Numerical study of cavitation regimes in flow over a circular cylinder

Abstract

Cavitating flow over a circular cylinder is investigated over a range of cavitation numbers (σ = 5 to 0.5) for both laminar (at Re = 200) and turbulent (at Re = 3900) regimes. We observe non-cavitating, cyclic and transitional cavitation regimes with reduction in freestream σ. The cavitation inside the Kármán vortices in the cyclic regime, is significantly altered by the onset of "condensation front" propagation in the transitional regime. At the transition, an order of magnitude jump in shedding Strouhal number is observed as the dominant frequency shifts from periodic vortex shedding in the cyclic regime, to irregular-regular vortex shedding in the transitional regime. In addition, a peak in pressure fluctuations, and a maximum in St versus σ based on cavity length are observed at the transition. Shedding characteristics in each regime are discussed using dynamic mode decomposition (DMD). A numerical method based on the homogeneous mixture model, fully compressible formulation and finite rate mass transfer developed by Gnanaskadan & Mahesh (Intl. J. Multiphase Flows, vol. 70, 2015, pp. 22-34) is extended to include the effects of non-condensable gas (NCG). It is demonstrated that the condensation fronts observed in the transitional regime are supersonic (referred as "condensation shocks"). In the presence of NCG, multiple condensation shocks in a given cycle are required for complete cavity condensation and detachment, as compared to a single condensation shock when only vapor is present. This is explained by the reduction in pressure ratio across the shock in the presence of NCG effectively reducing its strength. In addition, at σ = 0.85 (near transition from the cyclic to the transitional regime), presence of NCG suppresses the low frequency irregular-regular vortex shedding. Vorticity transport at Re = 3900, in the transitional regime, indicates that the region of attached cavity is nearly two-dimensional with very low vorticity, affecting Kármán shedding in the near wake. Majority of vortex stretching/tilting and vorticity production is observed following the cavity trailing edge. In addition, the boundary-layer separation point is found to be strongly dependent on the amounts of vapor and gas in the freestream for both laminar and turbulent regimes. 1 2 ρ∞U 2 ∞ , where p ∞ , ρ ∞ and U ∞ are pressure, density and velocity in the freestream respectively) in the freestream is sufficiently dropped, cavities develop †

Cavitation dynamics and vortex shedding in the wake of a bluff body

“…Figure 23(a Shedding mechanism S3 occurs for cavitation numbers below those corresponding to peak shedding frequency, with the Strouhal number decreasing with decreasing cavitation number. Such shock-induced collapse of partial cavities in the wake behind a cylinder was also reported by Brandao et al (2020). At such low cavitation numbers, the presence of cavitation within the separating shear layer affects the underlying flow to the extent that it deviates significantly from the near-wake flow observed at higher cavitation numbers.…”

“…Nevertheless, local compressibility of mixture still plays a role in this regime as inferred by the observation of propagating discontinuities resembling a propagating bubbly shock that causes cavity shedding, as seen in figure 24. Such shocks were also observed by Brandao et al (2020) on a cavitating cylinder. Compressibility effects are manifest in this regime by the formation of such propagating shocks.…”

“…Recently, Ganesh et al (2016) and Wu et al (2019) reported the presence of such bubbly shock waves, a phenomenon related to the compressibility of the bubbly cavitation flow, to be a dominant mechanism of shedding of partial cavities. The presence and the role of bubbly shock waves in partial-cavity shedding dynamics have been recently verified by Budich et al (2018), and Brandao et al (2020).…”

“…In this region, the observed Strouhal number increased, suggesting the importance of the presence of vapour in the near-wake cavity It is very likely that for this condition, the presence of large regions of cavitation within the shear layer can result in a flow scenario different from the two conditions discussed previously. Brandao et al (2020) presented averaged void-fraction profiles observed downstream of a cavitating cylinder wake. Upon comparison, the overall topology of void-fraction fields look similar to the present study, except for the location of a maximum void-fraction region.…”

“…More information on POD can be found in Taira et al (2017). Recently, Brandao et al (2020) used DMD to study the dynamics of partial cavitation behind a cylinder. The presence of noise in X-ray-based void-fraction measurements used in the current study made it difficult to use DMD.…”