Strength Of Wave Research Articles

Shock wave mitigation using zig-zag structures and cylindrical obstructions

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zig-zag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking.

Dynamics of blast wave and fireball after hydrogen tank rupture in a fire in the open atmosphere

This paper describes a CFD model of the blast wave and fireball dynamics after high-pressure hydrogen tank rupture in a fire in the open atmosphere. Experiments on rupture in a fire of tanks with nominal working pressure 35 MPa and 70 MPa are used to validate the model and get insights into underlying physical phenomena. Parametric studies are performed to understand the effect of different physical sub-models, numerical methods and other model parameters, e.g. instantaneous or inertial tank opening, on the convergence of simulations and closer reproduction of experiments. The model reproduces experiments well using different turbulence (RNG, Smagorinski-Lilly) and combustion (EDC, FRC) sub-models. It is demonstrated that hydrogen combustion at the contact surface between heated by starting shock air and cooled by expansion hydrogen at the initial stage of the process affects the blast wave strength, i.e. the peak pressure of the leading front and the blast wave impulse.

The Impact of SST Biases in the Tropical East Pacific and Agulhas Current Region on Atmospheric Stationary Waves in the Southern Hemisphere

AbstractClimate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3° and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.

Influence of Geometric Parameters on Overall Performance of High Bypass Ratio Turbofan Nacelle and Exhaust System

In order to understand the coupled effect of the nacelle and exhaust system and to improve their overall performance, we studied the aerodynamic performance and the flow characteristics of the high bypass ratio turbofan nacelle and exhaust system by numerical simulation. The geometric parameters of a nacelle and exhaust system (e.g., the contraction ratio of the cowl afterbody and the fan nozzle exit angle) were investigated to evaluate their influence on the overall performance of the nacelle and exhaust system. The related flow mechanism was explored as well. The results show that the flow field of the nacelle and exhaust system under the mid-cruise condition exhibits characteristics of transonic flow. A stagnation zone exits at the nacelle lip and there is a velocity peak at the nacelle forebody. There exist a number of complex flow phenomena (such as shockwave, expansion wave, shear flow and shock wave-boundary layer interaction) in the downstream of the fan nozzle exit plane. The magnitude of the fan nozzle thrust or the intake ram drag is much higher than that of the additional drag, the nacelle drag or the core nozzle thrust. And for the nacelle drag, the friction drag of the cowl is in the same order of magnitude as the pressure drag of the cowl, the core cowl and the plug. But it is much larger than the friction drag of the core cowl and the plug. The effective thrust increases by 4.7% as the contraction ratio of the cowl afterbody increases; and it increases by 2.4% as the fan nozzle exit angle increases. The expansion degree of the fanjet flow, the shock wave strength and location, and the existence of the flow separation or second shock wave are influenced by the contraction ratio of the cowl afterbody and the fan nozzle exit angle. These phenomena have effects on the pressure distribution of the core cowl and the surrounding fanjet flow velocity, and hence they further affect the nacelle drag. The increase in the fan nozzle exit angle can noticeably reduce the thrust of the fan nozzle.

A numerical study of the metal jet induced by a shock wave

In this work, a metal jet induced by a shock wave is studied numerically. Different from the previous works on metal jets, we apply a cut-cell based sharp interface numerical method for the study. The evolution of jets is simulated by the in house code CCGF [X. Bai and X. Deng, Adv. Appl. Math. Mech. 9(5), 1052–1075 (2017)], and the interfacial growth rate is computed and compared with some theoretical models. Various initial conditions, including disturbance amplitude and shock wave strength, are considered here. Based on the model of Karkhanis et al. [J. Appl. Phys. 123, 025902 (2018)], a modified model of the spike velocity is presented to achieve better consistency between the numerical simulation and the model formula under more wide initial conditions (here, the scaled perturbed amplitudes involved are 0.125 and 4, and the incident shock wave Mach number is from 2.5 to 8) in this paper. In order to extend the applicability of the empirical models, an approximate formula for the initial velocity V0 is also obtained; a direct prediction of the spike velocity will become possible when the initial perturbed amplitude and incident shock intensity are known. Relevant figures show that the modified model can estimate a more consistent result with the numerical simulation than the VK or GD model.

Reconstruction of nonstationary sound fields based on a time domain angular spectrum method.

