High Blockage Ratio Research Articles

Flow around a Rectangular Cylinder Placed in a Channel with a High Blockage Ratio under a Subcritical Reynolds Number

With the depletion of fossil energy sources, clean energy has become a growing concern for scholars. Vortex-Induced Vibration Aquatic Clean Energy (VIVACE), a device that uses water flow energy to generate electricity, has attracted much attention for its broad applicability and other advantages. Particle Image Velocimetry (PIV) experiments were conducted to improve the efficiency of the VIVACE device in low-velocity areas. The present study investigated the effects of the Blockage ratio (Br), Reynolds number (Re = ρU0D/μ), and Aspect ratio (Ar = B/D, width-to-height) of rectangular cylinders on flow characteristics. The influence of the Ar, Br, and Re on the flow field structure was systematically analyzed in terms of the time-averaged flow field, Reynolds shear stress, space–time correlation, vorticity field, and water pressure characteristics. The vorticity field was deconstructed by Proper Orthogonal Decomposition (POD). The results show that the first two orders of POD modal energy accounted for 75% of the total energy, indicating that the first two modes can be used to identify the large-scale vortex structure. The main water pressure frequency and vortex shedding frequency (f) had a high degree of consistency. Thus, vortex shedding was the main cause of wall water pressure fluctuations. Given the blockage effect, the shear layer’s development spanwise was restricted. Moreover, the blockage effect increased the local flow velocity and accelerated the vortex shedding. The dimensionless time-averaged flow velocity U/U0 increased to 1.5, and the frequency of vortex shedding increased by approximately 25% when the Br increased from 0.067 to 0.25. The frequency increased by 25% when the Ar decreased from 0.5 to 0.2. The experimental results also provide a new idea for optimizing the VIVACE device.

Experimental investigation of turbulent flow under different Reynolds numbers and blockage ratios in a heated rectangular channel

Particle Image Velocimetry (PIV) technology is used in current paper to investigated turbulent flow of central blocked accidents in a heated rectangular channel. Proper Orthogonal Decomposition (POD) method and Power Spectral Density (PSD) method are also used to expose the turbulent characteristics in the wake. The velocity field at different Reynolds numbers and blockage ratios is analyzed. During the blockage experiments, the movement of eddies are captured in detail in the instantaneous flow field. The vortex gradually decreases with the increase of Reynolds number, but gradually increases with the increase of blockage ratio. The dominant vortex structures in the first two POD modes gradually become sharper at higher blockage ratio. Subsequently, the peak frequencies of the instantaneous flow field are the same as those of POD modes at various blockage ratios.

Heat transfer and flow structure of a hot annular impinging jet

Infrared thermography and PIV have been used to investigate the flow and heat transfer of quasi-annular jets. The principal parameter studied is the blockage ratio corresponding to the ratio of the inner diameter over the outer Dint/D. The mass flow rate has been taken constant between the different configurations for comparison purpose and a circular jet have also been studied for reference purpose. Results show that the presence of inner diameter create inner shear layer structures that are bring below the jet main flow causing an increase in heat transfer rate. Moreover, higher blockage ratio also increase the ambient air entrainment into the jet before impingement. Finally, taking into account both heat transfer rate and pressure drop, an optimum blockage ratio has been determined.

Sedimentation behavior of a spherical particle in a Giesekus fluid: A CFD–DEM solution

Abstract The sedimentation of a spherical solid particle in viscoelastic fluids was studied using a Giesekus model by development of a direct numerical simulation solver based on fully-resolved and immersed boundary methods to understand the complex non-Newtonian fluid flow and spherical particle dynamics. Two benchmark problems were modeled to evaluate the accuracy of the developed solver and excellent agreement with prior experimental and simulation results were obtained. In addition, the temporal and spatial order of convergence of the transient and advection discretization schemes were studied. The aim is to provide a clearer picture for the effect of confinement and rheology of the viscoelastic fluid on the sedimentation phenomenon of a spherical particle in a Giesekus fluid. We report that the negative wake occurs at Weissenberg number Wi ≥ 2.45 at all blockage ratios a ∕ R = 0 . 243 , 0.390, 0.464, and 0.538. At Wi = 1 . 00 , the negative wake occurs mainly at blockage ratio a ∕ R ≥ 0.390 when the effect of confinement is enhanced. There is a linear relationship between the sphere overshoot/steady-state velocity and blockage ratio due to shear-thinning effects. The oscillations in the sphere velocity were reported at all Wi for blockage ratios a ∕ R ≤ 0.464. The sphere velocity oscillations disappear at the highest blockage ratio a ∕ R = 0 . 538 except at Wi = 0 . 01 and 1.00. The viscoelastic fluid velocity oscillations were reported at Wi ≥ 1.00 at all blockage ratios. The viscoelastic fluid velocity oscillations become more damped with blockage ratio a ∕ R which the mutual effects of shear-thinning and wall effects become more dominant.

