publications
publications in reverse chronological order. generated by jekyll-scholar.
2025
- Turbulence and mixing from neighbouring stratified shear layersAJK Yang , M-L Timmermans , J Olsthoorn , and 1 more authorPhysical Review Fluids, 2025
The vertical transport of sediment in marine settings can be influenced by a settling-driven convective instability and velocity shear. We conduct three-dimensional numerical simulations to investigate vertical sediment transport out of a surface mixed layer under different shear strengths. We show how this transport is determined by a competition between the growth of the settling-driven convective instability (Rayleigh-Taylor) and the stratified shear instabilities, and provide implications for marine carbon dioxide re- moval. In weak shear scenarios (characteristic velocity difference ΔU < 0.05 m/s), the Rayleigh-Taylor instability drives enhanced vertical sediment transport compared to Stokes particle settling; the sediment transport is weakly sensitive to the strength of shear, with the slowest effective settling velocity in the absence of shear. In the presence of strong shear scenarios (ΔU > 0.05 m/s), stratified shear instabilities grow rapidly and suppress the Rayleigh-Taylor instability, and the effective settling of particles is significantly reduced. Furthermore, with an increase in the strength of shear instabilities, the effective particle settling velocity and the rate of mass transport decrease. We show how these estimates for particle transport out of the surface mixed layer relate to the limited potential for atmospheric carbon dioxide draw down via approaches that rely on particle dissolution in the surface ocean.
@article{yang2025, title = {Turbulence and mixing from neighbouring stratified shear layers}, author = {Yang, AJK and Timmermans, M-L and Olsthoorn, J and Kaminski, AK}, journal = {Physical Review Fluids}, volume = {10}, pages = {014501}, year = {2025}, doi = {10.1103/PhysRevFluids.10.014501} }
2024
- Turbulence and mixing from neighbouring stratified shear layersC-L Liu , AK Kaminski , and WD SmythJournal of Fluid Mechanics, 2024
Studies of Kelvin–Helmholtz (KH) instability have typically modelled the initial mean flow as an isolated stratified shear layer. However, geophysical flows frequently exhibit multiple layers. As a step towards understanding these flows, we examine the case of two adjacent stratified shear layers using both linear stability analysis and direct numerical simulations. With sufficiently large layer separation, the characteristics of instability and mixing converge towards the familiar KH turbulence, and similarly when the separation is near zero and the layers add to make a single layer, albeit with a reduced Richardson number. Here, our focus is on intermediate separations, which produce new and complex phenomena. As the separation distance D increases from zero to a critical value D_c, approximately half the thickness of the shear layer, the growth rate and wavenumber both decrease monotonically. The minimum Richardson number is relatively low, potentially inducing pairing, and shear-aligned convective instability (SCI) is the primary mechanism for transition. Consequently, mixing is relatively strong and efficient. When D D_c, billow length is increased but growth is slowed. Despite the modest growth rate, mixing is strong and efficient, engendered primarily by secondary shear instability manifested on the braids, and by SCI occurring on the eyelids. Shear-aligned vortices are driven in part by buoyancy production; however, shear production and vortex stretching are equally important mechanisms. When D>D_c, neighbouring billow interactions suppress the growth of both KH instability and SCI. Strength and efficiency of mixing decrease abruptly as D_c is exceeded. As turbulence decays, layers of marginal instability may arise.
@article{liu2024, title = {Turbulence and mixing from neighbouring stratified shear layers}, author = {Liu, C-L and Kaminski, AK and Smyth, WD}, journal = {Journal of Fluid Mechanics}, volume = {987}, pages = {A8}, year = {2024}, doi = {10.1017/jfm.2024.387} }
2023
- Dynamics of asymmetric stratified shear instabilitiesJ Olsthoorn , AK Kaminski , and DR RobbPhysical Review Fluids, 2023
Most idealized studies of stratified shear instabilities assume that the shear interface and the buoyancy interface are coincident. We discuss the role of asymmetry on the evolution of shear instabilities. Using linear stability theory and direct numerical simulations, we show that asymmetric shear instabilities exhibit features of both Holmboe and Kelvin-Helmholtz (KH) instabilities, and develop a framework to determine whether the instabilities are more Holmboe-like or more KH-like. Further, the asymmetric instabilities produce asymmetric mixing that exhibits features of both overturning and scouring flows and that tends to realign the shear and buoyancy interfaces. In all but the symmetric KH simulations, we observe a collapse in the distribution of gradient Richardson number (Ri_g), suggesting that asymmetry reduces the parameter dependence of KH-driven mixing events. The observed dependence of the turbulent dynamics on small-scale details of the shear and stratification has important implications for the interpretation of oceanographic data.
