Academic literature on the topic 'Electric waves Measurement'

A project investigating the possibility to transmit electrical power (several mW) along the structure of an aircraft by using an intermediate medium of ultrasonic Lamb waves is being carried out at Cardiff University in conjunction with Airbus. This power supply method is aimed at wireless, surface-bonded sensor packages, primarily for the aircraft structural health monitoring (SHM) applications. It is expected to replace conventional batteries or energy harvesting devices. This paper presents methods of piezoelectric transducer characterisation, electric power measurement and electric circuit simulation that were developed in support of the ultrasonic power transmission project. The unique combination of low power and a high AC frequency range (up to 200 kHz) precluded the use of conventional power measurement instruments and called for a tailored system and software to be developed. Two approaches were developed: one relying on the measurement of the ultrasonic transducers impedance characteristics and their subsequent use in a circuit simulation; and another relying on the direct measurement of voltage waveforms in the power transmission setup. The two methods were found to be capable of producing closely matching results up to 300 kHz. Results of early power transmission trials are also presented. The optimum approach resulted in 1 mW of power transmission over a distance of 74 cm in a 1.5 mm thick aluminium plate.

New way for quantitative evaluation of any value of formation permeability based on numerical experiments with Pride's model of electrokinetic phenomena is presented. This approach is based on simultaneous measurement of pressure field and axial component of electric field excited by an acoustic source in fluid-filled borehole with help a set of receivers in borehole. Frequency dependence of ratio of the complex-valued amplitudes of the electric Stoneley wave to the pressure Stoneley wave obtained as a result of plane-wave decomposition of pressure field and mentioned component of electric field carries important information about permeability. In case of open and sealed pores on borehole wall were obtained enough simple analytical expressions for this ratio.

Terahertz wave (T-ray) technologies have become a popular topic in scientific research over the last two decades, and can be utilized in nondestructive evaluation (NDE) techniques. This study suggests an optimal scanning technique method for honeycomb sandwich composite panels, where skins were utilized with two different skins, namely, carbon fiber-reinforced plastic (CFRP) skin and glass fiber-reinforced plastic (GFRP) skin, as layers of the panel surfaces. Foreign objects were artificially inserted between the skins and honeycomb cells in the honeycomb sandwich composite panels. For this experiment, optimal T-ray scanning methods were performed to examine defects based on the angle between the one-ply thin fiber skin axis and the angle of the electric field (E-field) according to the amount of conductivity of the honeycomb sandwich composite panels. In order to confirm the fundamental characteristics of the terahertz waves, the refractive index values of the GFRP composites were experimentally obtained and analyzed, with the data agreeing with known solutions. Terahertz waves (T-rays) were shown to have limited penetration in honeycomb sandwich composite panels when utilized with a skin of carbon fibers. Therefore, T-rays were found to interact with the electrical conductivity and electric field direction of honeycomb sandwich composite panels with glass fiber skins. The T-ray images were obtained regardless of the electric field direction and the fiber direction. In the honeycomb sandwich composite panels with carbon fiber skins, the T-ray images with higher signal-to-noise (S/N) ratios depended on the scanning angle between the angle of the carbon fiber and the angle of the electric field. Thus, the angle of optimum detection measurement was confirmed to be 90° between the E-field and the fiber direction, particularly when using a carbon fiber skin.

Terahertz waves are located in the frequency band between radio waves and light, and they are being considered for various applications as a light source. Generally, the use of light requires focusing; however, when a terahertz wave is irradiated onto a small detector or a small measurement sample, its wavelength, which is much longer than that of visible light, causes problems. The diffraction limit may make it impossible to focus the terahertz light down to the desired range by using common lenses. The Bull’s Eye structure, which is a plasmonic structure, is a promising tool for focusing the terahertz light beyond the diffraction limit and into the sub-wavelength region. By utilizing the surface plasmon propagation, the electric field intensity and transmission coefficient can be enhanced. In this study, we improved the electric field intensity and light focusing in a small region by adapting the solid immersion method (SIM) from our previous study, which had a frequency-tunable nonconcentric Bull’s Eye structure. Through electromagnetic field analysis, the electric field intensity was confirmed to be approximately 20 times higher than that of the case without the SIM, and the transmission measurements confirmed that the transmission through an aperture had a gap of 1/20 that of the wavelength. This fabricated device can be used in imaging and sensing applications because of the close contact between the transmission aperture and the measurement sample.

