Changes In Flow Topology Research Articles

Large-Scale Measurements Using Robotic Volumetric LPT With Shake-The-Box Inside A Vehicle Cabin: Influence Of Different Fan Speeds On The Flow Topology

In this work, the flow topology inside a passenger car cabin for different fan speeds of the heat, ventilation and air conditioning (HVAC) system is investigated using robotic volumetric Lagrangian Particle Tracking (LPT). The coaxial volumetric velocimeter (CVV), also called MiniShaker, in combination with a robotic arm enables the measurement of the complete passenger's side. In total 37 CVV perspectives (sub-volumes) were measured. At each position 3000 double frame images were recorded. The interior was painted black and the mannequin was wrapped with black aluminum foil in order to keep reflections at a low level. Experimental results for three different fan speeds using helium-filled soap bubbles (HFSB) as tracers and Shake-the-Box (StB) algorithm for the three-dimensional LPT data analyses show that the flow topology changes significantly with increasing fan speeds. At higher fan speeds the flow fields show the highest complexity with four vortices.

Infrared thermography techniques for boundary layer state visualisation

The rapid decarbonisation of the power generation and aviation sectors will require a move away from incremental development, exposing designers and researchers to the risk of unexpected results from uncertainty in boundary layer state. This problem already exists for parts developed with fully turbulent assumptions, but in novel design spaces the risk increases for both real components, where previous knowledge of similar designs may be inapplicable, and particularly in experimental testing of scaled models, where reducing Reynolds number can result in a drastic change in flow topology that skews the conclusions of a test. Computational methods struggle to reliably predict boundary layer state so experimental techniques for diagnosing boundary layer state are needed. Infrared thermography (IR) is a non-invasive technique that offers simple, fast visualisation of boundary layer state with no additional instrumentation. IR is relatively uncommon in the literature and there is minimal information available on the best practices for its use. This paper aims to encourage the adoption of IR as a diagnostic tool by demonstrating routes for optimisation and pointing out pitfalls to avoid. A low-order model is developed and used to predict how the signal-to-noise ratio (SNR) of an IR visualisation changes depending on the thermal design of the test piece. It is shown that in low-speed flows with active heating from the surface the SNR is maximised through a suitable choice of surface insulation, while in high-speed flows, where passive temperature differences are used, there is a crossover between heat transfer and recovery temperature effects that results in an SNR of zero, an effect that can arise in both steady-state and transient experiments. Experimental validation of the 1D model in both flow regimes is shown alongside two case studies on the use of IR in sub-scale testing where uncertainty in boundary layer state results in critical differences from the full-scale flow.

Enhanced heat transfer and reduced flow reversals in turbulent thermal convection with an obstructed centre

We report an experimental study about the effect of an obstructed centre on heat transport and flow reversal by inserting an adiabatic cylinder at the centre of a quasi-two-dimensional Rayleigh–Bénard convection cell. The experiments are carried out in a Rayleigh number ( $Ra$ ) range of $2\times 10^7 \leq Ra \leq 2\times 10^9$ and at a Prandtl number ( $Pr$ ) of $5.7$ . It is found that for low $Ra$ , the obstructed centre leads to a heat transfer enhancement of up to 21 $\%$ , while as $Ra$ increases, the magnitude of the heat transfer enhancement decreases and the heat transfer efficiency ( $Nu$ ) eventually converges to that of the unobstructed normal cell. Particle image velocimetry measurements show that the heat transfer enhancement originates from the change in flow topology due to the presence of the cylindrical obstruction. In the low- $Ra$ regime the presence of the obstruction promotes the transition of the flow topology from the four-roll state to the abnormal single-roll state then to the normal single-roll state with increasing obstruction size. While in the high- $Ra$ regime, the flow is always in the single-roll state regardless of the obstruction size, although the flow becomes more coherent with the size of the obstruction. We also found that in the presence of the cylindrical obstruction, the stability of the corner vortices is significantly reduced, leading to a large reduction in the frequency of flow reversals.

