Direct Numerical Simulation of Hypersonic Flow over a Blunt Cone with Axisymmetric Isolated Roughness

Abstract

The ability of a finite roughness element to suppress the hypersonic second-mode instability on a cone is explored. A new code for simulating discrete roughness using a body-fitted grid over an analytic shape is developed. Linear stability analysis is performed on the Mach 8 meanflow of a 7 • half-angle straight cone. The resulting N-factor analysis determines the second-mode frequency of 240 kHz to most likely to lead to turbulent transition. A phase velocity plot of the hypersonic modes is obtained and the resulting synchronization location of s=0.2436 meters is determined. This led to the design and placement of a roughness element that will effectively suppress the targeted disturbance frequency. An unsteady simulation with a blowing-suction actuator, upstream of the roughness element, introduces a pulse with a nominal frequency content up to 1 MHz. FFT's of the pulse's history for a roughness case and a no-roughness case are computed and compared. Frequencies 218 kHz and higher are suppressed while lower frequencies are amplified, effectively showing that the roughness element is able to suppress the target disturbance frequency.

Joint Numerical and Experimental Investigation of Roughness Effect on Hypersonic 2nd Mode Instability and Transition

“…This is contrasted with a single roughness strip that will produce a distinct Mach wave whose angle will intersect with the shock boundary. 12 The temperature contours in Fig. 12b show a significant increase in temperature ratio over the smooth cone simulation in Fig.…”

“…By contrast on a single isolated roughness strip these compression and expansion regions lend themselves to creating a clearly defined Mach wave and expansion fan that extends downstream and into the shock layer. 12 Moreover the streamtraces in Fig. 13 evince a flowfield in which the streamtraces are influenced greatly by the presence of the roughness strips.…”

“…A simulation of the flowfield and design of a single transition-delaying roughness is detailed in Haley and Zhong. 12 The freestream conditions, blunt cone dimensions, and intermittent turbulence results were taken from a previous experimental case performed by Casper et al in Sandia's HWT-8. 17 The single roughness strip design was modified to include multiple strips in order to suppress all unstable frequencies.…”

“…The design of the transition-delaying roughness was carried out prior to the experiment with a numerical simulation of the test article and stability analysis of its meanflow. 12 The design is then implemented on the test article and the experiment is run. Several different freestream cases with the same roughness configuration are examined experimentally at Mach 8 and Mach 5.…”

Effect of a Roughness Element on the Receptivity of a Hypersonic Boundary Layer over a Blunt Cone Due to Pulse Entropy Disturbance with a Single Frequency

“…Fong and Tang [38,39] drew similar conclusions when they analyzed the effect of two-dimensional roughness element on the stability of the boundary layer on the flat plate at Mach 6. The roughness element resulted in an increase in the pressure in the upstream half region of the roughness element and a decrease in the downstream half region.…”