Turbulence Measurements Over a Heated Two-Dimensional Hill

2007 ◽  
Author(s):  
T. Houra ◽  
M. Tagawa ◽  
Y. Nagano
Keyword(s):  
Reverse Flow ◽  
Reynolds Shear Stress ◽  
Low Frequency ◽  
Flow Region ◽  
Outer Edge ◽  
Heated Wall ◽  
Leeward Side ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Turbulent Characteristics

Experimental investigation has been made on the flow and thermal fields over a heated two-dimensional hill with a cosine-squared shape. The detailed turbulent characteristics were measured by a backscatter-type two-component LDV, a PIV system, a fine thermocouple and a cold-wire probe. In the reverse-flow region on the leeward side of the hill, the turbulence intensities and the Reynolds shear stress show much larger values than that in a canonical wall-bounded shear flow. The mean temperature maintains a relatively high value below the location where the horizontal mean velocity rapidly decreases. At the outer edge of the reverse-flow region, there exists a second maximum intensity of temperature fluctuations. The instantaneous temperature waveforms near the heated wall show very large amplitude consisting of high-frequency fluctuations superimposed on the low-frequency motions.

Three-Dimensional Forced Convection in Plane Symmetric Sudden Expansion

2004 ◽  
Vol 126 (5) ◽  
pp. 836-839 ◽  
Author(s):  
J. H. Nie and ◽  
B. F. Armaly
Keyword(s):  
Nusselt Number ◽  
Forced Convection ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Flow Region ◽  
Outer Edge ◽  
Sudden Expansion ◽  
Recirculation Flow ◽  
Plane Symmetric ◽  
Laminar Forced Convection

Simulations of three-dimensional laminar forced convection in a plane symmetric sudden expansion are presented for Reynolds numbers where the flow is steady and symmetric. A swirling “jetlike” flow develops near the sidewalls in the separating shear layer, and its impingement on the stepped wall is responsible for the maximum that develops in the Nusselt number adjacent to the sidewalls and for the reverse flow that develops in that region. The maximum Nusselt number on the stepped wall is located inside the primary recirculation flow region and its location does not coincide with the jetlike flow impingement region. The results reveal that the location where the streamwise component of wall shear stress is zero on the stepped walls does not coincide with the outer edge of the primary recirculation flow region near the sidewalls.

Vortex Detachment and Reverse Flow in Pulsatile Laminar Flow Through Axisymmetric Sudden Expansions

1999 ◽  
Vol 121 (3) ◽  
pp. 574-579 ◽  
Author(s):  
S. Tavoularis ◽  
R. K. Singh
Keyword(s):  
Reynolds Number ◽  
Recirculation Zone ◽  
Reverse Flow ◽  
Flow Region ◽  
Vortex Rings ◽  
Mean Velocity ◽  
Zone Length ◽  
Deceleration Phase ◽  
Pulsatile Flows ◽  
Flow Through

Incompressible, steady and pulsatile flows in axisymmetric sudden expansions with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 ≤ Re ≤ 400 and Womersley number 0.1 ≤ W ≤ 50. For steady flow, the calculated recirculation zone length increased linearly with an increase in Re, in good agreement with earlier experiments. For pulsatile flows, particularly at higher values of W, the recirculation zone length correlated strongly with the acceleration of the flow and not with the instantaneous Reynolds number; it increased during the deceleration phase and decreased during the acceleration phase. The computed mean velocity and reattachment length were in general agreement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the vortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstream; at relatively high W, these vortices became, once more, attached.

Effects of Reynolds Number on Turbulent Characteristics of Surface Jet

2016 ◽  
Author(s):  
M. S. Rahman ◽  
M. F. Tachie
Keyword(s):  
Reynolds Number ◽  
Reynolds Shear Stress ◽  
Maximum Velocity ◽  
Reynolds Numbers ◽  
Mean Velocity ◽  
Potential Core ◽  
Turbulent Characteristics ◽  
Reynolds Number Effects ◽  
Surface Jet ◽  
Jet Characteristics

Experimental study was carried out to investigate the Reynolds number effects on surface jet characteristics. The surface jet was produced using orifice nozzle with offset height ratio of 2. Six different Reynolds numbers ranging from 2300 to 11900 were investigated. Potential core region of the jet decreased with Reynolds number up to the Reynolds number of 5500. Reattachment point was sensitive to Reynolds number within the range of the present study. The maximum velocity decay and jet spread were nearly independent of Reynolds number. The streamwise mean velocity, streamwise turbulence intensity and Reynolds shear stress distribution along surface-normal direction were affected by the free surface and showed Reynolds number independency at the Reynolds numbers beyond 5500.

Characteristics of Twin Jets in the Vicinity of a Free Surface

2016 ◽  
Author(s):  
M. S. Rahman ◽  
E. M. Nabess ◽  
M. F. Tachie
Keyword(s):  
Free Surface ◽  
Reynolds Shear Stress ◽  
Mean Velocity ◽  
Mixing Performance ◽  
Separation Ratio ◽  
Turbulent Characteristics ◽  
Velocity Decay ◽  
Twin Jets ◽  
Proper Orthogonal

Investigation of turbulent characteristics of twin jet near the free surface was carried out experimentally at four offset height ratios of 1, 2, 3 and 4. The experiments were conducted using square orifice nozzle pair with separation ratio of 2.6 at Reynolds number of 3890. The effect of the free surface as well as the effect of the offset height ratio on the streamwise mean velocity, streamwise turbulence intensity and Reynolds shear stress were discussed. The velocity decay and jet spread were quantified in order to characterize the effect of offset height on entrainment and mixing performance. The flow dynamics at the free surface was characterized by observing the variation of streamwise mean velocity and turbulence intensities at the free surface. Proper orthogonal decomposition was performed and the role of the energetic structures in the surface attaching twin jet was discussed both qualitatively and quantitatively.

