear evolution of mode S over a hump in Mach 4.5 and 5.92 flat plate and Mach 7.1 sharp cone boundary layers. Mean flow for stability analysis is obtained by solving the parabolized Navier-Stokes (PNS) equations. Hump with height smaller than local boundary layer thickness is used. The case of flat plate and sharp cone without the hump are also studied to provide comparable data. For flat plate boundary layers, destabilization and stabilization effect of hump located at upstream and downstream of synchronization point are confirmed. Results of parametric studies to examine the effect of hump height, location, etc. are also given. For sharp cone boundary layer, stabilization influence of hump is also identified for a specific range of frequency. Stabilization influence of hump on convective instability of mode S is found to be a possible cause of previous experimental observations of delaying transition in hypersonic boundary layers.

Nonlinear Stability Analysis of Boundary Layers by using Nonlinear Parabolized Stabiltiy Equations

Nonlinear Parabolized Stability Equations(NSPE) can be effectively used to study more throughly the transition process. NPSE can efficiently analyze the stability of a nonlinear region in transition process with low computational cost compared to Direct Numerical Simulation(DNS). In this study, NPSE in general coordinate system is formulated and a computer code to solve numerically the equations is developed. Benchmark problems for incompressible and compressible boundary layers over a flat plate are analyzed to validate the present code. It is confirmed that the NPSE methodology constructed in this study is an efficient and effective tool for nonlinear stability analysis.

PIV Measurement of Laminar Boundary Layer in a Streamwise Corner

Measurement of velocity profiles along a laminar boundary layer in a streamwise corner was carried out by using PIV in contrast to conventional hot-wire anemometry. To make the measurement successful, the laser light source for PIV was placed at a far downstream station and light sheet was emitted in parallel to the corner intersection line. The plate was tilted at a very small angle relative to the free-stream to realize laminar flow from the start of the corner. Based on the measured velocity profiles, effects of plate leading edge shape and angle of incidence of the corner plate were discussed. It was found that the velocity profiles measured correspond to the lower branch solution of the theoretical corner layer equations.

Transition Prediction of Boundary Layers over Airfoils based on Boundary Layer Stability Theory

Transition location of boundary layers over airfoils is predicted by using PSE(Parabolized Stability Equations) and -method. Growth rates of disturbances are obtained from the PSE analysis and the N-factor curves are calculated by integrating the growth rates. The computational code developed in the present study is validated by comparing the computed results with the well known data for the cases of flat plate boundary layers and airfoils. Predictions of transition location are made for the boundary layers over NACA0012, NLF(1)-0414F, and NLF(1)-0416 airfoil. Predicted transition locations are found to be in good agreement with the experimental data.