publications

2026 (In press)

1. 

   Reconstructing turbulent and orbital motions under wind-driven waves

   Daniel J. Ruth, Pim Bullee, Claudio Mucignat, and 1 more author

   International Journal of Multiphase Flow, 2026 (In press)

   DOI

2024

1. 

   Structure and energy transfer in homogeneous turbulence below a free surface

   Daniel J. Ruth and Filippo Coletti

   Journal of Fluid Mechanics, Dec 2024

   Abs DOI

   We investigate the turbulence below a quasi-flat free surface, focusing on the energy transport in space and across scales. We leverage a large zero-mean-flow tank where homogeneous turbulence is generated by randomly actuated jets. A wide range of Reynolds number is spanned, reaching sufficient scale separation for the emergence of an inertial sub-range. Unlike previous studies, the forcing extends through the source layer, although the surface deformation remains millimetric. Particle image velocimetry along a surface-normal plane resolves from the dissipative to the integral scales. The contributions to turbulent kinetic energy from both vertical and horizontal components of velocity approach the prediction based on rapid distortion theory as the Reynolds number is increased, indicating that discrepancies among previous studies are likely due to differences in the forcing. At odds with the theory, however, the integral scale of the horizontal fluctuations grows as the surface is approached. This is rooted in the profound influence exerted by the surface on the inter-scale energy transfer: along horizontal separations, the direct cascade of energy in horizontal fluctuations is hindered, while an inverse cascade of that in vertical fluctuations is established. This is connected to the structure of upwellings and downwellings. The former, characterized by somewhat larger spatial extent and stronger intensity, are associated with extensional surface-parallel motions. They thus transfer energy to the larger horizontal scales, prevailing over downwellings which favour the compression (and concurrent vertical stretching) of the eddies. Both types of structures extend to depths between the integral scale and the Taylor microscale.

2022

1. 

   Three‐Dimensional Measurements of Air Entrainment and Enhanced Bubble Transport During Wave Breaking

   Daniel J. Ruth, Baptiste Néel, Martin A. Erinin, and 4 more authors

   Geophysical Research Letters, Aug 2022

   DOI

2. 

   Experimental observations and modelling of sub-Hinze bubble production by turbulent bubble break-up

   Daniel J. Ruth, Aditya K. Aiyer, Aliénor Rivière, and 2 more authors

   Journal of Fluid Mechanics, Nov 2022

   DOI

3. 

   Capillary driven fragmentation of large gas bubbles in turbulence

   Aliénor Rivière, Daniel J. Ruth, Wouter Mostert, and 2 more authors

   Physical Review Fluids, Aug 2022

   DOI

4. 

   Speed and Acceleration of Droplets Generated by Breaking Wind‐Forced Waves

   M. A. Erinin, B. Néel, D. J. Ruth, and 4 more authors

   Geophysical Research Letters, Jul 2022

   DOI

2021

1. 

   The effect of nonlinear drag on the rise velocity of bubbles in turbulence

   Daniel J. Ruth, Marlone Vernet, Stéphane Perrard, and 1 more author

   Journal of Fluid Mechanics, Oct 2021

   DOI

2019

1. 

   Bubble pinch-off in turbulence

   Daniel J. Ruth, Wouter Mostert, Stéphane Perrard, and 1 more author

   Proceedings of the National Academy of Sciences, Dec 2019

   DOI

2018

1. 

   Using ducted fuel injection to attenuate or prevent soot formation in mixing-controlled combustion strategies for engine applications

   R.K. Gehmlich, C.J. Mueller, D.J. Ruth, and 3 more authors

   Applied Energy, Sep 2018

   Abs DOI

   Ducted fuel injection is a strategy that can be used to enhance the fuel/charge-gas mixing within the combustion chamber of a direct-injection compression-ignition engine. The concept involves injecting the fuel through a small tube within the combustion chamber to make the most fuel-rich regions of the micture in the autoignition zone leaner relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). This study is a follow-on to initial proof-of-concept experiments that also were conducted in a constantvolume combustion vessel. While the initial natural luminosity imaging experiments demonstrated that ducted fuel injection lowers soot incandescence dramatically, this study adds a more quantitative diffuse back-illumination diagnostic to measure soot mass, as well as investigates the effects on performance of varying duct geometry (axial gap, length, diameter, and inlet and outlet shapes), ambient density, and charge-gas dilution level. The result is that ducted fuel injection is further proven to be effective at lowering soot by 35–100% across a wide range of operating conditions and geometries, and guidance is offered on geometric parameters that are most important for improving performance and facilitating packaging for engine applications.

