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arXiv2018

On the Trajectory Dependence of Atmospheric Boundary Layer Turbulence Sensing using Small Unmanned Vehicles

Atmospheric turbulence, especially in the near-surface boundary layer is known to be under-sampled due to the need to capture a wide separation in length and time-scales and limitation in the number of sensors. Over the past decade, the use if Unmanned Aircraft Systems (UAS) technology is approaching ubiquitous proportions for a wide variety of applications, especially with the recent FAA relaxation of flying restrictions. From a geophysical sciences perspective, such technology would allow for sensing of large-scale atmospheric flows, particularly, atmospheric boundary layer (ABL) turbulence, air quality monitoring in urban settings where multitude of small, minimally-invasive and mobile sensors can drastically alter our ability to study such complex phenomena. Currently available observational data of atmospheric boundary layer physics is so sparse and infrequent which significantly limits analysis. With the quantity and resolution of the data that can be measured using a swarm of UAS, three-dimensional reconstruction and deduction of coherent structures in ABL turbulence may be feasible. However, key challenges remain in the form of identifying optimal trajectories to fly the UAS to obtain the relevant quantifications of the turbulence, interpretation of the sensor data from mobile sensors and understanding how representative are the sparse measurements of the overall turbulent boundary layer. This leads to many fundamentally interesting questions if which a few are explored in this article.

physics.flu-dyn↗

arXiv2018

Rheology of an inverted cholesteric droplet under shear flow

The dynamics of a quasi two-dimensional isotropic droplet in a cholesteric liquid crystal medium under symmetric shear flow is studied by lattice Boltzmann simulations. We consider a geometry in which the flow direction is along the axis of the cholesteric, as this setup exhibits a significant viscoelastic response to external stress. We find that the dynamics depends upon the magnitude of the shear rate, the anchoring strength of the liquid crystal at the droplet interface and the chirality. While for low shear rate and weak interface anchoring the system shows a non-Newtonian behavior, a Newtonian-like response is observed at high shear rate and strong interface anchoring. This is investigated both by estimating the secondary flow profile, namely a flow emerging along the out-of-plane direction (absent in fully Newtonian fluids, such as water), and by monitoring defect formation and dynamics which alter significantly the rheological response of the system.

cond-mat.soft↗

arXiv2018

Two-point spectral model for variable-density homogeneous turbulence

We present a study of buoyancy-driven variable-density homogeneous turbulence, using a two-point spectral closure model. We compute the time-evolution of the spectral distribution in wavenumber $k$ of the correlation of density and specific-volume $b(k)$, the mass flux $\bm{a}(k)$, and the turbulent kinetic energy $E(k)$, using a set of coupled equations. Under the modeling assumptions, each dynamical variable has two coefficients governing spectral transfer among modes. In addition, the mass flux $\bm{a}(k)$ has two coefficients governing the drag between the two fluids. Using a prescribed initial condition for $b(k)$ and starting from a quiescent flow, we first evaluate the relative importance of the different coefficients used to model this system, and their impact on the statistical quantities. We next assess the accuracy of the model, relative to Direct Numerical simulation of the complete hydrodynamical equations, using $b$, ${\bm a}$ and $E$ as metrics. We show that the model is able to capture the spectral distribution and global means of all three statistical quantities at both low and high Atwood number for a set of optimized coefficients. The optimization procedure also permits us to discern a minimal set of four coefficients which are sufficient to yield reasonable results while pointing to the mechanisms that dominate the mixing process in this problem.

physics.flu-dyn↗

arXiv2018

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: Characterisation of the flow-field

