Showing papers on "Velocity gradient published in 2023"

In-plane impact dynamics of thickness gradient honeycomb structures with negative Poisson’s ratio multi-arc concave cells

Abstract: Based on a novel multi-arc concave cell, four types of thickness gradient honeycomb structures with positive gradient, negative gradient, symmetric positive gradient, and symmetric negative gradient are constructed by changing cell wall thickness, and the relative density of the structure is deduced. The in-plane impact dynamics of gradient structure are revealed by numerical method. The deformation collapse mode, the dynamic response curve, the energy absorption effect, and the platform stress of different gradient arrangement and impact velocity are analyzed. It is found that gradient arrangement and impact velocity have significant effects on the impact properties of the thickness gradient honeycomb structures. When the impact velocity is low, the structure shows laminar shrinkage deformation, but this phenomenon will gradually disappear with the increase of impact velocity. The results show that the energy absorption capacity of the negative gradient structure is the best under different impact velocities. The platform stress is greatly affected by the impact velocity and less affected by the gradient arrangement.

Comparison of smooth- and rough-wall non-equilibrium boundary layers with favourable and adverse pressure gradients

Abstract: Abstract Measurements were made in rough-wall boundary layers subject to favourable, zero and adverse pressure gradients. Profiles of mean velocity and turbulence quantities were acquired and velocity fields were measured in multiple planes to document flow structure. Comparisons were made to equivalent smooth-wall cases with the same free stream velocity distributions. Outer layer similarity was observed between the rough- and smooth-wall cases in all quantities in the favourable and zero pressure gradient regions, but large differences were observed with adverse pressure gradients. In both the smooth- and rough-wall cases, the favourable pressure gradient reduced the turbulence in the boundary layer, and increased the size of turbulence structures relative to the boundary layer thickness in both the streamwise and spanwise directions, while lowering their inclination angle with respect to the wall. When the boundary layer was returned to a zero pressure gradient following the favourable pressure gradient region, the turbulence level and the size and inclination of the structures returned to their canonical zero pressure gradient condition. The response of the boundary layer was somewhat faster in the rough-wall case, causing it to reach equilibrium in a shorter streamwise distance after the changes in pressure gradient than in the smooth-wall case. The adverse pressure gradient increased turbulence levels relative to the wall friction velocity, reduced the size of turbulence structures relative to the boundary layer thickness and increased their inclination angle. The changes with the adverse pressure gradient were significantly larger with the rough wall than the smooth. The results suggest that similarity might be achieved with adverse pressure gradients if smooth- and rough-wall cases with the same Clauser pressure gradient parameter history are compared.

Modified Darcy’s law and couple stress effects on electro-osmotic flow of non-Newtonian nanofluid with peristalsis

Abstract: In this study, we focused on the heat transfer through a uniformly inclined rectangular duct caused by the electro-osmotic peristaltic flow of an unsteady non-Newtonian nanofluid. With couple stress, the fluid obeys the Papanastasiou model. The flow is through a porous medium that follows Darcy’s law in a modified form. In addition, Dufour and Soret effects, mixed convection, the impacts of a chemical reaction, and the effects of viscous couple stress dissipation are all considered. The governing equations that explain the velocity, temperature, and concentration of nanoparticles are simplified when wave transformation is used. The homotopy perturbation method was used to solve these equations analytically. Additionally, a collection of figures is used to discuss and visually illustrate the consequences of the physical characteristics. In fact, the modified Darcy’s law makes the velocity gradient appear in the momentum equation, which increases the contribution of the velocity gradient to the velocity profile. In addition, the electro-osmotic parameter and Helmholtz-Smoluchowski velocity have a significant impact on the velocity gradient’s direction, as well as the velocity gradient’s ability to be either positive or negative, depending on their values. In addition, in the case of forced convection, the values of the Nusselt number and the Sherwood number are highly affected by the value of Helmholtz–Smoluchowski velocity. The current findings have applications in biology and medicine, particularly in cancer therapy, which involves peristaltic blood pumps(arteries) and suspended gold nanoparticles (nanofluid). According to our knowledge, no prior studies have merged the couple stress Papanastasiou model and the modified Darcy’s law.