@article{olsthoorn2023, title = {Dynamics of asymmetric stratified shear instabilities}, author = {Olsthoorn, J and Kaminski, AK and Robb, DR}, journal = {Physical Review Fluids}, volume = {8}, pages = {024501}, year = {2023}, doi = {10.1103/PhysRevFluids.8.024501} } - The effects of boundary proximity on Kelvin-Helmholtz instability and turbulenceC-L Liu , AK Kaminski , and WD SmythJournal of Fluid Mechanics, 2023
Studies of Kelvin–Helmholtz (KH) instability have typically modelled the initial flow as an isolated shear layer. In geophysical cases, however, the instability often occurs near boundaries and may therefore be influenced by boundary proximity effects. Ensembles of direct numerical simulations are conducted to understand the effect of boundary proximity on the evolution of the instability and the resulting turbulence. Ensemble averages are used to reduce sensitivity to small variations in initial conditions. Both the transition to turbulence and the resulting turbulent mixing are modified when the shear layer is near a boundary: the time scales for the onset of instability and turbulence are longer, and the height of the KH billow is reduced. Subharmonic instability is suppressed by the boundary because phase lock is prevented due to the diverging phase speeds of the KH and subharmonic modes. In addition, the disruptive influence of three-dimensional secondary instabilities on pairing is more profound as the two events coincide more closely. When the shear layer is far from the boundary, the shear-aligned convective instability is dominant; however, secondary central-core instability takes over when the shear layer is close to the boundary, providing an alternate route for the transition to turbulence. Both the efficiency of the resulting mixing and the turbulent diffusivity are dramatically reduced by boundary proximity effects.
@article{liu2023, title = {The effects of boundary proximity on Kelvin-Helmholtz instability and turbulence}, author = {Liu, C-L and Kaminski, AK and Smyth, WD}, journal = {Journal of Fluid Mechanics}, volume = {966}, pages = {A2}, year = {2023}, doi = {10.1017/jfm.2023.412} } - Fluid dynamics challenges in predicting plastic pollution transport in the ocean: A perspectiveBR Sutherland , M DiBenedetto , A Kaminski , and 1 more authorPhysical Review Fluids, 2023
Plastic pollution has been observed throughout the world’s oceans and estuaries, whether floating at the surface, settled in bottom sediments, washed up on beaches, or ingested by marine life. However, the vast majority of discarded plastics are unaccounted for. The problem of predicting the fate of discarded plastics has spurred fundamental research into fluid-particle interactions in previously unexplored regimes. Through talks and focused discussion groups taking place during a February 2022 online workshop hosted by the Banff International Research Station, theorists, experimentalists, numerical modelers, and observational oceanographers presented recent advances and identified outstanding prob- lems in predicting plastic transport in the ocean, focusing on the role of fluid dynamics. The outcomes of this meeting and discussions that followed are reported upon here.
@article{sutherland2023, title = {Fluid dynamics challenges in predicting plastic pollution transport in the ocean: A perspective}, author = {Sutherland, BR and DiBenedetto, M and Kaminski, A and {van den Bremer}, T}, journal = {Physical Review Fluids}, volume = {8}, pages = {070701}, year = {2023}, doi = {10.1103/PhysRevFluids.8.070701} }
2022
- The butterfly effect and the transition to turbulence in a stratified shear layerC-L Liu , AK Kaminski , and WD SmythJournal of Fluid Mechanics, 2022
In a stably stratified shear layer, multiple competing instabilities produce sensitivity to small changes in initial conditions, popularly called the butterfly effect (as a flapping wing may alter the weather). Three ensembles of 15 simulated mixing events, identical but for small perturbations to the initial state, are used to explore differences in the route to turbulence, the maximum turbulence level and the total amount and efficiency of mixing accomplished by each event. Comparisons show that a small change in the initial state alters the strength and timing of the primary Kelvin-Helmholtz instability, the subharmonic pairing instability and the various three-dimensional secondary instabilities that lead to turbulence. The effect is greatest in, but not limited to, the parameter regime where pairing and the three-dimensional secondary instabilities are in strong competition. Pairing may be accelerated or prevented; maximum turbulence kinetic energy may vary by up to a factor of 4.6, flux Richardson number by 12 %-15 % and net mixing by a factor of 2.