A time domain angular spectrum method is proposed to reconstruct nonstationary sound fields. In this method, the sound field is expressed as a superposition of a series of plane wave bases, and the plane wave basis is constructed by an impulse response function that relates the time domain angular spectrum to the field point pressure. The impulse response function consists of two parts, the propagating plane waves and the evanescent plane waves, and their physical interpretation is provided. By discretizing the time convolution between the plane wave strength and the impulse response function, the reconstruction can be carried out at each time step, thus providing the advantage of real-time reconstructing sound fields. Since the real-time reconstruction process is non-recursive, it can provide a stable reconstruction. In the reconstruction process, the Tikhonov regularization is introduced at each time step to obtain an appropriate estimation of the plane wave strength. Numerical simulations with an unsteady excitation plate and an experiment with an impacted plate were carried out to demonstrate the feasibility of the proposed method on reconstructing nonstationary sound fields. The effect of numerical parameters on the reconstruction accuracy was also investigated in the numerical simulations.

Adjoint-based aerodynamic shape optimization including transition to turbulence effects

The inclusion of transition to turbulence effects in aerodynamic shape optimization makes it possible to use it as a tool for the design of airframes with laminar flow. Modified Reynolds-Averaged Navier–Stokes (RANS) models that consider transition to turbulence have gained traction in the computational fluid dynamics (CFD) community. These models enable the computation of transitional flows without the need for external modules. In this work, we use a smooth version of the amplification factor transport (AFT) model, called AFT-S, to perform gradient-based aerodynamic shape optimization (ASO) of airfoils in subsonic and transonic flow conditions. We investigate the benefits of including transition effects into the optimization process and assess the impact of losing laminar flow when early transition to turbulence occurs due to surface contamination. Our results indicate that our design optimization approach yields lower drag airfoils when transition effects are considered. For the transonic case, the optimizer trades between shock wave strength and laminar flow extension to minimize drag.

Turbulent exchanges between near-inertial waves and balanced flows

Abstract

Chislennoye modelirovaniye shtormovogo nagona 15 noyabrya 2019 goda na yuge ostrova Sakhalin

Purpose . Investigation of the storm surge in Korsakov in the southern part of the Sakhalin Island on November 15, 2019 and comparison of the results of its numerical simulation with the data of in situ measurements constitute the aim of the article. Methods and Results . In situ measurements of the storm surge in Korsakov (the Sakhalin region) were performed and the data on the flooded area dimensions were collected. A storm period on the Sakhalin Island is almost the annual event in an autumn-winter season. The severe storm that happened in the southern Sakhalin region on November 15, 2019 led to flooding of the port territory in Korsakov. Due to the NAMI-DANCE computational complex, the storm surge was numerically simulated within the framework of the system of shallow water equations in the spherical coordinates on the rotating Earth with the regard for the friction force and the atmospheric effect. The calculations included the data on temporal and spatial distribution of the wind speed at the altitude 10 m taken from the Climate Forecast System Reanalysis database. The data on the atmospheric pressure were not applied in simulations since the atmosphere pressure gradient at the area under study was small. The simulation was carried out in the course of three days. The simulations showed that in 20 hours after the wind forcing had started, the water level in the port increased up to its maximum values, and did not fall the whole day. The water level maximum heights were concentrated in the southwestern part of the Aniva Bay. At that the calculated current speeds reached 2 m/s. During the storm, at the wind speed up to 15 m/s, the storm surge height in the Korsakov port area constituted 1.7 m, whereas the width of the flooded zone was up to 200 m. These results are confirmed well by the in situ measurement data. Conclusions . The simulation values of the power characteristics for the above-mentioned storm are represented in the paper. The Froude number square reaches 0.03 in the Korsakov city port area, and spatial distribution of the wave strength moment is up to 1 m3/s2. Field measurements and eyewitness reports confirm the evidence of a powerful impact of a storm surge upon the port constructions.

Numerical Simulation of the Storm Surge at the Sakhalin Island Southern Part on November 15, 2019

