scholarly journals Predicting Bypass Transition: A Physical Model Versus Empirical Correlations

1997 ◽  
Author(s):  
Mark W. Johnson ◽  
Ali H. Ercan
Keyword(s):  
Boundary Layer ◽  
Adverse Pressure Gradient ◽  
Empirical Correlation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Empirical Correlations ◽  
Layer Transition ◽  
Model Versus ◽  
Current Modelling ◽  
Modelling Approach

A boundary layer transition model is presented which relates the near wall velocity fluctuations to the formation of turbulent spots. This model is used to determine the turbulent intermittency within a boundary layer integral code. Comparisons are made between the code predictions and established empirical correlations for the adverse pressure gradient transition experiments performed by Gostelow and co-workers. Similarly good accuracy was achieved by both the model and empirical correlation for start of transition. However, empirical correlations were less reliable than the model for predicting end of transition. The model was also able to predict the evolution of the measured intermittency considerably more accurately than the Narasimha empirical correlation. The current modelling approach is thus demonstrated to be more reliable than empirical correlation for the modelling of transitional boundary layers.

Effects of Reynolds Number and Free-Stream Turbulence on Boundary Layer Transition in a Compressor Cascade

2000 ◽  
Author(s):  
Heinz-Adolf Schreiber ◽  
Wolfgang Steinert ◽  
Bernhard Küsters
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
High Reynolds Numbers ◽  
High Turbulence ◽  
High Reynolds

An experimental and analytical study has been performed on the effect of Reynolds number and free-stream turbulence on boundary layer transition location on the suction surface of a controlled diffusion airfoil (CDA). The experiments were conducted in a rectilinear cascade facility at Reynolds numbers between 0.7 and 3.0×106 and turbulence intensities from about 0.7 to 4%. An oil streak technique and liquid crystal coatings were used to visualize the boundary layer state. For small turbulence levels and all Reynolds numbers tested the accelerated front portion of the blade is laminar and transition occurs within a laminar separation bubble shortly after the maximum velocity near 35–40% of chord. For high turbulence levels (Tu > 3%) and high Reynolds numbers transition propagates upstream into the accelerated front portion of the CDA blade. For those conditions, the sensitivity to surface roughness increases considerably and at Tu = 4% bypass transition is observed near 7–10% of chord. Experimental results are compared to theoretical predictions using the transition model which is implemented in the MISES code of Youngren and Drela. Overall the results indicate that early bypass transition at high turbulence levels must alter the profile velocity distribution for compressor blades that are designed and optimized for high Reynolds numbers.

Large Eddy Simulation of Boundary Layer Transition Mechanisms in a Gas-Turbine Compressor Cascade

2018 ◽  
Author(s):  
Ashley D. Scillitoe ◽  
Paul G. Tucker ◽  
Paolo Adami
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Suction Surface ◽  
Layer Transition ◽  
Turbulent Spots ◽  
Pressure Surface ◽  
Eddy Simulation ◽  
Large Eddy

Large Eddy Simulation (LES) is used to explore the boundary layer transition mechanisms in two rectilinear compressor cascades. To reduce numerical dissipation, a novel locally adaptive smoothing scheme is added to an unstructured finite-volume solver. The performance of a number of Sub-Grid Scale (SGS) models is explored. With the first cascade, numerical results at two different freestream turbulence intensities (Ti’s), 3.25% and 10%, are compared. At both Ti’s, time-averaged skin-friction and pressure coefficient distributions agree well with previous Direct Numerical Simulations (DNS). At Ti = 3.25%, separation induced transition occurs on the suction surface, whilst it is bypassed on the pressure surface. The pressure surface transition is dominated by modes originating from the convection of Tollmien-Schlichting waves by Klebanoff streaks. However, they do not resembled a classical bypass transition. Instead, they display characteristics of the “overlap” and “inner” transition modes observed in the previous DNS. At Ti = 10%, classical bypass transition occurs, with Klebanoff streaks incepting turbulent spots. With the second cascade, the influence of unsteady wakes on transition is examined. Wake-amplified Klebanoff streaks were found to instigate turbulent spots, which periodically shorten the suction surface separation bubble. The celerity line corresponding to 70% of the free-stream velocity, which is associated with the convection speed of the amplified Klebanoff streaks, was found to be important.

scholarly journals Improving Numerical Estimation of Cyclist Drag Area in Static Conditions Using Unsteady RANS

Proceedings ◽  
2020 ◽  
Vol 49 (1) ◽  
pp. 139
Author(s):  
Ardalan Javadi ◽  
Andrew J. M. Buckrell ◽  
Sean D. Peterson
Keyword(s):  
Boundary Layer ◽  
Correlation Coefficient ◽  
Turbulence Model ◽  
Computational Cost ◽  
Empirical Correlation ◽  
Boundary Layer Transition ◽  
Layer Transition ◽  
Shape Factors ◽  
Static Conditions ◽  
Drag Area

Herein, we compare the drag area estimated using unsteady Reynolds-averaged Navier-Stokes (URANS), using the γ−ReΘ transitional shear stress transport (SST) k−ω (SSTLM) turbulence model with published experimental measurements of a static full-scale cyclist mannequin in an open test section wind tunnel, with the left leg fully extended. The turbulence model employs a local empirical correlation based upon a classical Blasius boundary layer behavior to predict flow transition. For a given mesh density, we aim to improve drag area estimation by modifying the empirical correlation coefficient to better capture actual boundary layer transition location around the arms and legs, to facilitate computationally economical cyclist simulations. Large Eddy Simulation (LES), in conjunction with experimental wake data in the vicinity of the arms and legs, is used to assess boundary layer shape factors, which are related to the empirical coefficient. Overall, the drag area predicted by LES is within 3.7% of the measured results, while the original SSTLM is within 7.8%. By tuning the correlation coefficient, the drag area error is improved to 6.0% at no additional computational cost. The tuning was relatively coarse, and was only considered for the appendages. In other regions, the original SSTLM coefficient seems to perform better, suggesting that local coefficient selection may lead to further improvements in results over the currently employed global value.

