Local Pressure Fluctuations Research Articles

ERT-based fetus monitoring system using wearable conductive fabrics

Abstract During pregnancy, it is important to monitor the health of the fetus and fetal movement count is one of the key parameters that can be used to check the health of the fetus. Consequently, there is growing interest in developing non-invasive passive methods for fetal monitoring techniques that can be used outside of clinical settings. This study introduces a home-use system based on electrical resistance tomography that pregnant mothers can utilize for fetus monitoring. The setup utilizes a conductive fabric, functioning as electronic skin (e-skin), positioned on the mother’s abdomen to detect alterations in the fabric’s electrical characteristics caused by fetal movements. This method is validated through both numerical simulations and experimental investigations, which assess conductivity changes on the fabric’s surface in reaction to localized pressure fluctuations, mimicking fetal motions.

Using a Combination of High-Frequency Coastal Radar Dataset and Satellite Imagery to Study the Patterns Involved in the Coastal Countercurrent Events in the Gulf of Cadiz

This study exploits the combination of High-Frequency Coastal Radar (HFR) information with satellite-derived observations to characterize the patterns involved in the coastal countercurrents (CCCs) events in the Gulf of Cadiz (GoC), which is situated in the SW of the Iberian Peninsula. The westward alongshore currents are observed throughout the year, but the main drivers necessary to develop this flow and its extension in both parts of the basin are not fully clear. In order to identify the main physical processes (both local and remote) that induce the development of these countercurrents and to evaluate the connection of the circulation patterns between the eastern and the western part of the GoC, we make use of several data sources available for the region. First of all, a land-based system of HFR antennas located at four different sites of the GoC provides the velocity field of the surface circulation of the basin. To achieve a significant characterization of the CCCs in the Gulf, the dataset analyzed is processed by means of a series of operations, including the Empirical Orthogonal Functions (EOFs) analysis used to identify spatial and temporal variability of the flow, a low-pass filter used to isolate the sub-inertial signal of the current and temporal interpolation to fill in the missing values. Secondly, given the known importance of the zonal component of the local winds combined with the variations in the mean pressure at sea level over the Western Mediterranean during these events, time series of meteorological data are processed and correlated with the current velocity series via a statistical analysis. Finally, sea surface temperature fields and chlorophyll-a distribution patterns are used as tracers to obtain information on the extension of the countercurrents where HFR data are missing in four cases studied during the year 2017. The conducted analysis revealed the consistent occurrence of westward coastal currents throughout the year, driven in the most intense cases by a combination of the zonal component of the local wind and atmospheric pressure fluctuations over the Western Mediterranean Sea. During those events, CCCs reached the Portuguese side of the Gulf and facilitated the advection of biological material and warmer waters.

Simulation of fiber-induced melt pressure fluctuations within large scale polymer composite deposition beads

The process-structure-property relationship in Large Area Additive Manufacturing (LAAM) technology is an ongoing area of research as the inherent microstructural properties (chiefly fibers and voids) affect the performance of printed parts. Unfortunately, we currently lack adequate understanding of micro void nucleation and evolution during the LAAM and fused deposition modelling (FDM) processes. Modeling of the polymer melt flow during the extrusion process is important in understanding the underlying microstructural formation and associated properties of the print, that determines the part performance in service conditions. In this paper we compute fiber-induced local pressure fluctuations which may promote void formation in the bead’s microstructure. On a macro-scale, we determine flow fields of a purely viscous, Newtonian planar polymer deposition flow through a LAAM nozzle which are utilized on a micro-scale model where we simulate the evolution of a single ellipsoidal fiber along streamlines obtained from the macro-model. On the micro-scale, we determine instantaneous values of the translational and rotational velocities of the rigid ellipsoidal fiber that satisfies a balance of hydrodynamic forces and couples on the fiber’s surface based on a Newton Raphson algorithm and we track the fiber’s motion along the flow path via an explicit numerical iterative algorithm. Model verification is achieved by benchmarking results with solutions from well-known Jeffery’s equation of motion of a particle in homogeneous simple shear flow. We account for rotary diffusivity due to short-range fiber-fiber interaction in the FEA simulation by determining an effective fluid domain size representative of the interaction coefficient of the melt flow through a correlation analysis that yields an equivalent steady state orientation based on the Advani-Tucker equation. We also consider different possible motions of the fiber along individual LAAM flow paths from a given set of random initial fiber conditions to determine pressure bounds on the fiber surface along each streamline. For improved computational efficiency, calculations are carried out with respect to the fiber’s local coordinate axes to overcome the rigor of adaptive remeshing during the quasi-transient analysis. Results show low pressure extremes near the fiber’s surface which varies across the printed bead as well as through its thickness. Discussion is provided to gain insight into the effect of low-pressure extremes on micro void formation, particularly at the nozzle exit and during die swell/expansion.

