Investigation on onset of shock-induced separation

A great number of experimental data indicating shock wave/boundary layer interactions in internal or external supersonic flows were reviewed to make clear the mechanism of the interaction and to decide the onset of shock-induced separation. The interesting conclusions were obtained for the considerably wide range of flow geometries that the onset of separation is independent of the flow geometries and the boundary layer Reynolds number. It is found that the pressure rise necessary to separate the boundary layer in supersonic external flows could be applied to such internal flows as overexpanded nozzles or diffusers. This is due to the fact that the separation phenomenon caused by shock wave/boundary layer interactions is processed through a supersonic deceleration. The shock-induced separation in almost all of interacting flow fields is governed by the concept of free interaction, and the onset of shock-induced separation is only a function of the Mach number just upstream of shock wave. However, physical scales of the produced separation are not independent of the downstream flow fields.     

Dust entrainment in a shock-induced,turbulent air flow

Dust from a layer on the floor of a shock tube is entrained by the air flow behind the unsteady shock wave. The development of the dust mass concentration profiles is measured by means of an optical extinction method. The concentration profiles which can be described by an exponential law approach a stationary limit consistent with the results of pneumatic transport theory. A theoretical model simulating the dust entrainment by a diffusion process is evaluated numerically and compared with the experimental results.     

PIV study of large-scale flow organisation in slot jets

The paper reports on particle image velocimetry (PIV) measurements in turbulent slot jets bounded by two solid walls with the separation distance smaller than the jet width (5–40%). In the far-field such jets are known to manifest features of quasi-two dimensional, two component turbulence. Stereoscopic and tomographic PIV systems were used to analyse local flows. Proper orthogonal decomposition (POD) was applied to extract coherent modes of the velocity fluctuations. The measurements were performed both in the initial region close to the nozzle exit and in the far fields of the developed turbulent slot jets for Re ⩾ 10,000. A POD analysis in the initial region indicates a correlation between quasi-2D vortices rolled-up in the shear layer and local flows in cross-stream planes. While the near-field turbulence shows full 3D features, the wall-normal velocity fluctuations day out gradually due to strong wall-damping resulting in an almost two-component turbulence. On the other hand, the longitudinal vortex rolls take over to act as the main agents in wall-normal and spanwise mixing and momentum transfer. The quantitative analysis indicates that the jet meandering amplitude was aperiodically modulated when arrangement of the large-scale quasi-2D vortices changed between asymmetric and symmetric pattern relatively to the jet axis. The paper shows that the dynamics of turbulent slot jets are more complex than those of 2D, plane and rectangular 3D jets. In particular, the detected secondary longitudinal vortex filaments and meandering modulation is expected to be important for turbulent transport and mixing in slot jets. This issue requires further investigations.     

PIV measurements of a microchannel flow

 A particle image velocimetry (PIV) system has been developed to measure velocity fields with order 1-μm spatial resolution. The technique uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution of the PIV technique is limited primarily by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring the known flow field in a 30 μm×300 μm (nominal dimension) microchannel. The resulting velocity fields have a spatial resolution, defined by the size of the first window of the interrogation spot and out of plane resolution of 13.6 μm× 0.9 μm×1.8 μm, in the streamwise, wall-normal, and out of plane directions, respectively. By overlapping the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved PIV measurements published to date. Received: 29 October 1998/Accepted: 10 March 1999     

Large-eddy simulation of low-frequency unsteadiness in a turbulent shock-induced separation bubble

The need for better understanding of the low-frequency unsteadiness observed in shock wave/turbulent boundary layer interactions has been driving research in this area for several decades. We present here a large-eddy simulation investigation of the interaction between an impinging oblique shock and a Mach 2.3 turbulent boundary layer. Contrary to past large-eddy simulation investigations on shock/turbulent boundary layer interactions, we have used an inflow technique which does not introduce any energetically significant low frequencies into the domain, hence avoiding possible interference with the shock/boundary layer interaction system. The large-eddy simulation has been run for much longer times than previous computational studies making a Fourier analysis of the low frequency possible. The broadband and energetic low-frequency component found in the interaction is in excellent agreement with the experimental findings. Furthermore, a linear stability analysis of the mean flow was performed and a stationary unstable global mode was found. The long-run large-eddy simulation data were analyzed and a phase change in the wall pressure fluctuations was related to the global-mode structure, leading to a possible driving mechanism for the observed low-frequency motions.      

