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1. Forced oscillations of a cylinder in the flow of viscoelastic fluids

   https://doi.org/10.1017/jfm.2023.861  

   Patel, Umang N ; Rothstein, Jonathan P ; Modarres-Sadeghi, Yahya ( November 2023 , Journal of Fluid Mechanics)

   We investigate the effects of fluid elasticity on the flow forces and the wake structure when a rigid cylinder is placed in a viscoelastic flow and is forced to oscillate sinusoidally in the transverse direction. We consider a two-dimensional, uniform, incompressible flow of viscoelastic fluid at$Re=100$, and use the FENE-P model to represent the viscoelastic fluid. We study how the flow forces and the wake patterns change as the amplitude of oscillations,$A^*$, the frequency of oscillations (inversely proportional to a reduced velocity,$U^*$), the Weissenberg number,$Wi$, the square of maximum polymer extensibility,$L^2$, and the viscosity ratio,$\beta$, change individually. We calculate the lift coefficient in phase with cylinder velocity to determine the range of different system parameters where self-excited oscillations might occur if the cylinder is allowed to oscillate freely. We also study the effect of fluid elasticity on the added mass coefficient as these parameters change. The maximum elastic stress of the fluid occurs in between the vortices that are observed in the wake. We observe a new mode of shedding in the wake of the cylinder: in addition to the primary vortices that are also observed in the Newtonian flows, secondary vortices that are caused entirely by the viscoelasticity of the fluid are observed in between the primary vortices. We also show that, for a constant$Wi$, the strength of the polymeric stresses increases with increasing reduced velocity or with decreasing amplitude of oscillations.

    

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2. Coherent motions in a turbulent wake of an axisymmetric bluff body

   https://doi.org/10.1017/jfm.2023.231  

   Zhang, Fengrui ; Peet, Yulia T. ( May 2023 , Journal of Fluid Mechanics)

   The wake flow past an axisymmetric body of revolution at a diameter-based Reynolds number$Re=u_{\infty }D/\nu =5000$is investigated via a direct numerical simulation. The study is focused on identification of coherent vortical motions and the dominant frequencies in this flow. Three dominant coherent motions are identified in the wake: the vortex shedding motion with the frequency of$St=fD/u_{\infty }=0.27$, the bubble pumping motion with$St=0.02$, and the very-low-frequency (VLF) motion originated in the very near wake of the body with the frequency$St=0.002$–$0.005$. The vortex shedding pattern is demonstrated to follow a reflectional symmetry breaking mode, whereas the vortex loops are shed alternatingly from each side of the vortex shedding plane, but are subsequently twisted and tangled, giving the resulting wake structure a helical spiraling pattern. The bubble pumping motion is confined to the recirculation region and is a result of a Görtler instability. The VLF motion is related to a stochastic destabilisation of a steady symmetric mode in the near wake and manifests itself as a slow, precessional motion of the wake barycentre. The VLF mode with$St=0.005$is also detectable in the intermediate wake and may be associated with a low-frequency radial flapping of the shear layer.

    

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3. Criteria for dynamic stall onset and vortex shedding in low-Reynolds-number flows

   https://doi.org/10.1017/jfm.2024.753  

   Sudharsan, Sarasija ; Sharma, Anupam ( October 2024 , Journal of Fluid Mechanics)

   Dynamic stall at low Reynolds numbers,$Re \sim O(10^4)$, exhibits complex flow physics with co-existing laminar, transitional and turbulent flow regions. Current state-of-the-art stall onset criteria use parameters that rely on flow properties integrated around the leading edge. These include the leading edge suction parameter or$LESP$(Rameshet al.,J. Fluid Mech., vol. 751, 2014, pp. 500–538) and boundary enstrophy flux or$BEF$(Sudharsanet al.,J. Fluid Mech., vol. 935, 2022, A10), which have been found to be effective for predicting stall onset at moderate to high$Re$. However, low-$Re$flows feature strong vortex-shedding events occurring across the entire airfoil surface, including regions away from the leading edge, altering the flow field and influencing the onset of stall. In the present work, the ability of these stall criteria to effectively capture and localize these vortex-shedding events in space and time is investigated. High-resolution large-eddy simulations for an SD7003 airfoil undergoing a constant-rate, pitch-up motion at two$Re$(10 000 and 60 000) and two pitch rates reveal a rich variety of unsteady flow phenomena, including instabilities, transition, vortex formation, merging and shedding, which are described in detail. While stall onset is reflected in both$LESP$and$BEF$, local vortex-shedding events are identified only by the$BEF$. Therefore,$BEF$can be used to identify both dynamic stall onset and local vortex-shedding events in space and time.

