{"id":"6c27afb5-1fcd-4c0c-a8a6-39807661c6a7","arxiv_id":"2507.15135","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"high","formal_verification":"none","parameter_count":4,"one_line_summary":"Wall wettability, set by the contact angle, systematically shifts cavitation inception, cavity size, and flow unsteadiness around a Clark Y hydrofoil, with stabilizing effects at very low cavitation numbers.","lead":"Simulations of a Clark Y hydrofoil show that wall contact angle changes where and how vapor cavities form: higher contact angles trigger earlier and larger cavities at moderate cavitation, but at very low cavitation numbers they stabilize the attached cavity. The study suggests surface wettability could be a passive design knob for controlling cavitation and vibration in marine and hydraulic machinery.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The abstract's 'increasing WCA consistently promotes ... greater flow unsteadiness' is contradicted by the paper's own sigma=0.4 results, where WCA=160 stabilizes the cavity and reduces pressure fluctuations, making the central claim internally inconsistent as stated.","rationale":"Good-faith reading: the paper does a systematic parametric LES study, validates baseline lift, drag, and cavity thickness against experiment (Section 5), and reports plausible qualitative mechanisms. These are real supports for a restricted claim: WCA shifts inception and cavity thickness monotonically, and at low sigma it can stabilize an attached cavity. The load-bearing problem is that the headline claim overgeneralizes unsteadiness. The reader's weakest_assumption (2D LES) is a legitimate secondary concern, but it is not needed to show the current text is inconsistent: the contradiction is visible in the manuscript's own figures and text. A revised version that makes the sigma-dependence the central message and defines unsteadiness quantitatively would likely be publishable; therefore I keep CONDITIONAL/UNCHANGED. The 2D issue would still deserve an added limitation or 3D spot-check, but it is not the single most load-bearing concern because even in 2D the internal contradiction stands.","tokens_in":23171,"tokens_out":5781,"duration_ms":64656,"concrete_test":"Compute a common unsteadiness metric (e.g., RMS of fluctuating pressure at the probe locations of Fig. 7, or RMS of the lift coefficient) from the time series used for Figs. 12 and 13, for WCA=0 and 160 at sigma=0.4. If RMS(WCA=160) is lower than RMS(WCA=0), then the abstract's 'consistently greater flow unsteadiness' is false and the abstract/conclusion must be revised to present the low-sigma stabilization as the central regime-dependent finding. Also report the same metric for sigma=0.8 and 1.6 to verify for which regimes monotonicity in WCA actually holds.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim as stated in the abstract is internally inconsistent. The abstract says increasing WCA 'consistently promotes earlier cavitation inception, thicker cavity development, and greater flow unsteadiness,' and the conclusion repeats 'unsteadiness across all cavitation regimes.' But Section 6.2 (sigma=0.4) reports the opposite for the unsteadiness part: WCA=160 maintains a 'stable, wall-adhered vapor layer' and 'suppresses re-entrant jet activity,' with Fig. 13 showing more intense pressure fluctuations for WCA=0 and Fig. 12 showing strong temporal fluctuations for WCA=0 versus near-constant liquid fraction for WCA=160. The conclusion itself says superhydrophobic surfaces 'reduced unsteady loading.' A claim cannot be monotonic in WCA and also reverse at low sigma; one of these statements must be qualified. This matters because the advertised engineering value—'optimizing flow stability'—rests on the regime-dependent reversal at low sigma being real and correctly summarized. The contradiction is not a physics disagreement; it is a defect in the presentation of the central result. The paper should either define a single unsteadiness metric and report it for all cases, or explicitly state that WCA increases unsteadiness only at sigma=1.6 and 0.8 and decreases it at sigma=0.4.