Natural Convection of Air through an Asymmetrically Heated Vertical Channel with a Baffle

2016 ◽  
Vol 846 ◽  
pp. 7-11
Author(s):  
Cheng Wang Lei ◽  
Tuo Tian Wang
Keyword(s):  
Natural Convection ◽  
Reverse Flow ◽  
Air Flow ◽  
Vertical Channel ◽  
The Other ◽  
Heated Wall ◽  
Two Dimensional ◽  
Flow Through ◽  
Vertical Ventilation ◽  
Ventilation Performance

The buoyancy-induced air flow through a two-dimensional vertical ventilation channel is calculated. One of the channel walls is heated uniformly, and the other wall is adiabatic. A thin baffle is placed on the heated wall to manipulate the air flow through the channel. Numerical results are obtained for baffles of different lengths and placed at various heights along the heated wall. It is found that the baffle is effective in weakening a reverse flow at the exit of the channel, and significant enhancement of ventilation performance may be achieved with the presence of the baffle.

Comparison of Flying-Hot-Wire and Stationary-Hot-Wire Measurements of Flow Over a Backward-Facing Step

1999 ◽  
Vol 121 (2) ◽  
pp. 441-445 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Hot Wire Anemometry ◽  
Blunt Leading Edge

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

An experimental investigation of a turbulent shear flow with separation, reverse flow, and reattachment

1986 ◽  
Vol 163 ◽  
pp. 283-322 ◽  
Author(s):  
R. Ruderich ◽  
H. H. Fernholz
Keyword(s):  
Shear Flow ◽  
Shear Layer ◽  
Reverse Flow ◽  
Reynolds Shear Stress ◽  
Flow Region ◽  
Splitter Plate ◽  
Turbulent Shear ◽  
Spanwise Direction ◽  
Law Of The Wall ◽  
Wall Boundary Conditions

Experiments were performed in the highly turbulent and disturbed flow over a bluff plate with a long splitter plate in its plane of symmetry. The flow separates at the sharp bevelled edge of the bluff plate, forms a free shear layer on top of the reverse-flow region which is bounded on its other side by the splitter plate, and reattaches on the splitter plate over a narrow region curved in spanwise direction. Downstream of reattachment the shear flow adjusts slowly to the wall boundary conditions.Measurements of mean velocity, Reynolds-shear-stress and Reynolds-normal-stress distributions were carried out by hot-wire and pulsed-wire anemometry. The latter technique was used in those regions of the flow where reverse flow occurred or where the flow was highly turbulent. Spectra and integral lengthscales were measured to investigate the state and structure of the flow. The large-eddy structure in the inner region of the flow had lengthscales in the two cross-stream directions which were approximately equal, indicating a fast break-up of spanwise structures just downstream from separation.Mean and fluctuating quantities showed a self-similar behaviour in a short region upstream of reattachment and ‘profile similarity’ in the separated shear layer and along the splitter plate downstream from reattachment. Probability-density distributions of skin friction were measured and used to calculate mean and fluctuating values. No flapping of the reattaching shear layer could be observed. Pulsed-wire measurements revealed that the logarithmic law of the wall does not hold either in the reverse-flow region or in a region about half the length of the bubble downstream from reattachment.

Calculation of unsteady two-dimensional laminar and turbulent boundary layers with fluctuations in external velocity

1977 ◽  
Vol 355 (1681) ◽  
pp. 225-238 ◽  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Boundary Layers ◽  
Turbulent Flows ◽  
Reynolds Shear Stress ◽  
Low Frequency ◽  
Analytic Solutions ◽  
Turbulent Boundary Layers ◽  
Two Dimensional ◽  
Boundary Layer Equations

A numerical method is presented for calculating unsteady two-dimensional laminar and turbulent boundary layers with fluctuations in external velocity. The method used an eddy-viscosity formulation to model the Reynolds shear stress term appropriate to turbulent flow and an efficient two-point finite-difference method to solve the governing boundary-layer equations. The method is used to calculate phase angles between the wall shear stress and an oscillating external laminar boundary layer over a flat plate. The results are in excellent agreement with the analytic solutions of Lighthill for the high- and low-frequency limits and provide information in the region between. Similar calculations for turbulent flows are compared with experimental data and the method shown to be more precise than previously described attempts to represent flows of this type. The agreement between calculations and measurements is imperfect but probably within the resolution of the experiments and adequate for engineering purposes.

An experimental examination of the large-eddy equilibrium hypothesis

1966 ◽  
Vol 24 (1) ◽  
pp. 89-98 ◽  
Author(s):  
Ian S. Gartshore
Keyword(s):  
Reynolds Shear Stress ◽  
Turbulent Shear ◽  
Two Dimensional ◽  
Turbulent Shear Flow ◽  
Mean Velocity ◽  
Large Eddy ◽  
Energy Equilibrium ◽  
The Mean ◽  
Large Eddies ◽  
Made In

The large-eddy energy equilibrium hypothesis states that the largest eddies of a turbulent shear flow are in approximate energy equilibrium throughout a significant part of their lives. This hypothesis leads to a relationship between the mean rate of shear strain and the Reynolds shear stress which involves the scale of the large eddies. By assuming that the large-eddy scale is proportional to the standard deviation of the free turbulent boundary, or laminar superlayer, the validity of this hypothesis may be checked experimentally. Intermittency and mean velocity measurements made in five different two-dimensional shear flows are presented and these results, together with values calculated from Townsend's measurements in a two-dimensional wake, support the form of relationship suggested by the energy equilibrium hypothesis.

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