Experimental study on sub-regimes of air-water slug flow in a rectangular micro-channel

In mini/micro-channel two-phase flow, slug flow occurs easily because of the significant influence of surface tension and the low gravitational effect on the flow. Although an irregularity of slug flow has been reported, previous studies on mini/micro-channel slug flow have mainly concerned the slug flow with the alternate of film and liquid slug zones regularly. Therefore, the present study investigates sub-regimes of micro-channel slug flow with regular or irregular alternating film and liquid slug zones to understand their principal characteristics. Experiments are performed to visualize and simultaneously measure the transient local pressure of the air-water mixture flow in a horizontal rectangular channel with a 2 mm height and 1 mm width. Superficial Reynolds numbers for liquid and gas phases range from 56 to 1116 and 5.75 to 245, respectively. With the aid of visualization data, slug flow is categorized into two types of stable slug flow and four types of unstable slug flow. Present slug flow data is compared with previous correlations for predicting the regime of bubble formation mechanism, flow regime boundaries, and frictional pressure gradient. Measured temporal pressure records are analyzed using a fast Fourier transform and digital Butterworth filter, while a dominant frequency is identified from the normalized average power spectrum. Then, the local pressure fluctuation at the dominant frequency is examined for different types of slug flow using the visualization results. A morphological repetition is confirmed to be associated with the pressure fluctuation: the repeated film and liquid slug zones in both types of stable slug flow, the alternative between liquid-only flow and a group of gas bubbles and liquid slugs in type 3 unstable slug flow, and the periodic variation of bubble length in type 4 unstable slug flow. However, no correlation between morphological characteristics and pressure fluctuation is found in type 1 and 2 unstable slug flows.

A Method for Predicting the Aeroelastic Response of Radial Turbomachines

AbstractAircraft intermittent combustion engines often incorporate turbochargers adapted from ground-based applications to improve their efficiency and performance. These turbochargers can operate at off-design conditions and experience blade failures brought on by aerodynamic-induced blade resonances. A reduced-order model of the aeroelastic response of general fluid–structural configurations is developed using the Euler–Lagrange equation informed by numerical data from uncoupled computational fluid dynamic (CFD) and computational structural dynamic calculations. The structural response is derived from a method of assumed-modes approach. The unsteady fluid response is described by a modified version of piston theory that approximates the local transient pressure fluctuation in conjunction with steady CFD solution data. The reduced-order model is first applied to a classical panel flutter scenario and found to predict a flutter boundary that compares favorably to the boundary identified by existing theory and experimental data. The model is then applied to the high-pressure turbine of a dual-stage turbocharger. The model predictions are shown to reliably determine the lack of turbine blade flutter and rudimentary damping comparisons are performed to assess the ability of the model to ascertain the susceptibility of the turbine to forced response. Obstacles associated with the current experimental state of the art that impinge upon further numerical validation are discussed.