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports large-eddy simulations of cavitating flow around a Clark-Y hydrofoil at 8 degrees angle of attack, with wall contact angle varied from 0 to 160 degrees at three cavitation numbers, sigma = 1.6, 0.8, and 0.4. The central claim is that increasing WCA consistently promotes earlier cavitation inception, thicker cavity development, and greater flow unsteadiness, while the sigma = 0.4 results show a regime-dependent stabilization of an attached supercavity on superhydrophobic surfaces. The paper validates the solver against experimental lift/drag and cavity thickness data, reports a mesh independence study, and presents qualitative and semi-quantitative comparisons of pressure distributions, vapor volume fraction fields, velocity profiles, and pressure fluctuation histories across the WCA range.","tokens_in":23397,"tokens_out":5476,"duration_ms":60898,"significance":"If the reported trends are robust, the study would provide a useful demonstration that surface wettability can act as a passive control parameter for cavitation, with potential engineering relevance for marine and turbomachinery applications. The parametric LES dataset covering three cavitation regimes is a contribution, and the regime-dependent reversal at sigma = 0.4 is the most interesting finding. However, the significance as currently stated is not established because the abstract's monotonic 'greater flow unsteadiness' claim is contradicted by the paper's own sigma = 0.4 results, the grid independence evidence is non-convergent, and the dynamic contact angle model is not specified. The potential value of the work lies in a qualified, regime-dependent statement rather than the current universal claim.","major_comments":[{"comment":"The abstract and Section 7 claim that increasing WCA 'consistently promotes ... greater flow unsteadiness' and that unsteadiness increases 'across all cavitation regimes.' This is contradicted by Section 6.2, where WCA = 160 maintains a stable, wall-adhered vapor layer and suppresses re-entrant jet activity, while WCA = 0 shows strong temporal fluctuations in liquid volume fraction (Fig. 12) and more intense pressure fluctuations (Fig. 13). The conclusion itself states that superhydrophobic surfaces 'reduced unsteady loading.' Because a monotonic increase in unsteadiness cannot also reverse at low sigma, the paper must either define a single quantitative unsteadiness metric and report it for all cases, or explicitly restrict the monotonic claim to sigma = 1.6 and 0.8 and present the sigma = 0.4 behavior as a regime-dependent reversal.","section":"Abstract; Section 6.2; Fig. 13"},{"comment":"The grid independence table reports Cl values of 0.11, 0.46, 0.28, and 0.286 and cavity thickness values of 0.250, 0.100, 0.122, and 0.121 for very coarse to fine meshes. The sequence is non-monotonic, with a large outlier at the coarse mesh, and does not support the statement of 'clear convergence' or the claim that medium-to-fine differences are negligible as evidence of mesh independence. Since all production runs use the fine mesh, the quantitative results for cavity thickness and pressure fluctuations need either a properly converged grid study, a convergence curve with an uncertainty estimate, or a clear justification for why the non-monotonic coarse-mesh behavior does not affect the conclusions.","section":"Section 4, Table 1"},{"comment":"The dynamic wall contact angle model is mentioned in Section 2 as varying with contact line velocity, but no equation, model name, or parameter values are provided. Because WCA is the central control parameter, the simulation cannot be reproduced or fully assessed without specifying the contact angle model (e.g., Kistler, Shikhmurzaev, or a prescribed-angle variant), the static or advancing/receding angle inputs, and any relaxation or hysteresis parameters. In addition, the validation in Section 5 does not include any case with varying WCA, so there is no evidence that the model reproduces wettability-dependent behavior beyond the default hydrophilic setting.","section":"Section 2; Section 5"},{"comment":"The manuscript states that 'the hydrofoil spans the entire domain width, assuming a two-dimensional approximation.' Cloud cavitation shedding and re-entrant jet formation are strongly three-dimensional mechanisms involving spanwise vorticity and vortex stretching. The paper does not validate the 2D assumption against a 3D simulation or experimental spanwise measurements, and it does not discuss how this approximation might affect the reported WCA effects on shedding and re-entrant jet suppression. The central claims about cavity stability at sigma = 0.4 and intensified shedding at sigma = 0.8 could be numerical artifacts of suppressing spanwise degrees of freedom; the authors should either provide a 3D comparison for at least one regime or explicitly present all conclusions as conditional on the 2D approximation.","section":"Section 3"}],"minor_comments":[{"comment":"Section 3 states that simulations are performed for cavitation numbers of 0.8 and 0.4, but the abstract and Section 6.3 also include sigma = 1.6; this inconsistency should be corrected.","section":"Section 3"},{"comment":"The caption for Fig. 15 says 'The hydrophilic surface exhibits larger pressure fluctuation ranges due to unsteady cavity shedding and re-entrant jet activity,' while the text in Section 6.3 attributes localized