Downstream Ramp Research Articles

Flow Instabilities and Control Mechanisms in Cylindrical Cavities With Top Bounding Walls

Abstract Cavity flows studied over the past few decades have led to an increased understanding of the flow physics and instability modes in a range of configurations. While a large number of these studies focus on two-dimensional and three-dimensional rectangular/cuboidal cavities, significant variations are seen with three-dimensional cylindrical cavities with a top bounding wall. The present work details the flow physics of such cavities with a compressible (air) flow past the cavity at two pressure drops of 3850 Pa and 2000 Pa. Results from detached eddy simulations (DES) reveal the presence of the wake mode and shear layer instabilities, respectively, with modified dynamics and oscillatory modes owing to the top wall. In the interest of preventing mode switching with changes in operating conditions, which could lead to large-scale flow disruptions, a passive flow control technique is tested. The modified cavity (with a downstream ramp scaled by the size of shed structures) is seen to maintain the same mode throughout the range of operation offering valuable insights into design modifications for such cavities in practical settings.

Effects of ramp slope and discharge on hydraulic performance of submerged hump weirs

Hump weirs are amongst conventional hydraulic structures for water level control and discharge measurement. The slope in the upstream and downstream ramps is a design parameter that affects the performance of Hump weirs in both free and submerged flow conditions. A series of numerical simulations was performed to investigate the effects of ramp slope and discharge on discharge characteristics and water level variations of submerged Hump weirs. Five ramp slopes ranging from 1V:1H to 1V:5H were tested at different upstream discharges. The numerical results were compared with the laboratory data for verifications of the numerical model. The simulation discrepancies in prediction of water surface level and discharge coefficient were within ±10 % and ±5 % of the full range, respectively. The effects of ramp slope on variations of modular limit and discharge reduction factor were studied. It was found that the modular limit occurred at relatively higher submergence ratios as the ramp slope in Hump weirs increased. The onset of submergence was modeled by carefully increasing tailwater level with small increments and the results were compared with the classic definition of modular limit. It was found that the modular limit increases with increasing the ramp slope and discharge while the classic definition of modular limit indicated that the modular limit is independent of the discharge. The velocity and vortex fields in the downstream of Hump weirs indicated the formation vortex structure, which is controlled by the ramp slope. The energy losses were calculated from the numerical outputs, and it was found that the normalized energy losses decreased linearly with submergence.

Hydraulic investigation of modified semi-cylindrical weirs

The performance of semi-cylindrical weirs (SCWs) is evaluated and modified by considering a downstream ramp. Modified semi-cylindrical weirs (MSCWs) were constructed in 12 variations, with 4 different slopes (θ) and 3 radii (R), and were tested through laboratory experiments. Additionally, numerical studies were conducted to examine the performance of the MSCW models. The findings suggest that the θ significantly affects the hydraulic characteristics downstream of the MSCWs. In the traditional SCW (θ = 90°), the flow downstream of the weir is turbulent and irregular, and the flow depth increases with aeration. However, reducing θ decreases turbulence in the flow downstream of the weirs. The discharge coefficient (Cd) of the MSCWs is dependent on R (negative correlation) and independent of θ. When the ratio of the upstream flow depth (yup) to R changes from 0.306 to 1.36, the value of Cd varies within the range of 1.1–1.45. Examination of bed pressure distribution on the MSCW models reveals that reducing θ effectively controls the negative pressure on the crest surface.

Effect of ramp slope on the efficiency of hump weirs in free flow condition

A series of numerical simulations was performed to study the discharge capacity of symmetrical hump weirs in free flow condition. A wide range of ramp slopes was selected from 1 V:1 H to 1 V:5 H and the effects of flow discharge and ramp slope on variations of discharge coefficient were investigated. The variations of upstream water head with discharge indicated the existence of two distinct flow regimes on free flow over symmetrical hump weirs. It was found that the discharge coefficient increased with increasing the upstream water head until the maximum discharge coefficient was achieved. Further increase of water head caused a reduction in discharge coefficient and the flow regime became inefficient. The proposed models for prediction of discharge characteristics of sharp-crested weirs with an upstream and (or) downstream ramp(s) from the literature were also compared with the discharge capacity of sharp-crested weirs to study the effects of ramp slope. It was found that the available head–discharge models are acceptable for hump weirs with small ramp length and more accurate prediction models are required for symmetrical hump weirs with larger ramps. The boundary curve to determine the optimum performance in symmetrical hump weirs was introduced, and the head–discharge models for both efficient and inefficient discharge conditions were proposed.

