Tomography-like flow visualization of a hypersonic inward-turning inlet

Yiming LI, Zhufei LI, Jiming YANG

Department of Modern Mechanics, University of Science and Technology of China, Hefei 230027, China

Keywords:Counter-rotating Vortex Pair(CVP);Flow visualization;Inward-turning inlet;Planar Laser Scattering (PLS)method;Shock wave

Abstract The flow field in a typical inward-turning inlet was visualized using the Planar Laser Scattering (PLS) method in a shock tunnel with a nominal Mach number of 6. The opaque inlet,which is truncated at a series of sections, and the following transparent isolator, are combined to enable the optical access at different streamwise locations. The sequential PLS images provide a tomography-like flow visualization, which confirm the existence of streamwise Counter-rotating Vortex Pairs (CVPs) in both external and internal flow field of the inlet. Generation mechanisms of these CVPs are unraveled with the help of a numerical simulation, among which the cowl notch plays an important role in the generation of surface trailing CVPs along the centerline of the cowl.Moreover, the cowl shock sweeps the internal boundary layer towards the body side, which ultimately accumulates low-momentum flow on the body side in forms of a large CVP propagating downstream through the isolator. The CVPs formed in the shape-transition are responsible for the nonuniform flow field of the inward-turning inlet. This study indicates that the V-shaped cowl notch affects the downstream flow significantly and, therefore, should be examined thoroughly in practical applications.

1. Introduction

Three-dimensional inward-turning inlets have shown a great potential to improve the performance of hypersonic air breathing propulsion systems,1because of their high compression ratio, small wetted area, and the ability to be easily integrated with a circular cross-sectional engine,2and thus have attracted wide attention in both industrial and academic communities.To understand the complex three-dimensional internal flow of the inward-turning inlet, many efforts have been made and most of which were performed using numerical simulations. For instance, it was reported that the low-momentum streams easily accumulate on the body side in forms of a large streamwise Counter-rotating Vortex Pair (CVP),3-5resulting in an uneven distribution of the flow parameters.6Recently,Barth et al.7found that a small CVP is formed along the internal centerline of the cowl. These numerical findings indicate that the CVPs play an important role in the inward-turning inlet,which should be revealed thoroughly.On the other hand,the experimental results are relatively limited, although schlieren images of the external flow, surface pressure, and pitot pressure were acquired in some experiments.8-10In reality,the three-dimensional inlet surfaces inherently block visualization attempts of the internal flow field for a long time.

In recent years,the Planar Laser Scattering(PLS)technique based on nanoparticles has been successfully used for supersonic/hypersonic external flow visualizations. For instance,Zhang,11Tao,12and Wang13,14et al. observed supersonic Shock Wave/Boundary Layer Interactions (SWBLIs) and turbulent structures using the Nano-tracer PLS(NPLS)technique with elaborately prepared nano-titanium dioxide particles. In addition to the titanium dioxide particles, some easily accessible condensable-materials,such as water vapor(H2O)and carbon dioxide (CO2), which can spontaneously condense into nanoparticles as the gas undergoes the supersonic/hypersonic nozzle expansion process, are also used as tracers for the PLS.The condensed particles vaporize in the high temperature region, such as downstream of a shock wave and within a boundary layer, and thus, enable flow visualizations. For instance, Do15and Zhuang16et al. observed supersonic SWBLIs,whereas Poggie17and Zhang18et al.observed hypersonic boundary layers using the condensate-enhanced PLS.These works create an obvious motivation to observe the complex internal flow of a hypersonic inward-turning inlet. In a previous work by Li et al.,19the flow on streamwise sections of a circular cross-sectional isolator were observed, which demonstrated the reliability of the condensate-enhanced PLS technique in a shock tunnel. In contrast, a series of spanwise sections of a similar isolator were acquired by DiCristina et al.20to artificially construct the cross-sectional flow.Although some progress has been made to provide an illuminating insight into the internal flow,the flow structure between the cowl and the throat of the inward-turning inlet is still lacking because the violent cross-sectional shape transitions inherently distort the optical access.

In this paper, the flow field on a series of streamwise sections of a hypersonic inward-turning inlet were acquired,which provides a tomography-like flow visualization for a thorough understanding of the flow physics in the inwardturning inlet.

2. Methods

As shown in Fig.1,the inward-turning inlet model consists of an elliptical-like-to-circular shape transition inlet and an isolator of constant cross-sectional area.21The opaque inlet consists of aluminum and the transparent isolator consists of acrylic resin are connected at a circular throat with a diameter of 35 mm. The total length of the model is 720 mm and the throat locates atx=480 mm from the leading edge.The inlet has a design Mach number of 6.5 with the overall and internal contraction ratios of 5.7 and 1.6,respectively.The leading edge blunt radius of the inlet is 0.42 mm.To observe both the internal and external flow, the inlet is truncated at four sections labeled S1-S4 downstream of the cowl(see Fig.1(a))to enable the optical access for flow visualization. Moreover, four additional sections labeled S5-S8(see Fig.1(a))are visualized in the isolator using a transparent wall the same as that in Ref.15Streamwise locations of all sections are listed in Table 1,which can be used to construct a tomography-like flow visualization.

