Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Intermittency Behavior Along the Suction Surface of a Low Pressure Turbine Blade

Schobeiri, Meinhard T.; Öztürk, Burak; Ashpis, David E.

doi:10.1115/1.2219762

Cited by 21 publications

(5 citation statements)

References 32 publications

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“…Headley and Keffer [62] suggested the smoothing time period (Ts) to be equal to 4ΔT for a ZPG boundary layer, where ΔT is the sampling period of the velocity signals. Different smoothing periods have been reported by other investigators [43,73,74]. In the present work, we find that 4ΔT is not a sufficiently long smoothing period to get a reasonably good criterion function and prefer to use the large-eddy turnover time regions and hence can be identified with relative ease (Fig.…”

Section: Discussionsupporting

confidence: 64%

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Instability mechanisms and intermittency distribution in adverse pressure gradient attached and separated boundary layers

2021

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Direct Numerical Simulation of a boundary layer subjected to an adverse pressure gradient has been carried out to investigate the instability mechanisms and transition-zone characteristics, as the boundary layer changes its character from an attached to a separated flow. The freestream turbulence intensity is fixed at a moderately low value of 0.3% and the inlet Reynolds number (Re) is reduced to obtain one attached and two separated flow (small and large separation) cases. A detailed characterisation of the pre-transitional boundary layer has been carried out that reveals a "mixed-mode" instability, involving contribution from instability waves associated with inflectional instability and streamwise streaks generated by the lift-up effect. A unified picture is presented of the changes in the relative significance of these two modes, as Re is varied, leading up to the transition onset marked by a breakdown of spanwise vortical rollers for all the three cases. Next, we carry out a time-frequency analysis of the transitional velocity signals and show that as Re decreases, the character of the time traces evolves continuously from a "spotty" behaviour (i.e., exhibiting distinct turbulent spots) for the attached case to a "non-spotty" behaviour (i.e., involving a more "uniform" distribution of turbulent fluctuations in time) for the large separation case, encompassing the entire spectrum of transition scenarios. The intermittency factor (𝛾𝛾) has been calculated within the transition zone and its variation is seen to compare well with the "universal" 𝛾𝛾-distribution of Dhawan and Narasimha (J. Fluid Mech., Vol.3, pp.418, 1958) for all the three cases. One of the basic premises of the universal 𝛾𝛾-distribution is the concentrated breakdown hypothesis (requiring generation of spots randomly in time and span) and we investigate whether the transition 2 scenario in the large separation case conforms to this hypothesis. We find that although the time variation of velocity for large separation is non-spotty (or more "uniform"), the spanwise variation of velocity is "spotty" (or "random") in character showing a clear clustering of highwavenumber fluctuations separated by quasi-laminar regions. This suggests that the concentrated breakdown hypothesis is "partially" satisfied, which might explain why the 𝛾𝛾distribution compares reasonably well with the universal distribution for this case. On the other hand, for the attached-flow case, the velocity signal is "spotty" in both time and span, conforming more closely to the premise of this hypothesis. Finally, we present a physical cartoon for the transition scenarios for the attached and separated cases, using the ideas of vortex-wall interaction and the instability of spanwise vortical structures.

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Section: Discussionsupporting

confidence: 64%

“…A2 (h)). This can present difficulties in removing false zeros and ones, which again is a well-recognized fact in the literature [13,43,[72][73][74].…”

Section: Discussionmentioning

confidence: 96%

Instability mechanisms and intermittency distribution in adverse pressure gradient attached and separated boundary layers

2021

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“…The calmed region D is then also a zone of increased resistance against separation. That the transition evolution with a separated boundary layer, for paths under wakes and in between wakes, can be similar to the evolution under a statistically steady mean flow was further demonstrated by Schobeiri et al [32,33] for unsteady wake passages from cylindrical rods along the suction side of a low-pressure turbine profile. Turbulent spots generated by the wake impact suppress or reduce periodically the size and height of the separation bubble.…”

