Gravity currents, also known as density currents, or buoyancy currents, are happened by the density difference between the flow and its ambient fluid. Gravity current happen in geophysical environments. One of the most important types of these streams that occur in large-scale nature is density flow (Middleton 1993). These flows are happen due to gravity and its effect on the density gradient. When the gravity current arrived to ambient fluid, in the position that density of both gravity current and ambient fluid is equal the gravity current abandon the bed and flows in ambient fluid horizontally )He et al, 2016(. Some examples of gravity currents found in nature katabatic winds thunderstorms, dust storms (haboobs), turbidity currents running down the continental shelf, and advancing cold fronts )Thompson, 1986(. Examples of gravity currents found in industrial processes are the spread of heavy gases and also ventilation systems that utilize buoyancy to drive horizontal flows along the ceiling and floor )Samothrakis and cotel, 2006(. Other examples of gravity currents that are encountered in nature include snow avalanches )Hopfinger and Tochon-Danguy, 1977( and pyroclastic flows (Valentine, 1987). There are also examples of man-made gravity currents with industrial applications. Such an example is the instantaneous release of a dense gas in a less dense environment, after the failure of a containment tank (Baines, 2001). Simpson (1997) has detailed descriptions of these and other examples of gravity currents and also provides an overview of the research performed on the subjectIn dams reserve ambient fluid, usually has a vertical stratification. Therefore, the density current into this reservoirs may flow as interflow. In this study, the density current was investigated in ambient stratification fluid. For Experimental Investigation of VELOCITY PROFILE in interflow density current at stratification ambient, experiments were performed by 4 discharge 1, 1.5, 2 and 2.5 liters per second, and 4 concentrations 5, 10, 15 and 20 grams per liters on the bed slope 2.5, 3.25 and 4 percent. Stratification of ambient fluid was carried out by mixture water and salt with a deep concentration gradient. To create density flow silica particles with an average diameter of 8 microns was used.

A physical model of gabion overflow dams was studied to determine the VELOCITY PROFILE and Reynolds shear stress.Physical tests were done under two different conditions of dam crest, overflow dams with impermeable and with permeable crests.Instantaneous VELOCITY components over dam crest were measured by an ADV (Acoustic Doppler Velocimeter) instrument. This instrument is capable of measuring instantaneous VELOCITY components with frequencies up to 25 Hz. Average VELOCITY components and bed shear stress were extracted from ADV measurements. The results of this research show the effect of crest permeability on VELOCITY and Reynolds shear stress. The magnitude of Reynolds sheer stresses, horizontal VELOCITY components, and absolute value of vertical VELOCITY components under the permeable scenario are bigger than those of the impermeable scenario. VELOCITY distribution over the dam crest is different from the universal logarithmic PROFILE.

In fixed bed reactors, VELOCITY distributions have a direct effect on heat transfer properties. Therefore, in this study we first examined different flow models and solved the modified Brinkman flow model numerically where cannot be tackled analytically. Then, from a heat transfer point of view, plug and non-plug flow models with respect to velocities obtained from the modified Brinkman model have been solved and its effect on Biot number have been examined. In this paper, it has been shown that the correct choice of flow model can have a great impact on the heat transfer properties of such systems.    

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Understanding the ecological conditions of vegetation growth in water sources is vital to appraise the influence of vegetation on river engineering. Based on the experimental information that is accessible, the consequences of vegetation on flow resistance is described as an alteration in the drag coefficient and the planned area.  The current study analytically estimates the vertical distribution of stream-wise VELOCITY in open-channel flow while considering rigid and flexible vegetation. The flow is vertically separated into top free water layer and bottom vegetation layer using the projected deflection height of both vegetation. Related momentum calculations for each layer are then derived. Based on the gathered experimental data, a 3D numerical model with various simulation situations is used to model, calibrate, and evaluate the artificial cylinders. A considerable deflection analysis is utilised to calculate the VELOCITY-dependent stem height. This has proven to be more precise compared to formerly deflection investigation. The estimated outcomes show that precise predictions may be made for the vertical contours of vertical Reynolds shear stress based on mean horizontal VELOCITY. The numerical simulations demonstrate that plant flexibility reduces the vertical Reynolds shear stress and prompted flow resistance force of the vegetation.