This study examines the effects of angle of attack on the characteristics of the laminar separation bubble (LSB), its associated low-frequency flow oscillation (LFO), and the flow-field around a NACA-0012 aerofoil at $Re_c = 5\times 10^4$ and $M_{\infty} = 0.4$. In the range of the investigated angles of attack, statistics of the flow-field suggest the existence of three distinct angle-of-attack regimes: 1) the flow and aerodynamic characteristics are not much affected by the LFO for angles of attack $α< 9.25^{\circ}$; 2) the flow-field undergoes a transition regime in which the bubble bursting and reformation cycle, LFO, develops until it reaches a quasi-periodic switching between separated and attached flow in the range of angles of attack $9.25^{\circ} \leq α\leq 9.6^{\circ}$; and 3) for the angles of attack $α> 9.6^{\circ}$, the flow-field and the aerodynamic characteristics are overwhelmed by a quasi-periodic and self-sustained LFO until the aerofoil approaches the angle of full stall. A trailing-edge bubble (TEB) forms at $α> 9.25^{\circ}$ and grows with the angle of attack. The LSB and TEB merge and continue to deform until they form an open bubble at $α= 10.5^{\circ}$. On the suction surface of the aerofoil, the pressure distribution shows that the presence of the LSB induces a gradual and continues adverse pressure gradient (APG) when the flow is attached. The bursting of the bubble causes a gradual and continues favourable pressure gradient (FPG) when the flow is separated. This is indicative that a natural forcing mechanism keeps the flow attached against the APG and separated despite the FPG. The present investigation suggests that most of the observations reported in the literature about the LSB and its associated LFO are neither thresholds nor indicators for the inception of the instability, but rather are consequences of it.

physics.flu-dyn↗

arXiv2018

A Morphing Continuum Simulation of Transonic Flow over an Axisymmetric Hill

Finite volume simulations of turbulent boundary layer flow over an axisymmetric hill are performed for $Re_H$ = 6500 using Morphing Continuum Theory (MCT), and compared with DNS data from Castagna et. al. and experimental data obtained by Simpson. The inlet profile was specified by inputting values from the profile specified by Castagna et. al. Root- mean-square velocity fluctuations are inputted using the new variable of gyration from MCT. Additional terms introduced by MCT lead to a new formulation of the Q-criterion, which allows for the visualization of three-dimensional turbulent structures including a hairpin vortices. Streamline plots of the separation bubble show a more confined bubble than Castagna et. al., but agree well on the separation and reconnection points. The surface pressure coefficient matches Simpson much more closely than Castagna et. al. MCT data was obtained on a $6.72 \times 10^6$ cell mesh containing a smaller number of cells by an order of magnitude than that of the DNS mesh of $5.4 \times 10^7$ by Castagna et. al. The effects of the particle on the larger structures are inferred from the variation in the new variable of gyration.

physics.flu-dyn↗

arXiv2018

Turbulent channel flow of an elastoviscoplastic fluid

We present numerical simulations of laminar and turbulent channel flow of an elastoviscoplastic fluid. The non-Newtonian flow is simulated by solving the full incompressible Navier-Stokes equations coupled with the evolution equation for the elastoviscoplastic stress tensor. The laminar simulations are carried out for a wide range of Reynolds numbers, Bingham numbers and ratios of the fluid and total viscosity, while the turbulent flow simulations are performed at a fixed bulk Reynolds number equal to 2800 and weak elasticity. We show that in the laminar flow regime the friction factor increases monotonically with the Bingham number (yield stress) and decreases with the viscosity ratio, while in the turbulent regime the the friction factor is almost independent of the viscosity ratio and decreases with the Bingham number, until the flow eventually returns to a fully laminar condition for large enough yield stresses. Three main regimes are found in the turbulent case, depending on the Bingham number: for low values, the friction Reynolds number and the turbulent flow statistics only slightly differ from those of a Newtonian fluid; for intermediate values of the Bingham number, the fluctuations increase and the inertial equilibrium range is lost. Finally, for higher values the flow completely laminarises. These different behaviors are associated with a progressive increases of the volume where the fluid is not yielded, growing from the centerline towards the walls as the Bingham number increases. The unyielded region interacts with the near-wall structures, forming preferentially above the high speed streaks. In particular, the near-wall streaks and the associated quasi-streamwise vortices are strongly enhanced in an highly elastoviscoplastic fluid and the flow becomes more correlated in the streamwise direction.

