Nano-tracer Planar Laser Scattering Research Articles

Effect of wall-seeping gas film under different working mediums on conical hypersonic boundary layer stability

Wall-seeping gas film (WSGF) represents a promising method for transition control, drag reduction, and heat reduction in hypersonic vehicles. Experiments were conducted in a Mach 6 hypersonic quiet wind tunnel using nano-tracer planar laser scattering (NPLS) and high-frequency fluctuating pressure measuring techniques. This paper investigates the effects of wall-seeping helium, air, and carbon dioxide gas films under identical volume flow rate conditions on conical boundary layer thickness, disturbance wave structure, wavelength, frequency, amplitude, and nonlinear interaction. The experimental results reveal that WSGF significantly thickens the hypersonic boundary layer, with the thickest position appearing at the downstream boundary of the seeping zone. The boundary layer thickness is thinnest for helium gas film and thickest for carbon dioxide gas film. Generally, air gas film and carbon dioxide gas film induce the appearance of regular, rope-like, and interlaced second-mode waves in advance in the boundary layer. However, under a higher volume flow rate for carbon dioxide gas film, the disturbance wave structure resembles interface fluctuations, with a characteristic wavelength of approximately 18 mm and a peak frequency as low as about 35 kHz, without the rope-like interlaced characteristic. At this time, the influence of shear layer instability becomes significant. The disturbance waves do not exhibit second-mode wave characteristics for wall-seeping helium gas film, whose shape is irregular and undergoes deformation over time and space. Additionally, the power spectral density of wall fluctuating pressure exhibits insignificant variation with volume flow rate and flow direction, which is similar to the characteristics of power spectral density in the laminar boundary layer and has no peak frequency. The wavelength of second-mode waves is about 2~3 times the boundary layer thickness for air gas film while increasing to more than 3 times for carbon dioxide gas film. Compared to air gas film, the application of carbon dioxide gas film results in a smaller peak frequency and bandwidth of disturbance waves, larger characteristic wavelength and amplitude, longer propagation distance, and stronger nonlinear interaction. In the future, attention should be directed towards understanding disturbance wave characteristics in the boundary layer for helium gas film and shear layer instability under larger volume flow rates.

Experimental Study on Time Evolution of Shock Wave and Turbulent Boundary Layer Interactions

In this paper, a test system based on the Nano-tracer Planar Laser Scattering (NPLS) technique for studying time evolution of unsteady flow structures was finished. Based on this system, the experimental study on the interactions between the incident shock wave and the turbulent boundary layer of the incoming wall was performed. The experiments were performed in a Mach 3.4 supersonic low-noise wind tunnel at the unit Reynolds number of 6.30 × 106/m-1. For the first time, five frames of temporal-correlated fine structure images of transient flow field with shock wave and the turbulent boundary layer interactions (SWTBLI) were obtained under the experimental conditions, and the spatiotemporal evolution characteristics of the flow structure were analyzed. At the same time, the flow field characteristics of temporal-correlated images were studied when the density boundary layer thickness of incoming turbulent layer is δ1 = 0.55δ, δ2 = 0.72δ, δ3 = 0.87δ respectively. During the development of vortex structure in the boundary layer from turbulent boundary layer to separation bubble, the oscillation interval distribution law of induced shock wave was summarized, and the group velocity of vortex structure development in the boundary layer and the relationship between boundary layer thickness and physical space size growth law of separation bubble under different incoming turbulent boundary layer thicknesses were obtained. The results also show that with the increase of the incoming boundary layer thickness, the group velocity in the development process of vortex structure in the turbulent boundary layer does not change significantly. As the thickness of the boundary layer entering the separation bubble increases, the overall growth height of the separation bubble also increases.

Experimental Investigation of the Hypersonic Boundary Layer Transition on a 45° Swept Flat Plate

In this paper, the experimental study on the transition process of a boundary layer on a 45° swept flat plate is performed in the Mach 6.0 hypersonic low-noise wind tunnel. The instantaneous fine structures of the boundary layer on the swept flat plate in streamwise and spanwise planes have been investigated based on the Nano-tracer Planar Laser Scattering (NPLS) technique, respectively. The spatiotemporal evolution characteristics of the boundary layer translating from laminar to turbulence are analyzed. The transition process from laminar to turbulent flow is shown, and its spatiotemporal evolution characteristics are analyzed. It is found that the transitional plane is parallel to the leading edge of the 45° swept flat plate under the unit Reynolds number of 1.67 × 107 m–1. The distance from the turbulent zone to the leading edge of the swept flat plate is 18.9R. The laminar strip is at an angle of 9° to the streamwise plane. Using Temperature-Sensitive Paints (TSP) technique, the temperature gradient distribution on the surface of the 45° swept flat plate wall was obtained, and the boundary layer transition regular pattern obtained by temperature gradient was agree with the coherent structures obtained from the NPLS images. Compared the TSP result with the NPLS images, the correlation between flow structure and wall temperature gradient was obtained.

