Non-uniform Velocity Distribution Research Articles

A flame technique to isolate the detonation/product interface relevant to rotating detonation engines

A novel experimental technique is proposed to study the detonation propagation in a layer of non-reacted gas weakly confined by combustion products. This problem is relevant to rotating detonation engines, where transverse detonations are confined by products of a previous rotation cycle, and other applications such as industrial safety. The experimental technique utilizes a flame ignited along the top wall in a long channel. The preferential growth of the flame along the long direction of the channel creates a finger flame and permits to create a narrow layer of unburned gas. A detonation ignited outside of this layer then propagates through the layer. This permits to conduct accurate observations of the detonation interaction with the inert gas and determine the boundary condition of the interaction. The present paper provides a proof-of-concept demonstration of the technique in a 3.4 m by 0.2 m channel, in which long finger flames were observed in ethylene-oxygen mixtures. The flame is visualized by high-speed direct luminosity over its entire travel, coupled with pressure measurements. A direct simulation of the flame growth served to supplement the experiments and evaluate the role of the induced flow by the flame growth, which gives rise to a non-uniform velocity distribution along the channel length. Detonation experiments were also performed at various layer heights in order to establish the details of the interaction. The structure was visualized using high speed Schlieren video. It was found that an inert shock always runs ahead of the detonation wave, which gives rise to a unique double shock reflection interaction.

Curvature-induced secondary flow in 2D depth-averaged hydro-morphodynamic models: An assessment of different approaches and key factors

Curvature-induced secondary flows are ubiquitous in nature and have long attracted scientist attention. Modelling such kind of secondary flows is not straightforward. While full 3D models fit the purpose at the cost of great computational demand, simplified models often pose concerns about their effectiveness and the representation of key processes. In the present study, helical flow secondary currents are included in a two-dimensional depth-averaged hydro-morphodynamic model on cartesian unstructured meshes. The non-uniform vertical distribution of velocity in streamwise and spanwise directions is accounted for introducing dispersive terms in the shallow water equations, an anisotropic diffusivity tensor in the advection-diffusion equation, and a correction to the direction of bed shear stress and bedload transport. Different approaches available in the literature are recast in similar form and compared to each other in terms of flow field, tracer transport, and bed evolution, using data from laboratory experiments and real-world case studies. The model includes a novel, pure 2D implementation of the non-linear saturation mechanism that limits the growth of secondary flows in relatively sharp bends. A substantial part of the paper is devoted to discuss key factors in secondary flow modelling, including implementation tricks, guidelines to mesh design, the suitability of local and non-local approaches, and the role of bathymetry. The final goal is to provide useful guidelines for 2D hydro- and morphodynamic modelling in river bends.

Effect of Airflow Non-Uniformities on the Thermal Performance of Water–Air Heat Exchangers—Experimental Study and Analysis

The thermal performance of fin-and-tube heat exchangers (HX) is a crucial aspect in a multitude of applications and fields; several design and operational parameters influence this performance. This study focuses on the issue of flow maldistribution and its effect on the HX thermal performance. For this purpose, an experimental setup is designed and implemented to emulate the conditions under which an automotive heat exchanger operates in regard to the non-uniform upstream airflow velocity distribution over the HX surface. The setup allows obtaining various configurations of airflow velocity non-uniformity of some desired mean velocity and standard deviation. The experimental results reveal that a higher degree of non-uniformity (higher standard deviation of the velocity distribution) causes an increased deterioration of the HX thermal performance. For example, at a water flowrate of 200 L/h and a mean airflow velocity of 2 m/s, increasing the standard deviation from 0 to 2 m/s (i.e., moving from the lowest to highest degrees of non-uniformity) causes a total deterioration of 27% in the performance (3.78 to 2.75 kW, respectively), which can also be observed in the increased level of outlet water temperature (53.8 to 58.2 °C, respectively). The obtained results confirm the numerical results reported in the literature.