physics.flu-dyn↗

arXiv2018

Orthogonal and antiparallel vortex tubes and energy cascades in quantum turbulence

We investigate the dynamics of energy cascades in quantum turbulence by directly observing the vorticity distributions in numerical simulations of the Gross-Pitaevskii equation. By Fourier filtering each scale of the vorticity distribution, we find that antiparallel vortex tubes at a large scale generate small-scale vortex tubes orthogonal to those at the large scale, which is a manifestation of the energy cascade from large to small scales. We reveal the dynamics of quantized vortex lines in these processes.

cond-mat.quant-gas↗

arXiv2018

A paradox in Hele-Shaw displacements

We study the Hele-Shaw immiscible displacements when all surfaces tensions on the interfaces are zero. The Saffman-Taylor instability occurs when a less viscous fluid is displacing a more viscous one, in a rectangular Hele-Shaw cell. We prove that an intermediate liquid with a variable viscosity can almost suppress this instability. On the contrary, a large number of constant viscosity liquid-layers inserted between the initial fluids gives us boundless growth rates with respect to the wave numbers of perturbations. The same amount of intermediate liquid is used in both cases.

physics.flu-dyn↗

arXiv2018

On the Whitham system for the radial nonlinear Schrödinger equation

Dispersive shock waves (DSWs) of the defocusing radial nonlinear Schrödinger (rNLS) equation in two spatial dimensions are studied. This equation arises naturally in Bose-Einstein condensates, water waves and nonlinear optics. A unified nonlinear WKB approach, equally applicable to integrable or nonintegrable partial differential equations, is used to find the rNLS Whitham modulation equation system in both physical and hydrodynamic type variables. The description of DSWs obtained via Whitham theory is compared with direct rNLS numerics; the results demonstrate very good quantitative agreement. On the other hand, as expected, comparison with the corresponding DSW solutions of the one-dimensional NLS equation exhibits significant qualitative and quantitative differences.

nlin.PS↗

arXiv2018

Single-phase flow through a porous media revisited

The problem of the flow trough a porous media is formulated in terms of a pressure equation, based on arguments of volume conservation which state the mechanical equilibrium between the solid and the fluid phases. In the resulting governing equations, the conservation of mass is replaced by a transport-diffusion equation for the pressure field. When the deformation of the solid matrix is negligible, the pressure equation can be approximated by a non-linear diffusion equation. The behavior of the system of equations is tested under an oscillatory pressure gradient.

physics.flu-dyn↗

arXiv2018

Can Sound Waves be Slowed?

Dielectric fluids experience a striction force in the presence of an external electric field. Although the striction force on the entire body of the fluid is usually zero, it does contribute to its deformations. In this paper, I show that the speed of sound in dielectric fluids is reduced when they are exposed to uniform electric fields of sufficiently high magnitude. Ferrofluids also experience a similar striction force in presence of a magnetizing field. However, their low relative permeability makes the effect very small.

physics.flu-dyn↗

arXiv2018

Self similar flow under the action of monochromatic radiation behind a cylindrical shock wave in a self-gravitating, rotating axisymmetric dusty gas

