Blockage slope and ratio effects on box culvert hydraulics

A numerical study of magnetohydrodynamic flows and heat transfer past a circular cylinder in a duct under a strong magnetic field parallel to the cylinder axis is presented. In this configuration, the flow is quasi-two-dimensional and the modified Navier–Stokes equations are solved in a two–dimensional domain. The numerical simulations have been performed over a range of parameters including the Reynolds number 50 ≤ Re ≤ 3000, modified Hartmann number 50 ≤ Ha⋆ . 500, blockage ratio 0.1 ≤ β ≤ 0.5, offset ratio 0.25 ≤ γ ≤ 1, velocity amplitude 0 ≤ A ≤ 3 and forcing frequency 0 ≤ Ste ≤ 10. The primary aim of this study is to understand the fundamental mechanism that governs transition to unsteady flow in this system and exploit this for further improvements in heat transfer for MHD cooling duct flows. With this aim in mind, a spectral-element method is employed to compute the MHD flow and heat transfer past a confined circular cylinder in a rectangular duct. Meshes have been constructed to deal with the significant number of geometric flow parameters combinations. Thorough validation and grid resolution studies have been performed to ensure adequate domain sizes, and spatial and temporal resolutions to accurately resolve all flow and thermal features for the reported flow variable ranges. Studies show that the reported flow parameters are converged to better than 0.3% in terms of spatial and temporal accuracy and 1% with respect to the domain size. For the optimal growth studies, the dependence of energy growth on upstream domain length is also considered through the calculation of the energy growth over a fixed time span. This verifies that the effect of truncating the upstream length from 32d to 8d causes an error of less than 3% in the growth rate prediction. The critical Reynolds number Rec for the transition from steady to periodic flow is determined as a function of Ha⋆ and β, and this is found to increase with increasing Ha⋆ and β. In addition, the variation of the wake recirculation length in the steady flow regime is determined as a function of Reynolds number, Hartman number and blockage ratio, and a universal expression is proposed. The characteristics of heat transfer depend strongly on the proximity of the cylinder to the heated wall. For small blockage ratios, it increases significantly as the gap ratio decreases from 1 to 0.25. However, there is a substantial drop in heat transfer for high blockage ratio. Downstream cross-stream mixing induced by the cylinder wake is found to increase the heat transfer augmentation by more than a factor of two in some cases. The maximum gain in heat transfer generated by placing the cylinder in the channel near to the wall with that at the centerline was obtained for the lowest blockage ratio β = 0.1, as the cylinder is further approached the heated wall. For all β, a very significant transient energy growth is found in the subcritical regime below the onset of vortex shedding. This suggests a potential for the design of an actuation mechanism to invoke vortex shedding and then enhance heat transfer in these ducts. The energy amplification of the disturbances is found to decrease significantly with increasing Hartmann number and the peak growth shifts towards smaller times while it increases significantly with increasing blockage ratio. The structure of the optimal initial disturbance is found to be consistent across the all blockage ratios being tested. In line with similar problems, it convects along the separating region being amplified to the peak growth state downstream of the recirculation bubble. The maximum time for maximum energy growth τmax is found to increase significantly as recirculation length increases which demonstrates the amplifying nature of the separated shear layers in the wake. The critical Reynolds number for the onset of positive growth at different Hartmann numbers and blockage ratios is determined. It is found that it increases rapidly with increasing Hartmann number and blockage ratio. For all β, the peak energy amplification grows exponentially with Re from low Hartmann numbers. Direct numerical simulation studies in which the inflow is perturbed by random white noise confirms the predictions arising from the transient growth analysis: that is, the perturbation excites and feeds energy into the global mode. A considerable increase in heat transfer from the heated channel wall occurs from rotational oscillation of the cylinder, with a maximum enhancement of approximately 22% observed at higher amplitude over that for steady flow. The oscillation frequency range for effective enhancement is widened in both directions, while the frequency at which the peak of the Nusselt number occurs is shifted slightly to a lower frequency as A is increased. It is found that as the amplitude was reduced, the forcing frequency approaches the global frequency mode.

