scholarly journals Sound propagation in a lined nozzle with shear and bias flows

2018 ◽  
Vol 18 (1) ◽  
pp. 3-48
Author(s):  
LMBC Campos ◽  
C Legendre
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Boundary Layers ◽  
Lower Boundary ◽  
Cross Flow ◽  
Core Flow ◽  
The Core ◽  
Wide Range ◽  
Bias Flow ◽  
Number Formula

In this study, the propagation of waves in a two-dimensional parallel-sided nozzle is considered allowing for the combination of: (a) distinct impedances of the upper and lower walls; (b) upper and lower boundary layers with different thicknesses with linear shear velocity profiles matched to a uniform core flow; and (c) a uniform cross-flow as a bias flow out of one and into the other porous acoustic liner. The model involves an “acoustic triple deck” consisting of third-order non-sinusoidal non-plane acoustic-shear waves in the upper and lower boundary layers coupled to convected plane sinusoidal acoustic waves in the uniform core flow. The acoustic modes are determined from a dispersion relation corresponding to the vanishing of an 8 × 8 matrix determinant, and the waveforms are combinations of two acoustic and two sets of three acoustic-shear waves. The eigenvalues are calculated and the waveforms are plotted for a wide range of values of the four parameters of the problem, namely: (i/ii) the core and bias flow Mach numbers; (iii) the impedances at the two walls; and (iv) the thicknesses of the two boundary layers relative to each other and the core flow. It is shown that all three main physical phenomena considered in this model can have a significant effect on the wave field: (c) a bias or cross-flow even with small Mach number [Formula: see text] relative to the mean flow Mach number [Formula: see text] can modify the waveforms; (b) the possibly dissimilar impedances of the lined walls can absorb (or amplify) waves more or less depending on the reactance and inductance; (a) the exchange of the wave energy with the shear flow is also important, since for the same stream velocity, a thin boundary layer has higher vorticity, and lower vorticity corresponds to a thicker boundary layer. The combination of all these three effects (a–c) leads to a large set of different waveforms in the duct that are plotted for a wide range of the parameters (i–iv) of the problem.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

Buoyancy effects on large-scale motions in convective atmospheric boundary layers: implications for modulation of near-wall processes

2018 ◽  
Vol 856 ◽  
pp. 135-168 ◽  
Author(s):  
S. T. Salesky ◽  
W. Anderson
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Boundary Layers ◽  
Vertical Velocity ◽  
Amplitude Modulation ◽  
Large Scale ◽  
Streamwise Velocity ◽  
Small Scale ◽  
Convective Conditions ◽  
Wide Range

A number of recent studies have demonstrated the existence of so-called large- and very-large-scale motions (LSM, VLSM) that occur in the logarithmic region of inertia-dominated wall-bounded turbulent flows. These regions exhibit significant streamwise coherence, and have been shown to modulate the amplitude and frequency of small-scale inner-layer fluctuations in smooth-wall turbulent boundary layers. In contrast, the extent to which analogous modulation occurs in inertia-dominated flows subjected to convective thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number), has not been considered. And yet, these parameter values encompass a wide range of important environmental flows. In this article, we present evidence of amplitude modulation (AM) phenomena in the unstably stratified (i.e. convective) atmospheric boundary layer, and link changes in AM to changes in the topology of coherent structures with increasing instability. We perform a suite of large eddy simulations spanning weakly ($-z_{i}/L=3.1$) to highly convective ($-z_{i}/L=1082$) conditions (where$-z_{i}/L$is the bulk stability parameter formed from the boundary-layer depth$z_{i}$and the Obukhov length $L$) to investigate how AM is affected by buoyancy. Results demonstrate that as unstable stratification increases, the inclination angle of surface layer structures (as determined from the two-point correlation of streamwise velocity) increases from$\unicode[STIX]{x1D6FE}\approx 15^{\circ }$for weakly convective conditions to nearly vertical for highly convective conditions. As$-z_{i}/L$increases, LSMs in the streamwise velocity field transition from long, linear updrafts (or horizontal convective rolls) to open cellular patterns, analogous to turbulent Rayleigh–Bénard convection. These changes in the instantaneous velocity field are accompanied by a shift in the outer peak in the streamwise and vertical velocity spectra to smaller dimensionless wavelengths until the energy is concentrated at a single peak. The decoupling procedure proposed by Mathiset al.(J. Fluid Mech., vol. 628, 2009a, pp. 311–337) is used to investigate the extent to which amplitude modulation of small-scale turbulence occurs due to large-scale streamwise and vertical velocity fluctuations. As the spatial attributes of flow structures change from streamwise to vertically dominated, modulation by the large-scale streamwise velocity decreases monotonically. However, the modulating influence of the large-scale vertical velocity remains significant across the stability range considered. We report, finally, that amplitude modulation correlations are insensitive to the computational mesh resolution for flows forced by shear, buoyancy and Coriolis accelerations.

