{"id":"3f09d3ce-6b90-494d-8d00-3082bb04b51e","arxiv_id":"2501.10894","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Symmetric plasma forcing removes the antisymmetric screech tone in one symmetry component of a twin-rectangular supersonic jet, and bispectral analysis traces the remaining tones to an interconnected network of triads involving only antisymmetric guided-jet feedback modes.","lead":"This paper uses supercomputer simulations of twin rectangular supersonic jet nozzles to show that screech tones live only in specific mirror-symmetry patterns, and that firing plasma actuators in a symmetric pattern removes one of them. It explains the pattern using the symmetry of guided-jet feedback waves, pointing toward symmetry-based noise control for high-speed aircraft.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Antisymmetric-only G-JM claim rests on visual standing-wave signatures in forced-jet BMD modes and absence in SS/SA modes; with no dispersion/stability check and the paper's own 'G-JM or freestream acoustic waves' caveat, the symmetry-selection mechanism is not established.","rationale":"I read the paper as a careful, data-rich modal analysis whose main physical novelty is the symmetry argument: antisymmetric-only upstream G-JM explain why screech is antisymmetric about the major axis. I find no internal inconsistency in the D2 decomposition, the bounded BMD normalization, or the SPOD-from-BMD recovery; the latter has a plausible derivation and is checked against the LES pressure data in appendix B. The code availability and the use of parameter-free modal methods are genuine supporting evidence.\n\nThe load-bearing weakness is precisely the identification of the upstream waves. The manuscript's own sentence in Section 5.5 — 'guided-jet modes (G-JM) or freestream acoustic waves' — concedes that the standing-wave observation does not by itself determine the wave type. Moreover, the exclusivity claim (no symmetric G-JM) is inferred from the absence of standing waves in SS/SA BMD modes at selected triads. Because BMD modes are extracted for phase-coupled triads, an absent signature in a BMD mode is not equivalent to an absent mode in the linear spectrum. A dispersion or stability calculation on the LES mean flow would settle both issues: the wave type of the upstream component and the symmetry of the guided-mode branches.\n\nI therefore agree with the reader's weakest assumption. The concern is significant but does not invalidate the paper's empirical contributions: the natural-jet SPOD results, the symmetry-decomposed spectra, the forced-jet suppression of the AA tone, and the triad network are all presented in a coherent and reproducible manner. The reader's CONDITIONAL verdict appropriately captures that the central physical mechanism needs quantitative confirmation. No change to the verdict is needed.","tokens_in":32977,"tokens_out":8741,"duration_ms":111853,"concrete_test":"Perform a linear spatial stability analysis (or linearized-Euler solution) of the time-averaged natural and forced mean flows, decomposed into D2 symmetry components, at St = 0.29 and its harmonics. Compute the dispersion branches for upstream-propagating guided-jet modes with SS and AS symmetry. If an SS upstream branch exists, the antisymmetric-only claim fails; if only AS (and possibly AA) branches exist, it is supported. Complement this with a wavenumber-frequency spectrum of the LES pressure in the shear-layer region of the AS component to measure the phase speed of the upstream-travelling wave seen in the BMD modes, to distinguish G-JM from freestream acoustic waves.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that upstream-propagating guided-jet modes are antisymmetric about the major axis only, which is then used to explain why twin-rectangular jet screech is antisymmetric but not symmetric. The evidence, however, is indirect. In Section 5.5 the upstream-propagating component is identified from AS bispectral modes whose envelopes appear as standing waves, interpreted as interference between downstream Kelvin-Helmholtz waves and upstream G-JM or freestream acoustic waves (figures 13, 15; supplementary movie 2). The paper explicitly leaves open the freestream-acoustic alternative, and no dispersion relation, phase-speed estimate, or linear stability calculation for the LES mean flow is presented.\n\nThere are two distinct gaps. First, the standing-wave envelope demonstrates that an upstream-travelling wave exists in the AS component, but does not quantitatively distinguish a guided-jet mode from an acoustic wave; these have different propagation speeds and different symmetry properties. Second, the inference that no symmetric G-JM exist is based on the absence of visible standing waves in SS and SA bispectral modes. BMD modes are phase-locked, triad-specific structures, not the eigenmodes of the linearized problem; the absence of a standing-wave signature in a particular BMD mode does not establish absence of a symmetric upstream-guided mode in the linear spectrum. Since the symmetry-selection mechanism depends on which upstream modes exist, the headline physical conclusion is an interpretation rather than a demonstrated result. A further complication is that the G-JM evidence is drawn from the forced jet, whose mean flow is visibly deformed relative to the natural jet, while the claim is invoked to explain the natural jet's screech symmetries.