Publications
26 results found
Kataoka R, Winn SD, Touber E, 2022, Meteotsunamis in Japan associated with the Tonga Eruption in January 2022, SOLA : Scientific Online Letters on the Atmosphere, Vol: 18, Pages: 116-121, ISSN: 1349-6476
Large-amplitude meteotsunamis were observed in many areas in Japan, following the arrival of barometric Lamb waves emitted by an underwater volcanic eruption of Hunga Tonga-Hunga Ha‘apai in January 2022. We modeled the power spectra of the tidal level data obtained from 12 tide stations of the Geospatial Information Authority of Japan, based on a method of transfer function which converts the barometric pressure pulse spectra into the meteotsunami spectra. The obtained transfer functions are similar at 12 stations. The pressure pulse spectra are obtained from the ensemble average of ∼1500 Soratena weather sensors of Weathernews Inc. distributed over Japan. The observed meteotsunami spectra can be characterized by the enhanced seiche eigenmodes at each station excited by the mesoscale pressure pulse within the amplitude error of 50%, which contributes for accumulating the necessary knowledge to understand the potential dangers in various different areas over Japan.
Winn SD, Touber E, 2021, Non-ideal Gas Effects on Supersonic-Nozzle Transfer Functions, ERCOFTAC Series, Pages: 12-19
Flow disturbances traveling in a nozzle can be amplified or attenuated and generate excessive acoustic noise (e.g. jet engine exhaust) or interact with shocks to cause excessive loading on components (e.g. between turbine blades). Non-ideal gas-dynamic effects are investigated within the framework of linearised inviscid quasi-one dimensional nozzle flow. The transfer function of choked supersonic divergents is investigated when prescribing an inlet entropy perturbation. Initial results using a van der Waals gas highlight the contrast with ideal gas behaviour with and without a shock in the divergent. For the chosen conditions, in the shock-free configuration, a five-fold increase in amplification of pressure perturbations at higher wavelengths (relative to nozzle length) and stronger attenuation (over one order of magnitude lower) at lower wavelengths is observed when compared to an ideal gas. In the shocked configuration, greater amplification is again observed in the van der Waals case owing to the selectivity of the shock in amplifying the incoming density perturbations. Furthermore, up to an order of magnitude greater shock displacement is observed over the range of perturbation wavelengths in the van der Waals case.
Touber E, 2019, Small-scale two-dimensional turbulence shaped by bulk viscosity, Journal of Fluid Mechanics, Vol: 875, Pages: 974-1003, ISSN: 0022-1120
Bulk-to-shear viscosity ratios of three orders of magnitude are often reported in carbon dioxide but are always neglected when predicting aerothermal loads in external (Mars exploration) or internal (turbomachinery, heat exchanger) turbulent flows. The recent (and first) numerical investigations of that matter suggest that the solenoidal turbulence kinetic energy is in fact well predicted despite this seemingly arbitrary simplification. The present work argues that such a conclusion may reflect limitations from the choice of configuration rather than provide a definite statement on the robustness of kineticenergy transfers to the use of Stokes’ hypothesis. Two distinct asymptotic regimes (Euler– Landau and Stokes–Newton) in the eigenmodes of the Navier–Stokes equations are identified. In the Euler–Landau regime, the one captured by earlier studies, acoustic and entropy waves are damped by transport coefficients and the dilatational kinetic energy is dissipated, even more rapidly for high bulk-viscosity fluids and/or forcing frequencies. If the kinetic energy is initially or constantly injected through solenoidal motions, effects on the turbulence kinetic energy remain minor. However, in the Stokes–Newton regime, diffused bulk compressions and advected isothermal compressions are found to prevail and promote small-scale enstrophy via vorticity-dilatation correlations. In the absence of bulk viscosity, the transition to the Stokes–Newton regime occurs within the dissipative scales and is not observed in practice. In contrast, at high bulk viscosities, the Stokes– Newton regime can be made to overlap with the inertial range and disrupt the enstrophy at small scales, which is then dissipated by friction. Thus, flows with substantial inertial ranges and large bulk-to-shear viscosity ratios should experience enhanced transfers to small-scale solenoidal kinetic energy, and therefore faster dissipation rates leading to modific
Touber E, Alferez N, 2019, Shock-induced energy conversion of entropy in non-ideal fluids, Journal of Fluid Mechanics, Vol: 864, Pages: 807-847, ISSN: 0022-1120
From shaping cosmic structures in space to producing intense sounds in aircraft engines, shock waves in fluids ineluctably convert entropy fluctuations into swirling motions and sound waves. Studies of the corresponding conversion from internal energy to kinetic energy have so far been restricted to ideal (or idealised) fluids. Yet, many substances do not obey the ideal-gas law (including those in the above two examples). The present work demonstrates that non-ideal thermodynamic properties provide a remarkable degree of control over the conversion to solenoidal and dilatational kinetic energies. Of particular interest is the ability to suppress much of the emitted acoustic field whilst promoting mixing downstream of the shock. This is made possible by exploiting the convexity (or lack thereof) of the shock adiabats. Whilst illustrated here using dense vapours near the thermodynamic critical point, this ability to design and control specific shock-induced energy transfers extends beyond near-critical-point phenomena; e.g. shocked mixtures (high-speed dusty flows on Mars, nanoparticle formation in supersonic expanders for drug manufacturing), reacting fronts (supersonic combustion, rocket propulsion), ionising shocks (reentry systems, inertial confinement fusion) or fronts in active fluids (bacterial and crowd flows). This theoretical work, which demonstrates the predictive capabilities of linear theory, lays the foundation for future experimental investigations ultimately aimed at delivering novel shock-based flow-control strategies exploiting the thermodynamic properties of the fluid.
