This study presents a unified geometrical-theory framework for predicting the flow-characteristics of nozzle-flapper valves and annular restrictors employed in thrust-bearing applications. Building on the minor-loss formulation introduced in a previous work, the approach separates geometric effects from discharge phenomena, enabling accurate regression of the flow-coefficient K across both incompressible and compressible regimes. The theory incorporates the simulation of choking and a blended discharge-coefficient model that transitions smoothly between exponential and Busemann forms. Parameter identification is performed with a Levenberg-Marquardt algorithm, calibrated against experimental data from the Hayashi (nozzle-flapper) and Belforte (aerostatic restrictor) test cases. Results demonstrate that the unified model reproduces the measured K trends-including the decline of K at high Reynolds numbers and the onset of choking-more faithfully than the conventional equation proposed in literature, while requiring far fewer computational resources than full computational fluid dynamics (CFD) simulations. Sensitivity analyses reveal the dominant influence on both geometric loss coefficients and discharge behavior of the design parameters tuned in the previous work, i.e., flow path parameter and Reynolds number. The validated framework offers a rapid, physics-based tool for designers of precision pneumatic systems and aerostatic bearings, facilitating early-stage sizing, performance optimization, and integration with Reynolds-equation-based load calculations without resorting to intensive numerical modeling.
The linear stability of plane Poiseuille flow of a viscoelastic Navier-Stokes-Voigt (Kelvin-Voigt-type) fluid is analyzed under a transverse magnetic field. High-accuracy spectral Chebyshev collocation and Galerkin methods are employed to solve the resulting modified Orr-Sommerfeld stability equations. Unlike earlier studies, this work links energy budget analysis with nonmodal growth suppression to reveal distinct viscoelastic and magnetic dissipation mechanisms across the channel. The modal stability analysis, including eigenspectrum and temporal growth rate evaluations, reveals that both viscoelastic effects (parameterized by Lambda) and magnetic damping (via the Hartmann number M) exert strong stabilizing influences by suppressing Tollmien-Schlichting (TS) instabilities and shifting the critical Reynolds number to higher values. Neutral stability curves and growth-rate profiles confirm that increasing Lambda and M enlarges the stable parameter space, delaying the onset of linear instability. Beyond modal analysis, transient energy growth calculations capture the potential for nonmodal amplification in linearly stable regimes. Results show that both Lambda and M significantly reduce the peak transient energy growth and expedite its decay, thereby limiting the risk of subcritical transition. The epsilon-pseudospectrum analysis supports these findings, illustrating that the extent of spectral bulging into the unstable region diminishes with increasing damping. An in-depth energy budget analysis highlights spatially localized stabilization: viscoelasticity acts near the walls by attenuating shear production and enhancing Voigt-type regularization-induced dissipation, while magnetic damping dominates in the channel core via enhanced Lorentz force-induced dissipation. The results obtained using the Kelvin-Voigt-type model provide baseline insights that can guide future investigations employing more complex constitutive models such as Oldroyd-B or FENE-P.
Skin friction developments resulting from coaxial and noncoaxial collisions between a Re Gamma 0=3000 vortex ring and stationary spheres have been explored using large-eddy simulations (LES). The diameter ratio ( D/d), which is the ratio between sphere diameter (D) and initial vortex ring diameter (d), ranges from 1 to 4. Axis of the initial vortex ring is offset by delta/D=1/8- 1/2 to explore the effects of offset distance on skin friction evolution. For coaxial collisions, collision of primary vortex ring with the sphere usually produces the largest skin friction magnitude, which also increases with the diameter ratio. However, high-magnitude skin friction levels cover increasingly smaller regions as the diameter ratio increases. For noncoaxial collisions, skin friction evolutions differ significantly between the end closer to the sphere axis and the end further from it. Skin friction magnitude resulting from the collisions of primary vortex ring at the end closer to the sphere axis is considerably higher than that at the end further from the sphere axis, despite being more confined by the blockage imposed by the sphere top. Nevertheless, the skin friction induced by the primary vortex core at the end further from the sphere axis is often relatively lower but extends further downstream.
