Latest papers in fluid mechanics

Rheology-driven design of pizza gas foaming

Physics of Fluids - Tue, 03/22/2022 - 02:45
Physics of Fluids, Volume 34, Issue 3, March 2022.
This paper investigates the production of a yeast-free pizza by gas foaming and the use of rheology to guide the process design. The novel process relies on the use of a gaseous blowing agent and a pressure program to form and stabilize bubbles during baking, avoiding the use of yeast and the associated lengthy leavening stage. The evolution of the dough structure during baking has been studied by a rheological characterization at leavening and baking conditions. These experimental pieces of information have been used to evaluate the time available for blowing agent sorption under pressure during early baking stage, and to guide the pressure release during the final baking, to achieve an optimally foamed pizza.

On the scattering of focused wave by a finite surface-piercing circular cylinder: A numerical investigation

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
For nonlinear wave–structure interactions, the high-frequency scattered waves can be identified within the drag-inertia regime, especially in steep incident waves where viscous effects are not negligible. According to previous studies, this unexpected phenomenon is highly associated with the local flow field, posing challenges to the existing harmonic-based diffraction solutions (mostly up to second-order). To overcome these shortcomings in potential flows, we establish a high-fidelity numerical wave tank to solve this two-phase free surface flow in the open source computational fluid dynamics framework OpenFOAM. We implement the ghost fluid method to eliminate the spurious velocities, mostly reported in two-phase volume of fluid solvers, in the vicinity of the free surface and preserve a sharp air–water interface. A modified generating–absorbing boundary condition is employed to achieve high computational efficiency without passive relaxation zones. Good agreement with experimental data demonstrates the reliability and accuracy of the present numerical wave tank in extreme wave conditions. On this basis, this paper numerically investigates the wave scattering of the focused wave by a finite surface-piercing circular cylinder, with emphasis on the flow mechanism. Three types of high-frequency scattered waves are identified in the near field, namely, Type-1, Type-2, and Type-1* waves. The typical mechanisms of each type are analyzed in depth with detailed flow field data, which confirms and complements the observations from previous experiments. More importantly, the primary vortical structures involved in scattering are extracted by the Liutex vortex identification method. The behaviors of these vortical structures could characterize the evolution of the high-frequency scattered waves and provide new insights into this strongly nonlinear phenomenon. An overall schematic of the wave scattering evolution in this complex condition is summarized for a straightforward understanding.

On the similarities between the simplified Phan-Thien–Tanner model and the finitely extensible nonlinear elastic dumbbell (Peterlin closure) model in simple and complex flows

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
For many commonly used viscoelastic constitutive equations, it is well known that the limiting behavior is that of the Oldroyd-B model. Here, we compare the response of the simplified linear form of the Phan-Thien–Tanner model (“sPTT”) [Phan-Thien and Tanner, “A new constitutive equation derived from network theory,” J. Non-Newtonian Fluid Mech. 2, 353–365 (1977)] and the finitely extensible nonlinear elastic (“FENE”) dumbbell model that follows the Peterlin approximation (“FENE-P”) [Bird et al., “Polymer solution rheology based on a finitely extensible bead—Spring chain model,” J. Non-Newtonian Fluid Mech. 7, 213–235 (1980)]. We show that for steady homogeneous flows such as steady simple shear flow or pure extension, the response of both models is identical under precise conditions ([math]). The similarity of the “spring” functions between the two models is shown to help understand this equivalence despite a different molecular origin of the two models. We then use a numerical approach to investigate the response of the two models when the flow is “complex” in a number of different definitions: first, when the applied deformation field is homogeneous in space but transient in time (so-called “start-up” shear and planar extensional flow), then, as an intermediate step, the start-up of the planar channel flow; and finally, “complex” flows (through a range of geometries), which, although being Eulerian steady, are unsteady in a Lagrangian sense. Although there can be significant differences in transient conditions, especially if the extensibility parameter is small [math], under the limit that the flows remain Eulerian steady, we once again observe very close agreement between the FENE-P dumbbell and sPTT models in complex geometries.