The propagation of a cylindrical shock wave in a self-gravitating, rotating axisymmetric dusty gas under the action of monochromatic radiation with a constant intensity per unit area, which has variable azimuthal and axial components of fluid velocity, is investigated. The gas is assumed to be grey and opaque, and the shock is assumed to be transparent. The dusty gas is considered as a mixture of non-ideal gas and small solid particles, in which solid particles are continuously distributed. To obtain some essential features of shock propagation, small solid particles are considered as a pseudo-fluid, and it is assumed that the equilibrium flow condition is maintained in the entire flow-field. Similarity solutions are obtained as well as the effects of the variation of the radiation parameter, the gravitation parameter, the non-idealness parameter of the gas, the mass concentration of solid particles in the mixture, the ratio of the density of solid particles to the initial density of the gas are worked out in detail. The similarity solution exists under the constant initial angular velocity, and the shock strength is independent from the radiation parameter and the gravitation parameter. It is found that radiation parameter dominates the effect of dusty gas parameters on the variation of radiation heat flux. The total energy of the flow-field behind the shock front is not constant but varies as fourth power of the shock radius.

physics.flu-dyn↗

arXiv2018

Verification and Validation of a Numerical Wave Tank Using Waves2FOAM

Clean energy systems have been investigated recently by the researchers of the University of New Mexico to increase the efficiency specifically in the solar tower technology. Similar to solar energy, wave power harnesses energy that comes from the sun. Solar irradiation causes wind by changing the pressure, and wind gives its momentum to the ocean surface which produces waves. Modeling and simulation (M&S) of free surface flow can be a very useful tool for the wave energy industry. However, if these tools are to be relied upon, their accuracy must be tested and shown to be sound. In this investigation, we focus on verification and validation (V&V) of wave tank flow M&S. The Volume of Fluid (VOF) model was applied to perform transient computational fluid dynamics (CFD) analysis using the numerical simulation software OpenFOAM. To assess V&V, the height of the free surface flow was considered as the system response quantity (SRQ). Stokes theory (fifth order) was utilized as the most accurate available approximation of the exact solution to measure the order of accuracy of the discretized mathematical model which is known as code verification. Data collected during model-scale testing at the Naval Surface Warfare Center, Carderock Division (NSWCCD) Maneuvering and Seakeeping (MASK) basin were utilized to validate the computational results. The CFD domain consists of a two-dimensional plane representing the region of interest of the wave basin which has several wave height measurement probes. Our simulation results were compared to the data from one of these probes in order to conduct the V&V analysis. Code verification and comparison with experimental values demonstrate the accuracy and efficiency of our model.

physics.flu-dyn↗

arXiv2018

Viscous flow regimes in unit square: Part 3. Phenomena of leapfrogging and approaching among multiple vortex pairs

In the present note, we solved numerically the viscous vorticity equation of the initial-boundary value problem describing the classic Helmholtz phenomena of vortex interaction. In the leapfrogging of vortex pairs, we demonstrate the fact that there exists a variety of initial vortex configurations, such as the initial vortex core structures, the starting speeds, the lateral and longitudinal separations as well as the fluid viscosity. To simulate leapfrogging appears to be a straightforward task, based on the vorticity calculations for a number of initial vortices with different core structures. Indeed, the evaluation of the unsteady vortex interactions requires accurate numerical simulations, and the solutions are diverse and intriguing. In particular, the impact of two asymmetric approaching vortices can produce peeled-off fission vortices or cross-bred eddies of mushroom topology. Over the course of flow development, we are unable to define any unique consistent Reynolds number which may be used to classify the individual flows, because of the multiplicity of characteristic lengths and velocity scales. Our key observation is that the initial-boundary value problems of fluid motion do not necessarily imply the realization of dynamically similar flows.