Effects of upstream and downstream ramp on flow characteristics over a cylindrical weir

Cylindrical weirs are one of the most commonly employed hydraulic structures to measure flow, compare to other types of the same width they pass larger discharge. This paper presents an experimental work together with a CFD simulation to study the effects of the geometric characteristics of a cylindrical weir and a ramp placed upstream or downstream of the same weir on the discharge coefficient. Three different weir diameters and three ramp angles under three different discharges were utilized. The results show that the geometric characteristics represented by the diameter of the weir affects the discharge coefficient when there is no ramp. The discharge coefficient was observed to decrease as the slope of the upstream ramp was increased, however as the slope of the downstream ramp was increased the discharge coefficient was noted to increase. A mathematical relationship was developed in order to calculate the discharge of flow passes over the cylindrical weir depending on its diameter.

Effects of downstream ramp on the fuel diffusion of single cross jet at supersonic flow: Computational study

• Effect of downstream ramp on the mixing of cross jet at supersonic flow is studied. • The mixing performance of the air/fuel jets in various injector types is investigated. • Angle effect of downstream ramp on mixing performance is fully investigated. • CFD approach is applied for the simulation of the cross jet with downstream ramp. • The fuel mixing of the lobe injector is more efficient than rectangular and circular nozzle. In this research, all-inclusive efforts are performed to understand the impact of downstream ramp on fuel distribution of cross jet at high-speed flow inside the combustion chamber. Three different jet injectors have been examined to find the efficient injector (3-lobe, rectangular and circular nozzle) for this configuration. The simulation of the proposed configuration is done by Computational Fluid dynamic via solving RANS equations. SST turbulence model is applied for the computational modeling of the compressible flew inside the combustor. It is also found that the flow recirculation behind the ramp significantly augments vortex feature downstream of fuel jet and this increase the fuel mixing in combustion chamber. The archived outcomes display that rising the angle of the downstream ramp from 10 to 30˚ augments fuel mixing up to 30% inside the combustion chamber. According to our results, the flow circulation downstream of the 3-lobe injector in presence of downstream ramp (30˚) is more than 200% than other nozzle types. Our findings also show that the mixing performance of rectangular jet spanned in the depth of domain is more efficient than other nozzles.

Influence of downstream ramp on mass diffusion of various coaxial hydrogen and air injectors at supersonic combustion chamber

The mixing efficiency of injection system inside combustion chamber of the scramjet significantly influence on the total performance of the hypersonic vehicles. In this study, the existence of the downstream ramp on the flow structure of hydrogen cross jet at supersonic air stream is comprehensively investigated. Computational fluid dynamic method is used to simulate flow structure of sonic hydrogen jet at supersonic air cross flow in existence of ramp positioned downstream of annular injectors. Effects of different annular injectors on mixing and penetration of fuel jet are also disclosed. Besides, effects of ramp angles on the distribution of hydrogen jet are also explored. Comparison of the 3-lobe injector in two different downstream angles of 10 and 30 deg indicates that the mixing efficiency increase about 30% when the angle of the downstream ramp is increased from 10 deg to 30 deg. Moreover, our results show that fuel mixing of the 3-lobe injector with ramp angle of 30 deg is about 55% more than circular jet at the same condition.