Effect of Compressor Unsteady Wakes on a Gas Turbine Combustor Flow

Abstract In gas turbines, combustor inlets are characterized by significant levels of unsteady circumferential distortion due to compressor wakes and secondary flows, together with additional radial nonuniformity induced by the adverse pressure gradients in the prediffuser. This can cause nonuniform velocity distributions across the fuel injector, although the exact interaction mechanism, and the effects it has on the downstream air–fuel mixing, is not fully understood. This paper investigates the flow in an a single sector of a fully featured isothermal rig comprising of compression and combustion systems, exploiting the synchronous coupling of a compressible unsteady RANS simulation with a low-Mach large eddy simulation (LES). Validation against five-hole probe measurements shows that the coupled approach can correctly predict distortion onset and development, with no solution discontinuity at the coupling interface, and is able to preserve unsteady information. The coupled prediction is then compared against a standalone combustor simulation carried out using a circumferentially uniform inlet profile, showing that the additional turbulence from the wakes interacts with the injector, reducing the coherence of the precessing vortex core (PVC) and potentially affecting the air–fuel mixing characteristics.

Study on Critical Velocity of Sand Transport in V-Inclined Pipe Based on Numerical Simulation

The Yellow River has a high sand content, and sand deposition in the pipelines behind the pumping station occurs from time to time. It is of great significance to reasonably predict the critical velocity of the small-angled V-inclined water transportation pipes. In this study, a Eulerian multiphase model was employed to simulate the solid–liquid two-phase flow. Based on the conservation of the sand transport rate, the critical velocity of the V-inclined pipe was predicted. The effects of simulated pipeline length, pipe inclination and particle size were investigated. The results show that when the simulated pipeline length reached a certain value, it did not affect the prediction of the critical velocity of the overall pipeline. The ±2∘ pipe inclination had a negligible effect on the critical velocity for transporting small-sized particles, but it led to the nonuniform and asymmetrical distribution of liquid velocity and sand deposition at the different cross-sections. As the particle size increased, the critical velocity also increased. However, the influence of particle size on the critical velocity is currently complicated, resulting in a large difference between numerical simulation and empirical formulas when transporting large-sized particles. Accurate prediction of critical velocity is important for long-distance water transportation pipelines to prevent sand deposition and reduce costs.

Investigation of the Dynamic Characteristics of Muck during EPB Shield Tunnelling in a Full Chamber Model Using a CFD Method

For the safety and efficiency of earth pressure balance (EPB) shield tunnelling, the dynamic characteristics of the muck in the excavation chamber and the screw conveyor should be considered. However, the velocity and pressure distribution of the muck in the EPB shield, reflecting muck flowability and face stability, are rarely observed at the jobsite. Therefore, a computational fluid dynamics (CFD) model with a modified inlet boundary was established to investigate the muck characteristics in the chamber and the screw conveyor during tunnelling in a full chamber mode. A compressible Bingham model was adapted to describe the muck compressibility and rheology. The time-dependent tunnelling parameters obtained from the simulation excellently matched the field-measured data. The simulation results show the nonuniform velocity distribution of the muck at the excavation face and the low-velocity zones where clogging potentially occurs. The muck pressure reaches a peak at the excavation face and then decreases along the cutterhead openings, chamber and screw conveyor for inflow. The simulation also captures the pressure imbalance at the cutterhead and bulkhead, which may destabilize the excavation face. The proposed discharge efficiency is 56.7% in the simulated condition, indicating that tunnelling with these muck properties has no risk of muck spewing.

The study on the broadband thrust spectrum of propeller in anisotropic and inhomogeneous turbulence

Considering the effects of stretching of turbulent eddies and inhomogeneous distribution of turbulence parameters, such as turbulence length scale, a new analytical approach to calculating the unsteady broadband thrust of propeller in anisotropic and inhomogeneous turbulence was introduced based on the correlation methods conducted in isotropic turbulence. Stephens's experiment was used to verify the theoretical approach to modeling in anisotropic and inhomogeneous turbulence. The effects of turbulence parameters on propeller broadband thrust in anisotropic and inhomogeneous turbulent field were discussed. The results showed that the haystack amplitudes at blade passage frequencies increase with the nonuniform level of the turbulent intensity and stretching parameters of turbulence eddy, while the effects of nonuniform distribution of inflow velocity and turbulence length scale can be neglected. Additionally, the haystack frequencies of propeller thrust spectrum are only dependent on blade passage frequency.

