Search

In this work, we show that the Ghost Fluid Method (GFM) has actually zero-order accuracy if the gas-liquid interface is not in normal motion initially. The modified GFM (MGFM) is found to overcome this problem well. Examples are given to support the present analysis and conclusions obtained, and the MGFM is then applied to simulate compressible fluid-structure interaction. [ABSTRACT FROM AUTHOR]

In this paper, the cellular structure of a two-dimensional detonation wave in a low pressure H2/O2/Ar mixture calculated with a detailed chemical reaction model, high order scheme and high resolution grids is investigated. The regular cellular structure is produced about 1 ms after introducing perturbations in the reaction zone of a steady one-dimensional detonation wave. It is found from the present resolution study that the discrepancies concerning the structure type arising from the coarser grid employed can be resolved using a sufficiently fine grid size of 0.05 mm and below and shows a double-Mach-like strong-type configuration. During the structure evolution process, the structure configuration does not change much in the periods before and after the triple point collision. Through the triple point collision, three regular collision processes are observed and are followed by a quick change to the double-Mach-like configuration. The simulated structure tracks show that there are three different tracks associated with different triple points or the kink on the transverse wave. Comparisons with previous work and experiments indicate the presence of a strong structure for an ordinary detonation. [ABSTRACT FROM AUTHOR]

This work aims to model transonic airfoil-gust interaction and the gust response on transonic aileron-buzz problems using high-fidelity computational fluid dynamics (CFD) and the Long Short Term Memory (LSTM) based deep-learning approach. It first explores the rich physics associated with these interactions, which show strong flowfield nonlinearities arising from the complex shock-boundary-layer interactions using CFD. In the transonic regime, most linear reduced-order models (ROMs) fail to reconstruct the unsteady global parameters such as the lift, moment, and drag coefficients and the unsteady distributive flow variables such as velocity, pressure, and skin friction coefficients on the airfoil or in the entire computational domain due to the nonlinear shock-gust interaction. As it is well known that a deep-learning framework creates several hypersurfaces to generate a nonlinear functional relationship between the gust or structural input and the unsteady flow variables, an algorithm is proposed to overcome the limitations of linear ROMs. This algorithm consists of two integral steps, namely, a dimensionality reduction where the Discrete Empirical Interpolation Method based linear data compression approach is applied and the reduced state is trained using the LSTM based Recurrent Neural Network for the reconstruction of unsteady flow variables. The present work shows its potential for predicting transonic airfoil gust response and the aileron-buzz problem demonstrating several orders of computational benefit as compared to high-fidelity CFD. [ABSTRACT FROM AUTHOR]

In this study, an adaptive step size control algorithm incorporated in the continuation method (CM) is developed and combined with the filtered impulse function method based on aerostructural solvers and applied to two aeroelastic analysis tasks concerned with modes tracking and parameter variation. The nonlinear limit cycle oscillation (LCO) of a NACA0012 airfoil with control surface free-play behavior is considered to illustrate how this adaptive step can achieve a balance between the stability, accuracy, and efficiency in the modes crossing region in modes tracking, the flutter point region in modes tracking, and the detrimental LCO region in parameter variation. It is shown that the computation time for modes tracking using this approach can be reduced by more than 65% while achieving almost the same estimate of the flutter speed as when using a fixed step size. The study also addresses a few unique and important phenomena in parameter variation such as modes jumping in the LCO curve that occur frequently using this adaptive step size algorithm. Furthermore, a combination of the modes tracking results to provide an estimate for the restart point for the CM solver to overcome the occurrence of possible extraneous solutions or divergence is proposed. The results of flutter analysis for linear structural system and nonlinear LCO analysis are validated against the results from experiments and the classical Theodorsen theory. The effect of viscous damping on the occurrence of detrimental LCO phenomena is also discussed and compared with the corresponding nondamping cases. [ABSTRACT FROM AUTHOR]

This study aims to develop a linear nonintrusive reduced-order model (NIROM) for facilitating combined fast parametric analysis and reconstruction of unsteady aerodynamic flow variables by using a novel signal interpolation approach, which is combined with a subspace-identification-based method. The NIROM is then applied to airfoil flutter and gust interaction problems where the assumption of linearized dynamics gives reasonably accurate estimates. In the traditional subspace-identification method, the training datasets of force coefficient signals corresponding to each flow parameter (like the Mach number and angle of attack) are required to generate the linear time-invariant system. The generation of the training signal for each flow parameter is the most computationally expensive process in any parametric reduced-order model (ROM). In the present work, the discrete empirical interpolation method (DEIM) combined with the radial basis function (RBF) method is used for the interpolation of training signals at a specific flow parameter to reduce the overall computational cost. The conventional nonintrusive approaches are generally constrained by the number of output channels, and hence not suitable for the reconstruction of unsteady surface force distribution or flowfield variables. A few suitable sensor locations using the DEIM are selected as output channels of the subspace-identification method to realize the full datasets of surface force distribution over the airfoil. Finally, a suitable algorithmic structure is proposed to achieve unsteady distributive flow-variable reconstruction and parametric variation simultaneously. The current signal interpolation-based NIROM is applied to a wing section with two- and four-degree-of-freedom structural motions. [ABSTRACT FROM AUTHOR]