The presence of downstream ramp on fuel mixing of the multi micro jets at supersonic cross flow

Development of the fuel injection system in combustion chamber is greatly important for the overall thrust efficiency of the high-speed vehicles. Current article developed a three-dimensional model to discover the reality of downstream ramp on fuel mixing of the multi-jet at Ma>1. FVM is hired to scrutinize the impact of injector types (3-lobe, circular and rectangular shape) on the mixing productivity of downstream ramp in combustion chamber. Besides, the effects of ramp angle on fuel mixing are also analysed. Fuel mixing mechanisms in the selected models are investigated by comparing the Ma contour and mixing zone. Comparisons of the circulation strength downstream of these models confirm that the 3-lobe nozzles is more efficient than other styles. Our comparison indicates that overall mixing productivity of the circular jet is more than other cases.

Flow through a sluice gate over a broad crested weir under free- and submerged-flow conditions

ABSTRACT This paper describes the hydraulic characteristics of flow through a sluice gate over a broad-crested weir with a fixed upstream ramp slope of 1:2 and downstream ramp slopes of 1:2, 1:3, and 1:4. A relationship was proposed to determine the contraction coefficient of the flow passing through the gate, under free-flow conditions, in which the average value was 0.71 for all observed data. The developed relationship was used to calculate the head-loss coefficient from the studied structures under free-flow conditions, which averaged 0.31. The value of the mean absolute relative error for the proposed relationship to estimate the discharge coefficient under free-flow conditions was 1.7%. Examination of the different downstream slopes on the discharge coefficient under free-flow conditions showed that a downstream ramp slope of 1:3 is most appropriate when compared to the results for the other slopes. The value of the mean absolute relative error of the developed relationship to estimate the discharge coefficient under submerged flow conditions was 5.7%, while its corresponding value from the WinGate software was 13.1%. A survey of the effect of downstream ramp slope on the depth of submergence threshold showed that an increase in the downstream slope tends to increase the depth of the submergence threshold, whereby its value under low slopes was not significant.

Effects of different configurations of sloping crests and upstream and downstream ramps on the discharge coefficient for broad-crested weirs

Weirs are commonly used for controlling, measuring, and regulating water flow in conveyance structures such as canals or dams. In this study, hydraulic properties of flow over broad-crested weirs including the effects of different sloping crests and upstream and downstream ramps were characterized by using the finite volume method. Numerical simulations were validated by experimental results; the numerical simulations are very similar to the experimental results. The discharge coefficient (Cd) for the broad-crested weir with upstream and downstream ramps and positive slope of crest (BCW-UDR-PSC) and the broad-crested weir with upstream and downstream ramps and negative slope of crest (BCW-UDR-NSC) are higher than the broad-crested weir with upstream and downstream ramps and horizontal crest (BCW-UDR-HC). It was found that for both the BCW-UDR-NSC and the BCW-UDR-PSC, the Cd increases with steeper weir crest slope. This shows that the Cd is affected by the slope of the crest such that the negative sloped crest more significantly increases the Cd than a positive slope. The average increase in value of the Cd for the BCW-UDR-NSC with respect the BCW-UDR-PSC is approximately 8%.

Experimental and Numerical Investigations of Flow over and under Weir-Culverts with a Downstream Ramp

AbstractA series of experimental and numerical studies were conducted to investigate the hydrodynamics of weir-culverts with a downstream ramp in submerged-flow condition. A comparison of the numer...

Predictor variables influencing merging speed change lane crash risk by interchange type in urban areas

The objective of this research is to model and examine the influence of predictor variables on merging speed change lane crash risk by interchange type in urban areas. Data for selected merging speed change lanes in Charlotte, North Carolina, USA, for five-years, were collected and used in this research. Multinomial logistic regression models were developed to examine the risk of getting involved in a fatal or injury crash and property damage only (PDO) crash by interchange type. The findings indicate that merging speed change lanes at multi-lane ramps are safer than single-lane ramps (odds ratio > 50). The ramp average daily traffic (ADT) and the speed difference between the freeway and the ramp influence merging speed change lane crash risk at a cloverleaf interchange while the freeway annual average daily traffic (AADT) and the speed change lane length influence merging speed change lane crash risk at a diamond interchange (odds ratio > 1.4). The risk of getting involved in a fatal-injury crash in the merging speed change lane increases (odds ratio > 1.6) if an upstream or downstream ramp is too close to the subject cloverleaf interchange ramp or if a downstream ramp is too close to the subject diamond interchange ramp. Contrarily, the upstream ramp distance does not influence the merging speed change lane PDO crash risk at a cloverleaf or diamond interchange. It can be concluded that predictor variables influencing the merging speed change lane crash severity risk are different for cloverleaf, diamond, and other interchange types.

