Fluid Loading Conditions Research Articles

Computational modeling of orthostatic intolerance for travel to Mars

Astronauts in a microgravity environment will experience significant changes in their cardiopulmonary system. Up until now, there has always been the reassurance that they have real-time contact with experts on Earth. Mars crew however will have gaps in their communication of 20 min or more. In silico experiments are therefore needed to assess fitness to fly for those on future space flights to Mars. In this study, we present an open-source controlled lumped mathematical model of the cardiopulmonary system that is able simulate the short-term adaptations of key hemodynamic parameters to an active stand test after being exposed to microgravity. The presented model is capable of adequately simulating key cardiovascular hemodynamic changes—over a short time frame—during a stand test after prolonged spaceflight under different gravitational conditions and fluid loading conditions. This model can form the basis for further exploration of the ability of the human cardiovascular system to withstand long-duration space flight and life on Mars.

In vitro evaluation of an adaptive Starling-like controller for dual rotary ventricular assist devices.

Rotary ventricular assist devices (VADs) operated clinically under constant speed control (CSC) cannot respond adequately to changes in patient cardiac demand, resulting in sub-optimal VAD flow regulation. Starling-like control (SLC) of VADs mimics the healthy ventricular flow regulation and automatically adjusts VAD speed to meet varying patient cardiac demand. The use of a fixed control line (CL - the relationship between ventricular preload and VAD flow) limits the flow regulating capability of the controller, especially in the case of exercise. Adaptive SLC (ASLC) overcomes this limitation by allowing the controller to adapt the CL to meet a diverse range of circulatory conditions. This study evaluated ASLC, SLC and CSC in a biventricular supported mock circulation loop under the simulated conditions of exercise, sleep, fluid loading and systemic hypertension. Each controller was evaluated on its ability to remain within predefined limits of VAD flow, preload, and afterload. The ASLC produced superior cardiac output (CO) during exercise (10.1 L/min) compared to SLC (7.3 L/min) and CSC (6.3 L/min). The ASLC produced favourable haemodynamics during sleep, fluid loading and systemic hypertension and could remain within a predefined haemodynamic range in three out of four simulations, suggesting improved haemodynamic performance over SLC and CSC.

Diffusion of Supercritical Fluids through Single-Layer Nanoporous Solids: Theory and Molecular Simulations.

With the advent of graphene material, membranes based on single-layer nanoporous solids appear as promising devices for fluid separation, be it liquid or gaseous mixtures. The design of such architectured porous materials would greatly benefit from accurate models that can predict their transport and separation properties. More specifically, there is no universal understanding of how parameters such as temperature, fluid loading conditions, or the ratio of the pore size to the fluid molecular diameter influence the permeation process. In this study, we address the problem of pure supercritical fluids diffusing through simplified models of single-layer porous materials. Basically, we investigate a toy model that consists of a single-layer lattice of Lennard-Jones interaction sites with a slit gap of controllable width. We performed extensive equilibrium and biased molecular dynamics simulations to document the physical mechanisms involved at the molecular scale. We propose a general constitutive equation for the diffusional transport coefficient derived from classical statistical mechanics and kinetic theory, which can be further simplified in the ideal gas limit. This transport coefficient relates the molecular flux to the fluid density jump across the single-layer membrane. It is found to be proportional to the accessible surface porosity of the single-layer porous solid and to a thermodynamic factor accounting for the inhomogeneity of the fluid close to the pore entrance. Both quantities directly depend on the potential of mean force that results from molecular interactions between solid and fluid atoms. Comparisons with the simulations data show that the kinetic model captures how narrowing the pore size below the fluid molecular diameter lowers dramatically the value of the transport coefficient. Furthermore, we demonstrate that our general constitutive equation allows for a consistent interpretation of the intricate effects of temperature and fluid loading conditions on the permeation process.

