John M. Cimbala

Differential effect of steady versus oscillating flow on bone cells

Loading induced fluid flow has recently been proposed as an important biophysical signal in bone mechanotransduction. Fluid flow resulting from activities which load the skeleton such as standing, locomotion, or postural muscle activity are predicted to be dynamic in nature and include a relatively small static component. However, in vitro fluid flow experiments with bone cells to date have been conducted using steady or pulsing flow profiles only. In this study we exposed osteoblast-like hFOB 1.19 cells (immortalized human fetal osteoblasts) to precisely controlled dynamic fluid flow profiles of saline supplemented with 2% fetal bovine serum while monitoring intracellular calcium concentration with the fluorescent dye fura-2. Applied flows included steady flow resulting in a wall shear stress of 2 N m(-2), oscillating flow (+/-2 Nm(-2)), and pulsing flow (0 to 2 N m(-2)). The dynamic flows were applied with sinusoidal profiles of 0.5, 1.0, and 2.0 Hz. We found that oscillating flow was a much less potent stimulator of bone cells than either steady or pulsing flow. Furthermore, a decrease in responsiveness with increasing frequency was observed for the dynamic flows. In both cases a reduction in responsiveness coincides with a reduction in the net fluid transport of the flow profile. Thus. these findings support the hypothesis that the response of bone cells to fluid flow is dependent on chemotransport effects.

Large structure in the far wakes of two-dimensional bluff bodies

Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Karman-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased. The far-wake structure does not depend directly on the scale or frequency of Karman vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re [approximate] 6000), which do not initially shed Karman-type vortices into the wake. Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.

The effect of jet injection geometry on two-dimensional momentumless wakes

It is shown experimentally that a two-dimensional momentumless wake is strongly dependent on the jet injection configuration of the model. Namely, the decay rate of mean velocity overshoot ranged from X O . ~ * to x2.0 for three different configurations, while the spreading rate ranged from x O . ~ to x0.46 for those same configurations. The magnitude of axial turbulence intensity was also found to depend on model configuration. On the other hand, the rate of decay of axial turbulence intensity was the same (x-O.*l) for all three models. In all cases the mean shear and Reynolds stress decayed rapidly, leaving nearly isotropic turbulence beyond 30 or 40 model diameters. Appropriate lengthand velocity scales are identified which normalize the mean velocity profiles into self-similar form. The shape of t,he normalized profile, however, was different for each configuration, indicating again that t,he initial conditions are felt very far downstream.

AN EXPERIMENTAL INVESTIGATION OF THE TURBULENT STRUCTURE IN A TWO-DIMENSIONAL MOMENTUMLESS WAKE

Mean velocity profiles have been measured in the wake of a two-dimensional airfoil with and without mass injection through a slit along its rear end. In particular, four cases have been documented: ( a ) a pure wake (no injection), ( b ) a weak wake (some injection), ( c ) a momentumless wake (injection adjusted to provide a thrust which exactly cancels the models drag), and ( d ) a weak jet (more injection than necessary to cancel the drag). These mean velocity profiles clearly show the difference in momentum deficit for the four cases. When non-dimensionalized, the velocity profiles are self-similar. Smoke-wire flow visualizations are also presented for both the near wake (0 x/d x/d For the momentumless wake at Re = 5400, the axial, lateral, and transverse turbulence intensities as well as the Reynolds stress were measured. Similarity of the axial and the transverse turbulence intensities was observed; the overall shape of those profiles is Gaussian except for the very near-wake region. The mean centreline velocity difference decays much faster ( x −0.92 ) than the axial turbulent intensity ( x −0.81 ). Consequently, the mean shear practically disappears far downstream; the flow becomes nearly isotropic beyond about 45 body diameters from the model. This turbulence behaviour is quite different from that of plane wakes or jets but rather closer to the case of grid turbulence.

Indoor Air Quality Engineering : Environmental Health and Control of Indoor Pollutants

Contents Preface -- Contents -- Nomenclature -- Introduction -- The Respiratory System -- Design Criteria -- Estimation of Pollutant Emission Rates -- General Ventilation and the Well-Mixed Model -- Present Local Ventilation Practice -- Ideal Flow -- Motion of Particles -- Removing Particles from a Gas Stream -- Application of CFD to Indoor Air Quality -- Appendices -- References -- Index. Other Notes Written by experts, Indoor Air Quality Engineering offers practical strategies to construct, test, modify, and renovate industrial structures and processes to minimize and inhibit contaminant formation, distribution, and accumulation. The authors analyze the chemical and physical phenomena affecting contaminant generation to optimize system function and design, improve human health and safety, and reduce odors, fumes, particles, gases, and toxins within a variety of interior environments. The book includes applications in Microsoft Excel®, Mathcad®, and Fluent® for analysis of contaminant concentration in various flow fields and air pollution control devices.