The surface acoustic waves (SAWs) were used to study the effect of magnetic nanoparticles on nematic liquid crystal (NLC) behavior in weak magnetic and electric fields. The measurement of the attenuation of SAW propagating along the interface between piezoelectric substrate and liquid crystal is showed as an effective tool to study processes of structural changes. The magnetic nanoparticles Fe3O4 of nanorod shape and different low volume concentration were added to the NLC (4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT)) during its isotropic phase. In contrast to undoped liquid crystals the distinctive different SAW attenuation responses induced by both magnetic and also electric fields in studied NLC samples were observed suggesting both structural changes and the orientational coupling between both magnetic and electric moments of nanoparticles and the director of the NLC molecules. Experimental measurements including the investigation under linearly increasing and/or jumped magnetic and electrical fields, respectively, as well as the investigation of temperature and time influences on structural changes were done. The investigation of the SAW anisotropy gives supplemental information about the internal structure of nanoparticles in investigated NLCs. In addition, some magneto-optical investigations were performed to support SAW results and study their stability and switching time. The analysis of observed SAW attenuation characteristics confirmed the role of concentration of magnetic nanoparticles on the resultant behavior of investigated NLC compounds. Obtained results are discussed within the context of previous ones. The theoretical background of the presented SAW investigation is introduced, too.

Context. The Mutual Impedance Probe (RPC-MIP) carried by the Rosetta spacecraft monitored both the plasma density and the electric field in the close environment of comet 67P/Churyumov–Gerasimenko (67P), as the instrument was operating alternatively in two main modes: active and passive. The active mode is used primarily to perform plasma density measurements, while the passive mode enables the instrument to work as a wave analyzer. Aims. We are reporting electric field emissions at the plasma frequency near comet 67P observed by RPC-MIP passive mode. The electric field emissions are related to Langmuir waves within the cometary ionized environment. In addition, this study gives feedback on the density measurement capability of RPC-MIP in the presence of cold electrons. Methods. We studied the occurrence rate of the electric field emissions as well as their dependence on solar wind structures like stream interaction regions (SIRs) and coronal mass ejections (CMEs). Results. We are showing that strong electric field emissions at the plasma frequency near 67P were present sporadically throughout the period when Rosetta was escorting the comet, without being continuous, as the occurrence rate is reported to be of about 1% of all the measured RPC-MIP passive spectra showing strong electric field emissions. The Langmuir wave activity monitored by RPC-MIP showed measurable enhancements during SIR or CME interactions and near perihelion. Conclusions. According to our results, Langmuir waves are a common feature at 67P during the passage of SIRs. Comparing the plasma frequency given by the RPC-MIP passive mode during Langmuir wave periods with the RPC-MIP active mode observations, we conclude that the measurement accuracy of RPC-MIP depends on the operational submode when the cold electron component dominates the electron density.

Identification of hydro-acoustic waves emitted from floating units during mooring tests Measurements of hydro-acoustic noise emitted from vessels are a.o. a subject of the tests carried out in the control measurement ranges of the Navy. The measurements are performed both on anchored and floating vessels. Acoustic field of vessels is changing along with their speed changing and is associated with acoustic activity of wave sources installed in vessel's hull (main engines, electric generating sets, reduction gears, pumps, shaft-lines, piping, ventilating ducts etc) as well as hydro-dynamic sources such as screw propellers and water flow around the hull [5, 7]. Vibration energy generated by the onboard devices is transferred through ship structural elements to water where it propagates in the form of hydro-acoustic waves of a wide frequency band.

Numerical experiments based on Pride's model of electrokinetic phenomena have shown that electromagnetic Stoneley waves as well as pressure Stoneley waves are most sensitive to permeability variations. A new way for quantitative evaluation of any value of formation permeability is presented. It is based on simultaneous measurement of pressure field and axial component of electric field excited by an acoustic source in fluid-filled borehole with help from a set of receivers in borehole. Frequency dependence of ratio of the complex-valued amplitudes of the electric Stoneley wave to the pressure Stoneley wave obtained as a result of plane waves decomposition of pressure field and mentioned component of electric field carries important information about permeability. The ratio of the real part of this ratio to its imaginary part is very sensitive to permeability variations. The approximate analytical expressions for this ratio derived for open and sealed pores on borehole wall are base for construction of a new way of quantitative evaluation of formation permeability.