physics.flu-dyn↗

arXiv2018

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The underlying mechanism

The present investigation shows that a triad of three vortices, two co-rotating vortices (TCV) and a secondary vortex lies beneath them and counter-rotating with them, is behind the quasi-periodic self-sustained bursting and reformation of the laminar separation bubble (LSB) and its associated low-frequency flow oscillation (LFO). The upstream vortex of the TCV (UV) is driven by the gradient of the oscillating-velocity across the laminar portion of the separated shear layer and is faithfully aligned with it. The secondary vortex acts as a roller support that facilitates the rotation and orientation of the TCV. A global oscillation in the flow-field around the aerofoil is observed in all of the investigated angles of attack, including at zero angle of attack. The flow switches between an attached-phase against an adverse pressure gradient (APG) and a separated-phase despite a favourable pressure gradient (FPG) in a periodic manner with some disturbed cycles. When the direction of the oscillating-flow is clockwise, it adds momentum to the boundary layer and helps it to remain attached against the APG and vice versa. The transition location along the separated shear layer moves upstream when the oscillating-flow rotates in the clockwise direction causing early-transition and vice versa. The best description of the mechanism is that of a whirligig. When the oscillating-flow rotates in the clockwise direction, the UV whirls in the clockwise direction and store energy until it is saturated, then the process is reversed. The present investigation paves the way for formulating a time-accurate physics-based model of the LFO and stall prediction, and opens the door for control of its undesirable effects.

physics.flu-dyn↗

arXiv2018

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The dynamics of the flow-field

Detailed flow dissection using Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) is carried out to investigate the dynamics of the flow-field around a NACA-0012 aerofoil at a Reynolds number of $5\times 10^4$, Mach number of $0.4$, and at various angles of attack around the onset of stall. Three distinct dominant flow modes are identified by the DMD and the POD: 1) a globally oscillating flow mode at a low-frequency (LFO mode 1); 2) a locally oscillating flow mode on the suction surface of the aerofoil at a low-frequency (LFO mode 2); and 3) a locally oscillating flow mode along the wake of the aerofoil at a high-frequency (HFO mode). The LFO mode 1 features the globally oscillating-flow around the aerofoil, the oscillating-pressure along the aerofoil chord, and the process that creates and sustains the triad of vortices. The LFO mode 2 features the expansion and advection of the upstream vortex (UV) of the TCV. The HFO mode originates at the aerofoil leading-edge and features the oscillating mode along the aerofoil wake. The HFO mode exists at all of the investigated angles of attack and causes a global oscillation in the flow-field. The global flow oscillation around the aerofoil interacts with the laminar portion of the separated shear layer in the vicinity of the leading-edge and triggers an inviscid absolute instability that creates and sustains the TCV. When the UV of the TCV expands, it advects downstream and energies the HFO mode. The LFO mode 1, the LFO mode 2, and the HFO mode mutually strengthen each other until the LFO mode 1 and 2 overtake the HFO mode at the onset of stall. At the angles of attack $9.7^{\circ} \leq α\leq 10.0^{\circ}$ the low-frequency flow oscillation (LFO) phenomenon is fully developed. At higher angles of attack, the HFO mode overtakes the LFO mode 2, and the aerofoil undergoes a full stall.

physics.flu-dyn↗

arXiv2018

Turbulucid: A Python Package for Post-Processing of Fluid Flow Simulations

A Python package for post-processing of plane two-dimensional data from computational fluid dynamics simulations is presented. The package, called turbulucid, provides means for scripted, reproducible analysis of large simulation campaigns and includes routines for both data extraction and visualization. For the former, the Visualization Toolkit (VTK) is used, allowing for post-processing of simulations performed on unstructured meshes. For visualization, several matplotlib-based functions for creating highly customizable, publication-quality plots are provided. To demonstrate turbulucid's functionality it is here applied to post-processing a simulation of a flow over a backward-facing step. The implementation and architecture of the package are also discussed, as well as its reuse potential.

cs.CE↗

arXiv2018

The deformation of a flexible fiber settling in a quiescent viscous fluid

The equilibrium state of a flexible fiber settling in a viscous fluid is examined using a combination of macroscopic experiments, numerical simulations and scaling arguments. We identify three regimes having different signatures on this equilibrium configuration of the elastic filament: weak and large deformation regimes wherein the drag is proportional to the settling velocity as expected in Stokes flow and an intermediate elastic reconfiguration regime where the filament deforms to adopt a shape with a smaller drag which is no longer linearly proportional to the velocity.

physics.flu-dyn↗