Michael Pfister

Hydraulic design of A-type Piano Key Weirs

Piano Key Weirs (PKWs) are an alternative to linear overflow structures, increasing the unit discharge for similar heads and spillway widths. Thus, they allow to operate reservoirs with elevated supply levels, thereby providing additional storage volume. As they are relatively novel structures, few design criteria are available. Hence, physical model tests of prototypes are required. This study describes comprehensive model tests on a sectional set-up of several A-type PKWs, in which the relevant parameters were systematically varied. Considering data of former studies, a general design equation relating to the head–discharge ratio is derived and discussed. The latter is mainly a function of the approach flow head, the developed crest length, the inlet key height, and the transverse width. To extend its application range, case study model tests were analysed to provide a design approach if reservoir approach flow instead of channel flow is considered.

Chute Aerators. I: Air Transport Characteristics

Keywords: Chute Aerators Note: [849] Reference EPFL-ARTICLE-180342doi:10.1061/(ASCE)HY.1943-7900.0000189View record in Web of Science Record created on 2012-07-29, modified on 2016-08-09

Two-phase air-water flows: scale effects in physical modeling

Physical modeling represents probably the oldest design tool in hydraulic engineering together with analytical approaches. In free surface flows, the similitude based upon a Froude similarity allows for a correct representation of the dominant forces, namely gravity and inertia. As a result fluid flow properties such as the capillary forces and the viscous forces might be incorrectly reproduced, affecting the air entrainment and transport capacity of a high-speed model flow. Small physical models operating under a Froude similitude systematically underestimate the air entrainment rate and air-water interfacial properties. To limit scale effects, minimal values of Reynolds or Weber number have to be respected. The present article summarizes the physical background of such limitations and their combination in terms of the Morton number. Based upon a literature review, the existing limits are presented and discussed, resulting in a series of more conservative recommendations in terms of air concentration scaling. For other air-water flow parameters, the selection of the criteria to assess scale effects is critical because some parameters (e.g., bubble sizes, turbulent scales) can be affected by scale effects, even in relatively large laboratory models.

Closure of “Chute Aerators. I: Air Transport Characteristics

Keywords: Chute Aerators Note: [849] Reference EPFL-ARTICLE-180342doi:10.1061/(ASCE)HY.1943-7900.0000189View record in Web of Science Record created on 2012-07-29, modified on 2016-08-09

Chute aerators. II: Hydraulic design.

Chute aerators are applied if cavitation damage on spillways is expected or observed. The aerator efficiency is usually described with the ratio of the air discharge entrained through the air supply ducts and the water discharge, which does however not account for the resulting air concentration distribution within the flow or for air detrainment. The present study investigates the streamwise development of the air transport along the flow downstream of chute aerators. Based on an extensive test program in which six governing parameters were systematically varied, the development of the average and the bottom air concentrations is provided up to the self- aeration point. Based on this information, an optimization of aerators in terms of increased air entrainment and reduced detrainment rates is possible, by assuming minimum required air concentrations. The main parameters influencing the air transport downstream of aerators are the approach flow Froude number, the deflector angle and the chute bottom angle.

Discussion to Scale effects in physical hydraulic engineering models

Beside analytical approaches, physical modelling represents probably the oldest design tool in hydraulic engineering. It is thus a pleasure to see this Forum Paper in JHR. The Discussers focus on one aspect of the publication, thereby specifying the information of the Forum Paper. Free surface flows are typically scaled with the Froude similitude keeping identical F = V /(gh)0.5 both in the model and in the prototype. The air transport in models is affected by scale effects because the internal flow turbulence, represented by the Reynolds number R = Vh/ν, is underestimated, while surface tension, represented by the Weber number W = (ρV 2h)/σ , is overestimated (Chanson 2009), with V = flow velocity, g = gravity constant, h = flow depth, ρ = water density, σ = water surface tension, and ν = water kinematic viscosity. Because a strict dynamic similitude exists only at a full-scale, the underestimation of the air transport is minimized if limitations in terms of W or R are respected. The Forum Paper overlooks a number of aspects and probably recommends too optimistic limitations. As stated in Table D1, the literature mentions limitations around W = 110–170 and R = 1.0–2.5 × 105. These values focus on air entrainment at hydraulic jumps, general chute air entrainment and aerated stepped spillway flows, as well as the air entrainment coefficient β and the streamwise bottom air concentration Cb generated by chute aerators. Pfister and Hager (2010a, b) identified an underestimation up to one magnitude in terms of Cb if W < 140 (Fig. D1). There, the abscissa corresponds to the streamwise normalization given by these authors, and the trend lines correspond to the best fit of all Cb curves from tests with W ≥ 140, i.e. without significant scale effects. As can be noted from Table D1, two criteria are often applied relating to the herein discussed scale effects, i.e. limiting values for W and R for a range of air–water flow parameters. This results in an over-determined system, as the two numbers depend on each other, besides F and the Morton number M. The latter characterizes the shape of bubbles or drops moving in a surrounding medium, solely as a function of the fluid properties and the gravity constant (Wood 1991, Chanson 1997). With a negligible inner bubble density, as is typical for air– water flows, the Morton number is with μ = dynamic water viscosityKeywords: scale effects, engineering models Note: [830] Reference EPFL-ARTICLE-176040doi:10.1080/00221686.2012.654671View record in Web of Science Record created on 2012-04-05, modified on 2016-08-09