Alferez N, Touber E, 2017, Shock-induced energy transfers in dense gases, 1st International Seminar on Non-Ideal Compressible-Fluid Dynamics for Propulsion and Power (NICFD), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Touber E, Alferez N, 2017, One-dimensional refraction properties of compression shocks in non-ideal gases, Journal of Fluid Mechanics, ISSN: 1469-7645
Agostini L, Touber E, Leschziner MA, 2015, The turbulence vorticity as a window to the physics of friction-drag reduction by oscillatory wall motion, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 51, Pages: 3-15, ISSN: 0142-727X
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- Citations: 21
Agostini L, Touber E, Leschziner MA, 2014, Spanwise oscillatory wall motion in channel flow: drag-reduction mechanisms inferred from DNS-predicted phase-wise property variations at Re-tau=1000, Journal of Fluid Mechanics, Vol: 743, Pages: 606-635, ISSN: 0022-1120
A direct-numerical-simulation-based study is presented, which focuses on the response of near-wall turbulence and skin friction to the imposition of an oscillatory spanwise wall motion in channel flow. One point of contrast to earlier studies is the relatively high Reynolds number of the flow, namely Reτ=1000 in the unforced baseline flow. Another is the focus on transients in the drag that are in the form of moderate oscillatory variations in the skin friction and near-wall turbulence around the low-drag state at a sub-optimal actuation period. These conditions allow phase-averaged statistics to be extracted, during the periodic drag decrease and rise, that shed light on the interaction between turbulence and the unsteady Stokes strain. Results are presented for, among others, the phase-averaged second moments of stochastic fluctuations and their budgets, enstrophy components and joint probability density functions. The study identifies velocity skewness – the wall-normal derivative of the angle of the velocity vector – as playing a significant role in the streak-damping process during the drag-reduction phase. Furthermore, the phase-wise asymmetry in the skewness is identified as the source of a distinctive hysteresis in all properties, wherein the drag decrease progresses over a longer proportion of the actuation cycle than the drag increase. This feature, coupled with the fact that the streak-generation time scale limits the ability of the streaks to re-establish themselves during the low-skewness phase when the actuation period is sufficiently short, is proposed to drive the drag-reduction process. The observations in the study thus augment a previously identified mechanism proposed by two of the present authors, in which the drag-reduction process was linked to the rate of change in the Stokes strain in the upper region of the viscous sublayer where the streaks are strongest. Furthermore, an examination of the stochastic-stress budgets and the
Blesbois O, Chernyshenko SI, Touber E, et al., 2013, Pattern prediction by linear analysis of turbulent flow with drag reduction by wall oscillation, JOURNAL OF FLUID MECHANICS, Vol: 724, Pages: 607-641, ISSN: 0022-1120
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- Citations: 28
Hickel S, Touber E, Bodart J, et al., 2013, A parametrized non-equilibrium wall-model for large-eddy simulations
Wall-models are essential for enabling large-eddy simulations (LESs) of realistic problems at high Reynolds numbers. The present study is focused on approaches that directly model the wall shear stress, specifically on filling the gap between models based on wall-normal ordinary differential equations (ODEs) that assume equilibrium and models based on full partial differential equations (PDEs) that do not. We develop ideas for how to incorporate non-equilibrium effects (most importantly, strong pressure-gradient effects) in the wall-model while still solving only wall-normal ODEs. We test these ideas using two reference databases: an adverse pressure-gradient turbulent boundary-layer and a shock/boundary-layer interaction problem, both of which lead to separation and re-attachment of the turbulent boundary layer.
Agostini L, Touber E, Leschziner MA, 2013, Spanwise oscillatory wall motion in channel flow: Drag-reduction mechanisms inferred from DNS-predicted phase-wise property variations at Re<inf>τ</inf> = 1000
A DNS-based study is presented, which focuses on the response of near-wall turbulence and skin friction to the imposition of an oscillatory spanwise wall motion in channel flow. The main focus is on transients in the drag, at Reτ=1000, that are in the form of moderate oscillatory variations in the phase-averaged skin friction and near-wall turbulence around the low-drag state at non-optimal actuation conditions at which the drag reduction margin does not reach the highest possible level. The study reveals a distinctive hysteresis in the periodic fall and rise of the drag, and the results allow the interaction between drag and the turbulence response to the unsteady Stokes strain to be illuminated.
