Imperial College London

Dr Ali Mashayek

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Lecturer
 
 
 
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Contact

 

mashayek Website

 
 
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Location

 

336Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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30 results found

Mashayekhi A, Cael B, Cimoli L, Alford M, Caulfield Cet al., 2022, A physical-statistical recipe for representation of small scale oceanic turbulent mixing in climate models, Flow, ISSN: 2633-4259

It is well established that small scale cross-density (diapycnal) turbulent mixing induced by breaking of overturns in the interior of the ocean plays a significant role in sustaining the deep ocean circulation and in regulation of tracer budgets such as those of heat, carbon and nutrients. There has been significant progress in the fluid mechanical understanding of the physics of breaking internal waves. Connection of the microphysics of such turbulence to the larger scale dynamics, however, is significantly underdeveloped. We offer a hybrid theoretical-statistical approach, informed by observations, to make such a link. By doing so, we define a bulk flux coefficient, $\Gamma_B$, which represents the partitioning of energy available to an `ocean box' (such as a grid cell of a coarse resolution climate model), from winds, tides, and other sources, into mixing and dissipation. $\Gamma_B$ depends on both the statistical distribution of turbulent patches and the flux coefficient associated with individual patches, $\Gamma_i$. We rely on recent parameterizations of ~$\Gamma_i$~ and the seeming universal characteristics of statistics of turbulent patches, to infer $\Gamma_B$, which is the essential quantity for representation of turbulent diffusivity in climate models. By applying our approach to climatology and global tidal estimates, we show that on a basin scale, energetic mixing zones exhibit moderately efficient mixing that induces significant vertical density fluxes, while quiet zones (with small background turbulence levels), although highly efficient in mixing, exhibit minimal vertical fluxes. The transition between the less energetic to more energetic zones marks regions of intense upwelling and downwelling of deep waters. We suggest that such upwelling and downwelling may be stronger than previously estimated, which in turn has direct implications for the closure of the deep branch of the ocean meridional overturning circulation as well as for the associated tr

Journal article

Mashayek A, Caulfield CP, Alford MH, 2021, Goldilocks mixing in oceanic shear-induced turbulent overturns, Journal of Fluid Mechanics, Vol: 928, Pages: 1-32, ISSN: 0022-1120

We present a new, simple and physically motivated parameterization, based on the ratio of Thorpe and Ozmidov scales, for the irreversible turbulent flux coefficient ΓM=M/ϵ , i.e. the ratio of the irreversible rate M at which the background potential energy increases in a stratified flow due to macroscopic motions to the dissipation rate of turbulent kinetic energy ϵ . Our parameterization covers all three key phases (crucially, in time) of a shear-induced stratified turbulence life cycle: the initial, ‘hot’ growing phase, the intermediate energetically forced phase and the final ‘cold’ fossilization decaying phase. Covering all three phases allows us to highlight the importance of the intermediate one, to which we refer as the ‘Goldilocks’ phase due to its apparently optimal (and so neither too hot nor too cold, but just right) balance, in which energy transfer from background shear to the turbulent mixing is most efficient. The value of ΓM is close to 1/3 during this phase, which we demonstrate appears to be related to an adjustment towards a critical or marginal Richardson number for sustained turbulence ∼0.2--0.25 . Importantly, although buoyancy effects are still significant at leading order for the turbulent dynamics during this intermediate phase, the marginal balance in the flow ensures that the turbulent mixing of the (density) scalar is nevertheless effectively ‘locked’ to the turbulent mixing of momentum. We present supporting evidence for our parameterization through comparison with six oceanographic datasets that span various turbulence generation regimes and a wide range of geographical location and depth. Using these observations, we highlight the significance of parameterizing an inherently variable flux coefficient for capturing the turbulent flux associated with rare energetic, yet fundamentally shear-driven (and so not strongly stratified) overturns that make a disproportionate

Journal article

Cael BB, Mashayek A, 2021, Log-skew-normality of ocean turbulence, Physical Review Letters, Vol: 126, Pages: 1-6, ISSN: 0031-9007

The statistics of intermittent ocean turbulence is the key link between physical understanding of turbulence and its global implications. The log-normal distribution is the standard but imperfect assumed distribution for the turbulent kinetic energy dissipation rate. We argue that as turbulence is often generated by multiple changing sources, a log-skew-normal (LSN) distribution is more appropriate. We show the LSN distribution agrees excellently and robustly with observations. The heavy tail of the LSN distribution has important implications for sampling of turbulence in terrestrial and extraterrestrial analogous systems.

