Effect of Upstream Shear on Flow and Heat (Mass) Transfer Over a Flat Plate

2008 ◽  
Author(s):  
Kalyanjit Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Velocity Ratio ◽  
Leading Edge ◽  
Surface Velocity ◽  
Freestream Velocity ◽  
Inner Region

A parametric study is conducted to investigate the effect of wall shear on a two-dimensional turbulent boundary layer. The shear is imparted by a moving belt, flush with the wall, translating in the flow direction. Velocity and mass transfer experiments have been performed for four surface-to-freestream velocity ratios (0, 0.38, 0.52, 0.65) with a Reynolds number based on the momentum thickness between 770 and 1776. The velocity data indicate that the location of the ‘virtual origin’ of the turbulent boundary layer ‘moves’ downstream towards the trailing edge of the belt with increasing surface velocity. The highest velocity ratio represents a case which is responsible for the removal of the inner region of the boundary layer. Mass transfer measurements downstream of the belt show the presence of a local minimum in the variation of the Stanton vs. Reynolds number for the highest velocity ratio. Downstream of this minimum, approximately 1 cm from the leading edge of the mass transfer plate, the characteristics of the turbulent boundary layer are restored and the data fall back on the empirical variation of the Stanton number with Reynolds number.

Effect of Upstream Shear on Flow and Heat (Mass) Transfer Over a Flat Plate—Part II: Mass Transfer Measurements

2010 ◽  
Vol 132 (10) ◽  
Author(s):  
K. Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Heat Mass Transfer ◽  
Stanton Number ◽  
Freestream Velocity ◽  
Belt Speed ◽  
Inner Region

Mass transfer measurements on a flat plate downstream of a belt moving in the same direction of the freestream study the effect of the upstream shear on the heat (mass) transfer for four belt-freestream velocity ratios. With an increase in this ratio, the “virtual origin” of the turbulent boundary layer “moves” downstream toward the trailing edge of the belt. This is verified from the variation of the Stanton number versus the Reynolds number plots. As the “inner” region of the boundary layer is removed for a belt speed of uw=10 m/s (freestream velocity uin≈15.4 m/s), a corresponding local minimum in the variation of the Stanton number is observed. Downstream of this minimum, the characteristics of the turbulent boundary layer are restored and the data fall back on the empirical variation of Stanton with Reynolds number.

Effect of Upstream Shear on Flow and Heat (Mass) Transfer Over a Flat Plate—Part I: Velocity Measurements

2010 ◽  
Vol 132 (10) ◽  
Author(s):  
K. Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Ratio ◽  
Shear Velocity ◽  
Central Peak ◽  
Factor H ◽  
Surface Velocity ◽  
Wall Region ◽  
Velocity Measurements ◽  
Freestream Velocity

A parametric study investigates the effects of wall shear on a two-dimensional turbulent boundary layer. A belt translating along the direction of the flow imparts the shear. Velocity measurements are performed at 12 streamwise locations with four surface-to-freestream velocity ratios (0, 0.38, 0.52, and 0.65) and a momentum-based Reynolds number between 770 and 1776. The velocity data indicate that the location of the “virtual origin” of the turbulent boundary layer “moves” downstream toward the trailing edge of the belt with increasing surface velocity. The highest belt velocity ratio (0.65) results in the removal of the “inner” region of the boundary layer. Measurements of the streamwise turbulent kinetic energy show an inner scaling at locations upstream and downstream of the belt, and the formation of a new self-similar structure on the moving surface itself. Good agreement is observed for the variation in the shape factor (H) and the skin friction coefficient (cf) with the previous studies. The distribution of the energy spectrum downstream of the belt indicates peak values concentrated around 1 kHz for the stationary belt case in the near wall region (30<y+<50). However, with increasing belt velocity, this central peak plateaus over a wide frequency range (0.9–4 kHz).

Investigation of Flow and Mass Transfer on a Flat Plate Downstream of a Shear Inducing Moving Wall

2010 ◽  
Author(s):  
Kalyanjit Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Energy Content ◽  
Turbulent Energy ◽  
Wall Turbulence ◽  
Freestream Velocity ◽  
Moving Wall ◽  
Wall Boundary Layer ◽  
Logarithmic Region

The effects of an opposing (upstream-moving) wall-shear on a two-dimensional turbulent boundary layer are investigated. The shear at the boundary is imparted by a moving belt, flush with the wall. Boundary layer measurements are reported for four surface-to-freestream velocity ratios (0, −0.38, −0.51, −0.63) with the Reynolds number (based on the momentum thickness) between 922 and 1951. Velocity profiles downstream of the moving surface show an increased velocity deficit near the wall, which is more pronounced at higher (negative) belt velocity. Streamwise turbulence values downstream of the belt show the growth of a second peak in the logarithmic region of the boundary layer in addition to the normally-observed peak in the buffer region. This suggests the presence of larger length-scale turbulent eddies at locations away from the wall in the boundary layer. Spectral measurements indicate that the turbulent energy content is distributed over a wide portion of the logarithmic region. Mass transfer measurements using naphthalene sublimation provide the variation of Stanton with Reynolds number on the plate downstream of the moving belt. It shows little difference from the stationary belt case, which suggests that increased wall turbulence is balanced by an increase in the boundary layer thickness.

