Abstract: Shock wave interaction with obstacles of various geometric shapes has always attracted attention in a large number of experimental and numerical studies. During the interaction of a shock wave with an obstacle a very complex wave pattern is formed which affects the shock-wave induced flow. The interaction reduces the shock-wave strength and generates rotational flow behind the obstacle. The interaction of shock waves with rigid obstacles is of significant importance in aerodynamic science and other engineering applications. Whitham [1] formulated an approximate theory for the dynamics of two- and three-dimensional shock waves and applied this theory to the description of shock diffraction by wedges and corners. Bryson & Gross [2] broadened Whitham’s theory and applied it to two- and three-dimensional bodies such as cylinders and spheres. They carried out theoretical and experimental work to assess the analytical computations that were made by Whitham. One dominant direction in investigation of shock-cylinder interaction is finding the RR→MR transition criterion. When the shock wave strikes a cylinder, it is reflected as an RR and then transforms to a Mach reflection MR. Major RR→MR transition criteria were summarized and discussed in a scientific monograph by Ben-Dor [3]. Since 1970, due to progress in numerical techniques, very accurate simulations of shock wave propagation over obstacles have been achieved. In most of studies efforts to validate the Euler scheme were undertaken. In the numerical study of Drikakis et al. [4] viscous effects were examined at various Mach numbers during of shock-cylinder interaction by comparing the inviscid and viscous calculations. It was found that the flow field in the downstream half of the cylinder is influenced by viscosity. The main objective of the present study is to better understand the physical elements governing the flow induced by the shock wave and the elements affecting the shock wave strength after passing the obstacle. To carry out the overall research plan two different approaches have been utilized – experimental and numerical. In the present study we focused on the investigation of the reflected shock wave from a single cylinder for low Mach numbers (M S ~1 − 1.4) in order to characterize the physical factors affecting its propagation. The first part of a broad investigation of the shock wave interaction with complex geometries is presented.
Permalink: https://doi.org/10.1007/978-3-642-25685-1_96

Abstract: The interaction of shock waves with rigid obstacles is of significant interest in aerodynamic science and other engineering applications. During the interaction of a shock wave with an obstacle, a very complex wave pattern which affects the shockwave induced flow is formed. The interaction process depends on a variety of physical parameters such as the shape of the obstacle, the shock wave strength and the type of gas in which the interaction occurs. In the present paper, the interaction of a planar shock wave with a cylinder and a sphere is investigated. Our investigation follows closely the recentwork of Sadot et al. [1] which dealt with shock tube experiments with low Mach number shocks, in the range 1.1 to 1.4. An empirical relation was proposed for the trajectory of the reflected wave. This relation was expressed in terms of non-dimensional distance and time and was shown to be applicable for the investigated range of Mach numbers, cylinder diameters and a general ideal gas. The purpose of the present work is to focus on the backward reflected wave, and in particular, on its velocity change as it progresses away from the leading edge of the cylinder/sphere. It is expected that the reflected wave initially propagates at the velocity of shock reflection from a rigid wall, and asymptotically decelerates to the velocity corresponding to that of a sonic wave in the shocked region. This theoretical behavior is born out by fine mesh hydro-code computations of the interaction problem. The paper is organized as follows: in Section 2 a theoretical background for the limiting velocities of the reflected shock is given, followed by a brief description of the numerical codes and the problem setup (Section 3). The results of simulations for various cases by different CFD codes are given in Section 4. We conclude (Section 5) with a summary and suggestion of future work.
Permalink: https://doi.org/10.1007/978-3-642-25685-1_97

Abstract: The starting process of supersonic planar nozzles has been the subject of great number of the shock tube researches in the past. Initially this was motivated by the need to clearly separate the unsteady and quasi-steady parts of the expansion flow and thus specify the so-called ”test time” period of shock tube tunnels. Among other, the best known images illustrating the starting process were published by Amann [1]. It was clearly shown that the starting flow initiated by the primary shock wave (PS) includes the contact surface (CS) and the secondary shock (SS). Smith [2] was the first to show that the unsteady expansion wave (UEW) which follows the SS can also affect the total duration of the starting flow. Actually, the SS initiates flow separation and transient structure of the separation points (SP). Next, complex phenomenon which requires fundamental knowledge on the parameters of the external flow and the condition inside the boundary layer was discussed by Dussauge & Piponniau [3]. Flow separation may also cause significant effects on the trajectory of the SS and increase the total duration of the starting flow pattern. The renewed interest in nozzle starting phenomena appears due to wide application of the transient nozzle flow in different devices. The effect of separation, for example, becomes important inside the nozzles of rockets, missiles and/or supersonic aircrafts where it is usually undesirable since it may cause a dangerous lateral force which can damage the nozzle [4]. On the other hand, flow separation and the resulting instability of the exit plume could have positive effect when used in high speed mixing devices (Jonson & Papamoschou [5]). Despite a plentiful history and significant progress in the numerical as well as in the experimental investigations, many features of the nozzle starting, flow separation and its asymmetry are still open. Even a brief summary of the involved process clearly shows that the nozzle geometry, the viscous effects and the flow conditions at the entrance and exit are important parameters that must be examined [6]. The current investigation was conducted to evaluate how the nozzle starting process depends on the initial conditions and on the asymmetry of the nozzle installation. In order to assess the role of these factors the incident shock wave Mach number was varied between Ms  = 1.2 and Ms  = 1.9 and the nozzle asymmetry was introduced by the relative shifting of half of the nozzle.
Permalink: https://doi.org/10.1007/978-3-642-25685-1_111