River bed PROFILEs and depth-averaged velocities are used as basic data in empirical and analytical equations for estimating the longitudinal dispersion coefficient which has always been a topic of great interest for researchers. The simple model proposed by Maghrebi is capable of predicting the normalized isovel contours in the cross section of rivers and channels as well as the depth-averaged VELOCITY PROFILEs. The required data in Maghrebi’s model are bed PROFILE, shear stress, and roughness distributions. Comparison of depth-averaged velocities and longitudinal dispersion coefficients observed in the field data and those predicted by Maghrebi’s model revealed that Maghrebi’s model had an acceptable accuracy in predicting depthnaveraged VELOCITY.

Considering reservoir sedimentation phenomenon and turbidity current as the major factor of happening problems (such as reducing water storage capacity, water passages and gates clogging, depositing in water conveyance channels, reducing water resources quality and etc. ) which cause environmental and financial damages, it is essential to study the various aspects of turbidity currents in creating sustainable management of reservoirs as the main source of water and energy supply. Almost all of the research studies, so far, have been conducted under experimental conditions on rigid beds. In this study, in order to achieve more realistic results regarding to natural conditions in rivers and reservoirs, the experiments were conducted on a mobile bed. Since the body VELOCITY PROFILE of turbidity current has an important effect on generation and destruction of different types of bed forms, characteristics of the body VELOCITY PROFILEs were analyzed on a rigid and erodible bed for different effective hydraulic conditions. In this study, at first according to Special Specifications of turbidity current (very low VELOCITY in small experimental flumes), comprehensive study had been conducted on bed material properties and its required pre-experiments. Finally, the properties of the material used in this study were 450 µ m of average diameter (d50), 1070 kg/m3 specific gravity (ρ s) and 1. 45 of standard deviation (σ g). In order to simulate the turbidity current, an experimental model consisting of an inclined flume with length of 780 cm, height of 70 cm and width of 35 cm, was used. Bed material was situated at the bottom space of bed with height of 8 cm and length of 4. 5 m. VELOCITY PROFILEs were determined in four cross sections (at the certain distances of 1. 5, 2. 5, 3. 5 and 4. 5 m from the gate) using acoustic Doppler VELOCITY PROFILEr (DOP 2000). Overall 48 experiments were conducted which included 36 main experiments on mobile bed changing flow discharge (1, 1. 5, 2. 5 and 3. 5 l/s), bed slope (0, 1. 5%, 2. 5% and 3. 5%) and volumetric concentration of sediment (15%, 20% and 25%) and 12 control experiments on rigid bed at flow discharge of 1. 5 l/s. According to the results, with increasing in the flow concentration, due to increase in flow momentum, in both rigid and mobile bed condition, flow VELOCITY was increased. It is considerable that the percent increase in VELOCITY with change of concentration, was less in mobile than rigid bed condition (about 50%) which could be due to the resistance of the bed material particle and especially bed forms roughness against the flow as long as the bed form is generated. Exactly in experiment condition with bed slope of 2. 5 and 3. 5%, in which the bed forms were washed out and destructed due to increase in bed shear stress, the increasing trend of flow VELOCITY got improved. Also, in all the experiments, the values of maximum VELOCITY in the mobile bed condition decreased specifically and the percent of decrease increased with increasing in the bed slope. Since the bed material particle are active in mobile bed condition, therefore the bed roughness and resistance increases so that acts as resistant force against the flow and results in decrease in flow VELOCITY (up to about 30%). Comparison of dimensionless body VELOCITY PROFILEs in all experiments showed that the maximum ratio of z/h (the boundary between clear water and a concentrated flow) was more in rigid bed condition (with average value of 2) than the mobile bed condition (with average value of 1. 5). This decrease of z/h ratio in the mobile bed condition (about 25%) was due to decrease in average body VELOCITY PROFILE which caused increase in the elevation corresponding to the same VELOCITY of rigid bed condition. The value of z/h ratio was more in the experiments without bed form which were in accordance with results. In addition, the maximum value of u/U in mobile bed condition was less than rigid bed condition. This confirmed the body VELOCITY decrease in mobile bed condition which was more in the experiments with bed forms. The research data, especially dimensionless VELOCITY PROFILEs in supercritical flows, also were in good agreement with the results. In the present study generation of bed forms leaded to about 40% decrease in z/h ratio and 15% increase in elevation of the maximum of u/U location. Consequently by the results, erodible bed and especially generation of bed forms leaded to decrease the body VELOCITY of turbidity current up to about 30% and also to decrease the rate of increasing VELOCITY, due to increase of current density up to about 50%. Finally, comparison of dimensionless VELOCITY PROFILEs showed that the relative position of the interface of turbidity current was decreased about 25% on erodible bed and even up to 40%, where the bed forms generated.