Lewis number and preferential diffusion effects in lean hydrogen–air highly turbulent flames

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
Unsteady three-dimensional direct numerical simulations of highly turbulent, complex-chemistry, lean hydrogen-air flames were performed by changing the equivalence ratio [math], root mean square velocity [math], and turbulence length scale [math]. For each set of [math], to explore the influence of molecular transport coefficients on the turbulent burning velocity [math], four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity [math] of the mixture for all species; (iii) mixture-averaged diffusivities for all species with the exception of O2, whose diffusivity was equal to the diffusivity [math] of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with [math] to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in [math] due to differences in molecular transport coefficients even at Karlovitz number [math] as large as 565. The increase is documented in cases (i) and (iii) but is not observed in case (iv)—indicating that this phenomenon is controlled by Lewis number effects, whereas preferential diffusion effects play a minor role. The phenomenon is more pronounced in leaner flames, with all other things being equal. While the temperature profiles [math] conditionally averaged at the local value of the combustion progress variable [math] and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high [math], the [math]-profiles sampled from the leading edges of the same flame brushes show significant increase in the local temperature in cases (i) and (iii) characterized by a low Lewis number.

Self-diffusiophoresis of Janus particles that release ions

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
Catalytic Janus swimmers demonstrate a diffusiophoretic motion by self-generating the gradients of concentrations and electric potential. Recent work has focused on simplified cases, such as a release of solely one type of ions or low surface fluxes of ions, with limited theoretical guidance. Here, we consider the experimentally relevant case of particles that release both types of ions, and obtain a simple expression for a particle velocity in the limit of the thin electrostatic diffuse layer. Our approximate expression is very accurate even when ion fluxes and surface potentials are large and allows one to interpret a number of intriguing phenomena, such as the reverse in the direction of the particle motion in response to variations of the salt concentration or self-diffusiophoresis of uncharged particles.

Cavity and jet formation after immiscible droplet impact into deep water pool

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
The impact behavior of a droplet in a liquid pool is of fundamental importance in nature and industrial applications. While there are several reports on using the same fluid type for the droplet and liquid pool, there are a few reports on the use of different liquids. Moreover, the mixing process of the droplet and liquid pool is yet to be fully quantified. Herein, we present an experimental setup to study the effect of droplet solubility in water on the impact characteristics of a deep-water pool. In this study, we used three droplets (water, ethanol, and silicone oil) with different densities, surface tensions, viscosities, and solubilities in water and visualized the impact process using a high-speed camera. The diameter of the droplets ranged from 2.0 to 3.4 mm, and the impact velocities ranged from 1.4 to 3.2 m/s. The depth of the droplet pool was fixed at 30 mm. To better understand the impact characteristics, the obtained images were processed to quantify the created cavity and the subsequent liquid jet formed by the droplet impact. Energy analysis performed during the droplet impact process for the 1000 cSt silicone oil droplet revealed that approximately 70% of the impact energy was converted into cavity energy, and the remaining 30% was converted into flow loss. These experimental results provide physical insight into the immiscibility effect on droplet impact dynamics in a deep pool and pave the way for practical applications.

Effect of thermal and mechanical rejuvenation on the rheological behavior of chocolate

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
Chocolate is known to undergo solid–liquid transition upon an increase in temperature as well as under application of deformation field. Upon sudden reduction in temperature from a molten state (or thermal rejuvenation), the rheological properties of chocolate evolve as a function of time under isothermal conditions, a behavior reminiscent of physical aging in polymeric glasses. Then again, subsequent to cessation of shear flow (or mechanical rejuvenation), chocolate shows temporal evolution of the rheological properties, a behavior similar to physical aging in soft glassy materials. In this work, we evaluate three rheological properties—dynamic moduli, relaxation time spectrum, and characteristic relaxation time of chocolate—and compare their evolution after thermal as well as mechanical rejuvenation. We observe that the evolution of the rheological properties subsequent to mechanical rejuvenation is distinctly different from that of thermal rejuvenation, wherein the evolution is more gradual in the former case. On the one hand, this work provides unique insights into how shear affects the rheological behavior of chocolate. On the other hand, this work clearly suggests that chocolate explores different sections of the energy landscape after mechanical rejuvenation compared to that of thermal rejuvenation.