pressure fluctuations to the superhydrophobic WCA = 160 case; the caption should be reconciled with the text.","section":"Fig. 15 caption"},{"comment":"The sign conventions in the vaporization and condensation source terms should be checked against the liquid volume fraction transport equation, since alpha is the liquid fraction and the source term appears on the right-hand side with a positive sign.","section":"Eqs. (10) and (11)"},{"comment":"The horizontal axis of Fig. 7 is labeled 'Time (s)' but the text discusses low- and high-frequency content; if the plotted quantity is a temporal signal, the axis label is correct, but if it is a spectrum, the axis should be frequency.","section":"Fig. 7"},{"comment":"The x-axis label in Fig. 5 reads 'Normalized Position (C)' but the axis variable is x/C; please correct the label for clarity.","section":"Fig. 5"},{"comment":"Table 1 lists cavity thickness without units; specify whether delta_0.6C is normalized by chord length, and include the dimensional or normalized value consistently.","section":"Table 1"},{"comment":"The abstract describes sigma = 1.6 as producing 'smaller, detached vapor bubbles and localized pressure fluctuations,' while Section 7 states that hydrophilic surfaces at this sigma produce 'smoother pressure distributions and minimal pressure fluctuations'; these statements should be made consistent with respect to which WCA generates the fluctuations.","section":"Abstract and Section 7"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the scope of a CFD-oriented fluids journal, and the authors have assembled a substantial parametric dataset. My main concern is that the headline monotonic claim is not supported by the paper's own sigma = 0.4 results, and that the mesh study does not demonstrate convergence. These issues are fixable in revision by rephrasing the claims, adding a quantitative unsteadiness metric, and either strengthening the grid study or adding a 3D comparison. I do not see grounds for rejection, but the revision needs to be substantive rather than cosmetic."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know about this paper. The headline claim is self-contradictory: the abstract and conclusion say increasing WCA 'consistently' increases unsteadiness across all regimes, but the sigma=0.4 section shows WCA=160 stabilizing the cavity, suppressing re-entrant jets, and reducing pressure fluctuations compared to WCA=0. You cannot have both. That is a presentation flaw, not a physics disagreement.\n\nWhat is genuinely new: a systematic LES sweep of uniform contact angle from 0 to 160 degrees on a Clark-Y hydrofoil at three cavitation numbers, sigma=1.6, 0.8, and 0.4, using a dynamic contact angle model. Earlier work by the same group used hybrid wettability patterns; the uniform sweep with regime resolution is a useful extension. The validation against experimental lift and drag and cavity thickness at sigma=0.4 is reasonable, and the qualitative trends at sigma=0.8 (higher WCA -> earlier inception, thicker cavity, more shedding) are consistent across several diagnostics. The regime-dependent reversal at low sigma is the most interesting result — if it holds, it is exactly the kind of passive control lever that could be useful for hydrofoils or propellers.\n\nThe soft spots, in proportion. The internal contradiction is the biggest issue and must be fixed by reporting a single unsteadiness metric across all cases or explicitly qualifying the abstract. The grid independence table is non-monotonic (Cl goes 0.11, 0.46, 0.28, 0.286; thickness 0.250, 0.100, 0.122, 0.121 across very coarse to fine), so the claim of 'clear convergence' is overstated; the coarse-mesh outlier needs explanation. The whole study is 2D, and cloud cavitation shedding with re-entrant jets is known to be strongly three-dimensional. The paper notes the 2D approximation but does not validate it or discuss its impact on the central claims. The dynamic contact angle model is not specified (no equation, no parameters), and the Kunz mass transfer constants (Cv=Cc=1000, tinf=0.005 s) are taken from prior work without sensitivity tests. None of these are fatal on their own; they are the kind of things a serious referee would ask for.