REVIEW 3 major objections 3 minor 46 references
Effect of Gaussian wake amplitude on wake-induced transition for a T106A low pressure turbine cascade
T0 review · 3 major / 3 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Increasing the amplitude of periodic Gaussian wakes entering a T106A low-pressure turbine delays suction-surface separation and cuts skin friction by about 50%.
desk verdict A workmanlike 2D CFD sweep of wake amplitude on a T106A cascade, internally consistent on separation trends but carrying an applied profile-loss claim that its own momentum-thickness data appear to contradict. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The control parameter is the amplitude $a_{wake}$ of a periodic Gaussian wake superimposed on the inlet streamwise velocity, $u(w)=a_{wake}\exp[-\beta(\mathrm{mod}(y/y_{max}+2t/t_{wake}-1,2))^2]$, with $t_{wake}=0.35$ and $\beta=19$; nine amplitudes from 0.1 to 0.9 are compared. The numerical engine is an implicit large-eddy simulation of the two-dimensional compressible Navier-Stokes equations using dispersion-relation-preserving compact schemes and an optimized Runge-Kutta time integrator. The diagnostic that carries the energy argument is the compressible enstrophy transport equation (CETE), which splits the rate of change of enstrophy, a measure of rotational energy, into vortex stretching, compressibility, baroclinic, bulk-viscosity, and viscous terms; in two dimensions the stretching term is absent, so the remaining balance shows what the paper identifies as the dominant roles of baroclinicity and viscous stress. This decomposition, together with a turbulent kinetic energy production budget, is what connects the wake amplitude to separation suppression and profile loss.
What would settle it
Repeat the same nine Gaussian-wake cases in a spanwise-periodic three-dimensional simulation, or in a linear cascade experiment with pulsed wakes, and compare the time-averaged skin-friction coefficient along the suction surface: if the roughly 50% drop between $a_{wake}=0.1$ and $a_{wake}=0.9$, or the 23.3% shortening of the trailing-edge separation bubble, is not reproduced, the paper's central claim is falsified.
Extended reading notes
Core claim
On the suction surface of the T106A blade, increasing the nondimensional Gaussian wake amplitude $a_{wake}$ from 0.1 to 0.9 changes the time-averaged boundary layer in a consistent direction: the leading-edge separation bubble disappears for amplitudes above 0.4, the trailing-edge separation bubble is delayed and shortened by 23.3 percent in streamwise extent, reattachment moves upstream, and the time-averaged skin-friction coefficient falls by roughly 50 percent. At the same time the trailing-edge momentum thickness grows by 39.2 percent and the maximum unsteady separation-bubble half-height grows by 37 percent, so the wake does not simply thin the boundary layer. Enstrophy space-time maps show the wake-induced transition sequence: longitudinal puffs (compact turbulent patches) stretch into streaks, break into turbulent spots, and leave calmed regions behind them; the spot leading edge convects at about 83 percent of the free-stream speed and the calmed region at about 26 percent, both close to classic measurements. Turbulent kinetic energy production decreases with amplitude and its peak moves toward the wall, and the compressible enstrophy budget is dominated first by the viscous-stress term and second by the baroclinic term, with the baroclinic share increasing at higher amplitudes. The paper reads these trends as evidence that stronger wakes suppress separated flow and improve profile loss at the low Reynolds numbers relevant to low-pressure turbines.
Load-bearing premise
The central claim rests on the two-dimensional simulations being a faithful stand-in for a real three-dimensional blade flow; the paper itself notes that vortex stretching is absent in 2D and offers no three-dimensional or grid-convergence check, so if spanwise motions change the separation and reattachment response, the 50-percent drag reduction need not carry over to the engine.
Editorial extensions
If this is right
- If the 2D result represents the real flow, increasing upstream wake amplitude is a viable separation-control input: the suction surface sees about half the skin friction at $a_{wake}=0.9$ compared with $a_{wake}=0.1$.
- Calmed regions behind turbulent spots are the physical mechanism that periodically suppresses the separation bubble, so wake-passing design can be optimized for low-Reynolds-number operation rather than treated only as a disturbance source.
