Constructing the Shields Curve

2011 ◽  
Author(s):  
Sape A. Miedema
Keyword(s):  
Drag Force ◽  
Original Data ◽  
Main Flow ◽  
Simultaneous Occurrence ◽  
Turbulent Eddies ◽  
Effective Velocity ◽  
Sliding Mechanism ◽  
Drag And Lift ◽  
General Transport ◽  
Shields Curve

Prediction of the entrainment of particles is an essential issue for the study of erosion phenomena in many applications. The original Shields curve describes the entrainment of many particles at many locations and is thought critical to general transport. The mechanisms involved in general are sliding, rolling and lifting, new models of which have been developed. I will introduce concepts for the determination of the effective velocity and the acting point of the drag force, based on integration of the drag force over the cross section of the exposed particle (where most earlier models were based on integration of the velocity), the behavior of turbulence intensity very close to the virtual bed level and the factor of simultaneous occurrence of the small turbulent eddies. The resulting values of the Shields parameter, based on practical and reasonable properties, are compared with data, resulting in the best correlation for the sliding mechanism with the data of many researchers. The Shields parameter found for rolling and lifting overestimates the measurements from literature. Sliding seems to be the mechanism moving the top layer of the particles, while rolling and lifting are much more mechanisms of individual particles. In the new model it is considered that in the laminar region entrainment is dominated by drag and the influence of small turbulent eddies reducing the thickness of the viscous sub-layer, while in the turbulent region this is dominated by drag and lift. The transition region is modeled based on sophisticated interpolation. The model also describes the influence of exposure and protrusion levels and is compared with data of laminar main flow. The model correlates very well with the original data of Shields and data of others and also matches empirical relations from literature. The model is suitable for incorporating exposure and protrusion levels and laminar main flow.

REVIEW—Review of Flow Interference Between Two Circular Cylinders in Various Arrangements

1977 ◽  
Vol 99 (4) ◽  
pp. 618-633 ◽  
Author(s):  
M. M. Zdravkovich
Keyword(s):  
Flow Pattern ◽  
Drag Force ◽  
Lift Force ◽  
Flow Direction ◽  
Critical Survey ◽  
Tandem Arrangement ◽  
Critical Spacing ◽  
Gap Flow ◽  
Lift Forces ◽  
Drag And Lift

There are infinite numbers of possible arrangements of two parallel cylinders positioned at right angles to the approaching flow direction. Of the infinite arrangements, two distinct groups may be identified: in one group, the cylinders are in a tandem arrangement, one behind the other at any longitudinal spacing; and in the second group, the cylinders face the flow side by side at any transverse spacing. All other combinations of longitudinal and transverse spacings represent staggered arrangements. The tandem arrangement will be treated first. A critical survey of previous research revealed some “odd” features which had been observed and overlooked by various authors. The discontinuity of vortex shedding implies that a similar discontinuity should be expected for the drag force on both cylinders. The measurements of the front (gap) pressures of the downstream cylinder and the base pressures of both cylinders at various spacings reveal a discontinuous “jump” at some critical spacing. The discontinuity is caused by the abrupt change from one stable flow pattern to another at the critical spacing. A new interpretation is given for the existing data on the drag force for both cylinders. The effects of Reynolds number and surface roughness are treated in some detail. Following this, two cylinders arranged side by side to the approaching flow are considered. All the available data on measured forces are compiled together with additional measurements in the range of intermittent changes of drag and lift forces. The bistable nature of the asymmetric flow pattern around each cylinder produces two alternative values of the drag force coupled with two alternative values of the lift force. The introduction of the interference force coefficient exposes the physical origin of two different forces experienced by the cylinders when arranged side by side. Finally, the least reported arrangement of two staggered cylinders is reviewed. The various arrangements are grouped into classes according to the sign of the lift force, or whether the drag force is greater or less than that for a single cylinder. The measurements of drag and lift forces for various arrangements reveal two different regimes for the lift force. In one regime, the lift force directed toward the wake of the upstream cylinder is due to the entrainment of the flow into the fully developed wake of the upstream cylinder. The lift force in this regime reaches a maximum value when the downstream cylinder is near to the upstream wake boundary. In the second regime, at very small spacings, the lift force becomes very large due to an intense gap flow which displaces the wake of the upstream cylinder. The maximum lift force occurs with the downstream cylinder near to the horizontal axis of the upstream cylinder. A discontinuity in the lift force for some staggered arrangements is found and attributed to the bistable nature of the gap flow.

scholarly journals Reducing Drag and Oscillation of Spheres Used for Buoyancy in Oceanographic Moorings