The temperature of the pole windings is one of the main informative parameters, the deviation of which may indicate the presence of a number of common defects of electrical machines. However, the implementation of high-precision means for measuring the temperature of the pole windings of the electric machine rotor is associated with a number of technical difficulties. The aim is to increase the accuracy of measuring the temperature of the pole windings of the rotor of an electric machine by developing a contactless method and means of measuring the temperature of the pole windings of the rotor in real time. The paper shows that the existing most common methods of temperature measurement have significant disadvantages that limit the possibility of their use for the implementation of means of measuring the pole windings of the rotor of an electric machine. A bispectral pyrometric method of temperature measurement is proposed, which provides simultaneous conversion into voltage of functionally temperature-dependent spectral densities of radiation for two spectrally close electromagnetic waves with subsequent finding of their ratio, which allowed to remove the most significant components of methodical measurement characteristic of classical pyrometers. The design of the bispectral pyrometric measuring system of temperature of pole windings of a rotor of rotating electric machines realized on the basis of the offered method of measurement, suitable for work in a real-time mode together with systems of control of a technical condition and diagnostics is developed.

Thesis (DTech (Electrical Engineering))--Cape Peninsula University of Technology, 2016.
Geomagnetically induced currents (GIC) are currents induced in ground-based conductor networks in the Earth's surface. The GIC are driven by an electric eld induced by geomagnetic variations which are a result of time-varying magnetospheric-ionospheric currents during adverse space weather events. Several studies have shown that there is a likelihood of technological damage (the power grid) in the mid- and low-latitude regions that could be linked to GIC during some geomagnetic storms over the past solar cycles. The effects of GIC in the power system can range from temporary damage (e.g. protective relay tripping) to permanent damage (thermal transformer damage). Measurements of GIC in most substations are done on the neutral-to-ground connections of transformers using Hall-effect transducers. However, there is a need to understand the characteristics of GIC in the power lines connected to these transformers. Direct measurements of GIC in the power lines are not feasible due to the low frequencies of these currents which make current measurements using current transformers (CT) impractical. This thesis discusses two techniques that can be employed to enhance understanding GIC characteristics in mid-latitude regions. The techniques involve the measurement of GIC in a power line using differential magnetometer measurements and modelling GIC using the finite element method. Low frequency magnetometers are used to measure magnetic felds in the vicinity of the power lines and the GIC is inferred using the Biot-Savart law. A finite element model, using COMSOL-Multiphysics, is used to calculate GIC with the measured magnetic field and a realistic Earth conductivity profile as inputs. The finite element model is used for the computation of electric field associated with GIC modelling.

The limiter is a commonly used device in communication receiving systems to remove the amplitude variations of the received signal, but it is usually observed that limiter degrades the envelope detection performance of a single tone. In this thesis, it is demonstrated that the limiter-generated third harmonic can be used to improve the envelope detection performance of a single tone over that of the linearly processed fundamental. Differences in the probability distributions of the limiter-generated harmonics cause differences in their detection probabilities, which lead to differences in the performance of subsequent envelope detection. Comparison of the envelope detection performance of the limiter-generated third harmonic and the input to the limiter shows a maximum detection probability gain of 1.12 and also error probability gain of 2.4 in linear scale, whereas the fundamental harmonic does not show any practically significant gain. The envelope detection performance of the vector sum of the limiter-generated fundamental and third harmonics is also evaluated. This combination provides better performance than do the individual harmonics, especially for a large clipping parameter of the limiter. The combined harmonics shows a maximum detetction probability gain of 1.15 and error probability gain of 14, over that of the envelope detection performance of a tone without limiter. It is also observed that the envelope detection performance of a tone with limiter-generated higher harmonics shows non-monotonic behaviour as functions of either noise or the limiter-clipping parameter, which is a signature of stochastic resonance. The theoretical results from earlier research on the envelope detection of a single tone embedded in additive white Gaussian noise, both with and without a limiter, are presented and shown to match our simulation results. In previous work when a limiter was used in the envelope detection of a single tone, only the envelope of the fundamental harmonic was considered under very specific conditions. By contrast we also take notice of the limiter-generated higher harmonics and obtain improved envelope detection performance in the detection of a single tone.

