Flow-induced Instabilities Research Articles

Effect of the Tesla Valve on the heat transfer performance and the suppression of pressure drop oscillation in a liquid cooling loop

The suppression of pressure drop oscillation within a liquid cooling loop is necessary to maintain the system stability and develop a steady heat transfer performance. The Tesla Valve is proposed as a means of eliminating this instability by suppressing the backward flow, which in turn facilitates a stable heat transfer process. The steady-state analysis is performed to identify the conditions that would induce flow instability. The calculated results indicate that the flow instability will appear when the heat flux (q) reaches a threshold value of 28 W/cm2. Furthermore, the impact of the Tesla Valve on the suppression of instability and the heat transfer coefficient is revealed by comparison with the dynamic features observed in a common liquid cooling loop. Two flow instability regimes, with and without backflow, are observed in the common liquid cooling loop. In the case of the former instability, a violent fluctuation in both the inlet temperature and the heat transfer coefficient is captured. This phenomenon can be effectively eradicated by the Tesla Valve. In the latter instability regime, the hydrodynamic characteristics of pressure drop remain significantly oscillatory, while the heat transfer performance returns stable. Furthermore, this specific form of instability cannot be eliminated by the prototype I of Tesla Valve when q ≥ 30.4 W/cm2. In addition, the liquid cooling loop with prototype III displays a superior heat transfer performance, yet results in an additional pressure drop of 8.1 kPa in comparison to prototype II. To assess the suppression mechanism and efficiency, the Suppression Factor (Fs) and Coefficient of Suppression Performance (Fc) are proposed. It can be found that the pressure drop oscillation instability can be thoroughly suppressed by the Tesla Valve when Fs > 1. Meanwhile, the greater the Fc, the more effective the suppression performance. However, the incorporation of the Tesla Valve shows a negligible effect on the heat transfer performance in the two-phase region.

Kirigami sheets in fluid flow

Kirigami patterned materials have found several applications in recent years due to their ability to assume complicated shapes and exhibit emergent physical properties when exposed to external forces. Consisting of an array of cuts in a thin material, fabrication of these patterns can be quite simple. Here we show that when they are placed in fluid flow, kirigami cut sheets with various patterns produce a variety of flow patterns in the wake. Through several sets of experiments, we show that kirigami sheets placed in flow can undergo static or dynamic flow-induced instabilities as a result of which they can buckle or undergo limit cycle oscillations, or they can remain stable while undergoing very large elongations. These structural responses produce several different types of fluid patterns in the wake. We show that vortices both at small scales (scales comparable with the size of the individual kirigami cuts) and large scale (scales comparable with the size of the sheet) are formed in the wake of kirigami sheets. We also show that jets of different sizes can be formed and directed passively using kirigami sheets. These results show the potential of kirigami sheets for passive flow control.

Blind zones in radiating dispersion at high Péclet number driven by non-Newtonian fluids in porous media

A wide range of environmental, energy, medical and biological processes rely on dispersive transport through complex media. Yet, because of the stagnant and opaque nature of the microscopic system, the role of disordered flow and structure in the dispersive transport of solutes remains poorly understood. Here, we use a circular porous microfluidic system to investigate the radial dispersion in porous media driven by non-Newtonian fluids with strong advection rate (or at high Péclet number) and low-to-moderate Reynolds numbers. We observe for the first time the presence of diffusion ‘blind zones’ in the microstructure for high solution injection velocities. More specifically, an in-depth analysis uncovers that the circumferential flow frame, coformed by obstacles and vortices especially the ‘twin-vortex’ with same rotation direction, is responsible for the diffusion ‘blind zones’ and transport heterogeneity. The vortices are induced by the coupling of microfluidics and porous structures, and correlated to inertial flow-induced instabilities. The trade-off between diffusion efficiency and quality/completeness with respect to the high Péclet number (or high inlet velocity) serves to enhance our comprehension of intricate fluid dynamics and affords a set of principles to aid a diverse range of practical implementations.

