scholarly journals Wind-Tunnel Simulation of Stable Atmospheric Boundary Layers with an Overlying Inversion

2020 ◽  
Vol 175 (1) ◽  
pp. 93-112
Author(s):  
Philip E. Hancock ◽  
Paul Hayden
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Surface Condition ◽  
Surface Shear Stress ◽  
Atmospheric Boundary Layers ◽  
Surface Shear ◽  
Local Scaling ◽  
Obukhov Length ◽  
Boundary Layer Height ◽  
Inversion Condition

AbstractFour cases of an overlying inversion imposed on a stable boundary layer are investigated, extending the earlier work of Hancock and Hayden (Boundary-Layer Meteorol 168:29–57, 2018), where no inversion was imposed. The inversion is imposed to one or other of two depths within the layer: midway or deep. Four cases of changed surface condition are also investigated, and it is seen that the surface and imposed conditions behave independently. A change of imposed inversion condition leaves the bottom 1/3 of the layer almost completely unaffected; a change of the surface condition leaves the top 2/3 unaffected. Comparisons are made against two sets of local-scaling systems over the full height of the boundary layer. Both show some influence of the inversion condition. The surface heat flux and the reduction in surface shear stress, and hence the ratio of the boundary-layer height to surface Obukhov length, are determined by the temperature difference across the surface layer (not the whole layer), bringing all cases together in single correlations as functions of a surface-layer bulk Richardson number.

scholarly journals Wind-Tunnel Simulation of Approximately Horizontally Homogeneous Stable Atmospheric Boundary Layers

2021 ◽  
Author(s):  
Philip E. Hancock ◽  
Paul Hayden
Keyword(s):  
Boundary Layer ◽  
Reynolds Stresses ◽  
Atmospheric Boundary Layers ◽  
Mean Velocity ◽  
Local Scaling ◽  
Obukhov Length ◽  
Wind Tunnel Simulation ◽  
Roughness Lengths ◽  
The Mean ◽  
Vertical Height

AbstractTwo cases of an overlying inversion imposed on a stable boundary layer are investigated, extending the work of Hancock and Hayden (Boundary-Layer Meteorol 168:29–57, 2018; 175:93–112, 2020). Vertical profiles of Reynolds stresses and heat flux show closely horizontally homogeneous behaviour over a streamwise fetch of more than eight boundary-layer heights. However, profiles of mean temperature and velocity show closely horizontally homogeneous behaviour only in the top two-thirds of the boundary layer. In the lower one-third the temperature decreases with fetch, directly as a consequence of heat transfer to the surface. A weaker effect is seen in the mean velocity profiles, curiously, such that the gradient Richardson number is invariant with fetch, while various other quantities are not. Stability leads to a ‘blocking’ of vertical influence. Inferred aerodynamic and thermal roughness lengths increase with fetch, while the former is constant in the neutral case, as expected. Favourable validation comparisons are made against two sets of local-scaling systems over the full depth of the boundary layer. Close concurrence is seen for all stable cases for z/L < 0.2, where z and L are the vertical height and local Obukhov length, respectively, and over most of the layer for some quantities.

Prediction of the Current Structure Under Drifting Pack Ice

1988 ◽  
Vol 110 (4) ◽  
pp. 395-402
Author(s):  
D. Myrhaug
Keyword(s):  
Boundary Layer ◽  
Surface Shear Stress ◽  
Atmospheric Boundary Layers ◽  
Surface Shear ◽  
Eddy Viscosity Model ◽  
Ice Surface ◽  
Sea Bed ◽  
Pack Ice ◽  
Model Predictions ◽  
Rough Sea

A simple analytical theory which describes the motion in a turbulent planetary boundary layer near a rough sea bed by using a two-layer eddy viscosity model is presented. An inverted boundary layer similar to that at the sea bed is applicable under the pack ice. The vertical structure of the current in the boundary layer is presented, and comparisons are made with data from McPhee and Smith [1] obtained from measurements of the turbulent boundary layer under drifting Arctic pack ice. The present model predictions for the drag coefficient at the ice surface and the direction of the surface shear stress are also compared with corresponding formulas given in Davenport [2], which are obtained as fits to observations in atmospheric boundary layers.