- The measured spot and calmed-region convection speeds (about 83% and 26% of the free-stream speed) give quantitative anchors for transition models in turbomachinery codes.
- Because trailing-edge momentum thickness increases by 39.2% while skin friction drops by 50%, loss estimates must track boundary-layer growth and drag separately; a wake that reduces drag can still thicken the downstream boundary layer.
- The CETE term ordering (viscous first, baroclinic second) is roughly independent of amplitude, suggesting that a single enstrophy-based diagnostic can characterize wake-induced transition across a range of wake strengths.
Reading between the lines
- The 50% drag reduction is a two-dimensional result; because the paper itself notes that vortex stretching is absent in 2D and turbulent spots are intrinsically three-dimensional, I would not transfer the factor to engine blading until a spanwise-periodic 3D run at the same Reynolds number reproduces the trend.
- If wake amplitude is viewed as a control knob, the opposing trends in skin friction and momentum thickness imply a design trade-off: there may be an intermediate amplitude that minimizes total profile loss rather than the largest one.
- The CETE results hint at a reduced-order route: separation suppression at low Reynolds numbers could be predicted from a baroclinic-versus-viscous enstrophy balance without resolving the full broadband turbulence spectrum.
- Because the paper finds that wake amplitude mirrors the effects of Mach number and free-stream turbulence but more strongly, a combined scaling in amplitude, Mach number, and turbulence intensity might collapse separation-onset data on the T106A surface into one curve.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents two-dimensional implicit large-eddy simulations of the T106A low-pressure turbine cascade with periodically incoming Gaussian wakes, varying the wake amplitude awake from 0.1 to 0.9 in nine cases. The authors report that increasing wake amplitude delays suction-surface separation, moves reattachment upstream, reduces the separated-flow region, and lowers skin friction by about 50% at the highest amplitude. They also analyze vorticity spectra, space-time enstrophy maps, and TKE and compressible-enstrophy budgets, and conclude that calmed regions induced by wake passing suppress separation and improve profile loss at low Reynolds numbers.
Significance. If the quantitative claims were properly supported, the paper would provide a useful parametric database for wake-induced transition on a realistic LPT profile, and the application of the CETE budget to a wake-perturbed turbine cascade would be a novel diagnostic contribution. The systematic nine-case amplitude sweep and the connection to classical turbulent-spot and calmed-region convection speeds are valuable. However, the main applied claim—improved profile loss—is contradicted by the paper's own loss proxy, and the headline skin-friction reduction is not defined precisely enough to be evaluated. The two-dimensional framework is acknowledged but limits transfer of the conclusions to real turbine flows.
major comments (3)
- [Abstract; Sections 3.1 and 4; Eq. (13); Table 2] The claim that higher wake amplitude 'improve[s] profile loss' is contradicted by the paper's own loss model. Equation (13) states that profile loss is dominated (about 90%) by the term proportional to trailing-edge momentum thickness θTE. Section 3.1 and Table 2 report that increasing awake from 0.1 to 0.9 increases the maximum momentum thickness by 39.2% (from 0.0369 to 0.0514). Without a compensating decrease in Cpb and δTE, Eq. (13) predicts a substantial increase, not a reduction, in ζp. No direct ζp computation is presented anywhere, so the abstract's 'improve profile loss' and Section 4's 'potential benefit for reducing the profile loss' are unsupported; as written, the reported data point in the opposite direction.
- [Section 3.1; Fig. 4] The central quantitative claim of a 50% reduction in skin friction is not defined precisely. Figure 4 shows a strongly streamwise-varying local Cf, so the 50% figure is ambiguous: it could refer to a local value, a suction-surface integral, or a spatial average, and the manuscript does not state which. The claim also lacks uncertainty estimates and any grid-refinement or time-averaging convergence check, so the reader cannot determine whether the amplitude trend is numerical or physical.
