Unconfined Flow Conditions Research Articles

Influence of confinement on pressure and heat transfer distribution by impinging an air jet from a piccolo tube on a concave surface

Experimental work is carried out to understand the effect of longitudinal confinement on wall static pressure and heat transfer distribution on a concave surface by impingement of an air jet from a piccolo tube (orifice). The experiments are conducted for different parameters such as orifice diameter (d = 10–16 mm), curvature ratio (D/d = 2–5), circumferential angle (θ = 0°−40°), orifice to target surface distance (Z/d = 1–4), confinement distance to the orifice diameter ratio (l/d = 15–9.375) and the Reynolds number of flow (Re = 5000–50,000). It is observed that the maximum wall static pressure and the Nusselt number occur at a stagnation point (X/d = 0 and θ = 0°) for all Z/d and l/d ratios. For confined flow, the value of wall static pressure (Cp and Cpo ) and the Nusselt number (Nu and Nuo ) distribution are higher when compared to unconfined flow conditions. The results of the study can be used in optimising the cooling system design.

On the flow behaviour of confined finite-length wavy cylinders

Confined aspect-ratio of 6 wavy cylinders with a mean blockage-ratio of 0.5 were studied using time-resolved particle-image velocimetry at a sub-critical Reynolds number of 2700. Wavelengths and wave amplitudes of 2–4 and 0.1–0.3 mean diameters respectively were investigated. Results show that vortices are generally shed from the wavy cylinder and channel walls regularly, reminiscent of the unsteady symmetric flow configuration in confined non-wavy cylinders. Furthermore, vortex formation lengths for confined wavy cylinders are generally shorter than their unconfined counterparts, though their variations with respect to geometrical changes remain consistent with unconfined flow conditions. Gross cross-stream flow behaviour does not differ significantly between confined and unconfined wavy cylinders, indicating that finite-length effects are independent of the present confinement. Confined wavy cylinder wake regions are more sensitive towards geometrical changes and a combination of small wavelength and large wave amplitude leads to significant suppression of coherent cylinder and wall vortex-shedding. This is supported by phase-averaged flow reconstructions derived from Proper Orthogonal Decomposition analysis. Lastly, larger wave amplitudes lead to redistributions of dominant flow energy further downstream and to higher mode numbers, which suggests a causal link to the formation of stronger and more coherent streamwise vortices.

Groundwater Recharge Assessment in the Chateauguay River Watershed

The objective of this study was to evaluate groundwater recharge in the Chateauguay River watershed. The Hydrologic Evaluation of Landfill Performance (HELP) model was used to assess daily values of recharge, evapotranspiration and runoff. The study area was divided into a regular grid, 250 m × 250 m, for a total of 47,616 grid elements. The input parameters included soil physical properties, land use, vegetation and climate data. Calibration of HELP was carried out against runoff and baseflow estimates obtained from separation of five river hydrographs. Over a 39 year period, the mean annual recharge rate was estimated at 86 mm, or 9% of the total precipitation. Areas characterized by high water level elevations and unconfined flow conditions were identified as the main recharge areas. Daily estimates show that recharge takes place mainly in spring and fall. Over the observed period, the annual variations of evapotranspiration and runoff were directly related to changes in precipitation, whereas the annual recharge response was subdued, with much lower variations. HELP was also used to assess potential climate change scenarios using data for the driest and most humid years. The mean annual recharge was 51 mm for the driest year and 99 mm for the most humid year. Differences in the spatial distribution of recharge for the predictive scenarios indicate that the areas most sensitive to climate change correspond to the preferential recharge areas.

