Stagnation Pressure Distributions Research Articles

Four-Wall Flow Separation Control Using Microvortex Generators in Supersonic Duct

Suppression of shock-induced separation on all the walls of a rectangular duct using arrays of microvortex generators was experimentally investigated. The flowfield was analyzed using normal and inclined schlieren visualizations, surface oil flow visualization, wall static pressure distribution, and exit plane stagnation pressure distribution. The top wall and side wall array configurations were fixed, and only the bottom wall configuration was changed. It was found that control decreased the axial oscillations but increased the spanwise oscillations of the shock (oscillations in the spanwise length of the normal part of the shock). Among the control cases, as the control on the bottom wall increased, the exit plane stagnation pressure recovery, static pressure recovery across the shock, and area of the normal portion of the normal shock increased and then decreased. However, the attached flow percentage on the bottom wall increased with an increase in control. As the shock moved away from the microvortex generator arrays, the case with no control on any walls gave better results in terms of shock size and exit plane stagnation pressure recovery. Also, as the number of control devices on the bottom wall increased, the extent of flow separation on the bottom wall decreased; however, it had detrimental effects on the duct flow beyond some extent.

Numerical Simulation of Flow through a Centrifugal Impeller

The paper reports computational results of the flow through an impeller (backward curved) of a centrifugal compressor. 3D steady state investigations are performed at off-design and design mass flow rates. The static pressure as well as stagnation pressure distribution contours and velocity vector reveals the behavior of flow through the impeller at different flow coefficients. The flow pattern observed within the impeller passage is complex and influenced by several factors. Flow through the impeller is distorted due to presence of jet and wake. As flow happens through the impeller, energy is transferred from the impeller’s blades to the fluid. This creates the jet at pressure side region and wake at suction side region of the impeller. This fluid with different energy level gets mixed at the exit of the impeller causing mixing losses as well as secondary flows. These loss leads to a considerable fall in static pressure rise in the compressor and thereby affecting the overall efficiency of the centrifugal compressor. Also, existence of vortices in the flow field as flow turned from axial direction to radial is seen.

Performance Enhancement of a Low-Pressure Ratio Centrifugal Compressor Stage with a Rotating Vaneless Diffuser by Impeller Disk Extended Shrouds

Numerical simulations have been carried out to examine the performance and flow parameters of forced rotating vaneless diffuser obtained by the extension of impeller disks of a low-pressure ratio shrouded type centrifugal compressor stage with diffuser diameter ratio 1.40. Four different levels of shroud extensions (i.e., impeller disks alone) forming the rotating vaneless diffusers are analyzed at four different flow coefficients. The extension of impeller disks alone by 40% of impeller exit diameter leads to a fully forced rotating vaneless diffuser thereby replacing the existing stationary vaneless diffuser. The comparative studies are performed using the same impeller with a stationary vaneless diffuser also having a diffuser diameter ratio of 1.40. Static pressure rise in ES40 is found to be higher than SVD by around 9.84% at design flow coefficient and also at above off-design flow rates. Energy coefficient is highest for ES40, followed by ES30 compared to SVD. For ES40, the static pressure recovery coefficient also is higher compared to SVD. The efficiency of ES40 is lesser by around 5.40% to 3.43% compared to SVD, at design as well as at above off-design flow coefficients. The stagnation pressure losses for ES40 drastically reduced compared to SVD. The comparison of stagnation pressure contours and absolute velocity contours near the hub and shroud walls of ES40 and SVD configurations shows that the rotating diffuser walls as in ES40 causes further addition of energy to the fluid. This adds up the kinetic energy level of the fluid which due to better diffusion, results in gain of static pressure rise. Moreover, there is a net increase in stagnation pressure distribution at the exit of diffuser due to rotating vaneless diffuser. Also, the presence of a fully rotating vaneless diffuser (ES40) smooth out the distorted entry flow profiles, thereby improving the performance of the centrifugal compressor stage.