Microstructure and Cavitation Damage Characteristics of GX40CrNiSi25-20 Cast Stainless Steel by TIG Surface Remelting

Cavitation erosion degrades the surface of engineering components when the material is exposed to turbulent fluid flows. Under conditions of local pressure fluctuations, a nucleation of gas or vapor bubbles occurs. If the pressure suddenly drops below the vapor pressure, these bubbles collapse violently when subjected to higher pressure. This collapse is accompanied by the sudden flow of the liquid, which is manifested by stress pulses capable of causing plastic deformations on solid surfaces. Repeating these stress conditions can cause material removal and ultimately failure of the component itself. The present study aims to reduce the negative impact of this phenomenon on the mechanical systems components, using the TIG local surface remelting technique. Cavitation erosion tests were performed in accordance with the ASTM G32-2016 standard on samples taken from a cast high-alloy stainless steel. The alloy response for each melting current value was investigated by measuring mass loss as a function of cavitation attack time and by analyzing the damaged surfaces using optical and scanning electron microscopes. It was highlighted that the TIG remelted layers provide an increase in cavitation erosion resistance of 5-6 times as a consequence of the fine graining and microstructure induced by the technique applied.

Aeroacoustic control mechanism on near-wall-wing of Aero-train based on plasma jet

In this study, an aeroacoustic control mechanism of a plasma jet acting on a high-speed moving wing under a wing-in-ground effect is investigated. Moreover, a novel method is proposed to reduce the aeroacoustics of Aero-train wings. Numerical simulations of the aeroacoustics generated by flow around a National Advisory Committee for Aeronautics 4412 wing are performed under three different plasma excitation modes at four clearances with an incoming flow velocity of 0.3 Ma and an angle of attack of 5°. The results show that different plasma excitation modes interfere with the vortex generation and development in different ways to achieve aeroacoustic reduction. The UP excitation mode delays the airflow separation, delays the vortex generation and development, and reduces the vortex intensity. The BOTH excitation mode forces transverse vortices to transform into streamwise hairpin vortices and reduces the local pressure fluctuation intensity. Hence, plasma jets exhibit a good control effect on the peak aeroacoustics under different clearance conditions but result in the frequency shift effect of acoustic energy transfer to high frequencies. The modal analysis of the flow field of the three excitation conditions via a proper orthogonal decomposition method reveals that the trend of the modal change is similar for the three excitation conditions, and the change in each order of the modal corresponds to the energy decrease at the peak frequency and the energy increase at high frequencies.

An insight into direct water injection applied on the hydrogen-enriched rotary engine

The attractiveness of direct water injection advantages has made a comeback in the minds of designers due to the potential knock mitigation and lower NOx emissions. The current work established and validated an appropriate 3D model for hydrogen-enriched Wankel engines with direct water injection, and the used CFD simulation was mainly a consideration of combustion and flow phenomena inside the engine and securing the gained knowledge. The determinations of the knock operation and position were investigated, and the intrinsic mechanism of end-gas auto-ignition was clarified. Then, the effects of water injection strategies on knock mitigation and NOx reduction were analyzed in detail. Results showed that the end-gas auto-ignition caused the drastic variation of the local velocity field and the severe pressure oscillation in the vicinity of these regions, which led to knocking combustion. The highest knock propensity occurred at the trailing part of the recess, especially on the opposite side of the intake port. The pressure oscillation of water-enriched schemes was smaller than water-free schemes, and the knock intensity gradually decreased with the advance of water injection timing. When the injection timing was 250°EA, the propensity of end-gas auto-ignition was weakened obviously due to the reduced local temperature and pressure within the rotor chamber, and NOx formation was lower than the other schemes. The local pressure fluctuation was decreased by increasing water addition, whose maximum decrement occurred at the trailing part of the recess. The knock intensity gradually decreased as the amount of water injection increased. When the water injection ratio of 40%, there was no obvious auto-ignition occurrence, and NOx formation reduced significantly at the exhaust moment, which confirmed direct water injection was one of the most promising and effective methods to control knock propensity and NOx emissions simultaneously. In addition, the water droplets of a smaller water injection ratio (10%) scheme spread faster than the water-free scheme after spontaneous combustion, which could reach the fuel surface to present azeotropic phenomena and then enhance the combustion. This result shall give insights into the feasibility of a proper water addition for improving the thermal efficiency of the Wankel engine regime.

