Local Pressure Minimum Research Articles

Spatiotemporal characterization of turbulent channel flow with a hyperelastic compliant wall

Direct numerical simulations of turbulent flow in a channel with one rigid and one viscoelastic wall are performed. An Eulerian–Eulerian model is adopted with a level-set approach to identify the fluid–compliant material interface. Focus is placed on the propagation of Rayleigh waves in the compliant material, whose speed depends on the shear modulus of elasticity and whose dominant wavelength depends on the thickness of the viscoelastic layer. These parameters are selected to ensure coupling between the compliant surface and turbulence. When the phase speed of Rayleigh waves is commensurate with the advection velocity of near-wall pressure fluctuations, sheets of vorticity are lifted up and detached near the critical layer and lead to a local pressure minimum. These events are caused by the inflectional velocity profile near the troughs, and are controlled by the net vorticity flux at the elastic surface. This phenomenon is central to understanding the statistical characteristics of the flow, including the surface deformation–pressure correlation and enhanced stochastic Reynolds shear stresses. Finally, we discuss the influence of three-dimensionality of the surface topography on the generation of streamwise vorticity, secondary motions and lateral turbulent transport.

A CFD/experimental comparative database to feed a predictive model for ground vortex characteristics

Most modern aircraft engines rely on the use of high Bypass Ratios (BPR) turbofans to achieve both high thrust and low specific fuel consumption. However, such configurations are prone to the formation of ground vortices during low-speed operations. This phenomenon arises under specific combinations of wind direction, velocity or inlet air speed, generating engine vibrations and leading to the suction of damaging abrasive particles. Its characterization in early design stages is crucial. In this work, a joint experimental and numerical exploration of operating conditions leading to ground vortex presence is carried out on a scaled wind tunnel configuration. Flow details are investigated for several working points obtained from a specific set of input parameters (intake speed, wind speed, ground clearance). A methodology suitable for both experimental and Computational Fluid Dynamics (CFD) works is developed to extract vortex characteristic quantities, based on a local pressure minimum and Q-criterion contours topology. A very good agreement is obtained when comparing vortex predictions stemming from CFD and experiments. This database shall be used to transpose experimental data probed outside of the nacelle to data within the nacelle using data analytics techniques, paving the way for future data driven predictive models.

Velocity–pressure correlation in Navier–Stokes flows and the problem of global regularity

Abstract

The cavitating Taylor-Couette flow

This work presents an investigation of a new phenomenon of the Taylor-Couette flow: the onset of Taylor vortices in a cavitating fluid. This particular form of the Taylor-Couette flow develops if the shear flow between a rotating inner and a fixed outer cylinder approaches the critical Taylor number and the vapor pressure of the fluid simultaneously. This process is achieved by increasing the rotational speed of the inner cylinder, which causes an increase of the radial pressure gradient inside the laminar flow. The fully developed Taylor vortex flow is characterized by a pressure distribution in the azimuthal plane showing a local minimum adjacent to the wall of the inner cylinder between a pair of vortices that form a radial flow towards the outer cylinder. Thence, cavitation occurs simultaneously if the local pressure minimum drops below the vapor pressure of the fluid. This transition from a two-dimensional (Couette) into a three-dimensional (Taylor) flow triggered the idea to apply a newly developed unsteady 2-phase 3D-computational fluid dynamics code by computing the generation of vapor that is coinciding with the formation of Taylor vortices at the critical Taylor number. Whereas the results of a numerical simulation prove the existence of toroidal vapor caused by cavitation, the experimental validation demands additionally the development of a special fluid. Thus, the present work describes this specifically tailored fluid, which not only fulfills Taylor and pressure analogy but also features a favorable refractive index and a chemical suitability for the task.

