Session A53: Instabilities and Turbulence

Oil Induced Spontaneous Flow in Water- Bis(2-ethylhexyl)Sulfosuccinat (AOT) system

Instability and evaporation rates of oils within the layers of vesicles of a surfactants trigger the spontaneous (second flow) flow. The incorporation of oils into bis(2-ethylhexyl)sulfosuccinat (AOT) system remains incompletely characterized. We show that the second flow has a finite size that show a minimum at a particular concentration (mM) of surfactant solution. As a result, the layers are destabilized lead to “explode” and create the second flow. The fluorescence emission spectra and evaporation rates show that the oil diffuses into the layers of vesicles of a surfactant. We have characterized evaporation rates of oils on various concentrations (mM) of surfactant solution and observed that oils evaporation rates depend on volume and remain constant as the function of concentration of surfactant. We believe that second flow is new feature and brings a new insight into the fluid flow dynamics.   

Joule Heating Effects on Electrokinetic Flow Instabilities in Ferrofluids.

We have demonstrated in our earlier work that the application of a tangential electric field can draw fluid instabilities at the interface of a ferrofluid/water co-flow. These electrokinetic flow instabilities are produced primarily by the mismatch of electric conductivities of the two fluids. We demonstrate in this talk that the Joule heating induced fluid temperature rises and gradients can significantly suppress the electrokinetic flow instabilities. We also develop a two-dimensional depth-averaged numerical model to predict the fluid temperature, flow and concentration fields in the two-fluid system with the goal to understand the Joule heating effects on electric field-driven ferrofluid flow instabilities.   

Wall mode instability driven transition to turbulence in a soft microchannel

Transition to turbulence has been triggered due to structure fluid interaction at Reynolds number (Re) much lower than hard wall transition Re, in a soft walled micro channel of dimensions 40mm*1.5mm*0.16mm. Mixing index analysis indicates high degree of mixing accompanied by lower pressure drop as the channel deforms. Flow after transition velocity statistics has been extensively studied using Particle Imaging Velocimetry (PIV) along streamwise-wallnormal direction. The reduced plots of streamwise mean velocity are shown with the absence of viscous sublayer and presence of logarithmic layer with von Karman constants different from rigid wall channel. The one–point cross correlation between velocity fluctuations is non-zero at the soft surface which is in contrast to flow in hard walled channel. This indicates that the additional fluid stress exerted on the soft surface by the fluid velocity fluctuations result in net energy transfer due to shear work done at the interface. The structure fluid interface acts as a source of energy for the mean turbulent kinetic energy which is typically zero at the interface for hard walled channel. We also detect the onset of wall-oscillations primarily tangential to the surface at the transition Re.   

The effect of viscosity variation on the stability of a buoyantly unstable miscible layer in vertical porous media

We numerically show that in the absence of displacement a buoyantly unstable miscible layer with variable viscosity is less unstable than the constant viscosity layers. With the help of scaling analysis, we proved that the dynamics of variable viscosity layers with stable as well as unstable viscosity contrasts are identical in the absence of displacement. When the heavier fluid displaces the lighter one, the influence of viscosity contrast on the buoyantly unstable miscible layer is analogous to that in neutrally buoyant fluids. These findings of direct numerical simulations (DNS) in the fully nonlinear regime are consistent with the linear stability analysis (LSA). Furthermore, we perform a non-modal stability analysis of the linearized equations, which depicts the qualitative agreement with both LSA and DNS. In addition, the response of the linearized operator to external excitation has been studied through pseudospectra. The present findings are of great importance to understand the hydrodynamic mechanisms involved in geologic carbon sequestration.   

Untying vortex knots in fluids and superfluids

Recent work has demonstrated that vortex knots appear to always untie in fluids and superfluids. Should we expect the same behavior from these two very different systems? I will discuss this unknotting behavior, both quantitatively -- through helicity -- and qualitatively through the geometry and topology of the vortex lines as they evolve.   

Relaxation of Anisotropy in Superfluid Turbulence

We simulate superfluid turbulence on a 3-sphere rather than using the more common periodic boundary conditions. We find that our topology naturally leads to anisotropy in a steady-state vortex tangle. A fundamental assumption in turbulence studies is that any large-scale anisotropy due to a driving velocity can be ignored at small length scales. However, there are practical concerns over how quickly the anisotropy decreases with length scale, and whether isotropic turbulence is attained above the dissipation scale. Here we examine how the anisotropy decreases upon moving from large to small length scales.   