Visualization of Supersonic Low-Noise Flow Over Surface-Mounted Two-Dimensional Prisms

An experimental study of supersonic flow over two-dimensional surface-mounted prisms is carried out in a Mach 3 low-noise wind tunnel. The noise level of this supersonic wind tunnel, defined as the root mean-square Pitot pressure fluctuation normalized by the mean Pitot pressure, can be reduced to about 0.37%. The nanotracer planar laser scattering (NPLS) technique is used to analyze the influence of the prism geometry and the oncoming flow conditions on the typical flow structures including separation and reattachment shocks. With increase in the prism height the induced shocks move upstream. At a constant streamwise length L of a prism the timeaveraged NPLS images show that the length of the downstream recirculation region increases from 0.8L to 1.2L, when the prism height H changes from 3 to 5 mm. As compared with the flow structures occurring downstream of the prisms, the upstream flow structures are more susceptible to the oncoming boundary layer and are considerably different in laminar and turbulent flows. The separation shock wave is clearly visible in turbulent flow even for the 1-mm prism, whereas in the case of laminar flow there is no a distinct shock wave upstream of this prism. At the same time, the location of the flow reattachment and the angle of the reattachment shock wave in the downstream flow remain almost the same in both two flow regimes.

Effect of micro-ramps on supersonic boundary layer past a double wedge

The effect of micro-ramps on Mach 3 boundary layer structures past a double wedge was experimentally investigated in a supersonic low-noise wind tunnel using nano-tracer planar laser scattering (NPLS) technique. The fine structures of supersonic flow over the double wedge with and without the micro-ramps, as well as temporal and spatial evolutionary characteristics of the vortex structures with the micro-ramps, were revealed.

Experimental study on the density characteristics of a supersonic turbulent boundary layer

An experimental study on the density characteristics of a zero-pressure-gradient flat plate turbulent boundary layer at Ma=3.0 is performed by the density field measurement method based on Nano-tracer planar laser scattering (NPLS) technology. The mean and the fluctuating characteristics of the density field of the boundary layer are analyzed. And the spectrum analyses of density fluctuations are performed by utilizing Taylor's hypothesis to convert spatial measurements into pseudo-temporal measurements. The mean density profile increases away from the wall, which accords well with the density profile deduced from the mean velocity distribution by using the adiabatic Crocco-Busemann relation. The root mean square (RMS) of the density fluctuations increases in the logarithmic region with a peak value of 0.2ρ∞, and its probability density distribution follows a normal distribution. However, the RMS of density fluctuations decreases in the outer region of the boundary layer. According to the spectrum analysis, the density fluctuations are characterized in a wide range of frequencies throughout the boundary layer, with the maximum frequency on the order of 1 MHz. The low frequency fluctuations are predominant near the wall and in the outer region of the turbulent boundary layer. However, the proportion of high-frequency fluctuations is nearly equal to that of low-frequency fluctuations in the logarithmic region. The combined NPLS and PIV technique provide a simultaneous density and velocity measurements of the present turbulent boundary layer. The high frequency fluctuations in the supersonic turbulent boundary layer may be induced by the density fluctuations, which are caused by the convection of the turbulent structures with nonuniform density distributions. And the contribution of the velocity fluctuations only to the low frequency fluctuations is observed. There are good similarities between the density fluctuations and the mass flux fluctuations for both the probability density distribution and the spectrum characteristics. On the contrary, a large difference between the fluctuations of velocity and density is identified. Therefore, the strong density fluctuations inside supersonic turbulent boundary layers, as well as its difference between the velocity fluctuations, should be one of the most important differences between compressible and incompressible turbulent boundary layers.

Flow control of micro-ramps on supersonic forward-facing step flow**Project supported by the National Natural Science Foundation of China (Grant Nos. 11172326 and 11502280).

