Divergent Ducts Research Articles

Aerodynamics and aeroacoustics of ducted propellers: A study on the design and geometry effects

Ducted propellers present broad applicability in urban air mobility vehicles due to their enhanced operational safety, improved aerodynamic performance, and potential to mitigate noise emissions. This study proposes a numerical approach for designing adequate duct geometries, focusing on the duct's lip profile, expansion ratio, and tip clearance, aiming to provide valuable design guidance for ducted propellers. The simulations are validated through experimental data, showing reasonable agreement in terms of thrust generation and far-field noise. The mean flow and generated thrust are characterized with a parametric study using steady simulations, while delayed detached eddy simulations are employed to capture transient flow characteristics and investigate noise generation. The noise levels were computed using the integral solution of the Ffowcs-Williams and Hawkings equation. The lip geometry impacts the flow distribution and generated thrust, modifying the tonal noise. Furthermore, slightly divergent ducts can increase the total thrust by minimizing flow separation on the duct wall while increasing the suction on the duct lip. The primary noise sources are identified at the propeller's leading edge and tip. The results reveal that divergent ducts effectively reduce tonal noise at all observer angles but increase broadband noise, attributed to the noise sources at the leading edge of the propeller and the interaction with the duct lip. Additionally, reducing the tip clearance from 2 to 1 mm enhances the total thrust by more than 20% without causing extra noise generation.

Global classical solutions near vacuum to the initial-boundary value problem of isentropic flows through divergent ducts

Global classical solutions near vacuum to the initial-boundary value problem of isentropic flows through divergent ducts

Numerical Study on Thermal and Flow Characteristics of Divergent Duct with Different Rib Shapes for Electric-Vehicle Cooling System

The cooling performance of the air-conditioning system in electric vehicles could be enhanced through the geometrical optimization of the air ducts. Furthermore, it has been proven that the heat-transfer performance of divergent channels is better than that of conventional channels. Therefore, the present study investigates the thermal and flow characteristics of divergent ducts with various rib shapes for the cooling system of electric vehicles. The thermal and flow characteristics, namely, temperature difference, pressure drop, heat-transfer coefficient, Nusselt number and friction factor, are numerically studied. Divergent ducts comprising ribs with the different shapes of rectangle, isosceles triangle, left triangle, right triangle, trapezoid, left trapezoid and right trapezoid arranged symmetrically are modeled as the computational domains. The thermal and flow characteristics of divergent ducts with various rib shapes are simulated in ANSYS Fluent commercial software for the Reynolds-number range of 22,000–79,000. The numerical model is validated by comparing the simulated results with the corresponding experimental results of the Nusselt number and the friction factor, obtaining errors of 4.4% and 2.9%, respectively. The results reveal that the divergent duct with the right-triangular rib shape shows the maximum values of the heat-transfer coefficient and Nusselt number of 180.65 W/m2K and 601, respectively. The same rib shape shows a pressure drop and a friction factor of 137.3 Pa and 0.040, respectively, which are lower than those of all rib shapes, except for the trapezoidal and right-trapezoidal rib shapes. Considering the trade-off comparison between thermal and flow characteristics, the divergent duct with the right-triangular rib shape is proposed as the best configuration. In addition, the effect of various conditions of the inlet air temperature on the thermal characteristics of the best configuration is discussed. The proposed results could be considered to develop an air-duct system with enhanced efficiency for electric vehicles.

Global Continuous Sonic–Supersonic Flows in Two-Dimensional Semi-infinite Divergent Ducts

Global Continuous Sonic–Supersonic Flows in Two-Dimensional Semi-infinite Divergent Ducts

A Sonic Condition Model for Pseudo-Shock in Divergent Ducts

A Sonic Condition Model for Pseudo-Shock in Divergent Ducts

The Gas Flow Characteristics of A Helicopter Engine Exhaust Duct Using CFD Analysis

In this project is the gas flow characteristic of a helicopter engine exhaust and to derive an optimized geometry for the exhaust duct with minimum pressure loss using CFD analysis. In this project, the variables to be analyzed are the inlet static pressure, velocity and temperature of the exhaust gas at the exit of the exhaust duct. Cylindrical exhaust ducts of various dimensions are modeled by changing the primary and secondary inlet area and the exhaust geometry is optimized based on the CFD results. For the same optimized area of primary and secondary inlet, divergent exhaust duct is modeled and analyzed for various divergent angles. Parametric studies were carried out for a better comparison between cylindrical and divergent ducts for the better thermal efficiency.