Section: Wake-induced Transitionmentioning

confidence: 85%

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Dick

Kubacki

2017

IJTPP

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Current models for transition in turbomachinery boundary layer flows are reviewed. The basic physical mechanisms of transition processes and the way these processes are expressed by model ingredients are discussed. The fundamentals of models are described as far as possible, with a common structure of the equations and with emphasis on the similarities between the models. Tests of models reported in the literature are summarized and our own test is added. A conclusion on the performance of models is formulated.Keywords: turbomachinery flows; transition models; boundary layers; bypass transition; separationinduced transition; wake-induced transition; Reynolds-averaged Navier-Stokes; intermittency; laminar kinetic energy Transition MechanismsWith the objective of modelling with Reynolds-averaged Navier-Stokes (RANS) or URANS (unsteady or time-accurate RANS) description of a flow, generally, four types of transition from laminar state to turbulent state in turbomachinery boundary layer flows are distinguished [1,2]. These are: natural transition in attached boundary layer state, bypass transition in attached boundary layer state under a statistically steady mean flow, separation-induced transition in the free shear layer formed by separation of a boundary layer under a statistically steady mean flow, and wake-induced transition due to periodically unsteady impact of wakes on boundary layers in attached or separated state. Despite the vast amount of studies on these transition types during the last 4 or 5 decades, understanding of the mechanisms is still not complete, although large progress has been made in the last decade thanks to direct numerical simulation (DNS) and large-eddy simulation (LES). Hereafter, we summarise the mean mechanisms, taking into account the inherent limitations of modelling with Reynolds-averaged Navier-Stokes description of a flow. This means that secondary effects, which mostly are the least understood and sometimes still are a matter of controversy, are not discussed, because, anyhow these features cannot be represented by RANS or URANS. Natural TransitionUnder a statistically steady mean flow of low turbulence level, transition in an attached boundary layer is initiated by 2D viscosity-dependent Tollmien-Schlichting instability waves, followed by a 3D instability, leading to formation of spanwise periodic hairpin vortices, which, farther downstream, cause breakdown of the laminar layer with generation of turbulent spots. These spots finally merge resulting in the formation of a turbulent boundary layer [1,2]. This type of transition is called natural transition. It is a rather slow process and, under a very low mean-flow turbulence level, as in external Int.

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“…Heat transfer aspect of the gas turbine blades in high-pressure stages is thoroughly discussed and reported in many publications. On the other hand, not enough attention has been paid to heat transfer in LPT blades and especially in separated flow conditions while aerodynamics of such airfoils has been extensively studied in numerous publications, among others, Schobeiri et al [4][5][6] and Schulte et al 7 Schobeiri and Chakka 8 conducted heat transfer and boundary layer measurements along the suction and pressure surfaces of a high-pressure turbine (HPT) blade of the NASA Space Shuttle Main Engine (SSME). They studied flow parameters under periodic unsteady inlet flow conditions.…”

Section: Background Experimental Investigationmentioning

confidence: 99%

“…On the other hand, not enough attention has been paid to heat transfer in LPT blades and especially in separated flow conditions while aerodynamics of such airfoils has been extensively studied in numerous publications, among others, Schobeiri et al. 4–6 and Schulte et al. 7…”

Section: Introductionmentioning

confidence: 99%

Combined numerical and experimental investigations of heat transfer of a highly loaded low-pressure turbine blade under periodic inlet flow condition

Nikparto

Schobeiri

2018

Proceedings of the Institution of Mechanical Engineers, Part A:

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This paper experimentally and numerically investigates heat transfer characteristics of a low-pressure turbine blade under steady/unsteady flow conditions. Generally, the low-pressure turbine blades are not exposed to excessive temperatures that require detailed heat transfer predictions. In aircraft engines, they operate at low Re-numbers causing the inception of large separation bubbles on their suction surface. As documented in previous papers, the results of detailed aerodynamic simulations have shown significant discrepancies with experiments. It was the objective of the current investigation to determine the discrepancies between the experimental and numerical heat transfer results. It is shown that small errors in aero-calculation results in large deviations of heat transfer results. The characteristics of the blades mentioned above, make low-pressure turbine blades suitable candidates for evaluating the predictive capability of any numerical method. Documenting the scope of these discrepancies defines the framework of the current paper. The periodic flow inside the gas turbine engine was simulated using the cascade facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. In this study, the wakes that originate from stator blades were simulated by moving rods. The instrumented blade was covered with a liquid crystal sheet and it was used to measure heat transfer coefficient. Reynolds-averaged Navier-Stokes equations were used for numerical investigation purposes. Measurements and simulations were conducted at three different Reynolds numbers (110,000, 150,000, and 250,000). Furthermore, for unsteady flow condition, reduced frequencies of the incoming wakes were varied. The current paper includes a comprehensive heat transfer assessment of the predictive capability of Reynoldsaveraged Navier-Stokes based tools. The effect of the separation bubbles on heat transfer is thoroughly discussed in this paper. Comparisons of the experimental and numerical results detail the differences and identify the sources of error that leads to in accurate calculations in terms of predicting heat transfer calculation results.

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Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Intermittency Behavior Along the Suction Surface of a Low Pressure Turbine Blade

Cited by 21 publications

References 32 publications

Instability mechanisms and intermittency distribution in adverse pressure gradient attached and separated boundary layers

Instability mechanisms and intermittency distribution in adverse pressure gradient attached and separated boundary layers

Transition Models for Turbomachinery Boundary Layer Flows: A Review

Combined numerical and experimental investigations of heat transfer of a highly loaded low-pressure turbine blade under periodic inlet flow condition

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