Unstable two-phase flow rate in micro-channels and cracks under imposed pressure difference

This paper numerically investigates two-phase flow rate instabilities in micro-channels comprising localized geometrical restrictions (i.e., a convergent–divergent duct). A numerical Lattice Boltzmann method is used to model and simulate multiphase flow with an accurate treatment of the contact angle at the triple line. A pressure difference is imposed between the inlet and outlet sections of the channel. This induces flow and allows its time evolution to be followed. Indeed, the resulting flow exhibits strong time fluctuations as it is strongly influenced by the induced viscous forces acting at solid–fluid interfaces and by pressure discontinuities sustained across the curved fluid–fluid interfaces (i.e., across the curvatures of the menisci). Flow fluctuations are investigated parametrically for wetting and non-wetting fluids and for different micro-channel geometrical irregularities. For this purpose, an equivalent dimensionless formulation is adopted. Flow fluctuations are analyzed and related to the controlling dimensionless parameters (Reynolds number, Laplace number, Bejan number, contact angle and channel constriction ratio). This allows quantification of the coupled influence of physical fluid properties (surface tension and contact angle), channel geometry, and loading conditions (imposed pressure difference) on flow evolution. Numerical results show that, under particular conditions, capillary pressure jumps sustained across the fluid bridge (owing to flow rate, contact angle, and local orientation of channel walls) entirely compensate for the imposed pressure difference. This situation results in a no-flow state, i.e., the flow becomes impossible even under an imposed pressure difference.

Fluid loading effects on temporal profiles of cardiovascular responses to head-down tilt.

Susceptibility of healthy astronauts to orthostatic hypotension and presyncope is exacerbated upon return from spaceflight. Up to 64% of astronauts experience postflight orthostatic intolerance. A promising countermeasure for postflight orthostatic intolerance is fluid loading by giving crew salt tablets and water prior to re-entry. The primary goal of the current study was to determine the optimal time window prior to re-entry when individual crew-persons would initiate fluid loading. There were 16 subjects who were given two 6-h exposures, with and without fluid loading (conditions), to head-down tilt (HDT) to simulate the effects of microgravity. Pre- and post-HDT stand tests of orthostatic tolerance were given. Physiological measurements recorded included heart rate, blood pressure, peripheral blood volume, total peripheral resistance, and impedance cardiography. Echocardiography measures of stroke volume and cardiac output were also recorded. Data were analyzed with three-way repeated measures ANOVA (gender × condition × time). Only the condition × time interaction was significant for mean arterial pressure. Post hoc multiple comparison tests revealed significant increases in mean arterial pressure occurred between hours 1 and 3 of HDT after fluid loading (10 mmHg higher than no fluid). These findings indicate that the optimal time for crew to begin fluid loading is within 1 to 3 h prior to re-entry. Nonsignificant trends of multiple cardiovascular responses showed similar time profiles. The large amount of individual variability suggests that fluid loading alone may be an inadequate countermeasure for all crewmembers. Further research is needed on possible adjunct methods of tailoring countermeasures for individuals.

Tohoku Tsunami-Induced Building Failure Analysis with Implications for U.S. Tsunami and Seismic Design Codes

The structural details of numerous damaged buildings in the Tohoku region were documented soon after the 11 March 2011 Tohoku-oki earthquake and tsunami by a reconnaissance team sponsored by the American Society of Civil Engineers. Tsunami flow depths and velocities were determined based on analysis of video records and the observed effects on simple benchmark structures in the flow. Equations for various conditions of fluid loading were then validated through failure analyses completed for several buildings, using finite element modeling and LiDAR scans. These analysis tools were applied full-scale to buildings with clearly identified failure mechanisms to validate methodologies to be included in a new chapter on “Tsunami Loads and Effects” in the ASCE 7-2016 Standard, Minimum Design Loads for Buildings and Other Structures. These findings, together with an analysis of the inherent seismic inelastic capacities of mid-rise buildings, are relevant for establishing the loadings and performance objectives proposed for the new chapter on “Tsunami Loads and Effects” in the ASCE 7 Standard.

The effects of fluid loading and elastic supports on the transmission of low-frequency noise through a single-pane window

The transmission of low frequency noise (LFN) through a structural element is explored in the current study by coupling the finite element method (FEM) with the boundary element formulation (BEM). The techniques of component mode synthesis are applied to adjust the stiffness matrix in the FEM formulation. The effect of the elastic boundary conditions on the transmission of sound through a rectangular window is then investigated numerically. Parametric studies have been carried out to examine the effects of natural frequencies, fluid loading and dimensions of the elastic structures on the transmission of LFN. The coupled FEM/BEM formulation is particularly useful to study the structural responses due to LFN below 200 Hz because relevant parametric studies can be conducted numerically avoiding the need of performing tedious and expensive experimental measurements. It has been demonstrated in the numerical simulations that the effects of fluid loading and elastic mounting conditions are particularly important for computing the transmission of airborne excitations through elastic windows with frequency below 125 Hz.