Discussion of "Skimming, Nonaerated Flow on Stepped Spillways over Roller Compacted Concrete Dams" by Ines Meireles, Floriana Renna, Jorge Matos, and Fabian Bombardelli

AbstractThe nonaerated region may occupy a large portion of the skimming flow in steep, stepped spillways, particularly for relatively high unit flow rates. In spite of the numerous contributions on the hydraulic properties at both the inception point of air entrainment and the aerated region, much less is known regarding the flow in the nonaerated region. In this paper, new empirical evidence, based on an extensive data set obtained during several years in a large-scale facility, sheds light on the features of the nonaerated-flow region. Diverse ways to locate and estimate the main hydraulic properties at the inception point are first discussed and compared. Then, expressions capable of characterizing the main flow variables along the nonaerated region are presented, namely, the boundary-layer development, the velocity distribution, the equivalent clear-water depth, the characteristic depth taking into account the free-surface unsteadiness due to turbulence, and the energy dissipation. The energy dissipa...

Debris-Blocking Sensitivity of Piano Key Weirs under Reservoir-Type Approach Flow

The collection of floating woody debris at flow control structures, such as spillways and weirs, can potentially result in reduced discharge efficiency (higher upstream head for a given weir discharge). Compared to less hydraulically-efficient control structures, piano key weirs have higher discharge efficiency (lower upstream heads for a given discharge), which may increase the likelihood of woody debris collection. A systematic laboratory study was conducted to evaluate the interaction between various piano key weir geometries and woody debris types and sizes. The results of individual (noncumulative) debris tests indicated that floating debris blockage probability is highly influenced by trunk diameter and upstream head. The effects of debris accumulation on the upstream head varied with the value of the debrisfree reference upstream head condition. At lower upstream reference head values, the cumulative debris tests indicated a relative increase of the debris-associated upstream head of approximately 70%; higher upstream reference head values produced upstream head increases limited to approximately 20%.

Chute Aerators: Preaerated Approach Flow

Long spillways often include more than one chute aerator to assure an appropriate cavitation protection. The first aerator guarantees sufficient bottom air concentration along a limited streamwise distance attributed to deaeration. Further downstream, the air concentration may be insufficient to protect the chute from cavitation damage. There, a second aerator is required whose operation is affected by preaerated approach flow. The present investigation systematically model-tested typical chute aerators with various approach flow features including preaeration. The resulting air entrainment coefficient as the ratio of air discharge entrained into the flow by the water discharge remains practically unaffected. The jet length is also not affected. Downstream of these aerators, the streamwise development of the average and the bottom air concentrations was affected by preaeration: the average value increased, whereas the bottom value marginally decreased. The effect of preaeration was found to be only relevant for average approach flow concentrations exceeding some 20% of the corresponding uniform flow concentration.

Head losses in junction manholes for free surface flows in circular conduits

Former studies on combining flows resulted in an efficient layout of sewer junctions operated under supercritical approach flow conditions. Straight extensions allowed a reduction in the shock wave heights generated by the merging flows, so that the global discharge capacity was significantly increased. Herein, an extensive experimental campaign is presented on a physical model with the aforementioned layout, although with generalized geometrical conditions now including various conduit diameters. The effects of the main parameters governing the energy losses for combining flows were ascertained to enhance the information available from the literature. The results and their analysis provide a basis for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters and various flow conditions.