All experiments were conducted in the KDJB330 shock tunnel22of the University of Science and Technology of China with a nominal Mach number of 6, a total temperature of 957 K,a total pressure of 1.9 MPa,and a units Reynolds number of 4.8×106m-1. Preceding the experiments, water vapor was added into the air in the driven tube with a volume fraction of approximately 2%. When the water vapor passed through the tunnel nozzle,it condensed into tiny particles with an average diameter of approximately 67 nm19to implement the PLS. A double-pulse laser (Vlite-500, Beamtech Optronics Co., Ltd) with a wavelength of 532 nm was used as the light source. The duration of each pulse was 10 ns with an energy of 500 mJ. The laser was transformed into a light sheet via a set of lenses. A Charge Coupled Device (CCD) camera(Bobcat-B6620, Imperx, Inc.) with a resolution of 4400 pixel×6600 pixel was synchronized with the laser to record the PLS images. As shown in Fig. 1(b), when the planar laser illuminated a cross-section of the model, the instantaneous flow field was captured by the CCD camera with the help of a flat mirror placed downstream of the model. Intriguing flow images on the cross-sections were recorded by moving the planar laser along the streamwise direction.

Table 1 Locations of observed cross-sections (distance from leading edge).

Fig. 1 Schematics of inward-turning inlet model and optical path.

To better understand the complex flow, numerical simulation was performed at the same flow conditions as the experiments.The influence of H2O condensation is neglected because its volume fraction is minor.17A three-dimensional Reynoldsaveraged Navier-Stokes solver based on the finite volume method was employed with thek-ω SST (Shear Stress Transport) turbulence model. This solver has successfully predicted the inward-turning inlet flow in a previous work; for more detail, one can refer to Ref.23

3. Results and discussion

An overview of the flow field is shown in Fig. 2, where the Mach number contours on the symmetry plane and PLS images of S1-S8 are presented. As the freestream comes from the left, the leading edge shock (labeled 1) is first observed in Fig. 2(a). The impingement and reflection of the cowl shocks(labeled 2 and 3)generate a distinct separation near the shoulder.As a result,the following separation shock(labeled 4)and reattachment shock (labeled 5) reflect in the flow path. The instantaneous PLS images in Fig. 2(b) give a glimpse of the three-dimensional flow field. Generally, the scattered light is strong in the region containing a large number of tracers,resulting in a bright region in the PLS images. In contrast,the dark region contains less tracers mainly because the sufficiently high temperature of the local flow vaporize the tracers.17As expected, the high temperature gas in the boundary layer is shown as a dark region.Interestingly,the internal dark region thickens abruptly in a short distance and some small dark regions penetrate into the bright main flow. These flow features in combination with the numerical simulation are discussed as follows.

Fig. 2 Overview of flow field.

Fig. 3 Flow field on S1.

3.1. Flow features near cowl

It is of great importance to reveal the flow near the cowl at first because it affects the downstream flow significantly.As shown in the image of S1 in Fig. 2(b), a large bright arc and a dark region are formed along the external centerline of the cowl.Similarly,these features are also shown along the internal centerline of the cowl but with a much smaller size.To unravel the generation mechanisms of these features, eight pictures of S1 from four runs are superimposed and compared with the numerical result in terms of time-averaged structure in Fig. 3. It is obvious that the bright arcs (labeled 1 and 2) are cross-cutting lines of the three-dimentional cowl shock, which sweep the external and internal walls. Due to the concave internal wall of the cowl, another shock (labeled 3) is formed behind the cowl shock (labeled 2), which is identified on the flow field of S1.

Major vortices near the cowl are identified using contours of streamwise vorticity (ωx) in Fig. 3. The dark regions along the centerline of the cowl in Figs.3(a)and(b)nearly coincide with the high-temperature regions and the CVPs in Fig. 3(c).These regions of high fluid rotation roll up low-momentum fluid and protrude to the external and internal flow field.Interestingly,Barth et al.7reported that the CVP along the internal centerline of the cowl is formed when the streams of fluid being forced inward by the sidewall contraction converge downstream of the cowl notch.Although this argument seems plausible for the internal CVP,it cannot explain the appearance of the other CVP along the external centerline of the cowl where sidewall contraction does not exist. It is therefore of fundamental interest to understand the flow physics of the CVPs downstream of the cowl notch.As shown in Fig.4,the curved streamlines near the cowl notch undergo inward deflections by the interaction of the shocks(labeled 8 and 9;and displayed by iso-surface of pressure) from the same family. Due to the strong baroclinic effects of the cowl notch, these streamlines move towards the spanwise symmetry plane to form swirling flows.The generated vorticities are realigned in the streamwise direction to form the CVP downstream of the cowl notch.The streamlines confirm that the external CVP is originated from the cowl notch, rather than the sidewall contraction of the inlet.Likewise,the high rotation fluid enters the inlet in forms of the internal CVP. In previous works, Xiao24and Zhang25et al.demonstrated that a plate with a V-shaped blunt leading edge can generate CVPs converging to the symmetry plane.For more detail, one can refer to Ref.25, which can be seen as an additional support for the physics of the CVP downstream of the cowl notch as suggested by the current work.