Wave measurements from fixed gauges in a flume produce time-series of surface elevation. To characterize the waves and enable a comparison with theory, additional spatial information is needed. This is commonly obtained by simultaneous measurements at pairs of gauges with spacing much smaller than a typical wavelength, assuming the surface profile to be frozen between the gauges. Although a "wave" can be defined as the signal between successive positive or negative zero-crossings, between successive crests, or between successive troughs, experimental measurements are not agnostic to the choice of metric. We show that wave-by-wave measurements of phase velocity or wavelength are most stable and consistent over repeated experiments when using wave-by-wave cross-correlation, as opposed to the passage of zero-crossings, crests, or troughs. The high variability of these other metrics may contribute to some of the difficulty in measuring nonlinear wave properties in flume experiments.
Fighter aircraft should be capable of safe takeoff and landing under strong crosswind conditions; thus, studies on the aerodynamics of delta wings under the coupling of static ground effect (SGE) and sideslip are of considerable significance. In this paper, the aerodynamic characteristics and underlying flow physics of the standard delta wing model VFE-2 under SGE and sideslip coupling are systematically investigated using the computational fluid dynamics method and the full-process vorticity transport analysis method. At high flight heights (H/CR > 0.5), the sideslip is the primary contributor to the aerodynamic forces while the effect of SGE is negligible. Only at low flight heights (H/CR < 0.5) does the SGE-sideslip coupling become increasingly prominent. This coupling governs the aerodynamic forces and flow patterns on the lower wing surface: the SGE primarily increases pressure across the entire span via the ground blocking effect; while the sideslip primarily affects the local flow around the reattachment lines, moving the reattachment lines toward the windward edge, increasing the pressure along the reattachment line on the windward side and decreasing it on the leeward side. In contrast, the sideslip remains the dominant factor to the asymmetric evolution of leading-edge vortex (LEVs) on the upper wing surface as well as the corresponding aerodynamic forces. As the sideslip angle increases, the windward LEV breaks down prematurely before bending toward the freestream, while the leeward LEV remains concentrated and gradually straightens to align with the freestream.
A three-dimensional numerical model was developed to investigate the dynamic characteristics of two discontinuous gas jets emerging from two launch tubes of an unmanned underwater vehicle (UUV), aiming to predict the complex multiphase flow and its effect on impact loads applied to the UUV. The accuracy of the developed model was verified through experimental validation. Furthermore, the unsteady behaviors of the two discontinuous gas jets under various launch conditions (including simultaneous launch, inconsistent chamber pressure, and launch timing) were obtained through comparative analysis. The results show that two discontinuous jets emerging simultaneously from two jet bubbles, and then undergo repeated cycles of expansion and contraction following their coalescence. Resultantly, negative drags on the UUV are substantial. Inconsistent chamber pressure in the two launch tubes results in differences in the pulsation period and dimensions between the jet bubbles. Consequently, asymmetric radial loads are imposed on the UUV. Moreover, inconsistent launch timing alters the evolutionary patterns, causing the two jet bubbles to evolve independently and leading to the disappearance of coalescence. In this case, the loads of the UUV are governed by the phase difference between the two jet bubbles.
As the core component of CO2 heat pump air conditioning (HPAC) systems for electric vehicles, the performance of scroll compressors directly affects system energy efficiency and vehicle driving range. This study, for the first time, combines computational fluid dynamics (CFD) simulations with a one-dimensional dynamic model to systematically analyze the impact of unilateral axial clearance on the pressure characteristics of working chambers and the tangential gas force in CO2 scroll compressors. The research reveals that interstage leakage induced by unilateral axial clearance disrupts pressure uniformity in symmetrical working chambers, resulting in a maximum pressure difference exceeding 2 MPa. Dynamic simulations further demonstrate that positioning the unilateral axial clearance at the top of the orbiting scroll significantly improves compressor performance: the peak pressure difference in symmetrical working chambers is reduced by 8.09%, the maximum tangential gas force decreases by 818.29 N, and average power consumption drops by 30.43%. Additionally, when the unilateral axial clearance is located at the top of the orbiting scroll, it facilitates early leakage of high-pressure gas from the middle discharge ports through indirect flow paths, effectively suppressing pressure fluctuations. Therefore, optimizing the manufacturing tolerance matching between the orbiting and fixed scrolls to ensure the unilateral axial clearance is positioned at the top of the orbiting scroll represents a critical approach for enhancing the performance of CO2 scroll compressors.