@article{liu2022, title = {The butterfly effect and the transition to turbulence in a stratified shear layer}, author = {Liu, C-L and Kaminski, AK and Smyth, WD}, journal = {Journal of Fluid Mechanics}, volume = {953}, pages = {A43}, year = {2022}, doi = {10.1017/jfm.2022.985} } - Shear instabilities and stratified turbulence in an estuarine fluid mudJ Tu , D Fan , F Sun , and 2 more authorsJournal of Physical Oceanography, 2022
This study presents field observations of fluid mud and the flow instabilities that result from the interaction between mud-induced density stratification and current shear. Data collected by shipborne and bottom-mounted instruments in a hyperturbid estuarine tidal channel reveal the details of turbulent sheared layers in the fluid mud that persist throughout the tidal cycle. Shear instabilities form during periods of intense shear and strong mud-induced stratification, particularly with gradient Richardson number smaller than or fluctuating around the critical value of 0.25. Turbulent mixing plays a significant role in the vertical entrainment of fine sediment over the tidal cycle. The vertical extent of the billows identified seen in the acoustic images is the basis for two useful parameterizations. First, the aspect ratio (billow height/wavelength) is indicative of the initial Richardson number that characterizes the shear flow from which the billows grew. Second, we describe a scaling for the turbulent dissipation rate ε that holds for both observed and simulated Kelvin-Helmholtz billows. Estimates for the present observations imply, however, that billows growing on a lutocline obey an altered scaling whose origin remains to be explained.
@article{tu2022, title = {Shear instabilities and stratified turbulence in an estuarine fluid mud}, author = {Tu, J and Fan, D and Sun, F and Kaminski, A and Smyth, W}, journal = {Journal of Physical Oceanography}, volume = {52}, pages = {2257-2271}, year = {2022}, doi = {10.1175/JPO-D-21-0230.1} }
2021
- High-resolution observations of the North Pacific transition layer from a Lagrangian floatAK Kaminski , EA D’Asaro , AY Shcherbina , and 1 more authorJournal of Physical Oceanography, 2021
A crucial region of the ocean surface boundary layer (OSBL) is the strongly sheared and strongly stratified transition layer (TL) separating the mixed layer from the upper pycnocline, where a diverse range of waves and instabilities are possible. Previous work suggests that these different waves and instabilities will lead to different OSBL behaviors. Therefore, understanding which physical processes occur is key for modeling the TL. Here we present observations of the TL from a Lagrangian float deployed for 73 days near Ocean Weather Station Papa (508N, 1458W) during fall 2018. The float followed the vertical motion of the TL, continuously measuring profiles across it using an ADCP, temperature chain, and salinity sensors. The temperature chain made depth–time images of TL structures with a resolution of 6 cm and 3 s. These showed the frequent occurrence of very sharp interfaces, dominated by temperature jumps of O(1)8C over 6 cm or less. Temperature inversions were typically small (&10 cm), frequent, and strongly stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles. These struc- tures are consistent with scouring behavior rather than Kelvin–Helmholtz–type overturning. Their net effect, estimated via a Thorpe-scale analysis, suggests that these frequent small temperature inversions can account for the observed mixed layer deepening and entrainment flux. Corresponding estimates of dissipation, diffusivity, and heat fluxes also agree with previous TL studies, suggesting that the TL dynamics is dominated by these nearly continuous 10-cm-scale mixing struc- tures, rather than by less frequent larger overturns.