In a large and dense power network, the transmission lines, the generators and the loads are considered to be continuous functions of space. The continuum technique provides a macro-scale analytical tool to gain an insight into the mechanisms by which the disturbances initiated by faults and other random events propagate in the continuum. This dissertation presents one-dimensional and two-dimensional discrete models to illustrate the propagation of electromechanical waves in a continuum system. The more realistic simulations of the non-uniform distribution of generators and boundary conditions are also studied. Numerical simulations, based on the swing equation, demonstrate electromechanical wave propagation with some interesting properties. The coefficients of reflection, reflection-free termination, and velocity of propagation are investigated from the numerical results. Discussions related to the effects of electromechanical wave propagation on protection systems are given. In addition, the simulation results are compared with field data collected by phasor measurement units, and show that the continuum technique provides a valuable tool in reproducing electromechanical transients on modern power systems. Discussions of new protection and control functions are included. A clear understanding of these and related phenomena will lead to innovative and effective countermeasures against unwanted trips by the protection systems, which can lead to system blackouts.
Ph. D.

Thesis (M.S.)--West Virginia University, 2006.
Title from document title page. Document formatted into pages; contains xi, 89 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 88-89).

AbstractOtto Stern became famous for molecular beam physics, matter-wave research and the discovery of the electron spin, with his work guiding several generations of physicists and chemists. Here we discuss how his legacy has inspired the realization of universal interferometers, which prepare matter waves from atomic, molecular, cluster or eventually nanoparticle beams. Such universal interferometers have proven to be sensitive tools for quantum-assisted force measurements, building on Stern’s pioneering work on electric and magnetic deflectometry. The controlled shift and dephasing of interference fringes by external electric, magnetic or optical fields have been used to determine internal properties of a vast class of particles in a unified experimental framework.

Abstract Having already invested in understanding both the equation of state and the hydrodynamic equations, only straightforward algebraic manipulations will be required to derive the wave equation, justify its solutions, calculate the speed of sound in fluids, and derive the expressions for acoustic intensity and the acoustic kinetic and potential energy densities of sound waves. The “machinery” developed to describe waves on strings will be sufficient to describe one-dimensional sound propagation in fluids, even though the waves on the string were transverse and the one-dimensional waves in fluids are longitudinal. These results are combined with the thermal and viscous penetration depths to calculate the frequencies and quality factors in standing wave resonators. The coupling of those resonators to loudspeakers will be examined. The introduction of reciprocal transducers that are linear, passive, and reversible will allow absolute calibration of transducers using only electrical measurements (i.e., currents and voltages) by the reciprocity method, if the acoustic impedance that couples the source and receiver is calculable. Reflection and transmission at junctions between multiple ducts and other networks will be calculated and applied to the design of filters. The behavior of waves propagating through horns will provide useful impedance matching but introduce a low-frequency cut-off.

Abstract PMN-PT is one of promising materials for fabrication ultrasound sensors for flow metering, in that the relative permittivity and the dielectric loss factor are superior to PZT, the popular piezoelectric materials at present. However, there are not many studies focusing on characterizing the PMN-PT ultrasound sensors by acoustic measurements. This study introduces an acoustic bath to accommodate a pair of ultrasound sensors for measuring the relative acoustic sensitivity and the dispersion angle by ultrasound waves. Ultrasound sensors with resonance frequencies from 0.2 MHz to 1 MHz are also tested in terms of the relative permittivity, the dielectric loss factor, the electric impedance, and the electric admittance. From these measurements, it is found that PMN-PT is superior to PZT for increasing the relative acoustic sensitivity of ultrasound sensors. Impedance matching electronic circuits are fabricated by designing capacitance and inductance with the reflectance coefficients. Smith chart is used to calculate the capacitance and the inductance for the PMN-PT ultrasound sensors with resonance frequencies of 0.2 MHz and 0.5 MHz. As a results, it is also found that the impedance matching electronic circuits enhance the electrical admittance by 3.6 ∼ 25 times while keeping the electrical impedance between 50 Ω and 100 Ω.

This paper reports on the measurement of surface waves on a liquid lithium jet and results of the study of the Li target at the International Fusion Materials Irradiation Facility (IFMIF). The characteristics of the surface waves at the nozzle exit and just downstream of it were examined experimentally, since the initial growth of free surface waves exerts a definite influence on surface behavior in the downstream region. Experiments were carried out using the lithium circulation loop at Osaka University, with a focus on the free surface oscillations. These oscillations were measured using an electrocontact probe apparatus, which detects electric contacts between the probe tip and the Li surface. The apparatus was installed 15 mm downstream from the nozzle exit and was scanned along the liquid-depth direction. The contact signal recorded in the experiment was analyzed, and the wave amplitude and frequency of the surface waves were examined.