2. A methodology to analyze photovoltaic tracker uptime

   Dan Ruth and Matthew Muller

   Progress in Photovoltaics: Research and Applications, Jul 2018

   Abs DOI

   A metric is developed to analyze the daily performance of single‐axis photovoltaic (PV) trackers. The metric relies on comparing correlations between the daily time series of the PV power output and an array of simulated plane‐of‐array irradiances for the given day. Mathematical thresholds and a logic sequence are presented, so the daily tracking metric can be applied in an automated fashion on large‐scale PV systems. The results of applying the metric are visually examined against the time series of the power output data for a large number of days and for various systems. The visual inspection results suggest that overall, the algorithm is accurate in identifying stuck or functioning trackers on clear‐sky days. Visual inspection also shows that there are days that are not classified by the metric where the power output data may be sufficient to identify a stuck tracker. Based on the daily tracking metric, uptime results are calculated for 83 different inverters at 34 PV sites. The mean tracker uptime is calculated at 99% based on 2 different calculation methods. The daily tracking metric clearly has limitations, but as there is no existing metrics in the literature, it provides a valuable tool for flagging stuck trackers.

2017

1. Optimization of an Advanced Combustion Strategy Towards 55% BTE for the Volvo SuperTruck Program

   Jacqueline O’Connor, Meghan Borz, Daniel Ruth, and 12 more authors

   SAE International Journal of Engines, Mar 2017

   Abs DOI

   This paper describes a novel design and verification process for analytical methods used in the development of advanced combustion strategies in internal combustion engines (ICE). The objective was to improve brake thermal efficiency (BTE) as part of the US Department of Energy SuperTruck program. The tools and methods herein discussed consider spray formation and injection schedule along with piston bowl design to optimize combustion efficiency, air utilization, heat transfer, emission, and BTE. The methodology uses a suite of tools to optimize engine performance, including 1D engine simulation, high-fidelity CFD, and lab-scale fluid mechanic experiments. First, a wide range of engine operating conditions are analyzed using 1-D engine simulations in GT Power to thoroughly define a baseline for the chosen advanced engine concept; secondly, an optimization and down-select step is completed where further improvements in engine geometries and spray configurations are considered. Next, simultaneous high-fidelity simulation using StarCD and OpenFOAM as well as lab-scale unsteady jet mixing experiments are used to understand the interplay between fuel injection and piston bowl designs. A key to the success of this design and analysis process is the accuracy of both the 1-D and high-fidelity simulations; these tools have been significantly improved and benchmarked against a range of engine operating conditions throughout the program. We conclude by outlining future directions for this methodology.

2. Seasonal Trends of Soiling on Photovoltaic Systems

   Leonardo Micheli, Daniel Ruth, and Matthew Muller

   In 2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), Jun 2017

   Abs DOI

   This work investigates the seasonal variability of photovoltaic soiling losses over a 12-month period for 15 soiling stations deployed in the USA. We present a new parameter able to rank the sites according to the cumulative losses occurring over 3and 6- month periods. The relations between soiling losses and particulate matter are briefly discussed, as well. Moving from long-term to shorter-term data increases the complexity of the analysis: monthly correlations are found to have lower accuracy than the longer-term ones presented in the literature.

3. Ducted fuel injection: A new approach for lowering soot emissions from direct-injection engines

   Charles J. Mueller, Christopher W. Nilsen, Daniel J. Ruth, and 3 more authors

   Applied Energy, Oct 2017

   DOI
4. Development and Verification of Reduced-Order Model for Diesel Spray Penetration and Spreading during Wall Impingement

   Daniel Ruth and Jacqueline O’Connor

   In , Mar 2017

   DOI