The boundary layer over an impulsively started flat plate

1969 ◽  
Vol 310 (1502) ◽  
pp. 401-414 ◽  
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Surface Shear Stress ◽  
Unsteady State ◽  
Similarity Condition ◽  
Surface Shear ◽  
Boundary Layer Problem ◽  
Independent Variables ◽  
Selection Of ◽  
Layer Problem

A numerical solution has been obtained for the development of the flow from the initial unsteady state described by Rayleigh to the ultimate steady state described by Blasius. The usual formulation of the problem in two independent variables is dropped, and three independent variables, in space and time, are reverted to. The boundary-layer problem is unconventional in that the boundary conditions are not completely known. Instead, it is known that the solution should satisfy a similarity condition, and use is made of this to obtain a solution by iteration. A finite-difference technique of a mixed, explicit-implicit, type is employed. The iteration converges rapidly. It is terminated where the maximum errors are estimated to be about 0.04%. A selection of the results for the velocity profiles and the surface shear stress is presented. One striking feature is the rapidity of the transition from the Rayleigh to the Blasius state. The change is practically complete, at a given station on the plate, by the time the plate has moved a distance equal to four times the distance from the station to the leading edge of the plate.

scholarly journals Simultaneous Free Convection and Heat Generation on a Horizontal Isothermal Circular Cylinder

2015 ◽  
Vol 9 (12) ◽  
pp. 21 ◽  
Author(s):  
Sajjad Sedighi ◽  
Mohammad Saeed Aghighi
Keyword(s):  
Boundary Layer ◽  
Nusselt Number ◽  
Heat Generation ◽  
Heat Rate ◽  
Horizontal Cylinder ◽  
Surface Shear Stress ◽  
Surface Shear ◽  
Boundary Layer Equations ◽  
Linear Partial Differential Equations ◽  
Dimensional Boundary

<p class="zhengwen"><span lang="EN-GB">The linear boundary layer of the free flow around a circular horizontal cylinder with uniform surface temperature in the presence of heat generation was studied. Upon obtaining the non-dimensional boundary layer equations, the Runge-Kutta series method was used to solve the non-linear partial differential equations numerically. The surface shear stress results and surface heat rate were subsequently obtained in terms of the internal shell friction and the local Nusselt number respectively. The following heat generation parameters (C) were selected:  0.0, 0.2, 0.5, 0.8, and 1.0. The following results were obtained: 1) increasing C led to a corresponding increase u, v , VM , and θ , 2) Increasing i led to a corresponding increase in u, v , and VM, and 3) increasing C increased velocity variations and, naturally, the value of Cf, and 4) increasing i from i=0 to i=100 led to a decrease in the Nusselt number (Nu). </span></p>

The Effects of Localized Blade Endwall Suction on Secondary Flows and Heat Transfer

2010 ◽  
Author(s):  
Rebecca Hollis ◽  
Jeffrey P. Bons
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
High Surface ◽  
Surface Heat ◽  
Secondary Flows ◽  
Surface Shear Stress ◽  
Mean Velocity ◽  
Surface Shear ◽  
Surface Heat Transfer ◽  
High Heat Transfer

Two methods of flow control were designed to mitigate the effects of the horseshoe vortex structure (HV) at an airfoil/endwall junction. An experimental study was conducted to quantify the effects of localized boundary layer removal on surface heat transfer in a low-speed wind tunnel. A transient infrared technique was used to measure the convective heat transfer values along the surface surrounding the juncture. Particle image velocimetry was used to collect the time-mean velocity vectors of the flow field across three planes of interest. Boundary layer suction was applied through a thin slot cut into the leading edge of the airfoil at two locations. The first, referred to as Method 1, was directly along the endwall, the second, Method 2, was located at a height ∼1/3 of the approaching boundary layer height. Five suction rates were tested; 0%, 6.5%, 11%, 15% and 20% of the approaching boundary layer mass flow was removed at a constant rate. Both methods reduced the effects of the HV with increasing suction on the symmetry, 0.5-D and 1-D planes. Method 2 yielded a greater reduction in surface heat transfer but Method 1 outperformed Method 2 aerodynamically by completely removing the HV structure when 11% suction was applied. This method however produced other adverse effects such as high surface shear stress and localized areas of high heat transfer near the slot edges at high suction rates.

scholarly journals Dependency of particle size distribution at dust emission on friction velocity and atmospheric boundary-layer stability

2020 ◽  
Vol 20 (21) ◽  
pp. 12939-12953
Author(s):  
Yaping Shao ◽  
Jie Zhang ◽  
Masahide Ishizuka ◽  
Masao Mikami ◽  
John Leys ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Particle Size ◽  
Probability Distribution ◽  
Particle Size Distribution ◽  
Atmospheric Boundary Layer ◽  
Size Distribution ◽  
Turbulent Flows ◽  
Friction Velocity ◽  
Surface Shear Stress ◽  
Surface Shear