We describe the effect of Transverse Electric (TE) wave propagation in dielectric micro-spheres for optical resonances based on Whispering Gallery Mode (WGM). TE waves are characterized both theoretically and experimentally by considering the fact that the size parameter (pi times diameter divided by wavelength of light) of the micro-spheres is very large at optical wavelengths. Asymptotic expressions have been developed based on electromagnetic theory derivations at the large size parameter limits. The developed expressions for optical resonance condition of TE wave are very simple and can accurately characterize resonances in dielectric microspheres. The theoretical development is mathematically robust and significantly less complicated than existing approaches based on quantum physics presented in the literature. The theoretical result of size parameter for consecutive morphology dependent resonance (MDR) peaks are validated by experimental data obtained via this study using a tunable semiconductor laser and from the literature. The comparisons are shown to be very accurate for large size parameters. The quality factor of experimental resonance spectra observed in the laboratory is calculated approximately in the order of 104 which is sensitive enough to detect micro or nano level temperature changes in the surrounding medium. The sensitivity of the MDR temperature sensor is wavelength change of 10−9 meter for one degree centigrade change in temperature. This sensor could potentially be used for nano technology, Micro-Electro-Mechanical Systems (MEMS) devices, and biomedical applications.

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame-acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the detection of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For validation of this new method an experimental setup was developed capable of generating well defined entropy waves. As a reference temperature measurement technique a hot-wire anemometer is employed. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are detected for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adheres well to a Strouhal number scaling for different mean flow velocities. In addition, simultaneously to the entropy wave measurement, the equivalence ratio fluctuations in the mixing tube are detected using the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique.

Abstract We investigate electrokinetic behavior of colloidal particles in the vicinity of a solid-liquid interface. Colloidal liquids are expected to be used as thermal transport media for heat transfer applications such as nanofluids and phase change emulsions. They contain submicrometer-sized particles in liquid, and electrokinetic behavior of the solute particles should play an important role in the heat transfer between solid-liquid interfacing boundaries. However, experimental investigation of the behavior remains difficult due to the required spatial resolution beyond diffraction limit. We developed a measurement system based on laser Doppler principle using an interference of evanescent waves generated at total internal reflections of incident lasers at a solid wall. The system was developed for the measurement of velocities of colloidal particles at an interfacing boundary of colloidal liquid and a solid wall. The system has a unique advantage of a high spatial resolution in the direction perpendicular to the boundary due to the short penetration depth of an evanescent wave in the range of a few hundred nanometers. The principle and performance of the measurement system were investigated using a scanning probe in the measurement volume. We experimentally confirmed the validity of the measurement and characterized the uncertainty of velocity measurement. The system was further applied in a series of measurements of alumina particles dispersed in water in a square-shaped cell under induced electric fields. The measured velocities are proportional to the field strengths at different particle concentrations. The linear relationship is consistent with theoretical predictions, which demonstrates the feasibility of the system for the measurement of velocities of colloidal particles in the near wall region.

Entropy noise affects thermoacoustic stability in lean pre-mixed gas turbine combustion chambers. It is defined as acoustic noise that is emitted at the first turbine stage due to the acceleration of entropy waves that are advected from the reaction zone in the combustor to the turbine inlet. These non-isentropic temperature waves are caused by equivalence ratio fluctuations which are inherently present in a technically premixed combustion system. To experimentally study the generation and transport of entropy waves, an estimation of the spatial distribution of the entropy spots is highly valuable as it allows the accurate determination of the cross-section averaged entropy, which is the relevant quantity for the formation mechanism of entropy noise at the turbine stage. In this work, a time-of-flight based temperature measurement method is applied to a circular combustion test rig equipped with a premixed swirl-stabilized combustor. Downstream of the burner, an electric spark discharge is employed to generate a narrow acoustic pulse which is detected with a circumferentially arranged microphone array. The measured time of flight of the acoustic signal corresponds to the line-integrated inverse of the speed of sound between the acoustic source and each microphone. By modulating a share of the injected gaseous fuel, equivalence ratio fluctuations are generated upstream of the reaction zone and consequently entropy spots are advected through the axial measurement plane. The spark discharge is triggered at distinct phase angles of the entropy oscillation, thus allowing a time resolved-analysis of the thermo-acoustic phenomenon. Estimating the spatial temperature distribution from the measured line integrated inverse speed of sounds requires tomographic reconstruction. A Tikhonov regularized Onion Peeling is employed to deduce radial temperature profiles. To increase the number of independent data, the spark location is radially traversed, which enhances the resolution of the reconstructed temperature field. A phantom study is conducted, which allows the assessment of the capabilities of the reconstruction algorithm. By means of the reconstructed radial entropy field, spatially resolved entropy waves are measured and their amplitudes and phases are extracted. The characteristics of the entropy waves measured in this way correspond well to former studies.

