Pressure Coefficient Distribution Research Articles

Convective heat dissipation from a wheel-hub-mounted railway brake disc

Air flow and convective heat dissipation from a wheel-hub-mounted railway brake disc were studied using computational fluid dynamics (CFD). The analyses enabled detailed insight into air flow, temperature, speed, pressure, and convective heat transfer coefficient distributions, never before available to a brake designer. Such results are practically impossible to experimentally obtain at reasonable cost. Particularly interesting finding is the existence of relatively considerable secondary flow — circumferential flow behind the vanes. Although this effect may reduce air pumping, and therefore radial air speed, it increases turbulence by promoting airflow between channels. Average values of the convective heat transfer coefficients obtained using CFD are practically identical to the experimental values derived from cooling tests performed on dynamometer and spin rig. Compared with other railway disc designs, this disc demonstrates exceptionally good convective cooling characteristics, in particular considering its size and investigated operating condition (rotation in still air). The computational studies of air flow and heat dissipation characteristics are an excellent base for development work in improving existing and generating new disc designs with high heat dissipation characteristics. Future work is concentrated on conducting additional analyses of the current disc design, in order to investigate the most effective ways of maximizing convective cooling.

The interaction of an asymmetrical localised synthetic jet on a side-supported sphere

Abstract A localised synthetic jet offers promise of an optimum and cost-effective practical method of delaying separation and promoting reattachment in fluids with solid body interactions. The asymmetric flow that may result from its use may also be beneficial in improving the aerodynamic performance of a lifting body. There are insufficient studies of synthetic jets, particularly on three-dimensional bluff bodies that are more representative of complex flows in real situations. A comprehensive study on an 80 mm diameter sphere designed with localised synthetic jet orifices was, therefore, conducted in an 18 in×18 in open circuit closed test-section wind tunnel at a Reynolds number of 5×104. The coefficient of pressure distribution was measured by continuously varying the location of the synthetic jet and compared with the no synthetic jet condition. The three-dimensional effects on the flow over the sphere body are particularly made apparent through the growth and the effects of the boundary layer and the deviation from potential flow. Overall, the synthetic jet had the effect of delaying the separation point and extending it further downstream on the sphere surface concomitantly producing a significant reduction in drag, providing solid support to the viability of strategically located synthetic jet when higher lift or lower drag is desired. A surprising discovery was the ability of the synthetic jet to improve the flow at the junction of the sting support and sphere. This has promising implications in devising methods to reduce interference drag that are common in many practical applications such as near junctions between wing and the fuselage.

Steady Flow of Power Law Fluids over a Pair of Cylinders in Tandem Arrangement

The steady flow of incompressible power-law fluids over a pair of cylinders in tandem arrangement has been studied numerically. The field equations have been solved using a finite volume method based solver (FLUENT 6.2). In particular, the effects of the power-law index (0.4 ≤ n ≤ 1.8), Reynolds number (1 ≤ Re ≤ 40), and the gap ratio between the two cylinders (2 ≤ G ≤ 10) on the local and global flow characteristics such as streamline profiles, center line velocity, surface pressure coefficient, and individual and total drag coefficients, etc. have been studied in detail. The wake interference in conjunction with the power-law rheology exerts a strong influence on the flow dynamics at high Reynolds numbers, even in the steady-flow regime, whereas at low Reynolds numbers, the flow is influenced by the rheological behavior of the fluid. The increasing degree of shear-thinning behavior delays the flow separation, whereas early separation is seen in Newtonian and in shear-thickening fluids. The pressure coefficient distribution on the surface of the cylinders shows an intricate dependence on the power-law index, Reynolds number, and gap ratio. The shear-thinning behavior exerts stronger effects on the drag characteristics than those seen in Newtonian and shear-thickening fluids. Both upstream and downstream cylinders show smaller values of the individual and total drag coefficients than those for a single circular cylinder under otherwise identical conditions.

