Effect Of Hydraulic Resistance Research Articles

Resistance to Flow on a Sloping Channel Covered by Dense Vegetation following a Dam Break

AbstractThe effect of hydraulic resistance on the downstream evolution of the water surface profile h in a sloping channel covered by a uniform dense rod canopy following the instantaneous collapse of a dam was examined using flume experiments. Near the head of the advancing wavefront, where h meets the rods, the conventional picture of a turbulent boundary layer was contrasted to a distributed drag force representation. The details of the boundary layer around the rod and any interferences between rods were lumped into a drag coefficient Cd. The study demonstrated the following: In the absence of a canopy, the Ritter solution agreed well with the measurements. When the canopy was represented by an equivalent wall friction as common when employing Manning's formula with constant roughness, it was possible to match the measured wavefront speed but not the precise shape of the water surface profile. However, upon adopting a distributed drag force with a constant Cd, the agreement between measured and modeled h was quite satisfactory at all positions and times. The measurements and model calculations suggested that the shape of h near the wavefront was quasilinear with longitudinal distance for a constant Cd. The computed constant Cd(≈0.4) was surprisingly much smaller than the Cd(≈1) reported in uniform flow experiments with staggered cylinders for the same element Reynolds number. This finding suggested that drag reduction mechanisms associated with unsteadiness, nonuniformity, transient waves, and other flow disturbances were more likely to play a role when compared to conventional sheltering effects.

Bi-level Drainage System Design considering Upward Leakage from Semi-impermeable Barrier

Bi-level drainage system, in which alternate shallow and deep subsurface drains are laid, seems to be an economical option as compared to the level drainage system due to substantial reduction in cost of excavation. Mathematical solutions are available to describe fall of water table between two bi-level drains located at some distance above the horizontal impermeable barrier. But, if the barrier is semi-impermeable, little information is available to account for the vertical upward leakage in the soil system and its effect on spacing, fall of water table and discharge of drains. In the present study, a boundary value problem consisting of one dimensional linearized Boussinesq equation incorporating vertical upward leakage from semi-impermeable barrier with appropriate initial and boundary conditions was formulated. Analytical solution of such linearized equation has been obtained after devising a simple transformation through which boundary value problem was transformed to a heat transfer problem for which a solution was available. Spatial and temporal variation of water table between two bi-level drains was obtained employing this solution. A special case of the proposed solution (without vertical leakage) was verified with the existing solution and identical values of water table heights were obtained. The effect of hydraulic resistance of semi-impermeable barrier on spacing, water table heights between two bi-level and level drains and discharge of drains was studied by considering a numerical example based on assumed and experimental data. It was observed that with increase in the value of hydraulic resistance the spacing and fall of water table between two drains increases. Discharge of the drains is also influenced due to vertical upward leakage. An increase in hydraulic resistance, discharge of the drain decreases. The solution can be employed to determine discharge, spacing and fall of water table between two level or bi-level subsurface drains considering the semi impermeable barrier.

Effects of hydraulic resistance by vegetation on stage dynamics of a stormwater treatment wetland

Summary This work examined the potential effects of large-scale thinning of emergent vegetation on the stage dynamics in a very large (33.3 km2) constructed treatment wetland in South Florida. Dense vegetative biomass in treatment wetlands may restrict water flow and increase water levels, which may in turn have adverse effects on vegetative community structure. Here, we developed a physically-based, spatially-distributed hydrodynamic model of Stormwater Treatment Area 2, Cell 2 (STA2C2) to investigate the spatio-temporal variability of water level (stage) in response to management for thinning of emergent macrophytes (e.g., burning and/or herbicide treatments). The model was calibrated against stage measured at six monitoring stations for 1 year, and subsequently validated against 2 years of stage data from eight stations. Finally, the validated model was extended to simulate various vegetation management scenarios. The model provided an excellent fit to observed stage data in both calibration and validation periods (median model efficiency indices of 0.82 and 0.83, respectively). Higher stages in the treatment cell were dominantly associated with peak inflow magnitude and the timing of event intervals. Prolonged periods of sustained deep water conditions were observed when one flow peak was followed by consecutive peaks. A gradual stage gradient from the inlet to outlet was observed during peak flow periods, with a shift to a sharp gradient at approximately two-thirds distance from the inlet. Stages in the wetland were found to be controlled less by the hydraulic resistance, as indicated by a low sensitivity of simulated water levels for a ±50% perturbation in flow resistance parameter. Water depths were reduced by a maximum of 12 cm at the inlet region by thoroughly thinning the remaining emergent vegetation in STA2C2. Similarly, a maximum of only 2% of the total STA2C2 area was prevented from exceeding a water depth believed to be detrimental to Typha sp. (1.22 m) after the highest peak inflow. Collectively, our findings suggested that vegetation thinning may not be effective for minimizing deep water conditions in STA2C2.