Turbulence structure in a very sharp thermally stratified open-channel meander

Physics of Fluids - Mon, 03/21/2022 - 10:17
Physics of Fluids, Volume 34, Issue 3, March 2022.
Direct numerical simulation (DNS) results for turbulent open-channel flow through an idealized sine-generated meander with and without an internal heat source that models radiative heating from above are used to analyze the effect of a very sharp meander configuration and thermal stratification on the turbulence structure in the channel with friction Reynolds number [math]. Spatial distributions of temperature, mean velocities, vorticity, mean-flow kinetic energy, and turbulent kinetic energy (TKE) are presented. In both cases, the cross-sectional motion is characterized by three circulation cells: a center-region cell and two weaker outer bank and inner bank cells. However, there is also a small cell observed near the corner of the channel bed inner bank at the channel outlet and the channel bed outer bank at the channel inlet. The tri-cellular cross-stream motions control the distributions of temperature and kinetic energy. In the stratified case, two separated shear layers (SSLs) are found: the first one is formed before the bend apex, and the second one is observed in the wake after the bend apex. In the neutral case, only the first SSL is observed. Turbulent amplification can be seen in both cases; however, in the stratified case, the second SSL stretches out to the channel outlet and is introduced back to the channel inlet by an anti-symmetric periodic boundary condition and then follows the outer bank line. The two SSLs converge in the region before the bend apex and amplify the turbulence more strongly there than in the neutral case. The turbulence kinetic energy budget terms for the stratified case are analyzed to determine the characteristics of production, dissipation and transport of TKE in thermally stratified meandering flow.

Enstrophy change of the Reynolds-Orr solution in channel flow

Physical Review E - Mon, 03/21/2022 - 10:00

Author(s): Péter Tamás Nagy

The plane Poiseuille flow is one of the elementary flow configurations. Although its laminar-turbulent transition mechanism has been investigated intensively in the last century, the significant difference in the critical Reynolds number between the experiments and the theory lacks a clear explanati...


[Phys. Rev. E 105, 035108] Published Mon Mar 21, 2022

Stability analysis of a Newtonian film flow over hydrophobic microtextured substrates

Physical Review Fluids - Mon, 03/21/2022 - 10:00

Author(s): D. Pettas, G. Karapetsas, Y. Dimakopoulos, and J. Tsamopoulos

Our theoretical study reveals that air pockets inside the grooves of superhydrophobic surfaces may either stabilize the flow of liquid films or lead to film rupture depending on the wetting and geometrical characteristics of the substrate micro-texture.


[Phys. Rev. Fluids 7, 034004] Published Mon Mar 21, 2022

Deep spontaneous penetration of a water droplet into hot granular materials

Physical Review Fluids - Mon, 03/21/2022 - 10:00

Author(s): Fangye Lin, Stéphane Dorbolo, Wei Wang, and Jun Zou

The interaction between a liquid droplet and a hot granular material is explored in this work. Surprisingly, we found that the droplet deeply penetrated into the hot granular material when the temperature exceeds the boiling temperature of the liquid. The digging speed of the drop decreases with the temperature. A mechanism based on the Leidenfrost effect is proposed considering that the granular material can be modeled as a rough surface that can be eroded when the vapor speed is sufficient.


[Phys. Rev. Fluids 7, 034301] Published Mon Mar 21, 2022

Gradients in solid surface tension drive Marangoni-like motions in cell aggregates

Physical Review Fluids - Mon, 03/21/2022 - 10:00

Author(s): Vikrant Yadav, Md. Sulaiman Yousafzai, Sorosh Amiri, Robert W. Style, Eric R. Dufresne, and Michael Murrell

Gradients in the solid-like surface tension of model tissues, such as cell aggregates result in fast, internal cellular motions. The image shows the velocity (arrows) and vorticity (color) of cellular motions.


