水下爆炸中气泡射流壁压特性实验研究

水下爆炸气泡射流载荷是中近场水下爆炸壁压载荷的重要组成部分, 将水下爆炸气泡射流简化为一段高速水柱来研究水下爆炸气泡射流载荷特性是研究水下爆炸气泡射流载荷的主要手段。本文基于腔内爆炸提出了一种新的高速水射流实验方法,并给出了实验装置设计、实验方法以及实验系统。基于实验系统,开展了不同工况下高速水射流的实验研究,研究了腔口位置、腔深对水射流形态的影响,并对水射流的形态形成因素进行了分析。使用压电型壁压传感器测得了水射流冲击壁压,给出了水射流冲击壁压的特性及其特点。实验结果表明:腔口位置与腔深是影响水射流端面形态的重要因素;生成的高速水射流冲击壁压峰值满足水锤理论。基于腔内爆炸的高速水射流实验方法能够应用于包括水下爆炸气泡射流在内的高速水射流形态、壁压特性的研究。     

Extrudate swell for a round jet with large surface tension

The problem of extrudate swell of a viscoelastic fluid from a round pipe is studied by the method of domain perturbations. The perturbation problems are solved by a finite-element method through second-order in the flow rate parameter ∈ for small flow rates. The analysis extends the work of Sturges on swelling in two-dimensional channels to round capillary tubes. In perturbation studies for small ∈, the rheology of the fluid may be expressed by three parameters, the viscosity and the two constants α₁ and α₂ appearing at order two in the expansion of the extra stress around zero shear. Surface tension has an important influence on the shape of the jet at low speeds. The shape of the surface on a round jet depends on α₁ and α₂, in the plane jet only on α₁. The analysis predicts that no matter what the constitutive equation may be, the jet will first contract if the radius of the pipe is sufficiently small. The contraction takes place in a length less than $\text{1}\text{10}$ the diameter of the jet and is followed by a swell. The contraction is usually small and may be hard to observe. There are five different contributions to the jet shape at second-order but only the viscoelastic ones persist as the pipe radius goes to zero.     

Experimental analysis of high-speed helium jet flow using four-dimensional digital speckle tomography

A high-speed and initial helium jet flow has been analyzed by a developed four-dimensional digital speckle tomography. Multiple high-speed cameras have been used to capture movements of speckles in multiple angles of view simultaneously because a shape of a nozzle for the jet flow is asymmetric and the initial jet flow is fast and unsteady. The speckle movements between no flow and helium jet flow from the asymmetric nozzle controlled by a solenoid valve have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The four-dimensional density fields for the high-speed helium jet flow have been reconstructed from the deflection angles by a developed real-time tomography method.     

Effect of the Nozzle Edge Shape on the Acoustic Sensitivity of a Jet

The effect of the nozzle edge shape on the acoustic sensitivity of jets, that is, on the dependence of the jet parameters on the amplitude and frequency of the acoustic oscillations produced by an external source, is experimentally studied. The investigation was performed for nozzle edge configurations, the variation of which did not result in a change in the jet characteristics without external acoustic excitation. This means that the change in the edge shape alone had no influence on the flow pattern at the nozzle exit or the boundary layer flow regime on the nozzle walls. Measurements of the dependence of the mean velocity and the velocity fluctuation intensity on the jet axis on the distance from the nozzle exit showed that a change in the nozzle edge shape can lead to a change in the acoustic sensitivity of the jet when the jet is exposed to external acoustic action.     