A Hybrid BEM-CFD Virtual Blade Model to Predict Interactions between Tidal Stream Turbines under Wave Conditions

Tidal turbine array optimization is crucial for the further development of the marine sector. It has already been observed that tidal turbines within an array can be heavily affected by excessive aerodynamic interference, thus leading to performance deterioration. Small-scale experimental tests aimed at understanding the physical mechanisms of interaction and identifying optimal distances between machines can be found in the literature. However, often, the relatively narrow channels of laboratories imply high blockage ratios, which could affect the results, making them unreliable if extrapolated to full-scale cases. The main aim of this numerical study was to analyze the effects of the blockage caused by the laboratory channel walls in cases of current and also current surface waves. For this purpose, the performance predictions achieved for two turbines arranged in line for different lateral offsets in case of a typical laboratory scale were compared to the predictions obtained for a full scale, unconfined environment. The methodology consisted in the adoption a hybrid Blade Element Momentum–Computational Fluid Dynamics (BEM-CFD) approach, which was based on the Virtual Blade Model of ANSYS-Fluent. The results indicate that (1) the performance of a downstream turbine can increase up to 5% when this has a lateral separation of 1.5D from an upstream device in a full-scale environment compared to a misleading 15% calculated for the laboratory set-up, and (2) the relative fluctuations of power and thrust generated by waves are not significantly affected by the domain dimensions.

Some new characteristics of the confined flow over circular cylinders at low reynolds numbers

This study summarises some new characteristics of the fluid flow over a confined circular cylinder at low Reynolds numbers. Results from both two- and three-dimensional direct numerical simulations are presented at blockage ratio between 0.1 and 0.9 and Reynolds number between 120 and 500. Floquet stability analysis of selected cases will also be presented. From the two-dimensional simulations, it is found that the fluctuating lift forces decreases with blockage ratio and becomes zero (where the flow is steady) at blockage ratio of approximately 0.7–0.8. Upon further increasing the blockage ratio to 0.9, the simulations show a dramatic increase in the fluctuating lift forces, nearly an order of magnitude greater than previously reported for an unconfined cylinder flow. It is also found that for blockage ratio of 0.5, there is a long term two-dimensional instability that becomes more prominent with increasing Reynolds number. This instability has a time scale of approximately 105 time units (D/Umax) at Reynolds number of 500. In addition, the transition between two- and three-dimensional flow at blockage ratios up to 0.5 is investigated. It is shown that the transition Reynolds number decreases with increasing blockage ratio. At high blockage ratio of 0.5, as we increase the Reynolds number, the transition to three-dimensional flow is shown to go from unsteady two-dimensional to steady three-dimensional before transitioning to unsteady three-dimensional flow.

Impact of Sharp Versus Rounded Edge of Turbulator on Transport Enhancement

Considering the constraints of manufacturing sharp ribs, a systematic study of the aerothermal behavior of the sharp and rounded-edge ribs has been performed. The investigation was conducted in a square channel with 45 deg ribs over a wide range of Reynolds numbers ( to 135,000). The rounded-edge ribs are known to reduce heat transfer and friction compared to the sharp ribs. However, the studies at high blockage ratio show that rounded edges of the ribs have a smaller effect on the heat transfer but the substantial effect on the pressure drop, which is favorable for better cooling effectiveness. This new study with the lower blockage ratio (i.e., 0.0625) rounded-edge ribs offers an interesting insight. The result confirms that at the lower blockage ratio the rounded-edge ribs reduce the heat transfer capacity to a greater extent but have minimal effect on the pressure drop, especially at high Reynolds number. Numerical studies were also conducted using realizable , , and shear stress transport turbulence models. The results are compared with the experimental data to shed light on the prediction capability of these models for both kinds of the ribs at such a wide range of Reynolds numbers. The effect of the rounded-edge ribs on the flow behavior and the consequent impact on the local heat transfer distribution is also studied.