The Effects of Material Distribution and Flow Profile on the Stability of Cantilevered Axially Functionally Graded Pipes

Abstract This article investigates the influences of different material distribution types and flow profiles in the cross section on dynamics of cantilevered axially functionally graded (AFG) pipe. Functionally graded material as a designable material, its appliance in structures can enhance the stability of the structure by adequately choosing the material constituents and arranging constituents' distribution. The governing equation of the pipe system is derived based on the Euler–Bernoulli beam theory and numerically solved by the differential quadrature method (DQM). The influences of different volume fraction function and nonuniform flow velocity distribution on the natural frequencies and average critical flow velocities are discussed according to the numerical results. It can be concluded that the enhanced effect of the AFG material is mainly caused by an increment in the amount of stiffer constituent. With the same amount, pure distribution difference in exponential or power function type that brings stiffer fixed end results in slightly higher critical velocity against flutter. Ignoring the nonuniform flow velocity distribution leads to an overestimation of the pipe's stability and the overestimation is even apparent on AFG pipe. Nonuniform velocity distribution affects the stable flow velocity area and appearance of restabilizing phenomena.

Unidirectional steady-state inhomogeneous Couette flow with a quadratic velocity profile along a horizontal coordinate

The paper presents an exact solution to the boundary value problem describing the steady-state unidirectional flow of a viscous incompressible fluid. The fluid moves in an infinite horizontal strip (infinite fluid layer). The fulfillment of the no-slip condition is postulated at the lower boundary of the viscous fluid layer. At the upper boundary, which is assumed to be rigid, non-uniform velocity distribution is specified. The deformation of the free boundary is neglected due to the use of the rigid-lid boundary condition. The exact solution to the equations of the hydrodynamics of incompressible fluids automatically satisfies the continuity equation (the incompressibility equation). The velocity function is harmonic in this case. The simplest exact solution satisfying the Laplace equation is constructed, which takes into account the features of the velocity field along the transverse (vertical) coordinate and one of the longitudinal (horizontal) coordinates. The paper analyzes the topological properties of the velocity field, the tangential stress field, the vorticity vector, specific kinetic energy, and specific helicity.

Investigation of Flow Characteristics of Flow Passage Components in Francis Turbine under No-load and 50% Load operating conditions

Due to the varying demand of power grid for the electric supply of hydraulic power stations, the units are often under low-load operating condition and no-load standby state, causing a significant flow instability in hydraulic generator units. To further investigate the inside flow characteristics of hydraulic turbines under such operating conditions. In this study, a CFD (computational fluid dynamics) numerical simulation with applying RNG (Renorm alization Group) k-ε turbulence model is performed to study the inside flow characteristics of each flow passage components of a Francis turbine under no-load and 50% load conditions. The results indicate that under the two operating conditions, the flow distribution inside and outside the draft tube pier is significantly nonuniform. Simultaneously, the large-scale vortex in the runner area caused by the high-speed flow in the vane-free area with small opening and the nonuniform flow velocity distribution in the straight cone section of the draft tube are the main reasons for the increase of hydraulic loss and the decrease of efficiency.

