Net Pressure Force Research Articles

Logjams With a Lower Gap: Backwater Rise and Flow Distribution Beneath and Through Logjam Predicted by Two‐Box Momentum Balance

AbstractLogjams with a gap at the bed form naturally in small channels and are used in engineering practice to maintain river connectivity at base flow. Limited understanding of a jam's effect on backwater rise and flow velocity limits assessment of geomorphic and ecological impacts of natural logjams, design of river restoration interventions, and representation in flood models. The distribution of flow through and beneath the jam satisfies a two‐box, momentum‐based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. The model was validated with 68 flume experiments. Backwater rise is predicted from discharge beneath the jam following established models for solid sluice gates. As a result, backwater rise increases with jam resistance, which forces a greater discharge beneath the jam. Below‐jam velocity and Shields parameter increased with ratio of friction coefficient to slope and decreasing gap height.

Modeling water hammer in viscoelastic pipes using the wave characteristic method

For the first time, the wave characteristic method (WCM) is used to simulate transient flow in viscoelastic pipes. The WCM is based on Newton's second law equating the change in momentum and net pressure forces applied on the liquid, which leads to the Joukowsky equation. The friction head loss within a pipe segment is replaced by an imaginary friction orifice with the same head loss. Here the Joukowsky equation is rederived for elastic pipes on the basis of the conservation of energy principle. Then the same method is used to derive a quadratic equation for the conservation of energy in viscoelastic pipes. Under a relaxation assumption, the square root of the energy equation yields a linear equation that is applicable with the superposition principle. The method developed is studied numerically and verified with experimental data from the literature. The water hammer in a simple pipe system consisting of a reservoir, a pipe, and a valve is demonstrated. Parameters calibrated with the method of characteristics are taken from the literature and used as essential input data for the proposed WCM. A constant friction coefficient of the pipe is considered. Even if a small number of friction orifices are selected, good agreement is found between the experimental data and the simulation results especially for the first pressure head cycles. Finally, the numerical results obtained with both the WCM and the method of characteristics are compared to investigate the effectiveness of the WCM. The WCM shows superior computational efficiency in determining the maximum pressure.

Pumping effect of heterogeneous meniscus formed around spherical particle

HypothesisA disturbance such as a microparticle on the pathway of a spreading droplet has shown the tremendous ability to accelerate locally the motion of the macroscopic contact line (Mu et al., 2017). Although this ability has been linked to the particle-liquid interaction, the physical mechanisms behind it are still poorly understood despite its academic interest and the scope of numerous industrial applications in need of fast wetting. ExperimentsIn order to better understand the mechanisms behind the particle-liquid interaction, we numerically investigate the pressure and velocity fields in the liquid film. The results are compared to experiments assessing the temporal shape variation of the liquid-film meniscus from which pressure difference around the particle is evaluated. FindingsThe particle-induced acceleration of the film front depends both on the shape of the meniscus that forms around the particle foot and the liquid “reservoir” in the film that can be pumped thanks to the presence of the particle. The study validates the presence of three stages of pressure difference between the upstream and downstream regions of the meniscus around the particle, which leads to the local acceleration/deceleration of the macroscopic contact line. We indicate that asymmetric meniscus around the particle foot produces a net pressure force driving liquid and accelerating the liquid-film front.

A 1D fluid model of the Centaurus A jet

We implement a steady, one-dimensional flow model for the X-ray jet of Centaurus A in which entrainment of stellar mass loss is the primary cause of dissipation. Using over 260 ks of new and archival Chandra/ACIS data, we have constrained the temperature, density and pressure distributions of gas in the central regions of the host galaxy of Centaurus A, and so the pressure throughout the length of its jet. The model is constrained by the observed profiles of pressure and jet width, and conserves matter and energy, enabling us to estimate jet velocities, and hence all the other flow properties. Invoking realistic stellar populations within the jet, we find that the increase in its momentum flux exceeds the net pressure force on the jet unless only about one half of the total stellar mass loss is entrained. For self-consistent models, the bulk speed only falls modestly, from ~0.67c to ~0.52c over the range of 0.25-5.94 kpc from the nucleus. The sonic Mach number varies between ~5.3 and 3.6 over this range.

Numerical simulation of a compressible vortex–wall interaction

The wall interaction of isolated compressible vortices generated from a short driver section shock tube has been simulated numerically by solving the Navier–Stokes equations in axisymmetric form. The dynamics of shock-free (incident shock Mach number $$M = 1.36$$ ) and shock-embedded $$(M = 1.57)$$ compressible vortices near the wall has been studied in detail. The AUSM+ scheme with a fifth-order upwind interpolation formula is used for the convective fluxes. Time integration is performed using a low dissipative and dispersive fourth-order six-stage Runge–Kutta scheme. The evolution of primary and wall vortices has been shown using the velocity field, vorticity field, and numerical schlierens. The vortex impingement, shocklets, wall vortices, and their lift-off are clearly identified from the wall pressure time history. It has been observed that the maximum vorticity of the wall vortices reaches close to 30 % of the primary vortex for $$M = 1.36$$ and it reaches up to 60 % for $$M = 1.57$$ . The net pressure force on the wall due to incident shock impingement is dominant compared to the compressible vortex impingement and their evolution.

