Measurement of the Intermittency Factor in Diffuser-Flow

1971 ◽  
Vol 49 (23) ◽  
pp. 2917-2930 ◽  
Author(s):  
R. S. Azad ◽  
R. H. Hummel
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Time Derivative ◽  
Streamwise Velocity ◽  
Positive Pressure ◽  
Hot Wire ◽  
Diffuser Flow ◽  
Conical Diffuser ◽  
Intermittency Factor ◽  
Channel Analyzer

The intermittency factor in a thick axisymmetric boundary layer with positive pressure gradient (diffuser-flow) was evaluated with hot-wire anemometers and a multi-channel analyzer. The probability density of the signal S(t) (a space–time derivative of the streamwise velocity) is evaluated and then decomposed into three separate curves representing fluids in the turbulent, the transition, and the non-turbulent states respectively. Different values of the intermittency factor γ are calculated by taking different fractions of fluid in the transition state with the turbulent fluid. A model of flow in a conical diffuser is suggested.

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.

scholarly journals Similarity Solutions of the MHD Boundary Layer Flow Past a Constant Wedge within Porous Media

2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Ramesh B. Kudenatti ◽  
Shreenivas R. Kirsur ◽  
Achala L. Nargund ◽  
N. M. Bujurke
Keyword(s):  
Boundary Layer ◽  
Porous Medium ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Velocity Profiles ◽  
Similarity Solutions ◽  
Positive Pressure ◽  
Boundary Layer Equations ◽  
Similarity Transformations ◽  
Wide Range

The two-dimensional magnetohydrodynamic flow of a viscous fluid over a constant wedge immersed in a porous medium is studied. The flow is induced by suction/injection and also by the mainstream flow that is assumed to vary in a power-law manner with coordinate distance along the boundary. The governing nonlinear boundary layer equations have been transformed into a third-order nonlinear Falkner-Skan equation through similarity transformations. This equation has been solved analytically for a wide range of parameters involved in the study. Various results for the dimensionless velocity profiles and skin frictions are discussed for the pressure gradient parameter, Hartmann number, permeability parameter, and suction/injection. A far-field asymptotic solution is also obtained which has revealed oscillatory velocity profiles when the flow has an adverse pressure gradient. The results show that, for the positive pressure gradient and mass transfer parameters, the thickness of the boundary layer becomes thin and the flow is directed entirely towards the wedge surface whereas for negative values the solutions have very different characters. Also it is found that MHD effects on the boundary layer are exactly the same as the porous medium in which both reduce the boundary layer thickness.

The Evolution of Streamwise Counter-Rotating Vortices in Flat Plate Boundary Layer With Leading Edge Pattern

2015 ◽  
Author(s):  
Seyed Mohammad Hasheminejad ◽  
Hatsari Mitsudharmadi ◽  
S. H. Winoto ◽  
Kim Boon Lua ◽  
Hong Tong Low
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Plate Boundary ◽  
Leading Edge ◽  
Streamwise Vortex ◽  
Streamwise Velocity ◽  
Stream Velocity ◽  
Hot Wire ◽  
Layer Flow ◽  
Different Characteristics

The evolution of streamwise counter-rotating vortices induced by different leading edge patterns is investigated quantitatively using hot-wire anemometer. A notched and triangular leading edge with the same wavelength and amplitude were designed to induce streamwise vortices over a flat plate at Reynolds number (based on the wavelength of the leading edge patterns) of 3080 corresponding to free-stream velocity of 3 m/s. The streamwise velocity at different streamwise locations collected and analyzed using a single wire probe hot-wire anemometer showed reveal different characteristics of boundary layer flow due to the presence of these two leading edge patterns. The major difference is the appearance of an additional streamwise vortex between the troughs of the notched pattern. Such vortices increase the mixing effect in the boundary layer as well as the velocity profile.