Wall Effects in Two-Dimensional Axisymmetric Flow Over a Circular Disk Oriented Normal to Flow in a Cylindrical Tube

The effect of cylindrical walls on the drag of a neutrally buoyant disk (with zero inertia) oriented normal to the direction of flow has been investigated numerically. Using FLUENT, the field equations have been solved to obtain the values of the total, pressure and friction components of drag as a function of the Reynolds number and of the blockage ratio. The results presented herein encompass the following ranges of the conditions: Reynolds number: 1 to 100; and the blockage ratio (disk diameter/cylinder diameter): 0.02 to 0.5. As expected, the drag is found to be higher under the confined flow conditions than that in the unconfined flow conditions. However, the confining boundaries appear to exert only a weak influence on the formation and size of the wake region. The drag values are also weakly influenced by the thickness of the disk. However, this effect progressively disappears as the Reynolds number is gradually increased. The present numerical values corresponding to the unconfined flow conditions are in excellent agreement with the previous numerical and experimental results available in the literature.

Morphometric approaches to describing rip current behaviour

Rip currents are an integral component of intermediate beaches as described by the WRIGHT and SHORT (1984) model, but their morphologic character and behaviour at various states has yet to be suitably quantified. This study utilises field measurements encompassing an almost complete cycle of low-energy beach state evolution to describe morphometric approaches for quantifying rip current behaviour. First, a subjective analyses of the relative planimetric areal extent of rip feeder, rip-neck and non-rip (bar) components showed that active rip areas (AR) were never greater than 50% of the total system area (AT) with AR decreasing from 40-45% during the longshore bar-trough (LBT) state to less than 20% during the low tide terrace (LTT) state. Second, the concept of overfit and underfit rip currents (SHORT, 1985) is examined by describing topographically confined and unconfined flow conditions through the use of the Rip Fit Parameter; RF = (M-O)/Ar, where M = morphologic cross-sectional rip channel area; O = overbank cross-sectional rip area, and Ar = total cross-sectional rip area. In general, low-energy rips are unconfined at high tide for all intermediate states and are confined when RF > 0.5 during transverse bar-rip and low-tide terrace conditions at low-tide. Confined rips are also associated with maximum rip flow velocities and can be related to planimetric rip area. Both morphometric approaches provide new and useful quantification of intermediate beach state rip current morphology during accretionary conditions.

On the residence time distribution in idealized groundwatersheds

The relative cumulative frequency distribution of residence times F( T) is calculated for an entire groundwatershed under steady-state conditions and assuming Dupuit-Forchheimer flow. It appears that F( T) is always the same: F(T) = 1 - exp( −T T) , provided that the aquifer recharge rate and T are constant over the groundwatershed. T is the weighted mean residence time for the groundwatershed and is defined at T = n H N , where n is the aquifer porosity, H is the saturated aquifer thickness, and N the areal recharge rate owing to precipitation. In such an idealized groundwatershed the function F( T) appears independent of the groundwatershed size, shape, and nature of the stream network. It is also independent of regional variations in the hydraulic conductivity, provided the aquifer is locally homogeneous. Under unconfined flow conditions, where H varies, the relative frequency distribution of the residence time does depend on all of these parameters, but may be approximated by F( T), as demonstrated for the case of a one-dimensional groundwatershed. The frequency distribution of the residence times, dF dT , is the response curve for an instantaneous unit pulse of a conservative tracer, applied over the entire groundwatershed. The findings are of significance for studying the effects of non-point source pollution on the scale of one or more watersheds.

Estimation of Unsteady, Three‐Dimensional Drawdown in Single, Vertically Heterogeneous Aquifers

AbstractA quasi, a mixed, and a weighted three‐dimensional model are applied to approximate the three‐dimensional, unsteady drawdown in vertical pumping and observation wells, fully or partially penetrating a single, vertically heterogeneous, anisotropic aquifer. Cases of confined, semiconfined, and unconfined flow conditions are considered. Numerical examples are used to quantify the numerical error of the methods, introduced by the vertical heterogeneity of the aquifer. Results of pumping test data analysis in a three‐layer, anisotropic aquifer with multiple screens in the pumping well are presented.