[Phys. Rev. Fluids 7, L031101] Published Mon Mar 21, 2022

Intelligent reconstruction of the flow field in a supersonic combustor based on deep learning

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
The data-driven intelligent reconstruction of a flow field in a supersonic combustor aids the real-time monitoring of wave system evolution in a scramjet flow field structure, allowing the determination of the combustion state for active flow control. In this paper, a deep learning architecture based on a multi-branch fusion convolutional neural network (MBFCNN) is proposed to reconstruct the flow field in a supersonic combustor. Experiments on hydrogen-fueled scramjets with different equivalence ratios were carried out in a direct-connected supersonic pulse combustion wind tunnel with an inflow Mach number of 2.5 to establish a dataset for MBFCNN network training and testing. The trained model successfully reconstructed the flow field structure from measured wall pressure data. The flow field reconstruction model provided a rich information source for the evolution of the wave system structure under the self-ignition conditions of the hydrogen-fueled scramjet, greatly improving the detection accuracy. The proposed deep learning architecture method was compared with basic convolutional neural network and symmetric convolutional neural network methods. The three methods all accurately reconstructed the flow field of the supersonic combustor. However, the proposed MBFCNN provided the best reconstruction results, and its average linear correlation coefficient in the test set was 0.952. The proposed MBFCNN had a lower mean square error and higher peak signal-to-noise ratio than the other two methods, which verified that the proposed model is eminently able to reconstruct and predict the flow field of a supersonic combustor.

On the generalized Beltramian motion of the bidirectional vortex in a conical cyclone

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
This work presents an exact solution of Euler's incompressible equations in the context of a bidirectional vortex evolving inside a conically shaped cyclonic chamber. The corresponding helical flowfield is modeled under inviscid conditions assuming constant angular momentum. By leveraging the axisymmetric nature of the problem, a steady-state solution of the generalized Beltramian type is obtained directly from first principles, namely, from the Bragg–Hawthorne equation in spherical coordinates. The resulting stream function representation enables us to fully describe the ensuing swirl-dominated motion including its fundamental flow characteristics. After identifying an isolated singularity that appears at a cone divergence half-angle of 63.43°, two piecewise formulations are provided that correspond to either fluid injection or extraction at the top section of the conical cyclone. In this process, analytical expressions are readily retrieved for the three velocity components, vorticity, and pressure. Other essential flow indicators, such as the theoretically preferred mantle orientation, the empirically favored locus of zero vertical velocity, the maximum polar and axial velocities, the crossflow velocity, and other such terms, are systematically deduced. Results are validated using limiting process verifications and comparisons to both numerical and experimental measurements. The subtle differences between the present model and a strictly Beltramian flowfield are also highlighted and discussed. The conically cyclonic configuration considered here is relevant to propulsive devices, such as vortex-fired liquid rocket engines with tapered walls; meteorological phenomena, such as tornadoes, dust devils, and fire whirls; and industrial contraptions, such as cyclonic flow separators, collectors, centrifuges, boilers, vacuum cleaners, cement grinders, and so on.

Growth laws and self-similarity in the confined mixing zone of unstratified and strongly stably stratified isokinetic mixing-layers past a splitter plate

