Increase In Velocity Ratio Research Articles

Dynamic Response of Block Foundations Resting on Soil–Rock and Rock–Rock System Under Vertical Excitation

In the present study, the effect of various soil–rock and rock–rock foundation systems on dynamic response of block foundations of different mass and equivalent radius under vertical mode of vibration is investigated. The frequency dependent stiffness and damping of foundation resting on homogeneous soil and rocks are determined using the half-space theory. The dynamic response characteristics of foundation resting on the layered system considering soil–rock and rock–rock combinations are evaluated using finite element program with transmitting boundaries. The procedure to determine the frequency amplitude response of soil–rock and weathered rock–rock systems is discussed in detail and the equations are proposed for the same. The variation of natural frequency and resonant amplitude with shear wave velocity are investigated for different top layer thickness. The dynamic behaviour of foundation is also determined for different rock–rock systems by considering sandstone, shale and limestone underlain by basalt rock. The variation of the stiffness, damping and amplitudes of foundation with frequency are shown in this study for various rock–rock combinations. It has been observed that the natural frequency increases and the peak displacement amplitude decreases with increase in shear wave velocity ratio. The variation of natural frequency and peak displacement amplitude are also studied for different top layer thickness and eccentric moments. Finally the parametric study is carried out to determine the natural frequency and resonant amplitude for block foundations of size 4 m × 3 m × 2 m and 8 m × 5 m × 2 m using generalized relations proposed in the present study.

MHD stagnation point flow and heat transfer due to nanofluid towards a stretching sheet

In this paper we analyzed the effect of magnetic field on stagnation point flow and heat transfer due to nanofluid towards a stretching sheet. The transport equations employed in the analysis include the effect of Brownian motion and thermophoresis. The solution for the temperature and nanoparticle concentration depends on six parameters viz. velocity ratio parameter A, Prandtl number Pr, Lewis number Le, Brownian motion Nb, and the thermophoresis parameter Nt. Similarity transformation is used to convert the governing nonlinear boundary-layer equations to coupled higher order nonlinear ordinary differential equation. These equations were numerically solved using Runge–Kutta fourth order method with shooting technique. Numerical results are obtained for velocity, temperature and concentration distribution, as well as the skin friction coefficient, local Nusselt number and Sherwood number. The results indicate that the skin friction coefficient Cf and local Nusselt number increases with an increase in velocity ratio parameter A. Likewise, the local Sherwood number increases with an increase in both velocity ratio A and Lewis number Le. Besides, it is found that the heat transfer rate at the surface increases with the magnetic parameter when the free stream velocity exceeds the stretching velocity, i.e. A>1, and it decreases when A<1. A comparison with a previous study available in the literature has been done and we found an excellent agreement with them.

The effects of arbitrary injection angle and flow conditions on venturi-jet mixer

This paper describes the effect of jet injection angle, cross flow Reynolds number and velocity ratio on entrainment and mixing of jet with incompressible cross flow in venturi-jet mixer. Five different jet injection angles 45o, 60o, 90o, 125o, 135o are tested to evaluate the entrainment of jet and mixing performances of the mixer. Tracer concentration along the downstream of the jet injection, cross flow velocity, jet velocity and pressure drop across the mixer are determined experimentally to characterize the mixing performance of the mixer. The experiments show that the performance of a venturi-jet-mixer substantially improves at high injection angle and can be augmented still by increasing velocity ratio. The jet deflects much and penetrates less in the cross flow as the cross flow Reynolds number is increased. The effect could contribute substantially to the better mixing index with moderate pressure drop. Normalized jet profile, concentration decay, jet velocity profile are computed from equations of conservation of mass, momentum and concentration written in natural co-ordinate systems. The comparison between the experimental and numerical results confirms the accuracy of the simulations. Correlations for jet trajectory and entrainment ratio of the mixer are obtained by multivariate-linear regression analysis using power law.

