High-Speed Schlieren Imaging of Disturbances in a Transitional Hypersonic Boundary Layer

Abstract

A high-speed schlieren system was developed for the Sandia Hypersonic Wind Tunnel. Schlieren images were captured at 290 kHz and used to study the growth and breakdown of second-mode instabilities into turbulent spots on a 7 • cone. At Mach 5, wave packets would intermittently occur and break down into isolated turbulent spots surrounded by an otherwise smooth, laminar boundary layer. At Mach 8, the boundary layer was dominated by second-mode instabilities which would break down into larger regions of turbulence. Second-mode waves surrounded these turbulent patches as opposed to the smooth laminar flow seen at Mach 5.Detailed pressure and thermocouple measurements were also made along the cone at Mach 5, 8 and 14, in a separate tunnel entry. These measurements give an average picture of the transition behavior that complements the intermittent behavior captured by the schlieren system. At Mach 14, the boundary-layer remained laminar so the transition process could not be studied. However, the first measurements of second-mode waves were made in HWT-14. Nomenclatureδ boundary-layer thickness (mm) φ cone azimuthal angle ( • ) τ w nozzle wall shear stress (Pa) D model base diameter (m) M freestream Mach number M e Mach number at boundary-layer edge P 0 tunnel stagnation pressure (kPa) p root-mean-square pressure fluctuation (Pa) p pressure fluctuation, p − p e (Pa) p e boundary-layer edge pressure (Pa) q e dynamic pressure at boundary-layer edge (Pa) R cone radius (mm) Re freestream unit Reynolds number (1/m) Re D freestream Reynolds number based on model base diameter St Stanton number T 0 tunnel stagnation temperature (K) U c average convection velocity (m/s) U e boundary-layer edge velocity (m/s) x axial model coordinate measured from nose (m)

Simultaneous Pressure Measurements and High-Speed Schlieren Imaging of Disturbances in a Transitional Hypersonic Boundary Layer

“…Surface pressure fluctuations between 0-50 kHz under a laminar boundary layer were also measured to give an idea of this noise variation with M . 13 The surface pressure fluctuations do rise significantly with Mach number from 3% at Mach 5 to 14% at Mach 14. Table 3 summarizes the difference between these conditions.…”

“…An average convection velocity of 0.8 U e was used, which is in agreement with previous measurements on a cone. 13,21 The zero time origin for each case was picked in order to align the leading edges of the dominant disturbance atx = 0 m. This conversion invokes Taylor's hypothesis which assumes the flow is 'frozen' as it convects over the sensor. 22 This hypothesis is frequently used in the analysis of turbulent flow and has been shown to be a reasonable assumption for many cases, even for low-frequency fluctuations.…”

“…This cone has been used in previous experimental campaigns. [10][11][12][13] Two new inserts were fabricated to place closely spaced axial and spanwise arrays of pressure sensors on the cone. The axial array ( Fig.…”

“…9 To improve prediction of hypersonic pressure fluctuations, experiments were conducted on a 7 • sharp cone at zero angle of attack in two hypersonic tunnels under conventional and quiet noise levels. [10][11][12][13] Experiments under noisy flow were conducted in the Sandia National Laboratories Hypersonic Wind Tunnel (HWT) at Mach 5, 8, and 14. These previous measurements give an average picture of the transition process.…”

Experimental analysis on the effects of DC arc discharges at various flow regimes

“…However, an equivalent result can be achieved by narrowing the time of the illumination during the acquisition. 22 A novel concept used in this work was a PIV system for short exposure Schlieren imaging. PIV lasers usually produce pulses in the range of 5-9 ns with a time separation between two shots in the order of 0.6-2 µs.…”