Entrainment in a compressible turbulent shear layer

2016 ◽  
Vol 797 ◽  
pp. 564-603 ◽  
Author(s):  
Reza Jahanbakhshi ◽  
Cyrus K. Madnia
Keyword(s):  
Mach Number ◽  
Shear Layer ◽  
Surface Area ◽  
Mass Flux ◽  
Geometrical Shape ◽  
Turbulent Region ◽  
Irrotational Flow ◽  
Entrainment Velocity ◽  
Mass Fluxes ◽  
Convective Mach Number

Direct numerical simulations (DNS) of temporally evolving shear layers have been performed to study the entrainment of irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). Four cases with convective Mach number from 0.2 to 1.8 are used. Entrainment is studied via two mechanisms; nibbling, considered as vorticity diffusion across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. The mass flow rate due to nibbling is calculated as the product of the entrained mass flux with the surface area of the TNTI. It is found that increasing the convective Mach number results in a decrease of the average entrained mass flux and the surface area of the TNTI. For the incompressible shear layer the local entrained mass flux is shown to be highly correlated with the viscous terms. However, as the convective Mach number increases, the mass fluxes due to the baroclinic and the dilatation terms play a more important role in the local entrainment process. It is observed that the entrained mass flux is dependent on the local dilatation and geometrical shape of the TNTI. For a compressible shear layer, most of the entrainment of the irrotational flow into the turbulent region due to nibbling is associated with the compressed regions on the TNTI. As the convective Mach number increases, the percentage of the compressed regions on the TNTI decreases, resulting in a reduction of the average entrained mass flux. It is also shown that the local shape of the interface, looking from the turbulent region, is dominated by concave shaped surfaces with radii of curvature of the order of the Taylor length scale. The average entrained mass flux is found to be larger on highly curved concave shaped surfaces regardless of the level of dilatation. The mass fluxes due to vortex stretching, baroclinic torque and the shear stress/density gradient terms are weak functions of the local curvatures on the TNTI, whereas the mass fluxes due to dilatation and viscous diffusion plus the viscous dissipation terms have a stronger dependency on the local curvatures. As the convective Mach number increases, the probability of finding highly curved concave shaped surfaces on the TNTI decreases, whereas the probability of finding flatter concave and convex shaped surfaces increases. This results in a decrease of the average entrained mass flux across the TNTI. Similar to the previous works on jets, the results show that the contribution of the engulfment to the total entrainment is small for both incompressible and compressible mixing layers. It is also shown that increasing the convective Mach number decreases the velocities associated with the entrainment, i.e. induced velocity, boundary entrainment velocity and local entrainment velocity.

A study of compressibility effects in the high-speed turbulent shear layer using direct simulation

2002 ◽  
Vol 451 ◽  
pp. 329-371 ◽  
Author(s):  
C. PANTANO ◽  
S. SARKAR
Keyword(s):  
Growth Rate ◽  
Shear Stress ◽  
Mach Number ◽  
Shear Layer ◽  
Time Scale ◽  
High Speed ◽  
Turbulent Shear ◽  
Momentum Thickness ◽  
Turbulent Shear Layer ◽  
Convective Mach Number