Basin inversion: some consequences for drainage evolution and alluvial architecture

ABSTRACT Sedimentary basins developed in tectonically active areas commonly display dramatic changes in sedimentation rates, styles and spatial organization of alluvial facies. When deformation rates are relatively high these changes are frequently attributed to deflection and/or ponding of the fluvial systems. This paper examines the indirect modifications that less intense, epeirogenic uplift may instigate, during the latter stages of basin inversion, in the context of a fluvial system adjusting to the deformation. Field evidence is taken from the Sorbas Basin of south‐east Spain and data are employed from both the current drainage pattern and ancient (Plio‐Quaternary) sedimentary sequence. Major provenance and sedimentation changes in the ancient record (in terms of both rates and styles) are attributed to river capture. Intrinsic controls to the system (fan progradation and trenching) and the prevailing (semi‐arid) climate provided the overall framework of fan development (general coarsening upwards sequence and change from sheetflood to braided conditions in the lower part of the sequence). This sedimentation trend was then substantially modified (a change to small meandering channels and a change in provenance) by the capture of the fan feeder by an aggressive subsequent stream. This cut off the connection with the former source area and reduced sediment and water input into the system. Initially the sedimentation rates declined and minor incision was stimulated by the increase in the water to sediment ratio as a result of the reduction in easily erodible sand to the system that resulted from the source area change. This minor incision stimulated a rejuvenation of the streams sourcing the fan, temporarily increasing sediment supply and choking the small channels, leading to a return to unconfined flow conditions. The system rapidly exhausted the sediment from the reduced catchment area leading to a fining upwards and abandonment of the system. This was followed by incision of the modern drainage network stimulated by base level changes in the main Sorbas Basin drainage network, to which the study area was tributary.

Simulation of groundwater flow in complex multiaquifer systems: Performance of a quasi three-dimensional technique in the steady-state case

The ability of the classical quasi three-dimensional formulation to describe steady-state groundwater flow problems in complex multiaquifer environments is examined. In the present formulation, discontinuities in the aquifer and aquitard units can be accommodated along with partial or complete aquifer dewatering and confined or unconfined flow conditions. Some of the main assumptions underlying classical quasi three-dimensional schemes are scrutinized, including the requirement of a two orders of magnitude permeability contrast between aquifers and aquitards. Performance of the numerical scheme is studied through a series of test problems by comparing with results obtained from a conventional finite element model. A high degree of accuracy and flexibility is achieved with the extended quasi three-dimensional technique, yet the numerical efficiency inherent in the classical formulation is maintained. By dividing an aquifer into a series of horizontal sublayers, vertical resolution of the flow field can be achieved without resorting to a numerically intensive fully three-dimensional scheme. Because it is possible to compute a three-dimensional representation of the hydraulic head distribution in individual aquifers with the quasi three-dimensional formulation, even in the absence of layers of contrasting hydraulic conductivity, the technique provides a viable alternative to the much more complex fully three-dimensional schemes for a wide variety of groundwater flow problems. Key words: groundwater flow, multiaquifer, complex stratigraphy, numerical analysis, quasi three-dimensional, steady state.

Performance of the tolerant tunnel for bluff body testing

A new passive form of low-boundary-correction wind tunnel test section is under development. In the version designed for testing two-dimensional bluff bodies the two boundaries opposite the model under test consists of arrays of evenly spaced airfoils at zero incidence. The spacing is selected so that the outer streamlines of the test section flow can pass into an outer plenum and return to the test section downstream, in such a way that the overall streamline pattern closely approximates the corresponding free-air pattern. The design is passive in that a fixed optimized boundary configuration is used for all sizes and shapes of test model. The theoretical background of the design is described, and experimental pressure distributions are presented for several sizes each of normal flat plates, circular cylinders, and circular cylinders with splitter plates, over a range of boundary airfoil spacings. It is shown that a single optimal spacing exists for which the pressure distributions for all sizes of each bluff body shape collapse onto or very close to appropriate reference curves representing unconfined flow conditions.