The effects of the micro-ramps on supersonic turbulent flow over a forward-facing step (FFS) was experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of supersonic flow over the testing model were captured. The fine structures and their spatial evolutionary characteristics without and with the micro-ramps were revealed and compared. The large-scale structures generated by the micro-ramps can survive the downstream FFS flowfield. The micro-ramps control on the flow separation and the separation shock unsteadiness was investigated by PIV results. With the micro-ramps, the reduction in the range of the reversal flow zone in streamwise direction is 50% and the turbulence intensity is also reduced. Moreover, the reduction in the average separated region and in separation shock unsteadiness are 47% and 26%, respectively. The results indicate that the micro-ramps are effective in reducing the flow separation and the separation shock unsteadiness.

Fine structures and characteristics on supersonic flow over backward facing step with tangential injection

The fine flow structure over backward facing step with supersonic injection at the free-stream Mach number of 3.4 is investigated via nano-tracer planar laser scattering (NPLS). The Mach number of injection is measured to be 2.45 actually, even though designed to be 2.5 nominally. The shock wave, shear layer, mixing layer, Kelvin-Helmholtz vortex, horn-like vortex, coherent structures, etc, are clearly revealed. Flow images with the high spatiotemporal resolution are captured involving the streamwise and spanwise flow field in planes at different heights. Based on a large number of fine images, the spatial correlation analysis is conducted to reveal the structure scale and incline angle. The results indicate that with the flow developing, the structure angle tends to be larger and the structure scale becomes smaller. While the injection is working, the downstream surface of step will be covered by a thin film layer. In addition, the schlieren technique is used to compare with NPLS results, and the surface pressure coefficients are measured. In the downstream of injection, the coefficient is 0.0146. The fractal dimensions of different zones in NPLS image are calculated, showing that in the initial stage of flow the fractional dimension is approximate to 1 and the closer to downstream, the higher the dimension is.

Flow visualization of Mach 3 compression ramp with different upstream boundary layers

Experimental studies of supersonic flow over several compression ramps are carried out in a Mach 3.0 wind tunnel; the upstream boundary layers include laminar flow, transitional flow, and turbulent flow. Fine flow structures are obtained via nano-tracer planar laser scattering (NPLS) technique, typical flow structures such as boundary layer, shear layer, and shock wave are visible distinctly. The angle of shock wave and the development of boundary layer are measured by time-averaged flow field, the spatiotemporal evolutions of flow field are revealed by two time-dependent NPLS images. Results indicate that when ramp angles are 25°, a typical separation occurs in laminar flow, and a certain extent separation occurs in transitional flow, while separation does not occur in turbulent flow. When ramp angles are 28°, laminar flow separates further, a typical separation occurs in transitional flow, by comparison, separation region in turbulent flow is small. Significant differences of the three types of ramp flows can be found in shock wave structure and flow separation.

Flow visualization for the evolution of the slipstream in steady shock reflection

As an important part of the basic research on shock wave, shock reflection phenomena which appear in many practical situations, ranging from supersonic aircraft wings to hypersonic vehicles and engines, always receive a great deal of attention. Generally speaking, the slipstream which is an important part of shock reflection structures has a significant effect on the flow property. On the one hand, better understanding of the slipstream is beneficial to flow control which aims to reduce the stagnation pressure loss. On the other hand, it also provides a reasonable way in mixing enhancement. In view of these structures, the schlieren method and the shadow method are widely applied for flow visualization so far. However, the photographs which are obtained by the above methods merely reflect the flow field that is through temporal and spatial integration, rather than high-spatiotemporal resolution images. Also, the evolution of the slipstream cannot be fully investigated by means of the traditional methods of flow visualization. Garrison and Settles (1992) trace the evolution of the slipstream which emanates from the triple point of the k-shock system using the Planar Laser Scattering (PIS) technique. Afterwards, to describe the steady Mach reflection configuration approximately, Azevedo and Liu (1993) and Li and Ben-Dor (1997) investigate shape of the slipstream ignoring the viscous effect respectively. Then, Rikanati et al. (2006) indicate that the slipstream thickening of the pseudo-steady Mach reflection is caused by small-scale growth of the Kelvin–Helmholtz shear flow instability using the interferometry technique. Afterwards, Rubidge et al. (2013a, b; Rubidge and Skews 2014) point out that Kelvin–Helmholtz instability do form on the pseudo-steady Mach reflection slipstream, and describe the Kelvin–Helmholtz instability evolution using the schlieren images. However, little work has been conducted on the development of the slipstream in steady shock reflection. Hence, a series of experiments are performed to observe the fine structures of the steady shock reflection and investigate the evolution of the slipstream in this paper, using Nano-tracer Planar Laser Scattering (NPLS) technique.