楔形体入水时域解的复边界元数值分析

基于速度势理论,利用复数变量边界元法对二维楔形体常速入水冲击的时域解进行了数值研究。 以相似解作为数值计算的初始条件,采用时域解射流线性近似处理方法,利用复数变量边界元法进行求解,以 减少计算量和数值误差。深入讨论了扩展坐标系求时域解、射流处理、网格划分和网格更新等关键技术。最 后数值计算了不同斜升角楔形体入水时的自由液面隆起、射流飞溅和压力分布,经与相似解结果作比较,自由 液面隆起轮廓基本吻合,而压力分布更符合实际情况,从而证明了模型及分析方法的正确性。     

Near-field development of gas-phase horizontal laminar jets with positive and negative buoyancy measured with filtered Rayleigh scattering

Near-field mixing characteristics of horizontally issuing jets, alternatively positively and negatively buoyant, are explored. The cross-sectional mass fraction of a buoyant horizontal jet consisting of helium flowing into ambient air is measured using a non-intrusive technique, filtered Rayleigh scattering, for Reynolds numbers ranging from 50 to 1,200, Froude numbers ranging as low as 0.71, and Schmidt numbers on the order of unity for all tests. Several corresponding experiments were carried out using carbon dioxide in place of helium in order to determine whether the direction of the buoyancy changes the characteristic shape of the jet cross-section. Consistent with the literature, mixing rates were consistently higher on the side of the jet where instability, due to density stratification, was present. At jet Froude numbers ranging between 1.5 and approximately 3, the jet cross-section takes a shape consistent with a single plume of fluid being ejected from the core in a vertical direction—upward for a jet with positive buoyancy and downward for a jet with negative buoyancy. Remarkably, for Froude numbers less than unity, the distortion of the jet is quite different in that two separate plumes emanate from each side of the jet while ejection from the center is suppressed. Both the positively and negatively buoyant jet cross-sections exhibited this trait, suggesting that the mechanism that determines the cross-sectional shape of the jet core is only mildly influenced by centripetal effects brought about by streamline curvature. The location of the jet centroid at varied streamwise locations was computed from the mass fraction data, yielding jet trajectory.     

Mathematical simulation of unsteady flow past a wing with jets

The various approximate approaches to the investigation of the unsteady aerodynamic characteristics of an airfoil with jet flap [1–3] are applicable only for an airfoil, low jet intensity, and low oscillation frequencies. In the present paper, the method of discrete vortices [4] is generalized to the case of unsteady flow past a wing with jets and arbitrary shape in plan. The problem is solved in the linear formulation; the conditions used are standard: no flow through the wing and jet, finite velocities at the trailing edges where there is no jet, and also a dynamical condition on the jet. The wing and jet are assumed to be thin and the medium inviscid and incompressible.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 139–144, May–June, 1982.     

Dynamics and breakup of zigzag-like jets of polymeric liquids

Zigzag-like forms of the capillary jet breakdown are detected. The zigzag-like structures observed develop when a water jet with polymeric additives flows out from a transverse oscillating capillary. The factor responsible for the unusual behavior of the jet is the elasticity of the polymeric liquid. An analysis of the jet shape made it possible to find the elastic stress distribution in the disintegrating jet and to establish the laws of rheological behavior of polymeric solutions in intensive jet flows. An algorithm for the numerical calculation of the zigzag-like jet profiles is proposed.     

Viscous-fluid outflow from a vessel with account for jet formation

The results of a numerical simulation of slow free-boundary viscous-fluid outflow from a vessel are presented with account for jet formation. The problem is formulated in the creeping motion approximation. For solving the problem, a numerical algorithm for plane geometry, based on an indirect variant of the boundary-element method, is used. As a result of parametric studies, the evolution of the free surface inside the vessel and the jet shape is determined for different values of the governing parameters. Flow regimes with rapid funneling and film formation on the vessel walls are detected. The existence of an asymptotic flow regime is demonstrated using dimensional analysis and confirmed by calculation.     

Near field vorticity distributions from a sharp-edged rectangular jet

Experimental results on the near field development of a free rectangular jet with aspect ratio 10 are presented. The jet issues from a sharp-edged orifice attached to a rectangular settling chamber at Re_h ∼ 23,000, based on slot width, h. Measurements on cross plane grids were obtained with a two-component hot wire anemometry probe, which provided information on the three dimensional characteristics of the flow field. Two key features of this type of jet are mean axial velocity profiles presenting two off axis peaks, commonly mentioned as saddleback profiles, and a predominant dumbbell shape as described by, for example, a contour of the axial mean velocity. The saddleback shape is found to be significantly influenced by the vorticity distribution in the transverse plane of the jet, while the dumbbell is traced to two terms in the axial mean vorticity transport equation that diffuse fluid from the centre of the jet towards its periphery. At the farthest location where measurements were taken, 30 slot widths from the jet exit, the flow field resembles that of an axisymmetric jet.     

