Turbulent spots in hypersonic transitional planar and axisymmetric boundary layers

TL;DR

This study investigates the characteristics and generation rates of turbulent spots in hypersonic transitional boundary layers over flat and axisymmetric models at Mach 5.85, revealing differences in their development and convection speeds.

Contribution

It provides detailed measurements and analysis of turbulent spot properties in hypersonic boundary layers, comparing planar and axisymmetric configurations.

Findings

Leading edge of turbulent spots convects at 90% of boundary layer edge speed.

Trailing edge of spots on planar boundary layers moves slower than on axisymmetric layers.

Streamwise length scales grow faster in planar boundary layers.

Abstract

Experiments were conducted to investigate characteristics of turbulent spots formed in transitional boundary layers developed over a flat plate and an axisymmetric cone placed in similar hypersonic freestream environment of Mach number 5.85. The freestream Reynolds number in the present work varied between $3.0-6.0\times10^6$/m. Heat transfer measurement along the surface of both the test models was used to ascertain the state of boundary layer and to calculate the intermittency associated with transitional boundary layer. Turbulent spots generated in the transitional boundary layer were characterized in terms of their leading-trailing edge velocities, their streamwise length scales and their generation rates on both the test models. Leading edge of the turbulent spots developed over both the test models were found to be convecting at a speed equivalent to 90\% of the boundary layer edge speed. The trailing edge of the spots developed on a planar boundary layer traversed at a lower speed than its axisymmetric counterpart. Streamwise length scales of a turbulent spot developed in a planar boundary layer grew at a higher rate when compared with axisymmetric boundary layer. Turbulent spot generation rates for both planar and axisymmetric boundary layers was found to be in the range of $1000000-3000000$ spots/m/s.