Fig. 4 Flow features near cowl notch with contours of pressure on generatrix plane of cowl notch, contours of streamwise vorticity on cross-section of x =306 mm, and iso-surface of pressure in a quarter of domain.

3.2. Vortices in internal contraction

The contours of streamwise vorticity on S1 (see Fig.3(c)) also identify a region of high vorticity (labeled 6 and 7) near the roots of the cowl shock. This high vorticity structure grows with the sweep of the cowl shock in the internal contraction of the inlet. As shown in Fig. 5, the cowl shocks (labeled 2 and 3) arrive at the center of S2, which is accompanied by a thick, high temperature structure (labeled 10), along with a large vortical structure, near the roots of the cowl shock.Unsurprisingly, the cowl-side CVP on S2, being observed like a mushroom, grows larger than that on S1.

As shown in Fig.6,violent cross-sectional shape transitions are completed in the internal contraction of the inlet,in which the passage of sweeping cowl shock through the boundary layer induces a strong baroclinic torque, and generates the high vorticity near the roots of the cowl shock (CS1 and CS2).The coherent vortical structures sweep towards the body side as the flow approaches the throat (S4). As shown by the streamlines on S4 (see Fig. 6), the meeting streams result in vorticity which rolls up the low-momentum fluid into the body-side CVP.As a result,the internal body-side dark region observed in the PLS image thickens abruptly from S3 to S4(see Fig. 2). This finding is confirmed by examining the three-dimensional streamlines passing through the body-side CVP in Fig.6.The near-wall flow travels toward the body side and is then turned inward,inducing a rotation in the flow near the symmetry plane.It is worthwhile to note that despite lacking a long forebody upstream of the external compression surface, the inlet also form thick, high temperature body-side boundary layer with the sweeping of the cowl shocks.Interestingly, the strength of cowl-side CVP is diminished when it approaches S4 because of the destruction induced by multiple reflected shocks (see Fig. 2).

Fig. 5 Flow field on S2.

3.3. Flow features in isolator

The PLS images on the throat(S4)and the isolator outlet(S8)are compared in Fig.7.As shown in Fig.7(a),the mainstream,being observed as the bright region, occupies approximately 80%of the cross-section area at S4.However,the mainstream region shrinks gradually along the isolator. The reasons are discussed as follows. First, the accumulation of the lowmomentum flow involved in the body-side CVP (marked by cyan streamlines in Fig. 7 with the help of numerical simulation) make the body-side dark region enlarges along the flow direction. Second, the convective diffusion of the cowl-side CVP are intensified after the interference with the multiple reflected shocks in the isolator, which dramatically enlarges the dark region as the entrainment of the low-momentum flow.Third, the boundary layer further develops on the cowl-side wall, which is much thicker in the isolator (see Fig. 2(b)). As a result, the mainstream region occupies less than a half of the cross-section area at the isolator outlet (see Fig. 7(b));for more details, one can refer to Ref.19

Fig. 6 Formation of body-side CVP with pressure isolines overlaid on contours of streamwise vorticity on cross-sections of internal contraction of inlet, and streamlines overlaid on contours of streamwise vorticity on S4.

Fig.7 Instantaneous PLS images on S4 and S8(marked by cyan streamlines).

4. Conclusions

A tomography-like flow visualization in an inward-turning inlet is achieved at a flow Mach number of 6 using streamwise truncated model and the PLS method. This flow visualization technique provides a method to assess the inlet performance experimentally, which improves the understanding of the flow physics in the inward-turning inlet.Multiple streamwise CVPs are experimentally identified in internal and external flow of the inlet, which is consistent with the numerical simulation.It is demonstrated that the CVPs trailing along both the internal and external centerlines of the cowl are generated by the Vshaped cowl notch rather than the sidewall contraction.Moreover, the body-side CVP is confirmed to be originated in the shape-transition of the inlet, where the cowl shock sweeps the boundary layer towards the body side and the meeting streams roll up the low-momentum fluid. Both the cowl-side and body-side CVPs travel downstream through the isolator,intensifying the internal nonuniform flow. As the V-shaped cowl notch affects the downstream flow significantly,it should be examined thoroughly in practical applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Nos. 11772325, 11872356 and 11621202).