Mathematical Model of Fluid Flow Between Rotating Nonplane Disks

A mathematical model of viscous fluid flow in the gap of a disk pump with nonplane disks has been developed. The characteristics of the pump — flow velocity, pressure drop, stress tensor, and the hydraulic coefficient of pump efficiency — have been calculated. A local pressure minimum in the pump has been detected, which leads to local ″choking″ of flow and to a decrease in the pressure characteristics and in the pump efficiency. The parameters causing the local ″choking″ of flow have been found, and the means of overcoming this phenomenon are indicated. An optimum inner radius of the disk at which maximum pressure drop and efficiency of the disk pump are achieved has been established, which made it possible to calculate the optimum parameters of the pump′s disk packet.

Isothermal phase equilibria for the (xenon + cyclopropane) mixed-gas hydrate system

Isothermal three-phase equilibria of gas, aqueous, and hydrate phases for the {xenon (Xe) + cyclopropane ( c-C 3H 6)} mixed-gas hydrate system were measured at two different temperatures (279.15 and 289.15) K. The structural phase transitions from structure-I to structure-II and back to structure-I, depending on the mole fraction of guest mixtures, occur in the (Xe + c-C 3H 6) mixed-gas hydrate system. The isothermal pressure–composition relations have two local pressure minima. The most important characteristic in the (Xe + c-C 3H 6) mixed-gas hydrate system is that the equilibrium pressure–composition relations exhibit the complex phase behavior involving two structural phase transitions and two homogeneous negative azeotropes. One of two structural phase transitions exhibits the heterogeneous azeotropic-like behavior.

Volumetric three-component velocimetry measurements of the turbulent flow around a Rushton turbine

Volumetric three-component velocimetry measurements have been taken of the flow field near a Rushton turbine in a stirred tank reactor. This particular flow field is highly unsteady and three-dimensional, and is characterized by a strong radial jet, large tank-scale ring vortices, and small-scale blade tip vortices. The experimental technique uses a single camera head with three apertures to obtain approximately 15,000 three-dimensional vectors in a cubic volume. These velocity data offer the most comprehensive view to date of this flow field, especially since they are acquired at three Reynolds numbers (15,000, 107,000, and 137,000). Mean velocity fields and turbulent kinetic energy quantities are calculated. The volumetric nature of the data enables tip vortex identification, vortex trajectory analysis, and calculation of vortex strength. Three identification methods for the vortices are compared based on: the calculation of circumferential vorticity; the calculation of local pressure minima via an eigenvalue approach; and the calculation of swirling strength again via an eigenvalue approach. The use of two-dimensional data and three-dimensional data is compared for vortex identification; a ‘swirl strength’ criterion is less sensitive to completeness of the velocity gradient tensor and overall provides clearer identification of the tip vortices. The principal components of the strain rate tensor are also calculated for one Reynolds number case as these measures of stretching and compression have recently been associated with tip vortex characterization. Vortex trajectories and strength compare favorably with those in the literature. No clear dependence of trajectory on Reynolds number is deduced. The visualization of tip vortices up to 140° past blade passage in the highest Reynolds number case is notable and has not previously been shown.

Reynolds Number Effects on Low-Speed Aerodynamics of a Hypersonic Configuration

A Collaborative Research Center project is focused on fundamental design aspects of a hypersonic research conŽ guration that consists of a lifting body with a delta planform of aspect ratio 1.1 and rounded leading edges. The aerodynamics of the conŽ guration at Mach numbers below 0.3 are described. The conclusions are based on tests in different subsonicwind tunnels. Pressure distributionmeasurementswere carried out at aReynoldsnumber range of 3 £ 106 <– Re <– 40 £ 106 using different wind-tunnel models. Above an angle of attack of 8 deg, the  ow at the suction side changes and a concentrated vortex system (primary and secondary vortices) begins to form, causing local pressure minima in the pressure distribution.On rounded leading edges, the primary separation line is not a priori Ž xed and the vortex system has less intensity than for delta wings with sharp leading edges. The present investigations show that, in this case, the location of the separation line and the strength and location of the vortex system depend on the Reynolds number. Pressure distribution measurements at higher Reynolds numbers with a largemodel in theGerman–DutchWind Tunnel supported these observations. In particular, the high spatial resolution allowed a good physical interpretation of the  owŽ eld. Knowledge of the  ow processes was improved by surface oil- ow patterns, the laser-light-sheet technique, and particle image velocimetry.