Suppression of the wing-body junction vortex by body surface suction

A horseshoe-shaped vortex, known as a wing-body junction vortex or horseshoe vortex, forms when spanwise vorticity in the boundary layer along a surface wraps around a wing protruding from the surface. In the past, various techniques of suppressing the wing-body junction vortex have been attempted. Reported here is a novel approach whereby the oncoming wall boundary layer is removed by suction along the body surface immediately upstream of the wing. The idea is that elimination of the boundary layer essentially removes the spanwise vorticity, and inhibits the formation of a wing-body junction vortex. To test this concept experimentally, velocity data were acquired via five-hole probe surveys in a plane normal to one of the walls of a semi-infinite symmetrical airfoil. Differentiation of these data yielded mean stream wise vorticity contours and values of net circulation. In the unmodified (no suction) case, the streamwise leg of the horseshoe vortex was clearly identified. The addition of suction successfully reduced the size and circulation of the large scale vortex. In fact, at a suction volumetric flow rate of about twice that through the boundary layer, the large scale horseshoe vortex could no longer be found for our configuration.

Experimental Investigation of a Jet Impinging on a Ground Plane in Crossflow

An experimental investigation has been conducted in a wind tunnel to model the impingement of high-velocity jet exhaust flow on the ground, as encountered by V/STOL aircraft. A constant jet velocity was maintained while varying the wind tunnel crossflow velocity, upstream boundary-layer thickness, and height from the ground to the jet exit plane. The radial wall jet, when interacting with the crossflow, forms an oscillating horseshoe-shaped separation bubble, commonly referred to in the literature as a ground vortex. The streamwise distance of the separation point from the jet impingement point is documented here as a function of the flow parameters and geometry. Flow visualization of the flowfield and two-component laser Doppler velocimeter measurements taken through the separation bubble indicate that the separation bubble is highly unsteady and nonsymmetric. This unsteadiness may be related to shear-layer vortices shed from the lip of the jet. Thickening of the upstream boundary layer on the ground plane caused the wall jet to penetrate further upstream. The addition of a large plate flush-mounted to the jet exit caused the ground vortex to move downstream and also decreased the size of the ground vortex.

Analysis of Poultry House Ventilation Using Computational Fluid Dynamics

The airflow in and around poultry houses was studied numerically with the goal of determining the disease spread characteristics and comparing two ventilation schemes. A typical manure-belt laying hen egg production facility was considered. The continuity, momentum, and energy equations were solved for flow both inside and outside poultry houses using the commercial computational fluid dynamics (CFD) code FLUENT. The geometry was constructed by making some simplifying assumptions, such as two-dimensionality. The spread of virus particles was considered analogous to diffusion of a tracer contaminant gas, in this case ammonia (NH3). The effect of thermal plumes produced by the hens in the poultry house was also taken into consideration. Two ventilation schemes with opposite flow directions were compared. Contours of temperature and contaminant mass fraction for both cases were obtained and compared. The analysis shows that ventilation and air quality characteristics were much better for the case in which the airflow was from bottom to top instead of from top to bottom (top to bottom is how most current poultry houses are configured). This has implications for air quality control in the event of epidemic outbreaks. Decreased contaminant spread to downwind poultry houses was observed in the bottom-to-top airflow scheme.

Application of neutron radiography for fluid flow visualization

Real-time thermal neutron radiography has been applied to the visualization of fluid flows. Since neutrons can penetrate metal casings, the technique may be useful for the visualization of fluids flowing inside metal enclosures, such as valves, engine or transmission components, etc. The technique described involves shadowgraph imaging of neutron-opaque tracer materials (either solid or fluid particles) as they convert in a stream of neutron-transport ambient fluid. Real-time motion pictures of several simple flows have been recorded, from which velocities, regions of flow separation, rate of mixing, and other information about the flow field can be obtained. The neutron radiography facility at the Penn State Breazeale Nuclear Reactor and the studies performed to determine viable liquids useful in neutron radiography applications are described. Some samples of successful flow visualizations are also presented.

Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.