Investigating the occurrence of flow instabilities in coastal unconfined aquifers using sand tank experiments

Subterranean estuaries form the land-sea transition zone in coastal aquifers and are relevant for coastal freshwater management and coastal ecology. Tidal fluctuations typically lead to the formation of upper saline plumes, where denser saltwater circulates on top of the discharging, less dense freshwater. Extensive numerical simulations and only a few experimental trials previously demonstrated that these saline plumes may become unstable and exhibit salt fingering flow. This work used laboratory sand tank experiments to study the formation and evolution of tide induced flow instabilities in homogenous, unconfined coastal aquifers in a systematic manner, addressing the combined effects of relevant parameters such as sediment type, beach slope, fresh groundwater inflow, tidal amplitude, and tidal period variation. Some of these parameters have never been targeted in laboratory-based studies before and a comparable systematic and comprehensive approach has so far not been conducted. The resulting flow patterns were studied visually through the application of a colour tracer to the saltwater. The results show that the upper saline plume may be stable, unstable, or transitional, whereby the beach slope predominantly defines its state. When salt fingers form, they amplify the mixing zone between ground- and saltwater and force the freshwater to discharge at multiple locations along the beach. The processes were best visible in the coarsest sediment (1.02 × 10-2 m s−1). The finer grained the sediment was, the smaller the upper saline plume that formed. The present study further reveals the possibility of three-dimensional fingering flow, even though the physical model was intended to represent a two-dimensional cross-section. The application of a previously proposed non-dimensional stability diagram yielded reasonably accurate predictions of stability behaviour in coastal unconfined aquifers under laboratory settings. This further supports the plausibility of flow instabilities in subterranean estuaries within specific ranges of influencing hydrogeological factors.

Deep reinforcement learning-based active flow control of vortex-induced vibration of a square cylinder

We mitigate vortex-induced vibrations of a square cylinder at a Reynolds number of 100 using deep reinforcement learning (DRL)-based active flow control (AFC). The proposed method exploits the powerful nonlinear and high-dimensional problem-solving capabilities of DRL, overcoming limitations of linear and model-based control approaches. Three positions of jet actuators including the front, the middle, and the back of the cylinder sides were tested. The DRL agent as a controller is able to optimize the velocity of the jets to minimize drag and lift coefficients and refine the control strategy. The results show that a significant reduction in vibration amplitude of 86%, 79%, and 96% is achieved for the three different positions of the jet actuators, respectively. The DRL-based AFC method is robust under various reduced velocities. This study successfully demonstrates the potential of DRL-based AFC method in mitigating flow-induced instabilities.

Experimental investigation of power transient flow boiling

To advance the mechanistic understanding of power transients in flow boiling, experimental investigations are performed to capture the thermal response of four different kinds of cladding materials, including FeCrAls, to Fuchs reactivity initiated accident transients and linear ramp power transients. The transition boiling regime is phenomenologically reflected in power-transient flow boiling, which is different from steady-state boiling. In light of this, the critical heat flux that is usually featured with temperature overshooting does not indicate thermal safety margin of cladding materials as conservatively as the maximum heat flux, which is greater than critical heat flux in the power transient boiling curve. This is physically attributed to the thermal energy deposition on the cladding wall. In addition, the thermal energy released from the cladding wall results in different boiling heat transfer coefficient for the decreasing and increasing power stages respectively during the Fuchs power transient. The maximum heat flux difference gap, which appears due to changes in cladding materials and transient time scales, is appreciable under an intermediate heat convection regime. However, this difference gap is gradually weakened by the progressive increasing of mass flux and/or inlet subcooling due to the enhanced dominance of heat convection over heat conduction. The small difference gap of maximum heat flux is found to be insignificant under the weak heat convection because of power transient induced annular flow instability.

Investigation of Flow-Induced Instabilities in a Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load Conditions

The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR.

Flow-induced surface instabilities in a flexible-walled channel with a heavy wall

We consider the stability of laminar high-Reynolds-number flow through a planar channel formed by a rigid wall and a heavy compliant wall under longitudinal tension with motion resisted by structural damping. Numerical simulations indicate that the baseline state (with Poiseuille flow and a flat wall) exhibits two unstable normal modes: the Tollmien–Schlichting (TS) mode and a surface-based mode which manifests as one of two flow-induced surface instabilities (FISI), known as travelling wave flutter (TWF) and static divergence (SD), respectively. In the absence of wall damping the system is unstable to TWF, where the neutrally stable wavelength becomes shorter as the wall mass increases. With wall damping, TWF is restricted to long wavelengths through interaction with the most unstable centre mode, while for wall damping greater than a critical value the system exhibits an SD mode with a two branch neutral stability curve; the critical conditions along the upper and lower branches are constructed in the limit of large wall damping. We compute the Reynolds–Orr and activation energy descriptions of these neutrally stable FISI by continuing the linear stability analysis to the following order in perturbation amplitude. We find that both FISI are primarily driven by the working of normal stress on the flexible wall, lower-branch SD has negative activation energy, while upper-branch SD approaches zero activation energy in the limit of large wall damping. Finally, we elucidate interaction between TS and TWF modes for large wall mass, resulting in stable islands within unstable regions of parameter space.