Heat Transfer Augmentation in a Divergent Duct with Angled Ribs

Heat transfer characteristics in the rectangular divergent duct with parallel angled ribs are experimentally compared with the straight smooth circular duct. The ribs with four different parallel angles ([Formula: see text], 45[Formula: see text], 60[Formula: see text], and 90[Formula: see text]) are glued on the duct’s two opposite walls as well as on the duct’s one sided wall only, respectively. The 0.72[Formula: see text]-inclined walls are installed at the two opposite walls of the rectangular divergent duct. The test section of 1000[Formula: see text]mm long has the cross section of [Formula: see text][Formula: see text]mm2 at inlet and [Formula: see text][Formula: see text]mm2 at exit. The ribbed walls are manufactured with a rib height [Formula: see text][Formula: see text]mm and the ratio of rib spacing ([Formula: see text]) to height([Formula: see text]) [Formula: see text] 10. The main findings are summarized that the increase in the dimensionless Nusselt number for the flow attack angles can be seen in the order of 90[Formula: see text], 30[Formula: see text], 60[Formula: see text], and 45[Formula: see text] at the two opposite ribbed divergent wall ducts, in addition, the average Nusselt number in the divergent rectangular duct with two opposite ribbed walls is somewhat greater than in the ribbed straight cross-sectional rectangular duct.

Design of a ducted wind turbine for offshore floating platforms

Within the Marine Energy Laboratory project, funded by the Italian Ministry for Education, University and Research, one of the first offshore wind plants on floating hulls, hosting ducted wind turbines, has been considered. The confinement of horizontal-axis wind turbines inside divergent ducts is reconsidered, in light of material innovation and direct drive coupling. Ducted wind turbines can take advantage of the flow rate increase due to the effect of the divergent shrouds. The conventional blade element momentum theory has been reformulated in order to deal with ducted turbines. Furthermore, computational fluid dynamics simulations have been carried out based on the solution of the steady two-dimensional Reynolds-averaged Navier–Stokes equations for axisymmetric swirling flows. In order to avoid any expensive mesh refinement near the actual rotor blades, the turbine effect on the flow field is taken into account by means of source terms for the momentum equations solved inside the domain swept by the rotor. This technique allowed us to optimize the geometry of the ducted wind turbine in an extremely effective way.

Experimental study of developing turbulent flow and heat transfer in ribbed convergent/divergent rectangular ducts

The article represents an experimental investigation of friction and heat transfer characteristics of divergent / convergent rectangular ducts with an inclination angle of 1? in the y-axis. Measurements were taken for a convergent / divergent rectangular duct of aspect ratio AR at inlet1.25 and outlet in convergent channel 1.35; but in case of divergent duct it can be reversed. The four uniform rib heights, e = 3, 6, 9 and 12 mm the ratio between rib height to hydraulic mean diameter (e/Dm) are 34.8, 69.7, 104.6 and 138.7 a constant rib pitch distance, P = 60 mm has been used. The flow rate in terms of average Reynolds number based on the hydraulic mean diameter (Dm) is 86 mm of the channel was in a range of 20,000 to 50,000. The two ceramic heating strip of 10 mm thickness is used as a heating element have attached on top and bottom surfaces for the test sections. The heat transfer performance of the divergent / convergent ducts for 3, 6, 9 and 12 mm ribs was conducted under identical mass flow rate based on the Reynolds number. In our experiments has totally 8 different ducts were used. In addition, the acceleration / deceleration caused by the cross section area, the divergent duct generally shows enhanced heat transfer behavior for four different rib sizes, while the convergent duct has an appreciable reduction in heat transfer performance. From result point view divergent duct with 3 mm height ribbed square duct gets maximum heat transfer coefficient with minimum friction loss over the other convergent / divergent ducts.