Hydraulics of sharp-crested weir culverts with downstream ramps in free-flow, partially, and fully submerged-flow conditions

Laboratory experiments were conducted to investigate the hydraulics and discharge characteristics of sharp-crested weir culverts with downstream ramps for hydraulically smooth wall boundary and for free, partially submerged, and fully submerged-flow conditions. Four weir-culvert models were tested with different weir heights, ramp lengths, and culvert heights. The partially submerged-flow conditions were created when a slight increase in tailwater caused the headwater to rise due to partial submergence of the culvert. In this flow regime, supercritical flow over the ramp interacted with culvert outflow and this flow regime was classified as Supercritical Jet Interaction Regime or SJIR. Once tailwater reached the weir crest, the weir flow regime became submerged indicating fully submerged-flow condition. Based on the variations of water surface profiles, submerged flows were further classified into Surface Jump Regime (SJR), Surface Wave Regime (SWR), and Deeply Submerged Regime (DSR) and the boundaries between each flow regime were defined by regime plots. To predict discharge of weir culverts with downstream ramps, variations of discharge coefficient with weir geometry and flow regimes were studied and empirical formulations were developed. In addition, free and partially submerged flow discharges were predicted by implementing the interaction factor. It was found that the interaction factor was independent of the Froude number in SJR, while it correlated with the Froude number in SJIR. Under submerged-flow conditions, evaluation of the discharge characteristics of the proposed weir-culvert model showed correlations with water surface flow regimes and a three-stage prediction formula was proposed for estimation of submerged flow. The energy losses over weir-culvert models were also calculated and it decreased with submergence.

Predicting Lane-by-Lane Flows and Speeds for Freeway Segments

The Highway Capacity Manual is a major reference for evaluating the capacity and quality of service of road facilities. However, it holds the assumption that lanes perform equally, which can result in inaccuracies in performance estimation. The main purpose of this research is to develop a series of models for estimating flows and speeds by lane for various types of freeway segments, including basic, merge, and diverge types. These models consider the demand-to-capacity ratio, the presence of trucks, grade, and the presence of upstream and downstream ramps. To predict lane performance effectively, it is critical that capacity and free-flow speeds are also determined for individual lanes. Therefore, this study also investigates the relationship between segment average values and lane values for free-flow speeds and capacities, and proposes a method to estimate these parameters for each lane as a function of the segment average. Observed field data has shown that free-flow speeds and capacities have lowest values on the shoulder lanes and highest values on the median lanes. Speed and flow data were collected from 48 segments throughout the U.S.A., including basic, merge, and diverge segments, to develop flow and speed distribution models. A case example is provided to illustrate the application of the developed model and the predicted speed–flow relationship is compared with field data, with satisfactory results.

Effects of steady and pulsed discharge arcs on shock wave control in Mach 2.5 flow

The effects of the steady and pulsed arc discharge on the oblique shock wave control are explored through an experimental study in a supersonic wind tunnel with the maximum design Mach number of 2.5. The oblique shock is formed by a compression ramp with an angle of 7 degree mounted on the floor of the wind tunnel. Four electrodes are placed upstream of the compression ramp and spaced equally in the spanwise direction to generate the discharge arcs. The steady discharge is generated between the electrodes and downstream ramp corner, forming streamwise arcs. The arc length can be modulated between 10 mm and 40 mm by moving the electrodes in the streamwise direction. With a High Frequency Switch (HFS), the pulsed discharge is achievable with a frequency range from 5 kHz to 50 kHz. The pulsed discharge is generated between two adjacent electrodes, forming so-called transversal arcs, which are blown downstream by the main flow. Results show that the steady discharge arcs act like a uniformly distributed conductor. With an increase of the arc length, the arc power increases, and the weakening effect on the shock is enhanced. For the pulsed discharge arcs, the shorter arcs are observed with higher discharge frequency, which implies a lower arc power. Overall, the weakening effect of the steady arcs on the shock is more effective with shock strength reduced by 4%, compared to the pulsed ones with only 0.35% reduction.