Direct Numerical Simulation of Boundary Layers over Microramps: Mach Number Effects

Microvortex generators are passive control devices with heights below the boundary-layer thickness that have been proposed to mitigate the detrimental effects of shock-wave/boundary-layer interaction. Despite their demonstrated control effectiveness, several aspects of the flow induced in turbulent boundary layers still need to be characterized thoroughly. In this work, we present a campaign of direct numerical simulations of a turbulent boundary layer on a microramp, to investigate the effect of the Mach number, from subsonic to supersonic regime. We show that the flow topology changes significantly because of compressibility effects, and that typical wake features do not scale linearly with the geometry dimensions but rather depend on the incoming flow conditions. Moreover, we investigate the spectral content in time and space of the wake, which is dominated by the Kelvin–Helmholtz instability developing along the shear layer. For larger Mach numbers, the shedding onset is postponed and exhibits a lower peak frequency that evolves in space. Finally, we extract the spatially coherent structures convected in the wake by means of a dynamic mode decomposition along the characteristics, which represents effectively and efficiently the evolution of the entire field, despite the convective nature of the flow under consideration.

Physics of gust response mitigation in open-loop pitching manoeuvres

This paper experimentally investigates the flow field development and unsteady loading of three force-mitigating pitch manoeuvres during a transverse gust encounter. The manoeuvres are constructed using varying levels of theoretical and simulation fidelity and implemented as open-loop kinematics in a water towing tank. It is found that pitch actuation during a gust encounter results in two important changes in flow topology: (i) early detachment of the leading-edge vortex (LEV) and (ii) formation of an LEV on the pressure side of the wing upon gust exit. Each of the pitch manoeuvres is found to mitigate a significant portion of the circulatory contribution of the lift force while only manoeuvres with accurate modelling of the added-mass force are found to adequately mitigate the total lift force. The penalty of aerodynamic lift mitigation using pitch manoeuvres was a twofold increase in the pitching moment transients experienced by the wing for all cases. By quantifying changes in the vertical gust momentum before and after the encounter, lift-mitigating manoeuvres were found to reduce the disturbance to the gust's flow field, thereby reducing the momentum exchange between the gust and the wing.

Emergence of substructures inside the large-scale circulation induces transition in flow reversals in turbulent thermal convection

We present an experimental study of the reversal of the large-scale circulation (LSC) in quasi-two-dimensional turbulent Rayleigh–Bénard convection. It is found that there exists a transition in the Rayleigh number ($Ra$) dependence of the reversal rate $f$ with two distinct scalings: for $Ra$ less than a transitional value $Ra_{t}$, the non-dimensionalized reversal rate $ft_{E}\sim Ra^{-1.09}$; however, for higher $Ra$ the scaling changes to $ft_{E}\sim Ra^{-3.06}$, where $t_{E}$ is the turnover time of the LSC. Flow visualization shows that this regime transition originates from a change in flow topology from a single-roll state to a new, less stable, abnormal single-roll state with substructures inside the single roll. The emergence of the substructures inside the LSC lowers the energy barrier for the flow reversals to occur and leads to a slower decay of $f$ with $Ra$. Detailed analysis reveals that, although it is the corner rolls that trigger the reversal event, the probability for the occurrence of reversals mainly depends on the stability of the LSC. This is supported by a model we proposed to predict the critical condition for the transition, which agrees well with the experimental results.

On the scaling and topology of confined bluff-body flows

An experimental study of bluff bodies in confinement is presented. Two Reynolds matched rigs (pipe diameters: $D=40~\text{mm}$ and $D=194~\text{mm}$) are used to derive a picture of the flow topology of the primary-shedding mode (Kármán vortex, mode-I). Confined bluff bodies create an additional spectral mode (mode-II). This is caused by the close coupling of the shedder blockage and the wall and is unique to the confined bluff-body problem. Under certain conditions, modes-I and II can interact, resulting in a lock-on, wherein the modes cease to exist at independent frequencies. The topological effects of mode interaction are demonstrated using flow visualisation. Furthermore, the scaling of mode-II is explored. The two experimental facilities span Reynolds numbers (based on the shedder diameter, $d$) $10^{4}<Re_{d}<10^{5}$ and bulk Mach numbers $0.02<M_{b}<0.4$. Bluff bodies with a constant blockage ratio ($d/D$), forebody shape and various splitter-plate lengths ($l$) and thicknesses ($t$) are used. Results indicate that the flow topology changes substantially between short ($l<d$) and long ($l>d$) tailed geometries. Surface flow visualisation indicates that the primary vortex becomes anchored on the tail when $l\gtrsim 3h$ ($2h=d-t$). This criterion prohibits the development of such a topology for short-tailed geometries. When mode interaction occurs, which it does exclusively in long-tailed cases, the tail-anchored vortex pattern is disrupted. The onset of mode-II occurs at approximately the same Reynolds number in both rigs, although the associated dimensionless frequency is principally a function of Mach number. Accordingly, mode interaction is avoided in the larger-scale rig, due to the increased separation of the modal frequencies.

