Akshay Patil

he/him

Post-doctoral researcher

Publications

Thesis

Direct numerical simulations of wave-current interactions over bumpy walls

Published in Stanford University Digital Repository, 2023

Turbulence in estuarine bottom boundary layers influences a variety of biological and anthropogenic activities by driving the hydrodynamics and morphodynamic response of large-scale coastal ocean systems. Thus, a detailed understanding of these boundary layers enables accurate, long-term prediction of the fate of nutrients, pollutants, sediments, and other relevant quantities within estuarine environments. Typically, estuaries are driven by the mean turbulent currents and oscillatory wave motion that interact over naturally rough bottom boundaries. These wave-current interactions often occur non-linearly resulting in complex mean flow responses across the water column.

Recommended citation: Patil, A., Fringer O., Gorle C., Monismith S., and Stanford University (2023), Direct numerical simulations of wave-current interactions over bumpy walls. (Ph.D. Thesis)
Download Paper

Numerical investigation of nearshore wave transformation and surf zone hydrodynamics

Published in Delft University of Technology Education Repository (Thesis), 2019

Rapid climate change and the corresponding estimated sea level rise can affect the performance of the coastal defense structures such as breakwaters, seawalls, and dikes. In order to improve these coastal defenses, a detailed understanding of the processes which contribute to wave run-up and overtopping over the coastal defenses needs to be established. Following the exponential growth of computing capacity around 1970’s, a wide variety of computational models were developed to study fluid flow. Traditionally, three computational paradigms have existed in order to study wave transformation and surf zone hydrodynamics: phase averaged models, phase resolving models, and Computational Fluid Dynamics (CFD) models. Limitations posed by the underlying linear wave theory in phase averaged and other simplifications in the phase resolving models, may not provide sufficient de tail in wave breaking, wave energy dissipation, wave run-up, wave overtopping, and potentially other detailed hydrodynamic processes. This lack of resolution in depth-averaged models for wave-breaking, wave run-up, and wave overtopping processes motivates a detailed investigation using CFD based models, which can correctly mimic wave-breaking and other hydrodynamic processes.

Recommended citation: Patil, A., (2019), Numerical investigation of nearshore wave transformation and surf-zone hydrodynamics (MSc Thesis)
Download Paper

Journal Articles

Characterising the roughness in channel ﬂows using direct numerical simulations

Published in Journal of Hydraulic Engineering, 2023

Turbulent flows over bumpy walls are ubiquitous and pose a fundamental challenge to various engineering applications such as coastal boundary layers, drag on ships, hydraulic conveyance networks, and bluff body aerodynamics, to name a few. In this study, we used direct numerical simulations (DNS) along with a direct-forcing immersed boundary method (IBM) to understand the connection between the roughness geometry and the mean flow drag. A bumpy wall was constructed using an array of randomly oriented ellipsoids characterized by the Corey shape factor (Co). We found that our results exactly validated the experimental studies by Nikuradse for sand-grain type roughness (Co = 1.0). Additionally, we observed that the mean flow drag increased for decreasing Co through an increase in the form-drag contribution and a decrease in the viscous drag. We also developed a relationship between the statistics of the bottom height distribution and the roughness parameter (z0) that may help explain the spread observed in the drag coefficient predicted when using conventional tools such as the Moody diagram

Recommended citation: Patil, A., and Fringer O., (2023), Characterising the roughness in channel ﬂows using direct numerical simulations, Journal of Hydraulic Engineering
Download Paper

Drag enhancement by the addition of weak waves to a wave-current boundary layer over bumpy walls

Published in Journal of Fluid Mechanics, 2022

We present direct numerical simulation results of a wave-current boundary layer in a current-dominated flow regime (wave driven to steady current ratio of 0.34) over bumpy walls for hydraulically smooth flow conditions (wave orbital excursion to roughness ratio of 10). The turbulent, wave-current channel flow has a friction Reynolds number of 350 and a wave Reynolds number of 351. At the lower boundary, a bumpy wall is introduced with a direct forcing immersed boundary method, while the top wall has a free-slip boundary condition. Despite the hydraulically smooth nature of the wave-driven flow, the phase variations of the turbulent statistics for the bumpy wall case were found to vary substantially when compared with the flat wall case. Results show that the addition of weak waves to a steady current over flat walls has a negligible effect on the turbulence or bottom drag. However, the addition of weak waves to a steady current over bumpy walls has a significant effect through enhancement of the Reynolds stress (RS) accompanied by a drag coefficient increase of 11 % relative to the steady current case. This enhancement occurs just below the top of the roughness elements during the acceleration portion of the wave cycle: Turbulent kinetic energy (TKE) is subsequently transported above the roughness elements to a maximum height of roughly twice the turbulent Stokes length. We analyse the TKE and RS budgets to understand the mechanisms behind the alterations in the turbulence properties due to the bumpy wall. The results provide a mechanistic picture of the differences between bumpy and flat walls in wave-current turbulent boundary layers and illustrate the importance of bumpy features even in weakly energetic wave conditions.

