Surge Cycle Research Articles

Thermal structure of Svalbard glaciers and implications for thermal switch models of glacier surging

AbstractSwitches between cold‐ and warm‐based conditions have long been invoked to explain surges of High Arctic glaciers. Here we compile existing and new data on the thermal regime of six glaciers in Svalbard to test the applicability of thermal switch models. Two of the large glaciers of our sample are water terminating while one is land terminating. All three have a well‐known surge history. They have a thick basal layer of temperate ice, superimposed by cold ice. A cold terminus forms during quiescence but is mechanically removed by calving on tidewater glaciers. The other three glaciers are relatively small and are either entirely cold or have a diminishing warm core. All three bear evidence of former warm‐based thermal regimes and, in two cases, surge‐like behavior during the Little Ice Age. In Svalbard, therefore, three types of glaciers have switched from slow to fast flow: (1) small glaciers that underwent thermal cycles during and following the Little Ice Age (switches between cold‐ and warm‐based conditions), (2) large terrestrial glaciers which remain warm based throughout the entire surge cycle but develop cold termini during quiescence, and (3) large tidewater glaciers that remain warm based throughout the surge cycle. Our results demonstrate that thermal switching cannot explain the surges of large glaciers in Svalbard. We apply the concept of enthalpy cycling to the spectrum of surge and surge‐like behavior displayed by these glaciers and demonstrate that all Svalbard surge‐type glaciers can be understood within a single conceptual framework.

Large Eddy Simulation for a Deep Surge Cycle in a High-Speed Centrifugal Compressor With Vaned Diffuser

This paper presents a computational study for a high-speed centrifugal compressor stage with a design pressure ratio equal to 4, the stage consisting of a splittered unshrouded impeller and a wedged vaned diffuser. The aim of this paper is to investigate numerically the modifications of the flow structure during a surge cycle. The investigations are based on the results of unsteady three-dimensional, compressible flow simulations, using large eddy simulation (LES) model. Instantaneous and mean flow field analyses are presented in the impeller inducer and in the vaned diffuser region through one surge cycle time intervals. The computational data compare favorably with the measured data, from the literature, for the same compressor and operational point. The surge event phases are well detected inside the impeller and diffuser. The time-averaged loading on the impeller main blade is maximum near the trialing edge and near the tip. The amplitude of the unsteady pressure fluctuation is maximum for the flow reversal condition and reaches values up to 70% of the dynamic pressure. The diffuser vane exhibits high-pressure fluctuation from the vane leading edge to 50% of the chord length. High-pressure fluctuation is detected during the forward flow recovery condition as a result of the shock wave that moves toward the diffuser outlet.

Numerical investigation of rotating stall in centrifugal compressor with vaned and vaneless diffuser

This study presents a numerical simulation of the stall and surge in a centrifugal compressor and presents a description of the stall development in two different cases. The first case is for a compressor with vaneless diffuser and the second is for a compressor with vaned diffuser of the vane island shape. The main aim of this study is to compare the flow characteristics and behavior for the two compressors near the surge operating condition and provide further understanding of the diffuser role when back flow occurs at surge. Results showed that for a location near the diffuser entrance, the amplitude of the static pressure fluctuations for the vaneless diffuser case is higher than that for the vaned diffuser case near surge condition. These pressure fluctuations in the case of the vaneless diffuser appear with a gradual decrease of the mean pressure value as a part of the surge cycle. While for the case of the vaned diffuser, the pressure drop during surge occurs faster than the case of the vaneless diffuser. Also, results indicated that during surge in the case of vaneless diffuser, there is a region with low velocity and back flow that appears as a layer connecting all impeller passages near shroud surface and this layer develops in size with time. On the other hand, for the case of vaned diffuser during surge, the low velocity regions appear in random locations in some passages and these regions expand with time towards the shroud surface. Results showed that during stall, the impeller passages are exposed to identical impact from stall cells in the case of vaneless diffuser while the stall effect varies from passage to another in the case of the vaned diffuser.