Collision and reconnection of viscous elliptic vortex rings

It is well known that head-on collision of two identical circular vortex rings at moderate Reynolds numbers generates secondary vortex ringlets that propagate radially away from the center of the primary rings. In this paper, we show through numerical simulations that deforming vortex ring shape from a circle to an ellipse can lead to drastic changes in flow topology during the collision. The computations are performed for a range of flow parameters, including the aspect ratio of elliptic rings, the core size ratio (η), and the azimuthal angle between their major axes (θ). Results show that if two elliptic rings are identical and in the absence of azimuthal perturbation, the collision leads to the generation of two subelliptic vortex rings that propagate away from each other along a line approximately perpendicular to the original direction of approach of the primary rings. If, however, azimuthal perturbation is present, besides the two subelliptic rings, secondary vortex ringlets are generated on the remaining perimeter of the primary rings. On the other hand, if two primary elliptic rings are of unequal core sizes or if their major axes are not aligned to each other, the orientation and direction of travel of the two subelliptic rings generated deviate significantly from those involving two identical primary rings. If azimuthal perturbation is also present in these scenarios, more fine scale structures are generated and superimposed on the two subelliptic rings as well as the formation of secondary vortex ringlets. These findings may help us to explain some of the experimental observations reported in the literature and provide useful insights into the mixing processes of two directly opposed impinging elliptic jets.

On the peculiar structure of a helical wake vortex behind an inclined prolate spheroid

The self-similarity law for axisymmetric wakes has for the first time been examined and verified in a complex helical vortex in the far part of an asymmetric wake by means of direct numerical simulation (DNS). The helical vortex is the main coherent flow structure in the transitional non-axisymmetric wake behind an inclined 6:1 prolate spheroid at Reynolds number 3000 based on the minor axis. The gradual development of the complex helical vortex structure has been described in detail all the way from its inception at the spheroid and into the far wake. We observed a complex vortex composition in the generation stage, a rare jet-like wake pattern in the near wake and an abrupt change of helical symmetry in the vortex core without an accompanying change in flow topology, i.e. with no recirculation bubble.

Vortex breakdown in a truncated conical bioreactor

This numerical study explains the eddy formation and disappearance in a slow steady axisymmetric air–water flow in a vertical truncated conical container, driven by the rotating top disk. Numerous topological metamorphoses occur as the water height, Hw, and the bottom-sidewall angle, α, vary. It is found that the sidewall convergence (divergence) from the top to the bottom stimulates (suppresses) the development of vortex breakdown (VB) in both water and air. At α = 60°, the flow topology changes eighteen times as Hw varies. The changes are due to (a) competing effects of AMF (the air meridional flow) and swirl, which drive meridional motions of opposite directions in water, and (b) feedback of water flow on AMF. For small Hw, the AMF effect dominates. As Hw increases, the swirl effect dominates and causes VB. The water flow feedback produces and modifies air eddies. The results are of fundamental interest and can be relevant for aerial bioreactors.

Parameters influencing vortex growth and detachment on unsteady aerodynamic profiles

Experiments with a pitching and plunging airfoil are conducted in order to investigate the mechanisms responsible for the formation and detachment of leading edge vortices (LEVs). The chord length is varied from 90 to 180 mm, keeping all other non-dimensional parameters constant, specifically the Reynolds number (17 000), the Strouhal number (0.25), the reduced frequency (0.5) and the effective angle of attack history. It is shown that the mechanism of vortex detachment changes with chord length, evident in a corresponding change in flow topology. One mechanism scales with chord length, the other is attributed to viscous effects in the boundary layer. For the latter mechanism a new scaling of the LEV circulation is introduced. A second experiment investigates the influence of the reduced frequency on the LEV circulation and detachment mechanisms, again keeping all other non-dimensional parameters constant.