@article{kaminski2021, title = {High-resolution observations of the North Pacific transition layer from a Lagrangian float}, author = {Kaminski, AK and D'Asaro, EA and Shcherbina, AY and Harcourt, RR}, journal = {Journal of Physical Oceanography}, volume = {51}, pages = {3163-3181}, year = {2021}, doi = {10.1175/JPO-D-21-0032.1} } - Parameterizing eddy transport of biogeochemical tracersCJ Prend , GR Flierl , KM Smith , and 1 more authorGeophysical Research Letters, 2021
The distribution of oceanic biogeochemical tracers is fundamentally tied to physical dynamics at and below the mesoscale. Since global climate models rarely resolve those scales, turbulent transport is parameterized in terms of the large-scale gradients in the mean tracer distribution and the physical fields. Here, we demonstrate that this form of the eddy flux is not necessarily appropriate for reactive tracers, such as nutrients and phytoplankton. In an idealized nutrient-phytoplankton system, we show that the eddy flux of one tracer should depend on the gradients of itself and the other. For certain parameter regimes, incorporating cross-diffusion can significantly improve the representation of both phytoplankton and nutrient eddy fluxes. We also show that the efficacy of eddy diffusion parameterizations requires timescale separation between the flow and reactions. This result has ramifications for parameterizing subgrid scale biogeochemistry in more complex ocean models since many biological processes have comparable timescales to submesoscale motions.
@article{prend2021, title = {Parameterizing eddy transport of biogeochemical tracers}, author = {Prend, CJ and Flierl, GR and Smith, KM and Kaminski, AK}, journal = {Geophysical Research Letters}, volume = {48}, pages = {e2021GL094405}, year = {2021}, doi = {10.1029/2021GL094405} }
2020
- An experimental investigation of the Rossby two-slit problemAK Kaminski , KR Helfrich , and J PedloskyJournal of Fluid Mechanics, 2020
The problem of the transmission of wave energy through small gaps arises in a variety of physical contexts. Here we consider the problem of Rossby waves encountering a barrier with two small gaps. In contrast to waves encountering a barrier with one small gap, in which very little wave energy is predicted to transmit across the barrier, when there are two or more gaps linear theory predicts that the barrier may be surprisingly inefficient at blocking the transmission of Rossby wave energy, owing to the requirement that circulation be conserved around individual segments of the barrier. However, the theory neglects viscosity in the main basin interiors and nonlinear effects in the basins and the gaps. To examine these effects, here we present the results of a series of laboratory experiments in which Rossby basin modes interact with a barrier with zero, one or two gaps. We find that the large-scale waves are able to transmit across the barrier with two gaps as predicted by the theory. However, while the linear theory captures the large-scale flow structures, viscosity and nonlinearity significantly affect the flow along the boundaries and near the gaps in the barrier.
@article{kaminski2020a, title = {An experimental investigation of the Rossby two-slit problem}, author = {Kaminski, AK and Helfrich, KR and Pedlosky, J}, journal = {Journal of Fluid Mechanics}, volume = {893}, pages = {A4}, year = {2020}, doi = {10.1017/jfm.2020.224} } - Modal decomposition of polychromatic internal wave fields in arbitrary stratificationsAK Kaminski , and MR FlynnWave Motion, 2020
Internal waves such as those produced by tidal sloshing over seafloor topography play an important role in the energy budget of the oceanic overturning circulation. Understanding their spatial and temporal structure, which depend on both the details of the forcing topography and the forcing frequency, is relevant in predicting how and where wave breaking and mixing may occur. Past work has largely focused on the case of a monochromatic wave field; however, the forcing tides may be composed of multiple frequency constituents. Here we present an approach by which the vertical mode structure of a polychromatic internal wave field may be computed from velocity timeseries data without any a priori knowledge of the details of the forcing topography. We consider wave fields in both uniform and vertically-varying stratification, and show using synthetic data that our approach is able to accurately reconstruct the vertical mode strengths. The sensitivity of our approach to noise and vertical resolution is also examined.