Low-Cost Rotating Experimentation in Compressor Aerodynamics Using Rapid Prototyping

With the rapid evolution of additive manufacturing, 3D printed parts are no longer limited to display purposes but can also be used in structural applications. The objective of this paper is to show that 3D prototyping can be used to produce low-cost rotating turbomachinery rigs capable of carrying out detailed flow measurements that can be used, among other things, for computational fluid dynamics (CFD) code validation. A fully instrumented polymer two-stage axial-mixed flow compressor test rig was designed and fabricated with stereolithography (SLA) technology by a team of undergraduate students as part of a senior-year design course. Experiments were subsequently performed on this rig to obtain both the overall pressure rise characteristics of the compressor and the stagnation pressure distributions downstream of the blade rows for comparison with CFD simulations. In doing so, this work provides a first-of-a-kind assessment of the use of polymer additive technology for low-cost rotating turbomachinery experimentation with detailed measurements.

Pressure fluctuations and interfacial robustness in turbulent flows over superhydrophobic surfaces

Superhydrophobic surfaces can entrap gas pockets within their grooves when submerged in water. Such a mixed-phase boundary is shown to result in an effective slip velocity on the surface, and has promising potential for drag reduction and energy-saving in hydrodynamic applications. The target flow regime, in most such applications, is a turbulent flow. Previous analyses of this problem involved direct numerical simulations of turbulence with the superhydrophobic surface modelled as a flat boundary, but with a heterogeneous mix of slip and no-slip boundary conditions corresponding to the surface texture. Analysis of the kinematic data from these simulations has helped to establish the magnitude of drag reduction for various texture topologies. The present work is the first investigation that, alongside a kinematic investigation, addresses the robustness of superhydrophobic surfaces by studying the load fields obtain from data from direct numerical simulations (DNS). The key questions at the focus of this work are: does a superhydrophobic surface induce a different pressure field compared to a flat surface? If so, how does this difference scale with system parameters, and when does it become significant that it can deform the air–water interface and potentially rapture the entrapped gas pockets? To this end, we have performed DNS of turbulent channel flows subject to superhydrophobic surfaces over a wide range of texture sizes spanning values from $L^{+}=6$ to $L^{+}=155$ when expressed in terms of viscous units. The pressure statistics at the wall are decomposed into two contributions: one coherent, caused by the stagnation of slipping flow hitting solid posts, and one time-dependent, caused by overlying turbulence. The results show that the larger texture size intensifies the contribution of stagnation pressure, while the contribution from turbulence is essentially insensitive to $L^{+}$. The two-dimensional stagnation pressure distribution at the wall and the pressure statistics in the wall-normal direction are found to be self-similar for different $L^{+}$. The scaling of the induced pressure and the consequent deformations of the air–water interface are analysed. Based on our results, an upper bound on the texture wavelength is quantified that limits the range of robust operation of superhydrophobic surfaces when exposed to high-speed flows. Our results indicate that when the system parameters are expressed in terms of viscous units, the main parameters controlling the problem are $L^{+}$ and a Weber number based on inner dimensions; We obtain good collapse when all our results are expressed in wall units, independently of the Reynolds number.

Deeply undercritical microwave discharge excited by the field of a quasi-optical electromagnetic beam in a supersonic air jet

Electric discharge in a supersonic air jet is studied. It is ignited in a linearly polarized quasi-optical microwave electromagnetic beam the initial field intensity of which is much lower than the breakdown level. Electric breakdown is initiated by a tubular electromagnetic vibrator, one end of which has spikes and is covered by a quartz tube. Atmospheric air enters into a low-pressure working chamber through the inner channel of the vibrator. As a result, an immersed supersonic air jet forms in the chamber at the outlet from the quartz tube. A microwave discharge ignited in this jet is “attached” to aft spikes of the vibrator. The energy deposit into the discharge plasma and the effective area of energy interaction between the discharge and excited microwave field are estimated from the temperature and stagnation pressure distributions in the wake of the discharge.