Journal article

Reynard N, Ellison E, Wilson A, Williamson P, O-Niles J, Ransome E, Mashayekhi Aet al., 2020, The contribution of coastal blue carbon ecosystems to climate change mitigation and adaptation, The contribution of coastal blue carbon ecosystems to climate change mitigation and adaptation, www.imperial.ac.uk/Grantham, Publisher: The Grantham Institute, BP34

This briefing paper explores the potential for marine coastal ecosystems that store carbon, blue carbon ecosystems (BCEs), to help both limit climate change and adapt to the impacts of a changing climate. It also considers the range of benefits BCEs bring to coastal communities, and makes recommendations for policy approaches.

Report

Cimoli L, Caulfield CP, Johnson HL, Marshall DP, Mashayek A, Naveira Garabato AC, Vic Cet al., 2019, Sensitivity of deep ocean mixing to local Internal tide breaking and mixing efficiency, Geophysical Research Letters, Vol: 46, Pages: 14622-14633, ISSN: 0094-8276

There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the “efficiency” of diapycnal mixing, that is, the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is nontrivially sensitive to the covariation of these parameters. Varying these parameters one at a time can lead to significant errors in the patterns of diapycnal mixing-driven upwelling and downwelling and to the over and under estimation of mixing in such a way that the net rate of globally integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both parameters are allowed to covary and be spatially variable. These findings have important implications for the representation of oceanic heat, carbon, nutrients, and other tracer budgets in general circulation models.

Journal article

Mashayek A, Caulfield CP, Peltier WR, 2017, Role of overturns in optimal mixing in stratified mixing layers, Journal of Fluid Mechanics, Vol: 826, Pages: 522-552, ISSN: 0022-1120

Journal article

Mashayek A, Salehipour H, Bouffard D, Caulfield CP, Ferrari R, Nikurashin M, Peltier WR, Smyth WDet al., 2017, Efficiency of turbulent mixing in the abyssal ocean circulation, Geophysical Research Letters, Vol: 44, Pages: 6296-6306, ISSN: 1944-8007

Turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation. A key parameter used to characterize turbulent mixing is its efficiency, defined here as the fraction of the energy available to turbulence that is invested in mixing. Efficiency is conventionally approximated by a constant value near one sixth. Here we show that efficiency varies significantly in the abyssal ocean and can be as large as approximately one third in density stratified regions near topographic features. Our results indicate that variations in efficiency exert a first-order control over the rate of overturning of the lower branch of the meridional overturning circulation.

Journal article

Mashayek A, Ferrari R, Merrifield S, Ledwell JR, St Laurent L, Garabato ANet al., 2017, Topographic enhancement of vertical turbulent mixing in the Southern Ocean, Nature communications, Vol: 8, ISSN: 2041-1723

It is an open question whether turbulent mixing across density surfaces is sufficiently large to play a dominant role in closing the deep branch of the ocean meridional overturning circulation. The diapycnal and isopycnal mixing experiment in the Southern Ocean found the turbulent diffusivity inferred from the vertical spreading of a tracer to be an order of magnitude larger than that inferred from the microstructure profiles at the mean tracer depth of 1,500 m in the Drake Passage. Using a high-resolution ocean model, it is shown that the fast vertical spreading of tracer occurs when it comes in contact with mixing hotspots over rough topography. The sparsity of such hotspots is made up for by enhanced tracer residence time in their vicinity due to diffusion toward weak bottom flows. The increased tracer residence time may explain the large vertical fluxes of heat and salt required to close the abyssal circulation.