@article{kaminski2020b, title = {Modal decomposition of polychromatic internal wave fields in arbitrary stratifications}, author = {Kaminski, AK and Flynn, MR}, journal = {Wave Motion}, volume = {95}, pages = {102549}, year = {2020}, doi = {10.1016/j.wavemoti.2020.102549} } - Acoustic observations of Kelvin-Helmholtz billows on an estuarine lutoclineJ Tu , D Fan , Q Lian , and 4 more authorsJournal of Geophysical Research - Oceans, 2020
Kelvin-Helmholtz (KH) instability plays an important role in turbulent mixing in deep oceans, coastal seas, and estuaries. Though widely observed and studied in thermohaline-stratified waters, KH instability has only rarely been observed in sediment-stratified environments. For the first time, we present direct observations of KH billows on an estuarine lutocline by combining echosounder images with velocity and density measurements. The interaction between velocity shear and the density stratification induced by suspended sediments initiated shear instabilities near the bed, indicated by gradient Richardson number (Ri) < 0.25 in the early stages of the observed billows. Once formed, the instabilities enhanced the vertical mixing of momentum, reducing vertical shear and elevating Ri. Linear instability analysis using measured velocity and density profiles well predicts the vertical location and spatial characteristics of the observed billows. These instabilities are believed to contribute to the vertical mixing, entrainment, and transport of estuarine and coastal sediments.
@article{tu2020, title = {Acoustic observations of Kelvin-Helmholtz billows on an estuarine lutocline}, author = {Tu, J and Fan, D and Lian, Q and Liu, Z and Liu, W and Kaminski, A and Smyth, W}, journal = {Journal of Geophysical Research - Oceans}, volume = {125}, pages = {e2019JC015383}, year = {2020}, doi = {10.1029/2019JC015383} }
2019
- Stratified shear instability in a field of pre-existing turbulenceAK Kaminski , and WD SmythJournal of Fluid Mechanics, 2019
Turbulent mixing of heat and momentum in the stably-stratified ocean interior occurs in discrete events driven by vertical variations of the horizontal velocity. Typically, these events have been modelled assuming an initially laminar stratified shear flow which develops wavelike instabilities, becomes fully turbulent, and then relaminarizes into a stable state. However, in the real ocean there is always some level of turbulence left over from previous events. Using direct numerical simulations, we show that the evolution of a stably-stratified shear layer may be significantly modified by pre-existing turbulence. The classical billow structure associated with Kelvin–Helmholtz instability is suppressed and eventually eliminated as the strength of the initial turbulence is increased. A corresponding energetics analysis shows that potential energy changes and dissipation of kinetic energy depend non-monotonically on initial turbulence strength, with the largest effects when initial turbulence is present but insufficient to prevent billow formation. The mixing efficiency decreases with increasing initial turbulence amplitude as the development of the Kelvin–Helmholtz billow, with its large pre-turbulent mixing efficiency, is arrested.
@article{kaminski2019, title = {Stratified shear instability in a field of pre-existing turbulence}, author = {Kaminski, AK and Smyth, WD}, journal = {Journal of Fluid Mechanics}, volume = {862}, pages = {639-658}, year = {2019}, doi = {10.1017/jfm.2018.973} }
2017
- Nonlinear evolution of linear optimal perturbations of strongly stratified shear layersAK Kaminski , CP Caulfield , and JR TaylorJournal of Fluid Mechanics, 2017
The Miles-Howard theorem states that a necessary condition for normal-mode instability in parallel, inviscid, steady stratified shear flows is that the minimum gradient Richardson number, Rig,min, is less than 1/4 somewhere in the flow. However, the non-normality of the Navier-Stokes and buoyancy equations may allow for substantial perturbation energy growth at finite times. We calculate numerically the linear optimal perturbations which maximize the perturbation energy gain for a stably stratified shear layer consisting of a hyperbolic tangent velocity distribution with characteristic velocity U₀* and a uniform stratification with constant buoyancy frequency N*. We vary the bulk Richardson number Rib =N*²h*²/U*² (corresponding to Rig,min) between 0.20 and 0.50 and the Reynolds numbers Re = U₀*h*/ν* between 1000 and 8000, with the Prandtl number held fixed at Pr = 1. We find the transient growth of non-normal perturbations may be sufficient to trigger strongly nonlinear effects and breakdown into small-scale structures, thereby leading to enhanced dissipation and non-trivial modification of the background flow even in flows where Rig,min > 1/4. We show that the effects of nonlinearity are more significant for flows with higher Re, lower Rib and higher initial perturbation amplitude E₀ₚ. Enhanced kinetic energy dissipation is observed for higher-Re and lower-Rib flows, and the mixing efficiency, quantified here by εₚ/(εₚ + εₖ) where εₚ is the dissipation rate of density variance and εₖ is the dissipation rate of kinetic energy, is found to be approximately 0.35 for the most strongly nonlinear cases.