Wind Turbine Tip Vortices under the influence of Wind Tunnel Blockage Effects

The current paper describes the characteristics of the tip vortex in the near wake of a three-bladed upwind horizontal axis wind turbine with a rotor diameter of 3 m. Phase-locked stereo particle image velocimetry measurements were carried out under the influence of the wind tunnel walls that create a high blockage ratio. The location of the vortex, convection velocity, core radius, and strength were investigated and compared with similar investigations, including different blockages cases. Additionally, the same performance of the wind turbine model was simulated in the open source wind turbine tool QBlade, using the lifting line free vortex wake module in the absence of the walls.The results showed that the location of the tip vortices was more inboard the tip and more downstream the tunnel compared to the simulations and similar experiments. The convection velocity remained similar in the axial direction and changed in the lateral direction, contributing to the delay of the movement of the tip vortex outboard the tip. The strength, based on the circulation, was found with a difference of 4% between simulation and experiment.

Flame acceleration and deflagration-to-detonation transition in hydrogen-air mixture in a channel with an array of obstacles of different shapes

Numerical simulations were conducted to study the flame acceleration and deflagration-to-detonation transition (DDT) in a channel with a bank of aligned obstacles. The multidimensional, fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusion model for stoichiometric hydrogen-air mixture were solved using a high-order numerical algorithm. The results in the case with circular obstacles show that occurrence of DDT arises from shock focusing at flame front. Higher blockage ratio leads to faster flame acceleration and facilitates the formation of choked flame. Heat losses can significantly attenuate flame acceleration and delay detonation initiation in case of DDT, and even hinder occurrence of DDT for high blockage ratio. To understand the effect of shape of obstacle on the flame acceleration and DDT, circular, square, left triangular, and right triangular shapes, always maintaining the same formal blockage ratio, were simulated. The simulations indicate that there are significant differences in flame acceleration and DDT among these differently shaped obstacles, although the basic mechanism for detonation initiation is similar in all of the cases studied and involves interactions of shock wave with flame front and reactivity gradient. Square obstacles produce the largest growth rate of flame surface area in the flame-acceleration stage and thus result in the fastest flame acceleration and shortest detonation onset time compared to circular or triangular obstacles. Circular obstacles produce the weakest flame-surface growth, and thus they have the least effect on promoting flame acceleration and transition to detonation. A closer analysis shows that the sharp angles of the triangular obstacles, when compared to the circular obstacles, are more conducive to flame stretching and convolution and thus facilitate flame acceleration and DDT. The results also show that the effect of obstacle shape does not correlate with obstacle area.

Effect of Blockage Ratio on Heat Transfer and Pressure Drop in Rotating Ribbed Channels at High Rotation Numbers

At high rotation numbers, the rotational effects on heat transfer and flow could be diverse among the channels with different blockage ratios. However, most studies are conducted under low rotation number (less than 0.25) and selected blockage ratio. This paper experimentally investigates the effect of rib blockage ratio (ranges from 0 to 0.3) on pressure loss and heat transfer in a rotating square channel under high rotation number (up to 0.81). The ribs staggered on leading and trailing walls were oriented 90° to the mainstream flow. The Reynolds number and the wall-to-fluid temperature ratio varied from 20 000 to 40 000 and 0.08 to 0.2, respectively. The results showed that a larger blockage ratio resulted in a better heat transfer but a higher pressure drop. The optimum blockage ratio was 0.1 for the best thermal performance. The rotational effects were weakened in the passage with a higher blockage ratio, where the critical rotation number could not be observed. Moreover, the heat transfer enhancement induced by rotation was more significant when the temperature ratio increased. Finally, the correlations were developed for the pressure drop and the convective heat transfer on the leading and trailing edges.