The reduction and elimination of a closed separation region by free-stream turbulence

2001 ◽  
Vol 446 ◽  
pp. 271-308 ◽  
Author(s):  
M. KALTER ◽  
H. H. FERNHOLZ
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Reverse Flow ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Free Stream Turbulence ◽  
The Mean

This paper is an extension of an experimental investigation by Alving & Fernholz (1996). In the present experiments the effects of free-stream turbulence were investigated on a boundary layer with an adverse pressure gradient and a closed reverse-flow region. By adding free-stream turbulence the mean reverse-flow region was shortened or completely eliminated and this was used to control the size of the separation bubble. The turbulence intensity was varied between 0.2% and 6% using upstream grids while the turbulence length scale was on the order of the boundary layer thickness. Mean and fluctuating velocities as well as spectra were measured by means of hot-wire and laser-Doppler anemometry and wall shear stress by wall pulsed-wire and wall hot-wire probes.Free-stream turbulence had a small effect on the boundary layer in the mild adverse-pressure-gradient region but in the vicinity of separation and along the reverse-flow region mean velocity profiles, skin friction and turbulence structure were strongly affected. Downstream of the mean or instantaneous reverse-flow regions highly disturbed boundary layers developed in a nominally zero pressure gradient and converged to a similar turbulence structure in all three cases at the end of the test section. This state was, however, still very different from that in a canonical boundary layer.

Predicting Entropy Generation Rates in Transitional Boundary Layers Based on Intermittency

2006 ◽  
Author(s):  
Kevin P. Nolan ◽  
Edmond J. Walsh ◽  
Donald M. McEligot ◽  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Transition Region ◽  
Boundary Layers ◽  
Entropy Generation ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Time Averaging ◽  
Laminar And Turbulent Flow ◽  
Intermittency Factor

Prediction of thermodynamic loss in transitional boundary layers is typically based on time averaged data only. This approach effectively ignores the intermittent nature of the transition region. In this work laminar and turbulent conditionally-sampled boundary layer data for zero pressure gradient and accelerating transitional boundary layers have been analyzed to calculate the entropy generation rate in the transition region. By weighting the non-dimensional dissipation coefficient for the laminar conditioned data and turbulent conditioned data with the intermittency factor, the entropy generation rate in the transition region can be determined and compared to the time averaged data and correlations for laminar and turbulent flow. It is demonstrated that this method provides an accurate and detailed picture of the entropy generation rate during transition in contrast with simple time averaging. The data used in this paper have been taken from conditionally-sampled boundary layer measurements available in the literature for favorable pressure gradient flows. Based on these measurements a semi-empirical technique is developed to predict the entropy generation rate in a transitional boundary layer with promising results.

scholarly journals Studies on Wake-Disturbed Boundary Layers Under the Influences of Favorable Pressure Gradient and Free-Stream Turbulence: Part II — Effect of Free-Stream Turbulence

1997 ◽  
Author(s):  
Ken-ichi Funazaki ◽  
Takashi Kitazawa ◽  
Kazuyuki Koizumi ◽  
Tadashi Tanuma
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Transition Model ◽  
Hot Wire ◽  
Time Resolved ◽  
Free Stream Turbulence ◽  
Wire Probe ◽  
Favorable Pressure Gradient ◽  
Probe Measurements

This paper, as Part II of the study on wake-disturbed boundary layer, is aimed at investigation of the effects of free-stream turbulence on wake-induced transition of the boundary layer under a favorable pressure gradient. Hot-wire probe measurements are also made on the wake-disturbed boundary layer to obtain ensemble-averaged shape factor contours on the distance-time diagrams. These data are then used to examine how the favorable pressure gradient and the free-stream turbulence affects time-resolved behaviors of the boundary layer subjected to periodic wakes. In addition, likewise in Part I, the heat transfer data are compared with the transition model proposed by Funazaki (1996) in order to check the capability of the model under the favorable pressure gradient as well as the free-stream turbulence.