This study investigates the effects of runner blade trailing-edge material loss on the internal flow characteristics and pressure pulsations of a Francis turbine. Transient numerical results demonstrate that material loss weakens the flow-guiding capability of the blades, inducing kinematic slip in the circumferentially averaged outlet velocity triangle. This leads to an increase in the relative flow angle (beta), which significantly enhances the tangential velocity (V-u) and the swirl number (S-r). As the material loss b increases, the expansion of the low-velocity backflow zone exacerbates the blockage effect within the draft tube, resulting in a continuous decline in hydraulic efficiency. Frequency-domain analysis reveals that blade material loss significantly amplifies pressure pulsations at the 0.3f(n) characteristic frequency. Specifically, at monitoring point SC01, a notch width of 15 mm leads to increases in pressure pulsation amplitude of 361.6%, 490.6%, and 466.2% under 100%, 95%, and 90% load conditions, respectively. In contrast, medium-load conditions (75%-85%) exhibit a nonlinear response to the increase in V-u induced by material loss. Ultimately, these findings demonstrate that variations in 0.3f(n) serve as a robust indicator for monitoring runner blade material loss, with optimal diagnostic sensitivity achieved at S-r < 0.4 in the draft tube.
This research investigates the formation and dynamic behavior of a single gas bubble in de-ionized water subjected to negative pressure conditions (gauge pressure). Based on this framework, a systematic investigation was conducted to examine the influence of the intake pipe's inner diameter, liquid level height, and gas space volume on bubble formation and ascent dynamics. Experimental findings indicate that the bubble generation process can be divided into two distinct phases: a growth phase and a detachment phase. The size at detachment and the subsequent motion characteristics are governed by multiple factors, among which the intake pipe's inner diameter has the most pronounced effect. Specifically, an increase in pipe diameter corresponds to a significant enlargement in both the bubble detachment diameter and equivalent diameter, accompanied by intensified interface deformation and more pronounced fluctuations in the rising trajectory. Conversely, under conditions of smaller pipe diameters, bubbles tend to be smaller, and exhibit more stable motion. Increasing the liquid level height results in greater static pressure on the bubble, thereby extending the growth duration, augmenting the detachment, and equivalent diameters, and amplifying motion fluctuations during the ascent phase. An increase in gas space volume prolongs the duration of negative pressure extraction, enabling the bubble to accumulate more gas prior to detachment, which leads to a modest increase in detachment size; however, its impact on trajectory, velocity, and deformation during the rising phase remains comparatively limited. The research findings elucidate the fundamental dynamic mechanisms governing bubble evolution under conditions of negative pressure, providing novel experimental evidence that advances the understanding of bubble formation and motion in such environments.
Reciprocating cryogenic pumps can effectively enhance the energy efficiency of power systems. However, cavitation phenomenon caused by pressure drops can severely compromise the safety and reliability of the pump. In this regard, this study attempts to illustrate the inlet cavitation characteristics of a reciprocating liquid nitrogen pump based on computational fluid dynamics (CFD) simulations. The cavitation behavior under different inlet pressures, gas contents, and subcooling conditions is also discussed. It is found that under saturated inlet conditions, severe cavitation occurs in both the intake region and the cylinder, and the vapor volume fraction varies by up to 5.86% under different inlet pressures. Subsequently, it is displayed that the vapor volume fraction inside the cylinder is nearly independent of the inlet gas content. Furthermore, increasing the inlet subcooling gradually reduces the effect of cavitation, and at approximately 3 K subcooling, the cylinder is expected to remain in a pure-liquid state. This paper provides insights into the inlet cavitation characteristics to elucidate the actual suction progress of reciprocating cryogenic liquid nitrogen pumps and offers guidance for cavitation suppression.
