Inflow Froude Numbers Research Articles

Environmental Impact of Undular Tidal Bores in Tropical Rivers

A tidal bore impacts significantly on the estuarine ecosystem, although little is known on the flow field, mixing and sediment motion beneath tidal bores. In the absence of detailed systematic field measurements, a quasi-steady flow analogy was applied to investigate undular tidal bores with inflow Froude numbers between 1.25 and 1.6. Experimental results indicated that rapid flow redistributions occur beneath the free-surface undulations, with significant variations in bed shear stress between wave crests and troughs. Dynamic similarity was used to predict detailed flow characteristics of undular tidal bores. The effects of periodic loading on river sediments, scour of river bed and flow mixing behind the bore are discussed. A better understanding of these processes will contribute to better management practices in tidal bore affected rivers, including the Styx and Daly rivers in tropical Australia.

Physical modelling and similitude of air bubble entrainment at vertical circular plunging jets

When a plunging jet impinges into a pool of liquid, air bubble entrainment takes place if the inflow velocity exceeds a threshold velocity. This study investigates air entrainment and bubble dispersion in the developing flow region of vertical circular plunging jets. Three scale models were used and detailed air–water measurements (void fraction, bubble count rate, bubble sizes) were performed systematically for identical inflow Froude numbers. The results highlight that the modelling of plunging jet based upon a Froude similitude is affected by significant scale effects when the approach flow conditions satisfied We 1<1 E+3, while some lesser scale effect was noticed for V 1/ u r <10 and We 1>1 E+3. Bubble chord time measurements showed pseudo-chord sizes of entrained bubbles ranging from less than 0.5 mm to more than 10 mm with an average pseudo-chord size were between 4 and 9 mm . However, bubble size data could not be scaled properly.

Alvin Peterka—Hydraulic Engineer

An energy dissipator is a hydraulic structure used to convert hydraulic energy mainly into heat. These dissipators have a relatively short history because they were developed only after damage had occurred at the toe of overflow structures. The German Theodor Rehbock ~1864–1950! conducted the first detailed research into these structures, although hydraulic jumps had received attention as early as the 1820s by the Italian Giorgio Bidone ~1781–1839! by imposing constant energy head across the hydraulic jump. In 1838, Jean-Baptiste Belanger ~1790–1874! correctly applied the momentum equation for the prediction of sequent depths, and Henry Bazin ~1829–1917! verified this concept with limited observations. Nothing much happened until around 1910–1920 when Arnold Gibson ~1878–1959!, the successor of Osborne Reynolds at Manchester University, presented experiments that corroborated the predictions of Belanger. That concept obtained final acceptance by the 1916 paper of the American Karl R. Kennison ~1886–1977!, whose closure verified data of various sources ~Hager 1990!. At about the same time, Rehbock proposed his ‘‘dentated sill.’’ It was later realized that this device mainly deflects remaining bottom currents to the free surface and thus inhibits scour downstream of the end of the stilling basin. The development of stilling basins was then continued mainly by the Austrian Armin Schoklitsch ~1888–1969! with his two books ~Schoklitsch 1930, 1935! and by Hunter Rouse ~1906–1996! from the conceptual side by introducing the Froude number as the significant parameter for all quantities of that phenomenon ~Rouse 1934!. In 1933, the International Commission of Large Dams ICOLD was founded with exponents such as Andre Coyne ~1891–1960!, who in the late 1930s proposed the ski jump type dissipator as an alternative to the conventional hydraulic jump dissipators. The latter continued to be a standard design mainly for relatively small velocities up to a maximum of about 20 m/s and inflow Froude numbers essentially between three and six. A number of elements had been proposed to reduce the length of the stilling basin but without a definite consideration of the various pros and cons. By the end of World War II, hundreds of different types of energy dissipators were available, partly developed for particular site conditions and partly presented as an engineer’s design from the office table. What was definitely missing was a thorough scientific consideration of energy dissipators under a wide variety of flow conditions and proposals relating to engineering applications. Here, Peterka has contributed significantly and was even able to come up with a series of standard designs to be applied under well-defined flow conditions without the need for additional observations. The engineering community thus derived a great benefit from his services. For completeness, the contributions to that subject by Fred W. Blaisdell ~1911–1998!, of the Saint Anthony Falls Hydraulic Laboratory should also be mentioned.

Classical hydraulic jump: free surface profile

Based on a large number of experiments, a simple formula is developed for the time-averaged free surface profile of a classical hydraulic jump. This novel approach is based on the length of the roller. The resulting surface profile fits the data well for usual inflow Froude numbers in the range of 2 to 10. Key words: backwater, channel flow, hydraulics, open channel, surface profile.

Hydraulic Jump Type Stilling Basin for Low Froude Numbers

Hydraulic jumps for Froude numbers below 4.5 have oscillating charactertistics. Economical design of stilling basins at such low Froude numbers poses a problem. Laboratory models of common designs of hydraulic jump type stilling basins for low inflow Froude numbers were studied. As a result of the studies an efficient stilling basin design using chute blocks and wedge-shaped baffle piers of vertex angle 150° cut back at an angle of 90° can be developed.