Journal article

Ferrari R, Mashayek A, McDougall TJ, Nikurashin M, Campin Jet al., 2016, Turning ocean mixing upside down, Journal of Physical Oceanography, Vol: 46, Pages: 2239-2261, ISSN: 0022-3670

Journal article

Behzad M, Ashgriz N, Mashayek A, 2015, Azimuthal shear instability of a liquid jet injected into a gaseous cross-flow, Journal of Fluid Mechanics, Vol: 767, Pages: 146-172, ISSN: 0022-1120

Journal article

Mashayek A, Ferrari R, Nikurashin M, Peltier WRet al., 2015, Influence of enhanced abyssal diapycnal mixing on stratification and the ocean overturning circulation, Journal of Physical Oceanography, Vol: 45, Pages: 2580-2597, ISSN: 0022-3670

Journal article

Salehipour H, Peltier WR, Mashayek A, 2015, Turbulent diapycnal mixing in stratified shear flows: the influence of Prandtl number on mixing efficiency and transition at high Reynolds number, Journal of Fluid Mechanics, Vol: 773, Pages: 178-223, ISSN: 0022-1120

Journal article

Behzad M, Ashgriz N, Mashayek A, 2013, Azimuthal Shear Instability of a Liquid Jet in Gas Crossflow

Conference paper

Mashayek A, Peltier WR, 2013, Shear-induced mixing in geophysical flows: does the route to turbulence matter to its efficiency?, Journal of Fluid Mechanics, Vol: 725, Pages: 216-261, ISSN: 0022-1120

Journal article

Mashayek A, Caulfield CP, Peltier WR, 2013, Time-dependent, non-monotonic mixing in stratified turbulent shear flows: implications for oceanographic estimates of buoyancy flux, Journal of Fluid Mechanics, Vol: 736, Pages: 570-593, ISSN: 0022-1120

Journal article

Mashayek A, Ferrari R, Vettoretti G, Peltier WRet al., 2013, The role of the geothermal heat flux in driving the abyssal ocean circulation, Geophysical Research Letters, Vol: 40, Pages: 3144-3149, ISSN: 1944-8007

Journal article

Mashayek A, Peltier WR, 2012, The ‘zoo’of secondary instabilities precursory to stratified shear flow transition. Part 2 The influence of stratification, Journal of Fluid Mechanics, Vol: 708, Pages: 45-70, ISSN: 1469-7645

Journal article

Mashayek A, Peltier WR, 2012, The ‘zoo’of secondary instabilities precursory to stratified shear flow transition. Part 1 Shear aligned convection, pairing, and braid instabilities, Journal of Fluid Mechanics, Vol: 708, Pages: 5-44, ISSN: 1469-7645

Journal article

Mashayek A, Behzad M, Ashgriz N, 2011, Multiple injector model for primary breakup of a liquid jet in crossflow, AIAA journal, Vol: 49, Pages: 2407-2420, ISSN: 0001-1452

Journal article

Mashayek A, Ashgriz N, 2011, Dynamics of liquid droplets, Handbook of Atomization and Sprays, Publisher: Springer, Boston, MA, Pages: 97-123

Book chapter

Mashayek A, Ashgriz N, 2011, Atomization of a liquid jet in a crossflow, Handbook of Atomization and Sprays, Publisher: Springer, Boston, MA, Pages: 657-683

Book chapter

Mashayek A, Peltier WR, 2011, Three-dimensionalization of the stratified mixing layer at high Reynolds number, Physics of Fluids, Vol: 23, ISSN: 1070-6631

Journal article

Ashgriz N, Behzad M, Mashayek A, 2011, Multiple Injector Model for Primary Breakup of a Liquid Jet in Crossflow, ISSN: 0001-1452

Journal article

Mashayek A, 2011, Atomization of a liquid jet in a crossflow, Chap. 29 in Handbook of Atomization and Sprays (Ashgriz, N. ed.), Publisher: Springer

Other

Mashayek A, Peltier WR, 2011, Turbulence transition in stratified atmospheric and oceanic shear flows: Reynolds and Prandtl number controls upon the mechanism, Geophysical Research Letters, Vol: 38, ISSN: 1944-8007

Journal article

Behzad M, Mashayek A, Ashgriz N, 2010, A KIVA-based model for liquid jet in cross flow

Conference paper

Mashayek A, Ashgriz N, 2009, Model for deformation of drops and liquid jets in gaseous crossflows, AIAA journal, Vol: 47, Pages: 303-313, ISSN: 0001-1452

Journal article

Mashayek A, Jafari A, Ashgriz N, 2008, Improved model for the penetration of liquid jets in subsonic crossflows, AIAA journal, Vol: 46, Pages: 2674-2686, ISSN: 0001-1452

Journal article

Mashayek A, 2006, Experimental and numerical study of liquid jets in crossflow

Conference paper

Mashayek A, Jafari A, Ashgriz N, 2006, A Model for the Penetration of a Liquid Jet in Crossflow

Conference paper

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