리니어 압축기에서 그루브 형상 변화에 따른피스톤의 동특성 해석

It is possible to prevent a piston from contacting the cylinder by changing the shape of the piston or by applying micro-textures, such as micro-grooves or micro-holes, over the piston surface. Usually, the minimum radial clearance reaches its minimum value at the beginning of the suction stroke because the pressure around the piston is low and almost axisymmetric such that the net pressure force on the piston is not sufficiently high to support the piston from touching the cylinder. In this study, we apply a series of saw-tooth-shaped grooves on the piston surface, and numerically investigate the effects of groove depth, groove angle, and the number of grooves with radial clearance variations using a finite difference method. We conduct a dynamic analysis of the piston for various changes in groove geometries to obtain the minimum radial clearance variation for the entire compression cycle. The minimum radial clearance increases while friction loss decreases when we apply the series of saw-tooth-shaped grooves on the piston. In addition, we analyze the impact of the change in the groove shape variable due to changes in radial clearance. Leakage variations are relevant to radial clearance, but have almost no effect on the groove parameters.

Underexpanded Jet Development from a Rectangular Nozzle with Aft-Deck

An experimental study is reported of underexpanded supersonic jet plumes issuing from a high-aspect-ratio convergent rectangular nozzle. Schlieren visualization, Pitot probe, and Laser Doppler Anemometry measurements are carried out to capture the plume development in the near field, and in particular the effect on the plume flow of a finite-length extended shelf or aft-deck attached to the lower nozzle wall. This creates asymmetry in the inviscid shock cell pattern and the entrainment characteristics, both of which influence shear-layer growth and plume trajectory. A net pressure force is induced on the aft-deck wall, which leads to transverse deflection of the jet plume once it leaves the aft-deck, both upward and downward, depending on aft-deck length and nozzle pressure ratio. For sufficiently high nozzle pressure ratio and a sufficiently long aft-deck, separation and reattachment of the plume from the aft-deck is observed. Detailed mapping of both mean velocity and turbulence in the plume near field has been carried out, enabling comparison of flow behavior for a clean nozzle and a nozzle with aft-deck. The data provided are proposed as a suitable benchmark validation test case for computational fluid dynamics studies of rectangular nozzle plumes with aft-deck interaction effects.

Flow Visualisation Experiments on Sports Balls

Fluid dynamics plays a significant role in many sports, principally affecting the trajectory of the associated ball. Boundary layer theory can be used to explain why some of these effects take place, demonstrated here for the games of cricket and golf.The asymmetric nature of a cricket ball, due to the presence of a seam, causes the boundary layer to be tripped into turbulence on one side. On the other hemisphere, the smooth surface promotes laminar flow which separates at a smaller angle relative to the stagnation point. This results in a net pressure force and lateral movement known as swing. In golf inverted dimples are applied to the ball to reduce drag by promoting transition to turbulent flow, this in turn increases the maximum achievable range.In this study, scaled versions of a smooth sphere, a cricket ball and a golf ball were used to perform wind tunnel experiments in which these fluid dynamic effects were demonstrated. A novel infrared flow visualisation technique, in conjunction with measurements of pressure, highlighted the fluid mechanics at the representative conditions found in each sport. The results underlined the dependence on surface roughness, and provided qualitative visual evidence of the state of the boundary layer at a Reynolds number of 1 x 105.

Fluid dynamics of cricket ball swing

Swing describes the lateral deviation of a cricket ball in its trajectory towards the batsman. Conventional swing is effective with a new, or well-preserved, ball, and the fluid dynamics governing this phenomenon was first explained in 1957. In 2012, many test-match fast bowlers are able to swing, at high speed, an older ball in the reverse direction. This reverse swing of a ball aged under match conditions has never been explained fully. A cricket ball is asymmetric with six seams of 80–90 encircling stitches, protruding approximately 1 mm proud of the surface. Both conventional and reverse swings are a consequence of asymmetrical flow separation leading to a skewed wake and a net pressure force on the ball perpendicular to the flight trajectory. Here, experimental evidence is presented for the first time showing that the formation of a laminar separation bubble is the prominent flow feature creating the flow asymmetry for reverse swing. A new flow visualisation technique to capture the fluid dynamics of boundary-layer separation using an infrared camera is also introduced here.