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper studies the stationary, intermittent, and nonlinear dynamics of natural and forced supersonic twin-rectangular jets using large-eddy simulation data and spectral modal analysis. The flow is decomposed into four D2 reflectional symmetry components (SS, SA, AS, AA). In the natural jet, SPOD identifies two screech tones antisymmetric about the major axis: a steady AS tone and an intermittent AA tone. The authors then test the hypothesis that symmetric plasma forcing can disrupt these antisymmetric instabilities; the forcing removes the AA and AS tones and creates harmonic tones in SS and AS. The nonlinear dynamics are analyzed with an extended bispectral mode decomposition (BMD) that uses a normalized, unity-bounded bicoherence, includes mean-flow effects to recover SPOD along the zero-frequency axis, and enforces D2 symmetry triads. Three active symmetry triads are identified and assembled into an interconnected triad network. The central physical conclusion is that upstream-propagating guided-jet modes (G-JM) responsible for screech closure are antisymmetric about the major axis only, whereas downstream core modes can be symmetric or antisymmetric; the authors argue this symmetry dependence explains why the twin-rectangular jet exhibits antisymmetric but not symmetric screech modes.","tokens_in":33178,"tokens_out":4468,"duration_ms":52008,"significance":"If the central claim is correct, the paper provides a plausible symmetry-based mechanism for the selection of antisymmetric screech modes in twin-rectangular jets and demonstrates a control strategy that leverages D2 symmetry. The paper has clear methodological strengths: the D2 symmetry decomposition is exact and well explained; the proposed BMD normalization is proven to bound the mode bispectrum by unity; the mean-included recovery of SPOD along the zero-frequency axis is a useful and non-circular result; and the triad network framework is a natural extension of BMD to discrete spatial symmetries. The authors also make their Matlab BMD implementation publicly available. However, the decisive evidence for the G-JM symmetry claim is indirect, resting on visual identification of standing-wave envelopes in AS bispectral modes, and no dispersion or stability calculation is provided to distinguish G-JM from freestream acoustic waves. The bicoherence peaks are reported without uncertainty quantification, and the plasma forcing model is not validated against any forced experiment. These gaps prevent the paper from fully supporting its headline conclusion in its present form.","major_comments":[{"comment":"The central conclusion that upstream-propagating guided-jet modes are antisymmetric about the major axis only is inferred from standing-wave envelopes in AS bispectral modes, but the text explicitly leaves open the alternative interpretation that these upstream waves are freestream acoustic waves. No dispersion relation, phase-speed estimate, or linear stability calculation for the present LES mean flow is provided. Because the mechanism offered for antisymmetric-only screech depends on the existence and symmetry of these upstream modes, this identification must be strengthened before the headline claim can be accepted.","section":"§5.5, Figs. 13 and 15"},{"comment":"The absence of standing waves in SS and SA bispectral modes is used to conclude that symmetric G-JM do not exist. BMD modes are phase-locked, triad-specific structures and are not eigenmodes of the linearized problem; a null result in these modes does not establish the absence of a symmetric upstream-guided mode in the linear spectrum. The authors should either compute the linear modal spectrum of the mean flow, or provide an explicit symmetry argument for why symmetric G-JM cannot propagate upstream in this geometry.","section":"§5.5, Figs. 13 and 15"},{"comment":"The bicoherence magnitudes |β|=0.61, 0.37, and 0.29 are reported without uncertainty quantification. With only n_blk=18 blocks, sampling variability of bicoherence is substantial, and no statistical test against the null hypothesis of independent Fourier modes is given. The triad network in Figs. 10, 12, and 14 would be significantly more convincing with bootstrap confidence intervals or a significance threshold.","section":"§5.3, Table 3, Fig. 9"},{"comment":"The plasma actuation model parameters are adapted from voltage and current measurements, but the forced-jet results are not validated against any forced experiment. Since the control claim and the subsequent nonlinear