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
The velocity U and the mixing scalar field Θ of turbulent wake flows developing downstream of a slender splitter plate were studied experimentally for unstratified and strongly stably stratified conditions in a square channel with water as fluid. The experimental program comprises five different Reynolds numbers spanning [math]–60 000 for [math] (isokinetic conditions) between the two planar, initially separated streams and five relative density stratifications from [math] to 10%, since mixing studies for these stratification strengths are rather limited. This full-channel Reynolds number uses the hydraulic diameter [math] of the mixing section as the characteristic length scale. These experiments challenge the corresponding numerical calculations based on Reynolds-averaged Navier–Stokes approaches relying on the Boussinesq approximations of the first and second kind. The development of the mixing scalar field Θ was measured by two fundamentally different approaches: laser induced fluorescence (LIF) in the downstream x–y mid-plane and wire-mesh sensors (WMSs) in discrete lateral y–z planes. This allows for a direct comparison of both sensing techniques at locations where both planes intersect. For the unstratified conditions, it is shown that the downstream developing wake velocity deficit decay agrees well with the theoretical approach. The concentration fields based on LIF and WMS profiles of the mean and fluctuating (root mean square, RMS) data confirm self-similarity by introducing a mixing scalar Θ for both techniques. In the approximate limit, it is shown that the mean concentration field can be well described by an error function and the corresponding RMS data by a Gaussian profile in self-similar coordinates. By using the momentum thickness θm at the splitter plate as the normalization parameter, the downstream developing width of the mixing zone [math] becomes independent for [math]. The mean and the RMS values of the concentration field Θ for the stably stratified experiments show self-similarity in the core of the concentration field but also a departure from the error function and Gaussian profile in the outer parts. Introducing a local gradient Richardson number Rig, it is shown that both the growth and growth-suppression of the mixing zone are well described by the Miles–Howard criterion for all the stratified experiments considered, even though this criterion was originally developed for shear layers.

Flow control of wake around a wall-mounted cube using a horizontal hole of different diameters

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
This paper describes an experimental investigation of flow control for the wake around a wall-mounted cube using horizontal control holes (HCHs) of different diameters drilled from the center of the front surface to the rear surface of the cube. The cube has side lengths of D = 50 mm, and HCHs with diameters of d = 10, 12, and 14 mm are considered. The instantaneous velocity fields are measured at a Reynolds number of 7800 based on time-resolved particle image velocimetry in a water tunnel. The HCHs suppress the recirculation zone, turbulence intensity, and Reynolds stress, and these control effects gradually increase with the hole diameter. The issuing flow from the large-diameter HCH completely obstructs the development of the downwash flow to the bottom wall and decomposes the near-wake arch-type vortex into a double arch-type structure. As the hole diameter increases, the dominant frequency of the spectrum in the HCH wake increases. Proper orthogonal decomposition analysis indicates that the large-scale spanwise vortices are suppressed by HCH. In the dynamic evolution process of the cube wake, the HCH issuing flow hinders the interaction of shear flow on both sides, and the issuing vortices attract the spanwise vortices and accelerate their shedding.

Large-eddy simulation of turbulent natural convection in a cylindrical cavity using an off-lattice Boltzmann method

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
In the present work, a characteristic-based off-lattice Boltzmann method with the large-eddy simulation (LES) as the turbulence model is developed for the simulation of turbulent natural convection. A double-distribution-function approach is used to resolve flow and thermal fields, and the proposed framework is developed, in three-dimensional curvilinear coordinates. The solver is verified using three benchmark cases, namely, the turbulent Taylor–Green vortex flow, natural convection in a periodic tall cavity, and Rayleigh–Bénard convection. Due to the absence of an inlet in this kind of closed cavity flow, initial perturbations are proposed and verified, which accelerate transition to a turbulent state. The turbulent natural convection in a cylindrical cavity is simulated for a Rayleigh number of [math], and the flow and thermal characteristics are analyzed. A grid sensitivity study is conducted and an appropriate mesh resolution is selected, that is, further verified using the LES index of quality-of-resolution. The resulting turbulent flow and the associated thermal plume are analyzed using instantaneous and time-averaged mean and second-order statistics, vortical structures, turbulence anisotropy maps, energy budgets, frequency spectra, and the mean and root mean square of temperature and Nusselt numbers. The results indicate that the thermal plume region is highly anisotropic, whereas the rest of the annulus contains single-component axisymmetric turbulence. The production and convection of turbulence are dominant on top of the inner cylinder in the thermal plume region, whereas diffusion is dominant closer to the outer cylinder. The azimuthal profiles of mean Nusselt number for the inner and the outer cylinders are observed to be negatively correlated. Furthermore, natural convection in the cylindrical cavity is simulated for [math] to [math] and the effect of the Rayleigh number on the mean Nusselt number and flow patterns is studied.