Combustion waves in hydraulically resisted systems

The effects of hydraulic resistance on the burning of confined/obstacle-laden gaseous and gas-permeable solid explosives are discussed on the basis of recent research. Hydraulic resistance is found to induce a new powerful mechanism for the reaction spread (diffusion of pressure) allowing for both fast subsonic as well as supersonic propagation. Hydraulic resistance appears to be of relevance also for the multiplicity of detonation regimes as well as for the transitions from slow conductive to fast convective, choked or detonative burning. A quasi-one-dimensional Fanno-type model for premixed gas combustion in an obstructed channel open at the ignition end is discussed. It is shown that, similar to the closed-end case studied earlier, the hydraulic resistance causes a gradual precompression and preheating of the unburned gas adjacent to the advancing deflagration, which leads (after an extended induction period) to a localized autoignition that triggers an abrupt transition from deflagrative to detonative combustion. In line with the experimental observations, the ignition at the open end greatly encumbers the transition (compared with the closed-end case), and the deflagration practically does not accelerate up to the very transition point. Shchelkin's effect, that ignition at a small distance from the closed end of a tube facilitates the transition, is described.

Modeling turbulent flow from dam break using slow manifolds

We present a novel approach based on centre manifold techniques to describe the large scale dynamics of the mean turbulent dam-break flow. We avoid empirical assumptions about the cross-stream profile of the velocity; instead a solution is obtained using free surface and bed boundary conditions that accommodate constant turbulent shear as an approximately neutral mode. We describe the turbulent dynamics across the flow, and identify important factors affecting the turbulent dissipation in the lateral direction. Available experimental data verify the results. References J. Carr. Applications of centre manifold theory, volume 35 of Applied Mathematical Sciences. Springer--Verlag, 1981. D. J. Georgiev, A. J. Roberts, and D. V. Strunin. The dynamics of the vertical structure of turbulence in flood flows. ANZIAM Journal, 48(CTAC-2006):C573--C590, 2007. http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/124. D. J. Georgiev, A. J. Roberts, and D. V. Strunin. Nonlinear dynamics on centre manifolds describing turbulent floods: $k$-$\omega $ model. Discrete and Continuous Dynamical Systems Supplements, (Special):419--428, 2007. http://aimsciences.org/journals/pdfs.jsp?paperID=2848&mode=full. A. J. Hogg and D. Pritchard. The effects of hydraulic resistance on dam-break and other shallow inertial flows. Journal of Fluid Mechanics, 501:179--212, 2004. I. M. Janosi, D. Jan, K. G. Szabo, and T. Tel. Turbulent drag reduction in dam-break flows. Experiments in Fluids, 37:219--229, 2004. P. Keast and P. H. Muir. {EPDCOL}: A more efficient {PDECOL} code. ACM Transactions on Mathematical Software, 17(2):153--166, 1991. Y. A. Kuznetsov. Elements of applied bifurcation theory, volume 112 of Applied Mathematical Sciences. Springer--Verlag, 1995. N. K. Madsen and R. F. Sincovec. {PDECOL}: General collocation software for partial differential equations. ACM Transactions on Mathematical Software, 5(3):326--351, 1979. I. Nezu. Open-channel flow turbulence and its research prospects in the 21st century. Journal of Hydraulic Engineering, 131(4):229--246, April 2005. T. M. Oezgoekmen, T. Iliescu, P. F. Fischer, A. Srinivasan, and J. Duan. Large eddy simulation of stratified mixing and two-dimensional dam-break problem in a rectagular enclosed domain. Ocean Modelling, 16:106--140, 2007. A. J. Roberts. Low-dimensional modelling of dynamics via computer algebra. Computer Physics Communications, 100:215--230, 1997. A. J. Roberts. Computer algebra describes flow of turbulent floods via the Smagorinsky large eddy closure. Technical report, University of Southern Queensland, 2008. http://eprints.usq.edu.au/4008/. A. J. Roberts, D. J. Georgiev, and D. V. Strunin. Model turbulent floods with the Smagorinski large eddy closure. http://arxiv.org/abs/0805.3192/. P. K. Stansby, A. Chegini, and T. C. D. Barnes. The initial stages of dam-break flow. Journal of Fluid Mechanics, 374:407--424, 1998.

Effects of hydraulic resistance and heat losses on detonability and flammability limits

This paper presents an analysis of a one-dimensional combustion model capable of describing both deflagrations and detonations. Incorporating volumetric terms to account for hydraulic and thermal losses, the quenching diameters, below which each type of combustion wave cannot propagate, are calculated. The main conclusion is that, as expected, detonations have larger quenching diameters than deflagrations for sufficiently high activation energies. However, the opposite result is found for relatively low activation energies.