A Laplace pressure based microfluidic trap for passive droplet trapping and controlled release

Here, we present a microfluidic droplet trap that takes advantage of the net Laplace pressure force generated when a droplet is differentially constricted. Mathematical simulations were first used to understand the working range of the component; followed by finite element modeling using the CFD software package to further characterize the behavior of the system. Controlled release of the trapped droplets is also demonstrated through both a mechanical method and a chemical method that manipulates the total pressure exerted on the trapped droplet. The unique design of this trapping device also provides the capability for selection of a single droplet from a train, as well as droplet fusion.

Discontinuous Galerkin Method for 1D Shallow Water Flows in Natural Rivers

A numerical model is proposed for the solution of one-dimensional shallow water flow equations for natural rivers. This model is based on the total variation diminishing Runge-Kutta discontinuous Galerkin finite element method. In natural rivers, the cross-section shape and bed slope can be quite irregular, which requires a compatible discretization scheme for the bed slope term and net pressure force term. Therefore, in this model, the hydrostatic pressure force term and the wall pressure force term are combined and a new discretization for the resulting term is introduced. This formulation is shown to prevent unphysical flow due to improper treatment of bottom slope term. The mass and momentum flux term are calculated by HLL Riemann solver. A scheme is presented to model flow over dry bed. To evaluate the numerical scheme, tests are conducted for idealized dambreak problems in parabolic (with wet and dry beds), and rectangular channels, hydraulic jump in a rectangular channel, dambreak in the Teton River (Idaho, USA) and the Toce River (Northern Alps, Italy), and flooding event in the East Fork River (Wyoming, USA). The comparison of the computational results with analytical and laboratory results of dam break flows shows that the model is capable of handling flow over dry areas. The simulation results for hydraulic jump show the discharge conservation property and shock prediction capability of the model. The dambreak and flood simulations in natural channels show that the model is capable of handling flows in highly varying bed topography and channel geometry.

A Discontinuous Galerkin Method for Two‐Dimensional Shock Wave Modeling

A numerical scheme based on discontinuous Galerkin method is proposed for the two‐dimensional shallow water flows. The scheme is applied to model flows with shock waves. The form of shallow water equations that can eliminate numerical imbalance between flux term and source term and simplify computation is adopted here. The HLL approximate Riemann solver is employed to calculate the mass and momentum flux. A slope limiting procedure that is suitable for incompressible two‐dimensional flows is presented. A simple method is adapted for flow over initially dry bed. A new formulation is introduced for modeling the net pressure force and gravity terms in discontinuous Galerkin method. To validate the scheme, numerical tests are performed to model steady and unsteady shock waves. Applications include circular dam break with shock, shock waves in channel contraction, and dam break in channel with 45∘ bend. Numerical results show that the scheme is accurate and efficient to model two‐dimensional shallow water flows with shock waves.

Flow visualization experiments demonstrating the reverse swing of a cricket ball

Swing, the sideways deviation of the cricket ball in flight, is a phenomenon which can be explained in terms of fluid dynamics. Conventional swing has been used since the beginning of the twentieth century and is effective with a new or well-preserved ball. A controversial form of swing emerged in Pakistan in the 1980s, featuring an older worn ball which, at a high speed, swung in the reverse direction. A cricket ball is asymmetric because of the presence of a seam, which is made up of rows of prominent encircling stitches. For conventional swing, this seam trips into turbulence the boundary layer adjacent to one hemisphere of the ball which remains attached to a greater angle (about 120°) than does that on the other side (about 80°) where no seam is present to trip the laminar boundary layer. The result is asymmetrical separation, leading to a skewed wake and a net pressure force on the ball perpendicular to the flight trajectory. Reverse swing is thought to be a consequence of the fact that asymmetry inverts at a high speed if the seam thickens the turbulent boundary layer on one side of the ball. Although the fluid dynamics causes of both conventional and reverse swing are well known, this paper demonstrates clearly, by means of flow visualization and pressure measurements, the inversion of this pressure asymmetry at Reynolds numbers greater than 170×103.

Pacific trench motions controlled by the asymmetric plate configuration

We present a novel explanation for absolute trench‐normal motions of slabs surrounding the Pacific. Rapid subduction‐zone retreat on the eastern side of the Pacific and slow advance in the west can result from the large‐scale asymmetric plate configuration. We use simple fluid dynamics to explain the mechanical background of this hypothesis, and we use the results of a simple finite difference scheme to estimate the effect on trench motion velocities. The hypothesis is based on two key assumptions. First, we follow the concept of plate‐scale horizontal counterflow in the asthenosphere driven by accretion of asthenosphere into lithosphere and by plate motion. Second, we assume that horizontally wide slabs without large slab windows drift passively in the mantle flow field and do not retreat as a result of flow around the slab. If the asthenosphere transfers flow‐related horizontal shear stress into deeper levels of the mantle, an asymmetry in the plate configuration leads to different net pressure forces on the two slabs and thus affects the retreat behavior. In an ocean with an asymmetric ridge position, the slab of the smaller plate should retreat faster than the slab of the large plate, which may even advance. Also, the domain of a slower moving plate should collapse faster than the domain of the faster plate. Our model explains the counterintuitive negative correlation between slab age and retreat velocity observed in the Pacific. It also accords with the topographic asymmetry of the ridge flanks along the Pacific rise.