Turbulent Flow Separation Control over a Two-Dimensional Ramp Using Synthetic Jets

An experiment study of flow control using an array of three circular synthetic jets has been carried out in a separated turbulent flow over a two-dimensional ramp in a boundary-layer tunnel. The freestream velocity is 6:5 m=s and the Reynolds number based on the momentum thickness at a reference location upstream of the synthetic jet array is Re 1150. The synthetic array is operated at mean jet-to-freestream velocity ratios ranging from 0.2 to 0.5 and a reduced frequency based on the ramp height of F 0:2 and 0.6. Two-dimensional particle image velocimetry is employed to evaluate the control effect of synthetic jets at different actuator operating conditions in both timeand phase-averaged senses. Stereo particle image velocimetry is used to reveal the vortical structures thatwere responsible for theflow separation delay. It is found that at both reduced frequencies an increase in velocity ratio results in a further reduction in the height of the reverse-flow region, and somedifferences exist in the way inwhich the reverseflowresponds to jet actuation of the twodifferent reduced frequencies. It is also found that at the conditions tested in the present study, the interaction of synthetic jets with the turbulent boundary layer results in formation of hairpin vortices, which induce a further pair of streamwise vortices of the opposite sign of rotation directly underneath. It is believed that it is the streamwise vortices that produce a downwash bringing highmomentum fluid toward the wall, hence reducing the flow separation.

Monitoring the effect of admixtures on early-age concrete behaviour by ultrasonic, calorimetric, strength and rheometer measurements

The effect of mineral additions on concrete setting has already been investigated thoroughly using ultrasonic methods. However, growing interest in self-compacting and high-performance concrete has also increased the use of admixtures such as superplasticisers and air entrainers, which are less documented regarding their effect on ultrasound transmission. Self-compacting mixtures with different cement types and superplasticisers were therefore tested by means of ultrasonic, calorimetric, rheometer and strength measurements. The use of air entrainer was also investigated. Setting time was prolonged by the addition of PCE (polycarboxylic ethers) and PNS (polycondensates of naphthalene sulphonate) by 1·3–2·4 h and 2·4–4·4 h respectively, as determined by both traditional and ultrasonic methods. Postponed setting can be caused by hydration retardation (1–5 h) and cement particle dispersion. More air was entrained by PNS than by PCE, leading to lower initial ultrasonic wave velocities and lower compressive strengths. As for the air entrainer, a slower increase in velocity and energy ratio was noted, together with a lower value reached after 48 h, attributed to a lower stiffness of the samples and wave scattering on air inclusions.

Surface Grinding Force Model of Steel 55 Based on Undeformed Chip Thickness with CBN Electroplated Wheels

Undeformed chip thickness is one of the most important parameters in grinding process, which is related to the entire abrasive grains in grinding simultaneously and changed periodically with time. Simplifying the geometric shape of abrasive grains ,the paper modifies the mathematic models of undeformed chip thickness by analytic method, establishes an universal calculation model of grinding force based on undeformed chip thickness, then optimizes the parameters of the model by restrictive random direction method according to the measuring experiments of the inter-grain spacing about CBN electroplated wheels and the grinding experiments of steel 55 during surface grinding, analyses the influence factors of the friction ratio on the grinding force. The results show that under the same grinding depth， both of the ratio and the grinding force will be decreased with the increase of velocity ratio VS/VW, but the ratio increases and the grinding force decreases with the increase of inter-grain spacing.

