Maximum Drag Coefficient Research Articles

Two-dimensional simulations of vortex-induced vibration of a circular cylinder

In the present study, one of the cross-disciplinary problems known as vortex-induced vibration is numerically investigated. Effects of four different low mass-damping ratios; ζ = 0.013, 0.028, 0.074, and 0.124 of a smooth cylinder are taken into account for transition of shear layer 2 (TrSL2) type flow that falls between the Reynolds numbers from 2500 to 10,830 utilizing a two-dimensional cylinder that is free to move in normal-direction. Unsteady Reynolds-Averaged Navier–Stokes solutions indicate that the general trend is well captured with the adopted shear stress transport k-ω turbulence model, however, due to two-dimensional limitations some results are not consistent with experimental data. An inverse relation between the mass-damping ratio and the transition from the upper to the lower branch is detected. Change of drag and lift coefficients with the reduced velocities revealed that the maximum drag coefficient increases with reduced velocity until it reaches Ur = 5 and then decreases dramatically while the lift coefficients decrease consistently from the beginning.

Roughness sublayer flows over real urban morphology: A wind tunnel study

Atmospheric surface layer (ASL) consists of the inertial sublayer (ISL) and the roughness sublayer (RSL) with different dynamics. This study characterizes the ASL flows over real urban morphology using wind tunnel experiments. The building models of downtown Kowloon Peninsula, Hong Kong are fabricated by 3D printing. The vertical profiles (3 × 45 numbers) on three vertical planes in the streamwise direction are selected for velocity measurements by constant-temperature (CT) hot-wire anemometry (HWA). There exist noticeable variations of mean wind speed u‾, fluctuating velocities u’’ and w’’ together with vertical momentum flux u’’w’’ in the ISL and RSL over urban areas. On the plane with maximum drag coefficient Cd (25% higher than that on the other two), the RSL is thicker by 10% in which the winds evolve in response to individual buildings. A new analytical solution is derived to improve the prediction of ASL mean wind speeds whose root-mean-square error (RMSE) is reduced by 10 times. Its key variable, the RSL constant, converges asymptotically μ = 1.7 for urban setting. Unlike the ISL, turbulence statistics show that the RSL velocities are positively skewed (Su > 0) with high kurtosis (Ku > 3). The occurrence of ejection Q2 (detrainment; 32%–37%) is more frequent than sweep Q4 (entrainment; 30%–32%). Whereas, high-speed Q4 contributes most (0.2u’’w’’ to 0.4u’’w’’) to the RSL vertical momentum flux among the four quadrants. It is thus suggested that fresh air entry is driven by spurlike downdrafts while street-level aged air is removed by massive uprising current.

Experimental study on the drag forces on a twin-tube submerged floating tunnel segment model in current

In this paper, the drag forces on a rigid segment of a twin-tube submerged floating tunnel (SFT) under various Re number and submergence are experimentally investigated with a simplified rigidly connected tandem twin-cylinder model towed in a tank. The effects of the spacing distance between two cylinders and vortex-induced vibration (VIV) on the drag forces are investigated through stationary and self-oscillation towing tests. It is found that, in the stationary cases, the drag forces on the cylinders act in opposing directions when the spacing ratio is 2 and 3, which leads them attracting each other. Under VIV conditions, the drag forces are significantly amplified. The maximum mean drag coefficients of the up- and downstream cylinders increase from 1.27 to 4.1 and 0.43 to 1.7, respectively. As for the submergence effect, it is found that both the VIV response amplitude and mean drag coefficient decrease with the decreasing of the submerged depth, especially when the depth ratio is below 3.7.