Development of a Steady Vortex Generator Jet in a Turbulent Boundary Layer

2003 ◽  
Vol 125 (6) ◽  
pp. 1006-1015 ◽  
Author(s):  
Gregory S. Rixon ◽  
Hamid Johari
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Vortex Generator ◽  
Streamwise Vortex ◽  
Freestream Velocity ◽  
Local Boundary ◽  
Image Velocimetry ◽  
Particle Image Velocimetry Method ◽  
Vortex Generator Jet

The development of a vortex generator jet within a turbulent boundary layer was studied by the particle image velocimetry method. Jet velocities ranging from one to three times greater than the freestream velocity were examined. The jet was pitched 45 deg and skewed 90 deg with respect to the surface and flow direction, respectively. The velocity field in planes normal to the freestream was measured at four stations downstream of the jet exit. The jet created a pair of streamwise vortices, one of which was stronger and dominated the flow field. The circulation, peak vorticity, and wall-normal position of the primary vortex increased linearly with the jet velocity. The circulation and peak vorticity decreased exponentially with the distance from the jet source for the jet-to-freestream velocity ratios of 2 and 3. The wandering of the streamwise vortex can be as much as ±30% of the local boundary layer thickness at the farthest measurement station.

Development of Taylor-Go¨rtler Vortices Over the Pressure Surface of a Turbine Blade

2005 ◽  
Vol 127 (5) ◽  
pp. 540-543 ◽  
Author(s):  
H. P. Wang ◽  
S. J. Olson ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Turbine Blade ◽  
Flow Direction ◽  
Mass Transfer Rate ◽  
Leading Edge ◽  
Flow Region ◽  
Freestream Turbulence ◽  
Pressure Surface ◽  
Naphthalene Sublimation Technique

The naphthalene sublimation technique is used to investigate the development of Taylor-Go¨rtler vortices over the pressure surface of a simulated high performance turbine blade. Large spanwise variation in mass transfer is observed downstream on the pressure surface in the two-dimensional flow region for cases with low freestream turbulence, indicating the existence of Taylor-Go¨rtler vortices. Different average and local mass transfer rates for the same flow conditions suggest that roughness variation near the leading edge affects the initial formation of Taylor-Go¨rtler vortices. Larger and more uniformly distributed roughness at the leading edge produces much stronger Taylor-Go¨rtler vortices downstream and greatly enhances the mass transfer rate. The variation between the vortices does not change appreciably along the flow direction. The flow in the boundary layer is laminar over the entire pressure surface. In the presence of external disturbances such as high freestream turbulence or a boundary layer trip, no Taylor-Go¨rtler vortices are observed.

Numerical simulation of the effect of a moving wall on separation of flow past a symmetrical aerofoil

1998 ◽  
Vol 212 (1) ◽  
pp. 69-77 ◽  
Author(s):  
Y T Chew ◽  
L S Pan ◽  
T S Lee
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Lift Coefficient ◽  
Leading Edge ◽  
Surface Velocity ◽  
Freestream Velocity ◽  
Moving Wall ◽  
Rotating Circular Cylinder ◽  
Flow Past ◽  
Numerical Simulation Technique

This paper applies the numerical simulation technique based on the generalized conservation of circulation (GCC) method to investigate the effects of a leading-edge rotating circular cylinder on the suppression of stall flow past a symmetrical Joukowski aerofoil. The variables investigated were the angle of attack α and the ratio of the surface velocity of the cylinder to freestream velocity, CU. The Reynolds number based on chord length is 1.43 × 105. It was found that the separation point on the upper surface of the aerofoil shifts downstream with increasing CU and stall flow can be significantly suppressed even at α up to 30° when CU = 4. The lift coefficient CL increases and the drag coefficient Cd decreases with increasing CU and the optimum CL/ Cd occurs at α=80°. The maximum CL/ Cd obtained is about 60 at CU = 4.

Modeling mass transfer and substrate utilization in the boundary layer of biofilm systems

1998 ◽  
Vol 37 (4-5) ◽  
pp. 139-147 ◽  
Author(s):  
Harald Horn ◽  
Dietmar C. Hempel
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Film Theory ◽  
Substrate Utilization ◽  
The Other ◽  
Concentration Profiles ◽  
Hydrodynamic Conditions ◽  
Time Axis ◽  
Transfer Coefficients

The use of microelectrodes in biofilm research allows a better understanding of intrinsic biofilm processes. Little is known about mass transfer and substrate utilization in the boundary layer of biofilm systems. One possible description of mass transfer can be obtained by mass transfer coefficients, both on the basis of the stagnant film theory or with the Sherwood number. This approach is rather formal and not quite correct when the heterogeneity of the biofilm surface structure is taken into account. It could be shown that substrate loading is a major factor in the description of the development of the density. On the other hand, the time axis is an important factor which has to be considered when concentration profiles in biofilm systems are discussed. Finally, hydrodynamic conditions become important for the development of the biofilm surface when the Reynolds number increases above the range of 3000-4000.

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