Two dimensional featured one dimensional digital waveguide model for the vocal tract

A vocal tract model based on a digital waveguide is presented in which the vocal tract has been decomposed into a number of convergent and divergent ducts. The divergent duct is modeled by a 2D-featured 1D digital waveguide and the convergent duct by a one dimensional waveguide. The modeling of the divergent duct is based on splitting the volume velocity into axial and radial components. The combination of separate modeling of the divergent and convergent ducts forms the foundation of the current approach. The advantage of this approach is the ability to get a transfer function in zero-pole form that eliminates the need to perform numerical calculations on a discrete 2D mesh. In this way the present model named as a 2D-featured 1D digital waveguide model has been found to be more efficient than the standard 2D waveguide model and in very good comparison with it in the formant frequency patterns of the vowels /a/, /e/, /i/, /o/ and /u/. The model has two control parameters, the wall and glottal reflection coefficients that can be effectively employed for bandwidth tuning. The model also shows its ability to generate smooth dynamic changes in the vocal tract during the transition of vowels.

ACCRETION PROBLEM IN A KERR BLACK HOLE GEOMETRY VIEWED AS FLOWS IN CONVERGING–DIVERGING DUCTS

Accretion flow on a horizon is supersonic, no matter what the flow angular momentum or the spin of the black hole is. This means that a black hole accretion can always be viewed as a flow in a flat space–time through one or more convergent–divergent ducts. In this paper, we study how the area of cross-sections must vary in order that the flow has the same properties in both systems. We show that the accretion flow experiencing a shock is equivalent to having two ducts connected back-to-back, both with a neck where the flow becomes supersonic. We study the pressure and Mach number variations for corotating, contrarotating flows and flows around a black hole with evolving spin.

A study of the impulse waves discharged from convergent and divergent ducts

The present study describes experimental and computational works to investigate impulse waves that are generated by discharge of a compression pulse from the exit of convergent and divergent ducts. The objective of the present study is to compare the impulse waves discharged from a straight duct with those discharged from convergent and divergent ducts. Computational analysis is performed using the axisymmetric, unsteady, inviscid, compressible, Euler equations. A second-order total variation diminishing finite difference scheme is used to solve the governing equation system. Experiment is carried out in a simple shock tube with an open end. Convergent and divergent ducts are connected to the open end of the shock tube. Initial compression pulses with different overpressures and wavelengths are made at the entrance of the convergent and divergent ducts. The present computational method predicts the measured impulse waves well. The results obtained show that for a given duct the magnitude of the impulse wave decreases as the wavelength of the initial compression pulse increases and it is a weakly increasing function of the overpressure of the initial compression pulse. Compared with a straight duct, a convergent duct leads to a weaker impulse wave, while a divergent duct causes a stronger impulse wave. It is therefore believed that the convergent duct can be a passive control device used to reduce the magnitude of the impulse wave.

TURBULENT FLOW STRUCTURE IN A VARIABLE AREA DUCT

Turbulent structure of air flow field in a multi-convergent divergent duct is examined experimentally using a hot wire anemometer having an x-probe. The lateral profiles of the normalized mean axial and traverse velocities and the kinetic energy of the mean flow at selected axial stations are presented. The distributions of turbulence root mean square, shear stress, skewness and flatness are also presented. The present results improve the understanding of the interaction between the mean flow and turbulent quantities with periodic changes of flow cross-sectional area. This understanding is quite essential for the optimum engineering use of multi-convergent divergent ducts in either MHD or enhancement of heat transfer applications. The experimental data are also useful for developing suitable turbulence and numerical sub-models for prediction of the flow fields in variable area ducts.

EXPERIMENTAL STUDIES ON FLOW IN CONVERGENT AND DIVERGENT DUCTS OF RECTANGULAR CROSS SECTION

This paper describes the experimental examination of the pressure drop and heat transfer of the flow in convergent and divergent ducts of rectangular cross section. The aspect ratio based on the dimensions of the large end of the duct was 0⋅1. It has been found that at a given convergent or divergent angle pressure drop decreases while heat transfer increases with increasing Reynolds number. Along a given duct of small convergent angle, pressure drop increases while heat transfer decreases along the duct. However, heat transfer may increase near the downstream end of ducts of high convergent angle. At a given Reynolds number, both pressure drop and heat transfer increase with increasing convergent angle. As for flow in divergent ducts, the effects of Reynolds number on pressure drop and heat transfer are somewhat similar to those of flow in a convergent duct. © 1997 by John Wiley & Sons, Ltd.