Vortex formation and heat transfer in turbulent flow past a transverse cavity with inclined frontal and rear walls

The process of vortex formation, distributions of pressure coefficients, and convective heat transfer in a turbulent flow past a cavity with a low aspect ratio and inclined frontal and rear walls were experimentally studied. The angle of wall inclination φ was varied in the interval from 30° to 90°. Visualization techniques were applied to trace the evolution of the flow with the angle φ as the transverse cavity became more open. Pressure fields in the longitudinal and transverse sections on the bottom wall of the cavity, and on its frontal and rear walls, were measured. The measured distributions of temperature in the longitudinal and transverse sections on the three heated walls, and the obtained thermographic fields over the whole heated surface, were used to calculate local and average heat-transfer coefficients. It is found that in the interval of wall inclination angles φ = 60–70° the flow in the cavity becomes unstable, with the primary vortex changing its structure from single-cellular to double-cellular. As a result, the distributions of static pressure and surface temperature across and along the cavity suffer dramatic changes. At smallest angles φ the flow re-attachment point gets displaced into the cavity to cause an abrupt growth of pressure and heat-transfer coefficients on the rear wall, which leads to a slight increase of the surface-mean pressure and heat transfer inside the cavity. At the angle of instability, φ = 60°, the local heat-transfer coefficient decreases markedly over the cavity span from the end faces of the cavity toward its center, and a most pronounced intensification of heat transfer is observed.

PIV measurements around 2D and 3D building models

Particle image velocimetry (PIV) measurements for three Reynolds numbers were performed around two-dimensional and three dimensional model buildings that were placed inside an Eiffel type wind tunnel. For the 2D model, the height was half the length while for the 3D model the height equaled half the width and the length. Along with the velocity measurements, pressure measurements on the center plane of the models were also taken. Noticeable differences in the pressure coefficient distribution between the 2D and 3D models were observed. However, there are no significant Reynolds number effects in the range studied for both types of models.

Análise do Campo de Pressões em Ressalto Hidráulico Submergido a Jusante de uma Comporta

The hydraulic jump is widely used in energy dissipation downstream from hydraulic works. It is a dissipation process associated with velocity, water level and pressure fluctuations. Due to the damage caused to energy dissipation structures over the years by problems related to stress, cavitation and resonance, it is now important to understand the role of the hydraulic jump as a form of dissipation. The characterization of the pressure fields in the bottom of stilling basins is of practical use to the designers of hydraulic works seeking more efficient and economical sizing. There is a large bibliography on free hydraulic jumps, however, little has been written about the most common situation in stilling basins, the submerged hydraulic jump. This article analyses the pressure field in a submerged hydraulic jump downstream from a sluice gate in order to help understand the energy dissipation process and the design optimization of stilling basins. Through the use of non-dimensional parameters it was possible to evaluate and quantify the effects attributed to the submergence on the longitudinal distribution of average pressure, standard deviation, skewness and curtosis coefficients and on the power spectra of the pressure fluctuation

Experimental and Numerical Investigation of 65 Degree Delta and 65/40 Degree Double-Delta Wings

In this study, an experimental and numerical investigation was carried out to obtain lift, drag, and pitching moment data on 65 degree delta and 65/40 degree double-delta wings. The experimental tests were conducted at the King Fahd University of Petroleum and Minerals low-speed wind-tunnel facility, whereas the numerical tests were performed using the commercial computational fluid dynamics software FLUENT. Results from both experiments and numerical predictions were compared to other experimental data found in literature as well as to the theory of Polhamus. The results of comparison of surface pressure coefficient distribution and vortex breakdown location show good agreement with experiments. Overall, the comparison of result shows good agreement between different experimental studies as well as good agreement with the computational fluid dynamics predictions and the theoretical calculations.

Unsteady Flow Computations around Moving Airfoils by Overset Unstructured Grid Method

The study presents a computational method for unsteady flows around moving multiple objects using overset unstructured grid method. To capture unsteady flow characteristics, the dual time-stepping method coupled with the LU-SGS implicit scheme is applied to the time integration. Two or more moving boundaries can be treated easily by the overset unstructured grid method. In this study, Chimera-hole cutting is performed automatically along with movement of the objects. For validation, the numerical results for inviscid and viscous flow and the experimental results are compared using the lift coefficient and pressure coefficient distribution of a pitching NACA0012 airfoil. Both results show the tendency of the experimental data, confirming the validity of this method. Furthermore, the flow around several objects is examined and the generality of the overset grid method is verified. Two demonstrations of the unsteady computation flapping airfoil, and a section of a vertical-axis wind turbine with three rotors are also described.