High-performance displacement by microgel-in-oil suspension in heterogeneous porous media: Microscale visualization and quantification

Hypothesis: Preferential flow in porous media is commonly encountered and decreases the multiphase displacement efficiency. Here, we synthesized microgel-in-oil in suspension and demonstrated that microgel-in-oil as a novel additive could present self-adaptive transport behavior and introduce a novel multiphase displacement mode for improving displacement efficiency in heterogeneous porous media.Experiments: We investigated the microgel-in-oil formation process and characterized their morphology with fluorescence microscopy and Cryo-SEM. The suspension displacement performance in heterogeneous porous media was evaluated using a microfluidic chip containing a preferential flow pathway (PFP) and a parallel matrix region. The displacement results of microgel-in-oil were compared to plain microgel particles and analyzed from pore-scale particle transport behavior to macroscopic multiphase flow patterns.Findings: The results show that suspension with moderate microgel-in-oil yields the optimal displacement efficiency. Fewer microgel-in-oil cannot alter the flow direction, while too many microgel-in-oil would block the PFP region. The topological analysis identified that suspensions with moderate microgel-in-oil content could achieve the strongest sweeping and carrying abilities that contribute to the highest displacement efficiency. The synergistic transport of microgel-in-oil and plain microgel particles would result in local pressure fluctuations to divert displacing fluid from PFP into the matrix region, which explains the above flow behavior.

Steady thermal convection representing the ultimate scaling.

Nonlinear simple invariant solutions representing the ultimate scaling have been discovered to the Navier-Stokes equations for thermal convection between horizontal no-slip permeable walls with a distance [Formula: see text] and a constant temperature difference [Formula: see text]. On the permeable walls, the vertical transpiration velocity is assumed to be proportional to the local pressure fluctuations, i.e. [Formula: see text] (Jiménez et al. 2001 J. Fluid Mech., 442, 89-117. (doi:10.1017/S0022112001004888)). Two-dimensional steady solutions bifurcating from a conduction state have been obtained using a Newton-Krylov iteration up to the Rayleigh number [Formula: see text] for the Prandtl number [Formula: see text], the horizontal period [Formula: see text] and the permeability parameter [Formula: see text]-[Formula: see text], [Formula: see text] being the buoyancy-induced terminal velocity. The wall permeability has a significant impact on the onset and the scaling properties of the found steady 'wall-bounded' thermal convection. The ultimate scaling [Formula: see text] has been observed for [Formula: see text] at high [Formula: see text], where [Formula: see text] is the Nusselt number. In the steady ultimate state, large-scale thermal plumes fully extend from one wall to the other, inducing the strong vertical velocity comparable with the terminal velocity [Formula: see text] as well as intense temperature variation of [Formula: see text] even in the bulk region. As a consequence, the wall-to-wall heat flux scales with [Formula: see text] independent of thermal diffusivity, although the heat transfer on the walls is dominated by thermal conduction. This article is part of the theme issue 'Mathematical problems in physical fluid dynamics (part 1)'.

The Effect of Airfoil Camber on Pressure Fluctuation in Bidirectional Axial-Flow Pump

To obtain the influence of airfoil camber on the internal pressure fluctuation of a bidirectional axial-flow pump, the unsteady Reynolds time-averaged Navier–Stokes (URANS) equation was solved to predict the internal flow structure under three airfoil camber cases. The airfoil camber was quantitatively controlled by airfoil camber angle. The pressure standard deviation was used to define the local pressure fluctuation intensity (PFI) inside the impeller and guide vane. Fast Fourier transform was applied to analyze the frequency-domain characteristics of the pressure signal near the impeller–straight pipe interface and impeller–guide vanes interface. The results were validated by the external characteristic test. Under the forward condition, the area of high PFI near the outlet and leading edge of the impeller increased with a decrease in airfoil camber angle, and that near the leading edge of the guide vanes shifted to the middle section with a decrease in airfoil camber angle. The main frequency of the pressure signal near the impeller–guide vanes interface was the blade-passing frequency (BPF), and the main frequency amplitude increased with a decrease in airfoil camber angle. Under the reverse condition, the high PFI area near the inlet and the leading edge of the impeller declined with the decrease in airfoil camber angle. The main frequency of the pressure signal near the impeller–straight pipe interface and impeller–guide vanes interface was the BPF, and the main frequency amplitude decreased with a decrease in airfoil camber angle.