Numerical Simulation of Aerodynamic Instabilities in a Multistage High-Speed High-Pressure Compressor on Its Test Rig—Part II: Deep Surge

Aerodynamic instabilities such as stall and surge may occur in compressors, possibly leading to mechanical failures so their avoidance is crucial. A better understanding of those phenomena and an accurate prediction are necessary to improve both the performance and the safety. A surge event in a compressor threatens the mechanical integrity of the aircraft engine, and this remains true for a research compressor on a test rig. As a result, few experimental data on surge are available. Moreover, there are technological, restrictive constraints that exist on test rigs and limit severely the type of data obtainable experimentally. This partially explains why numerical simulation has become a usual, complementary and convenient tool to collect data in a compressor, as it does not disturb the flow nor does it encounter technological limits. Despite the inherent difficulties, an entire surge cycle has been simulated in a high-speed, high-pressure, multistage research compressor, using an implicit, time-accurate, 3D compressible unsteady Reynolds-averaged Navier–Stokes solver. First, the paper presents the main features of the surge cycle obtained, along with those from the experimental cycle, for a validation purpose. Four phases compose the surge cycle: surge inception, the reversed-flow phase, the recovery phase, and the repressurization of the compressor flow. All of them are described, and focus is put on surge inception and the reversed-flow phase, as they induce greater risk for the mechanical integrity of the machine.

Protracted active surge phase in the high arctic (svalbard): dynamic development and morphological impact

Glacial surges in valbard are protracted and characterized by individual dynamic evolution, in contrast to many other areas, which calls for a subdivision of the classic two‐phased surge cycle. A dominating part of the ice masses seem to have a surge potential and this represents a considerable challenge for palaeoclimatic studies. Glaciological and geological models therefore need to be coupled. The issue is discussed with Fridtjovbreen glacier as an example. This ice mass is one of few glaciers studied throughout a surge cycle. It was active for 12 years (1991–2002) and represents the most protracted surge documented. The maximum advance rate was 4.2 m day−1, its maximum extent was reached after seven years, its run‐out distance was 4 km, and the relocated ice filled 5 km2 of the fjord. Intense subglacial thrusting occurred during various stages, including part of the ice‐front retreat, as shown by sub‐bottom profiling data from 2002. A six‐stage model is presented and processes are discussed with emphasis on the ice‐front retreat with transition to the quiescent phase. Although the surge mechanism itself is unrelated to climate, climatic conditions obviously play a major role in the course of a surge. During the surge, the ice mass made a dramatic impression in the landscape, but 10 years after the maximum extent, there is little onshore evidence of the event.

Coexisting phenomena of surge and rotating stall in an axial flow compressor

The unsteady inner flow structure of a single-stage axial flow compressor under the coexisting conditions of surge and rotating stall was experimentally investigated via detailed measurements of the unsteady characteristics and the internal flow velocity fluctuations. The main relevant feature of the tested compressor is a shock tube with a capacity tank connected in series to the compressor outlet through slits and a concentric duplex pipe: surge and rotating stall can both be generated by connecting the shock tube. Research attention is focused on the unsteady behavior of a rotating stall during the surge cycle. The size of the rotating stall cell during the recovery process of an irregular surge cycle was experimentally determined by the circumferential flow velocity fluctuations ahead of the rotor blade. The results suggested that the size of the rotating stall cell at the switching point of the performance curve between large and small cycles is considered to be the key parameter in determining the following surge cycle. In addition, the surge cycle is largely influenced by the unsteady behavior of the rotating stall cell.

A Hybrid Approach for the Modelling of Axial Compressor Surge

This paper discusses the results of a hybrid approach, which couples the Moore–Greitzer potential flow model and the Navier–Stokes solution to predict the viscous effects on the axial compressor surge. The viscous effects are accounted by the inclusion of a duct between the compressor exit and the plenum inlet. The unsteady mass flow solution from the Moore–Greitzer model is supplied as an inlet boundary condition, to the duct, to solve the Navier–Stokes equation. The viscosity is seen to damp the surge oscillations and alter the character of oscillations significantly. A shortcoming of the Moore–Greitzer model in predicting proper phase relationship between the compressor flow and the plenum pressure is pointed out and the proposed hybrid approach is shown to overcome this flaw to a greater extent. It is depicted that this hybrid approach captures the existence of rotating stall disturbances in the combustor, during a typical classic surge cycle, as reported in the literatures. Thus this hybrid model is more physically appropriate in describing the plenum pressure oscillations than the Moore–Greitzer model. The combustor pressure oscillations from the hybrid approach shows two distinct time scales that can influence the combustor efficiency.