Flow Field in the Turbine Rotor Passagein an Automotive Torque Converter Based on the HighFrequency Response Rotating Five‐hole ProbeMeasurement Part I: Flow Field at the Design Condition (Speed Ratio 0.6)

The relative flow field in an automotive torque converter turbine was measured at three locations inside the passage (turbine 1/4 chord, mid‐chord, and 4/4 chord) using a highfrequency response rotating five‐hole‐probe. “Jet‐Wake” flow structure was found in the turbine passage. Possible flow separation region was observed at the core/suction side at the turbine 1/4 chord and near the suction side at the turbine mid‐chord. The mass averaged stagnation pressure drop is almost evenly distributed along the turbine flow path at the design condition (SR = 0.6). The pressure drop due to centrifugal and Coriolis forces is found to be appreciable. The rotary stagnation pressure distribution indicates that there are higher losses at the first half of the turbine passage than at the second half. The major reasons for these higher losses and inefficiency are possible flow separation and a mismatch between the pump exit and the turbine inlet flow field. The fuel economy of a torque converter can be improved through redesign of the core region and by properly matching the pump and the turbine. The Part I of the paper deals with the design speed ratio (SR = 0.6), and Part II deals with the off‐design condition (SR = 0.065) and the effects of speed ratio.

Vortex in a Supersonic Flow and its Influence on Blunt Body Flow and Heat Transfer

The dissipation of a tip vortex generated by a rectangular wing and the stagnation pressure and temperature distributions within the vortex are experimentally investigated. The interaction of the vortex with the bow shock ahead of a sphere and the heat flux distribution over the spherical surface are studied. The experiments were carried out at the Mach number M=3 and the unit Reynolds numbers Re1=1.1·107 and 3.7·107 1/m.

Flow Measurements in a Curved-Wall Annular Contraction

Flow development in an annular contraction is of fundamental and practical importance in various applications including the gas turbine systems. This paper describes an experimental study of flow characteristics in a curved wall annular contraction. The results are presented in terms of the velocity vectors, surface pressure coefficients, static and stagnation pressure distributions, and profiles of mean velocities, turbulence intensity, and Reynolds shear stress. The flow conditions at the entrance were varied to evaluate how they affected the flow development in the passage. Results show that the contraction produced uniform static pressure and axial velocity profiles at the exit plane. Higher inlet turbulence affected the Reynolds shear stress in the contraction although the change in the static and total pressure fields was insignificant. The overall stagnation pressure loss was only 2 to 3 percent of the dynamic head at the contraction exit plane. Results showing only typical data are included in this paper. More extensive data sets to validate computer codes are available from the authors.

Flow Investigation in an Inlet Duct of Water Jet Propulsor.

The flow in a flush type inlet duct of water jet propulsor is investigated both experimentally and numerically. In the experiment, a duct model is tested in a low speed wind tunnel facility. Stagnation pressure distribution at the exit of the inlet duct (i.e. pump inlet) is measured in detail. The measured data shows that the location of high loss region moves from ramp side to lip side as IVR (Inlet Velocity Ratio) increases. Meanwhile, 3-D Navier-Stokes code is apllied to this duct flow. The computed results reveal the complicated flow mechanisms inside and around the inlet duct, which is highly three-dimensional and strongly affected by viscouscity.

Numerical model of a transferred plasma arc

This article presents results of two-dimensional simulations for a transferred plasma arc. Calculations are performed for the internal plasma flow within the torch body, as well as the external plasma jet impinging on a surface. The governing equations describing the plasma flow are self-consistently coupled. These equations are the compressible Navier–Stokes equations for conservation of mass and momentum (in the radial and streamwise directions), conservation of energy, species continuity with finite-rate ionization and recombination kinetics, and the magnetic diffusion equation for the electromagnetics. The unsteady forms of these equations are time marched to steady state using the linearized block implicit method. This model does not employ any adjustable parameters, and therefore enables direct comparison with experiments. Model predictions are compared with experimental measurements of stagnation pressure distributions recorded on a water-cooled copper plate (workpiece), and indicate good agreement, given the lack of adjustable parameters.