2008 ◽  
Vol 25 (10) ◽  
pp. 1823-1833 ◽  
Author(s):  
Jane Hurley ◽  
Brad de Young ◽  
Christopher D. Williams
Keyword(s):  
Drag Force ◽  
Laboratory Measurements ◽  
Dynamical Calculation ◽  
Laboratory Results ◽  
Drag And Lift ◽  
Eddy Shedding

Abstract Numerical and laboratory results of the drag characteristics are presented for different configurations of an underwater buoyancy package. It is shown that the drag and oscillation of an underwater sphere can be reduced substantially with the addition of a shaped cowling. The influence of several different cowling shapes on the drag and lift are determined. The results from a numerical fluid dynamical calculation are compared and laboratory measurements are scaled. Both the dynamic and static components of drag and lift are presented. The drag force for an underwater sphere can be reduced by more than 80% for a full teardrop-shaped cowling. A truncated teardrop, more practical for real applications, will still reduce the drag by 60%–70%. In addition to the drag, the amplitude of oscillations driven by eddy shedding is similarly reduced.

Analysis of lift and drag coefficients of a GT2 racing car at various speeds with movable wheels and ground

2015 ◽  
Vol 12 (3) ◽  
pp. 261-270
Author(s):  
Albert Boretti
Keyword(s):  
Wind Tunnel ◽  
Drag Force ◽  
Computational Fluid Dynamic ◽  
Lift Force ◽  
Fluid Dynamic ◽  
Time Simulation ◽  
Rear Wing ◽  
Speed Range ◽  
Drag And Lift ◽  
Lift And Drag

The paper proposes a study of a GT2 racing car with a computational fluid dynamic (CFD) tool. Results of STAR-CCM+ simulations of the flow around the car in a wind tunnel with movable ground and wheels are presented for different air speeds to assess the different contributions of pressure and shear to lift and drag over the speed range. The rear wing contributes more than 85% of the lift force and 7-8% of the drag force for this particular class of racing cars. When reference is made to the low speed drag and lift coefficients, increasing the speed from 25 to 100 m/s produces an increase of CD of more than 3% and a reduction of CL of more than 2%. The resultsuggests modifying the constant CD and CL values used in lap time simulation toolsintroducing the tabulated values to interpolate vs. the speed of the car.

scholarly journals Aerodynamic performances of rounded fastback vehicle

2017 ◽  
Vol 231 (9) ◽  
pp. 1211-1221 ◽  
Author(s):  
Giacomo Rossitto ◽  
Christophe Sicot ◽  
Valérie Ferrand ◽  
Jacques Borée ◽  
Fabien Harambat
Keyword(s):  
Flow Field ◽  
Drag Force ◽  
Minor Effect ◽  
Near Wake ◽  
Vorticity Distribution ◽  
Aerodynamic Performances ◽  
Drag And Lift ◽  
A Minor ◽  
Yaw Angle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers.

scholarly journals Effect of turbulent fluctuations on the drag and lift forces on a towed sphere and its boundary layer

2013 ◽  
Vol 721 ◽  
pp. 155-179 ◽  
Author(s):  
Holger Homann ◽  
Jérémie Bec ◽  
Rainer Grauer
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Spherical Particle ◽  
Drag Force ◽  
Turbulent Regime ◽  
Mean Velocity ◽  
Turbulent Fluctuations ◽  
The Mean ◽  
Lift Forces ◽  
Drag And Lift

AbstractThe impact of turbulent fluctuations on the forces exerted by a fluid on a towed spherical particle is investigated by means of high-resolution direct numerical simulations. The measurements are carried out using a novel scheme to integrate the two-way coupling between the particle and the incompressible surrounding fluid flow maintained in a high-Reynolds-number turbulent regime. The main idea consists of combining a Fourier pseudo-spectral method for the fluid with an immersed-boundary technique to impose the no-slip boundary condition on the surface of the particle. This scheme is shown to converge as the power $3/ 2$ of the spatial resolution. This behaviour is explained by the ${L}_{2} $ convergence of the Fourier representation of a velocity field displaying discontinuities of its derivative. Benchmarking of the code is performed by measuring the drag and lift coefficients and the torque-free rotation rate of a spherical particle in various configurations of an upstream-laminar carrier flow. Such studies show a good agreement with experimental and numerical measurements from other groups. A study of the turbulent wake downstream of the sphere is also reported. The mean velocity deficit is shown to behave as the inverse of the distance from the particle, as predicted from classical similarity analysis. This law is reinterpreted in terms of the principle of ‘permanence of large eddies’ that relates infrared asymptotic self-similarity to the law of decay of energy in homogeneous turbulence. The developed method is then used to attack the problem of an upstream flow that is in a developed turbulent regime. It is shown that the average drag force increases as a function of the turbulent intensity and the particle Reynolds number. This increase is significantly larger than predicted by standard drag correlations based on laminar upstream flows. It is found that the relevant parameter is the ratio of the viscous boundary layer thickness to the dissipation scale of the ambient turbulent flow. The drag enhancement can be motivated by the modification of the mean velocity and pressure profile around the sphere by small-scale turbulent fluctuations. It is demonstrated that the variance of the drag force fluctuations can be modelled by means of standard drag correlations. Temporal correlations of the drag and lift forces are also presented.

