Gap Jet Research Articles

Design strategy and relevant flow mechanisms of highly loaded 3D compressor tandem cascades

To investigate the design strategy of highly loaded tandem cascades at both the midspan and endwall, the overall performance and flow mechanisms of four typical tandem cascades based on the optimization were analyzed from multiple perspectives numerically. The results show that the interference effects on the Front Blade (FB) and Rear Blade (RB) should not be overlooked during the design phase, and the design strategies at the midspan and endwall are completely different. At the midspan, the optimization aims to increase the interference effects and the strength of the gap jet while maintaining the same load on the FB and RB. However, the endwall optimal airfoil exhibits weakening interference effects, advancement of the gap jet location, and load transfer from the FB to RB. Through further analysis of flow characteristics, the midspan optimal airfoil is beneficial for inhibiting the low-energy fluid from interacting with the suction surface of RB under the design condition, but results in earlier occurrence of corner stall. The endwall optimal airfoil helps suppress the development of the secondary flow and delay the onset of corner stall. Furthermore, by combining the benefits of these two design approaches, additional forward sweep effects are achieved, further enhancing the performance of the tandem cascade.

Precision physics with the proton spectrometer and diffractive physics measurements from CMS

We describe recent results from CMS and TOTEM on hard diffraction, diffractive jets and jet gap jet events. We also give the first sensitivities and limits on quartic anomalous couplings and axion-like particles at high mass using the LHC as a \gamma \gammaγγ collider. The predicted sensitivities with 300 fb^{-1}−1 are better by two or three orders of magnitude compared to the more standard methods at the LHC without measuring intact protons after collision.

Complicated flow in tip flow field of a compressor tandem cascade using delayed detached eddy simulation

The complex flow characteristics in the tip region of a tandem cascade with tip clearance have been calculated and analyzed using Delayed Detached Eddy Simulation (DDES). The coherent mechanism of the vortex structures near the blade tip was discussed, and the unsteady behaviors and features in the tip flow field were analyzed. Additionally, the interaction between the tip leakage flow and the gap jet was revealed. The results show that, compared to the datum cascade, the blade tip load of the rear blade increases while that of the front blade decreases. Unsteady fluctuations of the tandem cascade are mainly caused by the interaction between the tip leakage flow and gap jet, and by the mixing of the vortex structures, but there is no essential change in the spectrum feature of the tip leakage flow. Finally, a detailed analysis of the development of vortices in the tip region is conducted by the topological structures of the flow field. Combined with the three-dimensional vortex structures, the schematic diagram of the vortex system of the datum single-row cascade and tandem cascade is summarized.

Design criteria of load split and chord length ratio for highly loaded compressor tandem cascades

To investigate the design strategy for load split (LS) and chord length ratio (CR) of highly loaded compressor tandem cascades, the parameterization study of the tandem cascades with different LS and CR was carried out. The parameterization results show that the LS is a crucial design parameter that can be used to regulate performance under off-design conditions. Specifically, lower LS enhances performance at positive incidence angles but comes at the cost of degraded performance at negative incidence angles. Moreover, relatively large CR (2–4) can improve performance under off-design conditions and also plays a role in achieving robust design, but it comes at the expense of optimal performance. To further explore the LS and CR effects on the flow field of the tandem cascades, three additional tandem cascades with LS = 0.5 and different CR were conducted. At large negative incidence angles (LS < 0.5), large CR (2.828) helps mitigate the accumulation of low-energy fluid on the pressure surface of the front blade and enhances the strength of the gap jet. Identically, at large positive incidence angles (LS > 0.5), the large CR mitigates the risk of corner stall and induces a transition in the stall mode of the front blade from corner stall to boundary layer separation over the entire span, further improving the tandem cascade performance. Based on the flow field analysis, the overall design strategy for LS and CR was summarized.

The Influence of Tip Clearance on the Performance of a High-Speed Inducer Centrifugal Pump under Different Flow Rates Conditions

The influence mechanism of the blade tip clearance (TC) of an inducer on the performance of a centrifugal pump at high speed was researched under different flow rate conditions in this work. An experiment on the pump’s external performance was carried out, and numerical calculation was also performed under four different TCs. The full characteristic performance curves, static pressure and pressure pulsation distributions of the pump were obtained. Through the research and analysis, it was found that the influence of the TC on the efficiency and the head of the centrifugal pump are related to the flow rate. Under the influence of a large flow rate, the increase in the TC is helpful to improve the efficiency and the head of the pump. The increase in the TC helps to weaken the gap jet effect on the inducer. The inlet jet of the inducer, caused by TC leakage, will form a low-pressure vortex zone at the inlet of the inducer. The splitter-bladed inducer’s pressure pulsation is affected by the TC. The peak pressure pulsation at the monitoring point at the short blades is larger than that at the long blades. With the increase in TC, the cavitation degree at the inlet of the long blade of the inducer is decreased, while the cavitation degree at the short blade is deepened. It is also found that the TC has little effect on the radial force of the inducer and the impeller. These results will provide the design basis for the tip clearance of an inducer.