Rheological & 3D printing properties of potato starch composite gels

Starch based gels can be three-dimensional (3D) printed to fabricate food products. Previously, researchers added sodium alginate to the starch gel to enhance shape retention of corresponding printed products. This inspired us to hypothesize that xanthan gum, which is also a common thickening agent, might play a similar role in structuring starch composite gel in 3D printing experiments. The geometric accuracy of products, as well as their rheological properties, water state distribution, and scanning electron microscopy images, were evaluated with varying amounts of added sodium alginate (4%, 5%, and 6% w/w, dry basis), xanthan gum (4%, 5%, and 6% w/w, dry basis), and sodium alginate/xanthan gum (2%/2%, 2.5%/2.5%, and 3%/3% w/w, dry basis) for extrusion-based 3D printing. Computation fluid dynamic simulation was used to model the printing process. The results showed that pseudoplastic gels with relatively high extrudability and shape retention were formed. The 3D-printed starch gel with an optimized formula with added sodium alginate and xanthan gum (2.5%/2.5% w/w, dry basis) exhibited the best geometric accuracy. The shape was retained with appropriate gel formation characteristics and physical properties. The addition of sodium alginate and xanthan gum at suitable levels contributed to the formation of 3D-printed starch based products with technical feasibility for potential food applications. Mathematical model of the 3D printing process was established. The model explained the non-uniformity of velocity distribution in the flow channel due to the varied diameter, and such non-uniformity led to jet expansion during the 3D printing process.

The formation mechanism of the chill fine-grain layer with high supersaturation and its influence on the mechanical properties of die casting Al-7Si-0.5Mg alloy

In this study, the chill layer with high supersaturation was observed on the surface of the Al-7Si-0.5Mg die casting tensile samples. The optional microstructure and electron backscatter diffraction (EBSD) results show that the chill layer has fine α-Al grains (≈5 μm). There is a solution gradient in die-casting aluminum alloy and the chill fine-grain layer is in a supersaturated state. The pre-solidified dendrites (PSDs) formed in the shot sleeve and the nonuniform velocity distribution of the melt are responsible for the formation mechanism of the chill fine-grain layer with high supersaturation. Under high pressure and high injection speed, the PSDs will aggregate in the core of the samples due to the Magnus effect. Moreover, the chill layer with solute enrichment can be obtained as a result of rapid cooling and incoordinate solute distribution. Consequently, the tiny Mg2Si phase could precipitate spontaneously from the supersaturated α-Al grains in the chill layer, simultaneously improving the of strength and ductility.

Design and experimental validation of a new condenser of an automotive air conditioning unit to address non-uniform air velocity distributions

• The front anti-collision beam in a car leads to air maldistribution. • A six-path condenser is an optimization of the traditional four-path condenser. • The new condenser shows better capacity with the anti-collision beam. • The cooling performance of automobile air conditioning system is improved. To improve the efficiency of automobile air conditioning and reduce the impact of non-uniform air velocity distributions caused by the front anti-collision beam on automobiles, we designed a new six-process condenser and tested it under different working conditions. A baffle was positioned to simulate the anti-collision beam during the experiment. We analyzed the condenser by itself and the system level performance of this newly designed condenser under three working conditions compared to a traditional four-process condenser. Condenser heat exchange capacity, condenser inlet pressure, system refrigerant flow rate, refrigerant flow resistance, thermal imaging of condenser outlet air, overall system cooling capacity, and coefficient of performance were compared. The coefficient of performance of the system with this new condenser improved by 8.5%, 7.8%, and 10.3% for the three tested working conditions.

In situ measurement on nonuniform velocity distribution in external detonation exhaust flow by analysis of spectrum features using TDLAS

Instantaneous and precise velocity sensing is a critical part of research on detonation mechanism and flow evolution. This paper presents a novel multi-projection tunable diode laser absorption spectroscopy solution, to provide a real-time and reliable measurement of velocity distribution in detonation exhaust flow with obvious nonuniformity. Relations are established between overlapped spectrums along probing beams and Gauss velocity distribution phantom according to the frequency shifts and tiny variations in components of light-of-sight absorbance profiles at low frequencies analyzed by the fast Fourier transform. With simulated optical measurement using H2O feature at 7185.6 cm−1 carried out on a phantom generated using a simulation of two-phase detonation by a two-fluid model, this method demonstrates a satisfying performance on recovery of velocity distribution profiles in supersonic flow even with a noise equivalent absorbance up to 2 × 10−3. This method is applied to the analysis of rapidly decreasing velocity during a complete working cycle in the external flow field of an air-gasoline detonation tube operating at 25 Hz, and results show the velocity in the core flow field would be much larger than the arithmetic average from traditional tunable diode laser doppler velocimetry. This proposed velocity distribution sensor would reconstruct nonuniform velocity distribution of high-speed flow in low cost and simple operations, which broadens the possibility for applications in research on the formation and propagation of external flow filed of detonation tube.