Inlet ground vortex aerodynamics under headwind conditions

Quantitative velocity flow field measurements of the vortex system for a small scale model intake have been taken using a three component PIV system along with fan face total pressure distortion measurements. The effect of velocity ratio, non-dimensional height and approaching boundary layer thickness have been assessed under quiescent and headwind conditions. A range of inlet vortex flow modes are identified which depend on the intake velocity ratio and are characterised by the primary vorticity source. As the headwind velocity increases, and the velocity ratio reduces, the vortex strength initially increases until a local maximum is reached. Further reductions in the velocity ratio leads to a strong reduction in the vortex strength until a ‘blow-away’ condition is achieved at which the vortex no longer forms. A clear link is established between the external vortical flow-field and the intake internal flow-field. Consequently, these characteristics are also observed for the total pressure distortion coefficient within the intake. As the ground clearance is increased the peak vortex strength reduces and the corresponding velocity ratio increases. At intermediate velocity ratios, the vortex strength is greater at higher ground clearances. At the selected external vortex measurement plane, the vortex strength is generally insensitive to the approaching boundary layer thickness. The intake total pressure distortion increases with boundary layer thickness due to increased total pressure loss for a given ingested mass-flow. The vortex strength and distortion distributions are shown to be self-similar when non-dimensionalised by the peak values and associated velocity ratio. This dataset establishes a quantitative map for headwind ground vortices which presents a new formation criterion as well as a measure of the vortex strength as a function of ground clearance and intake velocity ratio.

Effect of Velocity Ratio on Noise Source Distribution of Coaxial Jets

The noise source distribution of coaxial jets with a diameter ratio of 1.6 and variable velocity ratio is investigated with a small-aperture microphone phased array. The array design enables differentiation of noise emitted by large-scale and fine-scale turbulence structures, which have different directivities. The acoustic data are complemented by pitot surveys of the mean flow, which yield measurements of the primary and secondary cores of the jet. For zero velocity ratio (single-stream jet), the region near the nozzle emits strong high-frequency noise. Increasing the secondary-to-primary velocity ratio suppresses the near-nozzle noise and extends the location of the peak noise downstream, which increases moderately. The axial location of peak noise is approximately situated at the end of the primary core. The suppression of high-frequency noise is explained by the creation and elongation of the secondary core as the velocity ratio increases. The acoustic trends with velocity ratio are similar for small and large array observation angles from which large-scale and fine-scale turbulence noise, respectively, have been shown to radiate. However, the increase in peak noise is more pronounced for the large-scale noise.

Absolute lateral instability in capillary coflowing jets

We examine the stability of coflowing capillary jets under lateral (m=1) perturbations of small amplitude. Two models are considered for the perturbed basic flow: the Kelvin–Helmholtz (KH) and the outer boundary layer (OBL) models. We revisit the temporal analysis of the KH model and show that the flow is unstable if and only if the (conveniently defined) Weber number is greater than unity. On the contrary, the OBL flow becomes unstable for Weber numbers much smaller than unity, although the growth rate of the perturbations is very small in that case. The spatiotemporal analysis of the dispersion relations shows that both the KH and OBL flows become absolutely unstable (absolute whipping) for sufficiently large values of the Weber number and the ratio between the outer and inner stream velocities. Absolute whipping dominates the behavior of high-viscosity jets for large velocity ratios and prevents the jetting regime from being reached even when varicose perturbations are convected downstream. For sufficiently large values of the Reynolds number, the flow becomes absolutely unstable if the velocity ratio exceeds a critical value, which is almost independent of the Weber number. For small values of the velocity ratio, the flow is stable or at most convectively unstable independently of the Reynolds and Weber numbers. For sufficiently large values of the velocity ratio, there is a critical Reynolds number above which jetting can not be reached because the flow becomes absolutely unstable due to the modes m=0 and/or m=1. That critical Reynolds number decreases as the velocity ratio increases. These results have important implications in technological applications such as steady high-viscosity liquid microjet production and fiber spinning using coflowing gas conformation.

Numerical Study of Buoyancy-Opposed Wall Jet Flow

This paper describes a numerical study of the flow and thermal fields for opposed wall jet. The hot water injected from plane jet down one wall of a vertical passage of rectangular cross section into cooled water which moves leisurely to the upper. The flow is assumed to be two dimensional, steady, incompressible and turbulent. The finite volume scheme is used to solve the continuity equation, momentum equations, energy equation and κ-e model equations. The flow characteristics were studied by varying of Richardson number (0.0 ≤ Ri ≥ 0.052) and the ratio of back ground velocity to jet velocity (0.05 ≤ R ≤ 0.15). The results showed that, the buoyancy limited the downward penetration of the jet and its lateral spread when Richardson number increased. The shear layer formed at the interface between the two flow streams, and it becomes more concentrated at higher values of Richardson number. In this region, the intensity of the turbulence became stronger and the turbulent shear stress was a minimum value. When the velocity ratio increased, the penetration of jet decreases, its lateral spreading becomes less. Also the temperature difference decreases with the velocity ratio increases. The numerical results give good agreement with the experiment data of Ref. [1].