跨音速湿空气非平衡凝结流动的数值研究

跨音速流动条件下湿空气中的水蒸气由于快速膨胀而发生非平衡凝结,凝结潜热对跨音速气流进行加热,会显著改变气流的流动特性。通过对商用计算流体动力学软件FLUENT进行二次开发,建立了湿空气非平衡凝结流动的数值求解方法。该方法可用于二维或三维、粘性或无粘、内流或外流的求解中。采用该方法分剐对缩放喷管、透平叶栅以及绕CA-0.1圆弧翼型的湿空气非平衡凝结流动进行了数值分析。计算结果表明：湿空气凝结手l起缩放喷管中的凝结激波、导致叶橱流动中总压降低;对于翼型周围的流动,在相对湿度分别为50％、57．1％、64.1％时,依次计算得到了单激波、五激波、双激波。     

Scanning PIV measurements of a laminar separation bubble

Scanning PIV is applied to a laminar separation bubble to investigate the spanwise structure and dynamics of the roll-up of vortices within the bubble. The laminar flow separation with turbulent reattachment is studied on the suction side of an airfoil SD7003 at Reynolds numbers of 20,000–60,000. The flow is recorded with a CMOS high-speed camera in successive light-sheet planes over a time span of 1–2 s to resolve the temporal evolution of the flow in the different planes. The results show the quasi-periodic development of large vortex-rolls at the downstream end of the separation bubble, which have a convex structure and an extension of 10–20% chord length in the spanwise direction. These vortices possess an irregular spanwise pattern. The evolution process of an exemplary vortex structure is shown in detail starting from small disturbances within the separation bubble transforming into a compact vortex at the downstream end of the separation bubble. As the vortex grows in size and strength it reaches a critical state that leads to an abrupt burst of the vortex with a large ejection of fluid into the mean flow.     

跨超声速风洞扩散段流动分离控制设计参数研究

为抑制跨超声速风洞扩散段的分离,提出了一种较为完备的设计方法。由于影响扩散段性能的参数较多,完全通过试验方法进行设计的成本过高,该方法通过数值模拟,结合适当的边界条件,详细描述了扩散段角度、分流锥角度与长度、孔板开孔率对扩散段性能的影响;从数值模拟的结果可以看出,孔板开孔率和扩开角对扩散段性能有显著影响,通过比较得出较为合理的参数匹配,提高了扩散段的防分离性能,并改善了出口气流质量。数值结果与试验结果结论一致,表明本文所用的方法用于扩散段气动设计是可行的,为数值模拟方法应用于风洞部段气动设计创造了一定的条件。     

Numerical study of vortex-breakdown over a slender wing in transonic flow at high incidence

The transonic flowfields and vortex-breakdown over a slender wing with the angle of attack from 10° to 28° are studied numerically, and the emphasis is on the secondary separation and the charateristics of vortex-breakdown. The results indicated that: (a) TVD schemes have strong capability for capturing vortices in three-dimensional transonic separated flow at large angle of attack. (b) The development of secondary vortices is more complex than that of leading-edge ones, and is affected by wing's configuration, angle of attack and compressibility simultaneously, and the effect of compressibility is more severe at low angle of attack. (c) The starting angle of attack for vortex-breakdown (when vortex bursting point crosses trailing-edge) is about 18° forM∞=0.85, then the bursting point moves upstream quickly with increasing angle of attack. (d) At α=24°, breakdown occurs over most part of upper side, and the wing begins to stall. Therefore, there is a large lag of angle of attack between the beginning of vortex-breakdown and the stall of the wing. (e) This lag increase with the decreasing of Mach number.     

Oscillations of a plate cascade in a transonic gas flow

The transonic unsteady flow of a gas through a cascade of thin, slightly curved plates is quite complex and has received little study. The main difficulties are associated with the nonlinear dependence of the aerodynamic characteristics on the plate thickness. In [1] it is shown that, for a single thin plate performing high-frequency oscillations in a transonic gas stream, the variation of the unsteady aerodynamic characteristics with plate thickness may be neglected. For a plate cascade, the flow pattern is complicated by the aerodynamic interference between the plates, which may depend significantly on their shape. A solution of the problem of transonic flow past a cascade without account for the plate thickness has been obtained by Hamamoto [2].The objective of the present study is the clarification of the dependence of the aerodynamic characteristics of a plate cascade on plate thickness in transonic unsteady flow regimes. The nonlinear equation for the velocity potential is linearized under the assumption that the motionless plate causes significantly greater disturbances in the stream than those due to the oscillations. A similar linearization was carried out for a single plate in [3]. The aerodynamic interference between the plates is determined by the method presented in [4]. As an example, the aerodynamic forces acting on a plate oscillating in a duct and in a free jet are calculated.     

Bifurcations in the transonic flow past a symmetric airfoil

Transonic flow past an airfoil with a small curvature in its midchord region is numerically investigated. The branching of the stationary solutions of the Euler equations is established and attributed to flow instability at certain angles of attack and freestream Mach numbers. The dependence of the lift coefficient on these parameters is studied.     