Supersonic flow over circular protuberances on a flat plate

The nanotracer planar laser scattering (NPLS) technique is used to test the flow field of Mach 3.0 supersonic flow over circular protuberances. The protuberance heights (H) are 1, 2, and 4 mm, and the diameters (D) are all 4 mm with the Reynolds number of 7.7 × 106/m, while the thickness of the laminar boundary layer (δ) is about 1.2 mm at the mounting position, corresponding to H/δ values of 0.83, 1.67 and 3.33. The fine structures in the symmetrical plane of the flow field around the cylinders are imaged. In the NPLS images, expansion wave, three-dimensional bow-detached shock wave, reattachment shock wave and separation region ahead of the cylinders are clearly visualized. The spatial and temporal evolution characteristics of the vortex structures are analyzed. Special attention is paid to the study of the wake flow and the influence of the height of the protuberances on the wake flow, showing that vortices in the wake flow of the 1-mm high cylinder are isolated and simple, while to the 4-mm high cylinder are complex and mixed-up with less periodicity. After reattachment, the lower cylinder has a relatively longer laminar section as well as transition section. Fractal analysis is done on the wake flow, indicating that higher protuberances tend to have a larger fractal dimension and increasing slope. Based on the average fractal dimension of 50 NPLS images, the 2- and 4-mm cylinders have a close value while the 1-mm cylinder which is submerged in the boundary layer has a much smaller fractal dimension. Streamwise pressure distributions are measured both upstream and downstream, and the results show that the influence extent tends to depend little on the height of the cylinder. While the H/D values only influence on the size of the plateau region and the pressure increasing slope.

Experimental investigation of supersonic flow over elliptic surface

Abstract The coherent structures of flow over a compression elliptic surface are experimentally investigated in a supersonic low-noise wind tunnel at Mach Number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spacial resolution images and the average velocity profiles of both laminar inflow and turbulent inflow over the testing model were captured. From statistically significant ensembles, spatial correlation analysis of both cases is performed to quantify the mean size and orientation of large structures. The results indicate that the mean structure is elliptical in shape and structure angles in separated region of laminar inflow are slightly smaller than that of turbulent inflow. Moreover, the structure angle of both cases increases with its distance away from from the wall. POD analysis of velocity and vorticity fields is performed for both cases. The energy portion of the first mode for the velocity data is much larger than that for the vorticity field. For vorticity decompositions, the contribution from the first mode for the laminar inflow is slightly larger than that for the turbulent inflow and the cumulative contributions for laminar inflow converges slightly faster than that for turbulent inflow

Imaging of supersonic flow over a double elliptic surface

The coherent structures of flow over a double elliptic surface are experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of both laminar and turbulent inflows over the test model are captured. Based on the time-correlation images, the spatial and temporal evolutionary characteristics of the coherent structures are investigated. The flow structures in the NPLS images are in good agreement with the velocity fluctuation fields by PIV. From statistically significant ensembles, spatial correlation analysis of both cases is performed to quantify the mean size and the orientation of coherent structures. The results indicate that the mean structure is elliptical in shape and the structural angles in the separated region of laminar inflow are slightly smaller than that of turbulent inflow. Moreover, the structural angles of both cases increase with their distance away from the wall.

Visualization of Mach 3.0/3.8 Flow around Blunt Cone with Supersonic Film Cooling

Fine instantaneous flow structures of different scales around a blunt cone with or without supersonic film cooling were visualized via nanotracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. The Mach number of the freestream is 3.0 and 3.8 respectively and the air injection is at Mach 2.5. Lots of typical flow structures were visible clearly, such as shock wave, expansion fan, shear layer, mixing layer, K-H vortices and turbulent boundary layer. With injection, the model wall surface can be covered by a thin film layer. While no injection, the flow is similar to the supersonic flow over a backward-facing step and the structures are simpler relatively and there is a longer laminar region. Flow structures with or without film cooling at Mach 3.0 and 3.8 were compared.