Pore-scale study of coke combustion in a matrix-fracture system based on the micro-continuum approach

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
In situ combustion is an advanced recovery technique used to exploit heavy oil in the fractured reservoirs that make up approximately one-third of global heavy-oil resources. However, the mesoscopic mechanisms of coke combustion in the multiscale matrix-fracture system are not well understood because of the difficulty of performing pore-resolved simulations. In the present study, a pore-resolved micro-continuum approach was used to investigate fully coupled thermal and reactive flows through fractured media that contain nanometer-range coke pores, micrometer-range matrix pores, and sub-millimeter range natural fractures. Image-based simulations were implemented using synthetic geological models to mimic coke deposition patterns based on tomography images. The combustion regime diagram for the fractured media was mapped based on the ignition temperature and the air flux to exhibit three combustion regimes. The regime diagram was compared with that for unfractured media to address the impact of natural fractures on oxygen transport and the burning temperature. The oxygen diffusion mechanism dominated oxygen transport from the fracture into the matrix and led to a desirable smoldering combustion temperature regardless of the air injection rate. Effects of fracture geometries were quantified to demonstrate tortuous and discrete fractures, and matching air injection rates with fracture apertures can suppress air-channeling risk effectively. Possible discrepancies between lab measurements and field operations were demonstrated, and their potential to drive misinterpretation of experimental results was considered. The present pathway from tomography images to synthetic images and numerical simulations extends the “image and compute” technique to resolution of multiscale and nonlinear reactive transport.

Magnetic energy spectrum produced by turbulent dynamo: Effect of time irreversibility

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
We consider the kinematic stage of evolution of magnetic field advected by turbulent hydrodynamic flow. We use a generalization of the Kazantsev–Kraichnan model to investigate time irreversible flows. In the viscous range of scales, the infinite-time limit of the spectrum is a power law, but its slope is more flat than that predicted by the Kazantsev model. This result agrees with numerical simulations. The rate of magnetic energy growth is slower than that in the time-symmetric case. We show that for high magnetic Prandtl turbulent plasma, the formation of the power-law spectrum shape takes very long time and may never happen because of the nonlinearity. We propose another ansatz to describe the spectrum shape at finite time.

Droplet rebound and dripping during impact on small superhydrophobic spheres

Physics of Fluids - Fri, 03/18/2022 - 10:23
Physics of Fluids, Volume 34, Issue 3, March 2022.
While droplet impact processes on hydrophilic and hydrophobic spheres have been widely investigated experimentally and numerically, the impact behaviors of water droplets on small superhydrophobic spheres are studied numerically and theoretically in this research. The numerical model adopts the volume of fluid method (VOF) and is verified by comparing the simulation results with the experimental observations in the literature. The effects of Weber number and sphere-to-droplet diameter ratio on the droplet impact dynamics are discussed. The final outcomes of the impact droplets are classified into rebound and dripping types with the latter appearing at a larger Weber number or a smaller diameter ratio. As the Weber number and diameter ratio increase, droplet deformation during impact is reinforced with the maximum width factor of the rebound droplet becoming greater. The maximum width factor of the dripping droplet is nearly independent of the Weber number but is enlarged by the increasing diameter ratio. Moreover, a larger diameter ratio reduces the contact time of the rebound droplet but raises that of the dripping one. A theoretical model based on energy conservation is established to predict the boundary between the droplet rebound and dripping outcomes and is in good agreement with the simulation results. The diameter ratio limit for droplet dripping at a zero Weber number is also obtained. Our results and analyses provide insight into the interaction mechanism between the impact droplet and small spheres or particles.

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