Hot deformation behavior and microstructure evolution of the spray deposited and secondary hot extruded 2195 Al–Li alloy

This work aims to investigate the influence of hot deformation parameters on the microstructure evolution and deformation behavior of the extruded 2195 Al–Li alloy, as well as to explore its optimized hot deformation windows based on the hot compression tests carried out on a Gleeble-1500 thermo-mechanical simulator in the temperature range 370–520 °C and with the strain rate of 0.001–10 s−1. The results revealed that the extruded 2195 Al–Li alloy plate with a large aspect ratio had a lower deformation activation energy with respect to the spray deposited and as-cast 2195 Al–Li alloys owing to the existence of a large number of coarse second phases and LAGBs in the extruded plate. The optimized hot deformation temperature of the alloy ranged from 405 °C to 490 °C with a strain rate from 0.001 to 0.1 s−1. Hot deformation under the optimized conditions led to the occurrence of continuous dynamic recrystallization and resultant microstructure refinement. Although the occurrence of continuous dynamic recrystallization weakened the <101> deformation texture, but triggered another texture component with <001>//ND. Deformation at low temperature and high strain rate induced flow instability due to insufficient dynamic softening. In addition, the flow instability at a temperature higher than 490 °C arose from partial remelting of Al2Cu phase along grain boundaries. The work can provide an effective guidance for the designation of secondary hot processing of the extruded 2195 Al–Li alloy.

Instabilities and sensitivities in a flow over a rotationally flexible cylinder with a rigid splitter plate

This paper investigates the origin of flow-induced instabilities and their sensitivities in a flow over a rotationally flexible circular cylinder with a rigid splitter plate. A linear stability and sensitivity problem is formulated in the Eulerian frame by considering the geometric nonlinearity arising from the rotational motion of the cylinder which is not present in the stationary or purely translating stability methodology. This nonlinearity needs careful and consistent treatment in the linearised problem particularly when considering the Eulerian frame or reference adopted in this study that is not so widely considered. Two types of instabilities arising from the fluid–structure interaction are found. The first type of instabilities is the stationary symmetry breaking mode, which was well reported in previous studies. This instability exhibits a strong correlation with the length of the recirculation zone. A detailed analysis of the instability mode and its sensitivity reveals the importance of the flow near the tip region of the plate for the generation and control of this instability mode. The second type is an oscillatory torsional flapping mode, which has not been well reported. This instability typically emerges when the length of the splitter plate is sufficiently long. Unlike the symmetry breaking mode, it is not so closely correlated with the length of the recirculation zone. The sensitivity analysis however also reveals the crucial role played by the flow near the tip region in this instability. Finally, it is found that many physical features of this instability are reminiscent of those of the flapping (or flutter instability) observed in a flow over a flexible plate or a flag, suggesting that these instabilities share the same physical origin.

Influence of the elbow shape on the unsteady pressure field in decelerated swirling flows

The induced flow instabilities are developed in the draft tube cone of the hydraulic turbines leading to the pressure fluctuations with negative impact on the mechanical components lifetime. Usually, two components (rotating and plunging) are discriminated in the unsteady pressure field induced by the swirling flow in the draft tube cone. The rotating component is associated with the precession motion of the flow instabilities (e.g. vortex rope) while the plunging part corresponds to the interaction of the swirling flow with the elbow. Extensive experimental investigations were carried out on Timisoara Swirl Generator test case in order to examine the influence of the elbow shape on the unsteady pressure field. Two elbow shapes (90° S shape elbow and 90° sharp heel) are selected to investigate their influence on the unsteady pressure field. Several geometrical configurations with pipe extensions placed between the cone and the elbow are considered for each elbow shape to determine the response of the hydraulic circuit. Two unsteady pressure signals located in opposite positions are measured on four levels placed along the cone. The Fourier spectra are determined for discriminated time series identifying both rotating and plunging components. Then, the ratio between the frequencies of the plunging and the rotating components is obtained for both elbow shapes in relation to the pipe length placed between the cone and the elbow. It is clear that the shape of the elbow and the distance between the cone and the elbow lead to a significant influence on the response of the hydraulic circuit.

