Resonance-like phenomena in a submerged cylindrical shell subjected to two consecutive shock waves: The effect of the inner fluid

A submerged fluid-filled cylindrical shell subjected to a sequence of two shock waves originated at the same source is considered. It is demonstrated that, unlike in the case of a submerged evacuated shell, there exists a certain critical range of the values of the delay between the incident wavefronts where both the peak compressive and the peak tensile stress observed in the structure are significantly (60% or more) higher than the respective stresses in the same system subjected to a single-front loading. It is further demonstrated that the highest and the lowest hydrodynamic pressure attained in the system is also dramatically affected for certain values of the delay between the incident wavefronts, with the maximum double-front pressure being more than 30% higher than its single-front counterpart. The practical relevance of the findings is discussed in the context of the pre-design analysis of industrial systems subjected to shock loading.     

The response of circular composite plates to underwater blast: Experiments and modelling

We present a new experimental technique to allow laboratory-scale observation of underwater blast loading on circular plates, including dynamic deformation and failure of the plates as well as the sequence of cavitation events in water. The apparatus is used to measure and compare the responses of a quasi-isotropic glass/vinylester composite and of a woven carbon/epoxy plate. Dynamic explicit FE simulations are conducted and their predictions are found in good agreement with experiments. Measurements and FE predictions are used to validate a recently developed theoretical model for the response of elastic orthotropic plates to underwater blast.     

A Lagrangian approach for the coupled simulation of fixed net structures in a Eulerian fluid model

A Lagrangian approach for the coupled numerical simulation of fixed net structures and fluid flow is derived. The model is based on solving the Reynolds-averaged Navier–Stokes equations in a Eulerian fluid domain. The equations include disturbances to account for the presence of the net. For this purpose, forces on the net are calculated using a screen force model and are distributed on Lagrangian points to represent the geometry of the net. In comparison to previous approaches based on porous media representations, the new model includes a more physical derivation and simplifies the necessary numerical procedure. Hence, it is also suitable for arbitrary geometries and large scale simulations. An extensive validation section provides insight into the performance of the new model. It includes the simulation of steady currents through single and multiple fixed net panels and cages, and wave propagation through a net panel. Different solidities, inflow velocities and angles of attack are considered. The comparison of loads on and velocity reductions behind the net with available measurements indicates superior performance of the proposed model over existing approaches for a wide range of applications.     

Shock response of a system of two submerged co-axial cylindrical shells coupled by the inter-shell fluid

The shock response of a submerged system consisting of two co-axial cylindrical shells coupled with the fluid filling the inter-shell space is considered. The shock–structure interaction is modeled using a semi-analytical methodology based on the use of the classical apparatus of mathematical physics. Both the fluid and structural dynamics of the interaction is addressed, with special attention paid to the interplay between the two. It is demonstrated that the wave effects due to multiple reflections of the pressure waves travelling in the inter-shell fluid to a large degree determine the structural dynamics of the system, but have a more pronounced effect on the outer shell than on the inner one. It is also established that the effect of changing the thickness of the outer shell on the stress–strain state of the inner shell is incomparably more pronounced than vice versa. The investigation culminates with the results of a parametric study of the overall peak stress in the system, an example of utilizing the approach developed based on the introduced model and aiming at facilitating structural optimization of industrial systems at the pre-design stage in the context of shock resistance.     

Dynamic stability of a pipe conveying fluid with an uncertain computational model

This paper deals with the problem of a pipe conveying fluid of interest in several engineering applications, such as micro-systems or drill-string dynamics. The deterministic stability analysis developed by Paidoussis and Issid (1974) is extended to the case for which there are model uncertainties induced by modeling errors in the computational model. The aim of this work is twofold: (1) to propose a probabilistic model for the fluid–structure interaction considering modeling errors and (2) to analyze the stability and reliability of the stochastic system. The Euler–Bernoulli beam model is used to model the pipe and the plug flow model is used to take into account the internal flow in the pipe. The resulting differential equation is discretized by means of the finite element method and a reduced-order model is constructed from some eigenmodes of the beam. A probabilistic approach is used to model uncertainties in the fluid–structure interaction. The proposed strategy takes into account global uncertainties related to the noninertial coupled fluid forces (related to damping and stiffness). The resulting random eigenvalue problem is used to analyze flutter and divergence unstable modes of the system for different values of the dimensionless flow speed. The numerical results show the random response of the system for different levels of uncertainty, and the reliability of the system for different dimensionless speeds and levels of uncertainty.     