Investigation on flows in a supersonic isolator with an adjustable cowl convergence angle

This paper presents an investigation of supersonic isolator flow behavior considering the impacts from variable cowl convergence angles and blockage ratios at the exit. An isolator was designed with a cowl convergence angle varying from 0° to 16°, which can provide unsymmetrical incoming flow condition. A blockage adjustor was installed at the exit of the isolator to simulate the choked flow. Flow visualization using Nano-tracer Planar Laser Scattering (NPLS) technique and pressure measurement were employed to represent the characteristics of isolator flows from totally unblocked state to the choked state at different cowl convergence angles. Results show that flow structures such as supersonic boundary layer, separation flow, shock waves, Mach disk and slip lines could be revealed distinctly by NPLS. The starting performance of isolator with different cowl angles which greatly affected the entrance condition was evaluated. Flow structures and pressure profiles show that a favorable contraction ratio provided a moderate starting. While an excessive contraction or deficient contraction corresponding to the cowl convergence angle of 0° and 16° suffer a harder starting which resulted from the pressure rise and severe separation on the ramp wall at the inlet entrance. Both pressure profiles and flow visualization results come to a good agreement with each other. Pressure profiles indicated that isolator with a 8° cowl convergence angle had the largest pressure endurance. And flow decelerated to sonic at the exit when the blockage ratio increased to nearly chock the whole isolator.

Visualization of supersonic flow over double wedge

Flow visualization of supersonic flow over a double ramp is achieved by the nano-tracer planar laser scattering (NPLS) technique in a supersonic low noise wind tunnel at Mach number 3.0. The fine structures of the boundary layer on the model, as well as the temporal and spatial evolutionary characteristics of the coherent structures, are revealed and investigated. High-spatiotemporal resolution images of the incoming boundary layer with laminar flow over the model are presented. Boundary layer transition occurs at the first ramp and the boundary layer at the second ramp has already been turbulent flow. The hairpin vortex and hairpin packet can be observed at the boundary layer of the first ramp. The angles of the hairpin vortex and hairpin packet are larger than that in a flat-plate because of adverse pressure gradient of the ramp. NPLS images of the incoming boundary layer with turbulent flow over the model are shown. The coherent structures at the first ramp are similar to the incoming boundary layer, which exhibit an obvious hierarchy. In contrast, the coherent structures at the second ramp exhibit different characteristics.

The Effect of Micro-ramps on Supersonic Flow over a Forward-Facing Step

The effect of micro-ramp control on fully developed turbulent flow over a forward-facing step (FFS) is investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and supersonic particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and the average velocity profiles of supersonic flow over the FFS with and without the control of the micro-ramps are captured. The fine structures of both cases, including the coherent structures of fully developed boundary layer and the large-scale hairpin-like vortices originated from the micro-ramps as well as the interaction of shock waves with the large-scale structures, are revealed and compared. Based on the time-correlation images, the temporal and spatial evolutionary characteristics of the coherent structures are investigated. It is beneficial to understand the dynamic mechanisms of the separated flow and the control mechanisms of the micro-ramps. The size of the separation region is determined by the NPLS and PIV. The results indicate that the control of the micro-ramps is capable of delaying the separation and diminishing the extent of recirculation zone.

Simultaneous density and velocity measurements in a supersonic turbulent boundary layer

A novel technique for simultaneous measurements of instantaneous whole-field density and velocity fields of supersonic flows has been developed. The density measurement is performed based on the nano-tracer planar laser scattering (NPLS) technique, while the velocity measurement is carried out using particle image velocimetry (PIV). The present experimental technique has been applied to a flat-plate turbulent boundary layer at Mach 3, and the measurement accuracy of the density and velocity are discussed. Based on this new technique, the Reynolds stress distributions were also obtained, demonstrating that this is an effective means for measuring Reynolds stresses under compressible conditions.

Instantaneous and time-averaged flow structures around a blunt double-cone with or without supersonic film cooling visualized via nano-tracer planar laser scattering

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 μs revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K—H (Kelvin—Helmholtz) vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backward-facing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions.

Application of Nano Technique in Measuring Supersonic/Hypersonic Flow

Turbulence, universally exist in nature and human activities, is a kind of three-dimensional, irregular, unsteady flow. Ever since 19th century when people started to investigated turbulent flow technically, they have never dropped the po-tent and intuitionistic experimental method. Recently, with the development of aviation and aerospace industry， espe-cially with the increase desire of supersonic and hypersonic flight, the mechanism of high speed and compressible flow has become hot topic of fluid research, resulting in development of measurement method and technique. When encoun-tering compressible high flow, traditional techniques, such as schilieren, shadow and interference, cannot measure fine flow structures. Fortunately, multiple-discipline integration of nano technique, laser technique and imaging technique provides a new design for fluid measurement。Nano-tracer planar laser scattering (NPLS) is a new flow visualization technique, which was developed by the authors’ group in 2005, and it can visualize time correctional flow structure in a cross-section of instantaneous 3D supersonic flow at high spatiotemporal resolution. Many studies have demonstrated that NPLS is a powerful tool to study supersonic turbulence.