Three-dimensional Pressure–Rate-of-Strain, Pressure Diffusion, and Velocity–Pressure-Gradient Tensor Measurements in a Cavity Flow by Time-resolved Tomo-PIV

Abstract: Preliminary results on pressure-related turbulence statistics and the Reynolds stress tensor of a turbulent shear layer flow impinging on a cavity trailing corner measured by tomo-PIV at a Reynolds number of 40,000 is presented in this paper. The instantaneous three-dimensional pressure fields are obtained by means of parallel-ray omnidirectional integration method and the velocity-pressure-gradient as well as the pressure-rate-of strain were obtained from the 3D instantaneous velocity and pressure distributions. The results show that the time-averaged streamwise and wall-normal velocity components are roughly two-dimensional within the measurement volume while the time averaged spanwise velocity component, with a magnitude of no more than 2% of the freestream velocity, is three-dimensional. The streamwise and the spanwise Reynolds normal stresses are on the same order of magnitude, and the shear stress terms (u'w') and (v'w') show a three-dimensional profile. The terms Π13, Π23, R13, R23 from the velocity-pressure-gradient and pressure-rate-of-strain respectively also have three-dimensional profiles. Velocity measurement uncertainty in terms of the standard deviation of velocity divergence is less than 5% of the inflow wall shear in the free stream region, and the uncertainty of the pressure gradient estimated from the residual of the curl of the pressure gradient is below 10% for 90% of the measurement volume.

High-speed impact performance of a high-performance concrete with functionally gradient structure designs

Abstract: Abstract Ultra-high performance concrete is characterized by its high strength and high toughness, and ceramsite concrete can absorb energy and function as a buffer. Based on the mechanical properties of both, the two kinds of concrete materials are combined in the spatial structure, and three structural models, namely, gradient structure group target, double stacked structure group target, and single material structure group target, are designed and prepared. Then, the dynamic characteristics and protection performance of the three targets are compared and analyzed via experiments and numerical simulations under different projectile penetration velocities. The results show that because the two transition layers and the fiber layer between the interfaces are set in the gradient structure group target, which largely improves the resistance in the process of projectile penetration, the cratering area, cratering depth, and penetration depth of the gradient structure group target are the smallest under the same projectile penetration velocity. The larger the projectile penetration velocity is, the more outstanding the penetration resistance of the gradient structure group target is.

Homogeneity constraints on the mixed moments of velocity gradient and pressure Hessian in incompressible turbulence

Abstract: We derive a relation for the correlation between pressure Hessian and velocity gradient in homogeneous flows. This relation provides restrictions to the parameters in the closure models of pressure hessian in velocity gradient dynamics, and together with the Poisson equation, we could obtain an integral expression for the fourth-order moment of velocity gradient in isotropic flows. Furthermore, this relation could be generalized to shear flows, and it is approximately satisfied even in the presence of a shear and of a wall, as occurs in turbulent channel flows.

Influence of the Fragment Velocity Gradient on the Hit Density under the Dynamic Condition

Abstract: The influence of the fragment velocity gradient on the hit density under the dynamic missile-target meeting condition is discussed using theoretical analysis, and the analytical relationship is obtained. The results show that under static explosion conditions, the fragment hit density is independent of the velocity gradient of the fragment group. Under dynamic conditions, the distribution bandwidth of the fragments on the target becomes wider with the increase of the velocity of the missile-target meeting and the velocity gradient of the fragment group, and the increase of the fragment bandwidth decreases the fragment hit density.

High-resolution APEX/LAsMA $^{12}$CO and $^{13}$CO (3-2) observation of the G333 giant molecular cloud complex : I. Evidence for gravitational acceleration in hub-filament systems