A simple method to estimate the airfoil maximum drag coefficient

A relatively simple method is presented to predict the maximum two-dimensional drag coefficient of an airfoil only using its shape. The method is based on a contribution related to the leading edge thickness in terms of the y/c coordinate at x/c=0.0125 and a contribution related to the trailing edge flow angle which appears also to be sensitive to the leading edge thickness. The relations were deduced from measurements in the Delft low-turbulence wind tunnel. The first contribution was established using 3 airfoil models with systematically varying leading edge y/c coordinates and a zero trailing edge angle. The second followed from measurements of one of these airfoils equipped with sheet metal flaps of various flap deflections. Compared to measurements found in the public domain differences are found up to ± 2.3% with an average of about -0.2%

Experimental investigation of the heat transfer for non-circular tubes in a turbulent air cross flow

ABSTRACT The purpose of this experimental study is to measure the drag and heat transfer coefficients of the non-circular tubes including square, elliptic, and cam-shape tubes in a turbulent air cross flow. Studies have been made at 0° to 180° angles of attack () and Reynolds numbers (Re) based on the equivalent diameter of a circular tube within the range of 1.4 × 104 < Reeq < 7.5 × 104. The equivalent diameter Deq = 46 mm was chosen for the tubes. The results show that the minimum and maximum drag coefficients (Cd) belong to the cam-shape and square tubes at 0°, respectively. The (Cd) of the cam-shape and square tubes are 60% less and 25% more than the equivalent circular tube, respectively. Also, the ratio of the average Nusselt number () to the drag coefficient, /Cd, was used to compare non-circular tubes and with their equivalent circular ones. The results show that the maximum value belongs to the cam tube and the minimum occurs for the square tube at α = 0°. The /Cd ratio of the cam tube is 128% more than the equivalent circular tube, while the square tube yields a /Cd ratio that is13% less than its circular equivalent.

Magnification of hydrodynamic coefficients on a flexible pipe fitted with helical strakes in oscillatory flows

The features of hydrodynamic coefficients including drag and added mass coefficients on a flexible pipe fitted with helical strakes in an oscillatory flow are studied experimentally. The experiment of a flexible straked pipe was conducted in an oscillatory flow with the Keulegan-Carpenter (KC) number varying from 9 to 165 and the maximum reduced velocities ranging from 4 to 8. Strain response in both in-line and cross-flow directions are measured by fiber bragg grating sensors. Using displacement reconstruction and inverse analysis methods, displacement response and hydrodynamic force of the straked pipe are identified. Then, through the least squares method, corresponding drag and added mass coefficients are extracted. The asymmetric features at the acceleration and deceleration stages of hydrodynamic force on a flexible pipe in the in-line direction under oscillatory flow are first observed. Compared with a bare pipe, helical strakes can effectively reduce the higher frequency fluctuating force and enhance the wake effects. The hydrodynamic coefficients on the straked pipe are significantly magnified in the case of a relatively small KC number. The maximum mean drag coefficients can reach approximately 10. The presented work suggests that the risk of helical strakes application should be fully evaluated through flexible pipe experiments in both steady flow and unsteady flow.

Dynamics of dual-pulse laser energy deposition in a supersonic flow

Dynamics of plasma generated by the dual-pulse laser in a supersonic flow was studied numerically. The mathematical model includes species, momentum, electronic, vibrational and translational energy equations for the multicomponent ionized air. The model examines temporal dynamics of the formed air plasma and how it affects the drag, pressure signature and vorticity generation in a supersonic flow around a wedge. We observed that nonequilibrium plasmas is more effective in the drag reduction compared with the simultaneous thermal energy addition. The maximum drag reduction of around 50% and the maximum drag coefficient reduction of 30% was attained through the dual-pulse laser energy deposition. Variation of the plasma spot orientation did not significantly influence the drag reduction. We suggested that the surface pressure changes were not controlled by the vorticity generation but occurred due to the density changes and the formation of the low-density plasma spot.

