Predicting Transition Without Empiricism or DNS

2002 ◽  
Author(s):  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Isotropic Turbulence ◽  
Numerical Procedure ◽  
Length Scale ◽  
Arbitrary Orientation ◽  
Freestream Turbulence ◽  
Streamwise Direction ◽  
Laminar Boundary Layers ◽  
Experimental Values

A numerical procedure for predicting the receptivity of laminar boundary layers to freestream turbulence consisting of vortex arrays with arbitrary orientation has been developed. Results show that the boundary layer is most receptivity to those vortices which have their axes approximately in the streamwise direction and vortex wavelengths of approximately 1.2 δ. The computed near wall gains for isotropic turbulence are similar in magnitude to previously published experimental values used to predict transition. The new procedure is therefore capable of predicting the development of the fluctuations in the laminar boundary layer from values of the freestream turbulence intensity and length scale and hence determining the start of transition without resorting to any empirical correlation.

Predicting Transition Without Empiricism or DNS

2002 ◽  
Vol 124 (4) ◽  
pp. 665-669 ◽  
Author(s):  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Isotropic Turbulence ◽  
Numerical Procedure ◽  
Length Scale ◽  
Arbitrary Orientation ◽  
Freestream Turbulence ◽  
Streamwise Direction ◽  
Laminar Boundary Layers ◽  
Experimental Values

A numerical procedure for predicting the receptivity of laminar boundary layers to freestream turbulence consisting of vortex arrays with arbitrary orientation has been developed. Results show that the boundary layer is most receptivity to those vortices which have their axes approximately in the streamwise direction and vortex wavelengths of approximately 1.2 δ. The computed near wall gains for isotropic turbulence are similar in magnitude to previously published experimental values used to predict transition. The new procedure is therefore capable of predicting the development of the fluctuations in the laminar boundary layer from values of the freestream turbulence intensity and length scale, and hence determining the start of transition without resorting to any empirical correlation.

A Parametric Numerical Study of the Effects of Freestream Turbulence Intensity and Length Scale on Anti-Vortex Film Cooling Design at High Blowing Ratio

2013 ◽  
Author(s):  
Timothy W. Repko ◽  
Andrew C. Nix ◽  
James D. Heidmann
Keyword(s):  
Turbulence Intensity ◽  
Film Cooling ◽  
Numerical Study ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Cooling Effectiveness ◽  
Length Scales ◽  
Film Cooling Effectiveness ◽  
Blowing Ratio ◽  
Cooling Design

An advanced, high-effectiveness film-cooling design, the anti-vortex hole (AVH) has been investigated by several research groups and shown to mitigate or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low freestream turbulence levels. [1, 2] The effects of increased turbulence on the AVH geometry were previously investigated and presented by researchers at West Virginia University (WVU), in collaboration with NASA, in a preliminary CFD study [3] on the film effectiveness and net heat flux reduction (NHFR) at high blowing ratio and elevated freestream turbulence levels for the adjacent AVH. The current paper presents the results of an extended numerical parametric study, which attempts to separate the effects of turbulence intensity and length-scale on film cooling effectiveness of the AVH. In the extended study, higher freestream turbulence intensity and larger scale cases were investigated with turbulence intensities of 5, 10 and 20% and length scales based on cooling hole diameter of Λx/dm = 1, 3 and 6. Increasing turbulence intensity was shown to increase the centerline, span-averaged and area-averaged adiabatic film cooling effectiveness. Larger turbulent length scales were shown to have little to no effect on the centerline, span-averaged and area-averaged adiabatic film-cooling effectiveness at lower turbulence levels, but slightly increased effect at the highest turbulence levels investigated.