Purpose. Investigation of the storm surge in Korsakov in the southern part of the Sakhalin Island on November 15, 2019 and comparison of the results of its numerical simulation with the data of in situ measurements constitute the aim of the article. Methods and Results. In situ measurements of the storm surge in Korsakov (the Sakhalin region) were performed and the data on the flooded area dimensions were collected. A storm period on the Sakhalin Island is almost the annual event in an autumn-winter season. The severe storm that happened in the southern Sakhalin region on November 15, 2019 led to flooding of the port territory in Korsakov. Due to the NAMI-DANCE computational complex, the storm surge was numerically simulated within the framework of the system of shallow water equations in the spherical coordinates on the rotating Earth with the regard for the friction force and the atmospheric effect. The calculations included the data on temporal and spatial distribution of the wind speed at the altitude 10 m taken from the Climate Forecast System Reanalysis database. The data on the atmospheric pressure were not applied in simulations since the atmosphere pressure gradient at the area under study was small. The simulation was carried out in the course of three days. The simulations showed that in 20 hours after the wind forcing had started, the water level in the port increased up to its maximum values, and did not fall the whole day. The water level maximum heights were concentrated in the southwestern part of the Aniva Bay. At that the calculated current speeds reached 2 m/s. During the storm, at the wind speed up to 15 m/s, the storm surge height in the Korsakov port area constituted 1.7 m, whereas the width of the flooded zone was up to 200 m. These results are confirmed well by the in situ measurement data. Conclusions. The simulation values of the power characteristics for the above-mentioned storm are represented in the paper. The Froude number square reaches 0.03 in the Korsakov city port area, and spatial distribution of the wave strength moment is up to 1 m3/s2. Field measurements and eyewitness reports confirm the evidence of a powerful impact of a storm surge upon the port constructions.

The blast wave decay correlation for hydrogen tank rupture in a tunnel fire

This study presents a universal correlation for blast wave decay after hydrogen tank rupture in a tunnel fire. The validated CFD model is applied to perform numerical experiments in tunnels of cross-section area 24–139 m2, aspect ratio width-height 1.2–2.7, tunnel length 150–1500 m with tanks of volume 15–176 L, and pressure 35–95 MPa (mass 0.6–6.9 kg). A dimensionless correlation for transition distance from Zone 1, dominated by blast wave reflections, to Zone 2 of planar wave propagation is developed. The traditional models derived using high explosives are found to be non-appropriate to describe blast wave decay after hydrogen tank rupture in a tunnel fire. Therefore, an original correlation is developed using methods of similitude analysis and numerical experiments. The mechanical and fraction of chemical energy contributing to the blast wave strength are accounted for along with effects of tunnel aspect ratio and friction/minor losses.

Effects of micro-vortex generators on shock wave structure in a low aspect ratio duct, numerical investigation

The interaction between the shock wave and boundary layer and its induced separation is an undesirable phenomenon in the design of supersonic systems. The control of this phenomenon by active and passive methods is highly important in the design of aerodynamic systems. In the present work, different series of micro vortex generators were investigated as a passive control method (with the ability to increase the flow momentum), affecting the structure of a compressible flow at the Mach number of 2.05 in a low-aspect-ratio duct. In this regard, the finite volume method was used in the OpenFoam package. The obtained results were first validated against experimental data, and then four different vortex generators with a height of 10 μm were investigated. The results revealed that the use of vortex generators caused a reduction in the shock wave strength and energy loss across the flow. Furthermore, applying a vortex micro-generator with the outer body of national advisory committee for aeronautics 4412 airfoil led to a significant decrease in the strength of shock waves. Also, a vortex generator could shift the shock wave occurrence location to the downstream.

Damage to Polystyrene Polymer Film by Shock Wave Induced Bubble Collapse.

Metallic surfaces that are in contact with solutions are commonly used in numerous applications where these surfaces can be damaged by shock wave induced bubble collapse. Use of polymer films that coat such surfaces to prevent them from damage requires a better understanding of how much harm collapsing bubbles produce in the films. In this study, we report the results from coarse-grained molecular dynamics simulations to study the damage to polystyrene (PS) films coating a hard surface. The damage was caused by a collapsing nanobubble located in the proximity of the film and interacting with an impinging shock wave. This collapse produces a high-speed water jet that impacts the PS film with a greater force than the shock front and creates cavities/pits in the PS film. We observed that polymer molecules located in the jet vicinity undergo conformational extension in the direction perpendicular to the jet motion, while chain molecules in the rest of the film undergo compression. We also observed that damage to the film is sensitive to the strength of the shock wave.