Experimental study on wake flow structures of screen cylinders using PIV

Experiments were conducted in a water flume using Particle Image Velocimetry (PIV) to study the evolution of the vortical structures in the wakes of four types of screen cylinders at a Reynolds number of about 3200. The results were compared with that of a bare cylinder. The screen cylinders were made of stainless steel screen meshes of various porosities (37%, 48%, 61% and 67%) rolled into cylindrical shapes. Smoke wire flow visualisations in a wind tunnel were also conducted in support of the PIV tests. Depending on the porosity of the screen mesh, two vortex formation mechanisms for the screen cylinder wakes were identified. One was associated with wake instability and the other was associated with shear-layer (Kelvin-Helmholtz) convective instability which involved merging through pairing and tripling of small-scale vortices within the shear layers. The former was responsible for the formation of large-scale vortices in the bare cylinder and the screen cylinder wakes with 37% and 48% porosities, while the latter was responsible for the screen cylinder wakes with 61% and 67% porosities. The results also showed that with increasing porosity, the vortex formation region was extended farther downstream and the Reynolds shear stress, the Turbulent Kinetic Energy (TKE) and vortex intensity were decreased constantly.     

Formation of multiple shocklets in a transonic diffuser flow

Multiple shocklets are frequently generated in transonic diffuser flows. The present paper investigates the formation of these shocklets with a high-speed CCD camera combined with the schlieren method. It is observed that compression waves steepen while propagating upstream, and eventually become new shock waves. The ordinary shock wave is found to move upstream beyond the nozzle throat or to disappear while moving downstream depending on the pressure ratio across the nozzle. This phenomenon is also analyzed with the one-dimensional Euler equations by assuming a pressure disturbance given by the sine function at the channel exit. The calculated results are found to reproduce quite well the experimental behavior of the shocklets. The effect of the frequency of disturbance is also studied numerically, and it is shown that the multiple shocklet pattern appears when the amplitude of disturbance is not large and the diverging part of the channel downstream of the ordinary shock wave is long. Received 26 June 1998 / Accepted 15 March 1999     

Application of PIV in a Mach 7 double-ramp flow

The flow over a two-dimensional double compression ramp configuration is investigated by means of schlieren visualization, quantitative infrared thermography and particle image velocimetry (PIV) in a short-duration facility producing a free-stream flow at Mach 7. The study focuses upon the accuracy assessment of PIV in the hypersonic flow regime including flow facility effects such as repeatability of test conditions. The solid tracer particles are characterized by means of electron microscopy as well as by measuring the dynamic response across a planar oblique shock wave with PIV. The experiments display a strong variation in the light scattering intensity of the seeded flow over the flow field, due to the large flow compressibility. The mean velocity spatial distribution allows to clearly identify the shock pattern and the main features of the flow downstream of the shocks. However, the spatial resolution is insufficient to determine the wall flow properties. Furthermore the velocity data obtained with the PIV technique allow the determination of the spatial distribution of the Mach number under the hypothesis of adiabatic flow. The double ramp configuration with a variable second compression angle exhibits shock–shock interactions of Edney type VI or V for the lowest and highest ramp angle, respectively. A single heat transfer peak is detected with infrared thermography on the second ramp in case of a type VI interaction while for the type V shock interaction a double heat transfer peak is found. Shock wave angles measured with PIV are in good agreement with theory and the overall flow topology is consistent with schlieren visualization. Also in this respect the results are in agreement with compressible flow theory.     

Stability of transonic two-phase flow

The nature of a singular point in the stability of one-dimensional transonic flow of a vapor-drop mixture in a channel of variable cross section is considered within the framework of a two-lquid hydrodynamical model. It is shown that the singular point in the case of any lags of the drops preserves the nature of a saddle inherent to homogeneous gas flow, shifting only towards the divergent part of the channel if the content of condensed phase is not too high. Here the transition of subsonic two-phase flow into supersonic flow is stable and the predominance of drop agglomeration over fragmentation and the positive curvature of the channel profile are stabilizing factors. The saddle nature of the singularity is possible only if the lag of the drops is not too high in the case of flows with a higher content of condensed phase. In the opposite case, the point at which the speed of sound is attained loses the nature of a saddle point.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 163–171, January–February, 1976.     

Compression shock in the transonic flow around a convex corner

The interaction between mixed sub-and supersonic flow near a convex breakpoint of a profile with a rectilinear wall downstream of this breakpoint is investigated. If we start from the fact that the initial flow has the character of a singularity in the domain ahead of the last characteristic of the rarefaction node [1], then the solution in the interaction domain, obtained in the hodograph plane under the assumption of its continuity in the physical plane, is not realizable because of the presence of limit lines. This governs the hypotheses of the formation of the compression shock emerging from the corner point and having zero intensity there.