Farklı Rüzgar Yönlerinde Yere Monte Bir Güneş Panelinde Aerodinamik Mekanizmalar

Aerodynamics mechanisms on a solar panel were studied using Computational Fluid Dynamics methodology at different wind directions. The wind velocity was chosen as 10 m/s that corresponding turbulent flow. The inclination angle of the panel was fixed as 25°, while the wind directions were varied as 180°, 135°, 45°, and 0°. Governing equations were solved by utilizing a finite volume method with a realizable k-ε turbulence model and standard wall functions. The results showed that the recirculation area occurred for the straight wind directions, but it was not observed for the oblique wind directions. The highest pressure coefficients occurred at the leading edges of the solar panel and they reduced to the trailing edge for all wind directions. The maximum drag and uplift coefficient was obtained at the wind direction of 0° and 180°, respectively.

Effects of non-sinusoidal pitching motion on energy input performance of micro-linear turbine cascade

In this paper, a numerical simulation of a micro-linear turbine cascade is presented. The upstream stay vane and downstream guide vane were surrounded by incompressible flow, and the guide vane was modeled with forced pitching motions. The implicit immersed boundary method was used as the moving boundary flow solver. To seek a better understanding of the influence of the motion parameters on hydrodynamic characteristics and input power of the oscillating guide vane, the forced motion of a non-sinusoidal oscillating guide vane was first investigated for different reduced frequencies and profile parameters. Second, the instantaneous hydrodynamic behavior was simulated for different motion combinations and Reynolds numbers. The results showed that the reduced frequencies and profile parameters of the pitch movement had a large influence on the input power, and the peak of the input power at f∗ = 0.22 was nearly 9.41 times that at f∗ = 0.074, but the input power was not sensitive to the motion combination and Reynolds number. However, the Reynolds number had a large influence on the drag, and the maximum drag coefficient for Re = 6.65 × 105 was 4.566; however, it was only 0.5912 for Re = 6.65 × 104.

Experimental and numerical studies on improvement of drag force of a bus model using different spoiler models

The purpose of this paper is to reduce the drag force of a bus model by using four different spoiler models. The tests carried out at six different free stream velocities and in the range of 3.8 ×105 - 7.9 ×105 Reynolds numbers. The drag coefficient (CD) of the bus model was determined as 0.633 in a wind tunnel and 0.645 in Fluent® package program. The first spoiler model consists of a combination of airfoil and vortex generator mounted at five different locations (L/H) on bus model to improve the drag force. The drag reduction was obtained in the range of 12.25-4.35% in five different L/H rates by using spoiler 1 model. The spoiler 2 model has airfoil shape and maximum drag coefficient was improved by 3.34% experimentally. A 23.49% reduction was obtained by using spoiler 3 in L/H = 0.1 and 18.49% reduction by using spoiler 4 in L/H = 0.15. The numerical flow analyses were performed at the same wind tunnel conditions at six free stream velocities for the base model and best results for each spoiler model. The drag coefficients of best models were reduced by 11.48% and 8.59%, 15.90% and 19.05% respectively in Fluent®.

Effect of pitch on the flow behavior around a hovering wing

The effect of pitch on the evolution of flow and aerodynamic forces around a hovering flat-plate wing has been experimentally investigated in this study using particle image velocimetry and direct force measurements. The measurements are conducted on a wing at a reduced frequency of $$k=0.32$$ and Reynolds number of $$Re=220$$ . The Lagrangian finite-time Lyapunov exponent method is used to analyze the unsteady flowfields by identifying dynamically relevant flow features and their evolution. First, the effect of a change in the duration of pitch for a symmetric pitch is discussed. The flow stages based on the LEV emergence, growth, lift-off, and decay remain the same for the compared cases whereas the duration of flow stages varies. Second, we introduce a phase lead and lag with respect to the stroke timing and detailed flow development is discussed for these cases. This is further corroborated with the measured aerodynamic forces to highlight the effect of varying the phase-shift on the different characteristics of the hovering wing. Changing the pitching phase results in distinct flow changes that correlate with a higher lift production when the pitch precedes the stroke reversal and lower lift production when the pitch succeeds the stroke reversal. A schematic of differences in flow development based on the vorticity during advanced, symmetric, and delayed wing pitch. Blue features are indicative of the LEV and orange are indicative of the TEV. Maroon arrows indicate the magnitude and timing of the maximum lift in the half-stroke. Black arrows indicate the magnitude and timing of the maximum drag coefficients for three representative cases.