Understanding flow dynamics inside centrifugal pumps under off-design operating conditions is critical for their design and operation. Biglobal resolvent analysis of turbulent flow inside a model centrifugal pump at a high flow rate past nominal operating condition is presented. At this off-design condition, the flow near the volute tongue exhibits high level of unsteadiness, characterized by a dominant reverse flow on its volute side and impingement-induced vortex formation on its discharge side. On the discharge side of the tongue, the impeller-tongue interactions lead to the formation of an unsteady boundary layer and large-scale turbulent wake. By means of resolvent (input-output) analysis, we reveal the existence of key modes and frequencies that are responsible for the sustained flow unsteadiness. The optimal resolvent modes have the spatial and temporal scales that agree with the oscillations in the boundary layer, indicating a strong correlation between the impingement-induced vortex formation and the perturbation receptivity. The dominant forcing modes appear on the impeller side of the tongue and overlap with the region where reversed flow occurs. The dominant response modes appear on the discharge side of the tongue, containing both boundary layer and wake structures. These findings offer insights into the perturbation amplification mechanisms in centrifugal pumps at off-design operating conditions and highlight the sensitive nature of the tongue region, which can support improved pump designs and flow control strategies.
A detailed experimental investigation was carried out to evaluate the effect of elliptical sharp-tipped shallow (ESTS) lobed nozzles on ejector performance. The study included seven nozzle configurations: a baseline conical nozzle and six ESTS lobed variants, all identical up to the throat but differing in the number of lobes in the diverging section. Tests were conducted under consistent conditions with a primary stagnation pressure of 4 bar (gauge). To assess their integration into practical systems, the impact of mixing duct geometry was also examined, with variations in duct length aimed at achieving more compact ejector designs without compromising performance. Performance was evaluated using wall pressure measurements and secondary mass flow rates. The results revealed that lobed nozzles significantly improved mixing efficiency, with one configuration (E3) achieving up to 33% higher efficiency compared to the conical baseline. Furthermore, by optimizing the mixing duct length to match the higher mixing rate enabled by lobed nozzles, the overall ejector length was reduced by as much as 91%, demonstrating their potential in compact system applications. Importantly, the study underscores the need to redesign the mixing duct when adopting lobed nozzles. Without appropriate geometric modification, enhanced mixing causes the flow to experience frictional losses in the remaining duct length, leading to reduced compression performance. These findings highlight that while lobed nozzles offer substantial benefits, they must be integrated with tailored mixing duct geometries to realize their full performance potential.
Radial pumps and compressors are used in various engineering applications, including rocket turbopumps, automotive turbochargers, and refrigeration systems. Several physical effects, including viscous losses, flow separation, compressibility, and rotational dynamics, dominate radial impeller flow, making flow field prediction very difficult and requiring computationally expensive computational fluid dynamics (CFD). However, designers typically only require information at specific positions, resulting in most simulation data being unused. Accurately predicting the impeller exit flow field is often key to impeller design. Recent advances in reduced-order modeling and machine learning show promise for a priori flow field prediction. In this study, nine different reduced-order models (ROMs) were created to predict the dimensionless exit flow field of radial flow impellers in real-time. The ROMs consist of various linear and nonlinear dimensionality techniques paired with different regressors. Inputs include parameterized, dimensionless impeller geometry based on Bezier control points, number of blades, and dimensionless operating conditions. The ROMs were trained using over 1800 flow fields from high-fidelity CFD simulations representing a large radial flow compressor design space. ROMs were evaluated on relative error, training time, and evaluation time. Principal component analysis coupled with Gaussian process regression (PCA-GPR) emerged as the preferred ROM, training within 2 s, evaluating hundreds of cases in real-time, and matching the accuracy of computationally demanding nonlinear alternatives. PCA-GPR predictions show pressure, density, and velocity profiles within 5% average of CFD results. The ROM was validated through four test cases probing robustness across different operating conditions and geometries.