Direct simulations of the turbulent shear layer are performed for subsonic to supersonic Mach numbers. Fully developed turbulence is achieved with profiles of mean velocity and turbulence intensities that agree well with laboratory experiments. The thickness growth rate of the shear layer exhibits a large reduction with increasing values of the convective Mach number, Mc. In agreement with previous investigations, it is found that the normalized pressure–strain term decreases with increasing Mc, which leads to inhibited energy transfer from the streamwise to cross-stream fluctuations, to the reduced turbulence production observed in DNS, and, finally, to reduced turbulence levels as well as reduced growth rate of the shear layer. An analysis, based on the wave equation for pressure, with supporting DNS is performed with the result that the pressure–strain term decreases monotonically with increasing Mach number. The gradient Mach number, which is the ratio of the acoustic time scale to the flow distortion time scale, is shown explicitly by the analysis to be the key quantity that determines the reduction of the pressure–strain term in compressible shear flows. The physical explanation is that the finite speed of sound in compressible flow introduces a finite time delay in the transmission of pressure signals from one point to an adjacent point and the resultant increase in decorrelation leads to a reduction in the pressure–strain correlation.The dependence of turbulence intensities on the convective Mach number is investigated. It is found that all components decrease with increasing Mc and so does the shear stress.DNS is also used to study the effect of different free-stream densities parameterized by the density ratio, s = ρ2/ρ1, in the high-speed case. It is found that changes in the temporal growth rate of the vorticity thickness are smaller than the changes observed in momentum thickness growth rate. The momentum thickness growth rate decreases substantially with increasing departure from the reference case, s = 1. The peak value of the shear stress, uv, shows only small changes as a function of s. The dividing streamline of the shear layer is observed to move into the low-density stream. An analysis is performed to explain this shift and the consequent reduction in momentum thickness growth rate.

scholarly journals Entrainment at multi-scales across the turbulent/non-turbulent interface in an axisymmetric jet

2016 ◽  
Vol 802 ◽  
pp. 690-725 ◽  
Author(s):  
Dhiren Mistry ◽  
Jimmy Philip ◽  
James R. Dawson ◽  
Ivan Marusic
Keyword(s):  
Surface Area ◽  
Power Law ◽  
Mass Flux ◽  
Time Resolved ◽  
Axisymmetric Jet ◽  
Entrainment Velocity ◽  
Multi Scale ◽  
Planar Laser ◽  
The Mean ◽  
High Reynolds

We consider the scaling of the mass flux and entrainment velocity across the turbulent/non-turbulent interface (TNTI) in the far field of an axisymmetric jet at high Reynolds number. Time-resolved, simultaneous multi-scale particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to identify and track the TNTI, and directly measure the local entrainment velocity along it. Application of box-counting and spatial-filtering methods, with filter sizes $\unicode[STIX]{x1D6E5}$ spanning over two decades in length, show that the mean length of the TNTI exhibits a power-law behaviour with a fractal dimension $D\approx 0.31{-}0.33$. More importantly, we invoke a multi-scale methodology to confirm that the mean mass flux, which is equal to the product of the entrainment velocity and the surface area, remains constant across the range of filter sizes. The results, within experimental uncertainty, also show that the entrainment velocity along the TNTI exhibits a power-law behaviour with $\unicode[STIX]{x1D6E5}$, such that the entrainment velocity increases with increasing $\unicode[STIX]{x1D6E5}$. In fact, the mean entrainment velocity scales at a rate that balances the scaling of the TNTI length such that the mass flux remains independent of the coarse-grain filter size, as first suggested by Meneveau & Sreenivasan (Phys. Rev. A, vol. 41, no. 4, 1990, pp. 2246–2248). Hence, at the smallest scales the entrainment velocity is small but is balanced by the presence of a very large surface area, whilst at the largest scales the entrainment velocity is large but is balanced by a smaller (smoother) surface area.

scholarly journals The compressible turbulent shear layer: an experimental study

1988 ◽  
Vol 197 ◽  
pp. 453-477 ◽  
Author(s):  
Dimitri Papamoschou ◽  
Anatol Roshko
Keyword(s):  
Growth Rate ◽  
Mach Number ◽  
Shear Layer ◽  
Large Scale ◽  
Turbulent Shear ◽  
Plane Shear ◽  
Compressibility Effect ◽  
Pressure Profiles ◽  
Mach Numbers ◽  
Convective Mach Number