    

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4. Nonlinear electrophoretic velocity of a spherical colloidal particle

   https://doi.org/10.1017/jfm.2023.537  

   Cobos, Richard ; Khair, Aditya S. ( August 2023 , Journal of Fluid Mechanics)

   Electrophoresis is the motion of a charged colloidal particle in an electrolyte under an applied electric field. The electrophoretic velocity of a spherical particle depends on the dimensionless electric field strength$\beta =a^*e^*E_\infty ^*/k_B^*T^*$, defined as the ratio of the product of the applied electric field magnitude$E_\infty ^*$and particle radius$a^*$, to the thermal voltage$k_B^*T^*/e^*$, where$k_B^*$is Boltzmann's constant,$T^*$is the absolute temperature, and$e^*$is the charge on a proton. In this paper, we develop a spectral element algorithm to compute the electrophoretic velocity of a spherical, rigid, dielectric particle, of fixed dimensionless surface charge density$\sigma$over a wide range of$\beta$. Here,$\sigma =(e^*a^*/\epsilon ^*k_B^*T^*)\sigma ^*$, where$\sigma ^*$is the dimensional surface charge density, and$\epsilon ^*$is the permittivity of the electrolyte. For moderately charged particles ($\sigma ={O}(1)$), the electrophoretic velocity is linear in$\beta$when$\beta \ll 1$, and its dependence on the ratio of the Debye length ($1/\kappa ^*$) to particle radius (denoted by$\delta =1/(\kappa ^*a^*)$) agrees with Henry's formula. As$\beta$increases, the nonlinear contribution to the electrophoretic velocity becomes prominent, and the onset of this behaviour is$\delta$-dependent. For$\beta \gg 1$, the electrophoretic velocity again becomes linear in field strength, approaching the Hückel limit of electrophoresis in a dielectric medium, for all$\delta$. For highly charged particles ($\sigma \gg 1$) in the thin-Debye-layer limit ($\delta \ll 1$), our computations are in good agreement with recent experimental and asymptotic results.

    

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5. Comparing local energy cascade rates in isotropic turbulence using structure-function and filtering formulations

   https://doi.org/10.1017/jfm.2023.1066  

   Yao, Hanxun ; Schnaubelt, Michael ; Szalay, Alexander S ; Zaki, Tamer A ; Meneveau, Charles ( February 2024 , Journal of Fluid Mechanics)

   Two common definitions of the spatially local rate of kinetic energy cascade at some scale$\ell$in turbulent flows are (i) the cubic velocity difference term appearing in the ‘scale-integrated local Kolmogorov–Hill’ equation (structure-function approach), and (ii) the subfilter-scale energy flux term in the transport equation for subgrid-scale kinetic energy (filtering approach). We perform a comparative study of both quantities based on direct numerical simulation data of isotropic turbulence at Taylor-scale Reynolds number 1250. While in the past observations of negative subfilter-scale energy flux (backscatter) have led to debates regarding interpretation and relevance of such observations, we argue that the interpretation of the local structure-function-based cascade rate definition is unambiguous since it arises from a divergence term in scale space. Conditional averaging is used to explore the relationship between the local cascade rate and the local filtered viscous dissipation rate as well as filtered velocity gradient tensor properties such as its invariants. We find statistically robust evidence of inverse cascade when both the large-scale rotation rate is strong and the large-scale strain rate is weak. Even stronger net inverse cascading is observed in the ‘vortex compression’$R>0$,$Q>0$quadrant, where$R$and$Q$are velocity gradient invariants. Qualitatively similar but quantitatively much weaker trends are observed for the conditionally averaged subfilter-scale energy flux. Flow visualizations show consistent trends, namely that spatially, the inverse cascade events appear to be located within large-scale vortices, specifically in subregions when$R$is large.

    

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