Nonlinear stability of rarefaction waves for a viscous radiative and reactive gas with large initial perturbation

We investigate the time-asymptotically nonlinear stability of rarefaction waves to the Cauchy problem of an one-dimensional compressible Navier-Stokes type system for a viscous, compressible, radiative and reactive gas, where the constitutive relations for the pressure $p$, the specific internal energy $e$, the specific volume $v$, the absolute temperature $\theta$, and the specific entropy $s$ are given by $p=R\theta/v +a\theta^4/3$, $e=C_v\theta+av\theta^4$, and $s=C_v\ln \theta+ 4av\theta^3/3+R\ln v$ with $R>0$, $C_{v}>0$, and $a>0$ being the perfect gas constant, the specific heat and the radiation constant, respectively. For such a specific gas motion, a somewhat surprising fact is that, general speaking, the pressure $\widetilde{p}(v,s)$ is not a convex function of the specific volume $v$ and the specific entropy $s$. Even so, we show in this paper that the rarefaction waves are time-asymptotically stable for large initial perturbation provided that the radiation constant $a$ and the strength of the rarefaction waves are sufficiently small. The key point in our analysis is to deduce the positive lower and upper bounds on the specific volume and the absolute temperature, which are uniform with respect to the space and the time variables, but are independent of the radiation constant $a$.

Spin wave excitations in exchange biased IrMn/CoFe bilayers

Using an atomistic spin model, we have simulated spin wave injection and propagation into antiferromagnetic IrMn from an exchange coupled CoFe layer. The spectral characteristics of the exited spin waves have a complex beating behavior arising from the non-collinear nature of the antiferromagnetic order. We find that the frequency response of the system depends strongly on the strength and frequency of oscillating field excitations. We also find that the strength of excited spin waves strongly decays away from the interfacial layer with a frequency dependent attenuation. Our findings suggest that spin waves generated by coupled ferromagnets are too weak to reverse IrMn in their entirety even with resonant excitation of a coupled ferromagnet. However, efficient spin wave injection into the antiferromagnet is possible due to the non-collinear nature of the IrMn spin ordering.

A Modified Model for Predicting the Strength of Drying-Wetting Cycled Sandstone Based on the P-Wave Velocity

The drying-wetting cycles caused by operation of the Three Gorges Reservoir have considerable effect on the deterioration of reservoir bank rock mass, and the degradation of reservoir rock mass by the drying-wetting cycle is becoming obvious and serious along with the periodic operation. At present, the strength of the rock prediction research mainly focuses on the uniaxial strength, and few studies consider the drying-wetting effect and confining pressure. Therefore, in this paper, typical sandstone from a reservoir bank in the Three Gorges Reservoir area is taken as the research object, while the drying-wetting cycle test, wave velocity test and strength test are carried out for the research on the strength prediction of sandstone under the action of the drying-wetting cycle. The results show that the ultrasonic wave velocity Vp of the sandstone has an exponential function relation with the drying-wetting cycle number n, and the initial stage of drying-wetting cycles has the most significant influence on the wave velocity. Under different confining pressures, the compressive strength of sandstone decreases linearly with the increase of the drying-wetting cycle numbers, and the plastic deformation increases gradually. The damage variable of the sandstone has a power function relation with the increase of drying-wetting cycle numbers. A traditional strength prediction model based on P-wave velocity was established combined with the damage theory and Lemaitre strain equivalence hypothesis; in view of the defects of the traditional strength prediction model, a modified model considering both the drying-wetting cycle number and confining pressures was proposed, where the calculated results of the modified model are closer to the test strength value, and the prediction error is obviously decreased. This indicated that the modified model considering the drying-wetting cycle number and confining pressure is reasonable and feasible.

Study on the early cement hydration process in the presence of cationic asphalt emulsion

Cement-asphalt (CA) paste could be regarded as the Portland cement (PC) paste with enhanced flexibility due to the addition of asphalt emulsion (AE) and could be utilized as the semi-flexible pavement and repairing materials. In order to extensively explore the process of early cement hydration and the microstructure evolution of PC paste in the presence of cationic AE, various measurements including setting time, hydration heat, conductivity, compressive strength and ultrasonic pulse velocity, Fourier transform infrared spectroscopy (FTIR), chemically bound water, thermogravimetric (TG) and field emission scanning electron microscopy (FESEM) were adopted in the investigation. The experiment results showed that the setting process of PC paste was gradually prolonged with the increase of A/C, implying that the formation of microstructure of PC paste was inhibited by cationic AE, which is in agreement with the hydration heat and conductivity test. The compressive strength and pulse velocity of ultrasonic wave of cement mortar reduced with the incorporation of cationic AE, which thus implied the porosity increases with the addition of A/C, and in turn indicating the generation of microstructure of cement paste was suppressed gradually with the increase of cationic AE, this could also be confirmed by the chemically bound water, FTIR and FESEM results. Finally, the possible retardation mechanism of cement hydration, caused by the cationic AE, was proposed.