An Objective Cyclone Climatology for the Southern Hemisphere

Abstract An objective method is developed and used to derive a climatology of centers of cyclonic vorticity for the Southern Hemisphere, based on twice-daily European Centre for Medium-Range Weather Forecasts (ECMWF) 1000-hPa analyses during 1980–86. These centers were computed as local minima of geostrophic relative vorticity ζg, extending previous studies based on pressure minima. This use of ζg, avoids a bias favoring slower and/or deeper cyclones that occurs when pressure is used and includes a large number of additional mobile vorticity centers in the 45°–55°S band that are missed where a local pressure minimum cannot be found. An automated tracking algorithm similar to that of Murray and Simmonds is used to match predicted location, pressure, and ζg from previous track history with available centers. Large numbers of ζg centers were found south of 60°S and near the midlatitude continents, as in previous studies. These were a mixture of migratory centers and other fixed topographic features. The maxi...

Pulsatile pressure and flow in arterial stenoses simulated in a mathematical model

This paper presents the detailed pulsatile pressure and flow velocity patterns inside an axis symmetric stenosis model with 75% constriction. The pressure and velocities have been calculated by solving the Navier-Stokes equations by the finite element method, the velocity profile in a straight tube caused by a pulsating driving pressure has been calculated first and then used as a boundary condition for the stenosis calculations. The results of the mathematical simulations of the stenosis model have been obtained in terms of velocity vectors, streamlines and isobars at 16 different instances in time, each 15 degrees apart during a cardiac cycle. The calculated velocity field shows that a vortex is developed at the wall distal to the stenosis as the velocity decreases from the peak systolic value. At the site of the vortex, a local pressure minimum is found due to the conversion of pressure to kinetic energy. When the flow is reversed, the reversal occurs first along the wall, thus forcing the vortex toward the the centre of the tube. As the reverse flow velocity increases, a vortex is also developed at the proximal site of the stenosis.

Strong Downslope Winds at Boulder, Colorado

Abstract Twenty wind storms, with maximum gusts exceeding 33 m sec−1, are analyzed. At the height of the storms, mean hourly speeds in Boulder were 20 m sec−1 with mean maximum gusts of 36 m sec−1. At the same time wind speeds were about 50% lower along the lee slopes to the west and within 10 km to the east into the plains. The phenomena are non-stationary, and the result is a great diversity in surface wind speed patterns at the stations. Observational data show that the storms are the result of a deep lee trough or pressure jump. Air is accelerated down the slopes toward the local pressure minimum beneath the disturbance and is decelerated again rapidly to the east. Air mass characteristics that lead to the development of the disturbance are a stable layer or inversion at a critical level above mountain top and relatively strong winds at that level; strong winds in the upper troposphere are not necessary.

Magnetic Bubble Trapping in Liquids

In a high magnetic field a liquid is subject to forces due to its para- or diamagnetism which contribute to the hydrostatic pressure at each point. It is shown that this contribution may give rise to a local pressure minimum at which bubbles in the liquid will accumulate. This effect is often produced in commercial liquid nitrogen with a standard laboratory electromagnet, because of the paramagnetism caused by a small oxygen content, and can cause experimental problems. The corresponding effect caused by diamagnetism has now been observed in water with a high-field solenoid operated in the range 165–205 kG, and it is predicted that a bubble trap will likewise occur in liquid helium when a magnet of this design is operated at slightly higher fields. Such an effect can produce irregularities in the results of low-temperature experiments in high magnetic fields unless bubbles are prevented from forming.