analysis of the forced jet depend on the realism of this model, the authors should either provide a validation case against forced experiments or explicitly state the limitations of the model for quantitative predictions.","section":"§2.2, Table 2"}],"minor_comments":[{"comment":"The Reynolds number is printed as '1 .07× 106'; this should be '1.07×10^6' with consistent spacing and formatting.","section":"Table 1"},{"comment":"The data matrices in Eq. (5.5) appear to be duplicated in the typeset equation; please check and correct the display.","section":"§5.1.1, Eq. (5.5)"},{"comment":"Several reference names are missing spaces, e.g., 'Edgington-Mitchellet al. 2022' and 'Rodr´ıguez' with improper accent encoding; please proofread the bibliography for formatting errors.","section":"References"},{"comment":"The supplementary data URL is given as a placeholder ('https://doi.org/10.1017/jfm.2019...'); please update it to the actual DOI.","section":"Supplementary data"},{"comment":"The D2 decomposition is defined for the pressure field only; for the velocity components the decomposition is nontrivial and is deferred to Appendix D. Please add a sentence in §3 directing the reader to this treatment.","section":"§3"},{"comment":"The statement that mean-included BMD 'recovers' the SPOD is exact only for the pressure norm (and for the unity-replacement procedure); Fig. 17(b,c) shows that for the compressible-energy and schlieren norms the quantitative match is only approximate. Please qualify the wording accordingly.","section":"Appendix B, Fig. 17"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the journal's scope and the methodology is generally sound. My main concern is that the headline physical claim about G-JM symmetry is worded more strongly than the evidence supports; the identification of upstream G-JM is based on visual standing-wave envelopes and the paper itself acknowledges the acoustic-wave alternative. I would require either a quantitative dispersion/stability analysis or a softened claim. I have no concerns about attribution or overlap."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper is worth reading for its methodological additions and for the clear demonstration that symmetric forcing removes AA screech while leaving AS intact. The bounded BMD normalization is a genuine improvement, and the proof that mean-included BMD recovers SPOD along the abscissa/ordinate for a uniform mean is clean and honestly tested in the appendices. The D2-symmetry triad network is a new and useful organizing tool. There is no circular reasoning: no parameters are fitted to force the conclusion, and the interpretation imports guided-jet-mode theory from prior work. The authors are also candid about what their analysis cannot do, including the ambiguous nature of the forced AS tone and the fact that BMD does not quantify energy transfer. On those grounds the paper deserves a serious referee.\n\nThat said, the central physical claim — that upstream-propagating G-JM are antisymmetric about the major axis only, and that this explains antisymmetric-only screech in twin-rectangular jets — is not established. The identification rests on standing-wave envelopes in AS bispectral modes, and the paper itself states these could be G-JM 'or freestream acoustic waves'. No dispersion relation, phase-speed estimate, or linear stability calculation is presented for the LES mean flow. Without that, the upstream wave type is unconfirmed, and those two possibilities have different symmetry properties. The complementary claim — that no symmetric G-JM exist — is inferred from the absence of visible standing waves in SS/SA BMD modes. But BMD modes are phase-locked, triad-specific structures, not eigenmodes of the linearized problem; absence in those modes does not prove absence in the linear spectrum. A further complication is that the G-JM evidence comes from the forced jet, whose mean is visibly deformed relative to the natural jet, while the conclusion is meant to explain the natural jet's screech.\n\nOther soft spots are proportionally minor: bicoherence magnitudes are reported without uncertainty or significance thresholds from 18 blocks, and the plasma actuator model is not validated against the forced experiment. The authors acknowledge the latter; it weakens the control-design claim slightly but does not affect the modal analysis itself.