The Experts below are selected from a list of 14427 Experts worldwide ranked by ideXlab platform
G Bendor - One of the best experts on this subject based on the ideXlab platform.
-
similarity in mach stem evolution and termination in unsteady shock wave Reflection
Journal of Fluid Mechanics, 2020Co-Authors: E Koronio, G Bendor, O Sadot, M GevaAbstract:Shock-wave Reflection over concave surfaces poses a difficulty in its analysis due to the unsteady nature of the Reflection process and the occurrence of various types of Mach Reflections caused by it. In a pseudo-steady flow, the Reflection's configuration is self-similar since the shock wave reflects over a surface with constant inclination. The unsteady Mach Reflection introduces an additional complexity as it is affected by the changing inclination of the surface, forcing the Reflection to continuously adjust itself to the varying boundary condition. In this study, validated simulations of Mach Reflection (MR) over cylindrical concave surfaces with different radii were performed for three inviscid perfect gases with moderate incident shock Mach numbers (Ms) ranging from 1.3 to 1.5. The Reflection was investigated up to the point of transition from MR to transitioned Regular Reflection. A similar behaviour of the configuration and evolution of the Mach stem was observed, one that is independent of the surface radius and type of gas. With regards to different gases, the speed of sound a0 is a dominant factor since it dictates the propagation of wall disturbances. A universal condition of the rate of surface change was found, accounting for different radii, different gases and Ms variation. Analysis based on shock dynamics is employed to explain how disturbances caused by surface variations play a significant role in the behaviour of the Reflection. This method successfully supports the similarity that was demonstrated and facilitates a more informed perception of the MR process.
-
hysteresis processes in the Regular Reflection mach Reflection transition in steady flows
Progress in Aerospace Sciences, 2002Co-Authors: G Bendor, M S Ivanov, E I Vasilev, T ElperinAbstract:Abstract Ernst Mach recorded experimentally, in the late 1870s, two different shock-wave Reflection configurations and laid the foundations for one of the most exciting and active research field in an area that is generally known as Shock Wave Reflection Phenomena . The first wave Reflection, a two-shock wave configuration, is known nowadays as Regular Reflection , RR, and the second wave Reflection, a three-shock wave configuration, was named after Ernst Mach and is called nowadays Mach Reflection , MR. A monograph entitled Shock Wave Reflection Phenomena , which was published by Ben-Dor in 1990, summarized the state-of-the-art of the Reflection phenomena of shock waves in steady, pseudo-steady and unsteady flows. Intensive analytical, experimental and numerical investigations in the last decade, which were led mainly by Ben-Dor's research group and his collaboration with Chpoun's, Zeitoun's and Ivanov's research groups, shattered the state-of-the-knowledge, as it was presented in Ben-Dor (Shock Wave Reflection Phenomena, Springer, New York, 1991), for the case of steady flows. Skews's and Hornung's research groups joined in later and also contributed to the establishment of the new state-of-the-knowledge of the Reflection of shock waves in steady flows. The new state-of-the-knowledge will be presented in this review. Specifically, the hysteresis phenomenon in the RR↔MR transition process, which until the early 1990s was believed not to exist, will be presented and described in detail, in a variety of experimental set-ups and geometries. Analytical, experimental and numerical investigations of the various hysteresis processes will be presented.
-
stability of Regular and mach Reflection wave configurations in steady flows
AIAA Journal, 1996Co-Authors: A. Chpoun, D Passerel, G BendorAbstract:The stability of Regular Reflection and mach Reflection in the dual solution was investigated experimentally. A pressure-measurement-based method was applied.
-
reconsideration of oblique shock wave Reflections in steady flows part 1 experimental investigation
Journal of Fluid Mechanics, 1995Co-Authors: A. Chpoun, D Passerel, G BendorAbstract:The Reflection of shock waves over straight reflecting surfaces in steady flows was investigated experimentally using the supersonic wind tunnel of Laboratoire d'Aerothermique du CNRS, Meudon, France. The results for a flow Mach number M0 = 4.96 contradict the state of the art regarding the Regular [harr ] Mach Reflection transition in steady flows. Not only was a hysteresis found to exist in this transition, but, unlike previous reports, Regular Reflection configurations were found to be stable in the dual-solution domain in which theoretically both Regular and Mach Reflection are possible.
-
analytical prediction of Regular Reflection over rigid porous surfaces in pseudo steady flows
Journal of Fluid Mechanics, 1995Co-Authors: A Levy, G BendorAbstract:An analytical model for solving the flow field associated with Regular Reflections of straight shock waves over porous layers has been developed. The governing equations of the gas inside the porous material were obtained by simplifying the general macroscopic balance equations which were obtained by an averaging process over a representative elementary volume of the microscopic balance equations as originally done by Bear & Bachmat (1990). The analytical predictions of the proposed model were compared to experimental results of Skews (1992) and Kobayashi, Adachi & Suzuki (1993). Very good to excellent agreement was evident.
