Donald J. Bergstrom

Rough Wall Turbulent Boundary Layers in Shallow Open Channel Flow

An experimental study was undertaken to investigate the effects of roughness on the structure of turbulent boundary layers in open channels. The study was carried out using a laser Doppler anemometer in shallow flows for three different types of rough surface, as well as a hydraulically smooth surface. The flow Reynolds number based on the boundary layer momentum thickness ranged from 1400 to 4000. The boundary layer thickness was comparable with the depth of flow and the turbulence intensity in the channel flow varied from 2 to 4 percent. The defect profile was correlated using an approach which allowed both the skin friction and wake strength to vary. The wake parameter was observed to vary significantly with the type of surface roughness in contradiction to the wall similarity hypothesis. Wall roughness also led to higher turbulence levels in the outer region of the boundary layer. The profound effect of surface roughness on the outer region as well as the effect of channel turbulence on the main flow indicates a strong interaction, which must be accounted for in turbulence models

A dynamic nonlinear subgrid-scale stress model

In this paper, a dynamic subgrid scale (SGS) stress model based on Speziale’s quadratic nonlinear constitutive relation [C. G. Speziale, J. Fluid Mech. 178, 459 (1987); T. B. Gatski and C. G. Speziale, J. Fluid Mech. 254, 59 (1993)] is proposed, which includes the conventional dynamic SGS model as its first-order approximation. The closure method utilizes both the symmetric and antisymmetric parts of the resolved velocity gradient, and allows for a nonlinear anisotropic representation of the SGS stress tensor. Unlike the conventional Smagorinsky type modeling approaches, the proposed model does not require an alignment between the SGS stress tensor and the resolved strain rate tensor. It exhibits significant flexibility in self-calibration of the model coefficients, and local stability without the need for plane averaging to avoid excessive backscatter of SGS turbulence kinetic energy and potential modeling singularity problems. It also allows for variable tensorial geometric relations between the SGS str...

A study on the effects of wind on the air intake flow rate of a cooling tower: Part 2. Wind wall study

Abstract Cooling towers operating in western Canada are often subject to adverse weather, which can hinder their performance. In particular, during winter months, cooling towers may experience strong winds and sub-freezing temperatures, which can lead to large ice formations at the windward intake. These ice accumulations may be partly attributed to increased flow through the windward intake. Part 1 of this study showed that strong winds can increase the windward intake flow rate by as much as 45%. The present study examines the use of wind walls placed upstream of the cooling tower to control the flow rate entering the intakes. A 1:25 scale model cooling tower was tested in the simulated atmospheric boundary layer of a wind tunnel to investigate the effect of different wind wall configurations. The results show that a simple wind wall placed upstream of the windward intake can be used to balance the flow rate into the two intakes.

Application of power laws to low Reynolds number boundary layers on smooth and rough surfaces

Scaling laws for the overlap region of near-wall turbulent flows are of particular interest to turbulence researchers and engineers. For the mean flow at sufficiently high Reynolds numbers, the classical boundary layer theory proposes a logarithmic law for the overlap region. On the other hand, at low Reynolds numbers, refined measurements and direct numerical simulation results indicate that the log law region becomes negligibly small. Instead, power laws have received increasing attention as an alternative formulation for the overlap region at low Reynolds numbers. In the present study, we use open channel flow measurements to assess the ability of the power laws proposed by Barenblatt [J. Fluid Mech. 248, 513 (1993)] and George and Castillo [Appl. Mech. Rev. 50, 689 (1997)] to describe the overlap region in low Reynolds number boundary layers on smooth and rough surfaces. The skin friction laws derived from the power laws are also used to estimate the friction velocity, which values are then compared t...