Boiling Heat Transfer From an Oscillated Water Column Through a Porous Domain: A Simplified Thermodynamic Analysis

2016 ◽  
Author(s):  
Ersin Sayar
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Water Column ◽  
Thermodynamic Analysis ◽  
Bubbly Flow ◽  
Pool Boiling ◽  
Oscillating Flow ◽  
Transition Regime ◽  
Core Flow ◽  
The Core

Heat transfer in an oscillating water column in the transition regime of pool boiling to bubbly flow is investigated experimentally and theoretically. Forced oscillations are applied to water via a frequency controlled dc motor and a piston-cylinder device. Heat transfer is from the electrically heated inner surface to the reciprocating flow. The heat transfer in the oscillating fluid column is altered by using stainless steel scrap metal layers (made off open-cell discrete cells) which produces a porous medium within the system. The effective heat transfer mechanism is enhanced and it is due to the hydrodynamic mixing of the boundary layer and the core flow. In oscillating flow, the hydrodynamic lag between the core flow and the boundary layer flow is somehow significant. At low actuation frequencies and at low heat fluxes, heat transfer is restricted in the single phase flows. The transition regime of pool boiling to bubbly flow is proposed to be a remedy to the stated limitation. The contribution by the pool boiling on heat transfer appears to be the dominant mechanism for the selected low oscillation amplitudes and frequencies. Accordingly the regime is a transition from pool boiling to bubbly flow. Nucleate-bubbly flow boiling in oscillating flow is also investigated using a simplified thermodynamic analysis. According to the experimental results, bubbles induce highly efficient heat transfer mechanisms. Experimental study proved that the heater surface temperature is the dominant parameter affecting heat transfer. At greater actuation frequencies saturated nucleate pool boiling ceases to exist. Actuation frequency becomes important in that circumstances. The present investigation has possible applications in moderate sized wicked heat pipes, boilers, compact heat exchangers and steam generators.

Effects of the Axisymmetric Contraction Shape on Incompressible Turbulent Flow

1976 ◽  
Vol 98 (1) ◽  
pp. 58-68 ◽  
Author(s):  
A. K. M. F. Hussain ◽  
V. Ramjee
Keyword(s):  
Boundary Layer ◽  
Turbulence Structure ◽  
Exit Plane ◽  
Core Flow ◽  
The Core ◽  
Local Contraction ◽  
Contraction Ratio ◽  
Exit Boundary ◽  
The Mean ◽  
Equal Effectiveness

The performance characteristics of four different axisymmetric contraction shapes with the same contraction ratio are experimentally investigated for incompressible flow. The pre- and postcontraction mean and turbulent velocity profiles and spectra, and the variation of the mean and turbulent velocities along the axis as a function of local contraction ratio and axial length are presented in this paper. The results show that all the nozzles are of essentially equal effectiveness as far as the core flow in the exit plane is concerned. But the mean and turbulence characteristics of the exit boundary layer, the upstream influence of the contraction, and the departure from equipartition within the nozzle vary significantly with the contraction shape. The data demonstrate the inadequacy of the Batchelor-Proudman-Ribner-Tucker theory in predicting the effect of a contraction on the turbulence structure. These data are of interest in wind tunnel and nozzle design, and in boundary layer prediction.

scholarly journals Self-similar compressible turbulent boundary layers with pressure gradients. Part 1. Direct numerical simulation and assessment of Morkovin’s hypothesis

2019 ◽  
Vol 880 ◽  
pp. 239-283 ◽  
Author(s):  
Christoph Wenzel ◽  
Tobias Gibis ◽  
Markus Kloker ◽  
Ulrich Rist
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Mach Number ◽  
Direct Numerical Simulation ◽  
Boundary Layers ◽  
Turbulent Boundary Layers ◽  
Shear Stresses ◽  
Pressure Gradients ◽  
Mean Flow ◽  
Self Similar