The flow-induced sound of a wall-mounted finite airfoil

In this study, the aeroacoustic behavior of a wall-mounted finite airfoil is experimentally investigated. Compared to a semi-infinite or two-dimensional airfoil where end effects are not considered, a wall-mounted finite airfoil is more realistic, especially for applications such as wind turbine blades attached to a hub, submarine hydrofoils mounted to a hull, or stators connected to a hub or outer wall. Acoustic and aerodynamic measurements have been taken in an anechoic wind tunnel with single microphones, multiple acoustic beamforming arrays, and hot-wire anemometry. These measurements are used to examine changes in flow topology and radiated noise as a function of Reynolds number and airfoil aspect ratio. Additionally, the data gives insight into the influence of flow at the airfoil tip and wall junction on noise production.

Effect of exercise on patient specific abdominal aortic aneurysm flow topology and mixing

Computational fluid dynamics modeling was used to investigate changes in blood transport topology between rest and exercise conditions in five patient-specific abdominal aortic aneurysm models. MRI was used to provide the vascular anatomy and necessary boundary conditions for simulating blood velocity and pressure fields inside each model. Finite-time Lyapunov exponent fields and associated Lagrangian coherent structures were computed from blood velocity data and were used to compare features of the transport topology between rest and exercise both mechanistically and qualitatively. A mix-norm and mix-variance measure based on fresh blood distribution throughout the aneurysm over time were implemented to quantitatively compare mixing between rest and exercise. Exercise conditions resulted in higher and more uniform mixing and reduced the overall residence time in all aneurysms. Separated regions of recirculating flow were commonly observed in rest, and these regions were either reduced or removed by attached and unidirectional flow during exercise, or replaced with regional chaotic and transiently turbulent mixing, or persisted and even extended during exercise. The main factor that dictated the change in flow topology from rest to exercise was the behavior of the jet of blood penetrating into the aneurysm during systole.

Bistability and hysteresis of maximum-entropy states in decaying two-dimensional turbulence

We propose a theory that qualitatively predicts the stability and equilibrium structure of long-lived, quasi-steady flow states in decaying two-dimensional turbulence. This theory combines a maximum entropy principal with a nonlinear parameterization of the vorticity-stream-function dependency of such long-lived states. In particular, this theory predicts unidirectional-flow states that are bistable, exhibit hysteresis, and undergo large abrupt changes in flow topology; and a vortex-pair state that undergoes continuous changes in flow topology. These qualitative predictions are confirmed in numerical simulations of the two-dimensional Navier-Stokes equation. We discuss limitations of the theory, and why a reduced quantitative theory of long-lived flow states is difficult to obtain. We also provide a partial theoretical justification for why certain sets of initial conditions go to certain long-lived flow states.

Alteration of helical vortex core without change in flow topology

The abrupt expansion of the slender vortex core with changes in flow topology is commonly known as vortex breakdown. We present new experimental observations of an alteration of the helical vortex core in wall bounded turbulent flow with abrupt growth in core size, but without change in flow topology. The helical symmetry as such is preserved, although the characteristic parameters of helical symmetry of the vortex core transfer from a smooth linear variation to a different trend under the influence of a non-uniform pressure gradient, causing an increase in helical pitch without changing its sign.

Network calculations and ascendency based on eco-exergy

Ascendency is an index of activity and organization in living systems calculated in terms of flows. The concern here is with how that quantity behaves when the flows in question are measured in terms of eco-exergy. The storage of eco-exergy has served as a goal function in assessing parameter values for structurally dynamic models, but network magnitudes and topologies can change in response to significant changes in the forcing functions. As storages are relatively insensitive to such changes, it is advisable in such cases to explore how changes in a flow variable, like ascendency, might capture network adaptations. It happens that changes in ascendency calculated in terms of flows of simple energy are small in comparison to corresponding variations in the storages of eco-exergy. But when ascendency is reckoned in terms of flows of eco-exergy, its changes in response to network changes are more comparable to those in the storages. Ascendency seems to be more sensitive to changes in flow topology, however, so that a combination of eco-exergy storage and eco-exergy ascendency would probably be most appropriate for situations where changes in flow topology are significant.