Recovery in Large Diameter Wells

A closed-form approximate analytical solution for recovery in fully penetrating large diameter wells tapping a confined aquifer is presented. The solutions are also applicable for unconfined flow under conditions of small drawndown. The analysis predicts exponential decreases of inflow rate, a transient logarithmic recovery response, and an asymptotic increase of comulative recovery with time. The solution is verified against the field recovery response in a wide range of geological aquifer formations, e.g., fissured rock, lateritic as well as sandy alluvial deposits. The predicted response and the actual field response for all the types of aquifers mentioned are found to match extremely well. The available theoretical time-recovery relationship for unconfined flow conditions approaches to the present theory of confined aquifer under smaller drawdown.

Application of the Adaptive Wall Concept in Three Dimensions

The adaptive wall concept requires that to obtain unconfined flow conditions in a wind tunnel in the presence of any model it is necessary and sufficient to match independent measured flow variables on a surface near the tunnel boundary with flow variables which satisfy the unconfined, undisturbed boundary condition at infinity. The technique was employed at a limited number of test conditions to adjust individually the variable porosity walls of the AEDC Aerodynamic Wind Tunnel (4T) in order to minimize wall interference effects on a generalized transonic model configuration. Data improvement was demonstrated for each set of conditions. A procedural description and results of the experiment are presented. Nomenclature CD = drag coefficient CL = lift coefficient Cm = pitching moment coefficient CP = pressure coefficient CTS = captive trajectory system h = tunnel width k = relaxation factor MO, = Mach number P = pressure S = control surface s = model wing span v — perturbation velocity normal to top and bottom walls w = perturbation velocity normal to side walls x = longitudinal dimension y = lateral dimension, see Fig. 3 z = vertical dimension, see Fig. 3 a = angle of attack r = wall porosity, % = perturbation velocity potential (nondimensionalized)

Tunnel interference reduction on a finite airfoil

HE ventilated wind tunnel has been introduced and refined over the last two decades from the fixed geometry to the variable porosity perforated walls. Both theoretical results1 and experimental data2 have demonstrated that it was difficult to eliminate pitching-moment interference of an aircraft model simultaneously with lift interference when using walls with uniformly distributed porosity. The evidence3 in the experimental development of walls for V/STOL testing indicates that it becomes possible to simultaneously eliminate pitching moment and lift interference in a slotted tunnel with nonuniformly distributed porosity. The interference reduction concept has been demonstrated by the author's previous paper4 in which a mathematical technique is presented to predict the interference on an airfoil represented by a single singularity in a Gaussian-type distribution of porosity. The present paper extends the mathematical technique to the case of a finite chord airfoil to predict the proper porosity distribution to eliminate the interference. The present calculation becomes urgent and necessary for an experimental program developing a new wall. On the other hand, the self-correcting wind tunnel concept5'6 which refers to the adaptive wall, has been demonstrated by a numerical method7 to achieve interference-free flow conditions. This wind tunnel requires the provisions either to adjust the deflections of the wall surfaces, or to adjust the wall porosity and plenum pressure behind the wall to establish unconfined flow conditions. The present result of interference calculation is primarily for the improvement of the current existing nonadaptive wall wind tunnel. Analysis The airfoil is placed at the centerline of a perforated tunnel having walls with nonuniform distribution of porosities. The tunnel interference on a model is divided into two parts in the investigation, i.e. the blockage and lift interferences. Based on the thin airfoil theory, the airfoil thickness and camber may be treated separately. The blockage and lift interferences are induced by the presence of the airfoil thickness and camber in the tunnel, respectively. The formulation for both types of interferences is essentially similar, except for the different mathematical representations of the airfoil thickness and camber. The small perturbation theory for subsonic flow is used in the analysis with a homogeneous boundary conModel in a Tunnel