The effects of hydraulic resistance on dam-break and other shallow inertial flows

The effects of resistive forces on unsteady shallow flows over rigid horizontal boundaries are investigated theoretically. The dynamics of this type of motion are driven by the streamwise gradient of the hydrostatic pressure, which balances the inertia of the fluid and the basal resistance. Drag forces are often negligible provided the fluid is sufficiently deep. However, close to the front of some flows where the depth of the moving layer becomes small, it is possible for drag to substantially influence the motion. Here we consider three aspects of unsteady shallow flows. First we consider a regime in which the drag, inertia and buoyancy (pressure gradient) are formally of the same magnitude throughout the entire current and we construct a new class of similarity solutions for the motion. This reveals the range of solution types possible, which includes those with continuous profiles, those with discontinuous profiles and weak shocks and those which are continuous but have critical points of transition at which the gradients may be discontinuous. Next we analyse one-dimensional dam-break flow and calculate how drag slows the motion. There is always a region close to the front in which drag forces are not negligible. We employ matched asymptotic expansions to combine the flow at the front with the flow in the bulk of the domain and derive theoretical predictions that are compared to laboratory measurements of dam-break flows. Finally we investigate a modified form of dam-break flow in which the vertical profile of the horizontal velocity field is no longer assumed to be uniform. It is found that in the absence of drag it is no longer possible to find a kinematically consistent front of the fluid motion. However the inclusion of drag forces within the region close to the front resolves this difficulty. We calculate velocity and depth profiles within the drag-affected region, and obtain the leading-order expression for the rate at which the fluid propagates when the magnitude of the drag force is modelled using Chae#169;zy, Newtonian and power-law fluid closures; this compares well with experimental data and provide new insights into dam-break flows.

The transition from deflagration to detonation in thin channels

A numerical study of a two-dimensional model for premixed gas combustion in a thin, semi-infinite and thermally-insulated channel is performed. The work is motivated by recent theoretical advances revealing the important role of hydraulic resistance in deflagration-to-detonation transition, one of the central yet still poorly understood phenomena of gaseous combustion. The two-dimensional formulation reproduces the formation of the so-called tulip flame and its predetonational acceleration, well-known experimentally but unattainable within the quasi-one-dimensional approach employed previously. It is shown that the detonation first develops in the boundary layer where the effect of hydraulic resistance is stronger, and thereupon spreads over the channel's interior. However, the second stage of the transition does not proceed gradually but rather through a localized auto-ignition within the tulip.

On propagation limits in porous media combustion

Subsonic combustion in inert porous beds saturated by an explosive premixture is known to occur either as a slow nearly isobaric reaction wave (0.1–10 m s−1) or as a fast wave (100–300 m s−1) with a smooth rise in pressure. Whereas the slow wave is basically a corrugated deflagration driven by molecular transport, the fast wave is sustained by the drag induced diffusion of pressure. In general, the frictional drag, which is rather prominent in porous systems, promotes spontaneous transition from deflagrative to pressure-driven combustion. Practically, however, the transition does not always take place, and the deflagration may well endure without provoking the fast wave. Employing a reduced model for subsonic combustion in inert porous beds accounting for the combined effect of hydraulic resistance and heat losses, the transition-free deflagration is explained in terms of the disparity between the respective propagation limits.The model predicts a jumpwise (hysteretic) transition from the deflagrative to fast-subsonic-regime under gradual elevation of the initial pressure. This effect is likely to be related to an abrupt change-over from conductive to convective burning, known to occur in combustion of granular explosives.

Hydraulic resistance and multiplicity of detonation regimes

The paper considers the effect of hydraulic resistance on detonation of gaseous premixtures. It is shown that the multiplicity of detonation regimes occasionally observed for explosions in rough tubes and in inert porous media may be successfully described in the framework of Zeldovich’s (1940) quasi-one-dimensional formulation, provided the class of admissible solutions is not restricted exclusively to the solutions involving the transonic Chapman-Jouguet (CJ)-plane. Apart from the previously described high-velocity sub-CJ quasi-detonation, the extended analysis yields both the low-velocity, yet supersonic choking regime as well as shockless subsonic detonation, known to occur in porous media combustion.

Effect of hydraulic resistance on a dam-break wave in a channel of constant water depth

Effect of hydraulic resistance on a dam-break wave in a channel of constant water depth

The effects of hydraulic resistance in the dam-break problem

When resistance is neglected, the solution of the simple dam-break problem is readily obtained on the basis of shallow-water theory, and the results are well known. However, near the head of the wave, where the water surface meets the ground, resistance effects cannot be neglected; there is, in fact, a type of boundary layer near the wave-front. In this paper, the Pohlhausen method (which is used in conventional boundary-layer problems) is applied to a study of the effect of this ‘boundary layer’. In particular, the retardation of the wave-front behind the position predicted by the simple theory is found.