A Boundary Layer Transition Model

1996 ◽  
Author(s):  
Mark W. Johnson ◽  
Ali H. Ercan
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Transition ◽  
Test Cases ◽  
Length Scale ◽  
Transition Model ◽  
Freestream Turbulence ◽  
Layer Transition ◽  
Frequency Spectra ◽  
Low Frequencies ◽  
Turbulent Length Scale

A new boundary layer transition model is presented which relates the velocity fluctuations near the wall to the formation of turbulent spots. A relationship for the near wall velocity frequency spectra is also established, which indicates an increasing bias towards low frequencies as the skin friction coefficient for the boundary layer decreases. This result suggests that the dependence of transition on the turbulent length scale is greatest at low freestream turbulence levels. This transition model is incorporated in a conventional boundary layer integral technique and is used to predict eight of the ERCOFTAC test cases. Three of these test cases are for nominally zero pressure gradient and the remaining five are for a pressure distribution typical of an aft loaded turbine blade. The model is demonstrated to predict the development of the boundary layer through transition reasonably accurately for all the test cases. The sensitivity of start of transition to the turbulent length scale at low freestream turbulence levels is also demonstrated.

An Investigation of the Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

2008 ◽  
Author(s):  
Andrew R. Gifford ◽  
Thomas E. Diller ◽  
Pavlos P. Vlachos
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Turbulence Intensity ◽  
Physical Mechanism ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Stagnation Region ◽  
Heat Transfer Augmentation

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-Resolved Digital Particle Image Velocimetry (TRDPIV) and a new variety of thin film heat flux sensor called the Heat Flux Array (HFA) are used simultaneously to measure the spatio-temporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation region. Velocity measurements of grid generated freestream turbulence are first performed to quantify the turbulence intensity, integral length scale, and isotropy of the flow. Laminar flow and heat transfer at low levels of freestream turbulence (Tux ≅ 0.5–1.0%) are then examined to provide baseline flow characteristics and heat transfer coefficient. Similar experiments using the turbulence grid are then performed to examine the effects of turbulence with mean turbulence intensity, Tux ≅ 5.5%, and integral length scale, Λx ∼ 3.25 cm. At a mean Reynolds number of ReD = 21,000 an average increase in the mean heat transfer coefficient of 43% above the laminar level was observed. To better understand the mechanism of this augmentation, flow structures in the stagnation region are identified using a coherent structure identification scheme and tracked in time using a customized tracking algorithm. Tracking these structures reveals a complex flow field in the vicinity of the stagnation region. Filaments of vorticity from the freestream are amplified near the plate surface leading to the formation of counterrotating vortex pairs and single sweeping vortex structures. By comparing the transient heat flux measurements with the tracked vortex structures it is clear that heat transfer augmentation is due primarily to amplification of stream-wise vorticity and subsequent vortex formation near the surface. The vortex strength, length scale, and distance from the stagnation plate are key parameters affecting augmentation. Finally, a mechanistic model is examined which captures the physical interaction near the wall. Model results agree well with measured heat transfer augmentation.

Bypass Transition Modelling: A New Method Which Accounts for Free-Stream Turbulence Intensity and Length Scale

2000 ◽  
Author(s):  
Paul E. Roach ◽  
David H. Brierley
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Test Surface ◽  
Length Scale ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence

The publication of the present authors’ boundary layer transition data in 1992 (now widely known as the ERCOFTAC test case T3) has led to a spate of new experimental and modelling efforts aimed at improving our understanding of this problem. This paper describes a new method of determining boundary layer transition with zero mean pressure gradient. The approach examines the development of a laminar boundary layer to the start of transition, accounting for the influences of free-stream turbulence and test surface geometry. It is presented as a “proof of concept”, requiring a significant amount of work before it can be considered as a practically applicable model for transition prediction. The method is based upon one first put forward by G.I. Taylor in the 1930’s, and accounts for the action of local, instantaneous pressure gradients on the developing laminar boundary layer. These pressure gradients are related to the intensity and length scale of turbulence in the free-stream using Taylor’s simple isotropic model. The findings demonstrate the need to account for the separate influences of free-stream turbulence intensity and length scale when considering the transition process. Although the length scale has less of an effect than the intensity, its influence is, nevertheless, significant and must not be overlooked. This fact goes a long way towards explaining the large scatter to be found in simple correlations which involve only the turbulence intensity. Intriguingly, it is demonstrated that it is the free-stream turbulence at the leading edge of the test surface which is important, not that found locally outside the boundary layer. The additional influence of leading edge geometry is also shown to play a major role in fixing the point at which transition begins. It is suggested that the leading edge geometry will distort the incident turbulent eddies, modifying the effective “free-stream” turbulence properties. Consequently, it is shown that the scale of the eddies relative to the leading edge thickness is a further important parameter, and helps bring together a large number of test cases.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low Reynolds Numbers—Part II: Blade-Wake Interaction

