by Yongbao Xin. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 113-117). / by Yongbao Xin. / ABSTRACT --- p.I / ACKNOWLEDGMENTS --- p.II / LIST OF FIGURES --- p.V / LIST OF TABLES --- p.VIII / CHAPTERS / Chapter 1. --- INTRODUCTION --- p.1 / Chapter 1.1 --- Turbulence: A Long-standing and Unsolved Problem --- p.1 / Chapter 1.2 --- Rayleigh-Benard Convection --- p.3 / Chapter 1.3 --- High Rayleigh Number Rayleigh-Benard Convection --- p.5 / Chapter 1.3.1 --- The Rayleigh number dependence of Nu --- p.6 / Chapter 1.3.2 --- Large-scale flow in RB convection --- p.7 / Chapter 1.3.3 --- Thermal plumes in the high Ra convection --- p.9 / Chapter 1.4 --- Velocity Field and Viscous Boundary Layer Measurement in Rayleigh-Benard Convection --- p.11 / Chapter 2. --- SETUP OF RAYLEIGH-BENARD CONVECTION --- p.14 / Chapter 2.1 --- The Convection Cells --- p.14 / Chapter 2.2 --- Built Up the Convection --- p.18 / Chapter 2.2.1 --- Preparations of fluid --- p.18 / Chapter 2.2.2 --- Procedures to have a stable convective state --- p.19 / Chapter 2.3 --- Measurements of the Nusselt Number --- p.22 / Chapter 3. --- VELOCITY MEASUREMENT IN RAYLEIGH-BENARD CONVECTION --- p.25 / Chapter 3.1 --- Dual-beam Incoherent Cross-correlation Spectroscopy --- p.25 / Chapter 3.1.1 --- Theory of the technique --- p.25 / Chapter 3.1.2 --- Experimental setup --- p.31 / Chapter 3.1.3 --- Calibration and applications of the technique --- p.34 / Chapter 3.1.4 --- Summary --- p.41 / Chapter 3.2 --- Velocity Measurement in Turbulent Rayleigh-Benard Convection --- p.42 / Chapter 3.2.1 --- Cross-correlation functions in turbulent convection --- p.42 / Chapter 3.2.2 --- Some experimental details --- p.43 / Chapter 3.2.3 --- Some issues to be concerned in the velocity measurement --- p.47 / Chapter 4. --- LARGE SCALE CIRCULATION IN RAYLEIGH-BENARD CONVECTION --- p.50 / Chapter 4.1 --- Large Scale Circulation in Cylindrical Cells --- p.50 / Chapter 4.1.1 --- "Large scale circulation in cells of AR =1,2，and 44" --- p.50 / Chapter 4.1.2 --- Flow pattern in AR = 0.5 cell --- p.55 / Chapter 4.1.3 --- Velocity profiles along vertical central axis --- p.57 / Chapter 4.1.4 --- Prandtl number dependence of the boundary layer properties --- p.61 / Chapter 4.2 --- Large Scale Circulation in Cubic Cell --- p.65 / Chapter 4.2.1 --- Large scale flows near the plate and near the sidewall --- p.65 / Chapter 4.2.2 --- Velocity profiles along the vertical axis --- p.68 / Chapter 4.3 --- Summary --- p.73 / Chapter 5. --- SCALING PROPERTIES OF THE BOUNDARY LAYER QUANTITIES --- p.75 / Chapter 5.1 --- Ra Dependence of Boundary Layer Properties --- p.75 / Chapter 5.1.1 --- Scaled velocity and standard deviation profiles at different Ra --- p.75 / Chapter 5.1.2 --- Scaling properties of the maximum mean velocityvm and the maximum standard deviation σm --- p.76 / Chapter 5.1.3 --- Scaling properties of the shear rate γv and the gradient of the standard deviation yσ --- p.80 / Chapter 5.1.4 --- Scaling properties of boundary layer thickness --- p.84 / Chapter 5.2 --- Aspect-ratio Dependence of Boundary Layer Properties --- p.87 / Chapter 5.2.1 --- AR dependence of vm(Ra) and σm(Ra) --- p.87 / Chapter 5.2.2 --- AR dependence of the shear rate --- p.89 / Chapter 5.2.3 --- AR dependence of the viscous boundary layer thickness --- p.89 / Chapter 5.2.4 --- Vertical motion of the coherent structures in AR = 0.5 cell --- p.92 / Chapter 5.3 --- Summary --- p.93 / Chapter 6. --- CONCLUSION --- p.96 / APPENDIX / Chapter A1. --- NUMERICAL ANALYSIS OF THE CORRELATION FUNCTION EQ. 315 --- p.100 / Chapter A1.1 --- Analysis with Gaussian PDF Distribution --- p.100 / Chapter A1.2 --- Definition and Properties of Erfc(x) --- p.101 / Chapter A1.3 --- The Variable of Erfc in Eq. 315 --- p.104 / Chapter A1.4 --- Difference of Peak Position and t0 --- p.105 / Chapter A2. --- LASER HEATING EFFECTS IN THE LIGHT SCATTERING EXPERIMENT --- p.106 / Chapter A 2.1 --- Introduction --- p.106 / Chapter A 2.2 --- Experimental Technique --- p.106 / Chapter A 2.3 --- Results and Discussion --- p.108 / Chapter A 2.4 --- Summary --- p.109 / REFERENCES --- p.113