1.1 Macroeconomic summary Economic recovery has consistently outperformed the technical staff’s expectations following a steep decline in activity in the second quarter of 2020. At the same time, total and core inflation rates have fallen and remain at low levels, suggesting that a significant element of the reactivation of Colombia’s economy has been related to recovery in potential GDP. This would support the technical staff’s diagnosis of weak aggregate demand and ample excess capacity. The most recently available data on 2020 growth suggests a contraction in economic activity of 6.8%, lower than estimates from January’s Monetary Policy Report (-7.2%). High-frequency indicators suggest that economic performance was significantly more dynamic than expected in January, despite mobility restrictions and quarantine measures. This has also come amid declines in total and core inflation, the latter of which was below January projections if controlling for certain relative price changes. This suggests that the unexpected strength of recent growth contains elements of demand, and that excess capacity, while significant, could be lower than previously estimated. Nevertheless, uncertainty over the measurement of excess capacity continues to be unusually high and marked both by variations in the way different economic sectors and spending components have been affected by the pandemic, and by uneven price behavior. The size of excess capacity, and in particular the evolution of the pandemic in forthcoming quarters, constitute substantial risks to the macroeconomic forecast presented in this report. Despite the unexpected strength of the recovery, the technical staff continues to project ample excess capacity that is expected to remain on the forecast horizon, alongside core inflation that will likely remain below the target. Domestic demand remains below 2019 levels amid unusually significant uncertainty over the size of excess capacity in the economy. High national unemployment (14.6% for February 2021) reflects a loose labor market, while observed total and core inflation continue to be below 2%. Inflationary pressures from the exchange rate are expected to continue to be low, with relatively little pass-through on inflation. This would be compatible with a negative output gap. Excess productive capacity and the expectation of core inflation below the 3% target on the forecast horizon provide a basis for an expansive monetary policy posture. The technical staff’s assessment of certain shocks and their expected effects on the economy, as well as the presence of several sources of uncertainty and related assumptions about their potential macroeconomic impacts, remain a feature of this report. The coronavirus pandemic, in particular, continues to affect the public health environment, and the reopening of Colombia’s economy remains incomplete. The technical staff’s assessment is that the COVID-19 shock has affected both aggregate demand and supply, but that the impact on demand has been deeper and more persistent. Given this persistence, the central forecast accounts for a gradual tightening of the output gap in the absence of new waves of contagion, and as vaccination campaigns progress. The central forecast continues to include an expected increase of total and core inflation rates in the second quarter of 2021, alongside the lapse of the temporary price relief measures put in place in 2020. Additional COVID-19 outbreaks (of uncertain duration and intensity) represent a significant risk factor that could affect these projections. Additionally, the forecast continues to include an upward trend in sovereign risk premiums, reflected by higher levels of public debt that in the wake of the pandemic are likely to persist on the forecast horizon, even in the context of a fiscal adjustment. At the same time, the projection accounts for the shortterm effects on private domestic demand from a fiscal adjustment along the lines of the one currently being proposed by the national government. This would be compatible with a gradual recovery of private domestic demand in 2022. The size and characteristics of the fiscal adjustment that is ultimately implemented, as well as the corresponding market response, represent another source of forecast uncertainty. Newly available information offers evidence of the potential for significant changes to the macroeconomic scenario, though without altering the general diagnosis described above. The most recent data on inflation, growth, fiscal policy, and international financial conditions suggests a more dynamic economy than previously expected. However, a third wave of the pandemic has delayed the re-opening of Colombia’s economy and brought with it a deceleration in economic activity. Detailed descriptions of these considerations and subsequent changes to the macroeconomic forecast are presented below. The expected annual decline in GDP (-0.3%) in the first quarter of 2021 appears to have been less pronounced than projected in January (-4.8%). Partial closures in January to address a second wave of COVID-19 appear to have had a less significant negative impact on the economy than previously estimated. This is reflected in figures related to mobility, energy demand, industry and retail sales, foreign trade, commercial transactions from selected banks, and the national statistics agency’s (DANE) economic tracking indicator (ISE). Output is now expected to have declined annually in the first quarter by 0.3%. Private consumption likely continued to recover, registering levels somewhat above those from the previous year, while public consumption likely increased significantly. While a recovery in investment in both housing and in other buildings and structures is expected, overall