Publisher's Note: “Flow-induced instabilities of springs-mounted plates in viscous flows: A global stability approach” [Phys. Fluids 33, 034133 (2021)

Publisher's Note: “Flow-induced instabilities of springs-mounted plates in viscous flows: A global stability approach” [Phys. Fluids <b>33</b>, 034133 (2021)

Advancements in reactor stability analysis methods; personal perspective

Understanding the origins and propagation of flow-induced instabilities is very important for the operation and safety of the current and next generations of boiling water reactors (BWR), as well as for future new reactor types, such as the supercritical-pressure reactors, cooled by either water or CO2. Consequently, substantial (or even extensive) efforts have been made during the past 40+ years to investigate the physical phenomena governing such instabilities, and to develop models and computational tools to simulate the dynamics of marginally-stable/unstable boiling systems. The purpose of this paper is to present an updated perspective on, and discuss new aspects of, selected studies performed by the author and his collaborators, associates and graduate students, aimed at the development of various methods for the analysis of flow-induced instabilities in both boiling and supercritical-pressure systems. The impact of model and method selection on the results of predictions will also be discussed for a large class of systems, including the neutronically-coupled oscillations. Since the current paper is a part of the NED issue commemorating 40 years since the first NURETH–series conference, the list of references includes eleven (11) related papers published before in various NURETH proceedings, as well as other related journal articles and conference papers.

Flow-induced instabilities of springs-mounted plates in viscous flows: A global stability approach

The linear stability of a typical aeroelastic section, consisting in a rectangular plate mounted on flexion and torsion springs, is revisited here for low-Reynolds-number incompressible flows. By performing global stability analyses of the coupled fluid-solid equations, we find four types of unstable modes related to different physical instabilities and classically investigated with separate flow models: coupled-mode flutter, single-mode flutter, and static divergence at high reduced velocity U* and vortex-induced vibrations at low U*. Neutral curves for these modes are presented in the parameter space composed of the solid-to-fluid mass ratio and the reduced velocity. Interestingly, the flutter mode is seen to restabilize for high reduced velocities thus leading to a finite extent flutter region, delimited by low-U* and high-U* boundaries. At a particular low mass ratio, both boundaries merge such that no flutter instability is observed for lower mass ratios. The effect of the Reynolds number is then investigated, indicating that the high-U* restabilization strongly depends on viscosity. The global stability results are compared to a statically calibrated Theodorsen model: if both approaches converge in the high mass ratio limit, they significantly differ at lower mass ratios. In addition, the Theodorsen model fails to predict the high-U* restabilization of the flutter mode.

Efficient mixing enhancement by orthogonal injection of shear-thinning fluids in a micro serpentine channel at low Reynolds numbers

Viscoelastic fluids have been recently proposed to promote mixing effect in microfluidic systems, where limited success has been obtained. However, the flow and mixing mechanism of a viscoelastic fluid is still poorly understood, particularly regarding the roles of injection on mixing at low Reynolds numbers. In this work, an efficient mixer by orthogonal injection into a primary flow is proposed. The mixing behaviors of shear-thinning fluids in a serpentine channel with an orthogonal injection are investigated experimentally. Dye visualization and concomitant statistical analysis are conducted to quantitatively characterize the mixing performance and to reveal the flow kinematics. Enhanced mixing is achieved just over a short effective mixing length. The probability distribution functions (PDFs) analysis shows that the mixing efficiency, defined by the normalized concentration of dye intensity (Gan et al., 2007), can be significantly improved from 22% for the Newtonian fluids to 69% and 76% by using a shear-thinning fluid with polymer concentrations of 25 and 50 ppm, respectively. The decay exponents of shear-thinning fluids plateau at −3.4 indicates the occurrence of elastic instability. The relative-change-ratio fluctuations illustrates that the observation area can generate sufficient elastic stress to induce flow instabilities, resulting in effective mixing in the channel.

The analysis and prediction of pressure drop oscillation in phase-change cooling systems

Technologies utilizing flow boiling in microchannel evaporators promise high heat fluxes and system efficiencies. However, boiling in microchannels also face significant challenges due to the occurrence of critical heat flux and flow instabilities, such as pressure drop oscillations. Since several system parameters can cause these issues in a two-phase system, identifying these parameters in advance can help avoid these challenges. This study investigates pressure drop oscillation in a pumped liquid cycle using both experiments and numerical modeling. The model utilizes the moving boundary modeling approach to predict system stability and capture the oscillation characteristics. Both experiments and the model determine how different combinations of system parameters, like the valve settings, pump speed, and evaporator heat load, can affect system stability. The study also relates the change in flow oscillation characteristics to the system's supply and demand pressures. This study examines why certain combinations of system parameters induce flow instability and provide ways to mitigate this challenge. A general agreement between the model and experiments also demonstrates the accuracy of the moving boundary modeling approach. Consequently, this modeling approach can be useful in predicting instability, optimizing the system performance, and developing active control strategies for cooling technologies relying on flow boiling in microchannels.