Supersonic flow of hydrogen and air in a duct with variable area

A chemically nonequilibrium supersonic flow of hydrogen and air has been investigated in a duct with conically divergent or convergent walls. Elementary reaction schemes of radicals involved in reaction of hydrogen-air have been considered and solved through the CHEMKIN code. The aim was to promote an understanding of characteristics of chemically nonequilibrium supersonic flow by introducing a simple mathematical formulation. The temperature, pressure, and density all were found to decrease for divergent ducts as the flow was accelerated, whereas they increased for a slightly convergent duct or a constant cross-sectional area duct. For the divergent nozzle with a greater degree the flow became chemically frozen. But it was quite necessary to take account of the effect of chemical nonequilibrium in a moderately expanded or all convergent conical ducts. As was expected, it was found that the temperature, pressure and Mach number were reduced for a fuel-lean mixture.

FORCED CONVECTION IN CONVERGENT AND DIVERGENT DUCTS OF RECTANGULAR CROSS SECTION

Abstract This paper describes the simulation of forced convection in convergent and divergent ducts of rectangular cross section. The flow is modeled as being three-dimensional and parabolic, with constant wall temperature. The results show that the pressure drop decreases, while Nusselt number increases, with increasing Reynolds number or increasing aspect ratio. For convergent flows, both pressure drop and Nusselt number increase with convergent angle. For divergent flows, pressure drop may decrease with increasing divergent angle due to the recovery of the static pressure from dynamic pressure, but the variation of local Nusselt number is not so great as that of the convergent duct except in the entrance region.

Heat transfer and pressure drop characteristics of flow in convergent and divergent ducts

The heat transfer and pressure drop characteristics of the flow in convergent and divergent ducts of rectangular crosssection are obtained through the simulation of the flow by a three-dimensional parabolic model. The results show that in both convergent and divergent flows heat transfer decreases and pressure drop increases sharply near the entrance region of the ducts. Generally, the Nusselt number increases with increasing convergent/divergent angle, aspect ratio, or Reynolds number, and the pressure drop increases with increasing convergent/divergent angle or decreasing aspect ratio or Reynolds number in both flows. However, an increasing convergent/divergent angle may also result in a lower pressure drop owing to the recovery of static pressure from dynamic pressure. Furthermore, the pressure drop in a divergent flow is generally lower than that in a convergent flow except in the entrance region. For divergent flows with high divergent angle or high Reynolds number, flow separation may occur.

Possible Flow Regimes of Adiabatic One-Dimensional Compressible Fluid Flow with Friction in Convergent and Divergent Ducts

An analytical solution for the compressible one-dimensional flow in convergent and divergent ducts with friction is obtained. It is found that a non-dimensional parameter, N, can be formed using the friction factor, duct half-angle and the ratio of specific heats of the gas. Seven flow regimes are describable with the solution, based on certain bounds on the magnitude of N. The regimes are discussed and corollary data are presented graphically.

Impact Theory Calculations of the Lift and Drag of Ducted Bodies

SummaryNewtonian impact theory has been used to estimate the external pressure forces on ducted bodies of rectangular, circular and semi-circular cross section. With an allowance for skin friction (or other incidence-independent drag) it is shown that maximum lift/drag ratios of about three to four are possible with little effect of body geometry provided that short and very divergent, or tall and narrow rectangular ducts are avoided.Large changes occur in the separate lift and drag at maximum lift/drag ratio and these have to be made compatible with the weight and thrust capacity of the configurations. The maximum lift/drag ratio of a duct can be improved by the addition of wings, especially for a duct with poor lift/drag ratio, but the best overall performance is with a duct of good lift/drag ratio and low drag.It is anticipated that impact theory underestimates the pressure forces, and hence the lift/drag ratios, which would obtain at finite Mach numbers but the trends with geometrical changes should be reasonably reliable.