Fluid Forces and Vortex Patterns of an Oscillating Cylinder Pair in Still Water With Both Fixed Side-by-Side and Tandem Configurations

Abstract Models of cylinders in the oscillatory flow can be found virtually everywhere in the marine industry, such as pump towers experiencing sloshing load in a liquefied natural gas ship liquid tank. However, compared to the problem of a cylinder in the uniform flow, a cylinder in the oscillatory flow is less studied, let alone multiple cylinders. Therefore, we experimentally and numerically studied two identical circular cylinders oscillating in the still water with either a side-by-side or a tandem configuration for a wide range of Keulegan–Carpenter number and Stokes number β. The experimental result shows that the hydrodynamic performance of an oscillating cylinder pair in the still water is greatly altered due to the interference between the multiple structures with different configurations. In specific, compared to the single cylinder case, the drag coefficient is greatly enhanced when two cylinders are placed side-by-side at a small gap ratio, while dual cylinders in a tandem configuration obtain a smaller drag coefficient and oscillating lift coefficient. In order to reveal the detailed flow physics that results in significant fluid forces alternations, the detailed flow visualization is provided by the numerical simulation: the small gap between two cylinders in a side-by-side configuration will result in a strong gap jet that enhances the energy dissipation and increases the drag, while due to the flow blocking effect for two cylinders in a tandem configuration, the drag coefficient decreases.

Eulerian and Lagrangian Comparison of Wind Jets in the Tokar Gap Region

The Lagrangian and Eulerian structure and dynamics of a strong wind event in the Tokar Gap region are described using a Weather Research and Forecasting (WRF) model hindcast for 2008. Winds in the Tokar Gap reach 25 m s−1 and remain coherent as a jet far out over the Red Sea, whereas equally strong wind jets occurring in neighboring gaps are attenuated abruptly by jump-like hydraulic transitions that occur just offshore of the Sudan coast. The transition is made possible by the supercritical nature of the jets, which are fed by air that spills down from passes at relatively high elevation. By contrast, the spilling flow in the ravine-like Tokar Gap does not become substantially supercritical and therefore does not undergo a jump, and also carries more total horizontal momentum. The Tokar Wind Jet carries some air parcels across the Red Sea and into Saudi Arabia, whereas air parcel trajectories in the neighboring jets ascend as they cross through the jumps, then veer sharply to the southeast and do not cross the Red Sea. The mountain parameter Nh/U is estimated to lie in the range of 1.0–4.0 for the general region, a result roughly consistent with a gap jet having a long extension, and supercritical flows spilling down from higher elevation passes. The strong event is marked by the formation of a feature with a vertical cellular structure in the upstream entrance region of the Tokar Gap, a feature absent from the more moderate events that occur throughout the summer. The cell contains descending air parcels that are fed into the Tokar Gap and one of the neighboring gaps. An analysis of the Bernoulli function along air parcel trajectories reveals an approximate balance between the loss of potential energy and gain of internal energy and pressure, with surprisingly little contribution from kinetic energy, along the path of the descending flow. The winds in all gaps attain the critical wind speed nominally required to loft dust into the atmosphere, though only the Tokar Gap has a broad, silty delta region capable of supplying particulate matter for dust storms.

Cavitation control of centrifugal pump based on gap jet principle

Cavitation control of centrifugal pump based on gap jet principle

Inducer/Exducer Matching Characteristics inside Tandem Impellers of a Highly Loaded Centrifugal Compressor