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Typescript (photocopy) / ix, 240 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Mechanical Engineering, 1978

69 leaves : tables, graphs ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied Mathematics, 1978

The very stable boundary layer is a region of the atmosphere typified by large vertical gradients of temperature and momentum. Analysis of very stable atmospheric flows is complicated by the presence of nonlinear interactions among gravity waves, shear-driven overturning circulations, two-dimensional vortical modes and intermittent turbulence in various stages of development. This study examines the horizontal structure of a very stable atmospheric boundary layer, using data obtained primarily from terrain-following aircraft flights over central Oklahoma. Several diagnostic procedures are applied to the aircraft data, including classical and rotary spectral analysis, principal component analysis, and structure functions. Coherent structures with sharp boundaries are examined with a new conditional sampling technique which requires little a priori specification of sampling criteria. Because the flows involve sharp boundaries, spectral techniques do not provide as much useful information as other more localized procedures. The edges of the coherent structures are regions of significant vertical heat transport, a feature not often emphasized in studies of gravity waves and vortical modes in the stable boundary layer. The presence of significant turbulence even for large stability has implications for modelling of the very stable boundary layer. Forecasts of minimum temperature, boundary layer height, inversion characteristics, and pollutant dispersal are all significantly affected by turbulent mixing. Many models of the stable boundary layer artificially arrest the mixing under stable conditions, resulting in, for example, overestimates of nocturnal cooling. A new parameterization of the stable boundary layer is studied here by incorporating it into an existing model of the planetary boundary layer. The model is then run with one-dimensional sensitivity tests for an idealized atmosphere and with data from Wangara day 33. A simulation over snow cover is also examined. The tests substantiate the role of vertical mixing in ameliorating nocturnal cooling. An additional improvement is a more realistic boundary layer height for moderate wind speeds. / Graduation date: 1988

The influence of boundary layer pumping on an externally-forced synoptic-scale flow is examined. The results follow earlier theories of stratified incompressible Boussinesq flow. These theories state that the spin-down time scale and the penetration depth of the influence of boundary layer pumping are inversely proportional to the stratification and directly proportional to the horizontal length scale of the flow. However, the present development is performed in isentropic coordinates which allow estimates applicable to the atmosphere, and implicitly includes nonlinear influences due to tilting and vertical advection. This analysis indicates that boundary layer pumping could be important synoptically in the lower troposphere under conditions of significant surface stress and tropospheric stratification. The influence of stratification and accelerations on synoptic-scale, boundary layer production of vertical motion is examined for the case of oscillating boundary-layer flow driven by time-dependent, horizontally- periodic surface temperature perturbations. It is found that only very strong stratification can significantly reduce the boundary layer pumping through pressure adjustments within the boundary layer. As a step in understanding the complicated dynamics of the structure of accelerated stratified boundary layers, order-of-magnitude analyses of variables for each layer are examined. This structure depends on the relative magnitude of the non-dimensional forcing frequency and the product of the stratification parameter and Ekman number. Applications to both synoptic and diurnal atmospheric circulations are considered. / Graduation date: 1978

The numerical modeling of motions in the atmosphere's planetary boundary layer (PBL) is a challenging task. In general, the boundary layer interacts with both the overlying atmosphere and the underlying land or water surface in a complex manner. Random turbulence is also present in the PBL which precludes exact prediction by numerical models. Nonetheless, expensive three-dimensional numerical models have been developed which, with several parameterizations and assumptions, can give a good idea of the PBL structure in many situations. However, on certain occasions, there is strong mixing evident in the PBL which may enable one to describe the structure of the boundary layer in a much-simplified theoretical model. By eliminating the vertical dimension from consideration, this two-dimensional mixed-layer model can be applied to mesoscale phenomena (horizontal length scale < 100 km) at greatly-reduced costs. The equations for motion and mixed-layer height are derived for such a situation and methods appropriate to the numerical modeling of the atmospheric mixed layer are discussed. Using an energy-conserving finite-difference analog of the model equations, the model is integrated in time to simulate the motions which were associated with the atmospheric vortex street observed near Cheju-do, South Korea on 17 February 1975. Experiments were carried out which investigated the effects of lateral diffusion, horizontal resolution, and mixed-layer depth. It is concluded that, given proper representation of prognostic variables on a staggered finite-difference grid, only small, realistic values of eddy diffusivity need be utilized. It also appears evident from the numerical experiments and atmospheric observations that the vortex street will form only when the obstacle which triggers its formation protrudes above the mixed layer. Although the wind fields in the simulations sometimes lack clear, fully rotational cells well downstream of the island, the characteristic sinusoidal pattern observed in laboratory experiments and cloud photographs is explicitly resolved by the model. The simulated vortex street also compares favorably with the observed in that the dimensionless governing parameters of the simulated vortex street (the Reynolds number, Strouhal number, Lin's parameter, the spacing ratio, and the speed ratio) closely match the observed values. / Graduation date: 1981

