Eddy turnover time

This time scale is usually referred to as the eddy turnover time. 

Equation (3.82) illustrates the importance of the scalar spectral energy transfer rate in determining the scalar dissipation rate in high-Reynolds-number turbulent flows. Indeed, near spectral equilibrium, 7 (/cd, 0 (like Tu(kDi, 0) will vary on time scales of the order of the eddy turnover time re, while the characteristic time scale of (3.82) is xn <[Pg.99]

The scalar spectral energy transfer rate XA m)- t) will vary on time scales proportional to the eddy turnover time re. At high Reynolds numbers, (3.131) and (3.132) quickly attain a quasi-steady state wherein... 

The flame lift-off height, which is related to the ignition distance, was inversely affected by the excitation frequency. Since the flow time scale decreased with increasing frequency, the data were plotted as a function of the Damkohler number in Fig. 29.14, where the characteristic flow time scale was estimated by large-eddy turnover time as 1/17 and the characteristic chemical reaction time was computed using an ignition delay model [21] for ethylene jet. While the results did not show any evidence of critical Damkohler number, the range... 

To determine the turbulent reaction rate, conceptualize the reaction as a single-step process where the fuel and oxidant are converted to a product. Furthermore, visualize a turbulent flame as a collection of burning eddies. For a reaction to take place, the fuel and oxidant must be simultaneously available, and the reaction product must be removed properly. That is, the reaction can be limited by the availability of fuel, oxidant, or by the removal of product. Once the fuel and oxidant are mixed, combustion takes place, on average, within one eddy turnover time, k/e, so that the reaction rate can be written as... 

Unfortunately, the simplified engineering method doesn t provide much information about the structure of turbulence and the dynamics of the interactions between the turbulent vortexes. The only structural information extracted from the second moments in relation to the structure of turbulence is the length scale. A, and the only information in relation to the dynamics of turbulence is the time constant representing the eddy turnover time, A A A ... 

Using the dissipation rate (e) as the only relevant flow parameter in the v —> 0 limit, from dimensional considerations the typical fluctuations of the velocity field over a distance l and the corresponding characteristic timescales r(Z), usually interpreted as eddy turnover time , can be estimated as... 

This can be proven by correlating the droplet concentration c with the acceleration within the gas flow field AU JAx = 0). It can be stated that there is no significant correlation between the local droplet concentration and acceleration stagnation points. Thus, the basic cluster structure in the middle of the spray is formed over a period of time that is larger than the average eddy-turnover time of the large shear layer vortices. But nevertheless, the acceleration/deceleration mechanism is of major importance since it is repeated several times and thus is a factor for triggering the droplet cluster stmcture. 

Figure 34 Steady-state SOM mean residence times derived from eddy covariance data plotted against MAT. Bold solid curve is Lloyd and Taylor fit bold dashed curve is the 2io best fit and light dashed curve represents a linear regression wit, the common log of turnover time (source Sanderman et aL, 2003).

Francko and Heath (1979) studied the influence of UV radiation on the phosphate complexation properties of dissolved humic substances from Crazy Eddie Bog in the central United States. In this bog, filterable phosphorus compounds were largely associated with humic substances of high apparent molecular weight. These fulvic acid-phosphorus associations resisted hydrolysis by alkaline phosphatase, but released orthophosphate upon irradiation with low doses of UV radiation, " e turnover time of the fulvic acid-phosphorus compounds was calculated to be less than 1 hour at the surface of a lake on a cloudless day. 

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