The growth rate and turbulent structure of the compressible, plane shear layer are investigated experimentally in a novel facility. In this facility, it is possible to flow similar or dissimilar gases of different densities and to select different Mach numbers for each stream. Ten combinations of gases and Mach numbers are studied in which the free-stream Mach numbers range from 0.2 to 4. Schlieren photography of 20-ns exposure time reveals very low spreading rates and large-scale structures. The growth of the turbulent region is defined by means of Pitot-pressure profiles measured at several streamwise locations. A compressibility-effect parameter is defined that correlates and unifies the experimental results. It is the Mach number in a coordinate system convecting with the velocity of the dominant waves and structures of the shear layer, called here the convective Mach number. It happens to have nearly the same value for each stream. In the current experiments, it ranges from 0 to 1.9. The correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases rapidly with increasing convective Mach number, reaching an asymptotic vaue of about 0.2 for supersonic convective Mach numbers.

Large-scale structure and entrainment in the supersonic mixing layer

1995 ◽  
Vol 284 ◽  
pp. 171-216 ◽  
Author(s):  
N. T. Clemens ◽  
M. G. Mungal
Keyword(s):  
Mach Number ◽  
High Speed ◽  
Large Scale ◽  
Mixture Fraction ◽  
Mixing Layer ◽  
Mie Scattering ◽  
Planar Laser ◽  
The Cross ◽  
Convective Mach Number ◽  
Stream Direction

Experiments were conducted in a two-stream planar mixing layer at convective Mach numbers,Mc, of 0.28, 0.42, 0.50, 0.62 and 0.79. Planar laser Mie scattering (PLMS) from a condensed alcohol fog and planar laser-induced fluorescence (PLIF) of nitric oxide were used for flow visualization in the side, plan and end views. The PLIF signals were also used to characterize the turbulent mixture fraction fluctuations.Visualizations using PLMS indicate a transition in the turbulent structure from quasi-two-dimensionality at low convective Mach number, to more random three-dimensionality for$M_c\geqslant 0.62$. A transition is also observed in the core and braid regions of the spanwise rollers as the convective Mach number increases from 0.28 to 0.62. A change in the entrainment mechanism with increasing compressibility is also indicated by signal intensity profiles and perspective views of the PLMS and PLIF images. These show that atMc= 0.28 the instantaneous mixture fraction field typically exhibits a gradient in the streamwise direction, but is more uniform in the cross-stream direction. AtMc= 0.62 and 0.79, however, the mixture fraction field is more streamwise uniform and with a gradient in the cross-stream direction. This change in the composition of the structures is indicative of different entrainment motions at the different compressibility conditions. The statistical results are consistent with the qualitative observations and suggest that compressibility acts to reduce the magnitude of the mixture fraction fluctuations, particularly on the high-speed edge of the layer.

scholarly journals Significant effect of concentration ratio in synthesizing titania nanoflowers (TNF) powder for various application as additive

2018 ◽  
Vol 14 (3) ◽  
pp. 397-402 ◽  
Author(s):  
Faiz Hafeez Azhar ◽  
Zawati Harun ◽  
Muhamad Zaini Yunos ◽  
Azlinorazia Ahmad ◽  
Siti Hajar Mohd Akhair ◽  
...  
Keyword(s):  
Hydrochloric Acid ◽  
Surface Morphology ◽  
Band Gap ◽  
Surface Area ◽  
Concentration Ratio ◽  
Geometrical Shape ◽  
Facile Hydrothermal Method ◽  
Titanium Butoxide ◽  
Irregular Pattern ◽  
Shape Of Particles

