An Improved k-ε Model for Boundary Layer Flows

1990 ◽  
Vol 112 (1) ◽  
pp. 33-39 ◽  
Author(s):  
Y. Nagano ◽  
M. Tagawa
Keyword(s):  
Boundary Layer ◽  
Plate Boundary ◽  
Adverse Pressure Gradient ◽  
Turbulent Pipe Flow ◽  
Pressure Gradients ◽  
Boundary Layer Flows ◽  
Limiting Behavior ◽  
Diffuser Flow ◽  
Proposed Model ◽  
Made In

An improvement of the k-ε model has been made in conjunction with an accurate prediction of the near-wall limiting behavior of turbulence and the final period of the decay law of free turbulence. The present improved k-ε model has been extended to predict the effects of adverse pressure gradients on shear layers, which most previously proposed models failed to do correctly. The proposed model was tested by application to a turbulent pipe flow, a flat plate boundary layer, a relaminarizing flow, and a diffuser flow with a strong adverse pressure gradient. Agreement with the experiments was generally very satisfactory.

scholarly journals Effects of Periodic Wake Passing Upon Flat-Plate Boundary Layers Experiencing Favorable and Adverse Pressure Gradient

1998 ◽  
Author(s):  
Ken-ichi Funazaki ◽  
Eitaro Koyabu
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Plate Boundary ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Test Surface ◽  
Test Model ◽  
Pressure Gradients ◽  
Ensemble Averaging ◽  
Flow Acceleration

This paper deals with the investigation of wake-disturbed boundary layer on a flat-plate model with an elliptic leading edge. The wakes are generated by the transversely moving bars in front of the test model. Main focus of this paper is on how the wake passage affects the transitional behavior of the boundary layer under the influence of favorable and adverse pressure gradients over the test surface. Detailed measurements of the boundary layer are conducted by use of a hot-wire anemometry. An ensemble-averaging technique is also employed in order to extract the periodic events associated with the wake passage from the acquired data. The previously observed dependence of wake-induced transition on the movement of the wake generating bar is confirmed. It is also found that the wake passage induces a significant change in the flow structure downstream of the flow acceleration region.

Boundary Layer Flashback Model for Hydrogen Flames in Confined Geometries Including the Effect of Adverse Pressure Gradient

2020 ◽  
Author(s):  
Ólafur H. Björnsson ◽  
Sikke A. Klein ◽  
Joeri Tober
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flow Separation ◽  
Gradient Flow ◽  
Lewis Number ◽  
Adverse Pressure Gradient ◽  
Future Research ◽  
Diffuser Flow ◽  
Combustion Properties ◽  
Quenching Distance

Abstract The combustion properties of hydrogen make premixed hydrogen-air flames very prone to boundary layer flashback. This paper describes the improvement and extension of a boundary layer flashback model from Hoferichter [1] for flames confined in burner ducts. The original model did not perform well at higher preheat temperatures and overpredicted the backpressure of the flame at flashback by 4–5x. By simplifying the Lewis number dependent flame speed computation and by applying a generalized version of Stratford’s flow separation criterion [2], the prediction accuracy is improved significantly. The effect of adverse pressure gradient flow on the flashback limits in 2° and 4° diffusers is also captured adequately by coupling the model to flow simulations and taking into account the increased flow separation tendency in diffuser flow. Future research will focus on further experimental validation and direct numerical simulations to gain better insight into the role of the quenching distance and turbulence statistics.

Effects of distributed pressure gradients on the pressure–strain correlations in a supersonic nozzle and diffuser

2014 ◽  
Vol 742 ◽  
pp. 466-494 ◽  
Author(s):  
Somnath Ghosh ◽  
Rainer Friedrich
Keyword(s):  
Supersonic Nozzle ◽  
Turbulent Pipe Flow ◽  
Pressure Gradients ◽  
Reynolds Stresses ◽  
Streamline Curvature ◽  
Flow Parameters ◽  
Diffuser Flow ◽  
Remarkable Agreement ◽  
Distributed Pressure

AbstractDirect numerical simulation (DNS), based on high-order numerical schemes, is used to study the effects of distributed pressure gradients on the redistribution of fluctuating kinetic energy in supersonic nozzle and diffuser flow with incoming fully developed turbulent pipe flow. Axisymmetric geometries and flow parameters have been selected such that shock waves are avoided and streamline curvature remains unimportant. Although mean extra rates of strain are quite small, strong changes in Reynolds stresses and their production/redistribution mechanisms are observed, in agreement with findings of Bradshaw (J. Fluid Mech., vol. 63, 1974, pp. 449–464). The central role of pressure–strain correlations in changing the Reynolds stress anisotropy is highlighted. A Green’s function-based analysis of pressure–strain correlations is presented, showing remarkable agreement with DNS data.

Predicting Turbulent Boundary Layer Wall Pressure Fluctuations in Adverse Pressure Gradients

2005 ◽  
Author(s):  
Frank J. Aldrich
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Fluctuations ◽  
Adverse Pressure Gradient ◽  
Wall Pressure ◽  
Prediction Tool ◽  
Pressure Gradients ◽  
Wall Pressure Fluctuations ◽  
The Difference

A physics-based approach is employed and a new prediction tool is developed to predict the wavevector-frequency spectrum of the turbulent boundary layer wall pressure fluctuations for subsonic airfoils under the influence of adverse pressure gradients. The prediction tool uses an explicit relationship developed by D. M. Chase, which is based on a fit to zero pressure gradient data. The tool takes into account the boundary layer edge velocity distribution and geometry of the airfoil, including the blade chord and thickness. Comparison to experimental adverse pressure gradient data shows a need for an update to the modeling constants of the Chase model. To optimize the correlation between the predicted turbulent boundary layer wall pressure spectrum and the experimental data, an optimization code (iSIGHT) is employed. This optimization module is used to minimize the absolute value of the difference (in dB) between the predicted values and those measured across the analysis frequency range. An optimized set of modeling constants is derived that provides reasonable agreement with the measurements.

