Volume 27, Issue 7, July 2015
Index of content:
- Particulate, Multiphase, and Granular Flows
27(2015); http://dx.doi.org/10.1063/1.4922989View Description Hide Description
The mixing of solute tracers produced by a dilute suspension of spheres undergoing shear flow is examined in the limit that the gap between the sphere and the bounding walls becomes vanishingly small. Such tight confinement produces an envelope of flipping trajectories in the vicinity of the sphere giving rise to swift mixing of an anti-symmetric concentration distribution across the central plane of the gap. The size of this flipping envelope is demonstrated to be only weakly dependent on the angular velocity of the sphere and is accurately approximated by a two parameter model. This model can be used to calculate the additional mass transfer arising due to flipping. The degree of mixing in the system is directly related to the intensity of flipping which is shown to be a function of a single parameter χ given by where is the shear rate, a is the radius of the sphere, ϕs is the volume fraction of the spheres, and D is the diffusivity. As χ varies from 0 to ∞, the concentration distribution across the gap goes from being linear everywhere to being uniform across the flipping envelope, with the overall flux increasing by a factor of ∼3.
- Instability and Transition
Instability mechanisms in a low-Mach-number reacting flow from coupled convection-reaction-diffusion equations27(2015); http://dx.doi.org/10.1063/1.4923233View Description Hide Description
In this paper, instability mechanisms in a low Mach number reacting flow are investigated. Here, the emphasis is on the growth or decay of acoustic oscillations which arise from the acoustic-hydrodynamic interaction in a low Mach number reacting flow. Motivated by the studies in magnetohydrodynamics and atmospheric flows, we propose to investigate the acoustic-hydrodynamic coupling as a system of wave-mean flow interaction. For example, a comparison with the heat fluctuation modified hydrodynamics associated with magnetohydrodynamics is useful in understanding this coupling. The wavelike acoustic disturbance is introduced here as a compressibility correction to the mean flow. Accounting for the multiple scales introduced by the weak compressibility, we derive a set of equations governing wave-mean flow interaction in a reacting low Mach number flow. Sources such as volume expansion (which, in atmospheric flows arises due to the density variation with altitude) occur in reacting flows due to the heat release rate. This heat release rate, when coupled with the acoustic field, often leads to self-sustained thermo-acoustic oscillations. In the study of such oscillations, we discover a relation between the acoustic pressure and second order thermal fluctuations. Further, using this relation, we discover the nonlinear coupling mechanism that would lead to self-sustained oscillations in a reacting low Mach number flow. This mechanism, represented by a coupled convection reaction diffusion system, is presented here for the first time. In addition to the acoustic pressure and temperature fields, we also discover the role of acoustic velocity field in the acoustic-hydrodynamic interaction through a convective and lift-up mechanism.
- Turbulent Flows
27(2015); http://dx.doi.org/10.1063/1.4923234View Description Hide Description
A comparison between classical opposition control applied in the configuration of a fully developed turbulent channel flow and applied locally in a spatially developing turbulent boundary layer is presented. It is found that the control scheme yields similar drag reduction rates if compared at the same friction Reynolds numbers. However, a detailed analysis of the dynamical contributions to the skin friction coefficient reveals significant differences in the mechanism behind the drag reduction. While drag reduction in turbulent channel flow is entirely based on the attenuation of the Reynolds shear stress, the modification of the spatial flow development is essential for the turbulent boundary layer in terms of achievable drag reduction. It is shown that drag reduction due to this spatial development contribution becomes more pronounced with increasing Reynolds number (up to Re τ = 660, based on friction velocity and boundary layer thickness) and even exceeds drag reduction due to attenuation of the Reynolds shear stress. In terms of an overall energy balance, it is found that opposition control is less efficient in the turbulent boundary layer due to the inherently larger fluctuation intensities in the near wall region.
Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder27(2015); http://dx.doi.org/10.1063/1.4923063View Description Hide Description
The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).
- Compressible Flows
27(2015); http://dx.doi.org/10.1063/1.4922865View Description Hide Description
We examine the effects of compressibility, slip, and fluid inertia on the frequency response of particle-based velocimetry techniques for supersonic and hypersonic flows by solving the quasi-steady drag equation for solid, spherical particles. We demonstrate that non-continuum and fluid inertial effects significantly affect the particle response under all typical supersonic flow conditions. In particular, the particle frequency response obtained from a shock response test depends on the strength of the shock, decreasing with shock strength as non-continuum effects become more prominent. For weak disturbances, such as those typical of turbulence, the actual particle frequency response can therefore be much lower than that obtained from a typical shock response. The greatest variability in the response was found to occur at low supersonic Mach numbers. The results were found to be typical of solid particles used for velocimetry under a wide range of wind tunnel conditions, and so, previous particle frequency response analyses based solely on shock response tests may well have overestimated the response to turbulence.
Study of two supersonic streamwise vortex interactions in a Mach 2.5 flow: Merging and no merging configurations27(2015); http://dx.doi.org/10.1063/1.4923065View Description Hide Description
This work presents the detailed analysis of the flow fields resulting from two supersonic vortex interaction modes in a Mach 2.5 cold flow. The vortex interactions were selected beforehand by means of a reduced order modeling tool to obtain merging of the co-rotating structures in two counter-rotating vortex pairs in one case and to prevent their merging in the second. To experimentally target the flow physics of interest, expansion ramps of the same height but different width and mutual distance were placed on the surface of a strut injector. The resulting flow fields have been characterized using stereoscopic particle image velocimetry surveys conducted at selected streamwise planes. Accurate measurements of the velocity and vorticity fields, strain rates, circulation, and the rate of change of the area of each vorticity patch in the surveyed flow highlight the differences between merging and non-merging scenarios as well as the role of viscosity and turbulent diffusion in the resulting vortex dynamics. In particular, it is observed that, for the high Reynolds numbers typical of these supersonic flows and the associated time scales, an inviscid, non-diffusive formulation can be used for a reduced-order description that allows to capture the relevant dynamics related to the morphological distortion of the interacting streamwise vorticity patches introduced into the supersonic flow. The distribution of the strain rates in the flows resulting from the selected vortex configurations suggests that a faster mixing process may be expected in the merging case. However, the volumetric entrainment measurements show that the configuration in which merging was not attained yields a much larger entrainment of surrounding fluid compared to the merging case.
- Geophysical Flows
27(2015); http://dx.doi.org/10.1063/1.4923250View Description Hide Description
We present a numerical study on the mixing process between two stable states of a chemical reaction model. The two stable states of the reactions are found in practice not to coexist, and a single stable state of homogeneous scalar concentration is achieved over long time. With all other parameters fixed, we find the dependence of the final state on the rate of reaction. Interestingly, with the existence of coherent structures, at a range of intermediate rate of reaction, we find that the final state also depends on the initial locations of the scalar impurity. The exact dependence on initial condition is explored in detail. These results lead to the fundamental understanding on the variability of biogeochemical tracers in the environment induced by nonlinear fluid stirring.