A centrifugal compressor usually operates with low isentropic efficiency and a terrible stable operating range, resulting from the complex impeller flow structure companied with the intense interaction among the impeller and the diffuser downstream. In many studies, the potential of centrifugal compressor tandem-impeller configurations for improving the compressor has been demonstrated. Whereas, compared with the convincing results on the tandem-designed axial compressors, the results on tandem impellers are limited and contradictory. Very little insight has been provided into the flow mechanisms inside tandem impellers, which is considered to be the primary reason for the confusion in tandem impeller design and application. Tandem impellers are expected to exhibit a totally different behavior due to the intense aerodynamic interaction between the inducer and the exducer, which substantially contributes to the flow structure and the compressor performance change. In the present study, a numerical study of a highly-loaded centrifugal compressor with various tandem designs was conducted to explore the inducer/exducer matching characteristics and the underlying flow mechanism inside tandem impellers. Two tandem impeller design parameters, namely, the inducer/exducer clocking fraction and the axial gap (overlap), were considered in the tandem impeller design process. The tandem impeller was also compared to the existing conventional impeller which the tandem impeller was redesigned for. The results demonstrated that the tandem-designed impeller can improve the centrifugal compressor stage performance and intense inducer/exducer interaction can be observed with changes in the clocking fraction and the axial gap (overlap). The tandem impeller performance is sensitive to changes in axial gap (overlap) when the suction side of the exducer blade is circumferentially close to the inducer blade. The fundamental reason for the performance variation in the inducer and the exducer lies in the inducer pressure change in the blade trailing edge that is determined by the Kutta condition. Additionally, the correlation between the tandem impeller slip effect and the discharge flow quality should be emphasized in the inducer/exducer gap jet analysis, in which the jet injection angle and the Coanda effect of the exducer suction surface critically affect the discharge flow characteristics.

Probing BFKL dynamics at hadronic colliders in jet gap jet events

In this report, we give the Balitsky Fadin Kuraev Lipton formalism for jet gap jet events at hadronic colliders. We also discuss the case where in addition at least one proton is intact in the final state in diffractive events.

Effect of clearance jet on aerodynamic performance of centrifugal fan

The change of the depth of the inlet collector inserted into the impeller will change the gap shape between the collector and the impeller, and then affect the performance of the ventilator. Taking the small centrifugal ventilator as the object, the influence of the depth of the impeller inserted into the collector on the aerodynamic performance of the ventilator is studied in this paper. Firstly, the depth of the collector inserted into the front disk of the impeller is adjusted to 2 mm, 3 mm and 4 mm respectively to change the geometry of the clearance. Then, based on the computational fluid dynamics analysis method, the influence of the insertion depth of the above three different concentrators on the aerodynamic performance of the ventilator is analyzed. The analysis results show that the gap leakage is positively correlated with the depth of the impeller inserted into the collector, and the aerodynamic performance of the ventilator can be significantly improved by reducing the insertion depth of the collector under the conditions of design flow rate and large flow rate, but under the condition of small flow, the decrease of insertion depth weakens the effect of gap jet on the flow field in impeller, resulting in the decrease of fan efficiency and static pressure.

Downslope windstorms in the Isthmus of Tehuantepec during Tehuantepecer events: a numerical study with WRF high-resolution simulations

Abstract. Tehuantepecers or Tehuanos are extreme winds produced in the Isthmus of Tehuantepec, blowing south through Chivela Pass, the mountain gap across the isthmus, from the Gulf of Mexico into the Pacific Ocean. They are the result of the complex interaction between large-scale meteorological conditions and local orographic forcings around Chivela Pass, and occur mainly in winter months due to cold air damming in the wake of cold fronts that reach as far south as the Bay of Campeche. Even though the name refers mostly to the intense mountain gap outflow, Tehuantepecer episodes can also generate other localized extreme wind conditions across the region, such as downslope windstorms and hydraulic jumps, which are strong turbulent flows that have a direct effect on the Pacific side of the isthmus and the Gulf of Tehuantepec. This study focuses on investigating these phenomena using high horizontal and vertical resolution WRF (Weather Research and Forecasting) model simulations. In particular, we employ a 4-nested grid configuration with up to 444 m horizontal spacing in the innermost domain and 70 hybrid-sigma vertical levels, 8 of which lie within the first 200 m above ground. We select one 36 h period in December 2013, when favorable conditions for a strong gap wind situation were observed. The high-resolution WRF experiment reveals a significant fine-scale structure in the strong Tehuano wind flow, beyond the well known gap jet. Depending on the Froude number upstream of the topographic barrier, different downslope windstorm conditions and hydraulic jumps with rotor circulations develop simultaneously at different locations east of Chivela Pass with varied crest height. A comparison with observations suggests that the model accurately represents the spatially heterogeneous intense downslope windstorm and the formation of mountain wave clouds for several hours, with low errors in wind speed, wind direction and temperature.