The significant effect of concentration (TBut/HCl) ratio in synthesizing titania nanoflowers (TNF) towards powder morphologies, crystallographic phases, surface area and band gap were investigated. Various synthesized titania nanostructure were prepared via facile hydrothermal method using titanium butoxide (TBut) and hydrochloric acid (HCl) as a mixing composition. The morphologies of synthesizing titania powder was analyzed by using FE-SEM to observe the shape and geometry of the synthesized powder. XRD was used to determine the crystallographic phases of synthesized powder at 2θ angles of 25° to 75°. Each samples were then investigated under BET analyzer to observe the particles surface morphology and UV-Vis analyzer to determine the band gap. The results demonstrated that the concentration of TBut/HCl ratio gave a very significant effect in transforming the mixing solution into geometrical shape of microspheres, nanoflowers and nanorods of titania as increasing the ratio. At TN0.5, the synthesized powder was clearly showed a circle geometrical shape of particles. The shape was suddenly change into round nanoflowers form consist of tiny nanorods at TN1. At TN1.5, the powder morphologies shows the nanoflowers started to form in irregular pattern. As the TBut/HCl ratio increased, the nanoflowers form disappeared and nanorods begin to clumps. In addition, all synthesized powder was in rutile phases guided by XRD peaks and band gap value reported from previous works. The particles surface area was also different for each samples since the geometrical shape of powder was change by increasing the concentration (TBut/HCl) ratio. Thus, concentration ratio of the mixing composition plays a major role in transforming the overall morphologies and structures of hydrothermally titania synthesized particles.

scholarly journals Analysis of Convective Transport and Parameter Sensitivity in a Single Column Version of the Goddard Earth Observation System, Version 5, General Circulation Model

2009 ◽  
Vol 66 (3) ◽  
pp. 627-646 ◽  
Author(s):  
L. E. Ott ◽  
J. Bacmeister ◽  
S. Pawson ◽  
K. Pickering ◽  
G. Stenchikov ◽  
...  
Keyword(s):  
Mass Flux ◽  
General Circulation ◽  
Parameter Sensitivity ◽  
Convective Transport ◽  
Trace Gas ◽  
Gas Mixing ◽  
Mixing Ratios ◽  
Mass Fluxes ◽  
Single Column ◽  
Convective Mass Flux

Abstract Convection strongly influences the distribution of atmospheric trace gases. General circulation models (GCMs) use convective mass fluxes calculated by parameterizations to transport gases, but the results are difficult to compare with trace gas observations because of differences in scale. The high resolution of cloud-resolving models (CRMs) facilitates direct comparison with aircraft observations. Averaged over a sufficient area, CRM results yield a validated product directly comparable to output from a single global model grid column. This study presents comparisons of vertical profiles of convective mass flux and trace gas mixing ratios derived from CRM and single column model (SCM) simulations of storms observed during three field campaigns. In all three cases, SCM simulations underpredicted convective mass flux relative to CRM simulations. As a result, the SCM simulations produced lower trace gas mixing ratios in the upper troposphere in two of the three storms than did the CRM simulations. The impact of parameter sensitivity in the moist physics schemes employed in the SCM has also been examined. Statistical techniques identified the most significant parameters influencing convective transport. Convective mass fluxes are shown to be strongly dependent on chosen parameter values. Results show that altered parameter settings can substantially improve the comparison between SCM and CRM convective mass flux. Upper tropospheric trace gas mixing ratios were also improved in two storms. In the remaining storm, the SCM representation of CO2 was not improved because of differences in entrainment and detrainment levels in the CRM and SCM simulations.

Contributions of Convective and Orographic Gravity Waves to the Brewer–Dobson Circulation Estimated from NCEP CFSR

2020 ◽  
Vol 77 (3) ◽  
pp. 981-1000
Author(s):  
Min-Jee Kang ◽  
Hye-Yeong Chun ◽  
Byeong-Gwon Song
Keyword(s):  
Gravity Waves ◽  
Mass Flux ◽  
Convective Heating ◽  
Flux Divergence ◽  
Mass Fluxes ◽  
Time Period ◽  
Physically Based ◽  
Total Mass Flux ◽  
Convective Gravity Waves