On the scaling in sink-flow turbulent boundary layers

2013 ◽  
Vol 737 ◽  
pp. 329-348 ◽  
Author(s):  
Shivsai Ajit Dixit ◽  
O. N. Ramesh
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Physical Space ◽  
Streamwise Velocity ◽  
Turbulent Boundary Layers ◽  
Velocity Spectrum ◽  
Scaling Exponent ◽  
Order Of Magnitude

AbstractScaling of the streamwise velocity spectrum ${\phi }_{11} ({k}_{1} )$ in the so-called sink-flow turbulent boundary layer is investigated in this work. The present experiments show strong evidence for the ${ k}_{1}^{- 1} $ scaling i.e. ${\phi }_{11} ({k}_{1} )= {A}_{1} { U}_{\tau }^{2} { k}_{1}^{- 1} $, where ${k}_{1} $ is the streamwise wavenumber and ${U}_{\tau } $ is the friction velocity. Interestingly, this ${ k}_{1}^{- 1} $ scaling is observed much farther from the wall and at much lower flow Reynolds number (both differing by almost an order of magnitude) than what the expectations from experiments on a zero-pressure-gradient turbulent boundary layer flow would suggest. Furthermore, the coefficient ${A}_{1} $ in the present sink-flow data is seen to be non-universal, i.e. ${A}_{1} $ varies with height from the wall; the scaling exponent −1 remains universal. Logarithmic variation of the so-called longitudinal structure function, which is the physical-space counterpart of spectral ${ k}_{1}^{- 1} $ scaling, is also seen to be non-universal, consistent with the non-universality of ${A}_{1} $. These observations are to be contrasted with the universal value of ${A}_{1} $ (along with the universal scaling exponent of −1) reported in the literature on zero-pressure-gradient turbulent boundary layers. Theoretical arguments based on dimensional analysis indicate that the presence of a streamwise pressure gradient in sink-flow turbulent boundary layers makes the coefficient ${A}_{1} $ non-universal while leaving the scaling exponent −1 unaffected. This effect of the pressure gradient on the streamwise spectra, as discussed in the present study (experiments as well as theory), is consistent with other recent studies in the literature that are focused on the structural aspects of turbulent boundary layer flows in pressure gradients (Harun et al., J. Fluid Mech., vol. 715, 2013, pp. 477–498); the present paper establishes the link between these two. The variability of ${A}_{1} $ accommodated in the present framework serves to clarify the ideas of universality of the ${ k}_{1}^{- 1} $ scaling.

Predicting Entropy Generation Rates in Transitional Boundary Layers Based on Intermittency

2006 ◽  
Vol 129 (3) ◽  
pp. 512-517 ◽  
Author(s):  
Kevin P. Nolan ◽  
Edmond J. Walsh ◽  
Donald M. McEligot ◽  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Transition Region ◽  
Boundary Layers ◽  
Entropy Generation ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Time Averaging ◽  
Laminar And Turbulent Flow ◽  
Intermittency Factor

Prediction of thermodynamic loss in transitional boundary layers is typically based on time-averaged data only. This approach effectively ignores the intermittent nature of the transition region. In this work laminar and turbulent conditionally sampled boundary layer data for zero pressure gradient and accelerating transitional boundary layers have been analyzed to calculate the entropy generation rate in the transition region. By weighting the nondimensional dissipation coefficient for the laminar conditioned data and turbulent conditioned data with the intermittency factor, the entropy generation rate in the transition region can be determined and compared to the time-averaged data and correlations for laminar and turbulent flow. It is demonstrated that this method provides an accurate and detailed picture of the entropy generation rate during transition in contrast with simple time averaging. The data used in this paper have been taken from conditionally sampled boundary layer measurements available in the literature for favorable pressure gradient flows. Based on these measurements, a semi-empirical technique is developed to predict the entropy generation rate in a transitional boundary layer with promising results.

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