- [Sections 2 and 3.3] The simulations solve the two-dimensional compressible Navier-Stokes equations, and Section 3.3 acknowledges that the vortex stretching term is absent in 2D. Because the paper's applied conclusions—calmed regions 'suppress flow separation and improve profile loss'—are framed for LPT blades at low Re, and because turbulent spots and calmed regions are intrinsically three-dimensional, the 2D results cannot by themselves support those applied claims. The manuscript should either clearly scope the conclusions as two-dimensional only, with a discussion of expected three-dimensional effects, or provide a three-dimensional verification case.
minor comments (3)
- [Section 3.2; Fig. 9] The text refers to 'the spectrum in Fig. 9(c)' when describing the awake=0.3 case, but Fig. 9(c) is a time series and the spectrum is in Fig. 9(d); similar frame-number mismatches occur for the awake=0.5 and later cases.
- [Fig. 11 caption] The caption of Fig. 11 lists amplitudes awake=0.7 and 0.9, while the accompanying text describes awake=0.1, 0.3, and 0.5; the caption and text should be reconciled.
- [Section 2 versus Section 3.1] Section 2 states that 'five to six through-flows' are used to flush initial transients, while Section 3.1 says 'five through flows'; the numbers should be harmonized.
Circularity Check
No circular derivation: the wake-amplitude trends are direct CFD outputs and the CETE is a diagnostic; the profile-loss claim is an internal-consistency issue, not a circularity.
full rationale
The paper's central claims are direct outputs of nine 2D compressible Navier-Stokes simulations with a prescribed Gaussian wake (Eq. 12). The independent variable, awake, is imposed at the inflow, and the reported separation locations, skin friction, momentum thickness, and enstrophy budgets are all measured from the resulting flow field. No parameter is fitted to the claimed outcome: α = 19 and t_wake = 0.35 are taken from Karaca and Gungor [11], and the Strouhal number is stated after the fact. The compressible enstrophy transport equation (Eq. 15) is cited from the author's prior work [24], but it is a parameter-free transport equation derived from the Navier-Stokes equations and is used only as a diagnostic in Fig. 16; the budget trends are new computed outputs, not inputs. The turbulent-spot convection speed is compared with independent measurements [43], and the numerical framework is validated against external data [25, 15]. The acknowledged 2D limitation (Section 3.3, vortex stretching absent) is a modeling limitation, not a circular step. One genuine concern is that the conclusion that higher wake amplitude 'has a potential benefit for reducing the profile loss' is not supported by the paper's own loss model: Eq. (13) makes the dominant loss term proportional to trailing-edge momentum thickness, and Table 2 reports that this quantity increases by 39.2% with awake. That is an internal-consistency and correctness problem, because the claim is not derived from the paper's quoted loss proxy; it is not a circularity, since the claim is not equivalent to its own inputs. No self-definitional, fitted-input-called-prediction, or self-citation-chain circularity is present.
Assumptions & free parameters
free parameters (4)
- wake amplitude (awake) =
0.1 to 0.9 across nine cases
- Gaussian width exponent (alpha) =
19
- wake passing period (twake) =
0.35
- boundary-layer edge vorticity threshold =
minimum vorticity (exact value not specified)
assumptions (5)
- domain assumption The two-dimensional compressible Navier-Stokes equations, closed with the ideal gas law and Sutherland's viscosity, faithfully represent wake-induced transition on the T106A suction surface.
- domain assumption A Gaussian streamwise velocity deficit, with no imposed turbulence content, is an adequate model of an upstream stator wake for studying wake-induced transition.
- domain assumption Statistics obtained from 10-12 through-flows after 5-6 transient through-flows are converged for the reported time-averaged quantities.
- standard math The numerical schemes (DRP compact scheme, OUCS3, OCRK3, fifth-order filter with coefficient 0.47) provide accurate ILES results on the stated grid.
- standard math The compressible enstrophy transport equation (Eq. 15) is the correct budget for enstrophy in this flow.
