Study of density field measurement based on NPLS technique in supersonic flow

Abstract

Due to the influence of shock wave and turbulence, supersonic density field exhibits strongly inhomogeneous and unsteady characteristics. Applying traditional density field measurement techniques to supersonic flows yields three problems: low spatiotemporal resolution, limitation of measuring 3D density field, and low signal to noise ratio (SNR). A new method based on Nano-based Planar Laser Scattering (NPLS) technique is proposed in this paper to measure supersonic density field. This method measures planar transient density field in 3D supersonic flow by calibrating the relationship between density and concentration of tracer particles, which would display the density fluctuation due to the influence of shock waves and vortexes. The application of this new method to density field measurement of supersonic optical bow cap is introduced in this paper, and the results reveal shock wave, turbulent boundary layer in the flow with the spatial resolution of 93.2 μm/pixel. By analyzing the results at interval of 5 μs, temporal evolution of density field can be observed.

Visualization of coherent structures in a supersonic flat-plate boundary layer

“…The grayscale of the NPLS image is proportional to the local density as reported by Tian et al [17]. Thus, the dark (low grayscale) regions in Figure 4 should represent the boundary layer, where the local density is low.…”

“…Nanoparticles are used asracer particles and a pulse planar laser is used for illumination in NPLS; by recording images of particles in the flowfield with the CCD camera, imaging of supersonic flow with high spatiotemporal resolution is realized. Theflow-following ability of nanoparticles in supersonic flow has been validated in experiments, and high spatiotemporal resolution images of the shock wave, expansion, Mach disk, sliding line and mixing layer have been obtained with NPLS [14][15][16][17]. A detailed description of this technique has been given by Zhao et al [18].…”

Visualization of Hypersonic Flat-Plate Boundary Layer in Shock Tunnel

“…This strip structure can be seen as one half of the vortices or can be taken as asymmetric vortices structure. This is not the same as the transition process of supersonic boundary layer in [14]. The flow mechanism of hypersonic boundary layer transition needs to further study.…”

“…The grayscale of NPLS images stands for the density fluctuation in the flow [14]. Therefore, the boundary layer in Figure 2 is dark owing to low local density.…”

Fine Structures of Supersonic Laminar Flow over a Backward Facing Step at Ma=3.0,3.8

“…Based on NPLS, several new testing techniques have been developed. One of them is a new density testing technique, which named NPLS-DT, developed by TIAN L.F using an oblique shock wave calibration [7]. Fig 2 is the NPLS image of the Ma=3.0 transient and time-averaging flow-field, which distinctly reveals structures of the whole flow-field.…”