Can an industrial computational fluid dynamics (CFD) tool like ansys fluent perform direct numerical simulation (DNS) reliably? We answer by providing a combination of a feasibility report and a best-practice guide that will be useful for researchers and practicing engineers as pressure mounts to transition DNS from academia to industry. DNS has historically been the realm of efficient but somewhat restrictive spectral solvers. DNS has been validated on finite volume method (FVM) solvers, but many of these works have been on structured grids with high-order stencils. This paper explores canonical channel flow at Re tau=180 using ansys fluent, a commercial FVM code, with lower order methods. Additionally, for the first time, this paper provides direct comparisons of the diffusion and dispersion of fluent's momentum discretization schemes on six easily reproducible grids: two grid spacings, each of grids composed of triangular prisms, hexahedrons, and polygonal prisms. We also demonstrate the substantial increase in solution speed offered by GPUs. While velocity profiles are predicted with moderate error, wall-normal profiles of turbulent kinetic energy budgets exhibited significant error, and large amounts of spectral pileup were visible. Therefore, it can be concluded that, for a canonical flow configuration, ansys fluent is only partially "ready." While it cannot be said that all FVM DNS is problematic for all possible flows, we prove, through a simple flow scenario, that healthy skepticism should be applied when the term "DNS" is used in conjunction with ansys fluent and FVM codes, which use similar stencils.
This study examines the flow over a wall-bounded bump geometry using wall-modeled large-eddy simulations (WMLES) in the limit of very thin wall resolution. The geometry and flow conditions are based on an experimental investigation of a canonical geometry designed to replicate diffusion rates similar to those in low-pressure turbine blades. The high subsonic Mach and low Reynolds number upstream of the bump generate strong pressure gradients, both favorable and adverse. As the incoming turbulent boundary layer encounters the bump surface, it undergoes relaminarization, shock-wave interaction and subsequent separation. An inadequate grid resolution near the wall is found to suppress the formation of streamwise vortices during relaminarization, resulting in a different mixing process within the separated boundary layer and a longer separation bubble. To prevent this, extending the spatial region of high refinement to at least 3 delta 99 of the relaminarization region is recommended. The study also investigates the impact of freestream isotropic turbulence on the pressure distribution and separation, revealing that even a small variation in inflow turbulence intensity is sufficient to induce relevant changes in the mean flow results.
Conventional signal decomposition of flow visualization data with a limited field of view (FOV) fails to accurately identify cavitation instabilities in turbopump inducers. To overcome this limitation, a new deep learning-based framework is proposed to directly analyze visualization data and identify such instabilities. The framework employs a hybrid convolutional-recurrent neural network to extract spatiotemporal features of instability modes. The trained model correctly identifies two physically distinct instability modes-alternate blade cavitation (ABC) in a two-bladed inducer and supersynchronous rotating cavitation (RC) in a three-bladed inducer-consistent with ground-truth pressure measurements. Sensitivity analyses show that the model correctly classifies instability modes with input durations as short as one-sixth to one-half of the training sequence length and maintains over 90% accuracy when the field of view is reduced to approximately 80% of the original extent. These degraded input data conditions are not explicitly included during training, yet the model sustains its performance without retraining, demonstrating the robustness of the deep learning-based approach to limited observation conditions.