Numerical Simulation of Flow Past Multiple Porous Cylinders

The present study numerically investigates two-dimensional laminar flow past three circular porous cylinders arranged in an in-line array. Six approaches to face velocity (Vi/Vf) ratios are used and particle trajectories are computed for a range of velocities and particle diameters. Furthermore, the flow past a solid cylinder, which had similar geometry characteristics to the porous cylinders used in this study, is compared with the flow around multiple porous cylinders. For the same range of Reynolds number (312–520), the flow behavior around the solid cylinder differs from the flow around the porous cylinders. The flow characteristics around solid cylinders are determined by the Reynolds number, whereas the flow characteristics around the porous cylinders are detrained by the Vi/Vf ratio. Stagnation areas are found behind each porous cylinder, and the size of these areas increases as the Vi/Vf velocity ratio increases. Furthermore, for the particle ranges used in power plants (&lt;50 μm), the particles were uniformly distributed around the surface of the porous cylinders.

Influence of Leak Path Friction on Labyrinth Seal Inlet Swirl

Labyrinth seals in various kinds of rotating machinery often generate driving force components that may increase the unstable vibration of the rotor. The evaluation of labyrinth seal forces has been documented by CFD analysis but the leak path geometry influence on the resulting entry and reverse pumping flow prior to the labyrinth seal geometry has only recently been studied in detail. The labyrinth seal driving forces are known to increase with an increase of the inlet swirl velocity ratio. In a previous work, a commercial CFD program was used to simulate leak path and labyrinth seal flows of various centrifugal compressor eye seal leak flow geometries. For each case, 3D models with an eccentric rotor were solved to obtain the leakage flow, velocity vector, chamber pressure, and the average chamber swirl entering the seal. The current article will present the bulk flow analysis predicted entry swirl ratio for the same geometries and sealing conditions. The influence of the effective disk friction will be considered in the calculation of the bulk flow results by using modified turbulence factors. The CFD simulation showed that the inlet geometry and the resultant reverse pumping flow in the entry channel can have a significant influence on the calculated chamber average swirl velocity ratio. Generally, the bulk flow analysis gives a pessimistic estimate of the swirl velocities as compared to the CFD calculations.

Rotary tiller velocity effects on the distribution of wheat (Triticum aestivum) residue in the soil profile

Effect of velocity ratios of a forward rotational‐rotary tiller on the distribution of wheat (Triticum aestivum) stubbles incorporated into soil layers was investigated. A 2‐year experiment was conducted in a field with silt loam soil on an experimental site at Süleyman Demirel University located near Isparta, Turkey. Velocity ratios of the rotary tiller were maintained at 6.01, 7.51, 9.39, and 11.77. The soil depths studied were 0–5, 5–10, and 10–15 cm. Results showed that there were no significant differences among velocity ratios, and soil depths by velocity ratio interaction means in terms of stubble mass. However, there were significant differences among soil depths means in terms of stubble mass (P < 0.01). This indicates that there is no advantage of increasing velocity ratios of rotary tiller to bury more stubble mass into the soil since fuel consumption of the tractor and the power requirement of the rotary tiller increase with increased velocity ratios of the rotary tiller. There were significant differences among the soil depths means in terms of distribution of stubble length at the soil depths (P < 0.01). Longer stubbles accumulated at a soil depth of 0–5 cm whereas shorter stubbles were buried in a 5–15 cm soil layer regardless of velocity ratio.

