STO-Activities: (no title)
Activity title:
Assessment of vortex-vortex interaction and cavitation inception
Activity Reference:
AVT-386
Panel:
AVT
Security Classification:
NATO UNCLASSIFIED
Status:
Active
Activity type:
RTG
Start date:
2024-01-01T00:00:00Z
Actual End date:
2026-12-31T00:00:00Z
Keywords:
cavitation inception, experimental and computational fluid dynamics, vortex instabilities, vortexvortex interaction
Background:
In the NATO AVT-320 the state-of-the-art in computational fluid dynamics (CFD) for prediction of the flow characteristics of cavitating and non-cavitating tip vortex has been evaluated. The flow on an elliptical foil with a span of 1 m is investigated experimentally in two large test facilities, the wind tunnel of TUHH in Germany and large cavitation channel of INM of CNR in Rom. The high time resolution of the measurement data enabled direct comparison with the results of the Large Eddy Simulation (LES) simulations. The comparison is conducted over a relatively large distance behind the foil. In addition, the visualization of the flow in the wind tunnel provides comprehensive insights into the roll-up process of the vortices shedding from the trailing edge and their interaction with the tip vortex. In general, a good agreement is achieved for the predicted flow characteristics between the different CFD submissions and the experimental results. However, some discrepancies were observed regarding the turbulent kinetic energy level in the core of the tip vortex. The application of vortex wander correction leads to a significant reduction of these discrepancies. LES-based methods are able to simulate the complex interaction between different flow regimes in the region of tip vortices, vortex shedding from the trailing edge and outer flow.
The flows caused by the lifting surfaces of marine vessels, such as stabilizing fins, rudders, and propellers, are particularly susceptible to cavitation. These regions typically have multiple vortical structures with varying size, strength, and orientation. The interaction of these vortices can lead to unexpected results, namely that it is often not the strongest vortices in a flow that cavitate first. Rather, weaker vortices are stretched by stronger vortices and experience a temporary pressure drop in the vortex core that causes the onset of cavitation in the weaker vortices.
The proposed RTG will continue the work of AVT-320 by analyzing the cavitating flow around the large single hydrofoil, continuing to relate the underlying flow field to cavitation inception. The proposed RTG will extend the work of AVT-320 by examining the complex flow of two interacting vortices induced by marine control surfaces. The pressure drop associated with stretching in the weaker vortex can be strong enough to cause cavitation inception. A focus will be the prediction of the strength, meandering trajectory, core pressures in the vortices. The aim is to quantify the complex vortex-vortex interactions and cavitation experimentally and numerically.
New experiment set-ups will be developed to study the two isolated lifting surfaces to generate the required validation data to better evaluate the capabilities and limitations of CFD prediction methods with varying degrees of fidelity such as Reynolds-Averaged Navier-Stokes (RANS), hybrid RANS-LES and DES (detached eddy simulation) and wall-modeled LES.
Objectives:
The objective is to evaluate state-of-the-art of CFD methods for predicting cavitation inception and vortex instabilities due to vortex-vortex interactions. New experimental investigations of the flow on two lifting surfaces will be conducted in different test facilities to enable validation of numerical methods and provide physical insights on the dynamic interaction between the two vortices and cavitation inception mechanisms. Time resolved velocity distribution measurements will be conducted for non-cavitating flows. The results will allow the determination of all turbulence quantities and evaluation of the unsteady pressure field.
The experimental results will provide all participants a comprehensive data set to evaluate and improve the numerical methods applied and evaluate the applicability limits of the developed set-ups. A focus will be given for the evaluation of the experimental and numerical uncertainties in order to be able to evaluate the limitations of methods and approaches used. Recommendations will be given for the identification of the capabilities of applied numerical models, grid resolutions, turbulence modeling approach, and numerical set-ups.
Topics:
1. Conducting experimental investigations of the same physical problem in different test facilities, each with a different focus, in order to take advantage of the combination of capabilities of the test facilities involved in the experimental investigation
2. Performing comprehensive CFD simulations with scale-resolved simulations such as hybrid RANS/LES and wall-modelled LES to investigate the vortex-vortex interactions and the resulting instabilities and cavitation inception characteristics.
3. Evaluating of the performance of different approaches for the computation of the bubble dynamics and their interaction with vortex flow structures.
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Created at 18/10/2022 17:00 by System Account
Last modified at 16/05/2024 11:00 by System Account
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