A method for atomic force microscopy cantilever stiffness calibration under heavy fluid loading

This work presents a method for force calibration of rectangular atomic force microscopy (AFM) microcantilevers under heavy fluid loading. Theoretical modeling of the thermal response of microcantilevers is discussed including a fluid-structure interaction model of the cantilever-fluid system that incorporates the results of the fluctuation-dissipation theorem. This model is curve fit to the measured thermal response of a cantilever in de-ionized water and a cost function is used to quantify the difference between the theoretical model and measured data. The curve fit is performed in a way that restricts the search space to parameters that reflect heavy fluid loading conditions. The resulting fitting parameters are used to calibrate the cantilever. For comparison, cantilevers are calibrated using Sader's method in air and the thermal noise method in both air and water. For a set of eight cantilevers ranging in stiffness from 0.050 to 5.8 N/m, the maximum difference between Sader's calibration performed in air and the new method performed in water was 9.4%. A set of three cantilevers that violate the aspect ratio assumption associated with the fluid loading model (length-to-width ratios less than 3.5) ranged in stiffness from 0.85 to 4.7 N/m and yielded differences as high as 17.8%.

Automated pre-ejection period variation predicts fluid responsiveness in low tidal volume ventilated pigs

The respiratory variation in the pre-ejection period (Delta PEP) has been used to predict fluid responsiveness in mechanically ventilated patients. Recently, we automated this parameter and indexed it to tidal volume (PEPV) and showed that it was a reliable predictor for post-cardiac surgery, mainly paced, patients ventilated with low tidal volumes. The aims of the present animal study were to investigate PEPV's ability to predict fluid responsiveness under different fluid loading conditions and natural heart rates during low tidal volume ventilation (6 ml/kg) and to compare the performance of PEPV with other markers of fluid responsiveness. Eight prone, anesthetized piglets (23-27 kg) ventilated with tidal volumes of 6 ml/kg were subjected to a sequence of 25% hypovolemia, normovolemia, and 25% and 50% hypervolemia. PEPV, Delta PEP, pulse pressure variation (PPV), central venous pressure (CVP), and pulmonary artery occlusion pressure (PAOP) were measured before each volume expansion. Sensitivity was 89% and specificity was 93% for PEPV, 78% and 93% for Delta PEP, 89% and 100% for PPV, 78% and 93% for CVP, and 89% and 87% for PAOP. PEPV predicts fluid responsiveness in low tidal volume ventilated piglets.

Intraoperative myocardial ischemia during renal transplantation caused by anomalous origin of the right coronary artery

Anomalous origin of the right coronary artery (AORCA) is a rare congenital anomaly with an incidence of 0.92% during routine cardiac catheterization. Its presence raises an important concern to the anaesthesiologist because it can lead on to myocardial ischaemia manifesting as either angina pectoris or myocardial infarction, or sudden death in young patients with minimal exertion, even in the absence of atherosclerosis. Patients with AORCA may be intolerant to stress and the high cardiac output condition owing to volume loading. Such a therapeutic manoeuvre may be desirable during renal transplantation to enable better perfusion of the renal graft immediately after grafting the kidney, in order to improve its function. Hence, haemodynamic goals in renal transplant recipient with AORCA can be contradictory during surgery, thereby rendering anaesthetic management challenging. We report a case of acute myocardial ischemia precipitated by fluid loading conditions in a patient with AORCA during renal transplant that was successfully treated with emergent intra-aortic balloon pump therapy intraoperatively. Judicious intraoperative fluid replacement is recommended, and volume overload must be avoided in AORCA patients undergoing surgery.

Flow-induced vibrations of an elastic cylindrical shell conveying a compressible fluid

The paper addresses the effects of generation and transmission of the vibro-acoustic energy in an elastic cylindrical shell filled with water. The energy input is produced by velocity and pressure pulsations due to the presence of an obstacle in a flowing fluid. It is assumed that there is no back-reaction of the vibro-acoustic response on the driving turbulent field. The concept of modal Green's functions and modal boundary integral equations is introduced and the role of elastic deformations of a tube is highlighted in view of the energy re-distribution between the structural and the acoustical transmission paths. Another important aspect of the problem of ‘flow-induced sound generation’, which is addressed in this paper, is the treatment of a CFD pressure output for the modelling of vibro-acoustic field generation. For an elastic shell in heavy fluid loading conditions, it is indicated that only the pressure distribution at the surface of a shell provided by a CFD solver is requested in order to adequately predict its coupled vibro-acoustic response. The ‘inner’ and ‘outer’ acousto-elastic domains are introduced and the low-order system of linear algebraic equations is set up, which is constituted by modal boundary equations and continuity conditions. Several ‘generic’ excitation cases are studied and the results of analysis are explained.