The Adaptive Damping Technique: Improving the Simulation Accuracy of Hydraulic Transients

Hydraulic surges are transient events frequently observed in various industrial and laboratory flow situations. Understanding surge physics and its accurate numerical prediction is crucial to the safety of flow systems. The maximum accuracy achievable for transient surge simulations is limited by the inefficiencies in the mathematical models used. In this work, we propose a mathematical model that incorporates an adaptive damping technique for the accurate prediction of hydraulic surges. This model also takes the compressibility effects in the liquid during the surge process into account. The novel approach of using the local pressure fluctuation data from the flow to adjust the unsteady friction for controlling the dissipation is introduced in this paper. The adaptive-dissipation is actualized through a unique 'variable pressure wave damping coefficient' function definition. Numerical simulation of three different valve-induced surge experiments demonstrates the reliability and robustness of the mathematical model. Numerical results from the proposed model show an excellent match with the experimental data by closely reproducing both the frequency and the amplitude of transient pressure oscillations. A comparative study explains the improvement in the simulation accuracy achieved by replacing the constant damping coefficient with the proposed variable coefficient. The superiority of the new model with the adaptive damping capability over the similar models in literature and those used in commercial software packages is also well established through this study.

The ultimate state of turbulent permeable-channel flow

Direct numerical simulations have been performed for heat and momentum transfer in internally heated turbulent shear flow with constant bulk mean velocity and temperature, $u_{b}$ and $\theta _{b}$ , between parallel, isothermal, no-slip and permeable walls. The wall-normal transpiration velocity on the walls $y=\pm h$ is assumed to be proportional to the local pressure fluctuations, i.e. $v=\pm \beta p/\rho$ (Jiménez et al., J. Fluid Mech., vol. 442, 2001, pp. 89–117). The temperature is supposed to be a passive scalar, and the Prandtl number is set to unity. Turbulent heat and momentum transfer in permeable-channel flow for the dimensionless permeability parameter $\beta u_b=0.5$ has been found to exhibit distinct states depending on the Reynolds number $Re_b=2h u_b/\nu$ . At $Re_{b}\lesssim 10^4$ , the classical Blasius law of the friction coefficient and its similarity to the Stanton number, $St\approx c_{f}\sim Re_{b}^{-1/4}$ , are observed, whereas at $Re_{b}\gtrsim 10^4$ , the so-called ultimate scaling, $St\sim Re_b^0$ and $c_{f}\sim Re_b^0$ , is found. The ultimate state is attributed to the appearance of large-scale intense spanwise rolls with the length scale of $O(h)$ arising from the Kelvin–Helmholtz type of shear-layer instability over the permeable walls. The large-scale rolls can induce large-amplitude velocity fluctuations of $O(u_b)$ as in free shear layers, so that the Taylor dissipation law $\epsilon \sim u_{b}^{3}/h$ (or equivalently $c_{f}\sim Re_b^0$ ) holds. In spite of strong turbulence promotion there is no flow separation, and thus large-amplitude temperature fluctuations of $O(\theta _b)$ can also be induced similarly. As a consequence, the ultimate heat transfer is achieved, i.e. a wall heat flux scales with $u_{b}\theta _{b}$ (or equivalently $St\sim Re_b^0$ ) independent of thermal diffusivity, although the heat transfer on the walls is dominated by thermal conduction.

