Effect Of Boundary Layer Thickness Research Articles

Effect of boundary layer thickness before the flow separation on aerodynamic characteristics and heat transfer behind an abrupt expansion in a round tube

Results of numerical investigation of the boundary layer thickness on turbulent separation and heat transfer in a tube with an abrupt expansion are shown. The Menter turbulence model of shear stress transfer implemented in Fluent package was used for calculations. The range of Reynolds numbers was from 5·103 to 105. The air was used as the working fluid. A degree of tube expansion was (D2/D1)2 = 1.78. A significant effect of thickness of the separated boundary layer both on dynamic and thermal characteristics of the flow is shown. In particular, it was found that with an increase in the boundary layer thickness the recirculation zone increases, and the maximum heat transfer coefficient decreases.

Calculation of the viscous-interaction effects in the Euler/boundary-layer methods

A simple approach is proposed to incorporate the effect of viscous interaction in the calculation of aerodynamic heating of three-dimensional geometries by Euler/boundary-layer methods. In the present study, aerodynamic heating is calculated from the solution of a set of approximate convective-heating equations along the streamlines. These heating equations are derived using axisymmetric analogue. To determine streamline paths and flow properties along them, Euler equations are solved around the three-dimensional body. The effect of viscous interaction on the aerodynamic heating can be significant, especially if the Reynolds number is low. To incorporate this effect, the whole process should be repeated having considered the boundary-layer thickness all around the body. However, this approach causes different difficulties, and is not cost efficient as well. In the present work, the effect of boundary-layer thickness on the aerodynamic heating is incorporated through the correction of pressure distribution on the surface of the body. This approach of correcting pressure distribution is applied to spherical and elliptical blunted cones in laminar hypersonic flows. It is shown that the heating rates are improved in low Reynolds numbers, where viscous-interaction effect is significant. The present approach can result in a reduction of computational cost, when viscous interaction is significant in calculation of aerodynamic heating.

An examination of the effect of boundary layer thickness on vortex shedding from a square cylinder near a wall

A computational study has been undertaken to examine the effect of boundary layer thickness δ/ D on vortex shedding from a square cylinder in proximity to a solid wall. The computations were carried out in a second-moment turbulence modeling framework using a finite-volume technique. The computed results show that, in general, thickening of the wall boundary layer causes wake periodicity to persist for increasingly smaller cylinder-to-wall gap widths, S/ D. The nature of the periodic motion changes as S/ D approaches the critical value for complete suppression of vortex shedding. Similar to experimental observations, the location at which coupled shear layer motion is first observed shifts downstream of the base region. This shift is characterized by a rise in the shedding frequency and a drop in the time-averaged drag and lift on the cylinder. In addition, the pressure distribution along the lower wall is seen to change significantly due to the reduced size of the recirculation region in the cylinder wake.

Effect of Boundary-Layer Thickness on the Structure of a Near-Wall Flow with a Two-Dimensional Obstacle

Results of physical and numerical experiments on investigating the effect of the depth of immersion of a two-dimensional obstacle with a square cross section into a developed turbulent boundary layer on the length of the separated flow region are presented. The numerical simulation is based on solving averaged Navier–Stokes equations with the use of the k–e model of turbulence. The near-wall flow is visualized in the experiments, and the fields of mean and fluctuating velocities are measured. Flow regions where the results of numerical simulation agree with experimental data are determined. It is shown that the length of the recirculation flow region in the near wake increases with decreasing depth of immersion of the two-dimensional obstacle into the turbulent boundary layer.

직사각형 프리즘 주위의 유동구조에 대한 경계층 두께의 영향

Effect of boundary layer thickness on the flow characteristics around a rectangular prism has been investigated by using a PIV(Particle Image Velocimetry) technique. Three different boundary layers(thick, medium and thin)were generated in the Atmospheric Boundary Layer Wind Tunnel at Pusan National University. The thick boundary layer having 670 mm thickness was generated by using spires and roughness elements. The medium thickness of boundary layer(δ=270 mm) was the natural turbulent boundary layer at the test section floor with fairly long developing length(18 m). The thin boundary layer( δ = 36.5 mm) was generated on the smooth panel elevated 70cm from the wind tunnel floor. The Reynolds number based on the free stream velocity(3 m/s) and the height of the model(40 mm) was 7 .9x 10³. The mean velocity vector fields and turbulent kinetic energy distributions were measured and compared. The effect of boundary layer thickness was clearly observed not only in the length of separation bubble but also in the location of reattachment point. The thinner the boundary layer thickness, the higher the turbulent kinetic energy peak around the model roofbecame. It is strongly recommended that the height ratio between the model and the approaching boundary layer thickness should be encountered as a major parameter.