A nonlinear model of the dynamics of a large elastic plate with heavy fluid loading

In this paper, we derive weakly nonlinear equations for the dynamics of a thin elastic plate of large extent under conditions of heavy fluid loading. Two situations are then considered. First, we consider the case in which transverse motion of the plate generates a weaker in-plane motion, which is in turn coupled back to the evolution of the transverse motion. This results in the familiar nonlinear Schrödinger equation for the amplitude of a transverse plane wave, and we show that solitary-wave solutions are possible over the range of (non-dimensional) frequencies ω > ω c , which depends on the material properties. Dimensional values of ω c are physically realizable for a typical composite material underwater. Second, we consider the case in which the amplitudes of the transverse and in-plane motion are of the same order of magnitude, possible at a single resonant frequency, which leads to an evolution equation of rather novel type. We find a range of travelling-wave solutions, including cases in which incident in-plane waves can generate localized regions of transverse displacement.

Detachment and emission of airborne bacteria in gas‐phase biofilm reactors

Three laboratory scale biofilters filled with different packing materials (peat and sieved sugarcane bagasse) and operating with different microbial cultures (allochthonous and autochthonous bacteria) were run and monitored in parallel to assess the emission rate of airborne bacteria in the biofiltration of benzene-contaminated air streams. The effect of the fluid dynamic and loading conditions on the rate of microbial emission in the air environment was investigated by performing continuous experiments at different inlet benzene concentrations and superficial gas velocities. The experiments prove that the concentration of airborne bacteria in the effluent air from lab-scale biofilters is only slightly higher than in the ambient air. The emission rate is not dependent on superficial gas velocity because of low shear stress exerted by the gas flow. On the other hand, the loading conditions have a strong effect on the emission rate, which increases with increasing growth and degradation rate, and different packing media show remarkably different behaviors.

FORCED AND FREE VIBRATIONS OF RECTANGULAR SANDWICH PLATES WITH PARAMETRIC STIFFNESS MODULATION

An active control of the resonant vibrations of a rectangular sandwich plate performed by the parametric stiffness modulation is analyzed. The controlled vibrations are those of the dominantly flexural type excited by the transverse force acting at the first resonant frequency of dominantly flexural vibrations. The stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. The method of direct partition of motions is used that predicts an existence of the modal interaction between these two modes of vibrations due to the parametric stiffness modulation. It is shown that such a parametric control can provide a significant shift of the first eigenfrequency of a controlled plate (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled plate. Heavy fluid loading conditions are accounted for as well as the energy dissipation in the material of a plate. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. Dynamics of an elementary two-degree-of-freedom model mechanical system is considered to illustrate the mechanism of modal interaction, which is involved in the suggested way of an active control of vibrations of sandwich plates.

The active control of vibrations of composite beams by parametric stiffness modulation

The paper addresses an active control of the resonant vibrations of composite beams performed by a parametric stiffness modulation. A sandwich beam composition with the continuous core is considered. The stiffness modulation is introduced by some fairly small changes in an orientation of elements of the microstructure of a core ply. The controlled vibrations are those of the dominantly flexural type excited by a transverse force acting at a low resonant frequency, whereas the stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. This difference in time scales of the controlled vibrations and the input signal facilitates a use of the method of direct partition of motion that predicts an existence of the modal interaction between the low-frequency and the high-frequency motions due to so-called vibrational forces. It is shown that such a parametric control can provide a significant favourable shift of the first eigenfrequency of a controlled beam (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled beam. Heavy fluid loading conditions are accounted for as well as material losses in a structure. Then instead of analysis of eigenfrequencies, a problem of forced vibrations is posed and the forced frequency–amplitude response is analysed. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases.

On the existence of flexural edge waves on submerged elastic plates

The existence of flexural waves confined to the free edge of a fluid-loaded plate is established theoretically. Whereas analogous in vacuo edge waves exist for all parameter values, submerged plates are shown herein to support such waves only under very light fluid-loading conditions. For example, thin plates of aluminium, brass or Plexiglas will not support edge waves in water, although edge waves are permissible for each of these materials in air. The analysis is based on classical thin-plate theory and employs the Wiener-Hopf technique to derive the dispersion relation for the edge-wave wavenumber as a function of frequency In the limit of zero fluid loading the dispersion relation predicts the well-known result of Konenkov for edge waves on thin plates in vacuo.