Vortex-induced vibration of elastically-mounted spheres: A comparison of the response of three degrees of freedom and one degree of freedom systems

In published literature of the vortex-induced vibration of a sphere, limited attention has been devoted to spheres having three degrees of freedom (DOF) motion and the effect on the vibration response. In this study, the vortex-induced vibration response of a 3-DOF elastically-mounted sphere was examined numerically. The response of a sphere allowed to oscillate in all three spatial directions (3-DOF motion) was compared with the response of a sphere whose motion was restricted to only the transverse direction (1-DOF). Simulations were conducted over the reduced velocity range U∗≡U∕(Dfn)∈[3.5,16], using a sphere of mass ratio m∗≡ρs∕ρ=3, at a fixed Reynolds number of 2000, where U is the free-stream velocity of the flow, fn is the natural frequency of the system, ρs and ρ are the solid and fluid densities, respectively. When the sphere was allowed 3-DOF movement, it was initially excited to vibrate along a linear path in the transverse direction, synchronized with the two-sided shedding of vortex loops behind the sphere. As the simulation time advanced, the sphere trajectory eventually converted into a circular orbit with a spiralling wake behind it. The transition time between these two modes was found to increase with decreasing Reynolds number. The vibration amplitude was significantly smaller when the sphere motion was in all three spatial directions than when it was restricted only to the transverse direction. The maximum vibration amplitudes were approximately 0.6 and 0.8 diameters, for 3-DOF and 1-DOF motion, respectively. In the synchronization regime, the maximum time-averaged drag coefficient was approximately 100% greater than that of a stationary sphere, when the sphere had 3-DOF, similar to previous observations for a tethered sphere. However, the maximum change to the time-averaged drag coefficient was only 76% when the motion was restricted to the transverse direction.

Drag coefficients derived from ocean current and temperature profiles at high wind speeds

Two bottom-up methods based on the turbulence closure and bulk model were utilised to estimate drag coefficients at high wind speeds based on ocean current and temperature profiles observed by two subsurface buoys during Typhoon Megi in the South China Sea. A numerical experiment was conducted using the turbulence closure model to test the impact of missing measurements in the upper mixed layer on the wind stress estimate and reconstruction of the upper ocean current. The results were sufficiently robust after several time steps. The wind stresses derived from the two methods were consistent with each other. Wind stress increased quickly with increasing wind force, and then was constant. A parametric typhoon wind model was applied to obtain the wind field and estimate the corresponding drag coefficient. The results showed that the drag coefficient increased to a maximum at a critical wind speed of about 30 m s−1, and then levelled off with increasing wind speed. The uncertainty of the maximum drag coefficient was about ±0.5 due to neglect of nonlinear terms, the uncertainty of the model wind speeds and the drag coefficient. The critical wind speed for the maximum drag coefficient varied by at least ±5 m s−1. The uncertainty of the drag coefficient parameterisation was about 24% due to wind speed. These results indicate that a bottom-up method can be used to estimate drag coefficients at the forced stage during the passage of typhoons or hurricanes.

Numerical Investigation of Aerodynamic Braking for a Ground Vehicle

The purpose of this article is to observe the effect of an air brake on the aerodynamics of a ground vehicle and also to study the influence of change in the parameters like the velocity of the vehicle, the angle of inclination, height, and position of the air brake on the aerodynamics of the vehicle body. The test subject used is an Ahmed body which is a generic 3D car body as it retains all the aerodynamic characteristics of a ground vehicle. Numerical investigation has been carried out by RNG k-e turbulence model. Results are presented in terms of streamlines and drag coefficient to understand the influence of pertinent parameters on flow physics. It is found that with the use of an air brake, though the drag coefficient remains more or less constant with velocity, it increases with the increase in height and angle of inclination of the air brake. But the effect of position of air brake on the coefficient of drag is surprising since for certain heights of the air brake the drag coefficient is maximum at the foremost point and as the air brake moves towards the rear it is first observed to decrease and then increase. It is also observed that with the increase in height of the air brake the drag coefficient monotonically decreases as the position of the air brake is moved towards the rear. Taguchi method has been employed with L16 orthogonal array to obtain the optimal configuration for the air brake. For each of the selected parameters, four different levels have been chosen to obtain the maximum drag coefficient value. The study could provide an invaluable database for the optimal design of an airbrake for a ground vehicle.