\n\nMy take: the reader's conditional verdict is fair. The paper deserves peer review, not a desk reject, because the dataset and the regime-dependent finding are worth scrutiny. With a rewritten abstract, a consistent unsteadiness metric, and a transparent 2D limitation discussion, it could be a solid contribution. I would not cite it in its current form, but I would want to see the revised version.","headline":"A systematic WCA sweep with an interesting regime-dependent reversal at low sigma, buried under an internally contradictory abstract; deserves revision, not rejection.","tokens_in":24043,"tokens_out":4869,"would_cite":false,"duration_ms":45155,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Raising the wall contact angle from 0° to 160° consistently advances cavitation inception, thickens the vapor cavity, and increases unsteadiness, while at low cavitation numbers a high contact angle stabilizes the wall-attached cavity by…","keywords":["Cavitation Flow","Wall Contact Angle","Cavitation Number","Cavitation Regime","Large Eddy Simulation","Hydrofoil","Wettability","Re-entrant Jet"],"falsifier":"Repeat the contact-angle comparison in a spanwise-resolved three-dimensional simulation or a coated-hydrofoil experiment at cavitation number 0.8; if the 160° case no longer shows earlier inception, thicker cavities, and stronger pressure fluctuations relative to 0°, the two-dimensional assumption is the reason.","tokens_in":22891,"feed_emoji":"🌊","tokens_out":9189,"duration_ms":94833,"temperature":0.7,"pith_summary":"This paper argues that wall wettability—the contact angle between the liquid and the solid—can be used as a passive control parameter for cavitation around a Clark Y hydrofoil. Using large-eddy simulations with a dynamic contact angle model across three cavitation numbers, a dimensionless measure of how close the local pressure is to the vapor pressure, it finds a consistent ordering: as the wall contact angle rises from hydrophilic 0° to superhydrophobic 160°, cavitation starts earlier along the chord and the vapor cavity becomes thicker and more unsteady in the cloud-cavitation regime at σ=0.8. At σ=1.6 the higher contact angle produces small detached bubbles and localized wall-pressure fluctuations, while the hydrophilic wall keeps vapor attached and the pressure field smooth. At σ=0.4 the superhydrophobic wall suppresses the re-entrant jet and holds a stable, wall-adhered vapor layer, reducing unsteady loading. The authors conclude that contact angle is an effective passive mechanism for tailoring cavitation behavior and optimizing flow stability.","feed_headline":"Wetting angle controls when cavitation starts, sheds, and stabilizes","feed_subtitle":"Hydrophilic surfaces delay vapor formation; superhydrophobic surfaces stabilize attached supercavities.","key_machinery":"The machinery is the dynamic wall contact angle model embedded in the interface-capturing solver: instead of fixing the interface-wall intersection, the model lets the contact angle follow the contact-line velocity, and the resulting capillary force enters the momentum equation through the surface-tension term. This is the mechanism that lets the contact angle alter near-wall pressure, cavitation inception, vapor attachment, and re-entrant-jet formation in the simulations.","core_discovery":"The central claim is that wall contact angle is a first-order control on cavitation behavior: increasing it lowers the pressure needed for vapor formation, shifting inception upstream, enlarging vapor structures, and intensifying unsteadiness. The same increase has regime-dependent consequences—at σ=1.6 it yields smaller, mobile, detached bubbles with local pressure fluctuations; at σ=0.8 it produces thicker cavities, stronger shedding, and delayed wake recovery; at σ=0.4 it creates a stable attached supercavity that suppresses the re-entrant jet, the liquid jet that slices back through the cavity and drives shedding, and thereby reduces unsteady loading. The authors present these results as evidence that surface wettability can serve as a passive flow-control mechanism for cavitation.","pith_inferences":["Inference: the monotonic trend suggests a practical design rule—choose a hydrophilic surface to suppress cavitation onset and a superhydrophobic surface when a stable supercavity is wanted, though the sharp transitions between regimes, as between 120° and 160°, remain to be mapped.","Inference: the two-dimensional assumption probably weakens quantitative magnitudes such as shedding frequency and breakup scales more than it changes the ordering of contact-angle effects; a spanwise-resolved three-dimensional test would show how much of the reported stabilization survives spanwise instability.","Inference: combining wettability patterning with surface texture could provide a second knob for positioning cavity closure and mitigating erosion damage without changing the global geometry."],"forward_implications":["Hydrophilic wall treatments can delay incipient cavitation and keep pressure fluctuations low at moderate cavitation numbers.","Superhydrophobic wall treatments can stabilize an attached supercavity at low cavitation numbers, reducing cyclic loading from re-entrant-jet shedding.","Because wettability acts passively, contact-angle selection offers a no-moving-parts complement to active