Hydraulics of Submerged Triangular Weirs and Weirs of Finite-Crest Length with Upstream and Downstream Ramps

AbstractLaboratory tests were carried out to investigate the hydraulics of submerged flow over triangular weirs and weirs of finite-crest length with upstream and/or downstream ramp(s). Water surfa...

Numerical evaluation on the smoke control strategies and parameters for the road tunnel with converging junctions

Different smoke control strategies for the road tunnel with converging intersections are evaluated by the numerical method, and the optimum control strategy and longitudinal velocities are obtained. The results shown that when the fire occurs at the upstream to the intersection, longitudinal smoke control with the critical velocity can only prevent the backflow of the smoke at the main tunnel. The smoke can spread in the downstream ramp tunnel due to the stack effect, and this would be dangerous for the cars and the persons. The best smoke control strategy under this condition is that the longitudinal smoke control is implemented both in the main tunnel and the ramp tunnel, then the small longitudinal velocity in the ramp tunnel can prevent the smoke flowing in, this will be safe and economic.

Hydraulic Performance of an Embankment Weir with Rough Crest

AbstractAn experimental study was conducted on a large embankment weir with smooth and rough crests for a range of flow conditions 0.011≤Q≤0.226 m3/s. Close agreement between the rough and smooth crest experiments was observed in terms of flow patterns, free-surface profiles, and pressure distributions. The crest roughness had effects on the velocity distributions, the boundary layer properties, and the shear stress. On the rough crest, the velocity distributions were shifted upwards, increasing the growth rate of the boundary layer. The calculation of the shear stresses with the law of the wall, outer flow region, and momentum integral methods showed consistent results of about τ/(ρ g H)≈0.0015–0.0018 for the smooth crest and of τ/(ρ g H)≈0.0024 for the rough crest. The crest roughness reduced the discharge performance, while upstream and downstream ramps did not have any effects. The present observations highlighted the relevance of roughness and scaling on the hydraulic performances of weirs.

New Theoretical Solution of the Outflow Process for a Weir with Complex Shape

AbstractIn this paper, the outflow process of a weir with an irregular shape is studied using dimensional analysis and the incomplete self-similarity theory. The new unique stage–discharge relationship, which is deduced theoretically, represents a novel and comprehensive equation to describe the weir outflow process for a full range of flow and different geometric variables of weirs, including finite-crested weirs, broad-crested weirs with positive or negative crest slope, broad-crested weirs with an upstream, downstream, or both upstream and downstream crest ramps, and, triangular weirs with an upstream or downstream ramp. Finally the proposed stage–discharge relationship is calibrated using measurements available in literature.

Particle Image Velocimetry in a Nonreacting and Reacting High-Speed Cavity

Particle image velocimetry measurements were taken at the center plane of a high-speed cavity combustor in nonreacting and reacting conditions at fuel flows corresponding to medium, medium-high, and high fuel-loading conditions with supersonic core flow velocities. Calculation of the instantaneous and averaged pathlines, vorticity, swirling strength, and divergence of the velocity field revealed a highly unsteady three-dimensional flow with coherent eddy structures formed at the stagnation zone of the shear layer against the downstream ramp of the cavity, which appear to be convected upstream in the cavity. Comparison of the shear layer location, thickness, and impingement stagnation zone revealed a number of changes in the mean and unsteady velocity behavior that were dependent on the heat release in the cavity and shear layer. As combustion shifted from the cavity at medium fuel loading into the shear layer at high fuel loading, the volumetric expansion compressed the primary recirculation zone and thickened the downstream boundary layer at the cavity exit. Combustion in the cavity tended to attenuate cavity and shear layer unsteadiness. When the combustion shifted to the shear layer, velocity unsteadiness increased, though not to the amplitudes measured without combustion.