Farklı Ultrasonik Ses Dalgası Uygulamalarının Solanum torvum Tohumlarındaki Çimlenme ve Çıkış Değerleri Üzerine Etkisi

Bu çalışma, farklı ultrasonik ses dalgası uygulamalarının Solanum torvum tohumlarındaki çimlenme ve çıkış değerleri üzerine etkisini tespit etmek amacı ile yürütülmüştür. Bu amaçla tohumlar ultrasonik ses dalgası cihazı aracılığı ile 2 ultrasonik ses dalgası şiddeti (50% ve 100% amplitude) ve 3 ultrasonik ses dalgası süresi (5, 10 ve 15 dk) uygulamasına maruz bırakılmıştır. Bu uygulamaların Solanum torvum tohumlarının çimlenme oranı (%), ortalama çimlenme süresi (OÇS), çimlenme indeksi, çimlenme hızı G50 (gün), çıkış oranı (%), ortalama çıkış zamanı (OÇZ), çıkış indeksi ve çıkış hızı E50 (gün) üzerine etkileri ortaya konulmuştur. Araştırma sonucunda, tüm çimlenme ve çıkış değerleri açısından genel olarak 5A5D (50 amplitude-5 dakika) uygulamasının kontrole göre oldukça iyi sonuçlar verdiği tespit edilmiştir. Kontrol uygulamasında 60% civarında olan çimlenme ve çıkış oranları 5A5D uygulamasında sırası ile 96% ve 90% olarak belirlenmiştir. Ancak ortalama çimlenme süresi ve çıkış süresi açısından istatistiki olarak öne çıkan uygulama bulunmamıştır. Çimlenme hızı G50 ve çıkış hızı E50 değerleri yönünden 5A5D uygulamasının 4 gün kadar erken çimlenme ve çıkış sağladığı tespit edilmiştir. Ayrıca 5A5D ultrasonik ses dalgası uygulamasının, erken çimlenme ve yüksek oranda çimlenme ve çıkış oranları üzerine olumlu etkilerinin olduğu belirlenmiştir. Elde edilen sonuçların tohum çimlenme ve çıkış oranı açısından sorun yaşanan birçok bitki türünde kullanımlarına yönelik çalışmalar yapılması planlanmaktadır.

Microwave Field Distribution Uniformity Coefficient in the Grain Mass

The article considers the microwave electromagnetic fields as one of the options for improving the thermal drying of grain. Their application is limited by the high unevenness of the field propagation in the layer of the processed material. (Research purpose) The research purpose is in justifying the uniformity of distribution of microwave field in the layer of the processed grain. (Materials and methods) The article presents the scheme of computer models of microwave processing zones and waveguides, properties of materials for conducting a numerical experiment. (Results and discussion) A numerical experiment was performed to determine the uniformity coefficient of propagation of the microwave field in a layer of grain material. The article presents the dependencies. (Conclusions) It was found that the results of modeling the distribution of the electromagnetic field in the zone of microwave convective influence of the installation containing two sources of microwave power for processing the grain layer indicate a high level of its unevenness in the volume of the product pipeline. To assess the uniformity of the distribution of the electromagnetic field in the working area of a laboratory installation, there used a coefficient that is the ratio of the average value of the intensity in the zone of microwave convective action to its average value of the wave strength passing through the output of the waveguide. The values of the uniformity coefficient in the considered implementation options are in the range of 0.1757-0.4946 for a dense layer of wheat. To ensure a sufficient level of uniformity of the electromagnetic wave distribution in the volume of the microwave convective zone, the uniformity coefficient must be higher than 0.37. The article presents the dependence of the uniformity coefficient of the electromagnetic field on the humidity of the processed material by a third-degree polynomial with a coefficient of determination higher than 0.98.

Imprint of local environment on fast radio burst observations

ABSTRACT When fast radio burst (FRB) waves propagate through the local (${\lesssim}\rm 1\, pc$) environment of the FRB source, electrons in the plasma undergo large-amplitude oscillations. The finite-amplitude effects cause the effective plasma frequency and cyclotron frequency to be dependent on the wave strength. The dispersion measure and rotation measure should therefore vary slightly from burst to burst for a repeating source, depending on the luminosity and frequency of the individual burst. Furthermore, free–free absorption of strong waves is suppressed due to the accelerated electrons’ reduced energy exchange in Coulomb collisions. This allows bright low-frequency bursts to propagate through an environment that would be optically thick to low-amplitude waves. Given a large sample of bursts from a repeating source, it would be possible to use the deficit of low-frequency and low-luminosity bursts to infer the emission measure of the local intervening plasma and its distance from the source. Information about the local environment will shed light on the nature of FRB sources.