Geometric influence on the propagation of the quasi-detonations in a stoichiometric H2-O2 mixture

Combustion wave propagation was studied experimentally in an obstructed narrow channel filled with stoichiometric H2-O2 mixtures using a simultaneous schlieren and soot foil technique. The propagation modes uncovered were classified as fast-flames, discontinuous detonation, and continuous detonation. In this study blockage ratio (BR) was found to have a strong effect on the detonation limit. For BRs of 33% and 66%, the ratio of the obstacle opening and detonation cell size (d/λ) at the detonation limit was found to be 0.6 and 6.0, respectively. This order of magnitude change in the detonation limit was found to be governed by a change in propagation mechanism associated with the discontinuous detonation mode. For all BRs the discontinuous mode is characterized by detonation failure, as it diffracts through the obstacle opening, and then re-initiation. For the low BR obstacles detonation re-initiation is driven by the generation of a detonation Mach stem that propagates along the top and bottom walls following oblique reflection of the decoupled detonation leading shock wave. For the high BR obstacles shock reflection off the top and bottom walls does not produce a detonation wave, instead a detonation is initiated at the proceeding obstacle face following a roughly normal reflection of the leading shock. These propagation mechanisms are not governed by obstacle BR, but rather the obstacle height and spacing is the key parameter as they control the strength of the shock at the point of reflection.

Mathematical modeling of air pollution in city tunnels and evaluating mitigation strategies

A mаthemаtical model has been developed that is able to describe the environment of the city tunnels being affected by the road traffic, natural and forced air convective flows. The mathematical model describes the peculiarities of the traffic flows on one-lane roads with a satisfactory accuracy. The model well matches experimental data on traffic flows. Multidimensional calculations of the influence of cars on the airflow in tunnels are performed. The numerical model for simulating exhaust gas emissions by automobiles and their accumulation in a tunnel and evolution with traffic induced air flow was developed. The results of numerical investigations make it possible providing recommendations for transport researchers and policy-makers. In particular, it was shown that in the presence of long tunnels on automobile roads it is necessary to choose the traffic arrangement avoiding the necessity for vehicles to come to a full stop and then accelerate in tunnels. This could happen in the presence of traffic lights or other type of traffic regulation near the exit of the tunnel. Thus it is necessary in arranging traffic regulation avoiding placing traffic lights in the proximity of tunnel exit for the waiting cars line to be shorter than the distance from the tunnel exit. In venting the tunnel, the direction of the wind should coincide with the direction of traffic flow. Attempts to arrange venting in the opposite direction for high blockage ratio of a tunnel by vehicles could result in bringing to a worse situation with air pollution.

Experimental study of particulate fouling in partially filled channel with open-cell metal foam

This study experimentally investigates particle transport and deposition processes in a partially filled channel with an open-cell metal foam. Various blockage ratio (a ratio of foam height and channel height) in range of 0.05–0.39, and pore density (10 and 30 PPI) were used to determine the effects of the foam microstructure and exterior shape on the propensity of particulate fouling. In-situ observations were conducted using a high-speed camera to determine the particle trajectory and velocity in the partially filled channel. Result shows a complex transport and deposition process, which is significantly influenced by the particle size and pore diameter. If merely considering the exterior shape of the foam, a higher blockage ratio causes more deposition in the upstream region. While, a low blockage ratio causes more deposition on the top surface of the foam block, regardless of pore density. These results demonstrate that the foam arrangement and heights play primary roles in influencing the preferential deposition areas, which is originally associated with flow behaviours in the partially filled channel. Interestingly, the foam microstructure not only blocks and traps the flowing particles, yet causing the breakage of particle clusters. These particles either disperse within the incoming flow or cause more depositions on the foam surfaces. The build-up deposits would increase the pressure drops, as expected, but the effects of pore density become insignificant over time.

Optimization of Vortex Promoter Parameters to Enhance Heat Transfer Rate in Electronic Equipment

Abstract In this paper, optimization of the location and the geometry of a vortex promoter located above in a finned surface in a channel with eight heat sources is investigated for a Reynolds number of 12,500 < Re < 27,700. Heat transfer rates and the corresponding Nusselt number distributions are studied both experimentally and numerically using different vortex promoter geometries (square, circular, and triangular) in different locations to illustrate the effect of vortex promoter on the fluid flow. Optimization study considered a range of following parameters: blockage ratio of 0.30 < (y/C) < 0.45 and interpromoter distance ratio of 0.2277 < (x/L) < 0.3416. Results show that fins over which rectangular and circular promoters are integrated perform better in enhancing the heat transfer. According to the numerical and experimental results, higher blockage ratios cause significantly higher heat transfer coefficients. According to the observations, as the interpromoter distances increase, shedding gains strength, and more turbulence is created. All vortex promoters enhance heat transfer resulting in lower temperature values on the finned surface for different (y/C) and (x/L) values and Reynolds numbers. The use of promoters enhances the heat transfer, and the decrease in the maximum temperature values is recorded on the finned surface changing between 15% and 27%. The biggest decrease in maximum surface temperature value is 500 K–364 K and observed in circular promoter case with (y/C) = 0.43, (x/L) = 0.3416, and Reynolds numbers of 22,200.