High resolution data of moisture, temperature and wind velocity collected by aircraft during the 1975 Air Mass Transformation Experiment (ANTEX '75) provide information for detailed investigations of the phenomena occurring at the top of a cloud-topped mixed layer. Joint frequency diagrams of humidity and temperature reveal that for parts of the record the coldest temperatures occur in air near saturation while drier air or air with substantial liquid water is warmer. This suggests the possible occurrence of cloud-top entrainment instability (Deardorff, 1980). Using humidity as an indicator, the flight record is systematically searched for penetrative mixed layer elements and pockets of dry air penetrating into cloudy air (wisps). The separate phenomena are then composited to produce an "average" wisp and penetrating element. The composites show evidence of net cooling due to evaporation downstream from the penetrating element and upstream from the wisps. Sinking motion is associated with the cold temperatures. These results suggest the interaction of shear, penetrating elements and wisps, and the existence of cloud-top entrainment instability. / Graduation date: 1981

Four buoyant plumes were produced within a laboratory convectively mixed layer from a source height of about z[subscript s] = 0.15 h, where h is the height of the convectively mixed layer. The projected images of these plumes in the X-Z plane were analyzed using a densitometer (photomultiplier tube) to calculate dimensionless crosswind integrated concentration values. These values were examined at regular intervals of non-dimensionalized heights and downwind distances to calculate center-of-mass heights, approximate plume limits, and touchdown distances. The plume buoyancy values were expressed in dimensionless terms as the parameter F[subscript *]. The touchdown distances are greatest and the surface integrated crosswind concentrations are smallest for the three most buoyant plumes. The highest center-of-mass and plume limit positions are also associated with the most buoyant plumes. The surface crosswind integrated concentration values for even the least buoyant plumes are much smaller than for non-buoyant plumes from previous studies. Touchdown distances for buoyant plumes from this data set agree with data from Willis and Deardorff (1983); however, the centerline and lower plume limits are at greater heights for this study. Vertical profiles of crosswind integrated concentration values indicate that the least buoyant plume has a bimodal distribution near the stack then becomes uniform at greater distances. The vertical profiles for the three most buoyant plumes show the highest concentration values are present in the upper part of the mixed layer at most downwind distances examined in this study. This distribution of effluent is also indicated by the vertical center-of- mass heights being larger than the plume centerline calculated from the average of the lower and upper plume limits. / Graduation date: 1985

On-going efforts to reduce aircraft drag through transition delay focus on understanding the process of boundary-layer transition from a physics-based perspective. For swept-wings subject to transition dominated by a stationary crossflow instability, one of the remaining challenges is understanding how freestream disturbances and surface features such as surface roughness create the initial amplitudes for unstable waves. These waves grow, modify the mean flow and create conditions for secondary instabilities to occur, which in turn ultimately lead to transition. Computational methods that model the primary and secondary instability growth can accurately model disturbance evolution as long as appropriate initial conditions are supplied. Additionally, transition delay using discrete roughness arrays that exploit known sensitivities to surface roughness has been demonstrated in flight and wind tunnel testing; however, inconsistencies in performance from the two test platforms indicate further testing is required. This study uses detailed hotwire boundary-layer velocity scans to quantify the relationship between roughness height and initial disturbance amplitude. Naphthalene flow visualization provides insight into how transition changes as a result of roughness height and spacing. Micron-sized, circular roughness elements were applied near the leading edge of the ASU(67)-0315 model installed at an angle of attack of -2.9 degrees in the Klebanoff-Saric Wind Tunnel. Extensive flow quality measurements show turbulence intensities less than 0.02% over the speed range of interest. A survey of multiple roughness heights for the most unstable and control wavelengths and Reynolds numbers of 2.4 x 10⁶ 2.8 x 10⁶ and 3.2 x 10⁶ was completed for chord locations of 10%, 15% and 20%. When care was taken to measure in the region of linear stability, it was found that the disturbance amplitude varies almost linearly with roughness height. Naphthalene flow visualization indicates that moderate changes in already-low freestream turbulence levels can have a significant impact on transition behavior.