Abstract
Sheet cavitation inception can be influenced by laminar boundary layer flow separation under Reynolds numbers regimes with transitional flow. The lack of accurate prediction of laminar separation may lead to massive over-prediction of sheet cavitation under certain circumstances, including model scale hydrofoils and marine propellers operating at relatively low Reynolds number. For non-cavitating flows, the local correlation based transition model, γ−Reθ transition model, has been found to provide predictions of laminar separation and resulting boundary layer transition. In the present study, the predicted laminar separation using γ−Reθ transition model is bridged with a cavitation mass transfer model to improve sheet cavitation predictions on hydrofoils and model scale marine propellers. The bridged model is developed and applied to study laminar separation and sheet cavitation predictions on the NACA16012 hydrofoil under different Reynolds numbers and angles of attack. As a reference case, the open case of the PPTC VP1304 model scale marine propeller tested on an inclined shaft is studied. Lastly as an application case, the predictions of cavitation on a commercial marine propeller from Kongsberg is presented for model scale conditions. Simulations using the bridged model and the standard unbridged approach with k−ωSST turbulence model are performed using the open-source package OpenFOAM, both using the Schnerr–Sauer cavitation mass transfer model, and the respective results are compared with available experimental results. The predictions using the bridged model agree well compared to experimental measurements and show significant improvements compared to the unbridged approach.
Highlights
Many experimental studies show that seemingly local boundary layer separation is a pre-requisite for sheet cavitation inception, otherwise traveling bubble cavitation will develop that will not stay attached to the wall surface [2,3,4,5,6]
Franc and Michel [2], experiments were performed on a NACA16012 foil and an elliptical cylinder, and the major observation is that the cavity detachment point is not the minimum pressure point, but downstream of a laminar separation; cavitation is suppressed by laminar boundary layer upstream the sheet cavitation inception location where pressure is below saturation pressure
At 4◦ angle of attack (AoA) as shown in Figure 13, the bridged model predicted inception starts at 20% c while in the experiments the inception line varies between 13∼23% chord length
Summary
The single fluid homogeneous mixture approach is used to represent the two phases of water and vapor as ρ m = α l ρ l + (1 − α l ) ρ v , μ m = α l μ l + (1 − α l ) μ v , αl + αv = 1,. The transport equation of α with mass transfer source terms can be written as. The cavitation mass transfer model by Schnerr and Sauer [20] is used. The mass transfer term in Equation (2) is decomposed into two terms as ṁ = αl ṁ av + (1 − αl )ṁ ac ,. While the two terms ṁ av and ṁ ac are derived based on uniformly distributed spherical nuclei and simplified Rayleigh relations as ṁ ac ṁ av. In which Cv and Cc represent the vaporization and condensation constants
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