Comparisons of rotordynamic coefficients in stepped labyrinth seals by using Colebrook-White friction factor model

The objective of this work is to observe the effects of friction factors for the stepped labyrinth seals. The gas flow through the seals creates net pressure and shear forces acting on the rotor. It is necessary to predict these forces for reliably operating turbomachinery. So we investigated the effect of shear forces on the calculation of rotordynamic coefficients by comparing the results in the case shear forces are considered and in the case they are neglected. We also compared our results, obtained with the Colebrook–White friction factor model, with some reference experimental and computational results.

On the flow over cavities of large aspect ratio: A physical analysis

The flow over cavities of large aspect ratio is numerically investigated. The analysis is performed for both laminar and turbulent regimes. The results demonstrate that the opposite behavior on the displacement of the two vortices inside the cavity for turbulent and laminar regimes is linked to the net pressure force inside the cavity. The mathematical model corresponds to the incompressible, Reynolds-averaged, Navier-Stokes equations. A k– ε turbulence model is used to close the problem. Two numerical schemes were used. The SIMPLER algorithm was adopted for laminar flows. The turbulent analysis was based on a scheme, recently developed by the present authors.

Flow Mechanism for Stall Margin Improvement due to Circumferential Casing Grooves on Axial Compressors

A computational study is carried out to understand the physical mechanism responsible for the improvement in stall margin of an axial flow rotor due to the circumferential casing grooves. Computational fluid dynamics simulations show an increase in operating range of the low speed rotor in the presence of casing grooves. A budget of the axial momentum equation is carried out at the rotor casing in the tip gap in order to understand the physical process behind this stall margin improvement. It is shown that for the smooth casing the net axial pressure force at the rotor casing in the tip gap is balanced by the net axial shear stress force. However, for the grooved casing the net axial shear stress force acting at the casing is augmented by the axial force due to the radial transport of axial momentum, which occurs across the grooves and power stream interface. This additional force adds to the net axial viscous shear force and thus leads to an increase in the stall margin of the rotor.

Calculation of leakage and dynamic coefficients of stepped labyrinth gas seals

Rotor–fluid interactions can cause self-excited vibrations in high-density turbomachines. One of the most important sources for excitation is the flow through labyrinth seals. The gas flow through the seals creates net pressure and shear forces acting on the rotor. It is necessary to predict these forces exactly for reliably operating turbomachines. The labyrinth seal forces are characterized by rotordynamic coefficients. This paper presents a theoretical work on the calculation of leakage and dynamic coefficients for stepped labyrinth gas seals. The continuity and circumferential momentum equations are given for compressible flow in a stepped labyrinth seal. Periodic and analytic solutions of these equations are obtained for the case of time dependent flow generated by a non-axisymmetric rotation of the rotor using an axisymmetric solution based on different assumption about the flow coefficient. The rotordynamic coefficients are then calculated and the results of this analysis are compared with relatively recent experimental data.

Optically induced rotation of a trapped micro-object about an axis perpendicular to the laser beam axis

A strongly focused Nd:YAG laser beam has been used experimentally to achieve optical trapping and simultaneous rotation of a micro-object about an axis perpendicular to the trapping laser beam axis. The micro-object (12 μm in diameter), which has shape anisotropy in its interior, can be produced by oxygen reactive ion etching of a fluorinated polyimide film (refractive index n=1.53). This optically induced rotation of the micro-object in ethanol (n=1.36) is accomplished by utilizing the net radiation pressure force arising from the momentum change of light scattered at the micro-object’s inner walls.

The Mechanism of Buffeting Force on Tubes Immersed in Gas-Fluidized Beds.

Key mechanisms relating to buffeting forces on tubes in fluidized beds have been identified by analyzing results from simultaneous measurement of force and pressure on a horizontal tube. Careful experiments have been carried out in a two-dimensional fluidized bed with an accurate sensor tube for single bubble passage, bubble chains and free-bubbling. It is shown that the buffeting force in fluidized beds consists of the net pressure force, i.e., form drag and lift, and additional forces induced by particles become so large for high-velocity free-bubbling beds that it is impossible to estimate the total force only from the net pressure force, while for low velocity beds the total force is mostly accounted for by the net pressure force. The wake particles play an important role for large bubbles, and therefore the details of this component have also been investigated for single bubbles as well as for bubbles undergoing splitting and coalescence.