Evolution of texture in AA6111 aluminum alloy after asymmetric rolling with various velocity ratios between top and bottom rolls

Asymmetric rolling (ASR) was applied to aluminum alloy AA6111 with various velocity ratios between the top and bottom rolls from 1 to 4. The texture after ASR consists of the cube {0 0 1}〈1 0 0〉 and the β-fibre (Bs {0 0 1}〈2 1 1〉, S {1 2 3}〈6 3 4〉 and Cu {1 1 2}〈1 1 1〉 orientations) but rotates about the transverse direction for 9–14°, until the velocity ratio reaches a critical value of 4. With a velocity ratio of 4, the as-rolled texture is drastically weakened throughout the sheet thickness and a texture close to the rotated cube {0 0 1}〈2 1 0〉 becomes dominant in the sheet surface and centre. After solutionising at 560 °C, the recrystallization texture in the ASR processed sheet is similar to that of the conventionally rolled material, but the texture is more randomized with increasing velocity ratio. When the velocity ratio reaches the critical value of 4, the {1 1 1}〈1 1 0〉 texture component becomes dominant and the others are negligible.

Numerical simulation of turbulent impinging jet on a rotating disk

AbstractThe calculations of quasi‐three‐dimensional momentum equations were carried out to study the influence of wall rotation on the characteristics of an impinging jet. The pressure coefficient, the mean velocity distributions and the components of Reynolds stress are calculated. The flow is assumed to be steady, incompressible and turbulent. The finite volume scheme is used to solve the continuity equation, momentum equations and k–ε model equations. The flow characteristics were studied by varying rotation speed ω for 0⩽ω⩽167.6 rad/s, the distance from nozzle to disk (H/d) was (3, 5, 8 and 10) and the Reynolds number Re base on VJ and d was 1.45 × 104. The results showed that, the radial velocity and turbulence intensity increase by increasing the rotation speed and decrease in the impingement zone as nozzle to disk spacing increases. When the centrifugal force increases, the radial normal stresses and shear stresses increase. The location of maximum radial velocity decreases as the local velocity ratio (α) increases. The pressure coefficient depends on the centrifugal force and it decreases as the distance from nozzle to plate increases. In impingement zone and radial wall jet, the spread of flow increases as the angular velocity decreases The numerical results give good agreement with the experiment data of Minagawa and Obi (Int. J. of Heat and Fluid Flow 2004; 25:759–766). Copyright © 2006 John Wiley &amp; Sons, Ltd.

EVALUATION OF MIXING DOWNSTREAM OF TEES IN DUCT SYSTEMS WITH RESPECT TO SINGLE POINT REPRESENTATIVE AIR SAMPLING

Air duct systems in nuclear facilities must be monitored with continuous sampling in case of an accidental release of airborne radionuclides. The purpose of this work is to identify the air sampling locations where the velocity and contaminant concentrations fall below the 20% coefficient of variation required by the American National Standards Institute/Health Physics Society N13.1-1999. Experiments of velocity and tracer gas concentration were conducted on a generic "T" mixing system which included combinations of three sub ducts, one main duct, and air velocities from 0.5 to 2 m s (100 to 400 fpm). The experimental results suggest that turbulent mixing provides the accepted velocity coefficients of variation after 6 hydraulic diameters downstream of the T-junction. About 95% of the cases achieved coefficients of variation below 10% by 6 hydraulic diameters. However, above a velocity ratio (velocity in the sub duct/velocity in the main duct) of 2, velocity profiles were uniform in a shorter distance downstream of the T-junction as the velocity ratio went up. For the tracer gas concentration, the distance needed for the coefficients of variation to drop 20% decreased with increasing velocity ratio due to the sub duct airflow momentum. The results may apply to other duct systems with similar geometries and, ultimately, be a basis for selecting a proper sampling location under the requirements of single point representative sampling.