Vibration and damping analysis of flexible aluminum tube under the variation of support plate geometry using low speed water tunnel

Vibration response and damping effects on flexible aluminum tube placed in tube-bundle under fluid interaction velocity of 0.5 m/s using water tunnel, is analyzed experimentally for drag and lift direction. The study is undertaken to reduce the flow-induced instabilities in heat-exchanger, proposing low-vibrating design approach. Therefore, three tube bundles in triangular arrangement using distinct support plate thickness of 2 mm, 2.8 mm, and 3.6 mm under (P/D) ratio of 1.45 are developed, respectively. The vibrations are sensed through tri-axial accelerometer mounted at top of central-tube. The damping factors are assessed for monitored/central tube of each tube-bundle at tuned natural frequencies of 9, 8, 7, and 6 Hz, respectively. Whereas, tube-vibration response is further analyzed in terms of RMS-acceleration and dimensionless magnitude at fixed natural frequency of 8 Hz. The results highlight that, the damping factors (under all four natural frequencies) exhibit increasing and the tube-vibrations show decreasing trend with increase in support plate thickness, respectively. Moreover, successive decrease in natural frequency of tube (9-6 Hz) influence the damping factors to increase for each tube bundle. Based on assessment, it is noted that tube-bundle comprising 3.6 mm thick support plate along 8 Hz natural frequency, shows maximum stability for given setup.

A dislocation density-based model and processing maps of Ti-55511 alloy with bimodal microstructures during hot compression in α+β region

Hot compression features of Ti-55511 alloy are investigated by high-temperature compression tests in α+β region. It is found that the flow stress and softening mechanisms are obviously influenced by deformation conditions. The true stress decreases with the reduced strain rate or the raised temperature. The spheroidization of α phase and dynamic recrystallization (DRX) of β phase easily occur at low temperatures such as 973, 1003 and 1033 K, while the dynamic recovery (DRV) of β phase mainly occurs at high temperatures such as 1063 K because of the transformation from α phase to β phase at relatively high temperatures A dislocation density-based constitutive model, which is associated with DRV, work hardening mechanisms and the spheroidization of α phases, is established and validated to describe flow behavior. The correlation coefficient (R) and average absolute relative error (AARE) of the established model are 0.9924 and 6.8%, respectively. 3D power dissipation efficiency maps and processing maps are established to determine the appropriate processing window, i.e., too low temperatures (lower than 973 K) or too high strain rates (higher than 1 s−1) easily induce flow instability. Therefore, the medium temperature (1003–1063 K) and the low strain rate (0.001–0.1 s−1) are applicable for thermal compression of the studied titanium alloy.

Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Pérouse, a shallow seamount in the southwestern Indian Ocean

The La Pérouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Réunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Réunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Réunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Pérouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Pérouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.

Hemodynamic effects of blood clots trapped by an inferior vena cava filter.

The alteration of blood flow around an OPTEASE inferior vena cava filter with one or two blood clots attached was investigated by means of computational fluid dynamics. We used a patient-specific vein wall geometry, and we generated different clot models with shapes adapted to the filter and vein wall geometries. A total of eight geometries, with one or two clots and a total clot volume of 0.5 or 1 cm3 , were considered. A non-Newtonian model for blood viscosity was adopted and the possible development of turbulence was accounted for by means of a three-equation model. Two blood flow rates were considered for each case, representative for rest and exercise conditions. In exercise conditions, flow unsteadiness and even turbulence was detected in some cases. Pressure and wall shear stress (WSS) distributions were modified in all cases. Clots attached to the filter downstream basket considerably increased averaged WSS values by up to almost 50%. In all the cases a flow recirculation region appeared downstream of the clot. The degree of flow stagnation in these regions, an indicator of propensity to thrombogenesis, was estimated in terms of mean residence times and mean blood viscosity. High levels of flow stagnation were detected in rest conditions in the wake of those clots that were placed upstream from the filter. Our results suggest that one downstream placed big clot, showing a higher tendency to induce flow instabilities and turbulence, might be more harmful than two small clots placed in tandem.