Cite this review
Pith. "Pith review of Effect of Gaussian wake amplitude on wake-induced transition for a T106A low pressure turbine cascade." pith.science (2026). https://pith.science/paper/JRDCHZVQ
@misc{pith2026241112242,
author = {Pith},
title = {Pith review of: Effect of Gaussian wake amplitude on wake-induced transition for a T106A low pressure turbine cascade},
year = {2026},
howpublished = {\url{https://pith.science/paper/JRDCHZVQ}},
note = {Machine review of arXiv:2411.12242}
}
read the original abstract
The wake-induced transition on the suction surface of a T106A low-pressure turbine (LPT) blade is investigated through a series of implicit large eddy simulations, solving the two-dimensional (2D) compressible Navier-Stokes equations (NSE). The impact of the incoming Gaussian wake amplitude on the blade's profile loss and associated boundary layer parameters is examined, revealing a 50\% reduction in skin friction drag at the highest amplitude. The results indicate that increasing wake amplitude leads to delayed separation and earlier reattachment, resulting in reduced separated flow. The vorticity and enstrophy dynamics during the transition process under varying wake amplitudes reveal characteristic features of wake-induced transition, such as puffs, streaks, and turbulent spots. The periodic passing of wakes induces intermittent "calmed regions", which suppress flow separation and improve profile loss at low Reynolds numbers (Re), typically found in LPTs. The energy budget, accounting for both translational and rotational energy via the turbulent kinetic energy (TKE) and compressible enstrophy transport equation (CETE), respectively, shows trends with increasing wake amplitude. The relative contribution to TKE production and the roles of baroclinicity, compressibility, and viscous terms are explained.
Figures
Figures from the paper (13 more)
Reference graph
Works this paper leans on
-
[1]
M. M. Opoka, H. P. Hodson, Transition on the T106 LP Turbine Blade in the Presence of Moving Upstream Wakes and Downstream Potential Fields, in: Turbo Expo: Power for Land, Sea, and Air, Vol. 47934, 2007, pp. 1091–1104
work page 2007
-
[2]
D. E. Halstead, D. C. Wisler, T. H. Okiishi, G. J. Walker, H. P. Hodson, H. W. Shin, Boundary layer development in axial compressors and turbines: Part 1 of 4—composite picture (1997)
work page 1997
-
[3]
Sengupta, Numerical lnvestigation of disturbance environments in low pressure turbines, Ph.D
A. Sengupta, Numerical lnvestigation of disturbance environments in low pressure turbines, Ph.D. thesis (2020)
work page 2020
- [4]
-
[5]
M. R. Banieghbal, E. M. Curtis, J. D. Denton, H. P. Hodson, I. Hunstman, V. Schulte, N. W. Harvey, A. B. Steele, Wake passing in lp turbine blades, in: AGARD CONFERENCE PRO- CEEDINGS AGARD CP, AGARD, 1996, pp. 23–23
work page 1996
-
[6]
W. Lou, J. Hourmouziadis, Separation bubbles under steady and periodic-unsteady main flow conditions, J. Turbomach. 122 (4) (2000) 634–643
work page 2000
-
[7]
A. Sengupta, P. G. Tucker, Effects of forced frequency oscillations and free stream turbulence on the separation-induced transition in pressure gradient dominated flows, Physics of Fluids 32 (10) (2020)
work page 2020
-
[8]
R. Stieger, H. Hodson, The transition mechanism of highly loaded low-pressure turbine blades, J. Turbomach. 126 (4) (2004) 536–543. 22
work page 2004
Show all 46 references
-
[9]
J. G. Wissink, W. Rodi, H. P. Hodson, The influence of disturbances carried by periodically incoming wakes on the separating flow around a turbine blade, International journal of heat and fluid flow 27 (4) (2006) 721–729
2006
-
[10]
J. D. Coull, H. P. Hodson, Predicting the profile loss of high-lift low pressure turbines (2012)
2012
-
[11]
Karaca, A
S. Karaca, A. Gungor, Dns of unsteady effects on the control of laminar separated boundary layers, European Journal of Mechanics-B/Fluids 56 (2016) 71–81
2016
-
[12]
Addison, H
J. Addison, H. Hodson, Unsteady transition in an axial-flow turbine: Part 1—measurements on the turbine rotor (1990)
1990
-
[13]
Sengupta, P
A. Sengupta, P. Tucker, Effects of forced frequency oscillations and unsteady wakes on the separation-induced transition in pressure gradient dominated flows, Physics of Fluids 32 (9) (2020)
2020
-
[14]
Schulte, H
V. Schulte, H. Hodson, Unsteady wake-induced boundary layer transition in high lift lp turbines (1998)
1998
-
[15]
Wissink, DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes, International Journal of Heat and Fluid Flow 24 (4) (2003) 626–635
J. Wissink, DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes, International Journal of Heat and Fluid Flow 24 (4) (2003) 626–635
2003
-
[16]
Sengupta, N
A. Sengupta, N. Vadlamani, P. G. Tucker, Roughness induced transition in low pressure tur- bines, in: 55th AIAA Aerospace Sciences Meeting, 2017, p. 0303
2017
-
[17]
M. Alam, N. D. Sandham, Direct numerical simulation of ‘short’laminar separation bubbles with turbulent reattachment, Journal of Fluid Mechanics 410 (2000) 1–28
2000
-
[18]
Wissink, W
J. Wissink, W. Rodi, DNS of a laminar separation bubble affected by free-stream disturbances, in: Direct and Large-Eddy Simulation V: Proceedings of the fifth international ERCOFTAC Workshop on direct and large-eddy simulation held at the Munich University of Technology, Augus...