Multi-parton interactions and rapidity gap survival probability in jet–gap–jet processes

We discuss an application of dynamical multi-parton interaction model, tuned to measurements of underlying event topology, for a description of destroying rapidity gaps in the jet–gap–jet processes at the LHC. We concentrate on the dynamical origin of the mechanism of destroying the rapidity gap. The cross section for jet–gap–jet is calculated within LL BFKL approximation. We discuss the topology of final states without and with the MPI effects. We discuss some examples of selected kinematical situations (fixed jet rapidities and transverse momenta) as distributions averaged over the dynamics of the jet–gap–jet scattering. The colour-singlet ladder exchange amplitude for the partonic subprocess is implemented into the Pythia 8 generator, which is then used for hadronisation and for the simulation of the MPI effects. Several differential distributions are shown and discussed. We present the ratio of cross section calculated with and without MPI effects as a function of rapidity gap in between the jets.

The very strong coastal El Ni\xf1o in 1925 in the far-eastern Pacific

The 1925 El Niño (EN) event was the third strongest in the twentieth century according to its impacts in the far-eastern Pacific (FEP) associated with severe rainfall and flooding in coastal northern Peru and Ecuador in February–April 1925. In this study we gathered and synthesised a large diversity of in situ observations to provide a new assessment of this event from a modern perspective. In contrast to the extreme 1982–1983 and 1997–1998 events, this very strong “coastal El Niño” in early 1925 was characterised by warm conditions in the FEP, but cool conditions elsewhere in the central Pacific. Hydrographic and tide-gauge data indicate that downwelling equatorial Kelvin waves had little role in its initiation. Instead, ship data indicate an abrupt onset of strong northerly winds across the equator and the strengthening/weakening of the intertropical convergence zones (ITCZ) south/north of the equator. Observations indicate lack of external atmospheric forcing by the Panama gap jet and the south Pacific anticyclone and suggest that the coupled ocean–atmosphere feedback dynamics associated with the ITCZs, northerly winds, and the north–south SST asymmetry in the FEP lead to the enhancement of the seasonal cycle that produced this EN event. We propose that the cold conditions in the western-central equatorial Pacific, through its teleconnection effects on the FEP, helped destabilize the ITCZ and enhanced the meridional ocean–atmosphere feedback, as well as helping produce the very strong coastal rainfall. This is indicated by the nonlinear relation between the Piura river record at 5°S and the SST difference between the FEP and the western-central equatorial Pacific, a stability proxy. In summary, there are two types of EN events with very strong impacts in the FEP, both apparently associated with nonlinear convective feedbacks but with very different dynamics: the very strong warm ENSO events like 1982–1983 and 1997–1998, and the very strong “coastal” EN events like 1925.

The Tokar Gap Jet: Regional Circulation, Diurnal Variability, and Moisture Transport Based on Numerical Simulations

Abstract The structure, variability, and regional connectivity of the Tokar Gap jet (TGJ) are described using WRF Model analyses and supporting atmospheric datasets from the East African–Red Sea–Arabian Peninsula (EARSAP) region during summer 2008. Sources of the TGJ’s unique quasi-diurnal nature and association with atypically high atmospheric moisture transport are traced back to larger-scale atmospheric dynamics influencing its forcing. These include seasonal shifts in the intertropical convergence zone (ITCZ), variability of the monsoon and North African wind regimes, and ties to other orographic flow patterns. Strong modulation of the TGJ by regional processes such as the desert heating cycle, wind convergence at the ITCZ surface front, and the local land–sea breeze cycle are described. Two case studies present the interplay of these influences in detail. The first of these was an “extreme” gap wind event on 12 July, in which horizontal velocities in the Tokar Gap exceeded 26 m s−1 and the flow from the jet extended the full width of the Red Sea basin. This event coincided with development of a large mesoscale convective complex (MCC) and precipitation at the entrance of the Tokar Gap as well as smaller gaps downstream along the Arabian Peninsula. More typical behavior of the TGJ during the 2008 summer is discussed using a second case study on 19 July. Downwind impact of the TGJ is evaluated using Lagrangian model trajectories and analysis of the lateral moisture fluxes (LMFs) during jet events. These results suggest means by which TGJ contributes to large LMFs and has potential bearing upon Sahelian rainfall and MCC development.