O Sadot - One of the best experts on this subject based on the ideXlab platform.
-
similarity in mach stem evolution and termination in unsteady shock wave Reflection
Journal of Fluid Mechanics, 2020Co-Authors: E Koronio, G Bendor, O Sadot, M GevaAbstract:Shock-wave Reflection over concave surfaces poses a difficulty in its analysis due to the unsteady nature of the Reflection process and the occurrence of various types of Mach Reflections caused by it. In a pseudo-steady flow, the Reflection's configuration is self-similar since the shock wave reflects over a surface with constant inclination. The unsteady Mach Reflection introduces an additional complexity as it is affected by the changing inclination of the surface, forcing the Reflection to continuously adjust itself to the varying boundary condition. In this study, validated simulations of Mach Reflection (MR) over cylindrical concave surfaces with different radii were performed for three inviscid perfect gases with moderate incident shock Mach numbers (Ms) ranging from 1.3 to 1.5. The Reflection was investigated up to the point of transition from MR to transitioned Regular Reflection. A similar behaviour of the configuration and evolution of the Mach stem was observed, one that is independent of the surface radius and type of gas. With regards to different gases, the speed of sound a0 is a dominant factor since it dictates the propagation of wall disturbances. A universal condition of the rate of surface change was found, accounting for different radii, different gases and Ms variation. Analysis based on shock dynamics is employed to explain how disturbances caused by surface variations play a significant role in the behaviour of the Reflection. This method successfully supports the similarity that was demonstrated and facilitates a more informed perception of the MR process.
-
the Regular Reflection mach Reflection transition in unsteady flow over convex surfaces
Journal of Fluid Mechanics, 2018Co-Authors: Meital Geva, Omri Ram, O SadotAbstract:The non-stationary transition from Regular Reflection (RR) to Mach Reflection (MR) over convex segments has been the focus of many recent studies. Until recently, the problem was thought to be very complicated because it was believed that many parameters such as the radius of curvature, initial angle and geometrical shape of the reflecting surface influenced this process. In this study, experiments and inviscid numerical computations were performed in air ( ) at an incident shock-wave Mach number of 1.3. The incident shock waves were reflected over cylindrical and elliptical convex surfaces. The computations were validated by high-resolution experiments, which enabled the detection of features in the flow having characteristic lengths as small as 0.06 mm. Therefore, the RR →MR transition and Mach stem growth were successfully validated in the early stages of the Mach stem formation and closer to the surface than ever before. The evolution of the RR, the transition to MR and the Mach stem growth were found to depend only on the radius of the reflecting surface. The reflected shock wave adjusts itself to the changing angles of the reflecting surface. This feature, which was demonstrated at Mach numbers 1.3 and 1.5, distinguishes the unsteady case from the self-similar pseudo-steady case and requires the formulation of the conservation equations. A modification of the standard two-shock theory (2ST) is presented to predict the flow properties behind a shock wave that propagates over convex surfaces. Until recently, the determination of the time-dependent flow properties was possible solely by numerical computations. Moreover, this derivation explains the controversial issue on the delay in the transition from the RR to the MR that was observed by many researchers. It turns out that the entire RR evolution and the particular moment of transition to MR, are based on the essential ‘no-penetration’ condition of the flow. Therefore, we proposed a simple geometrical criterion for the RR →MR transition.
-
high spatial and temporal resolution study of shock wave Reflection over a coupled convex concave cylindrical surface
Journal of Fluid Mechanics, 2015Co-Authors: Omri Ram, Meital Geva, O SadotAbstract:Studying the nature of transient Reflections of shock waves from surfaces is important in many engineering fields, e.g. blast protection, supersonic flights, shock focusing, medical and industrial applications and more. The recent advancements in this field reveal that the major obstacle in better understanding this phenomenon by means of experimental investigations is the limited temporal and spatial resolution. An alternative approach to commonly used high-speed photography is based on the use of a single-lens reflex (SLR) camera that captures only one image per experiment. Using this method to study a transient Reflection process necessitates repeating each experiment many times while retaining extremely high repeatability. In the present study, we present a solution to this obstacle by means of a fully automated shock tube facility, which has been developed in the course of this study. A typical experiment can be executed a few hundred times with a repeatability of less than 0.01 in the incident-shock-wave Mach number at moderate shock strengths ( $M=1.2{-}1.4$ ). The system offers a very high spatial and temporal resolution description of the transient Reflection process of a shock wave over a coupled convex–concave surface. The study of this complex configuration using a fully automated shock tube enables one to observe, in greater detail than ever before, both the transient transition from Regular Reflection, RR, to Mach Reflection, MR, and the reverse transient transition from MR to RR. The geometry studied can also be found in blunt leading-edge reflectors in which higher pressures were recorded, and the results presented also describe in detail the shock Reflection process inside such a reflector. The results highlight and strengthen the recent understanding of the importance of high spatial and temporal resolution in determining the transition process from RR to MR over a coupled concave–convex surface. However, despite achieving very high statistical certainty in the experimental measurements, the question of the difference between the pseudo-steady transition criterion and the experimental results remains unresolved.