Stress overshoot/undershoot is an important phenomenon in structured fluids undergoing dynamic and transient flow. To accurately capture the flow process, it is important to have a better understanding of and include in the numerical modeling the microstructure evolution that leads to the stress overshoot/undershoot phenomenon. We present a procedure for incorporating a microstructure model into a Lagrangian framework based on the smoothed particle hydrodynamics fluid solver. The numerical simulation is performed for a typical structured fluid under an applied strain rate history flow. Good agreement between the numerical results and the experimental data lends credence to and validates the proposed procedure for simulations of complex mixture flows. Additionally, the interaction between a flow of structured fluid and a circular cylinder placed in a channel is investigated. The viscous force is found to overshoot together with the applied gradient pressure and decrease over time as the fluid approaches the equilibrium state. [ABSTRACT FROM AUTHOR]

The past research on the supercavity flow mostly focused on the homogeneous fluid medium, ignoring the influence of the sudden change of the medium density. We present results from numerical study of moving projectile passing density change zone, both from low-density zone to high-density zone and from high-density to low-density. Particular attention is given to the cavity variation when projectile voyaging through different liquid density areas. And the influence of the cavitation number on the cavity variation is also studied. Research results show that there is a 'diameter shrinkage' phenomenon when the projectile moving pass the density jump area. The location of 'diameter shrinkage' is relative to the density change model: moving from high density side to low, it appears at the interface of liquids; while moving from low density side to high side, it occurs at low density side not the interface. And the degree of 'necking' have something to do with the density difference of two sides. However the effect of cavitation number is much complex, the 'necking' phenomenon at the interface showed disorder maybe due to the sample limit, but the re-development of the cavity after passing through the interface showing obvious regularity with the cavitation number. [ABSTRACT FROM AUTHOR]

High-intensity focused ultrasound (HIFU) is a non-invasive technique and widely used in medical and dental treatment for reaching and affecting deep seated tissues within the body, such as the removal of fat tissues under the skin, destruction of malignant tumour cells or target volume for the purpose of tissue debulking. This study aims to explore the damage characteristics of elastic material (regarded as soft tissue) by HIFU separated by a thin membrane all immersed in a water medium, in which the latter is used to mimic the human skin in medical treatments. Measurements of acoustic pressure along the vertical central line of HIFU transducer with and without this membrane were conducted and effects of membrane on the focal region were investigated. Results showed that the existence of the membrane serves primarily to reflect partial energy of incident ultrasound wave, and has no effect on phases of wave propagating in water. Next, Polydimethylsiloxane (PDMS) was selected as the elastic material for the experiments. The process of PDMS damage through the membrane by HIFU were recorded and the distribution of temperature around and within the 'destroyed' PDMS block was captured by an infrared camera. The very high temperature measured within the PDMS leading to the formations of cavity and crack within the PDMS block, can be attributed to the accumulation of the energy from the ultrasound source. Finally, the effects of PDMS with different elasticity on their damage characteristics are discussed. [ABSTRACT FROM AUTHOR]

Underwater spark-discharge methods have been widely utilized for experimental studies in many fields such as material processing, water treatment, and cavitation bubble dynamics. However, the precise control of bubble size using this method has been difficult. This poses challenges to better understand the complex interactions of non-spherical cavitation bubble growth and collapse, which require fine and careful control of bubble size. A novel low-voltage (60.0 V) underwater spark-discharge method using a metal-oxide-semiconductor field effect transistor is presented here. We are able to repeatedly generate oscillating bubbles of consistent maximum radius, a. The dependency of the total circuit resistance to spark-generated bubble size in this method is discussed. [ABSTRACT FROM AUTHOR]

A rheological constitutive law is developed for a suspension of rigid rods in a Bingham fluid for volume fractions ranging up to the semiconcentrated regime. Based on a cell model approach, which allows expressing the shear stress on the particle surface, the particle stress contribution is derived and involves additional yield stress terms related to an ensemble average orientation distribution of the rods. As a first approach, a von Mises criterion is used to describe the composite flow threshold, which is found to be anisotropic in the sense that it depends on the rod orientation. A rod dynamics equation is also proposed and incorporates some diffusion/perturbation due to yielded regions encountered throughout the suspension. In parallel, an equivalent kinetic theory is also developed. The model provides good agreement with shear stress experiments for kaolin pastes filled with steel fibers of two different aspect ratios. [ABSTRACT FROM AUTHOR]

A preface to the journal "Modern Physics Letters B," is presented.

The modal analysis of pressure-driven flows in channels patterned with superhydrophobic surfaces containing periodic grooves and ribs aligned longitudinally to the flow direction has been performed. The effects of shear-free fraction (δ) and groove-rib spatial period normalized by full-channel height (L) on the linear flow stability of such flows have been explored. By performing a BiGlobal linear stability analysis via the pseudo-spectral method, such surfaces have been found to potentially exert a stabilizing or destabilizing effect on the base flow, depending predominantly on the normalized groove-rib spacing. For small values of L (i.e., L = 0.01 and 0.02), a stabilizing effect is predicted for flows over longitudinal superhydrophobic grooves, in agreement with the results obtained using a local stability analysis which employs a homogeneous slip condition along the walls. For a moderate value of normalized groove-rib spacing where the groove-rib periodic spacing is one-tenth of the channel height, the presence of longitudinal superhydrophobic grooves leads to flow instabilities at a lower critical Reynolds number. The redistribution of the base flow resulting from the vanishing shear rates along the liquid-gas interface could give rise to an inflectional instability that promotes temporal instability. The effects of patterning the superhydrophobic surfaces on one or both channel walls are also examined. [ABSTRACT FROM AUTHOR]