LES of complex flow

Propeller wakes:

The behavior of rotor wakes are of engineering and theoretical interest due to their use in many engineering applications such as propellers, wind turbines and helicopters. We used state-of-the-art simulation techniques to predict and understand the behavior turbulent wake of a five-bladed marine propeller. The predicted flow field and forces matched well with the available experimental data thereby demonstrating the predictive capability of our codes. The analysis of our results helped us understand the flow physics and propose a new mechanism of rotor wake instability explaining the complex wake evolution of a class of marine propellers.

Propeller flows generate sound. We used the LES flow field to compute the propeller generated sound a far away distance from the propeller. We demonstrated the usefulness of the employed flow simulation method by using just the force data to predict far field sound and its behavior without doing any further simulations. Moreover, the various acoustic methods of varying complexity available in literature were compared against each other in context of propeller sound prediction. The analysis found that the far field sound is broadband in nature and the contribution of hub and blades to the overall sound was quantified. The results were also used to compute physical quantities relevant to a wider range of applications in the noise modeling community.

Publications:

Jacob Keller, Praveen Kumar and Krishnan Mahesh, “Examination of propeller sound production using large eddy simulation”, Physical Review Fluids, 3(6) (2018): 064601.
Jacob Keller, Praveen Kumar and Krishnan Mahesh, “Large Eddy Simulation of Propeller in forward mode of operation”, Fifth International Symposium on Marine Propulsors, Finland, 2017.
Praveen Kumar and Krishnan Mahesh, “Large eddy simulation of propeller wake instabilities”, Journal of Fluid Mechanics, 814 (2017): 361-396.
Praveen Kumar and Krishnan Mahesh, “Analysis of Marine Propulsor in Crashback using Large Eddy Simulation”, Fourth International Symposium on Marine Propulsors, USA, 2015.

Flow over hull:

Computer simulations are a valuable tool to predict the flow field and the overall forces on engineering geometries. However, predicting complex turbulent flows requires the best available simulation methods and computational infrastructure. Hence, most past studies settled for a less accurate method. There is a great interest in industries to push our predictive capabilities for such flows, to augment experiments which are typically difficult and expensive to perform. Therefore, we undertook the challenge of pushing the limits of feasibility of large simulations using state-of-the-art simulation techniques and computational resources to accurately predict the turbulent flow behavior.

Our simulations were first of its kind to predict the turbulent flow field and the forces it exerts on a model submarine geometry, which matched well with the available experimental data. The results helped understanding the flow physics and answer some of the previously unknown questions regarding turbulent flows regarding wake evolution. Flow over bodies generates wakes, which evolve downstream in a complex manner. We used our simulation results to support past theoretical work on wake evolution. Past experiments have shown that flow over geometries with curved surface (cylindrical crossection) exerts higher friction than a planar surface like aircraft wing. However, the exact dependence of friction on the flow quantities was unknown. We obtained a new expression which gives the exact dependence and explains the increase in friction for geometries with curved surface. I also demonstrated the validity and versatility of the expression using past results. The simulation data is serving as a benchmark for my ongoing work to develop approximate models for similar problems which have potential to predict such flows at significantly reduced computational cost. The published LES results also augments the limited experimental data reported in literature and can be used by researchers to validate their results.

Publications:

Praveen Kumar and Krishnan Mahesh, “Large eddy simulation of flow over an axisymmetric body of revolution”, Journal of Fluid Mechanics, 853 (2018): 537-563.
Praveen Kumar and Krishnan Mahesh, “Large eddy simulation of flow over axisymmetric hull”, 32nd Symposium on Naval Hydrodynamics, Germany, 2018.
Praveen Kumar and Krishnan Mahesh, “Towards Large Eddy Simulation of Hull-attached Propeller in Crashback”, 31st Symposium on Naval Hydrodynamics, USA, 2016.
Krishnan Mahesh, Praveen Kumar, Aswin Gnanaskandan and Zane Nitzkorski, “LES Applied to Ship Research”, Journal of Ship Research, 59(4) (2015): 238-245.

Flow over elliptical hydrofoil:

Flow over a wing creates pressure difference between its two sides. This pressure difference causes a transverse flow which winds around the wing tip to form what is called tip vortex. Tip vortices are common in many engineering applications such as wings and propellers, thereby making them important and relevant to engineering applications. The behavior of tip vortices are significantly altered in the presence of walls in their vicinity, which is the case in ducted propellers and turbomachinery. Depending on the physical conditions, the pressure inside the tip vortices may drop causing the liquid to form vapor, a phenomenon called cavitation. Cavitation is undesirable for propellers as it causes performance degradation, material damage and produces noise. Therefore, Boulon et al. (1999) performed experiments to study the cavitation behavior of tip vortices under wall-confinement for the first time. Their experimental setup had side-walls as well as bottom wall which could be adjusted to a desired tip gap, the distance between the blade tip and the bottom wall. They performed several experiments varying the tip gap and other physical parameters of the flow and showed that the cavitation behavior of confined tip vortices is very different than that of unconfined one. However, their measurements were limited and they focused only on the cavitation behavior. The nature of confined tip vortices despite being relevant to many applications, is still not well-understood.

We investigated the nature of confined tip vortices by performing numerical simulation of the experimental setup of Boulon et al. (1999) using state-of-the-art technique. The tip vortex under side-wall confinement was simulated at two values of tip gaps, corresponding to negligible and severe bottom-wall confinement. My results matched the available experimental data well for both cases and revealed the drastic difference between the two cases. Future work will focus on cavitation behavior of the tip vortex in the presence of walls in its vicinity.

Publications:

Praveen Kumar and Krishnan Mahesh. "Large Eddy Simulation of flow over a confined elliptic hydrofoil", Sixth International Symposium on Marine Propulsors, Italy, 2019.