scholarly journals Numerical Analysis of Fairings for Bicycles

2021 ◽  
Author(s):  
Polamarasetty Teja Bhavani ◽  
P. Teja Bhavani ◽  
Y. Seetharama Rao ◽  
B. V. Ramana Murthy
Keyword(s):  
Numerical Analysis ◽  
Drag Force ◽  
Atmospheric Pressure ◽  
Lift Force ◽  
Flow Direction ◽  
Comfort Level ◽  
Oncoming Flow ◽  
Pressure Drag ◽  
Air Resistance ◽  
Drag And Lift

Abstract Aerodynamics is the study of moving air's properties and the interactions between moving air and solids. Rider gets slammed into air particles while riding that gets compressed once rider hit them and then become spaced out once they flow over the rider. The distinction in atmospheric pressure from your front to your back creates a retardant force. The force that's perpendicular to the oncoming flow direction is the lift force. It contrasts with the drag force. Aerodynamic shapes reduce this pressure drag and lift by minimizing that difference in pressure and allowing the air to flow more smoothly over your front, reducing the low-pressure wake behind the cyclist and reducing this drag, and increasing speed in this paper; fairings designed. NACA airfoil as a base, fairings are designed using CATIA.CFD analysis is carried out on the bicycle with a fairing to calculate drag and lift force. As the position of cyclists isn't modified and due to fairing, the air resistance reduces, which may increase the comfort level of cyclists. From this analysis, the economical fairing can be determined, facilitating additional drag and producing less lift.

scholarly journals NUMERICAL SIMULATION ON REAR SPOILER ANGLE OF MINI MPV CAR FOR CONDUCTING STABILITY AND SAFETY

SINERGI ◽  
2019 ◽  
Vol 24 (1) ◽  
pp. 23
Author(s):  
Alief Avicenna Luthfie ◽  
Dedik Romahadi ◽  
Hanif Ghufron ◽  
Solli Dwi Murtyas
Keyword(s):  
Fluid Dynamics ◽  
Numerical Simulation ◽  
Drag Force ◽  
Lift Force ◽  
Lift Coefficient ◽  
Air Resistance ◽  
Rear Part ◽  
Rear Spoiler ◽  
Computational Fluid Dynamics Cfd ◽  
Drag And Lift

Spoiler attached on the rear part of a car can generate drag force and negative lift force, called downforce. This drag force can increase air resistance to the car, meanwhile, a negative lift force can improve the car’s stability and safety. Refer to many researchers, the shape and the angle of the spoiler give different aerodynamic effects and therefore give a different value of drag force and lift force. Based on these facts, this study was focused on the analysis of different spoiler angle attached to a mini MPV car to drag and lift force generated by the spoiler. The method used in this study is a numerical simulation using the Computational Fluid Dynamics (CFD) technique. The analysis was carried out at different spoiler angle and car’s speed. The spoiler angles are -20o, -10o, 0o, 10o, and 20o. The car’s speeds are 40 km/h, 60 km/h, 80 km/h, 100 km/h, and 120 km/h. Then the drag and lift force and their coefficient generated by different spoiler angles were being investigated at specified speeds. The result shows that higher spoiler angles generate higher drag and lower lift. Spoiler angles higher than 0o generate negative lift force, otherwise generate positive lift force. Therefore, to increase a car’s stability and safety, it is recommended to use a spoiler angle higher than 0o. Based on the result, it is best to use spoiler angle 10o because it generates negative lift force with -0.05 lift coefficient and 0,68 drag coefficient.

scholarly journals Investigation of Drag and Lift Forces over the Profile of Car with Rearspoiler using CFD

2015 ◽  
Vol 1 (8) ◽  
pp. 331
Author(s):  
Naveen Kumar Velagapudi ◽  
Lalit Narayan K. ◽  
L. N. V. Narasimha Rao ◽  
Sri Ram Y.
Keyword(s):  
Drag Force ◽  
High Speed ◽  
Aerodynamic Drag ◽  
Fuel Utilization ◽  
Ansys Fluent ◽  
Total Drag ◽  
Rear Spoiler ◽  
Computational Fluid Dynamics Cfd ◽  
Lift Forces ◽  
Drag And Lift