B W Skews - One of the best experts on this subject based on the ideXlab platform.
-
dynamic transition from mach to Regular Reflection of shock waves in a steady flow
Journal of Fluid Mechanics, 2014Co-Authors: Kavendra Naidoo, B W SkewsAbstract:The steady, two-dimensional transition criteria between Regular and Mach Reflection are well established. Little has been done on the dynamic effect on transition due to a rapidly rotating wedge. Results from experiments and computations done on steady and unsteady shock wave transition from Mach Reflection to Regular Reflection, MR RR, are described. The measured motion and the initial shock incidence was used to mimic the experiment with a two-dimensional numerical code. The maximum rotation speed achieved at transition was approximately . Rapid wedge rotation was shown to have a significant measurable effect on transition. The code was also applied to the dependence of dynamic MR RR transition on other variables in the parameter space. These include rotation about the leading or trailing edge, initial incidence and rotation speed at two free-stream conditions. Impulsively started rotation in these cases was used with the rotation specified by where is constant angular velocity (negative anticlockwise), the distance from the edge considered to the pivot point and the free-stream sound speed. For the Mach numbers and range of rotation speeds tested, both the wedge and shock angle at transition decreased with increased rotation speed. The sensitivity of the transition angle to changing the rotation point from the trailing edge to the experimental model pivot point was investigated briefly at a free-stream Mach number of with . The wedge angle at transition increased by 1.5° and the shock angle at transition decreased by 1.5°, a significant variation. The effect of the initial incidence was also investigated. By reducing the initial wedge angle from 24.5 to 23.5° for these initial conditions the shock angle at transition decreased by approximately 1.8°, also a marked sensitivity. The flow field development for impulsive rotation about the wedge trailing and leading edges at for was analysed in some detail. The flow field development is very sensitive to the rotation centre, more especially at large rotation rates. Four phases of the Mach stem development were identified in both cases. For rotation about the wedge leading edge the Mach stem height remains constant until the expansion waves arrive at the triple point. This is followed by an increase in Mach stem height, which then remains constant for a short period after which it decreases until transition to RR. For rotation about the wedge trailing edge the impulsive start generates a disturbance on the incident wave which propagates down the wave, through the triple point and down the Mach stem. The stem height is constant until the arrival of the incident wave disturbance. This causes a sudden decrease in Mach stem height. Subsequently, the Mach stem height remains constant for a short time, before it decreases until transition to RR. Similar effects in the variation of stem height with wedge angle occur at the higher Mach number of 2.98. It was demonstrated that MR can be maintained for a while at zero wedge incidence with a sufficiently large rotation rate of , with , for both leading and trailing edge pivot points.
-
dynamic effects on the transition between two dimensional Regular and mach Reflection of shock waves in an ideal steady supersonic free stream
Journal of Fluid Mechanics, 2011Co-Authors: Kavendra Naidoo, B W SkewsAbstract:There have been numerous studies on the steady-state transition criteria between Regular and Mach Reflection of shock waves generated by a stationary, two-dimensional wedge in a steady supersonic flow, since the original shock-wave Reflection research by Ernst Mach in 1878. The steady, two-dimensional transition criteria between Regular and Mach Reflection are well established. There has been little done to consider the dynamic effect of a rapidly rotating wedge on the transition between Regular and Mach Reflection. This paper presents the results of an investigation on the effect of rapid wedge rotation on Regular to Mach Reflection transition in the weak-and strong-Reflection ranges with the aid of experiment and computational fluid dynamics. The experimental set-up includes a novel facility to rotate a pair of large aspect ratio wedges in a 450 mm x 450 mm supersonic wind tunnel at wedge rotation speeds up to 11000 deg s ―1 . High-speed images and measurements are presented. A numerical solution of the inviscid governing flow equations was used to mimic the experimental motion and to extend the investigation beyond the limits of the current facility to explore the influence of variables in the parameter space. There is good agreement between experimental measurements and numerical simulation. This paper includes the first experimental evidence of the Regular to Mach Reflection transition beyond the steady-state detachment condition in the weak- and strong-Reflection ranges. It also presents results of simulations for the dynamic Regular to the Mach Reflection transition which show a difference between the sonic, length-scale and detachment conditions. This paper includes experimental evidence of the Mach to Regular Reflection transition below the steady-state von Neumann condition.
