• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1996년도 한국자동제어학술회의논문집(국내학술편); 포항공과대학교, 포항; 24-26 Oct. 1996
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• pp.147-150
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• 1996

This paper describes controller design and implementation method for controlling the temperature distribution in a thermal stratified wind tunnel(TSWT) by using a neural network algorithm. It is impossible to derive a mathematical model of the relation between heat inputs and temperature outputs in the test section of the TSWT governed by a nonlinear turbulent flow. Thus inverse neural network models with a multi layer perceptron structure are used in a feedforward control loop and feedback control loop to generate an arbitrary temperature distribution in the test section of the TSWT.

• 대한기계학회논문집
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• 제18권3호
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• pp.690-700
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a circular cylinder were examined in a wind tunnel. In order to produce strong thermal stratifications, a compact heat exchanger type variable electric heater is employed. Linear temperature gradient of up to $250^{\circ}C/m$ can be well sustained. The velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured and the smoke wire flow visualization method was used to investigate the wake characteristics. It is found that the temperature field effects as an active contaminant, so that the mean velocity and temperature profiles can not sustain their symmetricity about the wake centerline when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower half of the wake in a stably stratified flow.

• 대한기계학회논문집B
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• 제23권2호
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• pp.218-225
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• 1999

Flow visualization study has been conducted to simulate the turbulent dispersion behavior of a crossflow jet physically under the conditions of various thermal stratification in a wind tunnel. A smoke jet with the constant ratio of the jet to freestream velocity is injected normally to the cross flow of the thermally stratified wind tunnel(TSWT) for flow visualization. The typical natures of the smoke dispersion under different thermal stratifications such as neutral, weakly stable, strongly stable, weakly unstable, strongly unstable and inversion layer are successfully reproduced in the TSWT. The Instantaneous velocity and temperature fluctuations are measured by using a cold and hot-wire combination probe. The time averaged dispersion behaviors, the centerline trajectories, the spreading angles and the virtual origins of the cross jet are deduced from the edge detected images with respect to the stability parameter. All the general characteristics of the turbulent dispersion behavior reveal that the definitely different dispersion mechanisms are inherent in both stable and unstable conditions. It is conjectured that the turbulent statistics obtained in the various stability conditions quantitatively demonstrate the vertical scalar flux plays a key role in the turbulent dispersion behavior.

• 대한기계학회논문집
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• 제18권9호
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• pp.2454-2462
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• 1994

The effects of thermal stratification on the flow of a stratified fluid past a heated circular cylinder were examined in a wind tunnel. Turbulent intensities, rms values of temperature and turbulent convective heat flux distributions in the heated cylinder wake with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. A phase averaging method was also used to estimated coherent motion in the near wake. It is found that the vertical turbulent motion in the stably stratified flow case dissipates faster than that of the neutral case, i.e., vertical growth of vortical structure is suppressed under the strongly stratified condition. The coherent motion of temperature makes a large contribution like velocity coherent motion. However, the coherent motions of temperature fluctuation become very different with the change of experimental conditions, though the velocity coherent motions are quite similar in all experimental conditions.

• 대한기계학회논문집
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• 제18권5호
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• pp.1322-1329
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• 1994

The effect of thermal stratification on the stratified flow past a circular cylinder was examined in a wind tunnel. Turbulent intensities, the rms values of temperature and turbulent convective heat flux as well as the velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured by using a hot-wire and cold-wire combination probe. It is found that the temperature field affects as an active contaminant, so that the vertical growth of vortical structure is suppressed and the strouhal number decreases with increasing the extent of stratification. And also, the wake structure can not sustain their symmetricity about the wake centerline and vertical turbulent motion dissipates faster than that of the neutral case when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower of the wake in a stably stratified flow because the turbulent intensities and convective heat flux in the upper half section are larger than those of the lower half of the wake.

• 대한기계학회논문집
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• 제19권5호
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• pp.1300-1307
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• 1995

The effect of thermal stratification on the turbulent dispersion from a fine cylindrical heat source was experimentally examined in a wind tunnel with and without a strong temperature gradient. A 0.5 mm dia. nichrome wire was used as a line heat source. Turbulent intensities, r.m.s. value of temperature and convective heat fluxes were measured by using a hot-wire and cold-wire combination probe. The results show that the peack value and the spread of the vertical turbulent intensity for the stratified case are far lower than those in the neutral case, which indicates that the stable temperature gradient suppresses the vertical velocity component. All of the third order moments including heat fluxes measured in the stable condition have very small values than those of the neutral case. This nature suggests that the decrease of scalar fluctuations in the stably stratified flow is mainly due to the suppression ofthe turbulent diffusion processes by the stable stratification. A simple gradient model with a composite timescale which has a simple weighted algebraic mean between dynamic and thermal time scale yields reasonably good numerical values in comparison with the experimental data.

• 대한기계학회논문집B
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• 제23권11호
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• pp.1434-1443
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• 1999

The turbulent fluctuations of temperature and two components of velocity have been measured with hot- and cold-wires in the Thermally Stratified Wind Tunnel(TSWT). Using the fin-tube heat exchanger type heaters and the neural network control algorithm, both stable ($dT/dz=109.4^{\circ}C$) and unstable ($dT/dz=-49.1^{\circ}C$) stratifications were realized. An ambient air jet was issued normally into the cross flow($U_{\infty}=1.0 m/s$) from a round nozzle(d = 6 mm) flushed at the bottom waII of the wind tunnel with the velocity ratio of $5.8(U_{jet}/U_{\infty})$. The characteristics of turbulent dispersion in the cross flow jet are found to change drastically depending on the thermal stratification. Especially, in the unstable condition, the vertical velocity fluctuation increases very rapidly at downstream of jet. The fluctuation velocity spectra and velocity-temperature cospectra along the jet centerline were obtained and compared. In the case of stable stratification, the heat flux cospectra changes Its sign from a certain point at the far field because of the restratification phenomenon. It is inferred that the main reason in the difference between the vertical heat fluxes is caused by the different length scales of the large eddy motions. The turbulent kinetic energy and scalar dissipation rates were estimated using partially non-isotropic and isotropic turbulent approximation. In the unstable case, the turbulent energy dissipation decreases more rapidly with the downstream distance than in the stable case.

• 대한기계학회논문집
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• 제19권5호
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• pp.1340-1350
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• 1995

The effects of thermal stratification on the flow past a heated circular cylinder with various heating rates were examined in a wind tunnel. Turbulent intensities, r.m.s.values of temperature and turbulent convective heat flux distributions in the cylinder wakes with and without thermal stratification were measured by using a hot-wire and cold-wire combination probe. The phase averaging method was also used to estimate coherent contributions to the turbulent flow field in the near wake. The results show that the scalar mixing process is very different according to the mean temperature fields especially in the upper part of the wake. The coherent structure of the temperature field makes a large contribution to the time mean value like velocity components. However, the coherency of the temperature fluctuation is very different with the change of mean temperature fields, though the velocity coherent motions are quite similar in all experimental conditions.