Expectations for computational fluid dynamics

This article presents a sampling of the author's expectations for the field of computational fluid dynamics (CFD) in the areas of research, development and application. The primary focus of the discussion herein is related to the non-linear transonic flow regime, and more specifically, for calculations about commercial transport aircraft. However, many of these topics are pertinent to all flow field regimes and aircraft designs. The underlying goal is to enable the automation of multi-disciplinary design processes, which utilize state-of-the-art numerical simulation methods. These include issues pertaining to accuracy, robustness, efficiency, ease-of-use, uncertainty requirements and other challenges.     

A multi-physics computational model of fuel sloshing effects on aeroelastic behaviour

A multi-physics computational method is presented to model the effect of internally and externally-carried fuel on aeroelastic behaviour of a pitch–plunge aerofoil model through the transonic regime. The model comprises three strongly coupled solvers: a compressible finite-volume Euler code for the external flow, a two-degree of freedom spring model and a smoothed particle hydrodynamics solver for the fuel. The smoothed particle hydrodynamics technique was selected as this brings the benefit that nonlinear behaviour such as wave breaking and tank wall impacts may be included. Coupling is accomplished using an iterative method with subcycling of the fuel solver to resolve the differing timestep requirements. Results from the fuel-structural system are validated experimentally, and internally and externally-carried fuel is considered using time marching analysis. Results show that the influence of the fuel, ignoring the added mass effect, is to raise the flutter boundary at transonic speeds, but that this effect is less pronounced at lower Mach numbers. The stability boundary crossing is also found to be less abrupt when the effect of fuel is included and limit cycles often appear. An external fuel tank is seen to exhibit a lower stability boundary, while the response shows a beating effect symptomatic of two similar frequency components, potentially due to interaction between vertical and horizontal fuel motion.     

A mathematical approach to study fluid-coupled vibration of eccentric annular plates

A mathematical method is proposed to study fluid-coupled vibration of axisymmetric plate structures with asymmetries due to either imperfection or practical reasons, e.g. the weight reduction of structure, natural frequency shifting, and accessibility. The suggested approach makes use of the separation of variables to determine general solutions of the partial differential equation of the plate transverse displacement, whilst defining multiple polar coordinate systems, each of which offers its own formulation of the plate deformation with respect to its coordinate system. Moreover, closed-form geometric equations and the chain rule for determining derivatives are implemented to move from one coordinate system to the other in order to satisfy boundary conditions. The mode shapes of the vibrating plate in the dry condition are determined and in turn used in the Rayleigh–Ritz method to characterize vibrational properties of the fluid-coupled plate structure. While implementing such an energy method, the fluid motion is formulated employing the velocity potential and solved using the separation of variables. Fluid–structure interaction is also taken into account satisfying the compatibility condition on the fluid–plate​ interface. The developed methodology to predict natural frequencies has been validated by comparison with results obtained by a commercial finite element program. It is also found that the eccentricity tends to reduce natural frequencies of the fluid-coupled system for the lower serial mode, but increases them for the higher serial modes regardless of the presence of liquid.     

Measurements of fluid fluctuations around an oscillating nuclear fuel assembly

In this paper, dynamic measurements of fluid velocity in the by-passes of a test-section representing a nuclear fuel assembly are presented. The test-section was designed to identify stiffness, damping and mass coefficients of a fuel assembly under axial flow, and previous studies have shown that the by-passes have an influence on the identified coefficients. The results presented in this paper show that the motion of the fuel assembly induces fluctuations in the axial fluid velocity in the by-passes. These fluctuations depend on the excitation frequency and position. A delay has been observed between the fuel assembly displacement and the fluid velocity fluctuations. The delay decreases when the axial velocity increases which means that it is a convection driven phenomenon.     

Experimental investigation of the wall dynamics of the A-pillar vortex flow

The influence of the A-pillar vortex on the wall flow of the side window of a car is investigated experimentally using a 30° dihedron model. The measurement of the unsteady pressure at the wall provides a map of the pressure fluctuation intensity, and a spectral analysis is performed to track the dominant frequencies of the wall pressure fluctuations. The wall flow generated by the vortex structure that develops parallel to the side-wall is characterised by means of particle image velocimetry (PIV). Its structure is analysed and compared to cross-sections of the A-pillar vortex in order to identify the different separation and reattachment lines. A comparison of the field of turbulent kinetic energy obtained by PIV with the map of the pressure fluctuations shows a correlation between the structure of the A-pillar vortex and the pressure fluctuations. It is found that the dominant wall pressure fluctuations are located at the secondary separation line, whereas the primary reattachment line does not show any significant pressure variations, that the A-pillar vortex will not naturally break down and that discrete vortices may be associated with the pressure fluctuations.     