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Ali Mahallati ◽  
Steen A. Sjolander
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Surface Boundary ◽  
Freestream Turbulence ◽  
Transition Flow ◽  
Suction Surface ◽  
Profile Losses ◽  
Wake Passing

Part II of this two-part paper presents the aerodynamic behavior of a low-pressure high-lift turbine airfoil, PakB, under the influence of incoming wakes. The periodic unsteady effects of wakes from a single upstream blade-row were measured in a low-speed linear cascade facility at Reynolds numbers of 25,000, 50,000 and 100,000 and at two freestream turbulence intensity levels of 0.4% and 4%. In addition, eight reduced frequencies between 0.53 and 3.2, at three flow coefficients of 0.5, 0.7 and 1.0 were examined. The complex wake-induced transition, flow separation and reattachment on the suction surface boundary layer were determined from an array of closely-spaced surface hot-film sensors. The wake-induced transition caused the separated boundary layer to reattach to the suction surface at all conditions examined. The time-varying profile losses, measured downstream of the cascade, increased with decreasing Reynolds number. The influence of increased freestream turbulence intensity was only evident in between wake-passing events at low reduced frequencies. At higher values of reduced frequency, the losses increased slightly and, for the cases examined here, losses were slightly larger at lower flow coefficients. An optimum wake-passing frequency was observed at which the profile losses were a minimum.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2005 ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed air flow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a Heat Flux Microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time average surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
A. C. Nix ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Stagnation Point ◽  
Turbulence Intensity ◽  
Integral Length Scale ◽  
Length Scale ◽  
Freestream Turbulence ◽  
Hot Wire ◽  
Velocity Signal ◽  
Heat Transfer Augmentation

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed airflow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a heat flux microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time-averaged surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Experimental and Numerical Investigation of Corner Stall in a Highly-Loaded Compressor Cascade

2014 ◽  
Author(s):  
Yanfeng Zhang ◽  
Ali Mahallati ◽  
Michael Benner
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Turbulence Intensity ◽  
Computational Study ◽  
Three Dimensional ◽  
Pressure Probe ◽  
Freestream Turbulence ◽  
Compressor Cascade ◽  
Numerical Predictions ◽  
Highly Loaded

Three-dimensional corner stall is one of the most important factors limiting compressor performance. This paper presents a complementary experimental and computational study of corner stall in a highly-loaded compressor cascade subjected to three inlet boundary layer thicknesses, two levels of freestream turbulence intensity and two Reynolds numbers. Experiments included seven-hole pressure probe traverses, airfoil loading and surface oil flow visualization. Measurements were supplemented with the numerical predictions from a commercially available CFD code. It was found that the low momentum boundary layer on the endwall was unable to overcome the large streamwise adverse pressure gradient in this high-lift profile and turned sharply towards the midspan due to the strong cross-passage pressure gradient. The corner stall, with distinct regions of three-dimensional reversed flow, started at 50% chord and occupied a large area of the suction surface as well as the downstream passage. Only a small region of the inlet boundary layer, very close to the endwall seemed to play a role in the corner stall. As such, the flow in the endwall region was found to be nearly independent of the inlet boundary layer thickness, freestream turbulence intensity and Reynolds number. Based on the endwall flow structures, a new topology of corner stall for compressor cascades with high airfoil diffusion factor and high flow turning has also been proposed.

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