@article{kaminski2017, title = {Nonlinear evolution of linear optimal perturbations of strongly stratified shear layers}, author = {Kaminski, AK and Caulfield, CP and Taylor, JR}, journal = {Journal of Fluid Mechanics}, volume = {825}, pages = {213-244}, year = {2017}, doi = {10.1017/jfm.2017.396} }
2015
- Axisymmetric gravity currents in two-layer density-stratified mediaRM Sahuri , AK Kaminski , MR Flynn , and 1 more authorEnvironmental Fluid Mechanics, 2015
Numerous studies have considered the flow of a rectilinear, high Reynolds number, Boussinesq gravity current through a two-layer stratified ambient, however, far less is known concerning the analogue axisymmetric problem. Whereas in both instances there is the possibility of a dynamic coupling between the gravity current front and the waves that are excited by its forward advance, axisymmetric gravity currents entail the added complexity of a radially-diverging flow. Because a steady-state formulation cannot then be developed, we instead present a one-layer shallow water model that describes the flow evolution for various initial conditions and ambient stratifications. We also report upon >30 full- and partial-depth lock release laboratory experiments that span a densitometric range 0 ≤ S < 0.8868 where S = (ρ1 − ρ2)/(ρc − ρ2) in which ρc, ρ1 and ρ2 denote, respectively, the densities of the gravity current and lower and upper ambient layers. Of principal interest is the initial front speed of the gravity current for which good agreement is observed between laboratory measurement and shallow water numerical simulation, despite the limiting assumptions of the latter. The horizontal distance over which the initial front speed is maintained may span several lock-lengths, however, this depends on whether or not the gravity current is substantially impacted by the interfacial wave(s). For example, when the lower ambient layer is moderate and S is large, the transfer of momentum from the gravity current front to the wave may lead to a deceleration so severe that gravity current fluid is swept in the −r direction. The connection between our analysis and problems of pollution dispersion is briefly outlined.
@article{sahuri2015, title = {Axisymmetric gravity currents in two-layer density-stratified media}, author = {Sahuri, RM and Kaminski, AK and Flynn, MR and Ungarish, M}, journal = {Environmental Fluid Mechanics}, volume = {15}, pages = {1035-1051}, year = {2015}, doi = {10.1007/s10652-015-9397-0} }
2014
- Transient growth in strongly-stratified shear layersAK Kaminski , CP Caulfield , and JR TaylorJournal of Fluid Mechanics, 2014
We investigate numerically transient linear growth of three-dimensional perturbations in a stratified shear layer to determine which perturbations optimize the growth of the total kinetic and potential energy over a range of finite target time intervals. The stratified shear layer has an initial parallel hyperbolic tangent velocity distribution with Reynolds number Re = U₀h/ν = 1000 and Prandtl number ν/κ = 1, where ν is the kinematic viscosity of the fluid and κ is the diffusivity of the density. The initial stable buoyancy distribution has constant buoyancy frequency N0, and we consider a range of flows with different bulk Richardson number Ri_b = N₀²h²/U₀², which also corresponds to the minimum gradient Richardson number Rig(z) = N₀2/(dU/dz)2 at the midpoint of the shear layer. For short target times, the optimal perturbations are inherently three-dimensional, while for sufficiently long target times and small Rib the optimal perturbations are closely related to the normal-mode ’Kelvin-Helmholtz’ (KH) instability, consistent with analogous calculations in an unstratified mixing layer recently reported by Arratia et al. (J. Fluid Mech., vol. 717, 2013, pp. 90–133). However, we demonstrate that non-trivial transient growth occurs even when the Richardson number is sufficiently high to stabilize all normal-mode instabilities, with the optimal perturbation exciting internal waves at some distance from the midpoint of the shear layer.
@article{kaminski2014, title = {Transient growth in strongly-stratified shear layers}, author = {Kaminski, AK and Caulfield, CP and Taylor, JR}, journal = {Journal of Fluid Mechanics}, volume = {758}, pages = {R4}, year = {2014}, doi = {10.1017/jfm.2014.552} }