Flow Field and Performance Characteristics of Combustor Diffusers: A Basic Study

Results of a detailed study concerning the influence of geometric as well as fluid mechanic parameters on the performance of a plane model combustor diffuser in cold flow are presented. For a qualitative insight into the complex flow field inside the prediffuser, the sudden expansion region, and the flow field around the flame tube dome, results of a flow visualization study with the hydrogen bubble method as well as with the ink jet method are presented for different opening angles of the prediffuser and for different flame tube distances. Also, quantitative data from detailed measurements with LDV and conventional pressure probes in a geometrically similar air-driven setup are presented. These data clearly demonstrate the effect of boundary layer thickness as well as the influence of different turbulence levels at the entry of the prediffuser on the performance characteristics of combustor diffusers. The possibility of getting an unseparated flow field inside the prediffuser even at large opening angles by appropriately matching the diffuser’s opening angle and the flame tube distance is demonstrated. Also, for flows with an increased turbulence level at the entrance—all other conditions held constant—an increased opening angle can be realized without experiencing flow separation. The comparison of the experimental data with predictions utilizing a finite-volume-code based on a body-fitted coordinate system for diffusers with an included total opening angle less than 18 deg demonstrates the capability of describing the flow field in combustor diffusers with reasonable accuracy.

Diamond deposition by atmospheric pressure induction plasma: effects of impinging jet fluid mechanics on film formation

The effect of boundary layer thickness on diamond film formation in an atmospheric pressure plasma was investigated by adding argon to the methane-hydrogen reactant jet so as to increase the jet momentum. It was found that argon addition significantly increased the range of conditions over which well-faceted, continuous diamond film could be grown. A numerical model was developed to predict the two-dimensional temperature and flow fields in the reactor and the one-dimensional chemical kinetics in the substrate boundary layer. An interesting outcome of these calculations is that the largest effect of thinning the boundary layer by adding argon to the central jet is a significant enhancement in the flux of monatomic carbon vapor to the substrate, and the experimentally observed results can be correlated with the calculated carbon fluxes.

Flow around a cube placed on a ground plane

The effects of boundary layer thickness upon the flow around a cube mounted on a ground plane, on which a turbulent boundary layer is developing, have been investigated .The height ratio h/δ, which indicates the ratio of cube height h to boundary layer thickness δ, is varied between 0.29 and 2. 86. In particular the measurements in h/δ=0.72 and 2 .86 on the surface pressure distribution and the drag coefficient of the cube, the reattachment length and the Strouhal number were compared in detail with the past experimental data .The following results are obtained.The reattachment distance corresponding to the tail of the separation bubble behind the cube, the minimum pressure and its position on the ground plane corresponding to the roots of the arch type vortex may be estimated by the experimental equation con-elated with h/δ, respectively. Both the drag coefficient CD and the Strouhal number St decrease with decreasing h/δ in the range of h/δ smaller than about 3, and become nearly the constant values of C0 =1.3 and St=0 .11 in the range of h/δ larger than about 3.

The Trailing Edge Loss of Transonic Turbine Blades

Trailing edge loss is one of the main sources of loss for transonic turbine blades, contributing typically 1/3 of their total loss. Transonic trailing edge flow is extremely complex, the basic flow pattern is understood but methods of predicting the loss are currently based on empirical correlations for the base pressure. These correlations are of limited accuracy. Recent findings that the base pressure and loss can be reasonably well predicted by inviscid Euler calculations are justified and explained in this paper. For unstaggered choked blading, it is shown that there is a unique relationship between the back pressure and the base pressure and any calculation that conserves mass, energy and momentum should predict this relationship and the associated loss exactly. For realistic staggered blading, which operates choked but with subsonic axial velocity, there is also a unique relationship between the back pressure and the base pressure (and hence loss) but the relationship cannot be quantified without knowing a further relationship between the base pressure and the average suction surface pressure downstream of the throat. Any calculation that conserves mass, energy and momentum and also predicts this average suction surface pressure correctly will again predict the base pressure and loss. Two-dimensional Euler solutions do not predict the suction surface pressure exactly because of shock smearing but nevertheless seem to give reasonably accurate results. The effects of boundary layer thickness and trailing edge coolant ejection are considered briefly. Coolant ejection acts to reduce the mainstream loss. It is shown that suction surface curvature downstream of the throat may be highly beneficial in reducing the loss of blades with thick trailing edges operating at high subsonic or low supersonic outlet Mach numbers.