Experiments on the aerodynamic behaviour of square cylinders with rounded corners at Reynolds numbers up to 12 million

The influence of the angle of incidence and corner radius on the aerodynamics of square-section cylinders is studied by means of wind tunnel experiments. Two different corner radii (r∕D=0.16 and 0.29) were investigated at three angles of incidence (α=0°, −22.5° and −45°). Steady and unsteady global forces and local surface pressures were measured in the high-pressure wind tunnel in Göttingen. The Reynolds number was varied up to values as high as 12×106, thereby spanning the known flow state regimes up to high transcritical. The results demonstrated that a decrease of the cylinder’s bluffness induced lower maximum drag coefficients and r.m.s. values, as well as higher Strouhal numbers in all flow state regimes. Furthermore, the critical Reynolds numbers shifted to significantly lower values. For the cylinder configurations at α=0° no upper transition or transcritical flow state was present up to ReD=12 million. A decrease in the angle of incidence resulted in a significant reduction of the length of the supercritical flow state, a shift of the drag force, Strouhal number and base pressure to higher values and an increase of the critical Reynolds numbers. The cylinders at non-zero angles of incidence all displayed a clear critical flow state, at which two discontinuous transitions were observed, accompanied by jumps in the CD and CL values and the Strouhal number. Only three out of six studied configurations experienced hysteresis, where for the high corner radius configuration at α=0° a particularly broad hysteresis effect was measured.

Impact of ambient wind on aerodynamic performance when two trains intersect inside a tunnel

In this study, the aerodynamic performance of two trains that intersect inside a tunnel under ambient wind conditions is investigated. A full-scale test is conducted to verify the computational method and mesh, and then a series of numerical simulations are performed to investigate the pressure variation and the aerodynamic force coefficient by comparing with the condition of no ambient air. The difference between the results with/without ambient wind in a tunnel is analysed. The results indicate that if Train A travels downwind, the positive pressure of train surface increases and the negative pressure decreases as the wind velocity increases, and the arrival time of the maximum positive pressure lags on average 0.024 s and 0.058 s for wind velocities of 20 m/s and 40 m/s, respectively. For the tunnel wall measuring points, the maximum positive pressure increases as the wind velocity increases. The maximum drag coefficient of downwind Train A decreases by 4.7% and 10.1% for wind velocities of 20 m/s and 40 m/s, respectively, whereas that of upwind Train B increases by 5.7% and 15.8%, respectively. The maximum positive side force coefficient of downwind Train A increases by 20.8% for an ambient wind velocity of 40 m/s, whereas the same coefficient decreases by 16.7% for upwind Train B.

Aerodynamic Drag Force Estimation of a Truck Trailer Model Using Artificial Neural Network

Prediction of the drag forces acting on truck trailer with/without spoiler is carried out by using artificial neural network (ANN). ANN model data set include the experiments of spoiler positions which have zero level to trailer front corner, -2 mm, -4.5 mm, -9 mm, +4.5 mm and +9 mm offset and truck trailer without spoiler. The experiments were carried out in the wind tunnel for number of ten free stream velocities of 4.6, 6.2, 7.8, 9.4, 11.0, 12.6, 14.2, 15.8, 17.45 and 19.3 m/s which are corresponds to Reynolds numbers range between 1.0x105 and 5.0x105. Mean absolute percentage error (MAPE) is obtained as 2.24% for training, 3.75% for validation and 7.58% for testing in the prediction of the drag forces. Prediction performance of the developed ANN model has very good accuracy. According to drag coefficients results, Reynolds independent for truck trailer model is obtained at Reynolds number between 1.97x105 and 4.89x105. For spoiler position cases, while minimum drag coefficient acting truck trailer with spoiler is seen at – 2mm offset, maximum drag coefficient is seen at -9 mm offset. The maximum drag reduction is 22.6 % at the position of -2 mm spoiler offset in Re = 3.2x105.

