Free-field Calibration of the Pressure Sensitivity of Microphones at Frequencies up to 80 kHz

A free-field (FF) substitution method for calibrating the pressure sensitivity of microphones at frequencies up to 80 kHz is demonstrated with both grazing and normal incidence geometries. The substitution-based method, as opposed to a simultaneous method, avoids problems associated with the non-uniformity of the sound field and, as applied here, uses a 1/2 -inch air-condenser pressure microphone as a known reference. Best results were obtained with a centrifugal fan, which is used as a random, broadband sound source. A broadband source minimizes reflection-related interferences that often plague FF measurements. Calibrations were performed on 1/4-inch FF air-condenser, electret, and micro-electromechanical systems (MEMS) microphones in an anechoic chamber. The accuracy of this FF method is estimated by comparing the pressure sensitivity of an air-condenser microphone, as derived from the FF measurement, with that of an electrostatic actuator calibration and is typically 0.3 dB (95% confidence), over the range 2-80 kHz

Measurement and characterization of infrasound from a tornado producing storm

A hail-producing supercell on 11 May 2017 produced a small tornado near Perkins, Oklahoma (35.97, –97.04) at 2013 UTC. Two infrasound microphones with a 59-m separation and a regional Doppler radar station were located 18.7 and 70 km from the tornado, respectively. Elevated infrasound levels were observed starting 7min before the verified tornado. Infrasound data below ~5Hz was contaminated with wind noise, but in the 5–50 Hz band the infrasound was independent of wind speed with a bearing angle that was consistent with the movement of the storm core that produced the tornado. During the tornado, a 75 dB peak formed at ~8.3 Hz, which was 18 dB above pre-tornado levels. This fundamental frequency had overtones (18, 29, 36, and 44 Hz) that were linearly related to mode number. Analysis of a larger period of time associated with two infrasound bursts (the tornado occurred during the first event) shows that the spectral peaks from the tornado were present from 4min before to 40 min after tornadogenesis. This suggests that the same geophysical process(es) was active during this entire window

Skin-Friction Drag Reduction within Turbulent Flows. Skin-Friction Drag Reduction within Turbulent Flows.

In nearly all transportation systems moving in a fluid, skin-friction drag is a major component to the total resistance to motion. Thus reduction of turbulent boundary layer (TBL) skin-friction in external flows is an ongoing research priority. This document describes experimental investigations into two active methods for reducing skin-friction drag (injection of air or polymer solution into a TBL). Direct measurement of TBL skin-friction was the primary diagnostic, but other flow measurements were acquired depending on the specific study (e.g. void fraction, concentration and velocity profiles, fluid rheology, etc.). PDR experiments were conducted at typical laboratory (2.72 cm diameter pipe and 0.9 m long flat plate) and at larger (12.9 m long flat plate) scale to bridge the gap in scale between experimental data (~ 1 m) and real world applications (~100 m). The major insights from these investigations are: (1) initial zone diffusion of polymer scales with the distance from the injector based Reynolds number, non-dimensional volumetric injection flux and injection concentration, (2) intermediate zone diffusion scales with flow and injection conditions (K) and the inner variable scaled roughness height (k+), (3) the percent drag reduction (%DR) equals 80[1 – exp(-0.08 S+)] where S+ is the effective slip when the drag is reduced with polymers or surfactants in channel, pipe or boundary layer flows (4) polymer degradation by chain scission within a TBL is important since at typical flow conditions an order of magnitude reduction in molecular weight is possible and (5) the chain scission can be scaled based on flow conditions. Air injection experiments were a continuation from work reported in Elbing et al. (2008) on a 12.9 m long flat test model. Two drag reduction regimes, bubble drag reduction (BDR) and air layer drag reduction (ALDR) were studied. BDR results showed that the %DR is linearly proportional to the near-wall void fraction. ALDR showed %DR between 80 and 100 and the critical volumetric flux of air required to achieve ALDR scaled with uτ/(νg)1/3 independent of surface condition, background water surface tension and injector design.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/62411/1/belbing_1.pd

Characterization of Bubble Size Distributions within a Bubble Column

The current study experimentally examines bubble size distribution (BSD) within a bubble column and the associated characteristic length scales. Air was injected into a column of water via a single injection tube. The column diameter (63–102 mm), injection tube diameter (0.8–1.6 mm) and superficial gas velocity (1.4–55 mm/s) were varied. Large samples (up to 54,000 bubbles) of bubble sizes measured via 2D imaging were used to produce probability density functions (PDFs). The PDFs were used to identify an alternative length scale termed the most frequent bubble size (dmf) and defined as the peak in the PDF. This length scale as well as the traditional Sauter mean diameter were used to assess the sensitivity of the BSD to gas injection rate, injector tube diameter, injection tube angle and column diameter. The dmf was relatively insensitive to most variation, which indicates these bubbles are produced by the turbulent wakes. In addition, the current work examines higher order statistics (standard deviation, skewness and kurtosis) and notes that there is evidence in support of using these statistics to quantify the influence of specific parameters on the flow-field as well as a potential indicator of regime transitions

Characterization of a Pulsating Drill Bit Blaster

The drill bit blaster (DBB) studied in this paper aims to maximize the drilling rate of penetration (ROP) by using a flow interrupting mechanism to create drilling fluid pulsation. The fluctuating fluid pressure gradient generated during operation of the DBB could lead to more efficient bit cutting efficiency due to substrate depressurization and increased cutting removal efficiency and the vibrations created could reduce the drill string friction allowing a greater weight on bit (WOB) to be achieved. In order to maximize these mechanisms the effect of several different DBB design changes and operating conditions was studied in above ground testing. An analytical model was created to predict the influence of various aspects of the drill bit blaster design, operating conditions and fluid properties on the bit pressure characteristics and compared against experimental results. The results indicate that internal tool design has a significant effect on the pulsation frequency and amplitude, which can be accurately modeled as a function of flowrate and internal geometry. Using this model an optimization study was conducted to determine the sensitivity of the fluid pulsation power on various design and operating conditions. Application of this technology in future designs could allow the bit pressure oscillation frequency and amplitude to be optimized with regard to the lithology of the formations being drilled which could lead to faster, more efficient drilling potentially cutting drilling costs and leading to a larger number of oil and natural gas plays being profitable

Modelling of a Drill Bit Blaster

The drill bit blaster (DBB) studied in this paper aims to maximize drilling rate by creating drilling fluid pulsation, which could lead to lower drill string friction, more efficient removal of cuttings underneath the bit and increased nozzle fluid pressure and velocity; all of which could ultimately lead to more efficient cutting and transport of cuttings while drilling. A testing procedure was established in order to measure the fluid pulsations in above ground testing and to create a repeatable method of testing further designs. An analytical model was created in an attempt to predict and describe the pulsation pressure characteristics as a function of operating conditions and internal geometry and compared against experimental results. The results indicated that internal tool design and operating conditions have a significant effect on the pulsation frequency and amplitude. Application of this technology in future designs could allow the bit pressure oscillation frequency and amplitude to be optimized with regard to the lithology of the formations being drilled. This could lead to faster, more efficient drilling which could cut drilling costs and lead to a larger number of oil and natural gas plays being profitable