investment levels in this case likely continued to be low, and gross fixed capital formation is expected to continue to show significant annual declines. Imports likely recovered to again outpace exports, though both are expected to register significant annual declines. Economic activity that outpaced projections, an increase in oil prices and other export products, and an expected increase in public spending this year account for the upward revision to the 2021 growth forecast (from 4.6% with a range between 2% and 6% in January, to 6.0% with a range between 3% and 7% in April). As a result, the output gap is expected to be smaller and to tighten more rapidly than projected in the previous report, though it is still expected to remain in negative territory on the forecast horizon. Wide forecast intervals reflect the fact that the future evolution of the COVID-19 pandemic remains a significant source of uncertainty on these projections. The delay in the recovery of economic activity as a result of the resurgence of COVID-19 in the first quarter appears to have been less significant than projected in the January report. The central forecast scenario expects this improved performance to continue in 2021 alongside increased consumer and business confidence. Low real interest rates and an active credit supply would also support this dynamic, and the overall conditions would be expected to spur a recovery in consumption and investment. Increased growth in public spending and public works based on the national government’s spending plan (Plan Financiero del Gobierno) are other factors to consider. Additionally, an expected recovery in global demand and higher projected prices for oil and coffee would further contribute to improved external revenues and would favor investment, in particular in the oil sector. Given the above, the technical staff’s 2021 growth forecast has been revised upward from 4.6% in January (range from 2% to 6%) to 6.0% in April (range from 3% to 7%). These projections account for the potential for the third wave of COVID-19 to have a larger and more persistent effect on the economy than the previous wave, while also supposing that there will not be any additional significant waves of the pandemic and that mobility restrictions will be relaxed as a result. Economic growth in 2022 is expected to be 3%, with a range between 1% and 5%. This figure would be lower than projected in the January report (3.6% with a range between 2% and 6%), due to a higher base of comparison given the upward revision to expected GDP in 2021. This forecast also takes into account the likely effects on private demand of a fiscal adjustment of the size currently being proposed by the national government, and which would come into effect in 2022. Excess in productive capacity is now expected to be lower than estimated in January but continues to be significant and affected by high levels of uncertainty, as reflected in the wide forecast intervals. The possibility of new waves of the virus (of uncertain intensity and duration) represents a significant downward risk to projected GDP growth, and is signaled by the lower limits of the ranges provided in this report. Inflation (1.51%) and inflation excluding food and regulated items (0.94%) declined in March compared to December, continuing below the 3% target. The decline in inflation in this period was below projections, explained in large part by unanticipated increases in the costs of certain foods (3.92%) and regulated items (1.52%). An increase in international food and shipping prices, increased foreign demand for beef, and specific upward pressures on perishable food supplies appear to explain a lower-than-expected deceleration in the consumer price index (CPI) for foods. An unexpected increase in regulated items prices came amid unanticipated increases in international fuel prices, on some utilities rates, and for regulated education prices. The decline in annual inflation excluding food and regulated items between December and March was in line with projections from January, though this included downward pressure from a significant reduction in telecommunications rates due to the imminent entry of a new operator. When controlling for the effects of this relative price change, inflation excluding food and regulated items exceeds levels forecast in the previous report. Within this indicator of core inflation, the CPI for goods (1.05%) accelerated due to a reversion of the effects of the VAT-free day in November, which was largely accounted for in February, and possibly by the transmission of a recent depreciation of the peso on domestic prices for certain items (electric and household appliances). For their part, services prices decelerated and showed the lowest rate of annual growth (0.89%) among the large consumer baskets in the CPI. Within the services basket, the annual change in rental prices continued to decline, while those services that continue to experience the most significant restrictions on returning to normal operations (tourism, cinemas, nightlife, etc.) continued to register significant price declines. As previously mentioned, telephone rates also fell significantly due to increased competition in the market. Total inflation is expected to continue to be affected by ample excesses in productive capacity for the remainder of 2021 and 2022, though less so than projected in January. As a result, convergence to the inflation target is now expected to be somewhat faster than estimated in the previous report, assuming the absence of significant additional outbreaks of COVID-19. The technical staff’s year-end inflation projections for 2021 and 2022 have increased, suggesting figures around 3% due largely to variation in food and