The propagation of a surge front on Bering Glacier, Alaska, 2001–2011

AbstractBering Glacier, Alaska, USA, has a ∼20 year surge cycle, with its most recent surge reaching the terminus in 2011. To study this most recent activity a time series of ice velocity maps was produced by applying optical feature-tracking methods to Landsat-7 ETM+ imagery spanning 2001-11. The velocity maps show a yearly increase in ice surface velocity associated with the down-glacier movement of a surge front. In 2008/09 the maximum ice surface velocity was 1.5 ±0.017 km a-1 in the mid-ablation zone, which decreased to 1.2 ±0.015 km a-1 in 2009/10 in the lower ablation zone, and then increased to nearly 4.4 ± 0.03 km a-1 in summer 2011 when the surge front reached the glacier terminus. The surge front propagated down-glacier as a kinematic wave at an average rate of 4.4 ±2.0 km a-1 between September 2002 and April 2009, then accelerated to 13.9 ± 2.0 km a-1 as it entered the piedmont lobe between April 2009 and September 2010. The wave seems to have initiated near the confluence of Bering Glacier and Bagley Ice Valley as early as 2001, and the surge was triggered in 2008 further down-glacier in the mid-ablation zone after the wave passed an ice reservoir area.

The propagation of a surge front on Bering Glacier, Alaska, 2001–2011

Bering Glacier, Alaska, USA, has a ∼20 year surge cycle, with its most recent surge reaching the terminus in 2011. To study this most recent activity a time series of ice velocity maps was produced by applying optical feature-tracking methods to Landsat-7 ETM+ imagery spanning 2001-11. The velocity maps show a yearly increase in ice surface velocity associated with the down-glacier movement of a surge front. In 2008/09 the maximum ice surface velocity was 1.5 ±0.017 km a-1 in the mid-ablation zone, which decreased to 1.2 ±0.015 km a-1 in 2009/10 in the lower ablation zone, and then increased to nearly 4.4 ± 0.03 km a-1 in summer 2011 when the surge front reached the glacier terminus. The surge front propagated down-glacier as a kinematic wave at an average rate of 4.4 ±2.0 km a-1 between September 2002 and April 2009, then accelerated to 13.9 ± 2.0 km a-1 as it entered the piedmont lobe between April 2009 and September 2010. The wave seems to have initiated near the confluence of Bering Glacier and Bagley Ice Valley as early as 2001, and the surge was triggered in 2008 further down-glacier in the mid-ablation zone after the wave passed an ice reservoir area.

A Simple Feedback Control Strategy for Controlling the Axial Compressor Surge

This paper presents the results of a feedback technique for controlling low frequency surge oscillations in an axial compressor. To simulate surge a hybrid model is developed, by coupling the Moore - Greitzer potential flow model and the Navier - Stokes solution, to result in more realistic flow predictions. This hybrid model incorporates the viscous effect on the flow behaviour, which is neglected in the Moore - Greitzer model. The present model is shown to capture the existence of rotating stall disturbances in the combustor during a typical classic surge cycle as has been reported in the experiments. After modelling the surge we propose a steady feedback control scheme to minimise the adverse effect of the surge oscillations on the combustor operation. The results for a four hole and six hole feedback control mechanisms are reported and it is seen that the existing high frequency fluctuations in the unsteady variation of plenum pressure before control are virtually eliminated in the presence of a stead...

Surge dynamics on Bering Glacier, Alaska, in 2008–2011

Abstract. A surge cycle of the Bering Glacier system, Alaska, is examined using observations of surface velocity obtained using synthetic aperture radar (SAR) offset tracking, and elevation data obtained from the University of Alaska Fairbanks LiDAR altimetry program. After 13 yr of quiescence, the Bering Glacier system began to surge in May 2008 and had two stages of accelerated flow. During the first stage, flow accelerated progressively for at least 10 months and reached peak observed velocities of ~ 7 m d−1. The second stage likely began in 2010. By 2011 velocities exceeded 9 m d−1 or ~ 18 times quiescent velocities. Fast flow continued into July 2011. Surface morphology indicated slowing by fall 2011; however, it is not entirely clear if the surge is yet over. The quiescent phase was characterized by small-scale acceleration events that increased driving stresses up to 70%. When the surge initiated, synchronous acceleration occurred throughout much of the glacier length. Results suggest that downstream propagation of the surge is closely linked to the evolution of the driving stress during the surge, because driving stress appears to be tied to the amount of resistive stress provided by the bed. In contrast, upstream acceleration and upstream surge propagation is not dependent on driving stress evolution.