In the context of CO2 emissions reduction, internal combustion engines are being replaced by electric motors in many applications. In hydraulic fluid power, this transition requires gear pumps to operate at much higher speeds than traditional systems, increasing the risk of incomplete chamber filling. This issue leads to significantly increased noise levels. Supplying the variable-volume chambers from both sides of the rotor is a well-known method for improving filling. This study investigates various inlet port placements for dual-sided feeding using a validated computational fluid dynamics (CFD) model of a gerotor pump. Two axial inlet positions, aligned with the shaft, and three radial positions are investigated. In addition, the axial-flow configuration is evaluated under varying inlet duct angles, while a slide is implemented in the radial configuration to assess its effect on flow direction. For radial feeding, the axial alignment of rotors relative to the housing space is evaluated, comparing symmetric and asymmetric configurations. Results indicate that axial feeding outperforms radial feeding, with optimal performance achieved when the duct is aligned with the variable-volume chambers. For radial feeding, a perfectly symmetrical configuration is preferable, though slightly asymmetrical setups yield comparable performance. Conversely, highly asymmetrical designs, while superior to single-side feeding, fail to fully exploit the advantages of dual-side feeding. This research demonstrates that, under high-speed operating conditions, CFD-based optimization of the suction volume can significantly improve volumetric efficiency by several percentage points.
Flow around two side-by-side circular cylinders is investigated by an implicit large eddy simulation (ILES) at the spacing ratio T / D = 2.0 and Reynolds number Re = 3900 . First, the results of the stationary case show that the flow of two stationary side-by-side circular cylinders is characterized by the biased flow pattern and the flip-flopping phenomenon. The generation mechanism is attributed to the relative development degree between the two gap-side shear layers and the merging tendency between the gap-side shear layer of one cylinder and the outside vortices from the other cylinder, which generates a strong pulling effect acting on the gap-side shear layer to pull it toward one side and deflects the gap flow. Then, two rotation control methods, namely, forward rotation and backward rotation, are introduced to control the biased flow pattern and the flip-flopping phenomenon. The numerical results show that the control mechanisms are different. The forward rotation enlarges the distance between the shear layers, so the merging process can hardly occur, and the gap flow is not biased. The backward rotation shortens the shear layer lengths and the distance between them, contributing to a weak interaction that can hardly pull the shear layer toward one side.
The current high-accuracy fluid-solid coupling algorithms based on immersed boundary method with a multidirect forcing scheme are not efficient due to the absence of a criterion to properly stop the cycles of the multidirect forcing scheme with acceptable precision to avoid redundant calculations. Therefore, a criterion based on the immersed forces on different cycles is proposed to end the unnecessary calculations after satisfactory results are obtained during the simulation. The numerical tests are based on immersed boundary-lattice Boltzmann method (IB-LBM) with a case of an oscillating sphere. The results have shown that the criterion proposed is more efficient than the fixed-number multidirect forcing scheme to achieve the same accuracy, and the time consumed by the multidirect forcing scheme with the criterion is significantly less. To extend this approach to other simulations, the threshold of error can be determined by the case configurations.
Actively monitoring pressure in fluids is essential in a variety of engineering applications. As industry trends point toward increased exploration and use of deep-sea and ocean environments, it is crucial to design and evaluate supporting pressure instrumentation. Of particular importance are micro-electromechanical-systems (MEMS) that combine reduced size and cost when compared to other relevant techniques. This work showcases the performance of MEMS-based piezoresistive pressure sensors; specifically, their multifunctional capabilities in monitoring static, acoustic, and turbulent events. Such multifunctional MEMS sensors were evaluated through extensive water tunnel testing. Two sensors were surface-mounted into a NACA 0018 hydrofoil and evaluated under a range of angles of attack (AoAs) and flow speeds. The results highlight the sensors' proficiency in monitoring static flow perturbations, acoustic events, and small-scale pressure fluctuations associated with turbulence. Static flow results were validated against flow visualization to analyze flow separation states. Acoustic events (produced from tonal noise) reveal key behaviors in the frequency domain and possess strong alignment with published literature. The acoustic response addresses the underexplored research area of tonal noise in underwater settings. Lastly, behavior consistent with turbulent decay was reported in high AoA flows, further emphasizing the sensors' multifunctional capabilities. The combined functionality of the MEMS sensor within a single sensing element invites the opportunity for use in real-world applications to streamline the performance of various engineering systems.