SAR-derived gap jet characteristics in the lee of the Philippine Archipelago

Gap jet characteristics were analyzed in the lee of the Philippine Archipelago during the 2004–2009 winter monsoon seasons (November–March) using wind fields derived from ENVISAT synthetic aperture radar (SAR) images. Jet characteristics were derived for the Philippine Island gap jets and compared to those from theoretical literature and experimental studies. It was found that 18 of 31 jets characterized from 13 SAR images followed the defined power laws, or classic case, and were similar to the defined scale equations. The remaining 13 jets did not follow the classic case and resulted from jet interaction with islands, adjacent jets, and variable atmospheric conditions. These non-classic jets were grouped based upon similarities and referred to as decreasing, increasing, and reverse classic cases. Distinct deviations from the classic case included downstream increases in velocity and decreases in jet width. It was found that the orientation of the wind field relative to topography significantly impacted jet classification, and thereby made it difficult to characterize jets by region. For all 31 jets, the initial jet width dictated the distance at which a jet became self-similar, wherein a larger initial jet width required a greater distance for self-similarity.

Numerical and observational case-study of a deep Adriatic bora

A detailed analysis of bora winds is presented that were observed on 28 March 2002 in the north-eastern part of the Adriatic Sea. Very high-resolution numerical simulations are compared with airborne, surface, and balloon observations. The key instrument for the verification of the numerical results is an aerosol backscatter lidar on board the DLR Falcon research aircraft. The high spatial resolution of the model and of the observational dataset allows several small-scale aspects of the bora winds to be explored. The study emphasises the great impact of boundary-layer effects including convective mixing and surface friction on the structure of the bora flow. The numerical model reveals that the diurnal cycle of the planetary boundary layer causes a diurnal variation of the gravity-wave amplitude and consequently a variation in the bora strength. The downslope flow is strongest during the night-time when the impinging air mass is stably stratified and exhibits a low-level jet. The bora strength weakens during the daytime due to the evolution of a neutrally stratified convective boundary layer. Lidar observations and simulations both suggest that the observed bora winds are driven by the dynamics of orographic gravity waves. In the deep north-easterly cross-mountain flow steeply amplified but non-breaking gravity waves as well as trapped lee waves are found throughout the troposphere. Vertical separation of the boundary layer over the steep leeward terrain slope prevents the bora flow from reaching the coast. As a result, the flow is kept aloft in a series of at least three unsteady trapped lee waves before it reattaches to the surface over the sea downstream of the mountains. No clear evidence is found for the existence of low-level rotors underneath these lee waves. The ability of the model to capture the trapped waves is found to depend on the horizontal model resolution. Bora winds are identified as jets emanating from several mountain gaps. The physical mechanism for the gap flow formation is boundary-layer separation controlled by surface friction. Flow separation is more effective over the highest terrain where a broad wake forms downstream with weak winds compared to the mountain gaps where the strong bora flow eventually reattaches to the surface. The strongest gap jet belongs to the Vratnik Pass upstream of the town Senj and forms the primary shear line in the northern Adriatic with a corresponding potential-vorticity (PV) banner. The numerical model reveals several secondary orographic PV banners and embedded PV filaments. The vertical PV distribution exhibits a two-layer structure with PV anomalies at lower (higher) elevations originating from mountain gaps (isolated mountain peaks). Surface friction has a minor direct effect on PV generation but has a major indirect effect in controlling the mechanism for the generation of PV. In a simulation without surface friction the primary PV banner forms as a result of gravity-wave breaking. In the more realistic simulation, however, the primary mechanism for PV generation is flow separation controlled by surface friction. Copyright © 2005 Royal Meteorological Society

Role of the shear layer instability in the near wake behavior of two side-by-side circular cylinders

Wakes, and their interaction behind two parallel cylinders lying in a plane perpendicular to the flow, have been investigated experimentally in the sub-critical Reynolds number regime. The experiments were performed in a water channel using laser Doppler velocimetry. The gap between the two cylinders was less than the cylinder diameter, a geometry referred to as strong interaction configuration. In this case the blockage is strong and a gap-jet appears between the cylinders. Two flow regimes of the near wake region have been identified: one below a critical Reynolds number Rec ∈]1000;1700[, where the gap jet is stably deflected to one side and the double near-wake becomes asymmetric; the other, above Rec, where the gap-jet deflection is unstable and a random flopping phenomenon takes place. When Re Rec, a third frequency appears in the near wake, related to the development of Kelvin-Helmholtz vortices in the separated shear layer of the cylinders [Prasad A, Williamson CHK (1997) J Fluid Mech 333:375]. The observed flopping behavior is attributed to the birth of these Kelvin-Helmholtz instabilities and their intermittent nature. Further downstream, beyond about five cylinder diameters, the random flopping flow phenomena disappear while a slightly asymmetric single wake persists. It is characterized by a Strouhal number St=0.13, a value that one would normally measure behind a single cylinder of twice its diameter.