Touber E, Leschziner MA, 2012, Near-wall streak modification by spanwise oscillatory wall motion and drag-reduction mechanisms, Journal of Fluid Mechanics, Vol: 693, Pages: 150-200
Touber E, Sandham ND, 2011, Low-order model for the low-frequency unsteadiness in oblique-shock/turbulent-boundary-layer interactions, Seventh International Symposium On Turbulence and Shear Flow Phenomena
Touber E, Leschziner MA, 2011, Near-wall streak modifications by spanwise oscillatory wall motions, Seventh International Symposium On Turbulence and Shear Flow Phenomena
Touber E, Sandham ND, 2011, Low-order stochastic modelling of low-frequency motions in reflected shock-wave/boundary-layer interactions, Journal of Fluid Mechanics, Vol: 671, Pages: 417-465
Touber E, Sandham ND, 2010, Unsteady Effects of Shock Wave Induced Separation, Unsteady Effects of Shock Wave Induced Separation, Editors: Doerffer, Hirsch, Dussauge, Babinsky, Barakos, Publisher: Springer Verlag, Pages: 263-285, ISBN: 9783642030031
The aim of this series is to publish promptly and in detailed form new material from the field of Numerical Fluid Mechanics and Multidisciplinary Design including the use of advanced computer systems.
Leschziner MA, Touber E, 2010, On the physics of streamwise friction-drag reduction by spanwise oscillatory wall motion – insight derived from turbulence simulations, 19th Polish National Fluid Dynamics Conference
Touber E, Sandham ND, 2010, Turbulence and Interactions, Turbulence and Interactions, Editors: Deville, Le, Sagaut, Publisher: Springer Verlag, Pages: 361-367, ISBN: 9783642141386
The aim of this series is to publish promptly and in detailed form new material from the field of Numerical Fluid Mechanics and Multidisciplinary Design including the use of advanced computer systems.
Sandham ND, Touber E, 2010, Numerical simulation of shock-wave/boundary-layer interaction phenomena, Direct and Large-Eddy Simulation 8 – ERCOFTAC Workshop
Touber E, 2010, Unsteadiness in shock-wave/boundary layer interactions
The need for better understanding of the low-frequency unsteadiness observed in shockwave/turbulent boundary layer interactions has been driving research in this area forseveral decades. This work investigates the interaction between an impinging obliqueshock and a supersonic turbulent boundary layer via large-eddy simulations. Specialcare is taken at the inlet in order to avoid introducing artificial low-frequency modesthat could affect the interaction. All simulations cover extensive integration times toallow for a spectral analysis at the low frequencies of interest. The simulations bringclear evidence of the existence of broadband and energetically-significant low-frequencyoscillations in the vicinity of the reflected shock, thus confirming earlier experimentalfindings. Furthermore, these oscillations are found to persist even if the upstreamboundary layer is deprived of long coherent structures.Starting from an exact form of the momentum integral equation and guided by datafrom large-eddy simulations, a stochastic ordinary differential equation for the reflectedshockfoot low-frequency motions is derived. This model is applied to a wide rangeof input parameters. It is found that while the mean boundary-layer properties areimportant in controlling the interaction size, they do not contribute significantly tothe dynamics. Moreover, the frequency of the most energetic fluctuations is shown tobe a robust feature, in agreement with earlier experimental observations. Under someassumptions, the coupling between the shock and the boundary layer is mathematicallyequivalent to a first-order low-pass filter. Therefore, it is argued that the observed lowfrequencyunsteadiness is not necessarily a property of the forcing, either from upstreamor downstream of the shock, but simply an intrinsic property of the coupled dynamicalsystem.
Touber E, Sandham ND, 2010, Stochastic low-order modelling of low-frequency motions in reflected shock-wave/turbulent-boundary-layer interactions, 45th Symposium of Applied Aerodynamics
Touber E, Sandham ND, 2009, Comparison of three large-eddy simulations of shock-induced turbulent separation bubbles, Shock Waves: an international journal on shock waves, detonations and explosions, Vol: 19, Pages: 469-478
Touber E, Sandham ND, 2009, Large-eddy simulations of an oblique shock impinging on a turbulent boundary layer: effect of the spanwise confinement on the low-frequency oscillations, 2nd International Conference on Turbulence and Interactions
Touber E, Sandham ND, 2009, Large-eddy simulations of low-frequency unsteadiness in a turbulent shock-induced separation bubble, Theoretical and Computational Fluid Dynamics, Vol: 23, Pages: 79-107
Touber E, Sandham ND, 2008, Large-eddy simulations of an oblique shock impinging on a turbulent boundary layer: low-frequency mechanisms, 18th International Shock Interaction Symposium
Touber E, Sandham ND, 2008, Oblique shock impinging on a turbulent boundary layer: low-frequency mechanisms, 38th AIAA Fluid Dynamic Conference
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