Geometric Changes in a Tidewater Glacier in Svalbard during its Surge Cycle

Fridtjovbreen, Svalbard, is a partially tidewater-terminating glacier that started a 7-year surge during the 1990s. Flow peaked during 1996 and no surge front was apparent. We use two pre-surge (1969 and 1990) and a post-surge (2005) digital elevation models (DEMs) together with a bed DEM to quantify volume changes and iceberg calving during the surge, calculate the changes in glacier hypsometry, and investigate the surge trigger. Between 1969 and 1990, the glacier lost 5% of its volume, retreated 530 m and thinned by up to 60 m in the lower elevations while thickening by up to 20 m in its higher elevations. During the surge, the reservoir zone thinned by up to 118 m and the receiving zone thickened by ∼140 m. Fridtjovbreen’s ice divide moved ∼500 m, incorporating extra ice into its catchment. Despite this volume gain, during 1990–2005 the glacier lost ∼ 10% of its volume through iceberg calving and 7% through surface melt. The surge occurred in a climate of decreasing overall ice volume, so we need to revise the notion that surging is triggered by a return to an original geometry, and we suggest Fridtjovbreen’s surge was triggered by increasing shear stresses primarily caused by increases in surface slope.

Unsteady behavior of surge and rotating stall in an axial flow compressor

Unsteady behaviors as well as unsteady cascade flow fields of a single-stage axial flow compressor were experimentally investigated by detail measurements of unsteady performance characteristics and casing wall pressure and internal flow velocity fluctuations. The main feature of the test compressor is a capacity tank connected directly to the compressor outlet in series through slits and a concentric duplex pipe, and also jet nozzles in order to inject compressed air toward the rotor tip region. Research attention is focused on the post-stall characteristics of surge and rotating stall which occur simultaneously. When the compressor was connected to the capacity tank, surge was generated with rotating stall in accordance with the capacitance increment of whole compressor system. The surge behavior changed irregularly with throttling valve installed behind the compressor, and several types of surge cycles were observed. In addition, the surge cycle changed by jet injection to the rotor tip region. The results suggested that the blockages of the cascade flow which were generated by a stall cell play an important role in deciding the surge behaviors.

Numerical Simulation of Surge in Axial Compressor

The object of this paper is to provide a reliable tool to simulate the stationary and transient operation performances in multistage axial compression systems, especially poststall behavior. An adapted version of the 1D Euler equations with additional source terms is solved by a time marching and control volume method. The equations are discretized at midspan both inside the blade rows and the nonbladed regions, along the real flow path geometry. The source terms express the blade-flow interactions and are estimated by calculating the velocity triangles for each blade row. Loss coefficient and deviation models are supplied by empirical correlations and are compared to experimental data in all flow regions. Transient simulations are carried and compared to the experimental results for several values of parameterB. The flow mechanism inside the compressor during a surge cycle is also shown.

Unsteady flow field under surge and rotating stall in a three-stage axial flow compressor

The unsteady flow structure between rotor blade-to-blade passages in a three-stage axial flow compressor is experimentally investigated by detailed measurements of unsteady performance characteristics, casing wall pressure fluctuations and their wavelet analyses. The main feature of the test compressor is a capacity tank facility connected in series to the compressor outlet in order to supply compression and/or expansion waves from downstream of the compressor. Research attention is focused on the post-stall characteristics of the surge and rotating stall which occur simultaneously. The influence of the compressor operating point on the unsteady performance curve shows that the surge cycle changes irregularly depending on the steady-state resistance characteristics, and the results of the wavelet analyses of the wall pressure fluctuations suggest that the surge cycle may selectively be determined by the rotating stall cell structure within the rotor cascade.

Svalbard glacier elevation changes and contribution to sea level rise

We compare satellite altimetry from the Ice, Cloud, and Land Elevation Satellite (ICESat, 2003–2007) to older topographic maps and digital elevation models (1965–1990) to calculate long‐term elevation changes of glaciers on the Svalbard Archipelago. Results indicate significant thinning at most glacier fronts with either slight thinning or thickening in the accumulation areas, except for glaciers that surged which show thickening in the ablation area and thinning in the accumulation areas. The most negative geodetic balances occur in the south and on glaciers that have surged, while the least negative balances occur in the northeast and on glaciers in the quiescent phase of a surge cycle. Geodetic balances are related to latitude and to the dynamical behavior of the glacier. The average volume change rate over the past 40 years for Svalbard, excluding Austfonna and Kvitøya is estimated to be −9.71 ± 0.55 km3 yr−1 or −0.36 ± 0.02 m yr−1 w. equivalent, for an annual contribution to global sea level rise of 0.026 mm yr−1 sea level equivalent.

Effect of piping systems on surge in centrifugal compressors

There is a possibility that the exchange of the piping system may change the surge characteristic of a compressor. The piping system of a plant is not always the same as that of a test site. Then it is important to evaluate the effect of piping systems on surge characteristics in centrifugal compressors. Several turbochargers combined with different piping systems were tested. The lumped parameter model which was simplified to be solved easily was applied for the prediction of surge point. Surge lines were calculated with the linearlized lumped parameter model. The difference between the test and calculated results was within 10%. Trajectory of surge cycle was also examined by solving the lumped parameter model. Mild surge and deep surge were successfully predicted. This study confirmed that the lumped parameter model was a very useful tool to predict the effect of piping systems on surge characteristics in centrifugal compressors, even though that was a simple model.