Abstract: The starting process of supersonic nozzles has been investigated numerically. The objective of this study is to identify the origin of side-loads during the start-up and shutdown phases of rocket nozzles. In such cases, a complex flow structure has been observed, with significantly separated boundary layer and strong moving shocks. Special attention has been given to the early phase of the starting process and to the appearance of a secondary shock wave and its interaction with the wall. The calculation results are compared with the experiments in terms of shock velocities. When turbulence is included in the simulation, the flow development has been shown to be more complex due to the strong the coupling between shock and turbulence, especially when large-scale turbulence is created in the mixing layer at the point of separation. This phenomenon is considered to be one of the fundamental the characteristics of shock stability in transient nozzle flows.
Permalink: https://doi.org/10.2514/6.2012-952

Abstract: The physical understanding and modeling of shock mitigation are important for the development of an effective barrier arrangement related to disaster management. While it is not currently feasible to simulate and analyze full configurations in detail, sufficient progress has been made to analyze the dynamics of simpler building block flows that provide useful insights into the underlying dynamics of these complex flows. Also, apart form the experimental study, numerical simulation has become quintessential tool for prediction of complex physics in solid/fluid interaction problems. Several authors dealt with experimental or numerical approaches in order to study the unsteady shock wave interaction with multiple obstacles, such as cylinders, spheres and triangular prisms [1, 2, 3, 4]. According to the recent findings of [5], the influence of different geometrical shapes on shock-wave attenuation is negligible for higher open passage. However, this finding requires a systematic study of the effects of different parameters for lower values of the open passage. In our previous works [6, 7], excellent agreement between experimental and numerical results is obtained for the case of shock-wave interaction with single cylinder and triangular prism. These validations prove the reliability of the computational techniques used for the present study. It is being observed that after the passage of the shock through the obstacle matrix, eddies of different length scales are generated, but the later stage of shock-vortex, shocklet-vortexlet interaction are different for inviscid and viscous computations [8].
Permalink: https://doi.org/10.1007/978-3-642-25685-1_74

Abstract: In this study, an immersed boundary (IB) method based on a direct forcing is coupled with a high-order weighted-essentially non-oscillatory (WENO) scheme to simulate fluid–solid interaction (FSI) problems with complex geometries. The IB is a general simulation method for FSI, whereas the WENO is an efficient scheme for fluid flow simulations and shock waves, and both of them work on regular cartesian grids. The effectiveness and the accuracy of the coupled scheme are first analyzed on well-documented supersonic test problems for a wide range of Mach numbers. The results are in good agreement with both analytical and experimental data. A comprehensive analysis of the interaction of the moving shock through an array of cylinder matrix is then conducted by varying the number of cylinders in the matrix block while keeping the same opening passage. The relaxation length between two adjacent columns of cylinders is kept identical to study uniquely the effect of surface-to-volume ratio of the obstacle matrix. It is shown that the configuration with higher surface-to-volume ratio produces more post-shock flow instabilities downstream of the matrix block. The complex shock/shock and shock/vortex interactions are well resolved by the present computation. It is being observed that after the passage of the shock through the cylinder matrix, eddies of different length scales are generated, but the later stage of shock/vortex and shocklet/vortexlet interactions are different for the two cases. The analysis of the PSD of the total kinetic energy globally conforms to Richardson’s inviscid cascade. An intermittent peaked PDF of downstream instantaneous vorticity field is obtained in the limit of Re →  ∞ . The baroclinic production of vorticity is found to be feeble as previously founded by Sun and Takayama (J. Fluid Mech. 2003; 478:237–256)
Permalink: https://doi.org/10.1002/nme.3271

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Abstract: Retrofitting of Kampen School in Norway has been a demonstration project where new concepts for energy efficient ventilation and lighting are integrated. Before retrofitting, the school had a mechanically balanced ventilation system that provided each classroom with approximately 120 liter/second of fresh air. After retrofitting, the school has demand controlled displacement ventilation controlled by a combined CO₂– and temperature sensor and energy efficient lighting system that utilize daylight. To examine the effects of the energy measures, we followed pupils at Kampen School over three to four years. We also followed pupils at nearby primary school (Lilleborg) over the same period as a control. Each year, performance tests, health and well-being questionnaires and technical measurements were carried out. We found that pupils at Kampen School in total had significant improvement of the concentration test scores and health and well-being questionnaires compared to Lilleborg.
Permalink: https://www.researchgate.net/profile/Mads-Mysen/publication/288362449_Kampen_School_-_Evaluation_of_pupils’_performance_and_perceived_health_and_well-being_before_and_after_school_retrofitting/links/5698f3ce08ae748dfaff2fc6/Kampen-School-Evaluation-of-pupils-performance-and-perceived-health-and-well-being-before-and-after-school-retrofitting.pdf