2003
-
[19]
Sengupta, V
A. Sengupta, V. K. Suman, T. K. Sengupta, Direct numerical simulation of vortex-induced in- stability for a zero-pressure-gradient boundary layer, Physical Review E 100 (3) (2019) 033118
2019
-
[20]
Sengupta, P
A. Sengupta, P. Sundaram, T. K. Sengupta, Nonmodal nonlinear route of transition to two- dimensional turbulence, Physical Review Research 2 (1) (2020) 012033
2020
-
[21]
J. H. Watmuff, Evolution of a wave packet into vortex loops in a laminar separation bubble, Journal of Fluid Mechanics 397 (1999) 119–169
1999
-
[22]
Sengupta, N
A. Sengupta, N. Gupta, B. Ubald, Separation-induced transition on a T106A blade under low and elevated free stream turbulence, Physics of Fluids 36 (2) (2024)
2024
-
[23]
X. Wu, R. G. Jacobs, J. C. Hunt, P. A. Durbin, Simulation of boundary layer transition induced by periodically passing wakes, Journal of Fluid Mechanics 398 (1999) 109–153
1999
-
[24]
V. K. Suman, P. Sundaram, J. K. Puttam, A. Sengupta, T. K. Sengupta, A novel compressible enstrophy transport equation-based analysis of instability during Magnus–Robins effects for high rotation rates, Physics of Fluids 34 (4) (2022). 23
2022
-
[25]
Stadtm¨ uller, L
P. Stadtm¨ uller, L. Fottner, A test case for the numerical investigation of wake passing effects on a highly loaded LP turbine cascade blade, in: Turbo Expo: Power for Land, Sea, and Air, Vol. 78507, American Society of Mechanical Engineers, 2001, p. V001T03A015
2001
-
[26]
Sengupta, P
A. Sengupta, P. Sundaram, Compressibility effects on the flow past a T106A low-pressure turbine cascade, Physics of Fluids 35 (10) (2023)
2023
-
[27]
Garai, L
A. Garai, L. Diosady, S. Murman, N. Madavan, DNS of flow in a low-pressure turbine cascade using a discontinuous-galerkin spectral-element method, in: Turbo Expo: Power for Land, Sea, and Air, Vol. 56642, American Society of Mechanical Engineers, 2015, p. V02BT39A023
2015
-
[28]
Ranjan, S
R. Ranjan, S. Deshpande, R. Narasimha, Direct numerical simulation of compressible flow past a low pressure turbine blade at high incidence, in: Fluids Engineering Division Summer Meeting, Vol. 46216, American Society of Mechanical Engineers, 2014, p. V01AT02A010
2014
-
[29]
De Vincentiis, K
L. De Vincentiis, K. Hurovi´ c, D. Lengani, D. Simoni, J. Pralits, D. S. Henningson, A. Hanifi, Effects of upstream wakes on the boundary layer over a low-pressure turbine blade, Journal of turbomachinery 145 (5) (2023) 051011
2023
-
[30]
Michelassi, L.-W
V. Michelassi, L.-W. Chen, R. Pichler, R. D. Sandberg, Compressible direct numerical simula- tion of low-pressure turbines—part II: effect of inflow disturbances, Journal of Turbomachinery 137 (7) (2015) 071005
2015
-
[31]
Pichler, V
R. Pichler, V. Michelassi, R. Sandberg, J. Ong, Highly resolved LES study of gap size effect on low-pressure turbine stage, in: Turbo Expo: Power for Land, Sea, and Air, Vol. 50787, American Society of Mechanical Engineers, 2017, p. V02AT40A006