Numerical evaluation of passive control of shock wave/boundary layer interaction on NACA0012 airfoil using jagged wall

Shock formation due to flow compressibility and its interaction with boundary layers has adverse effects on aerodynamic characteristics, such as drag increase and flow separation. The objective of this paper is to appraise the practicability of weakening shock waves and, hence, reducing the wave drag in transonic flight regime using a two-dimensional jagged wall and thereby to gain an appropriate jagged wall shape for future empirical study. Different shapes of the jagged wall, including rectangular, circular, and triangular shapes, were employed. The numerical method was validated by experimental and numerical studies involving transonic flow over the NACA0012 airfoil, and the results presented here closely match previous experimental and numerical results. The impact of parameters, including shape and the length-to-spacing ratio of a jagged wall, was studied on aerodynamic forces and flow field. The results revealed that applying a jagged wall method on the upper surface of an airfoil changes the shock structure significantly and disintegrates it, which in turn leads to a decrease in wave drag. It was also found that the maximum drag coefficient decrease of around 17 % occurs with a triangular shape, while the maximum increase in aerodynamic efficiency (lift-to-drag ratio) of around 10 % happens with a rectangular shape at an angle of attack of 2.26 $$^\circ $$ .

Shock interaction with deformable particles using a constrained interface reinitialization scheme

In this paper, we present axisymmetric numerical simulations of shock propagation in nitromethane over an aluminum particle for post-shock pressures up to 10 GPa. We use the Mie-Gruneisen equation of state to describe both the medium and the particle. The numerical method is a finite-volume based solver on a Cartesian grid, that allows for multi-material interfaces and shocks, and uses a novel constrained reinitialization scheme to precisely preserve particle mass and volume. We compute the unsteady inviscid drag coefficient as a function of time, and show that when normalized by post-shock conditions, the maximum drag coefficient decreases with increasing post-shock pressure. We also compute the mass-averaged particle pressure and show that the observed oscillations inside the particle are on the particle-acoustic time scale. Finally, we present simplified point-particle models that can be used for macroscale simulations. In the Appendix, we extend the isothermal or isentropic assumption concerning the point-force models to non-ideal equations of state, thus justifying their use for the current problem.

Vortex Shedding and Aerodynamic Performance of Airfoil with Multiscale Trailing-Edge Modifications

An experimental investigation was conducted into the nature of the vortex shedding generated by truncated and nonflat serrated trailing edges of a NACA 0012 wing section as well as the aerodynamic performance of such trailing edges. In line with previous findings, the truncated trailing edge generates a significant amount of vortex shedding, while increasing both the maximum lift and drag coefficients, resulting in an overall reduction in the maximum lift-to-drag ratio compared to a plain NACA 0012 wing section. By decreasing the chevron angle of the nonflat trailing-edge serrations (i.e., by making them sharper), the energy of the vortex shedding significantly decreases, and the lift-to-drag ratios increase compared to a plain NACA 0012 wing section. Fractal/multiscale patterns made of scaled-down repetitions of these serrations were also investigated with a view to further improve performance. It was found that the energy of the vortex shedding increases with an increasing fractal iteration if the chevron is broad () but decreases for sharper chevrons (). It is believed that if is too big, the multiscale trailing edges are too far away from each other to interact and break down the vortex shedding mechanism. Fractal/multiscale trailing edges are also able to improve aerodynamic performance compared to the NACA 0012 wing section.