Pressure Fluctuation Reduction of a Centrifugal Pump by Blade Trailing Edge Modification

The pressure fluctuation inside centrifugal pumps is one of the main causes of hydro-induced vibration, especially at the blade-passing frequency and its harmonics. This paper investigates the feature of blade-passing frequency excitation in a low-specific-speed centrifugal pump in the perspective of local Euler head distribution based on CFD analysis. Meanwhile, the relation between local Euler head distribution and pressure fluctuation amplitude is observed and used to explain the mechanism of intensive pressure fluctuation. The impeller blade with ordinary trailing edge profile, which is the prototype impeller in this study, usually induces wake shedding near the impeller outlet, making the energy distribution less uniform. Because of this, the method of reducing pressure fluctuation by means of improving Euler head distribution uniformity by modifying the impeller blade trailing edge profile is proposed. The impeller blade trailing edges are trimmed in different scales, which are marked as model A, B, and C. As a result of trailing edge trimming, the impeller outlet angles at the pressure side of the prototype of model A, B, and C are 21, 18, 15, and 12 degrees, respectively. The differences in Euler head distribution and pressure fluctuation between the model impellers at nominal flow rate are investigated and analyzed. Experimental verification is also conducted to validate the CFD results. The results show that the blade trailing edge profiling on the pressure side can help reduce pressure fluctuation. The uniformity of Euler head circumferential distribution, which is directly related to the intensity of pressure fluctuation, is improved because the impeller blade outlet angle on the pressure side decreases and thus the velocity components are adjusted when the blade trailing edge profile is modified. The results of the investigation demonstrate that blade trailing edge profiling can be used in the vibration reduction of low specific impellers and in the engineering design of centrifugal pumps.

Grid-free least-squares method for pressure evaluation from LPT data

Lagrangian particle tracking Shake-the-box (STB) method (Schanz et al., 2016) acquires the 3D positions of tracer particles from the temporal sequences of their 2D projection images even for rather high seeding densities. Approximation of tracks by analytical functions (Gesemann, 2015) provides an accurate evaluation of tracers’ local velocity and acceleration. This data, which is obtained on non-regular grid, can be used to estimate local pressure fluctuations based on the Navier–Stokes equation. The present paper describes a grid-free least-squares method for the gradients and pressure evaluation based on irregularly scattered LPT data with random noise minimization.

Pressure evaluation from Lagrangian particle tracking data using a grid-free least-squares method

The Lagrangian particle tracking shake-the-box (STB) method provides accurate evaluation of the velocity and acceleration of particles from time-resolved projection images for high seeding densities, giving an opportunity to recover the stress tensor. In particular, their gradients are required to estimate local pressure fluctuations from the Navier–Stokes equations. The present paper describes a grid-free least-squares method for gradient and pressure evaluation based on irregularly scattered Lagrangian particle tracking data with minimization of the random noise. The performance of the method is assessed on the basis of synthetic images of virtual particles in a wall-bound turbulent flow. The tracks are obtained from direct numerical simulation (DNS) of an initially laminar boundary layer flow around a hemisphere mounted on a flat wall. The Reynolds number based on the sphere diameter and free stream velocity is 7000, corresponding to a fully turbulent wake. The accuracy, based on the exact tracks and STB algorithm, is evaluated by a straightforward comparison with the DNS data for different values of particle concentration up to 0.2 particles per pixel. Whereas the fraction of particles resolved by the STB algorithm decreases with the seeding density, limiting its spatial resolution, the exact particle positions demonstrate the efficiency of the least-squares method. The method is also useful for extraction of large-scale vortex structures from the velocity data on non-regular girds.

Monitoring of the bubble columns hydrodynamics by recurrence quantification data analysis

Hydrodynamics of bubble column in the presence of different collectors was investigated by recurrence quantification analysis (RQA). By measuring the local pressure fluctuations of the gas-liquid bubble column by a piezo-resistive sensor and transforming the data to the state space, the recurrence plots were generated. Then, using the RQA, recurrence rate, determinism, and entropy of the plot were calculated. The RQA allows characterizing the flow regime and investigating the effect of adding various collectors, such as amine and ester, in square bubble columns. The results showed that determinism and entropy are sensitive parameters to detect the hydrodynamic changes in the bubble column. Furthermore, the transition range from homogeneous to heterogeneous regime was detected for tap water–air (4.4–5cm/s), 50ppm amine aqueous solution–air (4.4–5cm/s), and ester aqueous solution–air (2.5–3.5cm/s) systems. By examining the effect of adding collectors, it was found that entropy performs much better in detecting the changes in the hydrodynamics of the bubble column. For example, by adding 5ppm ester to water, entropy changed 2.5 times more than determinism. As a result, entropy can be considered as the proper recurrence parameter for investigating the hydrodynamics of bubble columns.