DIAMOND PORT JET INTERACTION WITH SUPERSONIC FLOW

Interaction flow field of the sonic air jet through diamond shaped orifices at different incidence angles （10 degrees, 27.5 degrees, 45 degrees and 90 degrees） and total pressures （0.10 MPa and 0. 46 MPa） with a Mach 5.0 freestream was studied experimentally. A 90 degrees circular injector was examined for comparison. Crosssection Mach number contours were acquired by a Pitot-cone five-hole pressure probe. The results indicate that the low Mach semicircular region close to the wall is the wake region. The boundary layer thinning is in the areas adjacent to the wake. For the detached case, the interaction shock extends further into the freestream, and the shock shape has more curvature, also the low-Mach upwash region is larger. The vortices of the plume and the height of the jet interaction shock increase with increasing incidence angle and jet pressure. 90 degrees diamond and circular injector have stronger plume vorticity, and for the circular injector low-Mach region is smaller than that for the diamond injector. Tapered ramp increases the plume vorticity, and the double ramp reduces the level of vorticity. The three-dimensional interaction shock shape was modeled from the surface shock shape, the center plane shock shape, and crosssectional shock shape. The shock total pressure was estimated with the normal component of the Mach number using normal shock theory. The shock induced total pressure losses decrease with decreasing jet incidence angle and injection pressure, where the largest losses are incurred by the 90 degrees, circular injector.     

Investigation on the collapse behavior of a cavitation bubble near a conical rigid boundary

The collapse stage of cavitation bubble development near a conical rigid boundary is investigated in detail by a finite-volume method and the volume of fluid method. The obtained results reveal the effect of the angle of the conical boundary on the bubble shape and the collapse time, as well as liquid jet formation. The degree of departure of the bubble shape from spherical one and the collapse time are found to increase with the increase of cone angle. The relationship between the prolongation factor of the collapse time near a conical boundary and the cone angle is proposed, and theoretical values of the collapse time are calculated. Good agreement is found between the theoretical values and the values obtained from simulations using a finite-volume method.     

On the length of annular jets interacting with an ambient medium

Annular jets of immiscible fluids are the subject of intense study. Particularly topical in applications are jets in the shape of a right circular cylinder. The space within annular jets may be reduced or increased by the influence of transverse forces and also by hydrodynamic instability of the jet flow. Twisting of the jet tends to make it close up. In the present paper, a study is made of ways of obtaining annular jets with nearly cylindrical shape and the greatest cavity length possible, allowance being made for gravity, capillary pressure surface forces, a pressure difference between the two sides of the phase interface, and the interaction with the ambient medium. A study is made of the influence of the velocity of the fluid and the medium in the initial section on the shape of the joint steady axisymmetric flow of immiscible viscous phases, including the shape of the middle surface of the annular jet.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 3–11, November–December, 1983.     

A computational analysis of the hydrodynamic instability of a liquid jet focused into a converging microchannel

The instability of a focused liquid jet is studied by semi-analytical methods and by methods of computational fluid dynamics. The semi-analytical approach relies on earlier work on the instability of an extending liquid thread and is based on the Stokes flow regime and small-amplitude perturbations. The evolution of different excitation modes is evaluated and compared. Through hydrodynamic focusing and the corresponding extensional flow an initially stable mode may become unstable and it depends on the position away from the inlet which mode is to be regarded as the most unstable one. When plotting a hypothetical jet decay length against the excitation wave number, a comparatively broad minimum is exhibited. The CFD simulations based on the volume-of-fluid method show that the jet may break up either in the conical focusing zone or in the attached capillary, depending on the flow velocity. When the deformation of the jet surface reaches a certain amplitude, the jet assumes a “beads-on-a-string” structure instead of a shape derived from a harmonic perturbation. A jet decay within the capillary produces elongated droplets with cusped ends. When comparing the results of the CFD and the semi-analytical model, it turns out that the CFD simulations predict more stable jets with a larger decay length. An analysis of the flow velocity field shows that the increased stability might be due to the interaction of the jet with the channel walls.     