A direct numerical simulation study of self-similar compressible flat-plate turbulent boundary layers (TBLs) with pressure gradients (PGs) has been performed for inflow Mach numbers of 0.5 and 2.0. All cases are computed with smooth PGs for both favourable and adverse PG distributions (FPG, APG) and thus are akin to experiments using a reflected-wave set-up. The equilibrium character allows for a systematic comparison between sub- and supersonic cases, enabling the isolation of pure PG effects from Mach-number effects and thus an investigation of the validity of common compressibility transformations for compressible PG TBLs. It turned out that the kinematic Rotta–Clauser parameter $\unicode[STIX]{x1D6FD}_{K}$ calculated using the incompressible form of the boundary-layer displacement thickness as length scale is the appropriate similarity parameter to compare both sub- and supersonic cases. Whereas the subsonic APG cases show trends known from incompressible flow, the interpretation of the supersonic PG cases is intricate. Both sub- and supersonic regions exist in the boundary layer, which counteract in their spatial evolution. The boundary-layer thickness $\unicode[STIX]{x1D6FF}_{99}$ and the skin-friction coefficient $c_{f}$, for instance, are therefore in a comparable range for all compressible APG cases. The evaluation of local non-dimensionalized total and turbulent shear stresses shows an almost identical behaviour for both sub- and supersonic cases characterized by similar $\unicode[STIX]{x1D6FD}_{K}$, which indicates the (approximate) validity of Morkovin’s scaling/hypothesis also for compressible PG TBLs. Likewise, the local non-dimensionalized distributions of the mean-flow pressure and the pressure fluctuations are virtually invariant to the local Mach number for same $\unicode[STIX]{x1D6FD}_{K}$-cases. In the inner layer, the van Driest transformation collapses compressible mean-flow data of the streamwise velocity component well into their nearly incompressible counterparts with the same $\unicode[STIX]{x1D6FD}_{K}$. However, noticeable differences can be observed in the wake region of the velocity profiles, depending on the strength of the PG. For both sub- and supersonic cases the recovery factor was found to be significantly decreased by APGs and increased by FPGs, but also to remain virtually constant in regions of approximated equilibrium.

An Approach to Three-Dimensional Turbulent Boundary Layers

1966 ◽  
Author(s):  
A. D. Carmichael
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Shape Factor ◽  
Basic Assumption ◽  
Three Dimensional ◽  
Cross Flow ◽  
Turbulent Boundary Layers ◽  
Factor H ◽  
Simple Method ◽  
The Cross

A relatively simple method for predicting some of the characteristics of three-dimensional turbulent boundary layers is presented. The basic assumption of the method is that the cross-flow is small. An empirical correlation of a basic shape factor of the cross-flow boundary layer against the streamwise shape factor H is provided. This correlation, together with data for the streamwise boundary layer, is used to predict the cross flow. The solution is very sensitive to the accuracy of the streamwise boundary-layer data which is predicted by conventional two-dimensional methods.

scholarly journals Blockage Development in a Transonic, Axial Compressor Rotor

1997 ◽  
Author(s):  
Kenneth L. Suder
Keyword(s):  
Boundary Layer ◽  
Axial Compressor ◽  
Flow Region ◽  
Tip Clearance ◽  
Surface Boundary ◽  
Core Flow ◽  
The Core ◽  
Compressor Rotor ◽  
Transonic Axial Compressor ◽  
The Impact

A detailed experimental investigation to understand and quantify the development of blockage in the flow field of a transonic, axial flow compressor rotor (NASA Rotor 37) has been undertaken. Detailed laser anemometer measurements were acquired upstream, within, and downstream of a transonic, axial compressor rotor operating at 100%, 85%, 80%, and 60% of design speed which provided inlet relative Mach numbers at the blade tip of 1.48, 1.26, 1.18, and 0.89 respectively. The impact of the shock on the blockage development, pertaining to both the shock / boundary layer interactions and the shock / tip clearance flow interactions, is discussed. The results indicate that for this rotor the blockage in the endwall region is 2–3 times that of the core flow region, and the blockage in the core flow region more than doubles when the shock strength is sufficient to separate the suction surface boundary layer.