Swirling flow in thrust nozzles

This paper investigates the effects of adding swirl to a dump combustor-nozzle propulsion system. The results of cold-flow testing are summarized and compared to a numerical analysis of the nozzle flowfield. The cold-flow testing included thrust, mass flow rate, and pressure measurements for two different conical nozzles in combination with four swirlers, one of which was a blank. Five-port probe measurements were made across the nozzle entrance. The measurements were used to determine the stagnation pressure and swirl distributions and to provide initial conditions for a nozzle analysis program. The measured results show that swirl has a strong effect on the stagnation pressure distribution. The largest losses occur near the swirl axis. Swirl also significantly reduces the system discharge coefficient. In contrast, the effect of swirl on the nozzle stream thrust efficiency was small, but measurable. Swirl reduced the nozzle stream thrust efficiency by about 0.5% for the highest swirl tested. Computed nozzle flowfields that do not account for the stagnation pressure distribution in the swirling flow can produce highly distorted and unrealistic results. Comparison of measured and computed results (which use measured stagnation pressure and swirl distributions) shows reasonable agreement.

The subsonic near-wake of an axisymmetric semielliptical afterbody

The near-wake of axisymmetric semielliptical afterbody was studied experimentally in a wind tunnel at 47 m/s using an upstream support system to minimize disturbances to the wake. The flowfield was surveyed with a variety of pressure and hot-wire probes. This paper presents the mean flowfield characteristics for the entire near-wake region. The results include static and stagnation pressure distributions as well as velocity profiles. Separation occurred at 0.38 model radii upstream of the base, while realignment was found at 0.2 model radii downstream. Centerline velocity recovery is slower than for blunt bases, but far-wake similarity was found as early as 0.4 radii downstream. The overall flowfield is heavily influenced by the wall geometry. The data presented will serve as a quantitative basis for the development and evaluation of analytical solutions.

Industrial jet noise: Coanda nozzles

Within the U.K. manufacturing industries noise from industrial jets ranks third as a major contributor to industrial deafness. Noise control is hindered because use is made of the air once it has exuded from the nozzle exit. Important tasks include swarf removal, paint spreading, cooling, etc. Nozzles which employ the Coanda effect appear to offer the possibility of significant noise reduction whilst maintaining high thrust efficiency when compared with the commonly used simple open pipe or ordinary convergent nozzle. In this paper the performance of Coanda-type nozzles is examined in detail and an index rating for nozzle performance is introduced. Results show that far field stagnation pressure distributions are Gaussian and similar in all cases with a dispersion coefficient σ = 0·64. Noise reduction and thrust efficiency are shown to be closely related to the design geometry of the central body of the nozzle. Performance is based on four fundamental characteristics, these being the noise level at 1 m from the exit and at a 90° station to the nozzle axis, and the thrust on a chosen profile, the noise reduction and the thrust efficiency. Physically, performance is attributed to flow near field effects where, although all nozzles are choked, shock cell associated noise is absent.

Re-evaluation of heat-transfer data obtained in flight tests of heat-sink shielded re-entry vehicles.

Stagnation point velocity and pressure distribution over heat-sink shielded reentry vehicle to test boundary layer heat transfer theories

Pressure effects of relaxation and bulk viscosity in gas motion

A comparative study is made of the pressure effect of relaxation and bulk viscosity in stationary flow. It is shown that both stagnation pressure defects (Kantrowitz effect) and pressure distribution in small nozzles in principle constitute methods for distinguishing between the two theories. A discussion of the experimental data on carbon dioxide shows that in this case only the relaxation interpretation is possible.