Effect of blockage ratio on the existence of multiple waves in rotating detonation engine

The effect of blockage ratio on the existence of multiple waves in rotating detonation engine was investigated through experimental approach. It has been shown experimentally that four typical detonation propagation modes (single wave, hybrid mode of single wave and double waves, double waves, longitudinal pulsed detonation) were obtained by varying blockage ratios and air mass flow rates at the global equivalence ratio of around 1.60. The results show that the increase of blockage ratio reduces the critical mass flow rate for the occurrence of multiple waves. The occurrence of multiple waves experiences coexistence of single wave and two co-rotating detonation waves, and steady two co-rotating detonation waves. The increase of blockage ratio intensifies the effect of reflected oblique shock waves on fresh reactants and elevates average pressure in the combustor, which play important role in the occurrence of multiple waves. Moreover, the increase of blockage ratio elevates the pre-combustion pressure. The high pre-combustion pressure resulting from high blockage ratio leads to the existence of volumetric explosion immediately after ignition, which extends the formation time of rotating detonation wave. Further increasing of blockage ratio triggers the longitudinal pulsed detonation (LPD), and the highly decreased injection pressure ratio is considered to be the main reason for the occurrence of LPD.

A numerical study of the train-induced unsteady airflow in a tunnel and its effects on the performance of jet fans

Analyzing the airflow generated by a moving train in tunnels ventilated by jet fans is of significant importance because of the piston effects and the alternating pressure and velocity fields inside the tunnel. Moreover, high blockage ratio and train velocity in these tunnels have caused serious concerns about the impact of train-induced airflow on the performance of jet fans. This study aims at simulating the train movement in such tunnels and investigating the above-mentioned effects. To this purpose, a finite-volume numerical approach with dynamic mesh technique is used to perform several simulations of a large tunnel with eight rows of jet fans and different train velocities. The results demonstrate that for the case in which the train moves in the opposite direction of the jet fans’ flow, their volume flow rates increase and decrease sharply, as the train head and tail reach the jet fans, respectively. However, the trend is opposite when the train travels along the direction of jet fans flow. From an electric perspective, it is shown that the generated flow field does not increase the motor torque and the current of the jet fans to a degree that causes the motors to shut down.

Flow of wormlike micellar solutions around microfluidic cylinders with high aspect ratio and low blockage ratio.

We employ time-resolved flow velocimetry and birefringence imaging methods to study the flow of a well-characterized shear-banding wormlike micellar solution around a novel glass-fabricated microfluidic circular cylinder. In contrast with typical microfluidic cylinders, our geometry is characterized by a high aspect ratio α = H/W = 5 and a low blockage ratio β = 2r/W = 0.1, where H and W are the channel height and width, and the cylinder radius r = 20 μm. The small cylinder radius allows access up to very high Weissenberg numbers 1.9 ≤ Wi = λMU/r ≤ 3750 (where λM is the Maxwell relaxation time) while inertial effects remain entirely negligible (Reynolds number, Re < 10-4). At low Wi values, the flow remains steady and symmetric and a birefringent region (indicating micellar alignment and tensile stress) develops downstream of the cylinder. Above a critical value Wic ≈ 60 the flow transitions to a steady asymmetric state, characterized as a supercritical pitchfork bifurcation, in which the fluid takes a preferential path around one side of the cylinder. At a second critical value Wic2 ≈ 130, the flow becomes time-dependent, with a characteristic frequency f0 ≈ 1/λM. This initial transition to time dependence has characteristics of a subcritical Hopf bifurcation. Power spectra of the measured fluctuations become complex as Wi is increased further, showing a gradual slowing down of the dynamics and emergence of harmonics. A final transition at very high Wic3 corresponds to the re-emergence of a single peak in the power spectrum but at much higher frequency. We discuss this in terms of possible flow-induced breakage of micelles into shorter species with a faster relaxation time.