Compressibility and Variable Density Effects in Turbulent Boundary layers

2006 ◽  
Vol 129 (4) ◽  
pp. 441-448 ◽  
Author(s):  
Kunlun Liu ◽  
Richard H. Pletcher
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Turbulent Boundary Layer ◽  
Boundary Layers ◽  
Wall Temperature ◽  
Stokes Equations ◽  
Variable Density ◽  
Turbulent Boundary Layers ◽  
Reynolds Analogy ◽  
Density Effects

Two compressible turbulent boundary layers have been calculated by using direct numerical simulation. One case is a subsonic turbulent boundary layer with constant wall temperature for which the wall temperature is 1.58 times the freestream temperature and the other is a supersonic adiabatic turbulent boundary layer subjected to a supersonic freestream with a Mach number 1.8. The purpose of this study is to test the strong Reynolds analogy (SRA), the Van Driest transformation, and the applicability of Morkovin’s hypothesis. For the first case, the influence of the variable density effects will be addressed. For the second case, the role of the density fluctuations, the turbulent Mach number, and dilatation on the compressibility will be investigated. The results show that the Van Driest transformation and the SRA are satisfied for both of the flows. Use of local properties enable the statistical curves to collapse toward the corresponding incompressible curves. These facts reveal that both the compressibility and variable density effects satisfy the similarity laws. A study about the differences between the compressibility effects and the variable density effects associated with heat transfer is performed. In addition, the difference between the Favre average and Reynolds average is measured, and the SGS terms of the Favre-filtered Navier-Stokes equations are calculated and analyzed.

Boundary Layers over Infinite Yawed Wings

1960 ◽  
Vol 11 (4) ◽  
pp. 333-347 ◽  
Author(s):  
J. C. Cooke
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Three Dimensional ◽  
Cross Flow ◽  
Turbulent Boundary Layers ◽  
Free Flow ◽  
Momentum Thickness ◽  
Displacement Thickness

SummaryA method of calculating turbulent boundary layers on infinite yawed wings is given, making use of a method of calculating turbulent boundary layers due to Spence and of an analogy between three-dimensional and axi-symmetric boundary layers. It is also shown that the displacement thickness is equal to that computed using chordwise components and that the streamwise momentum thickness is approximately equal to the chordwise momentum thickness. Shock-free flow and small boundary layer cross-flow are assumed.

Strongly nonlinear interfacial dynamics in core–annular flows

1995 ◽  
Vol 290 ◽  
pp. 131-166 ◽  
Author(s):  
V. Kerchman
Keyword(s):  
Surface Tension ◽  
Blow Up ◽  
High Surface ◽  
Spatiotemporal Chaos ◽  
Interfacial Dynamics ◽  
Core Flow ◽  
The Core ◽  
Wide Range ◽  
Lens Formation ◽  
Annular Film

Nonlinear stability of a pressure driven core-annular flow is analysed, and a study of the large-amplitude interfacial dynamics is reported in the limit of a small ratio β of the annular clearance to the radius. An asymptotic nonlinear evolution equation for the annular film thickness is derived as a general case which involves shear coupling with the core flow. We discuss the effects of the surface tension parameter and viscosity stratification of various orders in β. The governing equation is investigated by solving it on extended intervals. Long-term simulations in a wide range of parameters reveal rich dynamics of wave patterns and coherent structures. Only in a narrow window of the small control parameters can it be described by the weakly nonlinear dissipative-dispersive equation, exhibiting behaviour of strictly bounded solutions which varies from a spatiotemporal chaos to the quasi-steady wavetrains. For sufficiently high surface tension, some pulses (to which the primary instabilities saturate) can coalesce into stable larger structures. This leads to the formation of solitary humps via cascade absorption. Substantial thickness non-uniformities can cause collapse of the perfect CAFF owing to the lens formation or extreme film thinning. Our critical value of the control parameter is in good agreement with the experimental data by Aul & Olbricht. Under strong coupling of the core flow with a less viscous annular film the interfacial evolution settles to a train of inverted pulses. Long-time behaviour in the intermediate range of parameters is diversified from regular pulse trains, to the formation of wide multi-peak structures or blow-up, depending on the apparent involvement of the core.

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