arXiv subjects

Jason Olsthoorn

Publications and source records attributed to Jason Olsthoorn.

7 recordsLinked to original sources

arXiv2025

Swash flow due to obliquely incident bores

We present a new solution to the nonlinear shallow water equations and show that it accurately predicts the swash flow due to obliquely approaching bores in large-scale wave basin experiments. The solution is based on an application of Snell's law of refraction in settings where the bore approach angle $\theta$ is small. We use the weakly two-dimensional nonlinear shallow water equations [Ryrie (J. Fluid Mech., vol. 129, 1983, p. 193)], where the cross-shore dynamics are independent of, and act as a forcing to, the alongshore dynamics. Using a known solution to the cross-shore dynamics [Antuono (J. Fluid Mech., vol. 658, 2010, p. 166)], we solve for the alongshore flow using the method of characteristics and show that it differs from previous solutions. Since the cross-shore solution assumes a constant forward-moving characteristic variable, $\alpha$, we term our solution the `small-$\theta$, constant-$\alpha$' solution. We test our solution in large-scale experiments with data from fifteen wave cases, including normally incident waves and obliquely incident waves generated using the wall reflection method. We measure water depths and fluid velocities using in situ sensors within the surf and swash zones and track shoreline motion using quantitative imaging. The data show that the basic assumptions of the theory (Snell's law of refraction and constant-$\alpha$) are satisfied and that our solution accurately predicts the swash flow. In particular, the data agrees well with our expression for the time-averaged alongshore velocity, which is expected to improve predictions of alongshore transport at coastlines.

physics.flu-dyn↗

arXiv2024

Atmospheric cooling of freshwater near the temperature of maximum density

We perform three-dimensional direct numerical simulations of surface-driven convection near the temperature of maximum density $\tilde T_{md}$. A dynamic surface boundary condition couples heat flux through the surface to the induced convection, creating a dynamic equilibrium between the surface water temperature and the convection below. In this system, we identified three convective regimes: (1) free convection when the surface water temperature is above $\tilde T_{md}$, (2) penetrative convection when the surface water temperature is below $\tilde T_{md}$ and the convection is actively mixing the fluid layer, and (3) decaying convection when the convection weakens. We then predict the transitions between these regimes. Understanding these transitions is essential for the predicting timing of ice formation in natural systems.

physics.ao-ph↗

arXiv2023

Boundary layer instability beneath periodic internal solitary waves

We investigated the stability of the bottom boundary layer (BBL) beneath periodic internal solitary waves (ISWs) of depression over a flat bottom through two-dimensional direct numerical simulations. We explored the effects of variation in wave Reynolds number $Re_{ISW}$ and wave period on the nature of the instability, and energy production in the separated BBL. The instability characteristics and rate of vortex shedding of the BBL were strongly dependent on $Re_{ISW}$. The BBL was laminar and convectively unstable at $Re_{ISW}$ 90 and 300, respectively. At $Re_{ISW}=300$, the convective wave packet was periodically amplified by each successive ISW, until vortex-shedding occurred. This implies noise-amplification behavior and suggests that the discrepancies in the critical $Re_{ISW}$, for vortex shedding between lab and different numerical simulations, are due to differences in background seed noise. Instability energy decreased under the front shoulder of the ISW, analogous to flow relaminarization under a favourable pressure gradient. At larger $Re_{ISW}=900$, the BBL was initially convectively unstable, and then the instability tracked with the ISW, characteristic of global instability, regardless of the ISW periodicity. The simulated initial convective instability at both $Re_{ISW}$ 300 and 900 is in agreement with local linear stability analysis which predicts that the instability group speed is always lower than the ISW celerity. Increased free-stream perturbations and larger $Re_{ISW}$ shift the location of vortex shedding (and enhanced bed shear stress) closer to the ISW trough, thereby potentially changing the location of maximum sediment resuspension from the ISW, in agreement with field observations at higher $Re_{ISW}$.

physics.flu-dyn↗

arXiv2023

Accounting for surface temperature variations in Rayleigh-Bénard convection

Turbulent Rayleigh-Bénard convection is often modelled with a constant surface temperature. However, the surface temperature of many geophysical systems, such as lakes, is coupled to the atmospheric forcing. In this paper, we account for this dynamic surface temperature through an additional parameter $β$. Using an appropriately defined dynamical Rayleigh number $\RaD$, we recover many of the results from the standard Rayleigh-Bénard model. We hope that this work will simplify the application of Rayleigh-Bénard theory in geophysical contexts, such as lakes.

physics.flu-dyn↗

arXiv2022

The Dynamics of Asymmetric Stratified Shear Instabilities

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.

physics.flu-dyn↗

arXiv2022

Salt-fingering in seasonally ice-covered lakes

When ice forms on lakes, dissolved salts are rejected, which can lead to under-ice salt-finger formation. We performed a series of laboratory experiments to visualize these fingers. While we detected salt-fingers in our camera recordings, the signal of these fingers is nearly absent in the temperature record. We quantify the velocity of the salt plumes and measure the bottom salinity increase from these fingers. Further, we estimate that the salinity is often distributed evenly with depth. Comparing the salt fluxes in our experiments with a typical salt flux in lakes, we suggest that conditions are favorable for salt-fingering in most seasonally ice-covered lakes.

physics.geo-ph↗

arXiv2020

The Cooling Box Problem: Convection with a quadratic equation of state

We investigate the convective cooling of a fluid with a quadratic equation of state by performing three-dimensional direct numerical simulations of a flow with a fixed top-boundary temperature, which is lower than the initial fluid temperature. We consider fluid temperatures near the density maximum, where the nonlinearity is expected to be important. When the equation of state is nonlinear, the resultant vertical transport of heat is fundamentally different and significantly lower than the predictions derived for a linear equation of state. Further, three dimensionless groups parameterize the convective system: the Rayleigh number ($\hbox{Ra}_0$), the Prandtl number (Pr), and the dimensionless bottom water temperature $(T_B)$. We further define an effective Rayleigh number ($\hbox{Ra}_{eff} = Ra_0 \ T_B^2$), which is equivalent to the traditional Rayleigh number used with a linear equation of state. We present a predictive model for the vertical heat flux, the top boundary-layer thickness, and the turbulent kinetic energy of the system. We show that this model agrees well with the direct numerical simulations. This model could be used to understand how quickly freshwater lakes cool in high latitude environments.

physics.flu-dyn↗