Experimental and Numerical Study on Heat Transfer with Impinging Circular Jet on a Convex Hemispherical Surface

In this paper, the experimental and numerical study has been carried out to investigate the water jet impingement on a convex hemispherical surface. The pressure and skin friction coefficient distributions on the impingement surface were analyzed numerically. A great deal of attention was paid to analyze the effects of the jet impingement exit velocity and the nozzle-to-surface distance on the heat transfer characteristics. A comparison of the heat transfer coefficient was performed between the jet impingement on the convex surface and the flat plate. The results show that the Nusselt number for the convex surface is higher than that for the flat surface. The local Nusselt number is decreased monotonically from its maximum value at the stagnation point for lower Reynolds numbers. A secondary maxima occurs for higher Reynolds numbers. The experiment and simulation were performed with the following parameters: the jet impingement Reynolds number of Re = 1947–19478, and the nozzle-to-surface distance of L/D = 2.5–25.

Seismic passive resistance with vertical seepage and surcharge

Present paper focuses on the computation of the seismic passive earth pressure acting on a vertical rigid retaining wall by a soil mass subjected to vertical steady-state seepage and a uniform surcharge load. Based on the basic assumptions of Coulomb's theory and a pseudo-static method of analysis, a general solution for the passive earth pressure containing two coefficients is presented. In the solution, many parameters, such as unit weight of saturated soil, soil effective internal friction angle, soil/wall friction angle, water/soil unit weight ratio, surcharge intensity coefficient, horizontal and vertical seismic acceleration coefficients, Poisson's ratio of soil mass, hydraulic gradient, and coefficients of pore water pressure, are considered. The effects of hydraulic gradient and seismic forces on passive earth pressure coefficient and passive earth pressure distribution are investigated. The results indicate that passive earth pressure increases with increasing hydraulic gradient for downward water flow case, but decreases for upward water flow case, and that the presence of seismic forces induces a reduction in passive earth pressure.

냉각유로 내 곡관부 및 유로의 회전이 압력강하에 미치는 영향

The present study investigated local pressure drop in a rotating smooth square duct with turning region. The duct has a hydraulic diameter (D<SUB>h</SUB>) of 26.7 ㎜ and a divider wall of 6.0 ㎜ or 0.225 D<SUB>h</SUB>. The distance between the tip of the divider and the outer wall of the duct is 1.0 D<SUB>h</SUB>. The Reynolds number (Re) based on the hydraulic diameter is kept constant at 10,000, and the rotation number (Ro) is varied from 0.0 to 0.20. The pressure coefficient distribution (C<SUB>p</SUB>), the friction factor (f) and the thermal performance (ŋ) are presented on the leading, the trailing and the outer surfaces. It is found that the curvature of the 180°-turn produces Dean vortices that cause the high pressure drop in the turning region. The duct rotation results in the pressure coefficient discrepancy between the leading and trailing surfaces. That is, the high pressure values appear on the trailing surface in the first-pass and on the leading and side surfaces in the second-pass. As the rotation number increases, the pressure discrepancy enlarges. In the turning region, a pair of the Dean vortices in the stationary case transform into one large asymmetric vortex cell, and then the pressure drop characteristics also change.

Scaling of the Wall Pressure Field Around Surface-Mounted Pyramids and Other Bluff Bodies

The turbulent flow around square-based, surface-mounted pyramids, of height h, in thin and thick boundary layers was experimentally investigated. The influence of apex angle ζ and angle of attack α was ascertained from mean surface flow patterns and ground plane pressure measurements taken at a Reynolds number of 3.3×104 based on h. For both boundary layer flows, it was found that the normalized ground plane pressure distributions in the wakes of all the pyramids for all angles of attack may be scaled using an attachment length (Xa′) measured from the upstream origin of the separated shear layer to the near-wake attachment point on the ground plane. It was also shown that this scaling is applicable to data reported in the literature for other bluff body shapes, namely, cubes, cones, and hemispheres. The ground plane pressure coefficient distributions in the upstream separated flow region, for all the shapes and angles of attack examined, were found to collapse onto two curves by scaling their streamwise location using a length scale (Xu), which is a function of the frontal projected width of the body (w′) and the height of the body. These two curves were for cases where δ∕h&lt;1 (“thin” boundary layer) or δ∕h≥1 (“thick” boundary layer), where δ is the oncoming boundary layer thickness. Further work is required to provide a more detailed statement on the influence of boundary layer thickness (or state) on the upstream pressure field scaling.