Recommended citation: Patil, A., and Fringer, O. (2022). Drag enhancement by the addition of weak waves to a wave- current boundary layer over bumpy walls. Journal of Fluid Mechanics, 947, A3.
Download Paper

Conference Papers

Understanding the impact of varying geometry level of detail in multi-direction urban RANS simulations tailored for urban air-mobility viability

Published in EMS Annual Meeting, 2024

Wind flow predictions in realistic urban areas are sensitive to a wide range of governing parameters such as building resolution, wind incidence, urban morphology, and underlying topography to list a few. In this study, we quantify the impact of the level of detail (LoD) of the urban built environment and the inflow direction (θ) on wind-safety for urban-air mobility using a Reynolds-Averaged Navier-Stokes (RANS) simulation framework. To isolate the effect of LoD and θ, we chose the TU Delft campus (radius of ~ 1 km) and the city of Den Haag (radius of ~ 1.5 km) as representative urban environments that contain a variety of urban fabric and incident wind conditions.

Recommended citation: Patil, A., and C. García-Sánchez, (2024), Understanding the impact of varying geometry level of detail in multi-direction urban RANS simulations tailored for urban air-mobility viability, EMS Annual Meeting, Barcelona, Spain,
Download Paper | Download Slides

Numerical Investigation of breaking and broken regular wave forces on a shoal-mounted cylinder

Published in 16th OpenFOAM workshop, Book of Abstract, 2021

Shoal-mounted cylindrical structures such as lighthouses assist in the navigation of vessels, and act as warning systems. Many of these iconic lighthouses are continuously subjected to extreme sea-wave action, made acute by the effects of climate change [1], [2]. Studying the complex hydrodynamic loads acting on such structures surrounded by rocks or emerged shoals can provide valuable insights for addressing their structural integrity, and how much longer these heritage structures can withstand such harsh environments. In this work, the effectiveness of a finite volume-based numerical framework to predict wave loads on such structures is assessed. The proposed numerical investigation is supplemented by the STORMLAMP (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves) project's experimental data [3] that has been used as a benchmark case to validate the proposed computational framework.

Recommended citation: van Gorsel, J., Patil, A., Bricker, J., Pearson, S., Raby, A., Dassanayake. D., Antonini, A. (2021), Numerical Investigation of breaking and broken regular wave forces on a shoal-mounted cylinder
Download Paper

Effect of overflow nappe non-aeration on tsunami breakware failure

Published in Coastal Engineering Conference Proceedings, 2018

Sliding force and punching pressure were contributing factors to widespread breakwater damage caused during the 2011 Great East Japan Tsunami (Takagi and Bricker, 2015), and were dominant factors causing displacement of caissons from the world’s deepest breakwater: the Kamaishi bay-mouth composite tsunami breakwater (Arikawa et al., 2012; Bricker et al., 2013). The current study focuses on understanding the physics necessary to correctly model the problem of breakwater over-topping by tsunami. To effectively model the physical behavior of the system, scaled model studies were carried out by Mudiyanselage (2017). The earlier numerical investigations carried out by Bricker et al. (2013) and Mudiyanselage (2017), did not prove conclusive to numerically model tsunami breakwater overflow using OpenFOAM employing a 2-D modeling approach. This was shown to be a major hurdle in prediction of the sliding force on the caisson due to the inability of modeling the non-aerated overflow jet over the caisson. Validation of the numerical model would allow parametric study of the flow physics for varying overflow conditions. As a result, a threefold approach of experimental model, analytical model, and numerical model studies was proposed. To achieve sufficient reliability and have complete flexibility, OpenFOAM was chosen for the numerical setup. This numerical model was used to validate the experiments carried out by Mudiyanselage (2017). The numerical model validates and reproduces the flow physics very well. Overall, the numerical results indicate that non-aeration could provide about 8-19% additional force. It was observed that the force on the caisson has a periodic fluctuating behavior. Additionally, the aeration mechanism and overflow jet breakup during the flow was also investigated. It was observed that the highly 3-dimensional behavior of the overflow jet results in the aeration of the cavity underneath the jet. This also explains why the previous studies Bricker et al. (2013) and Mudiyanselage (2017) failed to correctly model the overflow jet using a 2-D modeling approach.

Recommended citation: Patil, A., Mudiyanselage, S. D., Bricker, J., Uijtewaal, W., Keetels, G., (2018), Effect of overflow nappe non-aeration on tsunami breakwater failure, Coastal Engineering Proceedings, 36, 18-18
Download Paper

Under Review

Understanding the impact of varying level of detail in urban areas on the wind prediction capabilities using Reynolds-averaged Navier-Stokes models with a focus on urban air-mobility viability.