2017
-
[32]
Lengani, D
D. Lengani, D. Simoni, R. Pichler, R. Sandberg, V. Michelassi, F. Bertini, On the identification and decomposition of the unsteady losses in a turbine cascade, Journal of Turbomachinery 141 (3) (2019) 031005
2019
-
[33]
Fiore, R
M. Fiore, R. Gojon, G. S´ aez-Mischlich, J. Gressier, LES of the T106 low-pressure turbine: Spectral proper orthogonal decomposition of the flow based on a fluctuating energy norm, Computers & Fluids 252 (2023) 105761
2023
-
[34]
J. D. Denton, Loss mechanisms in turbomachines, Vol. 78897, American Society of Mechanical Engineers, 1993
1993
-
[35]
Sagaut, V
P. Sagaut, V. Suman, P. Sundaram, M. Rajpoot, Y. Bhumkar, S. Sengupta, A. Sengupta, T. Sengupta, Global spectral analysis: Review of numerical methods, Computers & Fluids (2023) 105915
2023
-
[36]
Hirsch, Numerical computation of internal and external flows, Computational methods for inviscid and viscous flows 2 (1990)
C. Hirsch, Numerical computation of internal and external flows, Computational methods for inviscid and viscous flows 2 (1990)
1990
-
[37]
Sengupta, N
A. Sengupta, N. Shandilya, Thermal optimization of shock-induced separation in a natural laminar airfoil operating at off-design conditions, Physics of Fluids 36 (4) (2024)
2024
-
[38]
C. R. Doering, J. D. Gibbon, Applied analysis of the Navier-Stokes equations, no. 12, Cam- bridge university press, 1995. 24
1995
-
[39]
Sengupta, V
A. Sengupta, V. K. Suman, T. K. Sengupta, S. Bhaumik, An enstrophy-based linear and nonlinear receptivity theory, Physics of Fluids 30 (5) (2018)
2018
-
[40]
R. G. Jacobs, P. A. Durbin, Simulations of bypass transition, Journal of Fluid Mechanics 428 (2001) 185–212
2001
-
[41]
B. R. McAuliffe, M. I. Yaras, Transition mechanisms in separation bubbles under low-and elevated-freestream turbulence (2010)
2010
-
[42]
H. W. Emmons, The laminar-turbulent transition in a boundary layer-part i, Journal of the Aeronautical Sciences 18 (7) (1951) 490–498
1951
-
[43]
G. B. Schubauer, P. S. Klebanoff, Contributions on the mechanics of boundary-layer transition, Tech. rep. (1956)
1956
-
[44]
Joshi, A
B. Joshi, A. Sengupta, P. Sundaram, Exploring role of aspect ratio for compressible flow in a rectangular lid-driven cavity with a vertical temperature gradient, Physics of Fluids 35 (6) (2023) 066135
2023
-
[45]
Gostelow, G
J. Gostelow, G. Walker, W. Solomon, G. Hong, N. Melwani, Investigation of the calmed region behind a turbulent spot (1997)
1997
-
[46]
Sengupta, H
A. Sengupta, H. N. Ulloa, B. Joshi, Multi-layer Rayleigh-Taylor instability: Consequences for naturally occurring stratified mixing layers, Physics of Fluids 35 (10) (2023). 25 t ω 10 15 20 25 30 0 50 100 a) awake = 0.1: ω series t ω 10 15 20 25 30 0 50 100 c) awake = 0.3: ω ...
2023
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