Scaling of the turbulence transition threshold in a pipe

We report the results of an experimental investigation of the transition to turbulence in a pipe over approximately an order of magnitude range in $Re$. A novel scaling law is uncovered using a systematic experimental procedure which permits contact to be made with modern theoretical thinking. The principal result we uncover is a scaling law which indicates that the amplitude of perturbation required to cause transition scales as $O(Re^{-1})$.Comment: 4 pages, RevTex (submitted to Phys. Rev. Lett.

Oscillatory bubbles induced by geometrical constraint

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Multiple states of finger propagation in partially occluded tubes

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Moisture-Temperature Realtionships in a Sand Due to Outward, Radial Freezing

A "clean" sand is commonly specified as backfill around the evaporator section of thermosyphons designed to maintain the thermal regime of pernnially, frozen, thaw-unstable soils. A series of laboratory tests were performed to determine the magnitude of moisture migration. The test results indicate the moisture migration can result due to outward radial freezing in a nonfrost susceptible sand possessing a low to moderate degree of saturation. Moisture did not migrate when the sand was saturated prior to freezing. The redistribution of moisture changes the thermal properties of the soil system which effects the maximum radius of freezing by desiccating soil at the outermost radius of influence and increasing the degree of saturation around the evaporator. The desiccated soil region will experience an accelerated rate of thaw due to a lower volumetric latent heat of fusion. In addition, the radius of freezing is reduced as moisture migrates towards the evaporator section. These effects warrant additional considerations that must be addressed when designing refrigerated foundations with thermosyphons.List of Figures - vi List of Tables - x Acknowledgements - xi Chapter 1 Introduction - 1 Chapter 2 Heat Transfer in Soils - 5 2.1 Modes of Heat Transfer - 6 2.1.1 Conduction - 7 2.1.2 Convection - 8 2.2 Thermal Properties of Soils - 10 2.2.1 Thermal Conductivity - 10 2.2.2 Volumetric Latent Heat of Fusion - 14 2.2.3 Specific Heat - 16 2.2.4 Heat Capacity - 17 2.2.5 Thermal Diffusivity - 19 2.2.6 Enthalpy - 20 2.3 Heat Transfer in Cylindrical Coordinates - 23 2.3.1 Steady-State Heat Flow - 24 2.3.2 Heat Transfer with Phase Change - 29 2.3.3 Explicit Finite Difference Technique - 37 Chapter 3 Moisture Migration in Freezing Soils - 41 3.1 Soil-Water Properties Effecting Moisture Migration - 44 3.1.1 Grain Size and Grain Size Distribution - 45 3.1.1.1 Specific Surface - 48 3.1.1.2 Soil Fabric - 50 3.1.1.3 Capillarity - 51 3.1.2 Soil Density - 56 3.1.3 Freezing Point Depression and Unfrozen Water - 57 3.1.4 Hydraulic Conductivity - 65 3.2 Environmental Factors Effecting Moisture Migration - 67 3.3 Moistrue Migration Mechanisms - 72 3.3.1 Transport Along Continuous Films - 73 3.3.2 Vapor Transport - 79 Chapter 4 Experimental Setup - 83 4.1 Freezing Tank and Equipment - 84 4.2 Soil Properties - 87 4.3 Temperature Measurements - 94 4.4 Pore Pressure-Moisture Measurements - 102 4.5 Test Procedure - 105 Chapter 5 Experimental Results - 108 5.1 Test Properties and Controls - 109 5.2 Soil Temperatures - 112 5.2.1 Complete Phase Change - 118 5.2.2 Steady-State Temperatures - 121 5.2.3 Freezing Isotherm Velocities - 127 5.3 Soil Suction Measurements - 134 5.4 Moisture Redistribution - 140 Chapter 6 Summary - 149 6.1 Discussion - 149 6.2 Recommendations - 152 6.3 Conclusions - 153 References - 15

Peeling fingers in an elastic Hele-Shaw channel

Using experiments and a depth-averaged numerical model, we study instabilities of two-phase flows in a Hele-Shaw channel with an elastic upper boundary and a non-uniform cross-section prescribed by initial collapse. Experimentally, we find increasingly complex and unsteady modes of air-finger propagation as the dimensionless bubble speed, Ca, and level of collapse are increased, including pointed fingers, indented fingers and the feathered modes first identified by Cuttle et al.(J. Fluid Mech., vol. 886, 2020, A20). By introducing a measure of the viscous contribution to finger propagation, we identify a Ca threshold beyond which viscous forces are superseded by elastic effects. Quantitative prediction of this transition between 'viscous' and 'elastic' reopening regimes across levels of collapse establishes the fidelity of the numerical model. In the viscous regime, we recover the non-monotonic dependence on Ca of the finger pressure, which is characteristic of benchtop models of airway reopening. To explore the elastic regime numerically, we extend the depth-averaged model introduced by Fontana et al. (J. Fluid Mech., vol. 916, 2021, A27) to include an artificial disjoining pressure which prevents the unphysical self-intersection of the interface. Using time simulations, we capture for the first time the majority of experimental finger dynamics, including feathered modes. We show that these disordered states continually evolve, with no evidence of convergence to steady or periodic states. We find that the steady bifurcation structure satisfactorily predicts the bubble pressure as a function of Ca, but that it does not provide sufficient information to predict the transition to unsteady dynamics which appears strongly nonlinear.Comment: 28 pages, 15 figure