Abstract: Hub-filament systems are suggested to be the birth cradles of high-mass stars and clusters. We apply the FILFINDER algorithm to the integrated intensity maps of the 13CO (3-2) line to identify filaments in the G333 complex, and extract the velocity and intensity along the filament skeleton from moment maps. Clear velocity and density fluctuations are seen along the filaments, allowing us to fit velocity gradients around the intensity peaks. The velocity gradients fitted to the LAsMA data and ALMA data agree with each other over the scales covered by ALMA observations in the ATOMS survey. Changes of velocity gradient with scale indicate a ''funnel'' structure of the velocity field in PPV space, indicative of a smooth, continuously increasing velocity gradient from large to small scales, and thus consistent with gravitational acceleration. The typical velocity gradient corresponding to a 1 pc scale is ~1.6km/s/pc. Assuming free-fall, we estimate a kinematic mass within 1 pc of ~1190 M$_\odot$, which is consistent with typical masses of clumps in the ATLASGAL survey. We find direct evidence for gravitational acceleration from comparison of the observed accelerations to those predicted by free-fall onto dense hubs. On large scales, we find that the inflow may be driven by the larger scale structure, consistent with hierarchical structure in the molecular cloud and gas inflow from large to small scales. The hub-filament structures at different scales may be organized into a hierarchical system extending up to the largest scales probed, through the coupling of gravitational centers at different scales. We argue that the ''funnel'' structure in PPV space can be an effective probe for the gravitational collapse motions in molecular clouds. The large scale gas inflow is driven by gravity, implying that the molecular clouds in G333 complex may be in the state of global gravitational collapse.

Tracing of Magnetic field with gradients: Sub-Sonic Turbulence

Abstract: Recent development of the velocity gradient technique shows the capability of the technique in the way of tracing magnetic fields morphology in diffuse interstellar gas and molecular clouds. In this paper, we perform the numerical systemic study of the performance of velocity and synchrotron gradient for a wide range of magnetization in the sub-sonic environment. Addressing the studies of magnetic field in atomic hydrogen, we also study the formation of velocity caustics in the spectroscopic channel maps in the presence of the thermal broadening. We show that the velocity caustics can be recovered when applied to the Cold Neutral Medium (CNM) and the Gradient Technique (GT) can reliably trace magnetic fields there. Finally, we discuss the changes of the anisotropy of observed structure functions when we apply to the analysis the procedures developed within the framework of GT studies.

Tracing of Magnetic field with gradients: Sub-Sonic Turbulence

Abstract: Recent development of the velocity gradient technique shows the {\toreferee capability} of the technique in the way of tracing magnetic fields morphology in diffuse interstellar gas and molecular clouds. In this paper, we perform the numerical systemic study of the performance of velocity and synchrotron gradient for a wide range of magnetization in the sub-sonic environment. Addressing the studies of magnetic field in atomic hydrogen, we also study the formation of velocity caustics in the spectroscopic channel maps in the presence of the thermal broadening. We show that the velocity caustics can be recovered when applied to the Cold Neutral Medium (CNM) and the Gradient Technique (GT) can reliably trace magnetic fields there. Finally, we discuss the changes of the anisotropy of observed structure functions when we apply to the analysis the procedures developed within the framework of GT studies.

Probing Velocity Structures of Protostellar Envelopes: Infalling and Rotating Envelopes within Turbulent Dense Cores

Abstract: We have observed the three low-mass protostars, IRAS 15398$-$3359, L1527 IRS and TMC-1A, with the ALMA 12-m array, the ACA 7-m array, and the IRAM-30m and APEX telescopes in the C$^{18}$O $J=2$-1 emission. Overall, the C$^{18}$O emission shows clear velocity gradients at radii of $\sim$100-1000 au, which likely originate from rotation of envelopes, while velocity gradients are less clear and velocity structures are more perturbed on scales of $\sim$1000-10,000 au. IRAS 15398$-$3359 and L1527 IRS show a break at radii of $\sim$1200 and $\sim$1700 au in the radial profile of the peak velocity, respectively. The peak velocity is proportional to $r^{-1.38}$ or $r^{-1.7}$ within the break radius, which can be interpreted as indicating a rotational motion of the envelope with a degree of contamination of gas motions on larger spatial scales. The peak velocity follows $v_\mathrm{peak} \propto r^{0.68}$ or $v_\mathrm{peak} \propto r^{0.46}$ outside the break radius, which is similar to the $J/M$-$R$ relation of dense cores. TMC-1A exhibits the radial profile of the peak velocity not consistent with the rotational motion of the envelope nor the $J/M$-$R$ relation. The origin of the relation of $v_\mathrm{peak} \propto r^{0.46\operatorname{-}0.68}$ is investigated by examining correlations of the velocity deviation ($\delta v$) and the spatial scale ($\tau$) in the two sources. Obtained spatial correlations, $\delta v \propto \tau^{\sim0.6}$, are consistent with the scaling law predicted by turbulence models, which may suggest the large-scale velocity structures originate from turbulence.