Simulating transitional hydrodynamics of the cerebrospinal fluid at extreme scale

Chiari malformation type I is a disorder characterized by the herniation of cerebellar tonsils into the spinal canal through the foramen magnum resulting in obstruction to cerebrospinal fluid (CSF) outflow. The flow of pulsating bidirectional CSF is of acutely complex nature due to the anatomy of the conduit containing it - the subarachnoid space. We report lattice Boltzmann method based direct numerical simulations on patient specific cases with spatial resolution of $24\mu m$ amounting meshes of up to 2 billion cells conducted on 50000 cores of the Hazelhen supercomputer in Stuttgart. The goal is to characterize intricate dynamics of the CSF at resolutions that are of the order of Kolmogorov microscales. Results unfold velocity fluctuations up to $\sim10KHz$, turbulent kinetic energy $\sim 2$ times of the mean flow energy in Chiari patients whereas the flow remains laminar in a control subject. The fluctuations confine near the cranio-vertebral junction and are commensurate with the extremeness of pathology and the extent of herniation. The results advocate that the manifestation of pathological conditions like Chiari malformation may lead to transitional hydrodynamics of the CSF, and a prudent calibration of numerical approach is necessary to avoid overlook of such phenomena.   

Numerical Simulation of Parachutist Generated Turbulence on Parachute Inflation

Using the front tracking computational platform, we couple parachutists as rigid bodies with the spring-mass model for the parachute system. The rigid body generates turbulent flow which affect the parachute inflation and stability. In this talk, we will present our numerical method to solve the complex system and study the effect of the turbulence at the wake of the parachutist on the canopy opening and parachute descent. Several different turbulence models are used and compared with experiments.   

Tailoring boundary geometry to optimize heat transport in turbulent convection

Turbulent Rayleigh-B\'enard convection between planar horizontal boundaries is a classical example of the challenge posed by multiple interacting scales in fluid dynamics. Here, by tailoring the geometry of the upper boundary we manipulate the boundary layer -- turbulent interior flow interaction, and study the turbulent transport of heat in two-dimensional Rayleigh-B\'enard convection with numerical simulations using the Lattice Boltzmann method. By fixing the roughness amplitude of the upper boundary and varying the wavelength $\lambda$, we find that the exponent $\beta$ in the Nusselt-Rayleigh scaling relation, $Nu-1 \propto Ra^\beta$, is maximized at $\lambda \equiv \lambda_{\text{max}} \approx (2 \pi)^{-1}$, but decays to the planar value in both the large ($\lambda \gg \lambda_{\text{max}}$) and small ($\lambda \ll \lambda_{\text{max}}$) wavelength limits. The changes in the exponent originate in the nature of the coupling between the boundary layer and the interior flow. We present a simple scaling argument embodying this coupling, which describes the maximal convective heat flux. Results from simulations with both top and bottom rough boundaries showing a further enhancement of heat transport will also be presented.   

Tracking Coherent Structures and Source Localization in Geophysical Flows

There has been a steady increase in the deployment of autonomous underwater and surface vehicles for applications such as ocean monitoring, tracking of marine processes, and forecasting contaminant transport. The underwater environment poses unique challenges since robots must operate in a communication and localization-limited environment where their dynamics are tightly coupled with the environmental dynamics. This work presents current efforts in understanding the impact of geophysical fluid dynamics on underwater vehicle control and autonomy. The focus of the talk is on the use of collaborative vehicles to track Lagrangian coherent structures and to localize contaminant spills.   

Neutral equivalent surface stress

In laboratory turbulent flows, it has been observed that the eigen axes of the Reynolds stress tensor are oriented 17 deg from the streamwise coordinate system. This has also been observed in atmospheric flows over relatively flat terrain under thermally neutral conditions. The reliability of this relationship is examined, especially for locations very near the surface. The relationship is then used as the basis for a neutral equivalent momentum transport which can be calculated in complex environments, such as urban canyons, where the influence of multiple wall normals makes more conventional measurement of momentum transport problematic.