Abstract Contributions of convective gravity waves (CGWs) and orographic gravity waves (OGWs) to the Brewer–Dobson circulation (BDC) are examined and compared to those from resolved waves. OGW drag (OGWD) is provided by NCEP Climate Forecast System Reanalysis (CFSR), while CGW drag (CGWD) is obtained from an offline calculation of a physically based CGW parameterization with convective heating and background data provided by CFSR. CGWD contributes to the shallow branch of the BDC regardless of the season, while OGWD contributes to both the shallow and deep branches except for the summertime, when OGWs hardly propagate into the stratosphere. At 70 hPa, the annual-mean tropical upward mass fluxes from Eliassen–Palm flux divergence (EPD), OGWD, and CGWD are 68%, 7%, and 4% of the total mass flux, respectively. The tropical upward mass flux at 70 hPa shows an increasing trend during the time period from 1979 to 1998, with 28%, 18%, and 6% of the trend driven by EPD, OGWD, and CGWD, respectively. The width of the turnaround latitudes tends to narrow for the streamfunctions induced by OGWD and CGWD but tends to widen for that induced by EPD. The contributions of GWD from MERRA (MERRA-2) to the climatology and long-term trend of the BDC are 7% (7%) and 13% (4%), respectively, somewhat smaller than the contributions of CGWD plus OGWD, which are estimated from CFSR to be 12% and 20%, respectively.

Thermal-Hydraulic Performance of R-134a Boiling at Low Mass Fluxes in a Small Vertical Brazed Plate Heat Exchanger

2017 ◽  
Author(s):  
Hyun Jin Kim ◽  
Leon Liebenberg ◽  
Anthony M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Mass Flux ◽  
Nucleate Boiling ◽  
Plate Heat Exchanger ◽  
Two Phase ◽  
Mass Fluxes ◽  
Low Mass ◽  
Brazed Plate Heat Exchanger ◽  
R 134A

An experimental investigation was performed to study the heat transfer and pressure drop characteristics of refrigerant R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) at low mass flux. The heat transfer coefficient and pressure drop characteristics are analyzed in relation to varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase pressure drop shows a strong dependence on mass flux and significant saturation temperature drop at high mass flux. The two-phase heat transfer coefficient was both strongly dependent on heat flux (at vapor qualities below 0.4) and on mass flux (at vapor qualities above 0.4). There was also apparent dryout, as depicted by decreased heat transfer at high vapor qualities. These observations suggest that both nucleate and convective boiling mechanisms prevailed. Existing transition correlations however suggest that the experimental data is rather convection-dominant and not a mix of convection and nucleate boiling. The experimental data further strongly suggest the prevalence of both macrochannel and minichannel type flows. Several acknowledged semi-empirical transition criteria were employed to verify our observations. These criteria mostly support our observations that R-134a evaporating at low mass fluxes in a BPHE with a hydraulic diameter of 3.4 mm, has heat transfer and pressure drop characteristics typically indicative of macrochannel as well as minichannel flows. Disagreement however exists with accepted correlations regarding the prevalence of convective or nucleate boiling.

Interaction Between a Subsonic Air Jet and a Sharp Edge

1984 ◽  
Vol 8 (3) ◽  
pp. 126-132
Author(s):  
N.W.M. Ko
Keyword(s):  
Experimental Investigation ◽  
Mach Number ◽  
Shear Layer ◽  
Coherent Structure ◽  
Wake Flow ◽  
Sharp Edge ◽  
Spectral Measurements ◽  
Acoustic Feedback ◽  
Air Jet ◽  
Set Up

This paper describes an experimental investigation of a jet of Mach number 0.5 which is partially interrupted by an 180° sharp edge. Detailed Schlieren and pressure spectral measurements of the jet with the sharp edge located at different locations inside the jet have indicated the presence of the basic jet coherent structure, the axisymmetrical and azimuthal constituents and the resonances set up by the interaction of the jet flow and sharp edge. The resonances arc due not only to the interaction of the initial shear layer with the acoustic feedback from the basic coherent structure but also with the acoustic feedback from the wake vortices set up in the wake flow behind the sharp edge. For the former, dependence of the level of resonance on location of the sharp edge has also been found.

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