Now a days demand of a high speed car is increasing in which vehicle stability is of major concern. Forces like drag& lift,weight,side forces and thrust acts on a vehicle when moving on road which significantly effect the fuel consumption The drag force is produced by relative motion between air and vehicle and about 60% of total drag is produced at the rear end. Reduction of drag force at the rear end improves the fuel utilization. This work aims to reduce the drag force which improves fuel utilization and protects environment as well. In the stage of work a sedan car with different types of spoilers are used to reduce the aerodynamic drag force. The design of sedan car has been done on CATIA-2010 and the same is used for analysis in ANSYS-(fluent). The analysis is done for finding out drag and lift forces at different velocities, and spoilers. This study proposes an effective numerical model based on the computational fluid dynamics (CFD) approach to obtain the flow structure around a passenger car with a rear spoiler

Residual migration: Applications and limitations

Geophysics ◽  
1985 ◽  
Vol 50 (1) ◽  
pp. 110-126 ◽  
Author(s):  
Daniel H. Rothman ◽  
Stewart A. Levin ◽  
Fabio Rocca
Keyword(s):  
Seismic Data ◽  
Computational Cost ◽  
Velocity Model ◽  
Original Data ◽  
Migration Velocity ◽  
Data Sets ◽  
Dispersion Method ◽  
Effective Velocity ◽  
Low Dispersion ◽  
Residual Processing

The correct migration of seismic data depends on the accuracy of the chosen velocity model. Rocca and Salvador (1982) showed that small errors in the velocity model may be efficiently corrected by applying a residual migration to previously migrated data, rather than remigrating the original data with a corrected velocity field. The effective velocity used in this residual processing is usually small compared to the original migration velocity. This decreases computational cost relative to a full migration, and allows the initial migration to be done with a less accurate but faster algorithm than would otherwise be required. The possible advantages are many. The overall cost of migration may be reduced, a consideration especially important when migrating 3-D data sets. Migration quality may be improved, because the location of mispositioned reflectors can be corrected and because of the freedom to choose initial migration with a high dip, low dispersion method such as Stolt migration. Interactive residual sharpening of the migrated image also becomes feasible. We discuss the theoretical and practical limitations of residual migration and quantify the related reductions of effective dip, velocity, and frequency after initial migration. We determine how accurate the initial migration velocity must be to justify use of this approach and analyze aliasing and numerical artifacts. Field data examples using Kirchhoff summation and finite‐difference migration illustrate the features and drawbacks of the method.

scholarly journals Correlation Between Vibration Excitation Forces and the Dynamic Characteristics of Two-Phase Flow in a Rotated Triangular Tube Bundle

2006 ◽  
Author(s):  
C. Zhang ◽  
M. J. Pettigrew ◽  
N. W. Mureithi
Keyword(s):  
Void Fraction ◽  
Cross Flow ◽  
Main Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Drag Forces ◽  
Vibration Excitation ◽  
Tube Bundles ◽  
Lift Forces ◽  
Drag And Lift

Two-phase cross flow exists in many shell-and-tube heat exchangers. Flow-induced vibration excitation forces can cause tube motion that will result in long-term fretting-wear or fatigue. Detailed flow and vibration excitation force measurements in tube bundles subjected to two-phase cross flow are required to understand the underlying vibration excitation mechanisms. Some of this work has already been done. The distributions of both void fraction and bubble velocity in rotated-triangular tube bundles were obtained. Somewhat unexpected but significant quasi-periodic forces in both the drag and lift directions were measured. The present work aims at understanding the nature of such unexpected drag and lift quasi-periodic forces. An experimental program was undertaken with a rotated-triangular array of cylinders subjected to air/water flow to simulate two-phase mixtures. Fiber-optic probes were developed to measure local void fraction. Both the dynamic lift and drag forces were measured with a strain gage instrumented cylinder. The investigation showed that the quasi-periodic drag and lift forces are generated by different mechanisms that have not been observed so far. The quasi-periodic drag forces appear related to the momentum flux fluctuations in the main flow path between the cylinders. The quasi-periodic lift forces, on the other hand, are mostly correlated to oscillations in the wake of the cylinders. The relationships between the lift or drag forces and the dynamic characteristics of two-phase flow are established through fluid mechanics momentum equations. The quasi-periodic lift forces are related to local void fraction measurements in the unsteady wake area between upstream and downstream cylinders. The quasi-periodic drag forces correlate well with similar measurements in the main flow stream between cylinders.

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