Studies of the large-scale structure in adiabatic and moderately-wall-heated subsonic boundary layers

Simultaneous velocity-optical measurements in subsonic boundary layers were conducted in order to investigate the relationship between the instantaneous 2-D wavefronts, measured by different optical sensors, the Malley probe and 2-D Shack-Hartmann sensors, and the instantaneous large-scale structure along a wall-normal plane, using PIV in both incompressible and compressible subsonic boundary layers. These systematic studies of the instantaneous relation between the large-scale boundary layer structure and its aero-optical signature provide additional understanding of the instantaneous dynamics of the large-scale structure at subsonic and transonic speeds

Studies of the large-scale structure in adiabatic and moderately-wall-heated subsonic boundary layers

Simultaneous velocity-optical measurements in subsonic boundary layers were conducted in order to investigate the relationship between the instantaneous 2-D wavefronts, measured by different optical sensors, the Malley probe and 2-D Shack-Hartmann sensors, and the instantaneous large-scale structure along a wall-normal plane, using PIV in both incompressible and compressible subsonic boundary layers. These systematic studies of the instantaneous relation between the large-scale boundary layer structure and its aero-optical signature provide additional understanding of the instantaneous dynamics of the large-scale structure at subsonic and transonic speeds

Effect of Coherent Structures on Aero-Optic Distortion in a Turbulent Boundary Layer

The deflection of a small-aperture laser beam was studied as it passed through an incompressible turbulent boundary layer that was heated at the wall. The heating at the wall was sufficiently mild that the temperature and density fields acted as passive scalars with a Prandtl number of 0.71. Simultaneous particle image velocimetry and Malley probe laser deflection measurements were performed in overlapping regions of the boundary layer to identify correlations between coherent velocity structures, passive scalar transport, and optical beam deflection. Streamwise gradients in the streamwise and wall-normal velocity fields were observed to be correlated to the deflection of the optical beam and to streamwise density gradients. The passage of a large-scale motion through the beam path was shown to affect the statistics of the optical beam deflection as well as the local distribution of small-scale velocity features. The wall-normal small-scale velocity features were consistently correlated to the beam deflection, throughout different phases of the large-scale motion convection. The observations motivated a hypothesis that views the large scales as heat carriers, whereas the small scales modify the local sense of a velocity and density gradient toward a streamwise gradient that directly affects the optical beam deflection

Effect of Coherent Structures on Aero-Optic Distortion in a Turbulent Boundary Layer

The deflection of a small-aperture laser beam was studied as it passed through an incompressible turbulent boundary layer that was heated at the wall. The heating at the wall was sufficiently mild that the temperature and density fields acted as passive scalars with a Prandtl number of 0.71. Simultaneous particle image velocimetry and Malley probe laser deflection measurements were performed in overlapping regions of the boundary layer to identify correlations between coherent velocity structures, passive scalar transport, and optical beam deflection. Streamwise gradients in the streamwise and wall-normal velocity fields were observed to be correlated to the deflection of the optical beam and to streamwise density gradients. The passage of a large-scale motion through the beam path was shown to affect the statistics of the optical beam deflection as well as the local distribution of small-scale velocity features. The wall-normal small-scale velocity features were consistently correlated to the beam deflection, throughout different phases of the large-scale motion convection. The observations motivated a hypothesis that views the large scales as heat carriers, whereas the small scales modify the local sense of a velocity and density gradient toward a streamwise gradient that directly affects the optical beam deflection

Aero-Optical Characteristics of Compressible, Subsonic Turbulent Boundary Layers

An extensive experimental study of optical aberrations due to propagation through fully-developed turbulent boundary layers at high subsonic Mach numbers was performed. Time-resolved, high-bandwidth, direct optical measurements of the dynamic aberrations were made using a Malley probe. The probe was used to obtain the convective speeds of the optically-significant turbulence structures and to measure the optical path differences. Measurements were made over a range of boundary layer thicknesses and Mach numbers. Optical distortions were found to scale linearly with boundary layer thickness and freestream density, and to go as the square of the freestream Mach number