Mathematical modelling of blood flow in curved arteries

We study the mixed convection boundary layer heat transfer of power law fluid over a moving conveyor along an inclined plate. The effects of shear flow and power law viscosity on the temperature field are taken into account according to a modified Fourier law. Approximate analytical solutions are obtained by the homotopy analysis method (HAM). Results indicate that heat transfer is strongly dependent on the values of power law exponent, inclination angle, boundary velocity ratio and Prandtl number. Three distinct characteristics are found for power law exponents 0 < n < 1, n = 1 and n > 1, especially the nonlinear behavior due to skin friction and local Nusselt number shown in Figs. 4 and 17, which has never been reported before. The decrease of inclination angle causes the loss of velocity boundary layer but the gain of temperature boundary layer. Heat transfer efficiency is enhanced but skin friction is diminished with the increase in velocity ratio (the ratio of conveyor velocity/mean velocity of flow field). Critical ratio (with skin friction zero) is obtained which strongly depends on the power law exponent. The effects of involved parameters on the velocity and temperature fields are analyzed.

Dye visualisation of inclined and skewed synthetic jets in a cross flow

AbstractDye visualisation of both inclined synthetic jets and skewed synthetic jets was undertaken in a cross-flow experiment and the results were compared with those of normal synthetic jets. The process of vortex roll-up near the orifice exit and how the structure develops and interacts with the cross-flow as it propagates downstream was investigated so as to obtain an understanding of the effect of orifice orientation on the behaviour of synthetic jets. The effects of varying Reynolds number, velocity ratio and Strouhal number due to changes in diaphragm displacements and freestream velocities on the characteristics of synthetic jets were also examined. It is observed that in comparison to the normal jets vortical structures produced by both inclined and skewed jets tend to stay closer to the near wall region where maximum flow control effect is required. In both cases, at a relatively low Reynolds number and velocity ratio the active structures produced by the synthetic jet appear to be hairpin vortices which turn into vortex rings that migrate away from the wall as the Reynolds number and velocity ratio increase. These hairpin vortices are persistent in the near wall region hence are believed to be desirable structures for delaying flow separation.

Flows in T-Junction Piping System (1st Report, Flow Characteristics and Vortex Street Formed by Branch Pipe Flow)

Behavior of main and branch pipe flow in a T-junction piping system under the isothermal condition was examined experimentally using flow visualization techniques (the dye injection and smoke method). As a result, it was clarified that flows downstream of the T-junction are classified into three types : stratified flow, deflecting jet, and impinging jet. In the case of the deflecting jet, the flow issuing from the branch pipe into the main duct forms an arch vortex street. Next, the waveform of the axial velocity and its spectrum were measured by using hot wire, and the structure of the vortex was investigated. The frequency with which vortices are shed in an arch vortex street becomes higher with increasing Reynolds number under the condition of a constant velocity ratio and decreases with an increase in velocity ratio. Strouhal number is expressed by an approximate equation under the condition of Reynolds number Re=5000-50000, velocity ratio β=0.33-2.00 and diameter ratio α=2.04-5.20.

Laser optical investigation of turbulent transport of temperature ahead of the preheat zone in a premixed flame

This experimental study investigates temperature profiles upstream of localized thin reaction zones in a turbulent premixed flame stabilized on a low-swirl-burner. A simultaneous dual-sheet Rayleigh/OH-LIPF measurement technique was applied using two parallel laser light sheets to measure the temperature in the preheat zone for three lean (ϕ=0.7) turbulent methane/air flames. The ratio of the turbulence intensity to the laminar burning velocity v′/sL was varied from 3.5 to 9.2 and to 18.7. The first of these flames lies on the borderline between the “corrugated flamelets” regime and the “thin reaction zones” regime, while the other two are in the “thin reaction zones” regime. The results confirm the occurrence of strong temperature fluctuations ahead of the preheat zone for flames in the “thin reaction zones” regime. Single temperature images show a significant temperature increase ahead of the preheat zones of up to 700 K for flames at the highest turbulence intensity. For statistical analysis conditional mean temperature profiles and probability density functions conditioned on the distance from the flame contour were calculated from the experimental data. Only those portions of the flame front were included which were found to be approximately normal to the two laser sheets. The resulting probability density functions show that the effect of temperature rise ahead of the preheat zone becomes significant only for flames in the “thin reaction zones” regime. The mean temperature profiles show a much smaller temperature rise which, however, increases with increasing velocity ratios v′/sL.