\n\nMy recommendation: send it to peer review. The methodological core is sound and citable. But the referee should push the authors either to add a dispersion/stability check on the LES mean flow that distinguishes G-JM from freestream acoustic waves, or to soften the symmetry-selection claim to a conjecture. As written, Section 5.5 overstates a plausible interpretation as a mechanism.","headline":"A solid BMD/SPOD methods paper with a clear control outcome, but the headline symmetry-selection mechanism for screech is a visual interpretation, not an established result.","tokens_in":33883,"tokens_out":2219,"would_cite":true,"duration_ms":26410,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":["76F65","76Q05","76F06"],"pacs":["47.27.Rc","47.40.Ki","43.28.Ra"],"model":"deepseek-v4-flash","headline":"This paper claims that screech in twin-rectangular supersonic jets is closed by antisymmetric guided-jet modes only, so symmetric plasma forcing can remove one screech tone and explains why symmetric screech does not appear.","keywords":["supersonic twin-rectangular jet","jet screech","guided-jet modes","spectral proper orthogonal decomposition","bispectral mode decomposition","D2 symmetry","plasma actuation","large-eddy simulation"],"falsifier":"Compute the phase speed or dispersion relation of the upstream-travelling wavepacket in the antisymmetric bispectral mode from the LES snapshots, or run a linear stability analysis of the twin-rectangular mean flow: if the waves prove to be acoustic rather than subsonic guided-jet modes, or if an upstream-propagating symmetric guided-jet mode exists, the symmetry-based explanation of antisymmetric-only screech loses its mechanism.","tokens_in":32655,"feed_emoji":"🔊","tokens_out":6091,"duration_ms":56886,"temperature":0.7,"pith_summary":"Using large-eddy simulations of a Mach 1.5 twin-rectangular jet, the paper establishes that the symmetry of screech tones is set by the symmetry of the waves that close the screech feedback loop. In the natural jet, the two screech tones both flap antisymmetrically about the major axis: one steady (AS) and one intermittent (AA). When the jet is forced symmetrically at the screech frequency by modelled plasma actuators, the AA screech disappears entirely and tones appear only in the SS and AS components. Applying bispectral mode decomposition, the authors trace the harmonic tones to a network of triadic interactions and find that upstream-propagating guided-jet modes responsible for screech closure are antisymmetric about the major axis only, while downstream core modes can be symmetric or antisymmetric. This symmetry selection is the reason twin-rectangular jets show antisymmetric but not symmetric screech modes, and it is what makes symmetry-based forcing a viable control lever.","feed_headline":"Screech in twin jets rides only antisymmetric waves","feed_subtitle":"Symmetry-aware spectral analysis shows symmetric plasma forcing silences one screech tone by cutting its feedback path.","key_machinery":"The load-bearing objects are the four D2 reflectional-symmetry components (SS, SA, AS, AA) of the flow, obtained by quadrant-weighted sums of the pressure field, and the spectral decompositions applied within each component: spectral proper orthogonal decomposition (SPOD) for energetics and intermittency, and a normalized bispectral mode decomposition (BMD) whose mode bispectrum is bounded by unity and which recovers the leading SPOD eigenvalues and modes along the f_l=0 or f_k=0 axes when the mean is retained. BMD detects quadratic phase coupling between triadically compatible frequency and symmetry components; its modes educe the coherent structures, including Kelvin-Helmholtz wavepackets, trapped core modes, and guided-jet modes. The specific mechanism that carries the argument is the guided-jet mode: a subsonic instability wave with partial support in the slow ambient flow, which can therefore propagate upstream against the supersonic jet; the paper finds these upstream-propagating G-JM are antisymmetric about the major axis only.","core_discovery":"The central discovery is a symmetry selection rule for screech: the feedback loop of twin-rectangular jet screech is closed exclusively by upstream-propagating guided-jet modes that are antisymmetric about the major axis, whereas downstream-propagating core modes may be symmetric or antisymmetric. Consequently, screech modes themselves must be antisymmetric about the major axis, and the twin jet exhibits AS and AA screech but no symmetric screech. Symmetric (SS) plasma forcing at the screech frequency destroys the AA feedback path, eliminating that tone; the surviving tones are confined to the SS and AS components. The rule is presented as the translation of the azimuthal-symmetry dependence of guided-jet modes in round jets into the D2 dihedral symmetry of the twin-rectangular jet, and is supported by bispectral modes whose AS structures show standing-wave envelopes from counter-propagating Kelvin-Helmholtz and guided-jet waves, while SS structures show only trapped core modes.","pith_inferences":["If the symmetry selection is generic, forcing in any inactive symmetry component (SA or AS) should selectively suppress screech in the complementary component; the authors note such tests are outside their scope.","The visual identification of G-JM could be replaced by a quantitative test: computing the phase speed or dispersion relation of the upstream wavepacket from the LES database would settle whether the waves are guided-jet modes or freestream acoustic waves, as the paper itself flags.","The symmetry rule likely