Effects of cross-sectional aspect ratio of V-shaped ribs and blockage ratio on heat transfer in a channel at a low Reynolds number

V-shaped ribs were investigated with the objective of enhancing heat transfer at the high-temperature side of a fin-plate type heat exchanger used in a railway vehicle compressor. The high-temperature side supplied with compressed air has a Reynolds number of approximately 4000, which is not sufficiently high to generate strong turbulence. When a 3D printer is used to fabricate a heat exchanger, the cross-sectional aspect ratio or blockage ratio of the ribs should be greater than the optimum value in order to attain a certain value of strength. In this study, a series of numerical analyses were conducted to investigate whether increasing the aspect ratio or the blockage ratio was more advantageous for achieving high heat transfer. Ribs with cross-sectional aspect ratios ranging from 0.3 to 5 were investigated. The investigated blocking ratios were 0.1 and 0.3. The results of computational fluid dynamics calculations were used to investigate the influences of flow separation–reattachment, secondary flow, and turbulence on heat transfer. The results demonstrated that the introduction of V-shaped ribs with a high blockage ratio into the channel at a low Reynolds number resulted in greater heat transfer enhancement compared with the effect of introducing ribs with a high cross-sectional aspect ratio.

Numerical study of spheres settling in Oldroyd-B fluids

In order to simulate the motion of balls settling in Oldroyd-B fluids, we have generalized to three dimensions the distributed Lagrange multiplier based fictitious domain method we developed in the study by Hao et al. [“A fictitious domain/distributed Lagrange multiplier method for the particulate flow of Oldroyd-B fluids: A positive definiteness preserving approach,” J. Non-Newtonian Fluid Mech. 156, 95 (2009)] for viscoelastic particulate flow in two-dimensional channels. The effect of the fluid elastic number and that of the density ratio of the particle and fluid on the chaining of balls have been studied. For the cases of two balls released side-by-side at high elasticity numbers, the two balls attract each other first and then form a chain, and such a chain settles vertically. But at low elasticity numbers, the two balls either stay separate and interact periodically or attract each other, turn, and separate periodically. At high blockage ratios, a stronger wall effect enhances the formation of two ball chains. For the cases of three balls released side-by-side, the ball interaction is slightly more complicated due to its non-symmetrical initial configuration. At high elasticity numbers, either a three ball chain settles vertically or the leading two balls form a chain which leaves the third ball behind. But at low elasticity numbers considered in this article, only the leading two balls form a chain. For the vertical initial configuration, a three ball chain can be obtained at higher elasticity numbers and also the heavier balls can form a vertical chain easily.

Poiseuille flow-induced vibrations of a cylinder in subcritical conditions

Poiseuille flow-induced vibrations (PFIV, as a distinct type of vortex-induced vibrations, VIV) of a circular cylinder in subcritical conditions are numerically studied via a lattice Boltzmann method. A significant characteristic of the present PFIV model system is that the cylinder oscillation is excited by the Poiseuille-type fluid flow, rather than by the interaction of shedding vortices and structural elasticity for the classic VIV of an elastic structure. The effects of Reynolds number (10 ≤Re≤ 80), blockage ratio (0.4 ≤β≤ 0.667) and mass ratio (0.25 ≤m∗≤ 1024) on the behaviors of cylinders and fluid flows are investigated and special attention is paid to the critical phenomena. Based on the cylinder motion and the wake structure, three distinct regimes involving one steady and two vibration regimes are observed. For Re≥ 20, cylinder oscillation can be excited extensively in the branch of high blockage ratio. As β is decreased below a critical value, the cylinder suddenly ceases to oscillate and rests at the channel centerline. For Re≈ 20 with a high blockage ratio, the cylinder always undergoes large-amplitude vibration in the whole current range of mass ratio. When Re is increased, the phenomenon of critical mass ratio, beyond which large-amplitude oscillation ceases, is observed and the threshold decreases with increasing Re and decreasing β rapidly. By measuring the lift coefficient of a stationary cylinder placed at different transverse positions in a parallel-wall channel, the mechanisms for the observed phenomena are analyzed.