A novel method for the determination of frictional resistance coefficient for a plate with inhomogeneous roughness

Ship performance is largely influenced by roughness on immersed surfaces, which is mostly inhomogeneous. Therefore, the prediction of the effects of roughness on ship performance is of crucial importance. Even though it has several drawbacks, the Granville similarity law scaling method is widely reported in the literature for predicting the frictional resistance coefficient of a rough surface. Lately, the CFD approach based on a modified wall function approach has also been used, since it allows for a more comprehensive analysis of the effects of roughness. In this paper, a novel method for assessing the frictional resistance coefficient for rough surfaces is proposed. The proposed method can take into account the non-uniform distribution of friction velocity, as well as the roughness Reynolds number, and thus the non-uniform distribution of roughness function values can be predicted along a rough surface. Additionally, the proposed method can consider the non-uniform distribution of roughness and its effect on the frictional coefficient of a flat plate. Based on the proposed method, an in-house numerical code is developed, allowing for the rapid and accurate assessment of the frictional resistance coefficient for a rough surface. The applicability of the proposed method is demonstrated by comparison with the CFD approach for several surface conditions defined with different roughness function models and the longitudinal position of roughness. A comparison is made of two flat plates representing one containership and one tanker at two different speeds. The obtained relative deviations in the predicted frictional resistance coefficients for all surface conditions are lower than 2.5%. Therefore, it is demonstrated that the proposed method can be used for estimating the frictional resistance coefficient of various surface conditions defined with roughness function models and the longitudinal positions of roughness.

Non-Hydrostatic Transitional Open-Channel Flows from a Supercritical to a Subcritical State

Abstract In this study, a depth-averaged numerical model was employed to investigate the two-dimensional flow features of transitional open-channel flows from a supercritical to a subcritical state. Compared to a shallow-water model, the proposed model incorporates supplementary terms to account for the effects of non-uniform velocity and non-hydrostatic pressure distributions. The model equation was solved numerically by means of the Adams–Bashforth–Moulton scheme. A wide variety of transitional open-channel flow problems such as hydraulic jumps was considered for assessing the suitability of the numerical model. The results of the model for the free-surface profile, pressure distribution, and characteristics of the first wave of an undular jump were compared with the experimental data, and the agreement was found to be satisfactory. Despite the effects of the three-dimensional characteristics of the flow and the bulking of the flow caused by air entrainment, the model performed reasonably well with respect to the simulations of the mean flow characteristics of the curvilinear turbulent flow problems. Furthermore, the results of this investigation confirmed that the model is more suitable for analyzing near-critical turbulent flow problems without cross-channel shock waves.

Accurate Simulation of the Four Modes of Post-Die Extrudate Shape Distortion

Abstract A combined flow, thermal and structural analysis is employed to simulate post-die extrudate distortion in different profile dies. All four factors which can cause extrudate distortion, namely, nonuniform exit velocity distribution, extrudate shrinkage, extrudate draw down, and deformed shape of the calibrator or sizer profile, are simulated. To analyze the effect of exit velocity variation on extrudate distortion, the parameterized geometry of a simple profile die is optimized using an extrusion die optimization software. The simulation results presented for a bi-layer profile die successfully demonstrate how gradually changing profile shape in successive calibrators/sizers can be used to simplify the die design for extrusion of complex profiles. The predicted extrudate shape and layer structure for the bi-layer die are found to accurately match with those in the extruded product.