The Structure and Mesoscale Organization of Precipitating Stratocumulus

Abstract Large-eddy simulations are used to explore the structure and mesoscale organization of precipitating stratocumulus. The simulations incorporate a simple, two-moment, bulk representation of microphysical processes, which by varying specified droplet concentrations allows for comparisons of simulations that do and do not develop precipitation. The boundary layer is represented over a large (25.6 km × 25.6 km) horizontal domain using a relatively fine mesh, thereby allowing for the development of mesoscale circulations while retaining an explicit representation of cloud radiative, dynamical and microphysical interactions on scales much smaller than the dominant eddy scale. Initial conditions are based on measurements made as part of the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS-II). The simulations show that precipitation is accompanied by sharp reductions in cloudiness and changes in flow topology. Mesoscale features emerge in all of the simulations but are amplified in the presence of drizzle. A cloud albedo of near 75% in the nonprecipitating simulation is reduced to less than 35% in the precipitating case. The circulation transitions from a well-mixed, stationary stratocumulus layer with closed-cellular cloud planforms to a stationary cumulus-coupled layer, with incipient open-cellular cloud planforms and sustained domain-averaged surface precipitation rates near 1 mm day−1. The drizzling simulations embody many other features of observed precipitating stratocumulus, including elevated cloud tops in regions of precipitation and locally higher values of subcloud equivalent potential temperature. The latter is shown to result from the tendency for precipitating simulations to develop greater thermodynamic gradients in the subcloud layer as well as mesoscale circulations that locate regions of upward motion in the vicinity of precipitating cells.

Three-dimensional airway reopening: the steady propagation of a semi-infinite bubble into a buckled elastic tube

We consider the steady propagation of an air finger into a buckled elastic tube initially filled with viscous fluid. This study is motivated by the physiological problem of pulmonary airway reopening. The system is modelled using geometrically nonlinear Kirchhoff–Love shell theory coupled to the free-surface Stokes equations. The resulting three-dimensional fluid–structure-interaction problem is solved numerically by a fully coupled finite element method.The system is governed by three dimensionless parameters: (i) the capillary number, Ca=μU/σ*, represents the ratio of viscous to surface-tension forces, where μ is the fluid viscosity, U is the finger's propagation speed and σ* is the surface tension at the air–liquid interface; (ii) σ=σ*/(RK) represents the ratio of surface tension to elastic forces, where R is the undeformed radius of the tube and K its bending modulus; and (iii) A∞=A*∞/(4R2), characterizes the initial degree of tube collapse, where A*∞ is the cross-sectional area of the tube far ahead of the bubble.The generic behaviour of the system is found to be very similar to that observed in previous two-dimensional models (Gaver et al. 1996; Heil 2000). In particular, we find a two-branch behaviour in the relationship between dimensionless propagation speed, Ca, and dimensionless bubble pressure, p*b/(σ*/R). At low Ca, a decrease in p*b is required to increase the propagation speed. We present a simple model that explains this behaviour and why it occurs in both two and three dimensions. At high Ca, p*b increases monotonically with propagation speed and p*b/(σ*/R) ∝ Ca for sufficiently large values of σ and Ca. In a frame of reference moving with the finger velocity, an open vortex develops ahead of the bubble tip at low Ca, but as Ca increases, the flow topology changes and the vortex disappears.An increase in dimensional surface tension, σ*, causes an increase in the bubble pressure required to drive the air finger at a given speed; p*b also increases with A*∞ and higher bubble pressures are required to open less strongly buckled tubes. This unexpected finding could have important physiological ramifications. If σ* is sufficiently small, steady airway reopening can occur when the bubble pressure is lower than the external (pleural) pressure, in which case the airway remains buckled (non-axisymmetric) after the passage of the air finger. Furthermore, we find that the maximum wall shear stresses exerted on the airways during reopening may be large enough to damage the lung tissue.

Topology of laminar flow on a spheroid at incidence

Numerical solutions of the Navier-Stokes equations were obtained for steady, incompressible, laminar flow around a prolate spheroid of axes ratio six at incidences up to 30° to elucidate the changes in flow topology and separation patterns. The results clarify observations and speculations based on earlier flow visualizations, and provide details in the separated flow that were not accessible to previous calculations. In particular, a closed or singular separation, joining singular points of the skin-friction lines, is found at all incidences, and it is accompanied by one or more lines of the so-called open (or free-vortex, or ordinary) separation at higher incidences.