NON-LINEAR OSCILLATIONS OF A BAFFLED ELASTIC PLATE IN HEAVY FLUID LOADING CONDITIONS

Non-linear vibrations of an elastic plate in heavy fluid loading conditions are considered. The structural non-linearity is taken into account along with the fluid non-linearity in a full Bernoulli integral formulation for the contact acoustic pressure and with the non-linearity in the formulation of the continuity condition. The modal analysis in spatial co-ordinates is used along with the method of multiple scales to search for a stationary response in the time domain. Resonant frequencies of a fluid-loaded plate are detected in a coupled formulation of structural acoustics and typical excitation conditions (a weak resonant excitation, hard monochromatic sub- and super-harmonic excitations, an excitation by two driving forces) are explored. The roles of the structural non-linearity and the non-linearity in the formulation of the fluid response are compared.

Vibrations of and sound radiation from sandwich plates in heavy fluid loading conditions

A problem of forced vibrations of a sandwich plate submerged in a fluid (an acoustic medium) is formulated as a coupled problem of structural acoustics. Two-dimensional (plane) formulation relevant to cylindrical bending of a plate is explored. Dynamics of a sandwich beam (a sandwich plate in cylindrical bending) is described in the framework of the sixth-order theory of multi-layered plates. Asymptotic analysis of the dispersion polynomial is performed in the low frequency and the low module limits for an isolated infinitely long beam. Green's functions for flexural vibrations are obtained analytically and thus explicitly contain the principal parameters of beam's composition. Forced vibrations of a fluid-loaded beam in a rigid baffle are considered. Both the interaction between an acoustic medium and a plate and the interaction between a plate and its boundaries are described by boundary integral equations assembled in a two-level system. Eigenfrequencies of a fluid-loaded beam are detected by maximum of the radiated acoustic power. A semi-analytical sensitivity analysis of the objective function selected as a radiated acoustic power to the parameters of beam's composition is performed.

A non-contact technique for evaluation of elastic structures at large stand-off distances: applications to classification of fluids in steel vessels

A novel technique for non-contact evaluation of structures in air at large stand-off distances (on the order of several meters) has been developed. It utilizes a recently constructed air-coupled, parametric acoustic array to excite the resonance vibrations of elastic, fluid-filled vessels. The parametric array is advantageous for NDE applications in that it is capable of producing a much narrower beamwidth and broader bandwidth than typical devices that operate under linear acoustic principles. In the present experiments, the array operates at a carrier frequency of 217 kHz, and the sound field several meters from the source is described spectrally by the envelope of the drive voltage. An operating bandwidth of more than 25 kHz at a center frequency of 15 kHz is demonstrated. For the present application, the array is used to excite vibrations of fluid-filled, steel containers at stand-off distances of greater than 3 m. The vibratory response of a container is detected with a laser vibrometer in a monostatic configuration with the acoustic source. By analyzing the change in the response of the lowest order, antisymmetric Lamb wave as the interior fluid loading conditions of the container are changed, the fluid contained within the steel vessel is classified.

MODELLING OF NON-LINEAR OSCILLATIONS OF ELASTIC STRUCTURES IN HEAVY FLUID LOADING CONDITIONS

In a problem of structural acoustics with non-liner formulation of structural dynamics, a linearized compatibility condition at the fluid–structure interface is used along with the linear wave equation for the acoustic medium [1–7]. This approach is referred to as a light acoustic loading limit. Another approach is to formulate a compatibility condition at the moving boundary and solve a non-linear wave equation in a volume. As it is shown in references [3–5], a solution for this problem predicts shock wave formation at a certain distance from a vibrating surface. In the present paper, one more model of interaction between an acoustic medium and a non-linear structure is suggested for heavy fluid loading conditions. In this model, propagation of acoustic waves is described by a linear wave equation, but the continuity condition is formulated at the moving boundary and the contact acoustic pressure acting at the vibrating non-linear structure is calculated by the Bernoulli integral with a quadratic velocity term retained. Two model problems of coupled structural acoustics are considered—oscillations of a fluid-loaded piston and oscillations of an infinitely long periodically supported elastic plate. A method of multiple scales is used for analysis of the local non-linear dynamics of the model systems, whilst matched asymptotic expansions are used to model the fluid's motion. Several specific effects of structural vibrations generated by the non-linearity of fluid-structure interaction, rather than by structural non-linearity are demonstrated.