Doğal havalandırma, havadaki oksijenin suya olan transferi olarak tanımlanmaktadır. Doğal havalandırmayı sağlayan üç tane hidrodinamik süreç vardır: (1) hidrolik sıçrama, (2) su düşümü, (3) basamaklı kanallardan ilerleyen akım. Bu yapıların bir diğer ortak özelliği hidrolik yapılarda enerji kırıcı olarak kullanılmalarıdır. Bu noktadan hareketle, doğal havalandırma verimliliğiyle enerji kırılması arasında pozitif bir ilişkinin bulunması beklenmektedir. Bu çalışmada, bu hidrodinamik süreçlerden, hidrolik sıçrama olayı deneysel olarak incelenmiştir. Doğal havalandırmayı sağlayan bu hidrodinamik süreçlerde, termodinamiğin 1. prensibine dayanılarak, kaybolan enerjiden büyük ölçekli çevrilerin sorumlu olduğu ve bu çevrilerin yaptığı işin suya oksijen transferini sağlayan mekanizma olduğu yaklaşımında bulunulmuştur. Bu hipotezi desteklemek için laboratuvarda sistematik deneyler yapılmıştır. Deneyler, genişliği 0.5 m olan bir açık kanalda gerçekleştirilmiştir ve deneylerde arasında, ve arasında değerler almıştır. Transfer olan oksijen miktarıyla, kırılan enerji miktarı arasında kuvvetli bir ilişki bulunmuştur. Deneysel verilerin analizi sonucu, hidrolik sıçramanın havalandırma verimliliğini yük kaybının ve birim genişlikten geçen debinin fonksiyonu olarak tahmin eden bir formül geliştirilmiştir. Havalandırma verimliliği yük kaybının ¾’üncü, birim genişlikten geçen debinin ise ¼’üncü kuvvetiyle ilişkilendirilmiştir. Hidrolik sıçrama boyuncaki akım doğrultusundaki türbülans şiddeti eksponansiyel olarak azalan, türbülans kinetik enerjisi ise lineer olarak azalan bir fonksiyon sergilemiştir ve bu gidiş hidrolik sıçramadaki hava konsantarasyonu dağılımıyla uyuşmaktadır. Elde edilen deneysel bulgular, hidrolik sıçramdaki yük kaybının, ortamdaki türbülans kinetik enerjisini temsil ettiğini işaret etmektedir ve bu süreç yüzey yenilenme teoremini desteklemektedir. Anahtar Kelimeler: Doğal havalandırma, hidrolik sıçrama, su kalitesi, türbülans.Hydraulic jump takes place at the transition from supercritical regime to subcritical regime and characterized by highly turbulent flow, macro-scale vortices, kinetic energy dissipation and bubbly two-phase flow due to air entrainment (Leutheusser et al. 1973; Hager 1992). In the literature, studies about hydraulic jump aeration efficiency are very limited (Avery and Novak 1978; Wilhelms et al. 1981). The term self-aeration means transfer of oxygen from air towards water (Gulliver 1990) and it has important environmental and ecological implications for polluted streams. Aeration efficiency is calculated with a formula suggested by Gamenson (1957): where, E denotes aeration efficiency which has a range between 0, for no aeration, and 1, for total downstream saturation. Cu and Cd are dissolved oxygen concentrations at the upstream and downstream of a hydraulic structure, respectively and Cs is the mass concentration of dissolved oxygen under saturated conditions. Although, most of researches emphasized that turbulence structure is the main mechanism promoting to oxygen transfer, (Kobus 1991; Sene et al. 1994; Chanson 1996; Ervine 1998, El-Kamash et al. 2005) simultaneous measurements of turbulence quantities and dissolved oxygen are very limited. Indeed, hydrodynamic processes which ensure the self-aeration mechanism such as: (1) hydraulic jump, (2) plunging jet or water fall, and (3) stepped channels, have other common property: they are also used as energy dissipaters at hydraulic structures. The aim of this experimental study is to investigate aeration performance of hydraulic jumps in terms of energy loss approach and to reveal the turbulence?s important role in the process. Accordingly, experiments were carried out in a horizontal rectangular flume of glass sidewalls and concrete bottom 0.5 m wide, 0.45 m deep and 12.30 m long. Hydraulic jumps were generated downstream of a sluice gate placed 6.5 m distance from the beginning of the channel and location of the jumps were controlled by a tail gate installed at the outlet of the flume. Hydraulic jumps were grouped at seven different unit discharges with a range of and F1 took values between 2.1-6.4. During the experiments, a digital video camera was employed to determine the hydraulic jump and roller lengths. Dissolved oxygen (DO) measurements were conducted simultaneously at the upstream and downstream of the hydraulic jump with handheld oxygen meters (Model WTW Oxi 330i) which has a an accuracy of ±0.5% of oxygen concentration and ± 0.1% of temperature in the range of 0-19.99 mg/L DO and -5 to +50 oC of temperature. The air calibration technique was used in the experiments and the sampling rate of the oxygen probe was 1 Hz. In order to drop the DO content of the approaching flow, sodium sulfide and cobalt chlorite as a catalyst were added into chamber. Turbulence quantities were collected by a Nortek 10 Mhz type Acoustic Doppler Velocimeter (ADV) at 25 Hz frequency for two minute sampling time. The root mean squares (RMS) of the turbulent fluctuation velocities in the longitudinal, vertical and lateral directions, respectively, were determined by using WinADV32 software. Hydraulic jump turbulence structure was investigated at low Froude numbers: F1=1.9 and F1=3 because at higher Froude numbers, ADV does not make accurate measurements due to bubbly two-phase flow (Liu et al. 2004). Aeration efficiency took values between 0.01-0.1 and a strong correlation was found between the aeration efficiency and head loss indicating the large-scale surface renewal eddy's dominant role in the process. It was found that aeration efficiency is in proportion to the ¾ power of head loss and ¼ power of unit discharge. Turbulent characteristics exhibit decreasing exponential function through the hydraulic jump and this trend matches with the air concentration distribution. Moreover, the data reveals that head loss term represents the degree of turbulence related with the macro-scale vortices occurred in the roller length region. These findings are in agreement with the surface renewal theory, which states that the rate of mass transfer is a function of the surface renewal which is ensured by large-scale organized eddy motions near the free surface. Keywords: Self-aeration, hydraulic jump, energy dissipation, turbulence.