Wind effects on the world’s longest spatial lattice structure: Loading characteristics and numerical prediction

The 486-m long roof structure of Shenzhen Citizens’ Centre is the world’s longest spatial lattice structure. This paper presents some selected results from a combined wind tunnel and numerical simulation study of wind effects on the extra-long-span roof structure. In this study, simultaneous pressure measurements on its entire roof are made in a boundary layer wind tunnel, and the measured wind pressures, such as mean, root-mean-square (rms) and peak pressure coefficient distributions on the roof are presented and discussed. Based on the measured data from a number of pressure taps, a numerical simulation approach using backpropagation neural networks (BPNN) is developed for the predictions of wind-induced pressure time series at other roof locations which are not covered in the wind tunnel measurements. The BPNN is trained with the pressure data time series measured from adjacent pressure taps. The good performance of the developed neural network is demonstrated by comparing the predictions with the model test results, illustrating that the BPNN approach can serve as an effective tool for the design and analysis of wind effects on large roof structures in conjunction with wind tunnel tests.

低層建築物群内における陸屋根の低層建築物の風荷重について:その１ 実験条件および局部風圧係数の分布について

低層建築物群内における陸屋根の低層建築物の風荷重について:その１ 実験条件および局部風圧係数の分布について

Simulations of transonic flow fields around an elastic arrow wing

Transonic flows are characterised by the presence of adjacent regions of subsonic and supersonic flow, usually with shock waves present at the downstream supersonic-subsonic interface. An in-house fortran90 computer code, tranflow3d, which is robust and capable of simulating complex nonlinear flow fields around a double-swept-back semispan supersonic transport wing model (of high sweep angle) at transonic speeds is presented. The code has acceptable turnaround times on current high performance personal computers. The flow is governed by the general frequency, nonlinear, transonic small disturbance equation, subject to nonreflecting, far field, boundary conditions. Numerical solution procedure based on finite difference method, method of false transients and approximate factorisation technique is utilised. Chordwise distributions of steady pressure coefficient and local Mach numbers are computed, and compared with those obtained from wind tunnel measurements.

Turbulent flow past a transverse cavity with inclined side walls. 1. Flow structure

The process of vortex formation in a cavity with inclined walls, which has a moderate aspect ratio, is experimentally studied, and the distribution of pressure coefficients is measured. The angle of inclination of the side walls ϕ is varied from 30 to 90°. It is found that the flow in the cavity becomes unstable in the range of inclination angles ϕ = 60–70°. Flow reconstruction occurs, which substantially alters the surface-temperature and static-pressure distributions. Large changes in these characteristics and their nonuniform distributions for these angles are observed across the cavity on its frontal wall and on the bottom. For small angles (ϕ = 30 and 45°), the pressure on the rear wall drastically increases, which leads to a small increase in pressure averaged over the entire cavity surface.

NUMERICAL ANALYSIS OF TURBULENT FLOW IN A CONICAL DIFFUSER

A commercial CFD code, CFX-TASCflow, is used to predict turbulent flow in a conical diffuser. The computation was performed using a low-Reynolds number k–e model, low-Reynolds number k–ω model, a low-Reynolds number k–ω based non-linear algebraic Reynolds stress model, and a second moment closure with a wall-function. The experimental datasets of Singh [1] and Kassab [2] are used to validate the numerical results. The results show that all the turbulence models reproduce the static pressure coefficient distribution reasonably well. The low Reynolds number k–ω models give better prediction of the friction velocity than the second moment closure. The models also predict the Reynolds shear stress reasonably well but fail to reproduce the correct level of the kinetic energy.