Boundary Layer Transition on a Low Pressure Turbine Blade due to Downstream Potential Interaction

2012 ◽  
Author(s):  
Véronique Penin ◽  
Pascale Kulisa ◽  
François Bario
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Turbulence Intensity ◽  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Pressure Potential ◽  
Layer Transition ◽  
Transition Mode

Engine manufacturers wish to reduce the size and weight of their engines, and one way of achieving this is by reducing the rotor-stator gap. It follows that rotor-stator interactions become stronger, especially the influence of the pressure potential, which, despite its rapid spatial decay, becomes significant as the inter-row gap is reduced. Here we examine the upstream potential effect generated by downstream moving cylindrical rods on an upstream turbine blade. A large scale rectilinear blade cascade was constructed to improve access to the boundary layer. The Reynolds number was 1.6 × 105. Pressure measurements and two-dimensional Laser Doppler Anemometry around the blade were performed to study the boundary layer behavior. At low turbulence intensity (Tu−in = 1.8%), the laminar boundary layer experiences separation once per rod period. There are two transition modes which alternate during a rod period: separation transition mode and bypass mode. At high turbulence intensity (Tu−in = 4.0%), no boundary layer separation occurs. The boundary layer follows a bypass transition mode during an entire rod period.

Boundary Layer Transition Induced by Periodic Wakes

1998 ◽  
Vol 122 (3) ◽  
pp. 442-449 ◽  
Author(s):  
Xiaohua Wu ◽  
Paul A. Durbin
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Skin Friction ◽  
Plate Boundary ◽  
Boundary Layer Transition ◽  
Bypass Transition ◽  
Layer Transition ◽  
Benchmark Data ◽  
Flat Plate Boundary Layer

Turbulent wakes swept across a flat plate boundary layer simulate the phenomenon of wake-induced bypass transition. Benchmark data from a direct numerical simulation of this process are presented and compared to Reynolds-averaged predictions. The data are phase-averaged skin friction and mean velocities. The predictions and data are found to agree in many important respects. One discrepancy is a failure to reproduce the skin friction overshoot following transition. [S0889-504X(00)00503-1]

Boundary-layer transition by interaction of discrete and continuous modes

2008 ◽  
Vol 604 ◽  
pp. 199-233 ◽  
Author(s):  
YANG LIU ◽  
TAMER A. ZAKI ◽  
PAUL A. DURBIN
Keyword(s):  
Boundary Layer ◽  
Finite Amplitude ◽  
Secondary Instability ◽  
Boundary Layer Transition ◽  
Length Scale ◽  
Bypass Transition ◽  
Continuous Mode ◽  
Layer Transition ◽  
Boundary Layer Turbulence ◽  
Tollmien Schlichting

The natural and bypass routes to boundary-layer turbulence have traditionally been studied independently. In certain flow regimes, both transition mechanisms might coexist, and, if so, can interact. A nonlinear interaction of discrete and continuous Orr-Sommerfeld modes, which are at the origin of orderly and bypass transition, respectively, is found. It causes breakdown to turbulence, even though neither mode alone is sufficient. Direct numerical simulations of the interaction shows that breakdown occurs through a pattern of Λ-structures, similar to the secondary instability of Tollmien–Schlichting waves. However, the streaks produced by the Orr-Sommerfeld continuous mode set the spanwise length scale, which is much smaller than that of the secondary instability of Tollmien–Schlichting waves. Floquet analysis explains some of the features seen in the simulations as a competition between destabilizing and stabilizing interactions between finite-amplitude distortions.

Coherent structures in bypass transition induced by a cylinder wake

2008 ◽  
Vol 603 ◽  
pp. 367-389 ◽  
Author(s):  
CHONG PAN ◽  
JIN JUN WANG ◽  
PAN FENG ZHANG ◽  
LI HAO FENG
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Boundary Layer Transition ◽  
Wake Vortex ◽  
Bypass Transition ◽  
Cylinder Wake ◽  
Layer Transition ◽  
Vortical Structures ◽  
Free Stream Turbulence

Flat-plate boundary layer transition induced by the wake vortex of a two-dimensional circular cylinder is experimentally investigated. Combined visualization and velocity measurements show a different transition route from the Klebanoff mode in free-stream turbulence-induced transition. This transition scenario is mainly characterized as: (i) generation of secondary transverse vortical structures near the flat plate surface in response to the von Kármán vortex street of the cylinder; (ii) formation of hairpin vortices due to the secondary instability of secondary vortical structures; (iii) growth of hairpins which is accelerated by wake-vortex induction; (iv) formation of hairpin packets and the associated streaky structures. Detailed investigation shows that during transition the evolution dynamics and self-sustaining mechanisms of hairpins, hairpin packets and streaks are consistent with those in a turbulent boundary layer. The wake vortex mainly plays the role of generating and destabilizing secondary transverse vortices. After that, the internal mechanisms become dominant and lead to the setting up of a self-sustained turbulent boundary layer.

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