Submarine landforms characteristic of glacier surges in two Spitsbergen fjords

Well-preserved submarine landforms, all less than 100 years old, are imaged on high-resolution swath bathymetry obtained from Van Keulenfjorden and Rindersbukta (inner Van Mijenfjorden), Spitsbergen, Svalbard. Several tidewater glaciers in these fjords have surged in the last few hundred years. Streamlined landforms, found within the limits of known surges, are interpreted as mega-scale glacial lineations (MSGL) formed subglacially beneath actively surging ice. Large transverse ridges are terminal moraines formed by thrusting at the maximum position of glacier surges. Sediment lobes at the distal margins of terminal moraines are interpreted as glacigenic debris flows, formed either by failure of the frontal slopes of thrust moraines or from deforming sediment extruded from beneath the glacier. Sinuous ridges are eskers, formed after surge termination by the sedimentary infilling of subglacial conduits. Concordant ridges, parallel to former ice margins, are interpreted as minor push moraines, probably formed annually during winter glacier readvance. Discordant ridges, oblique to former ice margins, are interpreted as crevasse-squeeze ridges, forming when soft subglacial sediments are injected into basal crevasses. These submarine landforms have been deposited in the following sequence based on cross-cutting relationships between them, linked to stages of the surge cycle: (1) MSGL; (2a) terminal moraines and (2b) lobe-shaped debris flows; (3) isolated areas of crevasse-fill ridges; (4) eskers and (5) annual retreat ridges. A descriptive landsystem model for tidewater surge-type glaciers has been developed, whose wider applicability is emphasised by comparison with two areas in Isfjorden, Spitsbergen. The model also has a number of features in common with landsystem models for terrestrial surge-type glaciers, but is likely to be more complete since submarine landforms are particularly well preserved. The landforms discussed here may be produced and preserved in different proportions in the varied environmental settings where surging glaciers are found.

Time to surge concept and surge control for acceleration performance

Surge is a dangerous instability that can occur in compressors. It is avoided using a valve that reduces the compressor pressure. The control of this valve is important for the compressor safety but it also has a direct influence on the acceleration performance. Compressor surge control is investigated by first studying the surge phenomenon in detail. Experimental data from a dynamic compressor flow test bench and surge cycles measured on an engine is used to tune and validate a model capable of describing surge. A concept named time to surge is introduced and a sensitivity analysis is performed to isolate the important characteristics that influence surge transients in an engine. It is pointed out that the controller clearly benefits from a feed-forward term due to the small time frames associated with the transition to surge. In the next step this knowledge is used in the design of a novel surge controller. This surge controller is then compared to two other controllers and it is shown that it avoids surge and improves the acceleration performance by delivering both higher engine torque and turbo shaft speed after a gear change.

Slow surge of Trapridge Glacier, Yukon Territory, Canada

Trapridge Glacier, a polythermal surge‐type glacier located in the St. Elias Mountains, Yukon Territory, Canada, passed through a complete surge cycle between 1951 and 2005. Air photos (1951–1981) and ground‐based optical surveys (1969–2005) are used to quantify the modifications in flow and geometry that occurred over this period. Yearly averaged flow records suggest that the active phase began ∼1980, and lasted until ∼2000. The average velocity in the central area of the glacier went from 16 m yr−1 in 1974 to 39 m yr−1 in 1980; it peaked at 42 m yr−1 in 1984, and remained above 25 m yr−1 until 2001. Over that interval, the flow decelerated by steps, in 4‐year pulses. After a particularly vigorous acceleration in 1997–1999, the glacier gradually slowed to presurge velocities. In 2005, the flow was less than 9 m yr−1. Digital elevation models are generated by stereographic analysis of air photos for 1951, 1970, 1972, 1977, and 1981. These models are updated annually using ground‐based survey data and a novel implementation of Bayesian kriging. Over the course of the surge, the front of active ice advanced 450 m and the glacier area increased by 10%, with an associated thinning of the ice. The previous surge of Trapridge Glacier, starting before 1939 and ending before 1951, led to a terminus advance of ∼1 km. Comparison of the two surges suggests that the 1930s surge started with a slow progression similar to what we observed in the 1980s and 1990s, and switched to a faster flow mode after 1941. This second phase was never attained in the recent surge, probably owing to a lack of mass.