cavitation control.","The wake response—deeper velocity deficit and slower recovery at high contact angles—should be factored into predictions of drag and unsteady loads for hydrofoil-like bodies.","The same ordering of earlier inception and thicker cavities holds across regimes, so surface treatment can be chosen locally on a blade rather than globally."],"supporting_citations":[{"why":"This reference supplies the experimental lift, drag, and cavity-thickness data used to validate the LES setup.","marker":"[46]"},{"why":"This reference provides the Kunz cavitation model that sets the vaporization and condensation source terms in the simulations.","marker":"[45]"},{"why":"This reference provides the Volume-of-Fluid method used to track the liquid-vapor interface.","marker":"[44]"},{"why":"This reference provides the dynamic one-equation subgrid-scale model used for turbulence closure.","marker":"[43]"},{"why":"This reference establishes the LES/VOF approach for two-dimensional hydrofoil cavitation that the present study builds on.","marker":"[2]"},{"why":"This reference reports that hybrid surface wettability reduces cavitation volume and instability, the direct precedent for treating the contact angle as a control parameter.","marker":"[36]"},{"why":"This reference shows that the re-entrant jet direction changes when the contact angle crosses 90°, which is used to explain the suppression observed at 160°.","marker":"[39]"},{"why":"This reference reports contact-angle effects on wall shear stress and vortex structures during bubble collapse, supporting the proposed mechanism behind the observed trends.","marker":"[38]"}],"fun_headline_variants":["Wettability flips cavitation from shedding to stable supercavity","Contact angle steers bubble birth, shedding, and steadiness","Superhydrophobic walls suppress the jet that wrecks supercavities","Passive control: wall wettability dictates cavitation behavior"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the two-dimensional simulation geometry—the hydrofoil spans the full channel width, with the flow treated as two-dimensional—captures the same cavitation physics that would appear in a three-dimensional flow.","fun_headline_variants_meta":{"raw":{"variants":["Wettability flips cavitation from shedding to stable supercavity","Contact angle steers bubble birth, shedding, and steadiness","Superhydrophobic walls suppress the jet that wrecks supercavities","Passive control: wall wettability dictates cavitation behavior"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001065,"raw_usage":{"total_tokens":4454,"prompt_tokens":923,"completion_tokens":3531,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":539,"completion_tokens_details":{"reasoning_tokens":3455}},"tokens_in":539,"tokens_out":3531,"duration_ms":34041,"temperature":1.0,"reasoning_tokens":3455,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T15:40:08.216300+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Repeat the contact-angle comparison in a spanwise-resolved three-dimensional simulation or a coated-hydrofoil experiment at cavitation number 0.8; if the 160° case no longer shows earlier inception, thicker cavities, and stronger pressure fluctuations relative to 0°, the two-dimensional assumption is the reason.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference supplies the experimental lift, drag, and cavity-thickness data used to validate the LES setup."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference provides the Kunz cavitation model that sets the vaporization and condensation source terms in the simulations."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference provides the Volume-of-Fluid method used to track the liquid-vapor interface."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference provides the dynamic one-equation subgrid-scale model used for turbulence closure."},{"cited_title":"Roohi, A","cited_arxiv_id":null,"evidence_quote":"This reference establishes the LES/VOF approach for two-dimensional hydrofoil cavitation that the present study builds on."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"This reference reports that hybrid surface wettability reduces cavitation volume and instability, the direct precedent for treating the contact angle as a control parameter."},{"cited_title":"Saini, E","cited_arxiv_id":null,"evidence_quote":"This reference shows that the re-entrant jet direction changes when the contact angle crosses 90°, which is used to explain the suppression observed at 160°."},{"cited_title":"Huang, J","cited_arxiv_id":null,"evidence_quote":"This reference reports contact-angle effects on wall shear stress and vortex structures during bubble collapse, supporting the proposed mechanism behind the observed trends."}],"review_version":1}