Published in In Prep, 2025

Wind flow predictions in realistic urban areas are sensitive to a wide range of governing parameters such as building resolution, wind incidence, urban morphology, and underlying topography, to list a few. In this study, we use a Reynolds Averaged Navier-Stokes (RANS) computational framework to assess the impact of the geometric level of detail (LoD) of the urban built environment on wind safety tailored for urban air mobility. We develop a probabilistic risk metric (Pr) based on velocity and turbulence fields, that allows us to compare the efficacy of LoD1.2 and LoD2.2 in two different urban settings: the TUDelft campus (fairly open with varying height buildings) and Den Haag centrum (compact with similar height buildings). We found that LoD2.2 provides a more conservative prediction for high-risk areas compared to LoD1.2. Our results and methodology can help better predict the risk associated with urban air mobility and wind engineering applications with the appropriate tuning of the risk metrics.

Recommended citation: Patil, A., C. García-Sánchez, (in prep), Understanding the impact of varying level of detail in urban areas on the wind prediction capabilities using Reynolds-averaged Navier-Stokes models with a focus on urban air-mobility viability.

GenSDF: An MPI-Fortran based signed-distance-field generator for computational fluid dynamics applications.

Published in SoftwareX, 2025

This paper presents a highly efficient signed-distance field (SDF) generator designed specifically for computational fluid dynamics (CFD) workflows. Our approach combines the parallel computing power of Message Passing Interface (MPI) with the performance advantages of modern Fortran, enabling precise and scalable computations for complex geometric domains. The algorithm focuses on localized distance calculations to minimize computational overhead, ensuring efficiency across multiple processors. An adjustable input stencil width allows users to balance computational cost with the desired level of accuracy in distance approximation. Additionally, the generator supports the widely used Wavefront OBJ format, utilizing its encoded outward normal information to achieve accurate boundary definitions. Performance benchmarks demonstrate the tool’s ability to handle large-scale 3D models with high fidelity and reduced computational demands. This makes it a practical and effective solution for CFD applications that require fast, reliable distance field computations while accommodating diverse geometric complexities.

Recommended citation: Patil, A., U. C. K. Paranjothi, and C. García-Sánchez, (in review, SoftwareX), GenSDF: An MPI-Fortran based signed-distance-field generator for computational fluid dynamics applications.
Download Paper

Hydrodynamics of In-Canopy Flow in Synthetically Generated Coral Reefs Under Oscillatory Wave Motion.

Published in Journal of Geophysical Research: Oceans, 2025

The interaction of oscillatory wave motion with morphologically complex coral reefs showcases a wide range of consequential hydrodynamic responses within the canopy. While a large body of literature has explored the interaction of morphologically simple coral reefs, the in-canopy flow dynamics in complex coral reefs is poorly understood. This study used a synthetically generated coral reef over flat topography with varying reef height and density to understand the in-canopy turbulence dynamics. Using a turbulence-resolving computational framework, we found that most of the turbulent kinetic energy dissipation is confined to a region below the top of the reef and above the Stokes boundary layer. The results also suggest that most of the vertical Reynolds stress peaks within this region positively contribute to the down-gradient momentum flux during the forward phase of the wave cycle. These findings shed light on the physical relationships between in-canopy flow and morphologically complex coral reefs, thereby motivating a further need to explore the hydrodynamics of such flows using a scale-resolving computational framework.

Recommended citation: Patil, A. and C. García-Sánchez, (in review, JGR:Oceans), (2025), Hydrodynamics of In-Canopy Flow in Synthetically Generated Coral Reefs Under Oscillatory Wave Motion.

Synthetic turbulence to achieve swift converged turbulence statistics in a pressure-driven channel flows

Published in Physics of Fluids, 2025

In this study, we introduced a simple yet innovative application of the synthetic eddy method, the isotropic synthetic turbulence field generator (iSTFG) that leverages the homogeneity in the streamwise direction for channel flows, and triggers turbulence to achieve statistically stationary flow conditions faster than current standard community-used strategies. We compare this new method with two other well-established methods: linear profile superposed with random noise and descending counter-rotating vortices and log-law profile superposed with random noise and descending counter-rotating vortices. We found that iSTFG provides a computationally cheap and effective way to reduce simulation spin-up costs/time/emissions to achieve statistically stationary flow conditions when a precursor turbulent initial condition is unavailable. At a one-time cost between 1-10 Cental Processing Unit (CPU) hour(s) to generate the synthetic turbulent initial condition based on the target friction Reynolds numbers (1 CPU hour - Re = 500, 7 CPU hours - Re = 2000), the flow becomes statistically stationary within three eddy turnovers for all the parameters of interest in wall-bounded pressure-driven channel flow simulations when compared to other alternatives that can take more than ten eddy turnovers resulting in substantial savings in the computational cost. This method offers a practical and efficient solution for researchers studying turbulent channel flows, enabling faster convergence to a statistically stationary flow state.

Recommended citation: Patil, A. and C. García-Sánchez, (in review, Physics of Fluids), (2025), Synthetic turbulence to achieve swift converged turbulence statistics in a pressure-driven channel flows
Download Paper