Abstract. Particle size distribution of dust at emission (dust PSD) is an essential quantity to estimate in dust studies. It has been recognized in earlier research that dust PSD is dependent on soil properties (e.g. whether soil is sand or clay) and friction velocity, u∗, which is a surrogate for surface shear stress and a descriptor for saltation-bombardment intensity. This recognition has been challenged in some recent papers, causing a debate on whether dust PSD is “invariant” and the search for its justification. In this paper, we analyse the dust PSD measured in the Japan Australian Dust Experiment and show that dust PSD is dependent on u∗ and on atmospheric boundary-layer (ABL) stability. By simple theoretical and numerical analysis, we explain the two reasons for the latter dependency, which are both related to enhanced saltation bombardment in convective turbulent flows. First, u∗ is stochastic and its probability distribution profoundly influences the magnitude of the mean saltation flux due to the non-linear relationship between saltation flux and u∗. Second, in unstable conditions, turbulence is usually stronger, which leads to higher saltation-bombardment intensity. This study confirms that dust PSD depends on u∗ and, more precisely, on the probability distribution of u∗, which in turn is dependent on ABL stability; consequently, dust PSD is also dependent on ABL. We also show that the dependency of dust PSD on u∗ and ABL stability is made complicated by soil surface conditions. In general, our analysis reinforces the basic conceptual understanding that dust PSD depends on saltation bombardment and inter-particle cohesion.

Note on Water Drag on Sea Ice for Different Surface Roughness

1991 ◽  
Vol 113 (1) ◽  
pp. 67-69
Author(s):  
D. Myrhaug
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Shear Stress ◽  
Sea Ice ◽  
Turbulent Flow ◽  
Drag Coefficient ◽  
Turbulent Flows ◽  
Surface Shear Stress ◽  
Surface Shear ◽  
Ice Surface

The approach in Myrhaug [1], where a simple analytical theory describing the motion in a turbulent planetary boundary layer near a rough seabed was presented, is extended to smooth and transitional smooth-to-rough turbulent flow. An inverted boundary layer similar to that at the seabed is applicable under the sea ice. The water drag coefficient at the ice surface and the direction of the surface shear stress are presented for rough, smooth and transitional turbulent flows.

Experimental approaches towards interpreting dolphin-stimulated bioluminescence.

1998 ◽  
Vol 201 (9) ◽  
pp. 1447-1460 ◽  
Author(s):  
J Rohr ◽  
M I Latz ◽  
S Fallon ◽  
J C Nauen ◽  
E Hendricks
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Flow Visualization ◽  
Leading Edge ◽  
Field Studies ◽  
Flow Speed ◽  
The Body ◽  
Surface Shear Stress ◽  
Literature Report ◽  
Surface Shear

Flow-induced bioluminescence provides a unique opportunity for visualizing the flow field around a swimming dolphin. Unfortunately, previous descriptions of dolphin-stimulated bioluminescence have been largely anecdotal and often conflicting. Most references in the scientific literature report an absence of bioluminescence on the dolphin body, which has been invariably assumed to be indicative of laminar flow. However, hydrodynamicists have yet to find compelling evidence that the flow remains laminar over most of the body. The present study integrates laboratory, computational and field approaches to begin to assess the utility of using bioluminescence as a method for flow visualization by relating fundamental characteristics of the flow to the stimulation of naturally occurring luminescent plankton. Laboratory experiments using fully developed pipe flow revealed that the bioluminescent organisms identified in the field studies can be stimulated in both laminar and turbulent flow when shear stress values exceed approximately 0.1 N m-2. Computational studies of an idealized hydrodynamic representation of a dolphin (modeled as a 6:1 ellipsoid), gliding at a speed of 2 m s-1, predicted suprathreshold surface shear stress values everywhere on the model, regardless of whether the boundary layer flow was laminar or turbulent. Laboratory flow visualization of a sphere demonstrated that the intensity of bioluminescence decreased with increasing flow speed due to the thinning of the boundary layer, while flow separation caused a dramatic increase in intensity due to the significantly greater volume of stimulating flow in the wake. Intensified video recordings of dolphins gliding at speeds of approximately 2 m s-1 confirmed that brilliant displays of bioluminescence occurred on the body of the dolphin. The distribution and intensity of bioluminescence suggest that the flow remained attached over most of the body. A conspicuous lack of bioluminescence was often observed on the dolphin rostrum and melon and on the leading edge of the dorsal and pectoral fins, where the boundary layer is thought to be thinnest. To differentiate between effects related to the thickness of the stimulatory boundary layer and those due to the latency of the bioluminescence response and the upstream depletion of bioluminescence, laboratory and dolphin studies of forced separation and laminar-to-turbulent transition were conducted. The observed pattern of stimulated bioluminescence is consistent with the hypothesis that bioluminescent intensity is directly related to the thickness of the boundary layer.

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