On the possibility of shock-induced cavitation in submerged cylindrical shell systems

A circular cylindrical shell loaded by one or two fluids and responding to an external shock wave is analyzed in the context of the possible inception of shock-induced cavitation. Several scenarios of fluid contact are considered including a submerged evacuated shell and a submerged fluid-filled shell for three different combinations of the parameters of the internal and external fluids. A semi-analytical shell-shock interaction model is employed in order to predict the regions of the fluids where cavitation is likely to occur, and the respective cavitation development is hypothesized about. The most interesting and practically important finding is that when fluid is present both inside and outside the shell, there exist conditions when cavitation is expected to occur in both the internal and external fluid, resulting in a particularly complex and violent structural re-loading occurring upon the collapse of the respective cavitation regions. The inception of cavitation in the internal fluid alone and in the external fluid alone is also possible. The findings are summarized in a manner that is suitable for use at the pre-design stage as a guide for preliminary assessment of the possibility of shock-induced cavitation in fluid-interacting industrial systems.     

A theoretical computational model of a plate in hypersonic flow

A theoretical model of an elastic panel in hypersonic flow is derived to be used for design and analysis. The nonlinear von Kármán plate equations are coupled with 1st order Piston Theory and linearized at the nonlinear steady-state deformation due to static pressure differential and thermal loads. Eigenvalue analysis is applied to determine the system’s stability, natural frequencies and mode shapes. Numerically time marching the equations provides transient response prediction which can be used to estimate limit cycle oscillation amplitude, frequency and time to onset. The model’s predictive capability is assessed by comparison to an experiment conducted at a free stream flow of Mach 6. Good agreement is shown between the theoretical and experimental natural frequencies and mode shapes of the fluid–structure system. Stability analysis is performed using linear and nonlinear methods to plot stability, flutter and buckling zones on a free stream static pressure vs temperature differential plane.     

On the inflation and deflation dynamics of liquid-filled,hyperelastic balloons

We derive a reduced-order model describing the inflation and deflation dynamics of a liquid-filled hyperelastic balloon, focusing on inviscid laminar flow and the extensional motion of the balloon. We initially study the flow and pressure fields for dictated motion of the solid, which throughout deflation are obtained by solving the potential problem. However, during inflation, flow separation creates a jet within the balloon, requiring a different approach. The analyses of both flow regimes lead to a simple piecewise model, describing the fluidic pressure during inflation and deflation, which is verified by finite element computations. We then use a variational approach to derive the equation describing the interaction between the extensional mode of the balloon and the entrapped fluid, yielding a nonlinear hybrid oscillator equation. Analytical and graphical investigations of the suggested model are presented, shedding light on its static and dynamic behaviour under different operating conditions. Our simplified model and its underlying assumptions are verified utilizing a fully coupled finite element scheme, showing excellent agreement.     

Flutter of spring-mounted flexible plates in uniform flow

A fluid–structure interaction (FSI) system is studied wherein a cantilevered flexible plate aligned with a uniform flow has its upstream end attached to a spring mounting. This allows the entire system to oscillate in a direction perpendicular to that of the flow as a result of the mounting׳s dynamic interaction with the flow-induced oscillations, or flutter, of the flexible plate. We also study a hinged-free rotational-spring attachment as a comparison for the heaving system. This variation on classical plate flutter is motivated by its potential as an energy-harvesting system in which the reciprocating motion of the support system would be tapped for energy production. We formulate and deploy a hybrid of theoretical and computational modelling for the two systems and comprehensively map out their linear-stability characteristics at low mass ratio. Relative to a fixed cantilever, the introduction of the dynamic support in both systems yields lower flutter-onset flow speeds; this is desirable for energy-harvesting applications. We further study the effect of adding an inlet surface upstream of the mount as a means of changing the destabilising mechanism from single-mode flutter to modal-coalescence flutter which is a more powerful instability more suited to energy harvesting. This strategy is seen to be effective in the heaving system. However, divergence occurs in the rotational system for low spring natural frequencies and this would lead to its failure for energy production. Finally, we determine the power-output characteristics for both systems by introducing dashpot damping at the mount. The introduction of damping increases the critical speeds and its variation permits optimal values to be found that maximise the power output for each system. The addition of an inlet surface is then shown to increase significantly the power output of the heaving system whereas this design strategy is not equally beneficial for the rotational system.     