regulated items prices. The projection for inflation excluding food and regulated items also increased, but remains below 3%. Price relief measures on indirect taxes implemented in 2020 are expected to lapse in the second quarter of 2021, generating a one-off effect on prices and temporarily affecting inflation excluding food and regulated items. However, indexation to low levels of past inflation, weak demand, and ample excess productive capacity are expected to keep core inflation below the target, near 2.3% at the end of 2021 (previously 2.1%). The reversion in 2021 of the effects of some price relief measures on utility rates from 2020 should lead to an increase in the CPI for regulated items in the second half of this year. Annual price changes are now expected to be higher than estimated in the January report due to an increased expected path for fuel prices and unanticipated increases in regulated education prices. The projection for the CPI for foods has increased compared to the previous report, taking into account certain factors that were not anticipated in January (a less favorable agricultural cycle, increased pressure from international prices, and transport costs). Given the above, year-end annual inflation for 2021 and 2022 is now expected to be 3% and 2.8%, respectively, which would be above projections from January (2.3% and 2,7%). For its part, expected inflation based on analyst surveys suggests year-end inflation in 2021 and 2022 of 2.8% and 3.1%, respectively. There remains significant uncertainty surrounding the inflation forecasts included in this report due to several factors: 1) the evolution of the pandemic; 2) the difficulty in evaluating the size and persistence of excess productive capacity; 3) the timing and manner in which price relief measures will lapse; and 4) the future behavior of food prices. Projected 2021 growth in foreign demand (4.4% to 5.2%) and the supposed average oil price (USD 53 to USD 61 per Brent benchmark barrel) were both revised upward. An increase in long-term international interest rates has been reflected in a depreciation of the peso and could result in relatively tighter external financial conditions for emerging market economies, including Colombia. Average growth among Colombia’s trade partners was greater than expected in the fourth quarter of 2020. This, together with a sizable fiscal stimulus approved in the United States and the onset of a massive global vaccination campaign, largely explains the projected increase in foreign demand growth in 2021. The resilience of the goods market in the face of global crisis and an expected normalization in international trade are additional factors. These considerations and the expected continuation of a gradual reduction of mobility restrictions abroad suggest that Colombia’s trade partners could grow on average by 5.2% in 2021 and around 3.4% in 2022. The improved prospects for global economic growth have led to an increase in current and expected oil prices. Production interruptions due to a heavy winter, reduced inventories, and increased supply restrictions instituted by producing countries have also contributed to the increase. Meanwhile, market forecasts and recent Federal Reserve pronouncements suggest that the benchmark interest rate in the U.S. will remain stable for the next two years. Nevertheless, a significant increase in public spending in the country has fostered expectations for greater growth and inflation, as well as increased uncertainty over the moment in which a normalization of monetary policy might begin. This has been reflected in an increase in long-term interest rates. In this context, emerging market economies in the region, including Colombia, have registered increases in sovereign risk premiums and long-term domestic interest rates, and a depreciation of local currencies against the dollar. Recent outbreaks of COVID-19 in several of these economies; limits on vaccine supply and the slow pace of immunization campaigns in some countries; a significant increase in public debt; and tensions between the United States and China, among other factors, all add to a high level of uncertainty surrounding interest rate spreads, external financing conditions, and the future performance of risk premiums. The impact that this environment could have on the exchange rate and on domestic financing conditions represent risks to the macroeconomic and monetary policy forecasts. Domestic financial conditions continue to favor recovery in economic activity. The transmission of reductions to the policy interest rate on credit rates has been significant. The banking portfolio continues to recover amid circumstances that have affected both the supply and demand for loans, and in which some credit risks have materialized. Preferential and ordinary commercial interest rates have fallen to a similar degree as the benchmark interest rate. As is generally the case, this transmission has come at a slower pace for consumer credit rates, and has been further delayed in the case of mortgage rates. Commercial credit levels stabilized above pre-pandemic levels in March, following an increase resulting from significant liquidity requirements for businesses in the second quarter of 2020. The consumer credit portfolio continued to recover and has now surpassed February 2020 levels, though overall growth in the portfolio remains low. At the same time, portfolio projections and default indicators have increased, and credit establishment earnings have come down. Despite this, credit disbursements continue to recover and solvency indicators remain well above regulatory minimums. 1.2 Monetary policy decision In its meetings in March and April the BDBR left the benchmark interest rate unchanged at 1.75%.