-
three dimensional effects on Regular Reflection in steady supersonic flows
Shock Waves, 2003Co-Authors: Y Irving A Brown, B W SkewsAbstract:The Reflection of shock waves between two symmetrical wedges is investigated for the case of three-dimensional flows. Oblique shadowgraphs at various optical angles of yaw and pitch were used to examine the nature of fully three-dimensional flows, with wedge aspect ratios as low as 0.25 being considered. These images were used to construct surface models of the overall flow field for various Reflection patterns and aspect ratios, which provides a visual indication of the flow field shape. For a Mach number of 3.1, and suitable wedge angles, the flow field with Regular Reflection on the tunnel centreline and Mach Reflection further out is examined. The point of transition from Regular Reflection to the peripheral Mach surfaces is identified for various wedge angles and aspect ratios. It is shown that the transition points move outwards from the central plane as the aspect ratio decreases. This shows that three-dimensional flows favor Regular Reflection, because of the increasing curvature of the incident shock as the wedge becomes narrower, causing a decrease in the local angle of incidence. The height of the Mach stem is shown to be highly dependent on the geometry of the test wedge models. The Mach stem height decreases with aspect ratio due to the three-dimensional relieving effect, where the increase in lateral flow relieves the pressure over the surfaces of the wedges. Experimental evidence of the existence of the strong oblique shock solution in steady flows is presented.
Xueqiang Yuan - One of the best experts on this subject based on the ideXlab platform.
-
numerical study of cellular detonation wave Reflection over a cylindrical concave wedge
Combustion and Flame, 2019Co-Authors: Xueqiang Yuan, Xiaocheng Mi, Jin Zhou, Hoi Dick NgAbstract:Abstract Numerical simulations were performed to study Reflection of a stable detonation wave with Regular cellular patterns over a cylindrical concave wedge. The dynamics of this Reflection phenomenon was described by the two-dimensional reactive Euler equations with a two-step induction-reaction kinetic model and solved numerically using the adaptive mesh refinement code AMROC. The effects of various parameters on the Reflection evolution were analyzed in detail. The results indicate that the Reflection-type transition of a stable cellular detonation is similar to that of a planar shock wave over a concave wedge. The triple-point trajectory resulted from the Mach Reflection when the cellular detonation first encounters the concave wedge coincides with that of the planar shock propagating for the case with the same incident Mach number. As the effective wedge angle continuously increases, the Mach Reflection of cellular detonation deviates from that of a planar shock with a reduced Mach stem height, and the transition from Mach to Regular Reflection occurs at a smaller angle. This observation is further explored by adopting the length-scale (or “corner-signal”) concept, examining the velocity variation of corner signals generated by fluid particles around the wedge tip. The Reflection dynamics is described qualitatively by the ratio of two length scales characterizing the detonation structure, namely, the induction-zone and reaction-zone lengths. The increase of these length scales raises the Mach stem height and transition angle. Apart from the detonation length scales, the wedge curvature radius is found to have an opposite effect since the increase of radius expands the region where the corner signals are generated by the particles behind the induction zone, and makes the corner signals persist in a state with attenuating velocity.
Keming Cheng - One of the best experts on this subject based on the ideXlab platform.
-
theoretical study on Regular Reflection of shock wave boundary layer interactions
Journal of Fluid Mechanics, 2020Co-Authors: Longsheng Xue, F F J Schrijer, Bas Van Oudheusden, Chengpeng Wang, Zhiwei Shi, Keming ChengAbstract:In this paper the configurations of shock wave–boundary layer interactions (SWBLI) are studied theoretically and experimentally in Mach number 2 and 2.5 flows on test models with various wedge angles ranging from $9^\circ$ to $21^\circ$ . The proposed theoretical method couples the free interaction theory (FIT) with the minimum entropy production (MEP) principle to predict the appearance of separation shock, resulting in convex, straight and concave separation shock waves according to different solution combinations, which agree well with current experiments. Additionally, several influences on SWBLI are studied experimentally, in which the parameters related to theoretical solutions are found mostly determining the flow configuration, and SWBLI is much more sensitive to incident shock strength than incoming flow properties. Separation could be suppressed by incident shock when the MEP solution is smaller than the FIT, while it could be intensified when the MEP solution is larger than FIT; by contrast, the effects of separation position and model mounting height could be very weak.