extends to other discrete-symmetry jet geometries, such as twin-round or single-rectangular jets, where the feedback-wave symmetry rather than the shear-layer instability symmetry dictates which screech modes can exist.","The BMD-SPOD recovery along the zero-frequency axis gives a practical diagnostic for experiments that only record pressure or schlieren: bicoherence maps can double as spectra once the mean-removal caveats are handled."],"forward_implications":["Symmetric plasma forcing at the screech frequency is a demonstrated control strategy: it completely removes the AA screech component while leaving the SA component untouched.","Screech tones in twin-rectangular jets can only be antisymmetric about the major axis; any future detection of a symmetric screech tone would contradict the proposed mechanism.","Because only the (SS,SS,SS), (AS,AS,SS), and (SS,AS,AS) symmetry triads are active, nonlinear energy transfer at the forcing frequency is confined to the SS and AS components; SA and AA do not participate.","The BMD extension provides a single plot combining energetics (SPOD) and triadic phase coupling, applicable to pressure, density, and temperature data when the mean is nearly uniform.","The harmonic cascade in the forced jet is not an isolated chain but an interconnected triad network that also forms wavenumber triads, evidenced by doubling of the streamwise wavenumber between harmonics."],"supporting_citations":[{"why":"Establishes guided-jet modes as the upstream closure mechanism for screech; the paper's core claim extends their symmetry to D2.","marker":"Edgington-Mitchell et al. 2022"},{"why":"Predicts the three families of high-speed jet instability waves, including the subsonic guided-jet modes identified in the bispectral modes.","marker":"Tam & Hu 1989"},{"why":"Provides the Kelvin-Helmholtz shear-layer instability that energises the screech feedback loop.","marker":"Tam 1971"},{"why":"Original screech feedback-loop hypothesis that frames the present mechanism.","marker":"Powell 1953b"},{"why":"Experimental twin-rectangular jet set-up and plasma actuator data that the LES is validated against and from which actuator parameters are taken.","marker":"Samimy et al. 2023"},{"why":"Introduces bispectral mode decomposition, which this paper extends with bicoherence normalisation and mean-retained SPOD recovery.","marker":"Schmidt 2020"},{"why":"Supplies the SPOD theory and method used for the energetic and intermittency analysis.","marker":"Towne et al. 2018"},{"why":"Provides the D2 symmetry-component decomposition applied to the twin-jet flow field.","marker":"Rodríguez et al. 2018"}],"fun_headline_variants":["Twin jet screech rides only antisymmetric feedback waves","Symmetric plasma forcing kills one twin-jet screech tone","Why twin-jet screech is never symmetric: a symmetry rule","Antisymmetric guided modes close twin-jet screech loop"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The identification of the upstream-travelling waves in the AS bispectral modes as guided-jet modes rests on visual inspection of standing-wave envelopes and spatial support, and the paper itself notes they could be freestream acoustic waves; no dispersion relation or linear-stability check is performed.","fun_headline_variants_meta":{"raw":{"variants":["Twin jet screech rides only antisymmetric feedback waves","Symmetric plasma forcing kills one twin-jet screech tone","Why twin-jet screech is never symmetric: a symmetry rule","Antisymmetric guided modes close twin-jet screech loop"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000251,"raw_usage":{"total_tokens":1615,"prompt_tokens":1061,"completion_tokens":554,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":677,"completion_tokens_details":{"reasoning_tokens":483}},"tokens_in":677,"tokens_out":554,"duration_ms":5296,"temperature":1.0,"reasoning_tokens":483,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T18:52:13.695710+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Compute the phase speed or dispersion relation of the upstream-travelling wavepacket in the antisymmetric bispectral mode from the LES snapshots, or run a linear stability analysis of the twin-rectangular mean flow: if the waves prove to be acoustic rather than subsonic guided-jet modes, or if an upstream-propagating symmetric guided-jet mode exists, the symmetry-based explanation of antisymmetric-only screech loses its mechanism.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Predicts the three families of high-speed jet instability waves, including the subsonic guided-jet modes identified in the bispectral modes."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the Kelvin-Helmholtz shear-layer instability that energises the screech feedback loop."},{"cited_title":", Webb, N","cited_arxiv_id":null,"evidence_quote":"Experimental twin-rectangular jet set-up and plasma actuator data that the LES is validated against and from which actuator parameters are taken."}],"review_version":1}