Cerebrospinal Fluid Cavitation as a Mechanism of Blast-Induced Traumatic Brain Injury: A Review of Current Debates, Methods, and Findings.

Cavitation has gained popularity in recent years as a potential mechanism of blast-induced traumatic brain injury (bTBI). This review presents the most prominent debates on cavitation; how bubbles can form or exist within the cerebrospinal fluid (CSF) and brain vasculature, potential mechanisms of cellular, and tissue level damage following the collapse of bubbles in response to local pressure fluctuations, and a survey of experimental and computational models used to address cavitation research questions. Due to the broad and varied nature of cavitation research, this review attempts to provide a necessary synthesis of cavitation findings relevant to bTBI, and identifies key areas where additional work is required. Fundamental questions about the viability and likelihood of CSF cavitation during blast remain, despite a variety of research regarding potential injury pathways. Much of the existing literature on bTBI evaluates cavitation based off its prima facie plausibility, while more rigorous evaluation of its likelihood becomes increasingly necessary. This review assesses the validity of some of the common assumptions in cavitation research, as well as highlighting outstanding questions that are essential in future work.

An Adaptively-damped Compressible-liquid Model for Non-cavitating Hydraulic Surges

This research presents a compact and computationally-efficient two-equation compressible-liquid model. The model is specifically developed for the numerical computation of hydraulic surges in pipes under high fluid pressure where cavitation is absent. The proposed model aims to simplify the three-equation model of Neuhaus et al. for two-phase cavitational hammers. Compressible effects in liquid during the transients are considered by including a suitable equation of state into the model. A tunable function of the relative local pressure fluctuation called 'Variable Friction Coefficient' (VFC) for the flow transients is also incorporated into the model. For the accurate modeling of wave propagation, the split-coefficient matrix (SCM) method for characteristic-direction based splitting of eigenvalues is used in the study. The results show that the proposed two-equation model can reproduce the results from the three-equation model at a substantially reduced computational cost. The integration of the variable friction coefficient into the two-equation compressible-liquid model further improved the solver capability. The results computed using this aggregate solver are superior to the original three-equation model and the two-equation model without VFC. The results also suggest that the variable friction coefficient imparts adaptive damping capability to the solver model. This feature of the model is visible in the improved accuracy in the modeling of decaying pressure waves. The aggregate solver model, i.e., `the variable friction coefficient integrated two-equation compressible-liquid model,' offers a greatly simplified mathematical model and an inexpensive computational solver for the simulation of hydraulic surges in non -cavitating flow transients.

Effect on the drag coefficient of various spiked cylinders during buzz phenomenon subjected to hypersonic flows

Pulsation mode of transient flow behavior may occur in a violent manner in case of spiked cylinders depending on the geometry, type of spikes, and free stream conditions. Drastic pressure fluctuations near the face of the afterbody have been observed in most cases of pulsating flow mode and thus resulting in drastic variations in the coefficient of drag of the spiked body. The transient analysis of variations in Cd with spike length for various spiked cylinders have been carried out in the current work. The variations in Cd during one complete cycle as well as variations in time-averaged value of Cd with increase in L/D ratios have been analyzed. These variations have been explained with respect to the localized pressure fluctuations, shift in foreshock–aftershock interaction region, effect of effective shielding, and increasing spike lengths. It was found that aerodisk spiked cylinder had the least time-averaged Cd. The Cd for various other spiked bodies has also been summarized over a range of feasible spike lengths and an optimum design of the spiked body in terms of the spike shape and L/D has also been suggested.