Simulation of shear layers interaction and unsteady evolution under different double backward-facing steps

High-order accurate schemes are employed to numerically simulate the interaction of a supersonic jet and a co-directional supersonic inflow. A double backward-facing step model is proposed to investigate the interaction between the jet shear layer and the supersonic inflow shear layer. It is found that due to the interaction of the shear layer, a secondary jet is injected into the recirculation zone at the intersection of the two shear layers. The secondary jet produced by the interaction of the two shear layers has a periodicity because of shear layers interaction. The distinction in the shape of double backward-facing steps will induce changes in the period of the secondary jet. The analysis and discussion of the periodicity of the secondary jet are mainly focused in this letter.     

Free Surface of a High Speed Capillary Jet

A free surface shape of a viscous liquid jet is investigated at large Reynolds and Weber numbers. The jet is ejected into a vacuum from a cylindrical nozzle with a flat exterior surface. The liquid is completely wetting the nozzle material (zero contact angle). Free jet surface is non-cylindrical near the nozzle. There is a smooth connection between the flat external surface of the nozzle and the cylindrical surface of the jet away from the nozzle. The size of the connection region is estimated by means of the boundary layer technique.     

聚能射流水中侵彻行为的实验研究

对三种不同药型罩形成的射流在水中的侵彻行为进行了实验研究,采用电铜箔测速法及高速摄影法分别获取了聚能射流在水中的行进过程及图像。实验结果表明,水体的边界效应及聚能射流在水中的开坑行为对电铜箔的测量结果影响很大,且聚能射流的准定常侵彻行为在平均速度的变化上表现明显,在准定常侵彻阶段,聚能射流的速度可近似为线性递减,且药型罩构型对射流速度的衰减率影响较大,药型罩材料的影响较小。光测结果给出了聚能射流在水中的运动图像,且据此计算出的射流速度与电测结果吻合较好。     

Formation and stability of a charged jet in a strong electric field

The shape of a charged jet is determined in the approximation of a strong electric field. The stability of the jet with respect to both axisymmetric and nonaxisymmetric perturbations of the sinusoidal type is investigated in the linear approximation. The domains of predominance of the axisymmetric and bending modes and the longitudinal partition mode are determined. Experimental data on the longitudinal partition of a polymeric jet into several daughter jets are given.     

Stereo-PIV study of flow around a maneuvering fish

Stereoscopic PIV was used to measure the three-component velocity distribution around a live fish. At the same time, shadow images of the fish were captured by additional cameras in order to reconstruct the three-dimensional shape of the fish. The side jet shed against the swept tail during turning was evident in the results. The height of the jet was approximately equal to the height of the tail fin, and a vortex ring was observed around the side jet. Based on the circulation and diameter of the vortex ring, the impulse and time-averaged force in terms of the side jet were estimated.     

Fluid outflow from an orifice in a plane wall in the presence of a variable-strength source on the symmetry plane of flow

The unsteady jet outflow of an ideal incompressible weightless fluid from an orifice in a wall is considered in the presence of a variable-strength point source on the symmetry plane of flow. It is assumed that the velocity of the perturbed flow induced by the change of the source discharge is small compared to the velocity of the steady flow. The Gurevich–Haskind method is used to solve the problem. A boundary value problem for the complex potential of the perturbed flow is formulated and solved. The pressure distribution on hard walls is determined for the harmonic law of the source discharge variation. The evolution of the shape of the jet’s free boundary is studied.