Physical investigation of square cylinder array dynamical response under single-phase cross-flow

Fluid structure interaction and flow-induced vibration in square cylinder arrangement under single-phase incompressible laminar cross flow are investigated in the present paper. Dynamic instability governed by damping generation is studied without any consideration about mixing with turbulence effects. Conservative and non-conservative effects are pointed out and dynamical stability limit sensitivity to physical parameters is analyzed. Finally the influence of key physical parameters on fluid solid dynamics interaction is quantified.     

Modeling of share/soil interaction of a horizontally reversible plow using computational fluid dynamics

The horizontally reversible plow (HRP) is currently widely used instead of the regular mold-board plow due to its high operational performance. Soil pressure during HRP tillage generally has adverse effects on the plow surface, especially on either the plowshare or the plow-breast. This effect eventually shortens the tool’s service life. For this reason, this investigation used a three-dimensional (3D) computational fluid dynamics (CFD) approach to characterize the share/soil interaction and thus assess the effects of different tillage conditions on the interaction. To achieve this goal, a 3D model of the plowshare was first constructed in the commercial software SolidWorks, and soil from Xinjiang, China, was selected and subsequently characterized as a Bingham material based on rheological behaviors. Finally, 3D CFD predictions were performed using the control volume method in the commercial ANSYS code Fluent 14.0 in which the pressure distributions and patterns over the share surface were addressed under different tillage speeds in the range of 2–8 ms⁻¹ and at operational depths ranging from 0.1 to 0.3 m. The results show that the maximum pressure appeared at the share-point zone of the plowshare and that the increase in soil pressure was accompanied by either higher tool speed or greater operational depth. The calculated results qualitatively agreed with the preliminary experimental evidence at the same settings according to scanning electron microscopy (SEM). Once again, the CFD-based dynamic analysis in this study is demonstrated to offer great potential for the in-depth study of soil-tool interactions by simulating realistic soil matter.     

Investigation of passive oscillations of flexible splitter plates attached to a circular cylinder

This paper presents a numerical study to address wake control of a circular cylinder subjected to two-dimensional laminar flow regime using single and multiple flexible splitter plates attached to the cylinder. Three different cases are presented in the study, covering cylinders with one, two and three horizontally attached splitter plates while the locations of the plates around the cylinders are varied. The length of the splitter plates was equal to the cylinder diameter and Reynolds number was 100. Due to the flexibility of the plates, the problem was modeled as a Fluid–Structure Interaction (FSI) problem and the commercial finite element software, Comsol Multiphysics, was utilized to solve this problem using Arbitrary Lagrangian–Eulerian (ALE) method. Vortex shedding frequency and fluid forces acting on the cylinder are investigated, along with a comprehensive parametric study to identify the optimum arrangement of the plates for maximum drag reduction and maximum vortex shedding frequency reduction. The numerical results associated to the flexible splitter plates are also compared with the corresponding rigid splitter plate cases investigated in a previous study. Moreover, the tip amplitude of the plates and the maximum strains were measured in order to find an optimum position for placing a piezoelectric polymer to harvest energy from the flow.     

Flow field characteristics study of a flapping airfoil using computational fluid dynamics

The flow field of a flapping airfoil in Low Reynolds Number (LRN) flow regime is associated with complex nonlinear vortex shedding and viscous phenomena. The respective fluid dynamics of such a flow is investigated here through Computational Fluid Dynamics (CFD) based on the Finite Volume Method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier-Stokes (N-S) equations. The airfoil is a thin ellipsoidal geometry performing a modified figure-of-eight-like flapping pattern. The flow field and vortical patterns around the airfoil are examined in detail, and the effects of several unsteady flow and system parameters on the flow characteristics are explored. The investigated parameters are the amplitude of pitching oscillations, phase angle between pitching and plunging motions, mean angle of attack, Reynolds number (Re), Strouhal number (St) based on the translational amplitudes of oscillations, and the pitching axis location (x/c). It is shown that these parameters change the instantaneous force coefficients quantitatively and qualitatively. It is also observed that the strength, interaction, and convection of the vortical structures surrounding the airfoil are significantly affected by the variations of these parameters.