Effect of Boundary Layer Thickness on the Photoluminescence Spectra of GaAs Grown by MOCVD

Undoped GaAs epitaxial layers were grown by atmospheric MOCVD. The longitudinal distributions of photoluminescence spectra of these layers were studied. Mn, Cu, Si and C were found. The boundary layer thickness, which was expectd to develop on a heated substrate, showed no influence on the intensities of these metal impurities. The doping coefficient of the metal impurities may be unrelated to the boundary layer thickness. Where the boundary layer was thin, it was observed that a larger amount of carbon was incorporated in the epitaxial layers. Compared with a growth simulation, it is pointed out that the amount of the residual metalorganic gas which remained at the substrate surface is one of the significant factors regarding carbon incorporation.

Cross-Sectional Current Distribution in Coal Fired Diagonal Conducting Wall MHD Generator

Current distribution in the cross-sectional plane of a diagonal conducting wall MHD generator with slag coated walls is studied. The effects of boundary layer thickness and wall temperature on the current distribution and power output are investigated for magnetic fields of 2 and 6 T. The effect of sidewall boundar layer overshoot is also considered. For a cold wall there is substantial current flow to the sidewalls, whereas for a hot wall there is much less such a trend. The change in the thickness of the turbulent boundary layer has little effect on the current distribution and power output. Increasing the Hall parameter enhances the sidewall current significantly. Power output is strongly dependent on the wall temperature since the voltage drop increases rapidly as the wall temperature decreases. The temperature distribution in the slag layer varies with the current density for a specified wall temperature. There is no substantial current leakage through the slag layer between the various electrodes along the sidewall in all the cases considered. The poweroutput from the present investigation compares well with the experimental results and also with three-dimensional results.

Flow Past Cone Placed on Flat Plate : 2nd Report, Effects of Boundary Layer Thickness

This paper presents an experimental study on the effects of boundary layer thickness on the flow past a cone placed on a flat plate. The main results are as follows : (1) The flow around the cone with a dummy, which can be regarded as a flow without the effects of boundary layer thickness, is basically different from the flow around the cone placed on a flat plate because of the existence of necklace-like vortex around it. (2) The separation line on the cone surface curves with increasing of the vertex angle of the cone, and the angle of the separation point increases with boundary layer thickness inside the boundary layer.

Effect of Mainstream Variables on Jets Issuing from a Row of Inclined Round Holes

The effects of boundary layer thickness, Reynolds number and free stream turbulence intensity on jets issuing from a row of inclined holes are investigated from the viewpoint of film cooling. The local film cooling effectiveness and mean velocity and mean temperature distributions are measured. The turbulent eddy diffusivities are evaluated from calculations based on a heat source model. The boundary layer thickness and the free stream turbulence significantly influence the jet-mainstream interaction process and consequently the film cooling performance.

Streakline Flow Visualization of Discrete Hole Film Cooling for Gas Turbine Applications

Film injection from discrete holes in a three row staggered array with 5-dia spacing was studied for three hole angles: (1) normal, (2) slanted 30 deg to the surface in the direction of the mainstream, and (3) slanted 30 deg to the surface and 45 deg laterally to the mainstream. The ratio of the boundary layer thickness-to-hole diameter and the Reynolds number were typical of gas turbine film cooling applications. Results from two different injection locations are presented to show the effect of boundary layer thickness on film penetration and mixing. Detailed streaklines showing the turbulent motion of the injected air were obtained by photographing very small neutrally-buoyant helium filled “soap” bubbles which follow the flow field. Unlike smoke, which diffuses rapidly in the high turbulent mixing region associated with discrete hole blowing, the bubble streaklines passing downstream injection locations are clearly identifiable and can be traced back to their point of ejection.