Numerical Simulation of Transient Heat Transfer in a Protective Clothing System during a Flash Fire Exposure

A finite volume model was developed to simulate the transient heat transfer in a protective clothing system. The model domain consists of a fire-resistant fabric, the human skin, and the air gap between the fabric and the skin. The model uses a more sophisticated treatment of the air gap compared to previous models: it accounts for transient combined conduction-radiation heat transfer within the air gap and includes the variation in the air gap properties with temperature. Predictions were obtained for the temperature and heat flux distributions in the fabric, skin, and air gap as a function of time, as well as the time to receive skin burn injuries. The numerical model was used to explore the physics of heat transfer in protective clothing, which could potentially be used to improve the performance of this clothing. This study illustrates the dependence of the temporal behavior of the heat fluxes on the specific model assumptions, as well as the associated sensitivity of skin burn predictions to these assumptions.

Calculation of wall-mass transfer rates in separated aqueous flow using a low Reynolds number κ-ε model

Abstract Mass transfer in aqueous, turbulent flow through a sudden pipe expansion is simulated with a low Reynolds number (LRN) κ-e eddy viscosity model. The predicted wall-mass transfer rates are tested against experimental data, obtained with electrochemical measurements ( Re = 2.1−13 × 10 4 and Sc = 1460). LRN modifications to the turbulence model in the near-wall regions, coupled with the turbulent Schmidt number concept, enable successful predictions of wall-mass transfer rates to be obtained. For the specific case of a high Schmidt number fluid, the mass transfer boundary layer is much thinner than the hydrodynamic boundary layer. Furthermore, even low levels of turbulence in the near-wall region are shown to have significant influence on the overall wall-mass transport.

Predictive models for erosion-corrosion under disturbed flow conditions

Abstract Recent advances in the development of predictive models for erosion-corrosion in disturbed turbulent flows are reviewed. The application of turbulence models permits the structure (velocity, pressure, turbulence fields) of the complex flow to be determined along with the calculation of local mass transfer rates of reactants and products. Particle/wall interaction statistics that are required for the application of erosion models are also determined.

Skin friction correlation for smooth and rough wall turbulent boundary layers

In this paper, we propose a novel skin friction correlation for a zero pressure gradient turbulent boundary layer over surfaces with different roughness characteristics. The experimental data sets were obtained on a hydraulically smooth and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. The flow Reynolds number based on momentum thickness ranged from 3730 to 13,550. We propose a correlation that relates the skin friction, Cf, to the ratio of the displacement and boundary layer thicknesses, δ*∕δ, which is valid for both smooth and rough wall flows. The results indicate that the ratio Cf1∕2∕(δ*∕δ) is approximately constant, irrespective of the Reynolds number and surface condition.

New dynamic subgrid-scale heat flux models for large-eddy simulation of thermal convection based on the general gradient diffusion hypothesis

Three new dynamic tensor thermal diffusivity subgrid-scale (SGS) heat flux (HF) models are proposed for large-eddy simulation of thermal convection. The constitutive relations for the proposed modelling approaches represent the most general explicit algebraic formulations possible for the family of SGS HF models constructed using the resolved temperature gradient and SGS stress tensor. As a result, these three new models include a number of previously proposed dynamic SGS HF models as special cases. In contrast to the classical dynamic eddy thermal diffusivity SGS HF model, which strictly requires the SGS heat flux be aligned with the negative of the resolved temperature gradient, the three new models proposed here admit more degrees of freedom, and consequently provide a more realistic geometrical and physical representation of the SGS HF vector. To validate the proposed models, numerical simulations have been performed based on two benchmark test cases of neutrally and unstably stratified horizontal channel flows.

Power laws for rough wall turbulent boundary layers

An assessment of the ability of power laws to describe the mean velocity profile in the overlap region of a zero pressure gradient turbulent boundary layer is reported. The experiments were performed in a wind tunnel on smooth and four different types of rough surfaces at moderate Reynolds numbers. A novel modification to the power law velocity profile is proposed to account for the effect of surface roughness in the overlap region. This modification is analogous to the use of a roughness function to produce a downward shift in the logarithmic velocity profile. The roughness parameters in the proposed equation more accurately follow the effect of roughness on skin friction than does the roughness shift ΔU + . The present study shows that power laws can be used to effectively describe the mean velocity profile over a wider range than a log law for both smooth and rough surfaces.