Home > News > PhD Candidate Piyush Ranade - "Turbulence Amplitude Modulation in an Externally Forced Turbulent Boundary Layer"

PhD Candidate Piyush Ranade - "Turbulence Amplitude Modulation in an Externally Forced Turbulent Boundary Layer"

Start: 4/5/2016 at 9:30AM
End: 4/5/2016 at 12:00PM
Location: 103 Multidisciplinary Research Building
Event Type:
  • Ph.D. Defense
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The research presented in this dissertation describes an experimental investigation of an externally forced, high-Mach-number, subsonic turbulent boundary layer. The experimental study was motivated by the results of a previous experiment using wavefront data obtained by passing a laser through a high-Mach-number, regularized shear layer. While the results clearly showed that the wavefronts due to the shear layer were regularly repeated, there appeared to be synchronized turbulent “bursts” in the high-speed turbulent boundary layer on the wall of the shear-layer facility. The amplified turbulence appeared to be modulated by the systematic large-scale disturbance created in the shear layer. The modulation of small scales by larger scales in the turbulent boundary layer has been observed and reported in literature. Study of this phenomenon shows that a phase relationship between large and small scales exists in wall turbulence, and various statistical methods to quantify this relationship have been developed. More recently, experimental studies at CalTech have shown that the phase-organizing effect of large scales on small scales can be regularized by introducing a “synthetic” large scale using a spanwise oscillating rib internal to the boundary layer.

Using the same Compressible Shear Layer Facility at Hessert Aerospace Laboratory at Notre Dame where the wavefront “bursting” was first noted, the regularized shear layer was recreated. The response of the high-speed turbulent boundary layer on the upper wall to the unsteady pressure field was studied using hotwire anemometry. The effect of the shear layer was shown to impose an artificially created large scale in the boundary layer, but now due to external forcing. The relationship of this synthetic large turbulence scale to the small-scale turbulence was then studied. Through statistical analysis, it was determined that the amplitude modulation phenomenon reported in the literature is also present in the perturbed boundary layer. Analyzing the externally forced boundary layer using the same methods as the internally forced boundary layer reported in literature revealed that the synthetic large scale imposed by the regularized shear layer organizes the small scales of turbulence similarly in both cases, and leads to the amplified turbulence which was previously seen using the wavefronts. The research presented in the dissertation adds to a growing database of flow conditions in which the modulation of small scales has been observed. It is the first experimental study of its kind conducted at Mach 0.6, and has implications for the favorable manipulation of boundary layer structures.

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