Challenges in the Experimental Measurement and High-Fidelity Simulation of Turbulent Cavitation

Turbulent cavitation, particularly in the inception and sheet to cloud transition regimes, presents formidable challenges to both controlled experimentation and high-fidelity simulations. Cavitating flows are of great practical interest since the highly unsteady flow can induce significant fluctuations in the thrust and torque of marine propellers, while the violent collapse of vapour clouds can cause material damage to the blades. We describe a study of sheet to cloud transition over a wedge by collaborative experiments performed at the University of Michigan and large-eddy simulation performed at the University of Minnesota. Observations of partial cavitation forming on the apex of a wedge was studied both experimentally and with numerical simulations. High-speed visualization and time resolved X-ray densitometry measurements were employed to examine the cavity dynamics, including the time resolve void fraction fields within the cavity, as described by Ganesh (2015) and Ganesh et al. (2016). Both UNRANS and LES were used to compute the cavity flow. The LES uses a novel numerical method developed by Gnanaskandan and Mahesh (2015); the homogeneous mixture model represents the multiphase mixture, a characteristic based filtering captures discontinuities and a dynamic Smagorinsky model represents small-scale turbulence. The paper will elaborate on details of the experiment, simulations, and their comparison. It will then outline the challenges posed in the validation and make recommendations for future experimental measurements that are essential in developing the reliable computational prediction of cavitating flows. STO-MP-AVT-246 9 1 PUBLIC RELEASE PUBLIC RELEASE Challenges in the Experimental Measurement and High-Fidelity Simulation of Turbulent Cavitation 1.0 BACKGROUND Cavitation can occur when the pressure in a liquid falls below the vapour pressure as a result of hydrodynamically or acoustically generated pressure fields. In many practical instances, vapour forms when small bubbles (i.e. nuclei) are present in a region of low or negative pressure (i.e. tension) and rapidly grow. If such vapour bubbles are then exposed to a recovering pressure, they can collapse violently. This leads to the production of strong noise emission, and the creation of shock waves and mechanical impulses at nearby flow boundaries. Cavitation has been associated with a variety of flow mechanisms that lead to steady or unsteady reductions in the fluid pressure. Turbulent flows are associated with regions of coherent vorticity and strong unsteady pressure fluctuations, and these flows can exhibit complex cavitating behaviour. Both wall bounded and free shear flows can cavitate. Under incipient conditions, the number and frequency of cavitating nuclei are few. But, as the cavitation number is reduced, the likelihood that regions of tension form in the turbulent flow increases, and the cores of the low-pressure vortices may begin to fill with vapour. Prediction of incipient and developed cavitating turbulent flows remains a challenge. A key reason for the limited success to-date is that the onset of cavitation involves non-linear and often explosive interactions between free-stream and/or surface nuclei and unsteady pressure fields. Consequently, prediction of the conditions cavitation inception and development as well as the physical mechanisms associated with the creation of noise and pressure impulses requires quantitative understanding of the pressure field of the flow, which is spatially inhomogeneous for most flows of naval interest, and in the case of turbulent flows, unsteady and scale dependent. The so-called “water quality”, which encompasses the nature, as well as the spatial and size distribution of free stream nuclei, along with the dissolved gas content, and surface tension is also an important factor, especially for incipient cavitation. These underlying physical processes can vary substantially for similar flows that span wide range of Reynolds numbers. Moreover, inception is often associated with the smallest (e.g. secondary) unsteady flow features that can be difficult to resolve in simulation across of wide range of Reynolds numbers. Computation of cavitating flows is particularly challenging, especially for high Reynolds number turbulent flows. Steady–state RANS has traditionally been the simplest turbulence model and has been used to predict steady state quantities like cavity length and pressure. Unsteady RANS (URANS) has been successfully used in predicting some quantities in unsteady cavitation. For example, URANS predicts cloud-shedding frequency in sheet to cloud cavitation transition to a reasonable accuracy. However, even URANS does not predict all unsteady quantities and LES can potentially be used as a predictive tool in unsteady cavitation problems (Mahesh et al., 2015). In the present study, the authors experimentally examine and numerically compute a particular turbulent cavitating flow: partial cavitation. Partial cavitation occurs when the low-pressure regions produced by separated shear flows are filled with vapour, forming a cavity. Flow scenarios exhibiting partial cavitation are separated shear layers on the suction side of lifting surfaces at high attack angles, blades of turbo machinery, inducers of cryogenic rocket motors, and the passages of a diesel fuel injectors. Once formed, partial cavities can be stable, with a nominally constant length. But, partial cavities can also undergo auto oscillations of cavity length that are characterised by the shedding of vapour clouds, termed as cloud cavitation. The length of oscillation can be intermittent or periodic. Moreover, the presence of cloud cavitation can lead to unsteady loading, pressure pulsations, erosion, and severe degradation of system performance. The authors and their collaborators developed a canonical geometry to study the formation and dynamics of a nominally two-dimensional partial cavity forming at the apex of a wedge. The experiments and computations were conducted simultaneously and jointly, and the preliminary results of each effort informed the other. Both Challenges in the Experimental Measurement and High-Fidelity Simulation of Turbulent Cavitation 9 2 STO-MP-AVT-246 PUBLIC RELEASE