A comparative study of natural and ventilated supercavitation across two closed-wall water tunnel facilities

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires

Effects of trapezoidal bluff bodies on blow out limit of methane/air combustion in a micro-channel

Effect of blade thickness on the hydraulic performance of a Francis hydro turbine model

Debris accumulations upstream and through crossing hydraulic structures such as culverts cause the upstream water level and the downstream scour depth to increase, which can lead to structure failure. This experimental study aimed to investigate the effects of various inlet blockage ratios on culvert efficiency and scour hole depth. In a non-blocked case, various submergence ratios (S = 1.06, 1.33, 1.60, and 1.90) were tested with different discharge rates. In a blocked case, the effects of inlet blockage with various blockage ratios (Ar = 10%, 20%, and 30%) were seen as sediment blockage on the pipe bed or floating debris upstream of the culvert. The results show that as the submergence ratio increases, the maximum scour depth decreases at the same discharge rate, and the relative energy loss also decreases in the non-blocked case. In the sediment blockage (Ar d) case, the relative maximum depth increases with increasing densimetric Froude number and with an increasing blockage ratio. An empirical equation was developed to predict the relative scour depth under the present study conditions.

Blockage effect on wakes of various bluff bodies: A review of confined flow

An adapted blockage factor correlation approach in wind tunnel experiments of a Savonius-style wind turbine

Computational investigation of wind tunnel wall effects on buffeting flow and lock-in for an airfoil at high angle of attack

This article proposes an experimental apparatus design to measure the power of a cross-flow marine hydrokinetic turbine system operating in a laboratory water tunnel. Data, from one Hall sensor output signal, was processed to capture the three types of torque exerted on the turbines: mechanical loss, brake, and hydrodynamic torque. The method was then applied to compare the power of a twin turbine system in different counter-rotating configurations. Controlled by a hysteresis brake, the tip-speed-ratio was varied in a constant freestream velocity of 0.316 m/s. While the braking torque was independent of the speed, the mechanical loss was found to depend on the system rotational speed and the amount of mass mounted on the mechanical support. In a counter-rotating configuration, the turbines were synchronized through a pair of spur gears and timing pulleys. Operating at the average chord based Reynolds number of 8000, each turbine had three NACA0012 blades mounted at 15∘ pitch angle. The power coefficient results of 8 turbine configurations showed the tendency of power enhancement of counter-rotating configurations due to blade interaction and increase in blockage ratio. Comparison of the results suggested direct application in a river flow scenario and manipulation of the blade interaction for optimal power production.

Evaluation of buffer block effectiveness in tsunami force reduction: An experimental approach

With increasing inlet temperature of gas turbines, turbine blades need to be effectively protected by using cooling technologies. However, the deposition from the fuel impurities and dust particles in the air is often found inside film holes, which results in partial hole blockage. In this paper, the deposition geometry is simplified as a rectangular channel, and the effect of three blockage ratios is investigated by using the computational fluid dynamics. In addition, water droplets are also released from the coolant inlet to provide a comparison of the results with and without mist injection. It is found that the lateral film cooling effectiveness is reduced with increasing blockage ratio. For all the cases with the blowing ratio 0.6, 1% mist injection provides an improvement of the cooling performance by approximately 10%.

A new CFD modeling method for flow blockage accident investigations

This paper focuses on the effects of wall confinements (or blockage ratios) on the flow and heat transfer characteristics around a long equilateral triangular bluff body placed in a horizontal channel for Reynolds number (Re) range 1 to 80 and blockage ratio range 0.1 to 0.5 for air as the working fluid. The governing continuity, Navier–Stokes and energy equations along with appropriate boundary conditions are solved by using a finite volume method-based commercial solver Ansys Fluent. The total drag coefficient decreases with an increasing value of Re for a fixed value of the blockage ratio; however, it increases with an increasing value of the blockage ratio for a fixed value of Re due to the fact that the channel walls exert an extra retardation force on the obstacle. The onset of flow separation is delayed as the value of the blockage ratio increases. The critical Re (i.e., the transition to a time-periodic regime) exists between 45 and 46, 46 and 47, 58 and 59, and 79 and 80 for the blockage ratios of 0.1, 0.125, 0.25, and 0.5, respectively. The simple correlations for wake length, total drag coefficient, and average obstacle Nusselt number are obtained for the range of conditions covered.

Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) developed an analytical model to predict backwater rise by log jams, using the size and packing density of logs and the jam length, as well as river slope and bed roughness. We show that the model formulas can be rewritten using the Froude number instead of river slope and roughness, thus improving their applicability in engineering practice. The equation terms and results of Follett et al. (2020a, https://doi.org/10.1029/2020gl089346) are found to be similar to those of the empirically derived formula by Schalko et al. (2018, https://doi.org/10.1061/(asce)hy.1943‐7900.0001501). However, some differences are identified, calling for further study. Most notably, these distinctions pertain to the effect of accumulation porosity, with additional minor differences in the exponent of the Froude number. Lastly, model implications for some broader applications are explored, showing a methodology to calculate the representative log size for log mixtures, and the expected effect of log orientation on backwater rise.