Numerical Study to Improve Fluid Flow Patterns in HRSG by using a Turning Vane and a Combination of using a Turning Vane with Transition Zone Geometry Modification

Since operated from 1995 until 2018, the total steam production of HRSG 1.1 has decreased by 6.34 % and exhaust gas outlet temperature increased by 40.86 °C. From previous studies on HRSG it can be assumed that flue gas flows at HRSG 1.1 has nonuniform velocity and temperature distribution. This research is carried out by modeling and simulating HRSG as porous media using Computational Fluid Dynamic (CFD) software. The simulation will be carried out on a 3D model with the steady-state condition, use energy model, a heat exchanger (HE) model, and standard k-ε as a turbulence viscous model. Energy absorption in HE will be modeled by an ungroup macro model with fixed inlet temperature and NTU model. Simulations in HRSG are implemented at a corner angle of 6 degrees (original), adding a turning vane on the inlet duct and combination adding a turning vane on inlet duct with 12 degrees corner angle on transition zone. Model validation was accomplished comparing simulation data to power plant data. The result shows adding a corner angle in the transition zone and installing a turning vane in the curve bend minimizes secondary flow in the transition zone. Heat absorption increases 0.008% and exhaust gas outlet temperature decrease 0.04% compared to the existing model.

Research on viscoelastic fluid unsteady flow model based on torque loss correction

To address the complexity of unsteady shear flow in the initial start-up stage of rheometers, a model considering torque losses in motors and fixtures was developed based on the true torque balance relationship at the rotor boundary of the rheometer and the constitutive equation of the viscoelastic rheological model. The viscoelastic fluid transient flow model was established, and the real wall stress expression of unsteady shear flow of the viscoelastic fluid in the initial stage was obtained. The reliability of the model was verified by comparing the result from the analytical model with numerical calculation. The impact of the geometric dimensions of the test system and the material viscoelastic characteristics on the transient test was analyzed. The results show that the non-uniform velocity distribution has little effect on the initial strain response in the gap of the actual test system of the rheometer. The oscillation curves of strain and stress obtained by numerical and analytical methods coincide completely, proving the reliability of the model in characterizing the transient flow. For a Jeffery fluid, the elastic modulus G mainly affects the oscillation period and amplitude, the parallel viscosity pots η1 mainly impact the amplitude and oscillation attenuation speed, and the series clay pots η2 mainly influence the slope of the strain curve. The greater the system inertia, the more significant the creep oscillation in the unsteady flow process. The coupling between the test system inertia and the viscoelasticity fluid is the essential cause of the creep oscillation. Combined with the mechanical response characteristics of rheometer, a Jeffery model based on short term correction and torque loss was established. The model can accurately describe the strain time curve of gelled crude oil in the initial unsteady flow stage. The results of this study can help understand the rheological properties of materials in the transient start-up process.

Image-guided simulation in comparison with laser speckle contrast imaging for full-field observation of blood flow in a microvasculature model

The in vitro reconstruction of the microvascular network model provides a reproducible platform for hemodynamic study with great biological relevance. In the present study, microvascular models with different parametric features were designed under the guidance of Murray's law and derived from representative natural vascular network topography in vivo. Computational fluid dynamics (CFD) was used to numerically simulate blood velocity distributions inside of the designed microvasculature models. Full-field blood flow in the vascular network was visualized in vivo using a laser speckle contrast imaging (LSCI) system, from which the measured relative velocity was compared with CFD computed flow distribution. The results have shown that, in comparison with the simplified flow patterns obtained from idealized geometries, the irregular vascular topography is expected to lead to nonuniform and poor regional blood velocity distribution. The velocity distribution acquired by in vivo LSCI experiment is in good agreement with that of numerical simulation, indicating the technical feasibility of using biomimetic microchannels as a reasonable approximation of the microcirculatory flow conditions. This study provides a new paradigm that can be well suited to the study of microvascular blood flow properties and can further expand to mimic other in-vivo scenarios for accurately recapitulating the physical and hemodynamic environment of the microcirculation.