The Effect of Real Turbine Roughness With Pressure Gradient on Heat Transfer

2003 ◽  
Author(s):  
Jeffrey P. Bons ◽  
Stephen T. McClain
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Combined Effects ◽  
Pressure Gradients ◽  
Smooth Wall ◽  
Reynolds Analogy ◽  
Integral Boundary ◽  
Favorable Pressure Gradient

Experimental measurements of heat transfer (St) are reported for low speed flow over scaled turbine roughness models at three different freestream pressure gradients: adverse, zero (nominally), and favorable. The roughness models were scaled from surface measurements taken on actual, in-service land-based turbine hardware and include samples of fuel deposits, TBC spallation, erosion, and pitting as well as a smooth control surface. All St measurements were made in a developing turbulent boundary layer at the same value of Reynolds number (Rex≅900,000). An integral boundary layer method used to estimate cf for the smooth wall cases allowed the calculation of the Reynolds analogy (2St/cf). Results indicate that for a smooth wall, Reynolds analogy varies appreciably with pressure gradient. Smooth surface heat transfer is considerably less sensitive to pressure gradients than skin friction. For the rough surfaces with adverse pressure gradient, St is less sensitive to roughness than with zero or favorable pressure gradient. Roughness-induced Stanton number increases at zero pressure gradient range from 16–44% (depending on roughness type), while increases with adverse pressure gradient are 7% less on average for the same roughness type. Hot-wire measurements show a corresponding drop in roughness-induced momentum deficit and streamwise turbulent kinetic energy generation in the adverse pressure gradient boundary layer compared with the other pressure gradient conditions. The combined effects of roughness and pressure gradient are different than their individual effects added together. Specifically, for adverse pressure gradient the combined effect on heat transfer is 9% less than that estimated by adding their separate effects. For favorable pressure gradient, the additive estimate is 6% lower than the result with combined effects. Identical measurements on a “simulated” roughness surface composed of cones in an ordered array show a behavior unlike that of the scaled “real” roughness models. St calculations made using a discrete-element roughness model show promising agreement with the experimental data. Predictions and data combine to underline the importance of accounting for pressure gradient and surface roughness effects simultaneously rather than independently for accurate performance calculations in turbines.

The Isobars in Boundary Layers at Supersonic Speeds

1968 ◽  
Vol 19 (2) ◽  
pp. 105-126 ◽  
Author(s):  
D. F. Myring ◽  
A. D. Young
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Equations Of Motion ◽  
Normal Pressure ◽  
Simple Extension ◽  
Pressure Gradients ◽  
Boundary Layer Flows ◽  
Hyperbolic Flow ◽  
The Individual ◽  
Momentum Integral

SummaryFor boundary layer flows over curved surfaces at moderately high supersonic speeds the existence of normal pressure gradients within the boundary layer becomes important even for small curvatures and they cannot be ignored. The describing equations are basically parabolic in form so that the simplifications inherent in hyperbolic flows would not at first sight seem to be relevant. However, the equations of motion for a two-dimensional, supersonic, rotational, viscous flow are analysed along the lines of a hyperbolic flow and the individual effects of viscosity and vorticity are examined with regard to the isobar distributions. It is found that these two properties have compensating effects and the experimental evidence presented confirms the conclusion that inside the boundary layer the isobars follow much the same rules as those which determine the isobars in the external hyperbolic flow. Since for turbulent boundary layers the fullness of the Mach number profile produces almost linear Mach lines in the boundary layer, this provides a simple extension to the methods of analysis, and the momentum integral equation is reformulated using a swept element bounded by linear isobars. The final equation is similar in form to the conventional one except that the momentum and displacement thicknesses are now defined by integrals along the swept isobars, and all normal pressure gradients due to centrifugal effects are accounted for.

scholarly journals Investigation of the Calmed Region Behind a Turbulent Spot

1996 ◽  
Author(s):  
J. P. Gostelow ◽  
G. J. Walker ◽  
W. J. Solomon ◽  
G. Hong ◽  
N. Melwani
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Axial Flow ◽  
Adverse Pressure Gradient ◽  
Turbulent Spot ◽  
Boundary Layer Flows ◽  
Turbulent Spots ◽  
Turbulent Layer ◽  
Controlled Diffusion ◽  
Flow Compressor

Measurements are presented of the calmed region behind triggered wave packets and turbulent spots under a controlled diffusion adverse pressure gradient in a wind tunnel. Similar measurements are also presented from the stator blades of an axial flow compressor, where turbulent spots are induced by the passing of rotor wakes. The purpose is to gain an appreciation of turbulent spot behavior under a strong adverse pressure gradient as a foundation for the more accurate modeling of spots and their environment in predictions of transitional boundary layer flows. Under an adverse pressure gradient the calmed region behind the spot is extensive; its interaction with the surrounding turbulent layer is complex and is dependent on whether the surrounding natural boundary layer is laminar or turbulent. Some insights are gleaned concerning the behavior of the calmed region which will subsequently be used in attempts to model the calmed region. Although these fundamental investigations of the calmed region have been extensive much remains to be understood.

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