Data-driven model for Lagrangian evolution of velocity gradients in incompressible turbulent flows

Abstract: Velocity gradient tensor, $A_{ij}\equiv \partial u_i/\partial x_j$, in a turbulence flow field is modeled by separating the treatment of intermittent magnitude ($A = \sqrt{A_{ij}A_{ij}}$) from that of the more universal normalized velocity gradient tensor, $b_{ij} \equiv A_{ij}/A$. The boundedness and compactness of the $b_{ij}$-space along with its universal dynamics allows for the development of models that are reasonably insensitive to Reynolds number. The near-lognormality of the magnitude $A$ is then exploited to derive a model based on a modified Ornstein-Uhlenbeck process. These models are developed using data-driven strategies employing high-fidelity forced isotropic turbulence data sets. A posteriori model results agree well with direct numerical simulation (DNS) data over a wide range of velocity-gradient features.

The velocity of climate change revisited: Smooth velocity field and ecological relevance

Abstract: Describing climate change in terms of spatial velocity is essential to assess the ability for ecosystems or individual species to migrate at a sufficient pace to keep environmental conditions allowing their survival. While climate models provide a temporal evolution of a number of  variables at each point of their computational grid, Loarie et al. introduced a velocity of climate change, defined as the ratio of the temporal derivative to the spatial gradient of temperature, or any other variable such as precipitations [1]. This amounts to assume that isotherms shift along the temperature gradient. Although intuitive, this idea is mathematically correct only for straight isotherms parallel to each other [2]. Whenever this condition is not met, e.g., due to complex topography or coastlines, the gradient-based velocity field will display artefacts in the form of local convergence or divergence that are likely to bias the analysis.We show that these artefacts can be fixed by defining a much more regular velocity field. This alternative approach to the velocity of climate change determines the direction of the velocity vector by minimising the local vorticity rather than by the gradient. From a fundamental point of view, the resulting smoother velocity field allow an analysis at finer temporal and spatial scales. It also allows to define the climate trajectory of a given origin point. Our approach also provides tools to estimate the stability of climate trajectories depending on the behaviour of their "return" trajectory obtained by reversing time [3].  From an ecological point of view, we discuss preliminary results on the relevance of each definition of the velocity of climate change, based on comparisons of the obtained climate trajectories with ecological trajectories from observational data relative to species distribution areas.References 1. S. R. Loarie et al., Nature 462, 1052 (2009) 2. J. Rey, G. Rohat, M. Perroud, S. Goyette, J. Kasparian, Env. Res. Lett. 15, 034027 (2020) 3. I. Gaponenko, G. Rohat, S. Goyette, P. Paruch, J. Kasparian, Sci. Rep., 12, 2997, (2022)

Probing Velocity Structures of Protostellar Envelopes: Infalling and Rotating Envelopes within Turbulent Dense Cores

Abstract: We have observed the three low-mass protostars, IRAS 15398−3359, L1527 IRS, and TMC-1A, with the ALMA 12 m array, the ACA 7 m array, and the IRAM-30 m and APEX telescopes in the C18O J = 2–1 emission. Overall, the C18O emission shows clear velocity gradients at radii of ∼100–1000 au, which likely originate from the rotation of envelopes, while velocity gradients are less clear and velocity structures are more perturbed on scales of ∼1000–10,000 au. IRAS 15398−3359 and L1527 IRS show a break at radii of ∼1200 and ∼1700 au in the radial profile of the peak velocity, respectively. The peak velocity is proportional to r −1.38 or r −1.7 within the break radius, which can be interpreted as indicating the rotational motion of the envelope with a degree of contamination by gas motions on larger spatial scales. The peak velocity follows v peak ∝ r 0.68 or v peak ∝ r 0.46 outside the break radius, which is similar to the J/M–R relation of dense cores. TMC-1A exhibits a radial profile of the peak velocity that is not consistent with the rotational motion of the envelope nor the J/M–R relation. The origin of the relation of v peak ∝ r 0.46–r 0.68 is investigated by examining correlations of the velocity deviation (δ v) and the spatial scale (τ) in the two sources. The obtained spatial correlations, δ v ∝ τ ∼0.6, are consistent with the scaling law predicted by turbulence models, which may suggest that large-scale velocity structures originate from turbulence.