Dynamic modelling of a wind catcher/tower turret for natural ventilation

This paper discusses experimental and theoretical investigations and Computational Fluid Dynamics (CFD) modelling considerations to evaluate the performance of a square section wind catcher system connected to the top of a test room for the purpose of natural ventilation. The magnitude and distribution of pressure coefficients (Cp) around a wind catcher and the air flow into the test room were analysed. The modelling results indicated that air was supplied into the test room through the wind catcher's quadrants with positive external pressure coefficients and extracted out of the test room through quadrants with negative pressure coefficients. The air flow achieved through the wind catcher depends on the speed and direction of the wind. The results obtained using the explicit and AIDA implicit calculation procedures and CFX code correlate relatively well with the experimental results at lower wind speeds and with wind incidents at an angle of 08. Variation in the Cp and air flow results were observed particularly with a wind direction of 458. The explicit and implicit calculation procedures were found to be quick and easy to use in obtaining results whereas the wind tunnel tests were more expensive in terms of effort, cost and time. CFD codes are developing rapidly and are widely available especially with the decreasing prices of computer hardware. However, results obtained using CFD codes must be considered with care, particularly in the absence of empirical data. Practical application: There exist various modelling techniques for the investigation of the performance of natural systems such as wind catchers. These modelling techniques include simple calculation procedures, wind tunnel testing, salt bath, Computational Fluid Dynamics (CFD) and real building performance (POE studies). The calculation procedural models are simple to use, however, due to their simplicity they do not provide a full picture of the performance of the natural ventilation system and air movement inside rooms. Other models such as wind tunnels and CFD are more comprehensive but expensive and time consuming to use. Various commercial CFD models are available in the market today and not many of them are specifically designed for modelling of natural ventilation. Results obtained using CFD models should be considered with care specially in the absence of empirical data and if the results were obtained by novice users. Wind catchers are innovative techniques for the application of natural ventilation in buildings in temperate climates such as that of the UK. Their performance greatly depends on wind conditions. However, they should be designed as an integral part of the overall design of the HVAC system in a hybrid or mixed mode operation. The natural ventilation system of wind catchers should be exploited whenever possible, particularly in the hot summer months to reduce the energy and environmental cost of full operation of an air-conditioning system.

Steady Flow of Power Law Fluids across a Circular Cylinder

AbstractThe momentum equations describing the steady cross‐flow of power law fluids past an unconfined circular cylinder have been solved numerically using a semi‐implicit finite volume method. The numerical results highlighting the roles of Reynolds number and power law index on the global and detailed flow characteristics have been presented over wide ranges of conditions as 5 ≤ Re ≤ 40 and 0.6 ≤ n ≤ 2. The shear‐thinning behaviour (n &lt; 1) of the fluid decreases the size of recirculation zone and also delays the separation; on the other hand, the shear‐thickening fluids (n &gt; 1) show the opposite behaviour. Furthermore, while the wake size shows non‐monotonous variation with the power law index, but it does not seem to influence the values of drag coefficient. The stagnation pressure coefficient and drag coefficient also show a complex dependence on the power law index and Reynolds number. In addition, the pressure coefficient, vorticity and viscosity distributions on the surface of the cylinder have also been presented to gain further physical insights into the detailed flow kinematics.

Peak Wind Load Comparison: Theoretical Estimates and ASCE 7

A new procedure to derive the distribution of peak pressure and load coefficients from individual sample records is applied to wind tunnel records obtained from a generic flat-roof model tested at the Univ. of Western Ontario (UWO). The initial step of this procedure requires the identification of the appropriate marginal probability of these records. The corresponding distribution of the peaks is then obtained with the use of the standard translation process. Predicted load coefficients over variable tributary areas as determined from UWO records and based on a preset probability of non-exceedence are compared with the provisions of the ASCE 7 standard. Based on the open-terrain observations, the code provisions generally correspond to relatively low levels of nonexceedence of 84% or less. The ASCE 7 suggested method to estimate the peak pressure coefficients for the suburban environment based on those for the open terrain could be successfully applied to the UWO observations. This, however, is generally not true when performing a similar estimation procedure for peak load coefficients from the Clemson experiments.